Sage Journals: Discover world-class research

Abstract

Breast cancer (BC) is among the most prevalent type of malignancy affecting females worldwide. BC is classified into different types according to the status of the expression of receptors such as estrogen receptor (ER), human epidermal growth factor receptor 2 (HER2), and progesterone receptor (PR). Androgen receptor (AR) appears to be a promising therapeutic target of BC. Binding of 5α-dihydrotestosterone (DHT) to AR controls the expression of microRNA (miRNA) molecules in BC, consequently, affecting protein expression. One of these proteins is the transmembrane glycoprotein cluster of differentiation 44 (CD44). Remarkably, CD44 is a common marker of cancer stem cells in BC. It functions as a co-receptor for a broad diversity of extracellular matrix ligands. Several ligands, primarily hyaluronic acid (HA), can interact with CD44 and mediate its functions. CD44 promotes a variety of functions independently or in cooperation with other cell-surface receptors through activation of varied signaling pathways like Rho GTPases, Ras-MAPK, and PI3K/AKT pathways to regulate cell adhesion, migration, survival, invasion, and epithelial–mesenchymal transition. In this review, we present the relations between AR, miRNA, and CD44 and their roles in BC.

Keywords

Breast cancer CD44 miRNA hyaluronic acid

1. Introduction

Breast cancer (BC) is the most prominent cause of mortality among women in most developed countries [1]. It is deemed to be a greatly heterogeneous group of cancers that develop from varied cell types, each having its particular clinical consequences [2] and is composed of several biologically various entities with discrete pathological features and response to treatment [3,4]. Therefore, the precise grouping of BC into clinically related subtypes is of real importance for therapeutic decision [5]. Conventional Immunohistochemistry (IHC) markers including estrogen receptor (ER), human epidermal growth factor receptor 2 (HER2), and progesterone receptor (PR) are applied to patient prognosis and management [6,7]. The forthcoming of high-throughput schemes of gene expression profiling have revealed that tumor cell response to treatment is not entrenched using anatomical predicted features, rather fundamental molecular features that can be explored using molecular techniques [4,8]. Therefore, during the past 15 years, modulation of BC classifications has been in progress from histopathological typing to the molecular classification [5]. For instance, ER-positive and ER-negative BCs comprise different diseases [9]. Besides, the existence of four essential subtypes of BC including luminal A, luminal B, HER2-enriched, and basal-like has been established [10]. The Cancer Genome Atlas Network (TCGA) has designated a broad profiling for BC at the DNA, RNA, and protein levels [10] whereby the luminal BC subtype can be divided into at least two subgroups, each with a distinctive mRNA profile [11]. Each subtype manifests different prognosis, incidence, treatment response, preferred metastatic sites, and disease-free survival rates [12].

Table 1
Effect of different exon number on CD44v characteristics

Variant exon number Features added to CD44

V1 Not expressed in human CD44 variants.

V2 Gives additional sites for O- and N-glycosylation [122].

V3 - Gives a motif for GAGs addition (as heparan sulfate that confers CD44 the ability to bind to heparin-binding growth factors and chemokines) [122]. - Mediates cell-cell adhesion that was dependent on HA [123]. - Mediates Met receptor activation and binding to its ligands (SF/ HGF) [124]

V4 - Enhances phosphorylation of CD44 cytoplasmic tail [26]

V5 - Enhances malignancy and invasiveness of some tumors [125]. - Enhances phosphorylation of CD44 cytoplasmic tail [26].

V6 - Broadens the ligand specificity to include GAGs other than HA [126]. - Makes CD44 able to form a complex with hepatocyte growth factor (HGF) and its receptor Met [14]. - Enhances phosphorylation of CD44 cytoplasmic tail [26].

V7 - Mediates cell survival by osteopontin [127]. - Enhances phosphorylation of CD44 cytoplasmic tail [26].

V8 - Gives additional sites of O-lycosylation (its extensive O-glycosylation inhibits CD44 binding HA [122].

V9 - Gives additional sites of O-lycosylation (its extensive O-glycosylation inhibits CD44 binding HA [122].

V10 - Gives additional sites of O-lycosylation (its extensive O-glycosylation inhibits CD44 binding HA [122].

Different variant exon numbers and their features add to CD44.

Variant exon number	Features added to CD44
V1	Not expressed in human CD44 variants.
V2	Gives additional sites for O- and N-glycosylation [122].
V3	- Gives a motif for GAGs addition (as heparan sulfate that confers CD44 the ability to bind to heparin-binding growth factors and chemokines) [122]. - Mediates cell-cell adhesion that was dependent on HA [123]. - Mediates Met receptor activation and binding to its ligands (SF/ HGF) [124]
V4	- Enhances phosphorylation of CD44 cytoplasmic tail [26]
V5	- Enhances malignancy and invasiveness of some tumors [125]. - Enhances phosphorylation of CD44 cytoplasmic tail [26].
V6	- Broadens the ligand specificity to include GAGs other than HA [126]. - Makes CD44 able to form a complex with hepatocyte growth factor (HGF) and its receptor Met [14]. - Enhances phosphorylation of CD44 cytoplasmic tail [26].
V7	- Mediates cell survival by osteopontin [127]. - Enhances phosphorylation of CD44 cytoplasmic tail [26].
V8	- Gives additional sites of O-lycosylation (its extensive O-glycosylation inhibits CD44 binding HA [122].
V9	- Gives additional sites of O-lycosylation (its extensive O-glycosylation inhibits CD44 binding HA [122].
V10	- Gives additional sites of O-lycosylation (its extensive O-glycosylation inhibits CD44 binding HA [122].

CD44 is an integral membranous protein with a multi-structure and multi-functions. It been found to play a role in tethering cells to the extracellular matrix (ECM) via mediating complex formation between extracellular components and intracellular cytoskeletal elements. This role is no less important than its function in cellular signaling and cell-cell communication [13]. It senses the changes in ECM and cell’s microenvironment that in turn influences many cell aspects, for example and not as a limitation: cell survival, growth, differentiation and motility [14].

2. CD44 structure

As mentioned previously, CD44 consists of three domains: the extracellular domain (ECD), transmembrane domain (TMD), and intracellular domain (ICD) [15]. The ECD includes the amino terminal globular domain where HA, glycosaminoglycans (GAGs) and other ligands bind such as hyaluronic acid (HA) [16]. The TMD is sponsible for CD44 oligomerization [17]. Palmitoylation can occur reversibly for that domain. Several studies reported that this modification could impair signal transduction in lymphocytes and promote CD44 association with ankyrin and ezrin, radixin, and moesin (ERM) proteins [18,19]. The cytoplasmic domain, also called the ICD, has two configurations, short- and long-tail configurations [20]. This domain is essential in many cellular aspects including subcellular localization of CD44 into the basolateral side of polarized epithelia [21] and leading edge of lamellipodia during cell migration, signaling, cytoskeletal organization and gene transcription [22]. A number of factors, such as mitogens and membrane-type 1 matrix Matrix metallopeptidases (MT-1 MMP), promote the cleavage of cytoplasmic tail [20]. Translocation of that cleaved tail into the nucleus influences the transcription of many genes including CD44 itself in a positive feedback pattern [23]. The cytoplasmic domain displays a divergent role in signaling. It alters CD44 affinity to bind ligands “inside-out signaling” and also mediates downstream signaling upon CD44 binding ligands “outside-in signaling” [19].

3. CD44 isoforms

The mechanisms that account for heterogeneity of CD44 are alternative splicing, N-glycosylation, O-glycosylation and palmitoylation [24,25]. CD44 gene is composed of 20 exons. Exons (1–5), (16–18) and (20) produce CD44s (the long-tail configuration), which is the smallest compared to other isoforms. Exons 6–15, designated as (v1-10), are variable with various combinations produced by alternative splicing The addition of one or more of the exons (v2-v10) to CD44s generates other CD44 isoforms with larger molecular mass (up to 381 amino acids long). Take into consideration that expressing v1 exon would yield a truncated protein due to presence of a stop codon and therefore, it is not expressed in human CD44v [26]. Table 1 displays the different features of CD44v depending on its exon number.

Whether post translation modification or the alternative splicing affects the binding affinity of CD44 or increases its affinity for its ligands is not clear. However, what is clear is that both processes are dependent on cell type, cell growth condition, environment, and oncogenic pathways such as the Ras-MAPK cascade [24,27]. Though, CD44s is expressed remarkably in most tissues, whereas CD44 variants are expressed mainly in tumor cells [15,24]. On the other hand, other studies have reported that CD44 variants are expressed in normal and the tumor cells at different levels suggesting that they are also necessary for normal cellular functions [28,29]. CD44s and its variants take part in cancer invasiveness. For instance, CD44v8-10 and CD44v9 are expressed in high amounts in pancreatic and colorectal cancers respectively [20]. CD44v6 and soluble (cleaved) CD44 were seen in prostate cancer cell derived from bone metastasis. Switching between CD44 isoforms, as observed in prostate cancerous cells when CD44s is replaced by CD44v6, promotes adhesion and survival [30]. Comparing benign with the malignant forms of prostate cancer, CD44v5 and CD44s are highly expressed in these forms, respectively [31]. A few studies showed that CD44v promotes some cancer types progression and metastasis [32,33], while others indicated that they are tumor suppressors in other types of cancers [34,35]. This discrepancy of findings may stem from applying different means for detecting CD44 (e.g. polymerase chain reaction or IHC) or using different primary anti-CD44 antibodies that may not recognize the CD44v epitopes [29]. For more detailed information regarding some cancer type’s relation with CD44v, see Table 2.

Table 2
Contribution of CD44v in different type of cancer

Cancer type CD44 isoform Isoform contribution

Colorectal Cancer CD44v3 - Activates invasion and resistance to apoptosis.

CD44v6 - Associated with tumor metastasis. - Essential for their migration and generation of metastatic tumors [30,128,129].

CD44v8–10 - Has a role in metastasis [130].

CD44v2 - Is overexpressed & upregulated in CRC. - Significantly worse prognosis [131].

Lung cancer CD44v5 CD44v6 - Promotes squamous cell carcinoma metastasis In [132,133].

CD44v7 CD44v3 - Are expressed in non-small cell lung carcinomas [134,135].

CD44v5 CD44v6 - CD44v6 is associated with lymph node metastasis and poor survival [136,137].

Breast Cancer CD44v3, CD44v5, and CD44v6 - Promotes metastasis [138].

CD44v5, CD44v6, and CD44v7-8 - Are associated with axillary lymph node metastasis [139].

CD44v3 - Enhances metastases to the lymph nodes [140].

CD44v2–10 CD44v3–10 - High expression is correlated to positive steroid receptor status, low proliferation, and luminal A subtype [114].

CD44v8-10 - Is correlated to positive EGFR, negative/low HER2 status, and basal-like subtype [114].

Leukemia CD44v3, CD44v6, CD44v9, and CD44v10 - Are expressed in bone marrow progenitors [141].

CD44v3, CD44v6, and CD44v9 - Are expressed in lymphocytes and monocytes following stimulation with inflammatory cytokines [141].

Non-Hodgkin’s lymphoma and myeloma CD44v6, CD44v9, and CD44v10 - Are overexpressed and associated with poor prognosis [142].

Acute myeloid leukemia CD44v6 - Is associated with poor prognosis and shorter survival rate [142].

Pancreatic Cancer CD44v6 - Is expressed in primary and metastatic human pancreatic CA [143].

CD44v5 - Is strongly expressed on adenocarcinomas [144].

CD44v6 And CD44v2 - Are expressed on tumor cells and correlated with decreased overall survival [145].

CD44v6, CD44v9 - Promotes metastasis - risk factor affecting overall survival [146].

CD44v9 - Their expression is upregulated [147].

Head and Neck Cancer CD44v3 - Leads to a significant increase in migration [148].

CD44v3, CD44v6, and CD44v10 - Are associated with lymph node metastasis, perineural invasion, decreased survival, distant metastasis and radiation failure [149,150].

CD44v6 - Is correlated with tumor invasion, lymph node metastasis, and shorter survival [151].

Prostate cancer CD44v6 - Promotes cell survival [30]

Contribution of CD44v in different type of cancer.

Cancer type	CD44 isoform	Isoform contribution
Colorectal Cancer	CD44v3	- Activates invasion and resistance to apoptosis.
	CD44v6	- Associated with tumor metastasis. - Essential for their migration and generation of metastatic tumors [30,128,129].
	CD44v8–10	- Has a role in metastasis [130].
	CD44v2	- Is overexpressed & upregulated in CRC. - Significantly worse prognosis [131].
Lung cancer	CD44v5 CD44v6	- Promotes squamous cell carcinoma metastasis In [132,133].
	CD44v7 CD44v3	- Are expressed in non-small cell lung carcinomas [134,135].
	CD44v5 CD44v6	- CD44v6 is associated with lymph node metastasis and poor survival [136,137].
Breast Cancer	CD44v3, CD44v5, and CD44v6	- Promotes metastasis [138].
	CD44v5, CD44v6, and CD44v7-8	- Are associated with axillary lymph node metastasis [139].
	CD44v3	- Enhances metastases to the lymph nodes [140].
	CD44v2–10 CD44v3–10	- High expression is correlated to positive steroid receptor status, low proliferation, and luminal A subtype [114].
	CD44v8-10	- Is correlated to positive EGFR, negative/low HER2 status, and basal-like subtype [114].
Leukemia	CD44v3, CD44v6, CD44v9, and CD44v10	- Are expressed in bone marrow progenitors [141].
	CD44v3, CD44v6, and CD44v9	- Are expressed in lymphocytes and monocytes following stimulation with inflammatory cytokines [141].
Non-Hodgkin’s lymphoma and myeloma	CD44v6, CD44v9, and CD44v10	- Are overexpressed and associated with poor prognosis [142].
Acute myeloid leukemia	CD44v6	- Is associated with poor prognosis and shorter survival rate [142].
Pancreatic Cancer	CD44v6	- Is expressed in primary and metastatic human pancreatic CA [143].
	CD44v5	- Is strongly expressed on adenocarcinomas [144].
	CD44v6 And CD44v2	- Are expressed on tumor cells and correlated with decreased overall survival [145].
	CD44v6, CD44v9	- Promotes metastasis - risk factor affecting overall survival [146].
	CD44v9	- Their expression is upregulated [147].
Head and Neck Cancer	CD44v3	- Leads to a significant increase in migration [148].
	CD44v3, CD44v6, and CD44v10	- Are associated with lymph node metastasis, perineural invasion, decreased survival, distant metastasis and radiation failure [149,150].
	CD44v6	- Is correlated with tumor invasion, lymph node metastasis, and shorter survival [151].
Prostate cancer	CD44v6	- Promotes cell survival [30]

4. CD44 ligands

Several ligands can interact with CD44 and mediate its functions. Osteopontin, collagens, growth factors, fibronectin and MMP can bind to CD44, although HA is the principal one [20].

4.1. HA

HA is one of the mucopolysaccharides or GAGs found mainly in the ECM, pericellular matrix, and can be found intracellularly [36]. It is an unbranched, non-sulfated, anionic heteropolysaccharide with a molecular weight up to 10,000 KDa [24]. HA is composed of a repeated units of disaccharide; β-D-glucuronic acid and D-N-acetylglucosamine and is abundant in soft connective, epithelial and neural tissues [36]. It is synthesized by an integral membrane enzyme called hyaluronic acid synthase (HAS) followed by a direct release into the ECM [37]. Many cells have the ability to synthesize HA, but mesenchymal cells are the main ones [38]. Using CD44-nul COS-7 cells transfected with human CD44s plasmids, cells gained HA-binding ability and were able to internalize a conjugated HA [39,40]. This experiment strengthens the idea that HA turnover and internalization correlate with CD44 expression level. Researchers exploit that notion in their attempt to improve cancer treatment by conjugating drugs to HA to improve its delivery and internalization into cancerous cells that have high expression of CD44 [41].

The enzymes that are involved in HA degradation are hyaluronidases (HYALs), β-d-glucuronidase, and β-N-acetyl-hexosaminidase [37]. These enzymes are found intracellularly and in the serum. HYALs cleave high molecular weight HA (HMW) into smaller lower molecular weight (LMW) oligosaccharides while β-d-glucuronidase and β-N-acetylhexosaminidase additionally degrade the oligosaccharide fragments [36]. Some studies defined HMW HA as HA molecule with a molecular mass >500 KDa. Human HYALs are encoded by six genes: hyall1, hyal2, and hyal3 [37]. HYAL1 and HYAL2 are the main hyaluronidases in most tissues; HYAL2 is responsible for cleaving HMW HA that is bound to CD44 receptor. The resulting HA fragments are further degraded by HYAL1 generating HA oligosaccharides [42].

HA can perform many functions. It binds to a number of keratin sulfate and chondroitin sulfate molecules and from proteoglycan aggregates in the ECM. If it is not linking with other molecules, it will bind to water and maintains tissue hydration and osmotic balance [36]. HA contributes in many other physiological and pathological cellular processes such as cell adhesion, migration, cancer progression and inflammation [36,38]. CD44 can bind HA and mediates a host of cellular functions [36]. Furthermore, most of HA functions in most tissues are related to CD44-HA interactions [36]. Based on CD44 affinity to HA, CD44 can be classified into three forms: the low affinity form that is expressed in normal cells [43], the inflammation-induced high affinity form, and the constitutive high affinity form that is expressed all the time in cancerous cells [29,44].

Table 3
Functions of HMW-HA and LMW-HA

LMW-HA roles HMW-HA roles

Reduces CD44 clustering [24,45]. Induces CD44 clustering [24,45].

Provokes an inflammatory response and stimulates expression of genes required for inflammation [48,49]. Has anti-inflammatory role and supports tissue integrity [48].

Induces wound site inflammation and scar formation [38]. Induces wound healing with minimal inflammation and scar formation [38].

Induces tumor regression [53]. Inhibits tumor growth in schwannomas [47].

Interrupts HA-CD44 interaction in breast CA → Inhibits cell growth, motility, and invasion [55]. Bridges high number of receptors over large cell area [36].

Enhances CD44 break down and angiogenesis [27]. Inhibits neovascularization [152].

Suppresses anti-apoptotic signaling pathways in cancer cells [50,51]. Stimulates Ras-CD44 in ovarian tumor cell [46].

Inhibits HA synthesis [52]. Limits viral infections of further neighboring T- cells [48].

Inhibits multidrug resistances transporter [50,51].

Disassembles CD44-multidrug transporter complex and tyrosine kinase receptor complexes [52].

Different functions of HMW-HA and LMW-HA.

LMW-HA roles	HMW-HA roles
Reduces CD44 clustering [24,45].	Induces CD44 clustering [24,45].
Provokes an inflammatory response and stimulates expression of genes required for inflammation [48,49].	Has anti-inflammatory role and supports tissue integrity [48].
Induces wound site inflammation and scar formation [38].	Induces wound healing with minimal inflammation and scar formation [38].
Induces tumor regression [53].	Inhibits tumor growth in schwannomas [47].
Interrupts HA-CD44 interaction in breast CA → Inhibits cell growth, motility, and invasion [55].	Bridges high number of receptors over large cell area [36].
Enhances CD44 break down and angiogenesis [27].	Inhibits neovascularization [152].
Suppresses anti-apoptotic signaling pathways in cancer cells [50,51].	Stimulates Ras-CD44 in ovarian tumor cell [46].
Inhibits HA synthesis [52].	Limits viral infections of further neighboring T- cells [48].
Inhibits multidrug resistances transporter [50,51].
Disassembles CD44-multidrug transporter complex and tyrosine kinase receptor complexes [52].

In addition, several studies have reported that many of HA effects are related to its molecular weight. HMW-HA provokes distinct effects compared to those mediated by LMW-HA on the same cell. For example, LMW-HA reduces CD44 clustering and increases cell adhesion, while HMW-HA induces CD44 clustering [24,45]. It is the vast length of HMW-HA that enables it to bind and bridge higher number of receptors over large cell area [36]. HMW-HA stimulates Ras-CD44 in ovarian tumor cell [46], inhibits tumor growth in schwannomas [47] as well as inhibits neovascularization in breast cancerous cells [20]. Furthermore, HMW-HA fragments have anti-inflammatory role [48] and support tissue integrity [36,48]. In contrast, LMW-HA stimulates expression of genes required for inflammation [49]. In cancer cells, LMW-HA suppresses anti-apoptotic signaling pathways and inhibits the activity of transporters that enhances multidrug resistances to some chemotherapeutic agents [50,51], inhibits HA synthesis [52] and induces tumor regression [53]. Moreover, LMW-HA leads to the “degradation of CD44-multidrug transporter complex and receptor tyrosine kinase (TRK) complexes, internalization of these disassembled structures, and sapping of their role” [52]. Furthermore, LMW-HA blocks dissemination of an ovarian cancer cells who are induced artificially to metastasize [54]. Disaccharides of HA compared to HMW-HA and other HA oligosaccharides fragments/length shows the ability to inhibit cell growth, motility, and invasion significantly in BC via disturbance of the interaction of endogenous HA-CD44 complex [55]. LMW-HA enhances CD44 break down and angiogenesis and attenuates cell adhesion in breast cancer [20]. Note that studies that discussed LMW- and HMW-HA cellular effects did not define the type of receptor that is involved in their functions [24]. Table 3 summarizes functions of both HMW-HA and LMW-HA.

5. Role of CD44 and HA in cancer signaling

CD44 promotes a variety of functions independently or in cooperation with various cell-surface receptors through triggering a number of signaling pathways via Rho GTPases, Ras-MAPK, and PI3K/AKT among others [24,56]. CD44 functions as a growth or arrest sensor according to signals from the microenvironment that can influence cancer progression [57]. The protein directly intercedes signal transduction pathways through triggering of LMW-HA when bound to CD44. It also recruits signaling mediators to the cytoplasmic tail of CD44 that controls the activity of regulatory signaling molecules such as Tiam1, p115, Rac1, Rho GEFs, Rho-associated protein kinase, and cSrc [27]. Interactions with these signaling proteins lead to induction of the PI3K pathway that activates a number of cellular behavior including cell survival and invasion [58]. In addition, CD44 stimulates RhoA independently of HA binding, which triggers CD44 involvement with ankyrin resulting in creation of membrane projections and stimulation of migration [59]. Furthermore, CD44 mediates actin cytoskeleton remodeling and invasion by interaction with ERM protein complex. The latter complex induces the re-formation of the actin cytoskeleton and facilitates cell adhesion cell motility [60]. The ERM proteins compete with another protein known as merlin, which acts as a tumor suppressor. Through binding to CD44 cytoplasmic tail, merlin can either facilitate or inhibit cell growth and motility. In addition to HMW-HA, merlin combines with CD44 shifting ERM and, subsequently, suppressing Ras pathway [61]. Activation of PI3K leads to phosphorylation and deactivation of merlin by p21-associated kinase (Pak2), which obstructs merlin binding to CD44. As a result, ankyrin and ERM proteins become free to bind CD44 cytoplasmic tail to the actin cytoskeleton enhancing cell invasion [62]. Several studies revealed that collagen-embedded HMW-HA can hinder the induction of epidermal growth factor receptor (EGFR) and inhibit filopodia formation in MDA-MB-231 cells grown on collagen [63]. Merlin attaches to CD44 and prevents ERM, N-Wasp, and Grb2 complexing, which leads to inactivation of Ras pathway and suppression of cell invasion, motility, and growth [27]. Alteration of CD44-mediated biology is a result of the expression of differentially spliced isoforms; alternative expression has been proposed to be associated with amplified metastatic behavior [27,64]. Furthermore, CD44 inhibits cancer progression, whereby when it attaches to merlin, it functions as a growth/arrest sensor in relation to signals from the microenvironment and has a function in contact repression [65].

6. AR

AR is a member of the steroid hormone nuclear receptors family that also includes ER and PR [66–68]. Although the main functions of androgens and their receptor are associated with male sexual differentiation, AR and its ligands, testosterone and its more potent product, DHT, are critical in the development of the mammary gland. Androgens effect various sites throughout the body: the hypothalamus and amygdala in the brain, breast, bone, skin, adipose tissues, skeletal muscles, vascular and genital tissues, and in women ovaries and extra-gonadal tissues in which androgens are the essential precursors for estrogen biosynthesis [69,70]. Thus, androgens influence sexual desire and function, mineral density in bones, muscle mass and strength, the distribution of adipose tissue, energy, mood and psychological well-being as well [71].

7. AR expression in BC

AR expression has been identified in 60–85% of breast tumors [72]. However, the expression of AR is different among the different subtypes. Approximately 70 to 90% of ER-positive cancers are AR-positive as well [73,74]. Indeed, there is a significant association between the expression of both ER and AR. In addition, approximately 60% of ERBB2-positive breast cancers overexpress AR [73,75]. On the other hand, the prevalence of AR expression in triple-negative BC (TNBC) is less frequently reported, ranging from 13 to 65% [76]. This variability may be due to the differences in the techniques or criteria used to define AR positivity [76,77].

8. AR as a therapeutic target

Historically, androgens such as fluoxymesterone, testolactone, and calusterone were used for the treatment of advanced breast cancer resulting in about 18–39% clinical responses [78]. However, the undesirable masculinity side effects of these agents have limited their routine use in the treatment of breast cancer especially in the advent of newer, less toxic endocrine agents. There is a renewed interest in the use of AR-targeted agents in the treatment of breast cancer given the improvement in treating prostate cancer using such agents and the significance of AR signaling pathway in breast cancer. This is true particularly for TNBC. The fact that AR-positive TNBCs have preserved androgenic signaling suggests that AR can be used as a possible therapeutic target similar to ER-positive breast cancer targeting [79,80]. However, the use of AR as a potential therapeutic target in breast cancer has yet to be established due to the difficulties in both the identification of the patients who might be benefit from AR-targeted therapies and the development of the right combination of therapeutic agents based on AR-targeted therapies [73,75,81].

9. Androgen regulation of miRNA

miRNAs are short, single-stranded, non-coding RNA molecules of 20–25 nucleotides that are widely conserved among species [82]. They regulate cell function via negatively targeting the stability of mRNAs and/or their use in protein synthesis. Interestingly, a correlation has been found in the expression of miRNAs in androgen-dependent and independent prostate cancers, in addition to benign and malignant prostate tissues [83]. In addition, miRNAs have been found to regulate the long AR 3’untranslated region (UTR) [84], suggesting a functional regulation by AR. This is not surprising given that primary function of AR as a transcription regulatory protein.

More than 50% of all human gene translation is thought to be regulated by miRNA [85]. This extra level of gene regulation is involved in the cell signaling pathways in both normal and tumor tissues [86,87]. Moreover, each miRNA can regulate numerous target genes, and, vice versa, the same target gene can also be regulated by several types of miRNAs, creating a complex network [88–90]. It is of note that miRNA is not always associated with inhibitory or down regulatory effects. In rare circumstances, depending on the cell cycle and protein co-factors, a miRNA can activate mRNA translation and, hence, increase protein levels [91]. The inherent complexity of this regulatory system allows miRNAs to control the global activity of the cell, including cell differentiation, proliferation, stress response, metabolism, cell cycle, apoptosis, and angiogenesis.

Different miRNA expression profiles between cancerous cells and paired normal tissues from the same organ and cancer types have been documented in a number of studies [92,93]. Some miRNAs are down-regulated in a number of different tumors, and their re-introduction weakens the viability of cancer cells since they function as tumor-suppressor genes with an anti-proliferative and pro-apoptotic activity role [92,93]. In contrast to the tumor-suppressor miRNAs, oncogenic miRNAs, oroncomiRs, display an antiapoptotic activity and are over-expressed in cancer cells. Therefore, alterations in the expression of these miRNAs can stimulate tumorigenesis [56].

In 2011, Waltering et al. examined androgen regulation of miRNAs using one of the first miRNA microarray showing that DHT positively regulates 17 miRNAs in metastatic prostate VCaP tumor cells and causes high expression in 42 miRNAs [94]. Further works by several independent groups have demonstrated that miRNAs such as miR-19a, miR-148, miR-27a, miR-125b, miR-135a, MiR-32 and miR-21 are androgen inducible [95–101]. Other miRNAs are down-regulated by androgens such as the well-established oncogenes miR-221 and miR-222, which are located on the X chromosome and are over-expressed in pancreatic, breast, liver, and lung cancer [100,102,103]. Other miRNAs that are down-regulated by androgens include miR-126-5p, miR-146b, miR-219-5p, miR181b-1, miR-181c, and miR-221 [104].

On the other hand, many studies have examined the role of miRNAs in controlling the AR pathway. Using a miRNA library, Ostling et al. reported the ability of 71 unique miRNAs to influence the function of AR with 52 decreasing and 19 increasing its RNA and protein levels [84]. Since then, several miRNAs have been described to have a role in the regulation of AR activity directly or indirectly via co-regulators [105,106]. MiR-205 was found to exhibit a negative correlation to AR through binding to the receptor’s 3’UTR and decrease its transcript and protein levels [107], and there is a statistically significant inverse association exists between miR-34a and AR [84]. In addition to those examples, let-7c determines prostate tumor suppression through AR, and this mechanism is linked to the capability of this tumor-suppressing miRNA to target c-MYC, a molecule that is required for the correct transcription of AR [108].

However, miRNAs do not only have direct effects, rather they can use alternative routes to control androgen signaling such as those mediated by the ERBB-2 and PI3K/AKT. The tyrosine receptor ERBB-2 is often elevated in prostate cancer, whereas the activation of PI3K/AKT signaling is associated with proliferation, metastasis, apoptosis resistance and angiogenesis in prostate cancer [109]. The latter study has shown that the 3^′-UTR of ERBB-2 mRNA contains two specific target sites for miR-331-3p, which suppresses the expression of ERBB-2 at both the transcript and protein levels. The same group also illustrated that miR-331-3p is involved in the downstream PI3K/AKT signaling in multiple prostate cancer cell lines [109]. MiR-488* directly affects AR signaling by targeting the 3^′-UTR of AR and down-regulates AR protein expression in both androgen-sensitive and -insensitive prostate cancer cells inhibiting cellular growth and inducing apoptosis [110]. Another miRNA miR-17-5p, has been shown to target PCAF, a coactivator of AR, and to support the development of prostate tumor [111].

Table 4
miRNAs that regulate CD44 expression in BC cells

miRNA Role in BC Experimental model Reference

miR-143 ∙ Increase in S-phase population of cancer cells ∙ Reduces stem cell properties such as: self-renewal and sphere formation abilities ∙ Reduces cancer cell proliferation (in vivo) ∙ Suppresses epithelial-mesenchymal transition and metastasis • Tested on BT474, MDA231, MDA468, MCF7, SUM102 and SKBR3 human BC cell line in vitro [120]

miR-373 ∙ Increase invasion and migration by suppressing CD44, promoting metastasis ∙ No effect on cell cycle or cell proliferation ∙ Tested on MCF-7 human BC cell line in vitro and in vivo (transplanted into SCID mice) ∙ Expressed by MDA-MB-435 human breast cancer cell line and DU-145 human prostate cancer cell line and is responsible for their migratory ability ∙ Overexpressed by lymph-node metastatic breast cancer samples compared to primary tumors [153]

miR-520c ∙ Increase invasion and migration by suppressing CD44, promoting metastasis ∙ No effect on cell cycle or cell proliferation ∙ Tested on MCF-7 human BC cell line in vitro and in vivo (transplanted into SCID mice) [153]

miR-34a ∙ Inhibits tumor growth and survival ∙ Induces cell cycle arrest and senescence ∙ Reduces metastasis (in vivo) and invasion ∙ Inhibits stem cell properties like holoclone and sphere formation ∙ LAPC4, LAPC9 &Du145 xenograft models of human prostate cancer in vitro ∙ In vivo models of prostate cancer in NOD-SCID mice [154]

SCID: severe combined immunodeficiency

NOD: non-obese diabetic

Different roles of miRNAs in BC and their experimental models.

miRNA	Role in BC	Experimental model	Reference
miR-143	∙ Increase in S-phase population of cancer cells ∙ Reduces stem cell properties such as: self-renewal and sphere formation abilities ∙ Reduces cancer cell proliferation (in vivo) ∙ Suppresses epithelial-mesenchymal transition and metastasis	• Tested on BT474, MDA231, MDA468, MCF7, SUM102 and SKBR3 human BC cell line in vitro	[120]
miR-373	∙ Increase invasion and migration by suppressing CD44, promoting metastasis ∙ No effect on cell cycle or cell proliferation	∙ Tested on MCF-7 human BC cell line in vitro and in vivo (transplanted into SCID mice) ∙ Expressed by MDA-MB-435 human breast cancer cell line and DU-145 human prostate cancer cell line and is responsible for their migratory ability ∙ Overexpressed by lymph-node metastatic breast cancer samples compared to primary tumors	[153]
miR-520c	∙ Increase invasion and migration by suppressing CD44, promoting metastasis ∙ No effect on cell cycle or cell proliferation	∙ Tested on MCF-7 human BC cell line in vitro and in vivo (transplanted into SCID mice)	[153]
miR-34a	∙ Inhibits tumor growth and survival ∙ Induces cell cycle arrest and senescence ∙ Reduces metastasis (in vivo) and invasion ∙ Inhibits stem cell properties like holoclone and sphere formation	∙ LAPC4, LAPC9 &Du145 xenograft models of human prostate cancer in vitro ∙ In vivo models of prostate cancer in NOD-SCID mice	[154]
SCID: severe combined immunodeficiency
NOD: non-obese diabetic

10. AR and CD44

Treatment of MDA-MB-231 BC cells with DHT alters CD44 expression at both the RNA and protein levels [112]. The expression of the different variants of CD44 is regulated at the post-transcriptional level. Moreover, MDA-MB-231 cells express elevated levels of CD44 in contrast to other BC cell lines [113]. The high levels of CD44 could be due to the large number of miRNA types such as miR-512-3p, miR-328, miR-491, and miR-671 that bind to 3’UTR of CD44 in these cells [86]. The levels of these miRNAs could also be regulated by DHT. Understanding the role of CD44 in cancer progression must take in consideration the existence of different CD44 isoforms as the antibodies in the majority of the earlier studies identifies an epitope located in a non-variable region of CD44 of cancer stem cells, so they cannot differentiate between different CD44 isoforms [114]. Not all CD44 proteins bind to HA. In facts, three isoforms of CD44 cannot interact with HA unless activated by physiological stimuli, and constitutive binding [115]. CD44 switching from the inactive state (low-affinity incapable of binding and internalizing HA) to the active state (high-affinity with ability to bind and internalize HA) requires posttranslational modification including glycosylation of the extracellular domain and/or phosphorylation of specific serine residues in the cytoplasmic domain [26,116]. This modification is vital for cellular migration that allows CD44 to be inserted into the leading border of the cells and lamellipodia [117].

11. miRNA and CD44

Seventy miRNAs control CD44 expression levels in different tissues [118]. Different types of miRNAs regulate the expression of CD44 (Table 4). A study on MCF-7 BC cells showed another HA/CD44 signaling pathway that is initiated by activation of protein kinase C (PKC). The latter kinase promotes phosphorylation of the stem cell marker Nanog, which is a transcriptional growth factor that plays a role in the self-regeneration of pluripotency in embryonic stem cells. Phosphorylated Nanog can then translocate from the cytosol into the nucleus and associate with RNAase III (DROSHA) and the RNA helicase (p68), which together lead to up-regulation of miRNA-21. The latter molecule then down-regulates the tumor suppressor protein, program cell death 4 (PDCD4), induces expression of inhibitor of apoptosis protein (IAPs) as well as X-linked inhibitor of apoptosis protein (XIAP), and promotes chemoresistance by stimulating the expression of MDR1 (P-gp) [119].

Recently, numerous miRNAs have been identified to affect cancer progression; miRNA-143 directly binds the 3’ UTR region of CD44 resulting in reduction of its expression and the capability of BC cells to metastasize. These effects are as a result of the tumor suppressing activity of miRNA-143 [120]. Furthermore, it has been found that miRNA-520a-3p acts as a tumor suppressor and CD44 gene as one of its targets in BC tissues and numerous cell lines including MCF-7 and MDA-MB-231 cells [121].

12. Conclusion

The prevalence of BC indicates that it is a major problem that must be effectively tackled. Making things worse is the heterogeneity and lack of effective therapeutic strategies in treating TNBC. AR is a promising molecules at various levels. First, it can be used as a prognostic indicator of the disease. In addition, it is considered a promising therapeutic target. However, the previous point necessitates better understanding of the biological roles of AR in BC. The three-way link between AR, CD44, and miRNA opens the window towards clarification of this role. It can be surmised that since AR can regulate CD44 via miRNA, it can also do the same for other proteins and, consequently, various cellular processes. Elucidation of these intricate molecular networks can lead to better treatments of BC.

Footnotes

Conflict of interest

There is no conflict of interest to declare.

References

Childers

, National estimates of genetic testing in women with a history of breast or ovarian cancer, J Clin Oncol , 35(34): 3800, 2017.

Grunewald

, Understanding tumor heterogeneity as functional compartments-superorganisms revisited, Journal of Translational Medicine , 9(1): 79, 2011.

Weigelt

, Baehner

, Reis-Filho

, The contribution of gene expression profiling to breast cancer classification, prognostication and prediction: a retrospective of the last decade, The Journal of Pathology: A Journal of the Pathological Society of Great Britain and Ireland , 220(2): 263–280, 2010.

Reis-Filho

, Molecular profiling: moving away from tumor philately, Science Translational Medicine , 2(47): 47ps43, 2010.

Dai

, Breast cancer intrinsic subtype classification, clinical use and future trends, Am J Cancer Res , 5(10): 2929, 2015.

Vallejos

, Breast cancer classification according to immunohistochemistry markers: subtypes and association with clinicopathologic variables in a peruvian hospital database, Clin Breast Cancer , 10(4): 294–300, 2010.

Cheang

, Ki67 index, HER2 status, and prognosis of patients with luminal B breast cancer, JNCI: Journal of the National Cancer Institute , 101(10): 736–750, 2009.

Sotiriou

, Pusztai

, Gene-expression signatures in breast cancer, N Engl J Med , 360(8): 790–800, 2009.

Yan

, Zuo

, Wei

, Concise review: emerging role of CD44 in cancer stem cells: a promising biomarker and therapeutic target, Stem Cells Translational Medicine , 4(9): 1033–1043, 2015.

10.

C.G.A. Network. Comprehensive molecular portraits of human breast tumours, Nature , 490(7418): 61, 2012.

11.

Sørlie

, Gene expression patterns of breast carcinomas distinguish tumor subclasses with clinical implications, Proc Natl Acad Sci , 98(19): 10869–10874, 2001.

12.

Parker

, Supervised risk predictor of breast cancer based on intrinsic subtypes, J Clin Oncol , 27(8): 1160, 2009.

13.

Waugh

Adhesion and penetration: two sides of CD44 signal transduction cascades in the context of cancer cell metastasis. Hyaluronan in Cancer Biology . . Elsevier; 109–125. 2009.

14.

Orian-Rousseau

, CD44 is required for two consecutive steps in HGF/c-Met signaling, Genes Dev , 16(23): 3074–3086, 2002.

15.

Miwa

, Isoform switch of CD44 induces different chemotactic and tumorigenic ability in gallbladder cancer, Int J Oncol , 51(3): 771–780, 2017.

16.

Bajorath

, Molecular organization, structural features, and ligand binding characteristics of CD44, a highly variable cell surface glycoprotein with multiple functions, Proteins: Structure, Function, and Bioinformatics , 39(2): 103–111, 2000.

17.

Liu

, Sy

M-S

, Phorbol myristate acetate stimulates the dimerization of CD44 involving a cysteine in the transmembrane domain, J Immunol , 159(6): 2702–2711, 1997.

18.

Bourguignon

, Rho-kinase (ROK) promotes CD44v3, 8–10- ankyrin interaction and tumor cell migration in metastatic breast cancer cells, Cytoskeleton , 43(4): 269–287, 1999.

19.

Thorne

, Legg

, Isacke

, The role of the CD44 transmembrane and cytoplasmic domains in co-ordinating adhesive and signalling events, J Cell Sci , 117(3): 373–380, 2004.

20.

Senbanjo

, Chellaiah

, CD44: a multifunctional cell surface adhesion receptor is a regulator of progression and metastasis of cancer cells, Front Cell Dev Biol , 5: 18, 2017.

21.

Neame

, Isacke

, The cytoplasmic tail of CD44 is required for basolateral localization in epithelial MDCK cells but does not mediate association with the detergent-insoluble cytoskeleton of fibroblasts, J Cell Biol , 121(6): 1299–1310, 1993.

22.

Cywes

, Stamenkovic

, Wessels

, CD44 as a receptor for colonization of the pharynx by group A Streptococcus, J Clin Invest , 106(8): 995–1002, 2000.

23.

Okamoto

, Proteolytic release of CD44 intracellular domain and its role in the CD44 signaling pathway, J Cell Biol , 155(5): 755–762, 2001.

24.

Ponta

, Sherman

, Herrlich

, CD44: from adhesion molecules to signalling regulators, Nature Rev Mol Cell Biol , 4(1): 33, 2003.

25.

Wang

, Expression of CD44 standard form and variant isoforms in human bone marrow stromal cells, Saudi Pharmaceutical Journal , 25(4): 488–491, 2017.

26.

Naor

, Sionov

, Ish-Shalom

. CD44: structure, function and association with the malignant process. Advances in Cancer Research . . Elsevier; 241–319. 1997.

27.

Louderbough

, Schroeder

, Understanding the dual nature of CD44 in breast cancer progression, Mol Cancer Res , 9(12): 1573–1586, 2011.

28.

Wang

, Bourguignon

, Role of hyaluronan-mediated CD44 signaling in head and neck squamous cell carcinoma progression and chemoresistance, Am J Pathol , 178(3): 956–963, 2011.

29.

Thapa

, Wilson

, The importance of CD44 as a stem cell biomarker and therapeutic target in cancer, Stem Cells Int , 20162016.

30.

Todaro

, CD44v6 is a marker of constitutive and reprogrammed cancer stem cells driving colon cancer metastasis, Cell Stem Cell , 14(3): 342–356, 2014.

31.

Gupta

, Promising noninvasive cellular phenotype in prostate cancer cells knockdown of matrix metalloproteinase 9, Sci World J , 20132013.

32.

Döme

, Expression of CD44v3 splice variant is associated with the visceral metastatic phenotype of human melanoma, Virchows Archiv , 439(5): 628–635, 2001.

33.

X-D

, Clinical significance of CD44 variants expression in colorectal cancer, Tumori J , 99(1): 88–92, 2013.

34.

Sato

, Reduced expression of CD44 variant 9 is related to lymph node metastasis and poor survival in squamous cell carcinoma of tongue, Oral Oncol , 36(6): 545–549, 2000.

35.

Diaz

, CD44 expression is associated with increased survival in node-negative invasive breast carcinoma, Clin Cancer Res , 11(9): 3309–3314, 2005.

36.

Necas

, Hyaluronic acid (hyaluronan): a review, Veterinarni Medicina , 53(8): 397–411, 2008.

37.

Karbownik

, Nowak

, Hyaluronan: towards novel anti-cancer therapeutics, Pharmacological Reports , 65(5): 1056–1074, 2013.

38.

Jordan

, The role of CD44 in disease pathophysiology and targeted treatment, Front Immunol , 6: 182, 2015.

39.

Ghosh

, Neslihan Alpay

, Klostergaard

, CD44: a validated target for improved delivery of cancer therapeutics, Expert Opinion on Therapeutic Targets , 16(7): 635–650, 2012.

40.

Rios de la Rosa

, The CD44-mediated uptake of hyaluronic acid-based carriers in macrophages, Advanced Healthcare Materials , 6(4)2017.

41.

Cai

, Cellular uptake and internalization of hyaluronan-based doxorubicin and cisplatin conjugates, Journal of Drug Targeting , 22(7): 648–657, 2014.

42.

Chanmee

, Ontong

, Itano

, Hyaluronan: a modulator of the tumor microenvironment, Cancer Lett , 375(1): 20–30, 2016.

43.

Maiti

, Maki

, Johnson

, TNF-𝛼 induction of CD44-mediated leukocyte adhesion by sulfation, Science , 282(5390): 941–943, 1998.

44.

Cichy

, Puré

, The liberation of CD44, J Cell Biol , 161(5): 839–843, 2003.

45.

Yang

, The high and low molecular weight forms of hyaluronan have distinct effects on CD44 clustering, J Biol Chem , 287(51): 43094–43107, 2012.

46.

Bourguignon

, Hyaluronan promotes CD44v3-Vav2 interaction with Grb2-p185HER2 and induces Rac1 and Ras signaling during ovarian tumor cell migration and growth, J Biol Chem , 276(52): 48679–48692, 2001.

47.

Morrison

, The NF2 tumor suppressor gene product, merlin, mediates contact inhibition of growth through interactions with CD44, Genes Dev , 15(8): 968–980, 2001.

48.

Turville

. Blocking of HIV Entry through CD44–Hyaluronic Acid Interactions . . Nature Publishing Group; 2014.

49.

Ohkawara

, Activation and transforming growth factor-𝛽 production in eosinophils by hyaluronan, American Journal of Respiratory Cell and Molecular Biology , 23(4): 444–451, 2000.

50.

Toole

, Ghatak

, Misra

, Hyaluronan oligosaccharides as a potential anticancer therapeutic, Current Pharmaceutical Biotechnology , 9(4): 249–252, 2008.

51.

Toole

, Slomiany

, Hyaluronan, CD44 and Emmprin: partners in cancer cell chemoresistance, Drug Resistance Updates , 11(3): 110–121, 2008.

52.

Slomiany

, Abrogating drug resistance in malignant peripheral nerve sheath tumors by disrupting hyaluronan-CD44 interactions with small hyaluronan oligosaccharides, Cancer Res , 69(12): 4992–4998, 2009.

53.

Gilg

, Targeting hyaluronan interactions in malignant gliomas and their drug-resistant multipotent progenitors, Clin Cancer Res , 14(6): 1804–1813, 2008.

54.

Ween

, Versican induces a pro-metastatic ovarian cancer cell behavior which can be inhibited by small hyaluronan oligosaccharides, Clinical & Experimental Metastasis , 28(2): 113–125, 2011.

55.

Urakawa

, Inhibition of hyaluronan synthesis in breast cancer cells by 4-methylumbelliferone suppresses tumorigenicity in vitro and metastatic lesions of bone in vivo, Int J Cancer , 130(2): 454–466, 2012.

56.

Marhaba

, Zöller

, CD44 in cancer progression: adhesion, migration and growth regulation, J Mol Histol , 35(3): 211–231, 2004.

57.

Hanahan

, Weinberg

, The hallmarks of cancer, Cell , 100(1): 57–70, 2000.

58.

Bourguignon

, Hyaluronan-CD44 interaction promotes c-Src-mediated twist signaling, microRNA-10b expression, and RhoA/RhoC up-regulation, leading to Rho-kinase-associated cytoskeleton activation and breast tumor cell invasion, J Biol Chem , 285(47): 36721–36735, 2010.

59.

Bourguignon

, Hyaluronan-mediated CD44 interaction with RhoGEF and Rho kinase promotes Grb2-associated binder-1 phosphorylation and phosphatidylinositol 3-kinase signaling leading to cytokine (macrophage-colony stimulating factor) production and breast tumor progression, J Biol Chem , 278(32): 29420–29434, 2003.

60.

Bretscher

, Edwards

, Fehon

, ERM proteins and merlin: integrators at the cell cortex, Nature Rev Mol Cell Biol , 3(8): 586, 2002.

61.

Morrison

, The NF2 tumor suppressor gene product, merlin, mediates contact inhibition of growth through interactions with CD44, Genes Dev , 15(8): 968–980, 2001.

62.

Kissil

, Merlin phosphorylation by p21-activated kinase 2 and effects of phosphorylation on merlin localization, J Biol Chem , 277(12): 10394–10399, 2002.

63.

Louderbough

, Lopez

, Schroeder

, Matrix hyaluronan alters epidermal growth factor receptor-dependent cell morphology, Cell Adh Migr , 4(1): 26–31, 2010.

64.

Brown

, CD44 splice isoform switching in human and mouse epithelium is essential for epithelial-mesenchymal transition and breast cancer progression, J Clin Invest , 121(3): 1064–1074, 2011.

65.

Herrlich

, CD44 acts both as a growth- and invasiveness-promoting molecule and as a tumor-suppressing cofactor, Ann N Y Acad Sci , 910: 106–118, 2000.

66.

Lubahn

, Sequence of the intron/exon junctions of the coding region of the human androgen receptor gene and identification of a point mutation in a family with complete androgen insensitivity, Proc Natl Acad Sci , 86(23): 9534–9538, 1989.

67.

Olsen

, Context dependent regulatory patterns of the androgen receptor and androgen receptor target genes, BMC Cancer , 16(1): 377, 2016.

68.

Toocheck

, Mouse spermatogenesis requires classical and nonclassical testosterone signaling, Biology of Reproduction , 94(1): 11, 2016. 1–14.

69.

Davis

, Androgen replacement in women: a commentary, The Journal of Clinical Endocrinology & Metabolism , 84(6): 1886–1891, 1999.

70.

Simpson

, Role of aromatase in sex steroid action, J Mol Endocrinol , 25(2): 149–156, 2000.

71.

Somboonporn

, Bell

, Davis

, Testosterone for peri and postmenopausal women, Cochrane Database Systematic Reviews (4), 2005.

72.

Niemeier

, Androgen receptor in breast cancer: expression in estrogen receptor-positive tumors and in estrogen receptor-negative tumors with apocrine differentiation, Modern Pathology , 23(2): 205, 2010.

73.

Collins

, Androgen receptor expression in breast cancer in relation to molecular phenotype: results from the Nurses’ Health Study, Modern Pathology , 24(7): 924, 2011.

74.

Huo

, Androgen receptor expression in breast cancer in relation to molecular phenotype: results from the Nurses’ Health Study: Collins LC, Cole KS, Marotti JD, et al (Beth Israel Deaconess Med Ctr and Harvard Med School, Boston, MA; Dartmouth-Hitchcock Med Ctr, Lebanon, NH) Mod Pathol 24: 924–931, 2011 §, Breast Diseases: A YB Quarterly , 23(2): 155–156, 2012.

75.

Park

, Androgen receptor expression is significantly associated with better outcomes in estrogen receptor-positive breast cancers, Ann Oncol , 22(8): 1755–1762, 2011.

76.

Asano

, Expression and clinical significance of androgen receptor in triple-negative breast cancer, Cancers , 9(1): 4, 2017.

77.

Mohammadizadeh

, Androgen receptor expression and its relationship with clinicopathological parameters in an Iranian population with invasive breast carcinoma, Adv Biomed Res , 3, 2014.

78.

Ingle

, Combination hormonal therapy with tamoxifen plus fluoxymesterone versus tamoxifen alone in postmenopausal women with metastatic breast cancer. An updated analysis, Cancer , 67(4): 886–891, 1991.

79.

Lehmann

, Identification of human triple-negative breast cancer subtypes and preclinical models for selection of targeted therapies, J Clin Invest , 121(7): 2750–2767, 2011.

80.

Gucalp

, Phase II trial of bicalutamide in patients with androgen receptor–positive, estrogen receptor–negative metastatic breast cancer, Clin Cancer Res , 19(19): 5505–5512, 2013.

81.

Elebro

, Combined androgen and estrogen receptor status in breast cancer: treatment prediction and prognosis in a population-based prospective cohort, Clin Cancer Res , 21(16): 3640–3650, 2015.

82.

Christodoulatos

, Dalamaga

, Micro-RNAs as clinical biomarkers and therapeutic targets in breast cancer: Quo vadis?, World J Clin Oncol , 5(2): 71, 2014.

83.

Shi

G-M

, Identification of side population cells in human hepatocellular carcinoma cell lines with stepwise metastatic potentials, J Cancer Res Clin Oncol , 134(11): 1155, 2008.

84.

Östling

, Systematic analysis of microRNAs targeting the androgen receptor in prostate cancer cells, Cancer Res , 71(5): 1956–1967, 2011.

85.

Shu

, Dynamic and modularized microRNA regulation and its implication in human cancers, Sci Rep , 7(1): 13356, 2017.

86.

Rutnam

, Yang

, The non-coding 3^′ UTR of CD44 induces metastasis by regulating extracellular matrix functions, J Cell Sci , 125(8): 2075–2085, 2012.

87.

Subtil

, Carbon ion radiotherapy of human lung cancer attenuates HIF-1 signaling and acts with considerably enhanced therapeutic efficiency, FASEB J , 28(3): 1412–1421, 2014.

88.

Esteller

, Non-coding RNAs in human disease, Nature Rev Genet , 12(12): 861, 2011.

89.

Spizzo

, Long non-coding RNAs and cancer: a new frontier of translational research?, Oncogene , 31(43): 4577, 2012.

90.

Mendell

, Olson

, MicroRNAs in stress signaling and human disease, Cell , 148(6): 1172–1187, 2012.

91.

Vasudevan

, Tong

, Steitz

, Switching from repression to activation: microRNAs can up-regulate translation, Science , 318(5858): 1931–1934, 2007.

92.

, MicroRNA expression profiles classify human cancers, Nature , 435(7043): 834, 2005.

93.

Zhu

, Different miRNA expression profiles between human breast cancer tumors and serum, Front Genet , 5: 149, 2014.

94.

Waltering

, Androgen regulation of micro- RNAs in prostate cancer, The Prostate , 71(6): 604–614, 2011.

95.

Murata

, miR-148a is an androgen-responsive microRNA that promotes LNCaP prostate cell growth by repressing its target CAND1 expression, Prostate Cancer and Prostatic Diseases , 13(4): 356, 2010.

96.

Fletcher

, Androgen-regulated processing of the oncomir miR-27a, which targets Prohibitin in prostate cancer, Hum Mol Genet , 21(14): 3112–3127, 2012.

97.

, Identification of novel AR-targeted microRNAs mediating androgen signalling through critical pathways to regulate cell viability in prostate cancer, PloS one , 8(2): e56592, 2013.

98.

Ribas

, miR-21: an androgen receptor–regulated microRNA that promotes hormone-dependent and hormone-independent prostate cancer growth, Cancer Res , 69(18): 7165–7169, 2009.

99.

Takayama

, Integration of cap analysis of gene expression and chromatin immunoprecipitation analysis on array reveals genome-wide androgen receptor signaling in prostate cancer cells, Oncogene , 30(5): 619, 2011.

100.

Jalava

, Androgen-regulated miR-32 targets BTG2 and is overexpressed in castration-resistant prostate cancer, Oncogene , 31(41): 4460, 2012.

101.

Kroiss

, Androgen-regulated microRNA-135a decreases prostate cancer cell migration and invasion through downregulating ROCK1 and ROCK2, Oncogene , 34(22): 2846, 2015.

102.

Garofalo

, miR221/222 in cancer: their role in tumor progression and response to therapy, Curr Mol Med , 12(1): 27–33, 2012.

103.

Gui

, Androgen receptor-mediated downregulation of microRNA-221 and-222 in castration-resistant prostate cancer, PloS One , 12(9): e0184166, 2017.

104.

Ambs

, Genomic profiling of microRNA and messenger RNA reveals deregulated microRNA expression in prostate cancer, Cancer Res , 68(15): 6162–6170, 2008.

105.

Gao

, Alumkal

, Epigenetic regulation of androgen receptor signaling in prostate cancer, Epigenetics , 5(2): 100–104, 2010.

106.

Shih

J-W

, Non-coding RNAs in castration-resistant prostate cancer: regulation of androgen receptor signaling and cancer metabolism, Int J Mol Sci , 16(12): 28943–28978, 2015.

107.

Hagman

, miR-205 negatively regulates the androgen receptor and is associated with adverse outcome of prostate cancer patients, Br J Cancer , 108(8): 1668, 2013.

108.

Nadiminty

, MicroRNA let-7c suppresses androgen receptor expression and activity via regulation of Myc expression in prostate cancer cells, J Biol Chem , 287(2): 1527–1537, 2012.

109.

Epis

, miR-331-3p regulates ERBB-2 expression and androgen receptor signaling in prostate cancer, J Biol Chem , 284(37): 24696–24704, 2009.

110.

Sikand

, miR 488* inhibits androgen receptor expression in prostate carcinoma cells, Int J Cancer , 129(4): 810–819, 2011.

111.

Gong

A-Y

, miR-17-5p targets the p300/CBP-associated factor and modulates androgen receptor transcriptional activity in cultured prostate cancer cells, BMC Cancer , 12(1): 492, 2012.

112.

Al-Othman

, Hammad

, Ahram

, Dihydrotestosterone regulates expression of CD44 via miR-328-3p in triple-negative breast cancer cells, Gene , 675: 128–135, 2018.

113.

Smith

, Cai

, Cell specific CD44 expression in breast cancer requires the interaction of AP-1 and NFkappaB with a novel cis-element, PLoS One , 7(11): 30, 2012.

114.

Olsson

, CD44 isoforms are heterogeneously expressed in breast cancer and correlate with tumor subtypes and cancer stem cell markers, BMC Cancer , 11(1): 418, 2011.

115.

Lesley

, Hyaluronan binding by cell surface CD44, J Biol Chem , 275(35): 26967–26975, 2000.

116.

Skelton

, Glycosylation provides both stimulatory and inhibitory effects on cell surface and soluble CD44 binding to hyaluronan, J Cell Biol , 140(2): 431–446, 1998.

117.

Zohar

, Intracellular osteopontin is an integral component of the CD44- ERM complex involved in cell migration, J Cell Physiol , 184(1): 118–130, 2000.

118.

G.T.H.G. Database, GeneCards®: The Human Gene Database, 2019. [cited 2019; Available from: https://www.genecards.org.

119.

Bourguignon

, Hyaluronan-CD44 interaction with protein kinase C𝜖 promotes oncogenic signaling by the stem cell marker Nanog and the production of microRNA-21, leading to down-regulation of the tumor suppressor protein PDCD4, anti-apoptosis, and chemotherapy resistance in breast tumor cells, J Biol Chem , 284(39): 26533–26546, 2009.

120.

Yang

, MicroRNA- 143 targets CD44 to inhibit breast cancer progression and stem cell-like properties, Mol Med Rep , 13(6): 5193–5199, 2016.

121.

, Suppressing role of miR-520a-3p in breast cancer through CCND1 and CD44, American Journal of Translational Research , 9(1): 146, 2017.

122.

Bennett

, Regulation of CD44 binding to hyaluronan by glycosylation of variably spliced exons, J Cell Biol , 131(6): 1623–1633, 1995.

123.

Milstone

, Epican, a heparan/chondroitin sulfate proteoglycan form of CD44, mediates cell-cell adhesion, J Cell Sci , 107(11): 3183–3190, 1994.

124.

Kurz

, The impact of c- met/scatter factor receptor on dendritic cell migration, Eur J Immunol , 32(7): 1832–1838, 2002.

125.

Cheng

, Sharp

, Regulation of CD44 alternative splicing by SRm160 and its potential role in tumor cell invasion, Mol Cell Biol , 26(1): 362–370, 2006.

126.

Sleeman

, Variant exons v6 and v7 together expand the repertoire of glycosaminoglycans bound by CD44, J Biol Chem , 272(50): 31837–31844, 1997.

127.

Lee

J-L

, Osteopontin promotes integrin activation through outside-in and inside-out mechanisms: OPN-CD44V interaction enhances survival in gastrointestinal cancer cells, Cancer Res , 67(5): 2089–2097, 2007.

128.

Kuhn

, A complex of EpCAM, claudin-7, CD44 variant isoforms, and tetraspanins promotes colorectal cancer progression, Mol Cancer Res , 5(6): 553–567, 2007.

129.

Kuniyasu

, Heparan sulfate enhances invasion by human colon carcinoma cell lines through expression of CD44 variant exon 3, Clin Cancer Res , 7(12): 4067–4072, 2001.

130.

Yamaguchi

, Expression of a CD44 variant containing exons 8 to 10 is a useful independent factor for the prediction of prognosis in colorectal cancer patients, J Clin Oncol , 14(4): 1122–1127, 1996.

131.

Ozawa

, Prognostic significance of CD44 variant 2 upregulation in colorectal cancer, Br J Cancer , 111(2): 365, 2014.

132.

Pirinen

, Reduced expression of CD44v3 variant isoform is associated with unfavorable outcome in non–small cell lung carcinoma, Hum Pathol , 31(9): 1088–1095, 2000.

133.

Mizera-Nyczak

, Isoform expression of CD44 adhesion molecules, Bcl-2, p53 and Ki-67 proteins in lung cancer, Tumor Biol , 22(1): 45–53, 2001.

134.

Zhao

, Prognostic value of CD44 variant exon 6 expression in non-small cell lung cancer: a meta-analysis, Asian Pac J Cancer Prev , 15(16): 6761–6766, 2014.

135.

Ruibal

, Cell membrane CD44v6 levels in squamous cell carcinoma of the lung: association with high cellular proliferation and high concentrations of EGFR and CD44V5, Int J Mol Sci , 16(3): 4372–4378, 2015.

136.

Miyoshi

, The expression of the CD44 variant exon 6 is associated with lymph node metastasis in non-small cell lung cancer, Clin Cancer Res , 3(8): 1289–1297, 1997.

137.

Jiang

, Zhao

, Shao

, Prognostic value of CD44 and CD44v6 expression in patients with non-small cell lung cancer: meta-analysis, Tumor Biol , 35(8): 7383–7389, 2014.

138.

Kaufmann

, CD44 variant exon epitopes in primary breast cancer and length of survival, The Lancet , 345(8950): 615–619, 1995.

139.

Tempfer

, Prognostic value of immunohistochemically detected CD44 isoforms CD44v5, CD44v6 and CD44v7–8 in human breast cancer, Eur J Cancer , 32(11): 2023–2025, 1996.

140.

Rys

, The role of CD44v3 expression in female breast carcinomas, Pol J Pathol , 54(4): 243–247, 2003.

141.

Bendall

, Gottlieb

, CD44 and adhesion of normal and leukemic CD34+ cells to bone marrow stroma, Leukemia & Lymphoma , 32(5–6): 427–439, 1999.

142.

Legras

, A strong expression of CD44-6v correlates with shorter survival of patients with acute myeloid leukemia, Blood , 91(9): 3401–3413, 1998.

143.

Rall

, Rustgi

, CD44 isoform expression in primary and metastatic pancreatic adenocarcinoma, Cancer Res , 55(9): 1831–1835, 1995.

144.

Gansauge

, Differential expression of CD44 splice variants in human pancreatic adenocarcinoma and in normal pancreas, Cancer Res , 55(23): 5499–5503, 1995.

145.

Gotoda

, Expression of CD44 variants and its association with survival in pancreatic cancer, Cancer Sci , 89(10): 1033–1040, 1998.

146.

, CD44v/CD44s expression patterns are associated with the survival of pancreatic carcinoma patients, Diagn Pathol , 9(1): 79, 2014.

147.

Kiuchi

, Pancreatic cancer cells express CD44 variant 9 and multidrug resistance protein 1 during mitosis, Experimental and Molecular Pathology , 98(1): 41–46, 2015.

148.

Reategui

, Characterization of CD44v3 containing isoforms in head and neck cancer, Cancer Biol Ther , 5(9): 1163–1168, 2006.

149.

Wang

, CD44 variant isoforms in head and neck squamous cell carcinoma progression, The Laryngoscope , 119(8): 1518–1530, 2009.

150.

Wang

, Wreesmann

, Bourguignon

, Association of CD44 V3- containing isoforms with tumor cell growth, migration, matrix metalloproteinase expression, and lymph node metastasis in head and neck cancer, Head & Neck , 29(6): 550–558, 2007.

151.

Kawano

, Expression of E-cadherin, and CD44s and CD44v6 and its association with prognosis in head and neck cancer, Auris Nasus Larynx , 31(1): 35–41, 2004.

152.

Slevin

, Hyaluronan-mediated angiogenesis in vascular disease: uncovering RHAMM and CD44 receptor signaling pathways, Matrix Biol , 26(1): 58–68, 2007.

153.

Huang

, The microRNAs miR-373 and miR-520c promote tumour invasion and metastasis, Nature Cell Biol , 10(2): 202, 2008.

154.

Liu

, Identification of miR-34a as a potent inhibitor of prostate cancer progenitor cells and metastasis by directly repressing CD44, Nature Med , 17(2): 211–215, 2011.

Role of CD44 in breast cancer

Abstract

Keywords

1. Introduction

3. CD44 isoforms

4.1. HA

6. AR

7. AR expression in BC

8. AR as a therapeutic target

9. Androgen regulation of miRNA

11. miRNA and CD44

12. Conclusion

Footnotes

Conflict of interest

References