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Abstract

Leucine-rich repeat–containing G-protein-coupled receptors regulate stem cell activity and tissue homeostasis within female reproductive organs, primarily through their interaction with the Wnt/β-catenin signaling pathway. LGR4–6 are increasingly recognized for their roles in organ development, regeneration, and cancer. This review aims to provide a comprehensive overview of the roles of LGR4–6 in female reproductive organs, highlighting their significance in normal physiology and disease states, specifically in the context of ovarian cancer. LGR4 is essential for the proper development of the female reproductive system; its deficiency leads to significant reproductive abnormalities, including delayed menarche and follicle development issues. LGR5 is a well-established marker of stem cells in the ovary and fallopian tubes. It has been implicated in the pathogenesis of high-grade serous ovarian cancer. LGR6, while less studied, shares functional similarities with LGR5 and can maintain stemness. It contributes to chemoresistance in ovarian cancer. LGR6 is a marker for fallopian tube stem cells and is involved in stem cell maintenance and differentiation. LGR4–6 regulate the pathophysiology of female reproductive tissues. LGR4–6 are promising therapeutic targets for treating reproductive cancers and other related disorders. Molecular mechanisms underlying the functions of LGR4–6 should be studied.

Graphical Abstract

Keywords

leucine-rich repeat–containing G-protein-coupled receptors ovary cancer reproductive organ breast

Introduction

The Wnt/β-catenin signaling pathway is crucial for various cellular processes, including cell proliferation, differentiation, and self-renewal¹. Wnt proteins bind to a receptor complex of Frizzled (FZD) and low-density lipoprotein receptor–related protein 5/6 (LRP5/6) and inhibit the β-catenin destruction complex². Consequently, β-catenin accumulates in the cytoplasm, translocates to the nucleus, and activates transcriptional complexes, further driving the expression of genes involved in cellular proliferation and maintenance³.

Leucine-rich repeat–containing G-protein-coupled receptors (LGRs) are a family of G protein-coupled receptors (GPCRs) characterized by a large extracellular domain (ECD) containing leucine-rich repeats (LRRs)⁴. They are divided into three types: A, B, and C. Type A LGRs include receptors for follicle-stimulating hormone, luteinizing hormone, and thyroid-stimulating hormone⁵. They have seven to nine LRRs and a long hinge region essential for receptor activation⁴. Type B LGRs (LGR4–6) are receptors for R-spondins (RSPOs) and have roles in stem cell differentiation and cancer⁶. They typically have 16 to 18 LRRs and a medium-length hinge region⁶. Type C LGRs, which include RXFP1 and RXFP2, are unique in having an N-terminal low-density lipoprotein class A (LDLa) module essential for activation⁴. They have about 10 LRRs and a shorter hinge region⁴. All types of LRR domains serve as ligand-binding sites⁷. While Type A and C LGRs function through canonical GPCR pathways, Type B LGRs regulate Wnt signaling upon ligand binding⁸.

LGR4–6, along with RSPOs 1 to 4, positively regulates Wnt signaling⁹. Upon the binding of RSPOs to LGRs, they inhibit receptor-mediated downregulation of Wnt signaling, further enhancing the stability and activity of β-catenin¹⁰. LGR4–6 are critical in various organs because they modulate stem cell activity and tissue regeneration⁹.

LGR5 is a remarkable marker of organ-specific stem cells in various tissues such as the intestine¹¹, hair follicles¹², and ovary¹³. LGR6 is a marker of stem cells in the epidermis¹⁴, mammary gland¹⁵, and fallopian tube^13,16. LGR4, although not a direct stem cell marker, is essential for the developmental processes in the intestine¹⁷, hair¹⁸, prostate¹⁹, and mammary gland²⁰, and has been implicated in carcinogenesis and metastasis^20,21.

Herein, we review the roles of LGR4–6 in female reproductive organs, highlighting their significance in normal physiology and disease states, particularly in ovarian cancer.

Mechanism of Interaction Between LGR4–6 and Wnt/β-Catenin Signaling Pathway

LGR4–6 interact with the Wnt/β-catenin signaling pathway primarily through their roles as receptors for RSPO proteins, which are potent Wnt pathway agonists²². The Wnt/β-catenin pathway is crucial for cell proliferation, differentiation, and stem cell maintenance, making it essential for both normal tissue homeostasis and tumorigenesis¹.

LGR4–6 as receptors for RSPO ligands

LGR4–6 serve as receptors for RSPO proteins (RSPO1–4), which bind specifically to these receptors on the cell membrane²³. RSPO binding activates the receptors and enhances the Wnt/β-catenin signaling pathway²⁴. RSPO binding induces LGR4–6 to interact with the Wnt co-receptors LRP5/6 and FZD receptors, enhancing Wnt signal strength²⁵.

Stabilization of FZD and LRP5/6 co-receptors

When RSPO binds to LGR4–6, it recruits E3 ubiquitin ligases RNF43 and ZNRF3, which normally function to ubiquitinate and degrade FZD and LRP5/6, thereby attenuating Wnt signaling²⁶. The binding of RSPO to LGR4–6 inhibits the activity of RNF43 and ZNRF3, preventing the degradation of FZD and LRP5/6²⁷. This stabilizes the Wnt receptors on the cell surface, amplifying the Wnt signal.

Activation of Wnt/β-catenin signaling

The stabilized FZD and LRP5/6 receptors allow Wnt ligands to bind more effectively, activating downstream signaling. Upon Wnt binding, the FZD receptor forms a complex with LRP5/6, triggering the recruitment of the Disheveled (DVL) protein to the membrane²⁸. DVL activation inhibits the β-catenin destruction complex (composed of APC, Axin, CK1, and GSK-3β), which normally phosphorylates β-catenin, marking it for ubiquitination and degradation²⁹. By inactivating this destruction complex, β-catenin accumulates in the cytoplasm and eventually translocates into the nucleus³.

Nuclear translocation and gene activation

In the nucleus, β-catenin associates with transcription factors, mainly TCF/LEF (T-cell factor/lymphoid enhancer factor), to activate the transcription of Wnt target genes³⁰. These target genes include those involved in cell cycle progression, proliferation, and stem cell maintenance, which are critical in tissues where LGR4–6 are expressed, such as the intestines, hair follicles, and mammary glands³¹.

Role in stem cell maintenance and regeneration

By potentiating Wnt/β-catenin signaling, LGR4–6 supports the maintenance of adult stem cell populations and facilitates tissue regeneration, as seen in tissues with high regenerative capacity³². LGR5, for instance, is a well-established stem cell marker in the intestinal crypts, where it helps maintain the pool of intestinal stem cells required for rapid epithelial turnover³³.

Implications in cancer progression

The amplification of Wnt signaling through LGR4–6 can also contribute to cancer progression²¹. Aberrant activation of Wnt/β-catenin signaling due to mutations or overexpression of LGR4–6 can lead to uncontrolled cell proliferation and resistance to apoptosis, promoting tumorigenesis³⁴. LGR5, in particular, is often overexpressed in colorectal cancer and serves as a cancer stem cell (CSC) marker, suggesting that its interaction with Wnt signaling supports the survival and proliferation of CSCs, contributing to tumor initiation, growth, and metastasis³⁵.

The Temporal Expression Patterns of LGR4–6 in Reproductive Tissues and Stem Cells

LGR4 in ovarian and uterine tissues

Ovarian development and folliculogenesis: LGR4 is highly expressed in primordial and primary ovarian follicles, where it plays a critical role in early follicle development^36,37. During folliculogenesis, LGR4 levels increase, especially in response to RSPO2 signaling, which enhances follicle maturation³⁸. LGR4’s expression tends to decrease as follicles reach later stages, suggesting its primary role is in the early growth and establishment of follicular structure³⁹.

Reproductive cycle: In the uterus, LGR4 expression fluctuates in coordination with hormonal changes across the estrous cycle⁴⁰. High LGR4 levels in the endometrium are observed during the proliferative phase, potentially facilitating Wnt signaling that promotes uterine epithelial cell proliferation⁴¹. These levels decline during the secretory phase, aligning with the shift from proliferation to preparation for potential embryo implantation⁴⁰.

LGR5 in fallopian tubes and ovarian epithelium

Embryogenesis and early development: LGR5 shows early expression during embryonic development in the ovarian surface epithelium and fallopian tubes, indicating its role in establishing these structures⁴². The expression of LGR5 in these tissues during organogenesis is essential for forming epithelial stem cell niches, which maintain tissue integrity.

Adult ovulation repair and regeneration: In adult fallopian tubes and the ovarian surface epithelium, LGR5 is expressed in stem cells involved in tissue maintenance and repair, particularly following ovulation, which causes minor trauma to the ovarian epithelium⁴². LGR5’s role here is to promote rapid regeneration of the epithelial layer, thus protecting against repeated damage that could lead to inflammation or malignancy.

Reproductive aging: LGR5 expression tends to decline with age, reflecting the reduced regenerative capacity in reproductive tissues⁴³. This decline is associated with diminished stem cell function in the ovarian and fallopian tube epithelia, which could contribute to increased vulnerability to age-related conditions, such as ovarian cancer^42,44.

LGR6 in mammary glands and skin stem cells

Mammary gland development: LGR6 expression is temporally regulated during key developmental windows, such as puberty and pregnancy, where it supports mammary gland branching and alveolar expansion¹⁵. During pregnancy, LGR6+ stem cells contribute to the rapid growth and differentiation of mammary tissue required for lactation¹⁵.

Wound healing and injury response: In the skin and mammary glands, LGR6 is upregulated following injury, signaling its role in wound healing^14,45. Temporally increased LGR6 expression after epithelial damage aids in activating stem cell populations that promote tissue repair, demonstrating a role in rapid response to injury¹⁴.

Postlactational involution: Following lactation, LGR6 expression declines, correlating with the reduction of stem cell activity as the mammary gland undergoes involution⁴⁶. This reduction in LGR6 levels aligns with tissue remodeling and the return to a prepregnancy state.

Distinct temporal roles and implications in cancer progression

LGR4–6 in tumor initiation and progression: Dysregulation of the temporal expression of LGR4–6 can contribute to cancer initiation, especially in tissues with high regenerative demands^13,47,48. For instance, inappropriate persistence of LGR5 expression in ovarian and colorectal tissues can lead to the continuous proliferation of cells that otherwise would not proliferate postinjury or postovulation, increasing the risk of malignancy^13,49.

LGR4 Function in Ovarian Epithelial Stem Cells

LGR4 plays a significant role in ovarian epithelial stem cells²⁰. Knocking out LGR4 in mice leads to the failure of the development of the female reproductive system, highlighting its importance in reproductive health⁵⁰. Carriers of an LGR4 nonsense mutation show delayed menarche, further underscoring its role in female reproductive maturation⁵¹. Intense LGR4 expression has been observed in primordial and primary follicles, and RSPO2 promotes the development of primary follicles in mice, suggesting that the RSPO2-LGR4 axis is crucial for follicle development⁵⁰.

LGR4 is crucial for ovarian epithelial stem cell function and female reproductive health, as evidenced by the failure of reproductive system development in mice with LGR4 knockout, delayed menarche in carriers of LGR4 mutations, and its intense expression in primordial and primary follicles. RSPO2 enhances primary follicle development through the RSPO2-LGR4 axis.

RSPO1 and LGR5 Function in Ovarian Epithelium Stem Cells

LGR5 in ovarian stem cells is essential from embryogenesis through adulthood in mice⁴¹. It is co-expressed with stem cell markers, such as CD133, ALDH, and Ki67, in the ovarian surface epithelium, indicating its role in stem cell regeneration and repair, particularly after ovulation⁴⁴. LGR5-positive ovarian epithelial cells have been linked to increased tumorigenicity in TP53- and RB1-deficient mice, suggesting its involvement in ovarian cancer⁵².

RSPO1, widely expressed during embryonic ovarian development, is critical for the development of female gonads^53,54. Postnatally, RSPO1 expression is limited to specific cells, including the ovarian epithelium. Increased RSPO1 expression is associated with granulosa cell tumors, indicating its potential role as an oncogene in the ovary^23,55.

LGR5 is crucial for ovarian stem cell regeneration and repair, and its expression is linked to increased tumorigenicity in ovarian cancer. RSPO1, essential for ovarian development, may function as an oncogene in granulosa cell tumors.

The Function of LGR5 in Fallopian Tube Stem Cells

LGR5 is a marker of stem cells in the fallopian tubes^16,56. In vivo models and single-molecule fluorescent in situ hybridization (FISH) experiments have demonstrated the expression of LGR5 in the ovarian surface and fallopian tube epithelium during organogenesis and adulthood⁵². This expression is critical for maintaining epithelial homeostasis and facilitating ovulation repair⁵². Flow cytometry studies have confirmed that a significant proportion (54%–80%) of fallopian tube stem cells (FTSCs) express LGR5⁵⁷.

LGR5 is a key stem cell marker in the fallopian tubes, which is essential for maintaining epithelial homeostasis and supporting ovulation repair. Its expression has been observed in both ovarian surface and fallopian tube epithelium throughout organogenesis and adulthood and confirmed in 54% to 80% of FTSCs.

Functions of RSPO1 and LGR6 in FTSCs

The fallopian tube epithelium comprises secretory and ciliated cells, with peg cells considered stem cells of the fallopian tubes⁵⁸. CD44 and cytokeratin 5 (KRT5) are stem cell markers for peg cells. Their expressions have been implicated in cells of origin for ovarian tumors⁵⁸.

Isolated Ki67+ fallopian tube epithelial cells can form organoids that recapitulate the fallopian tube morphology and cellular composition when grown in media containing Wnt3a, FGF, and EGF^16,59. Adding RSPO to the culture media enhances Wnt signaling and increases the size and number of organoids⁶⁰. Notably, gene analysis revealed increased expression of LGR6, but not LGR5, in these organoids⁶¹. Inhibition of LGR6 expression by Notch signaling blockade leads to differentiation of the organoids, highlighting the importance of LGR6 in maintaining stemness of the fallopian tube epithelium⁶¹.

The fallopian tube epithelium contains stem-like peg cells marked by CD44 and KRT5, and LGR6 is crucial for maintaining its stemness, as evidenced by increased expression in Ki67+ cell-derived organoids, whereby its inhibition induces differentiation.

LGR5 and LGR6 in High-Grade Serous Ovarian Cancer

In high-grade serous ovarian cancer (HGSOC), LGR5 and LGR6 play pivotal roles¹³. Although The Cancer Genome Atlas (TCGA) reports low-frequency amplification of LGR5 (4%) and LGR6 (8%) with no significant copy number variation (CNV), both genes are highly expressed in HGSOC compared with other tumors¹³. LGR5-positive ovarian surface epithelium is considered a potential cell of origin for HGSOC. Studies have demonstrated that LGR5-expressing ovarian epithelial cells can generate HGSOC⁵².

Among the ligands of LGRs, RSPO1 (9%), and RSPO2 (24%) show low-frequency amplification and lack mutations in HGSOC⁶². RSPO1 mRNA is highly expressed in epithelial tumors, and polymorphisms in RSPO1 are risk factors for HGSOC⁶³. The correlation between RSPO1 expression and LGR6 in HGSOC suggests a potential oncogenic role of this signaling axis, although the exact mechanisms remain to be fully elucidated⁶².

Despite the low-frequency amplification of LGR5 and LGR6, both genes are highly expressed and linked to HGSOC development. LGR5-positive ovarian epithelial cells are potential cells of origin, and the RSPO1-LGR6 signaling axis is a likely underlying oncogenic pathway.

Recent Studies on LGR6 and Ovarian Cancer

Recent studies have explored the role of LGR6 in ovarian cancer. Co-expression analysis of stem cell markers (OCT4, SSEA4) and CSC markers (ALDH1/2, CD44, LGR5) with the proliferation marker Ki67 in HGSOC tumors has unveiled the presence of dual-positive proliferating stem cells and CSCs, suggesting a link between stemness and tumor proliferation⁶⁴.

Ruan et al. reported that silencing LGR6 decreased stemness and chemoresistance in ovarian cancer cells by inhibiting the Wnt/β-catenin signaling pathway. Their findings suggest that LGR6 is upregulated in ovarian cancer and associated with poor survival outcomes. Targeting LGR6 in vitro and in vivo could reduce chemoresistance and improve treatment efficacy⁶⁵.

RSPO1 and LGR6 are co-expressed in HGSOC as well as in the normal tissues from which this tumor originates. Their expression is associated with the Wnt signaling pathway and crucially regulates cell fate and proliferation⁶².

The above studies have highlighted the significance of LGR6 in ovarian cancer, particularly its association with stemness and tumor proliferation. Analysis of HGSOC tumors has revealed the presence of dual-positive proliferating stem cells and CSCs, further linking stemness to tumor growth. Silencing LGR6 in ovarian cancer cells decreases stemness and chemoresistance by inhibiting Wnt/β-catenin signaling, suggesting that LGR6 upregulation is linked to poor survival outcomes. RSPO1 and LGR6 are co-expressed in HGSOC and the normal tissues from which this tumor originates, and their expression is associated with Wnt signaling pathways that regulate cell fate and proliferation.

Recent Studies on LGR5 and Ovarian Cancer

LGR5 has been implicated in the progression of ovarian cancer. Sun et al. reported that high LGR5 expression correlates with advanced The International Federation of Gynecology and Obstetrics (FIGO) staging, higher tumor grading, and poor prognosis in patients with ovarian cancer⁶⁶. Liu et al. demonstrated that LGR5 promoted proliferation, metastasis, and epithelial-mesenchymal transition (EMT) through the Notch1 signaling pathway in ovarian cancer cells. Their research highlights the significant association between aberrant LGR5 expression and patient age, tumor histologic type, and distant metastasis⁶⁷.

The above findings underscore the importance of aberrant LGR5 expression in the aggressiveness of ovarian cancer, further highlighting its potential as a therapeutic target.

Recent Studies on LGR4–6 in Other Female Reproductive Organs

Cervix

LGR5 promotes proliferation and tumor formation in cervical cancer cells through the Wnt/β-catenin signaling pathway⁶⁸. LGR5 is associated with chemoresistance and CSC traits in cervical cancer, further supporting its role in cancer progression. Downregulation and overexpression of LGR5 in cervical cancer cell lines resulted in enhanced stem cell characteristics and chemoresistance⁶⁹. LGR5 is a potential prognostic marker in patients with cervical carcinoma⁷⁰. LGR5 is positively expressed in cervical cancer tissues and serves as an independent prognostic biomarker for patients with cervical cancer⁷¹. LGR6 activates the Wnt/β-catenin signaling pathway. A β-catenin/TCF7L2/LGR6 feedback loop is established in the LGR6^high cervical CSCs⁷².

LGR5 has a significant role in mediating the progression of cervical cancer by promoting proliferation, tumor formation, and chemoresistance through the Wnt/β-catenin signaling pathway. It enhances CSC traits and is an independent prognostic biomarker of cervical cancer. LGR6 establishes a feedback loop with β-catenin in cervical CSCs.

Endometrium

LGR4 and LGR5 are expressed in the human endometrium. LGR4 shows increased expression after ovulation, indicating its potential role in endometrial regeneration and function during the menstrual cycle⁷³. LGR5 is expressed in the luminal endometrium and correlates with Ki67 expression, suggesting a role in cellular proliferation⁷⁴. Hormonal regulation, such as progesterone, decreases LGR5 expression in the endometrium, highlighting the dynamic regulation of this receptor during the menstrual cycle⁷⁴. A previous study developed a risk model based on stemness-related genes. High-risk patients had shorter overall survival, and LGR5 was a key factor linked to poor prognosis and accelerated cell proliferation in endometrial cancer⁷⁵. Wnt activation, particularly through the APC/β-catenin pathway and its interaction with hormonal and other signaling pathways, contributes to the development of endometrial cancer⁷⁶.

LGR4 and LGR5 are involved in endometrial regeneration and cellular proliferation, with LGR5 linked to poor prognosis in endometrial cancer. These findings highlight the significance of Wnt signaling and hormonal regulation in endometrial function and cancer development.

Breast

Emerging research on LGR4–6 in the breast tissue has identified their roles in mammary gland development and carcinogenesis^{31,50,77–79}. LGR5 is specifically expressed in the regenerative myoepithelium of human mammary ducts and is associated with higher-grade, triple-negative breast cancer (TNBC). Its expression suggests involvement in postinjury regeneration rather than homeostasis, with potential implications for the prognosis of patients with TNBC⁸⁰. The prevalence of high LGR5 expression in breast cancer, particularly in TNBC, is significant because it hints at a promising therapeutic target for breast cancer treatment⁸¹. However, another study has shown LGR5 expression in a notable percentage of patients with breast cancer but without a significant correlation with tumor characteristics⁸². LGR5 overexpression is associated with reduced relapse-free survival in patients with breast cancer⁸³. These receptors may influence stem cell activity and tumor progression through Wnt/β-catenin signaling, although further studies are needed to fully understand their functions in breast tissue.

Emerging research highlights the role of LGR4–6 in mammary gland development and breast cancer. LGR5 is particularly significant in TNBC and is associated with poorer relapse-free survival. However, its correlation with tumor characteristics remains unclear.

The Roles of LGR4 in Reproductive Tissues or Cells Beyond the Wnt Pathway

LGR4, a GPCR, interacts with multiple ligands, including norrin, which activates Wnt signaling through LGR4 but not LGR5/6⁸⁴. Norrin, an important Wnt ligand, forms a unique dimer that binds and activates the FZD 4 receptor, inducing a ternary complex with Lrp5/6 coreceptors⁸⁵. In the oviduct, LGR4 modulates a WNT-NR5A2 signaling cascade, facilitating epithelial secretory cell maturation and steroidogenesis⁸⁶. LGR4 interacts with Norrin, suggesting a potential role in retinal vascularization⁸⁷. In bone cells, LGR4 acts as an alternative receptor for RANKL, inhibiting osteoclast differentiation⁸⁸. A naturally occurring splice variant, Lgr4-ED, encodes only the ectodomain and functions as a soluble antagonist. Administration of recombinant Lgr4-ED in mice and rats decreased expression of estrogen receptor alpha, aquaporin 1, and steroidogenic genes in gonads, suggesting a role in fine-tuning RSPO/norrin-mediated Lgr4-Wnt signaling during gonadal development⁸⁹. Nidogen-2, a basement membrane protein, serves as an endogenous inhibitor of vascular calcification by binding to the GPCR LGR4 and activating a biased Gαq-PKCα-AMPKα1 signaling pathway⁹⁰. In addition, LGR5 interacts with IQGAP1, promoting cell–cell adhesion through the IQGAP1-Rac1 pathway, independent of its role in Wnt signaling⁹¹. These findings highlight LGRs’ complex functions in reproductive tissues or cells beyond canonical pathways.

Conclusion and Perspectives

LGR4–6 play crucial roles in regulating stem cell activity and tissue regeneration in female reproductive organs (Figure 1). Their involvement in the Wnt/β-catenin signaling pathway highlights their pivotal roles in regulating stem cell activity and tissue regeneration in female reproductive organs.

Figure 1.

Roles of LGR4–6 in female reproductive organs and breast tissue.

Summary of Key Roles of LGR4–6 in Female Reproductive Organs (Table 1)

LGR4: Critical for the development of female reproductive organs. Knockout of LGR4 in mice leads to developmental failures, and its presence is essential for normal follicle development and reproductive health.

LGR5 is a marker for stem cells in the ovary and fallopian tubes, facilitating tissue regeneration after ovulation. It has been implicated in ovarian carcinogenesis, where its overexpression correlates with increased tumorigenicity and poor prognosis in HGSOC. LGR5 is involved in cellular proliferation and potentially in tumor progression in other female reproductive tissues, such as the cervix and endometrium.

LGR6: Similar to LGR5, LGR6 is involved in stem cell regulation within the fallopian tubes and may contribute to chemoresistance in ovarian cancer. It can maintain the stemness in FTSCs, and its inhibition can decrease chemoresistance and stemness traits in ovarian cancer cells.

Table 1.

Summary of the Roles of LGR4–6 in Female Reproductive Organs and Ovarian Cancer.

LGR	Function in female reproductive organs and breast	Role in ovarian cancer	Role in breast cancer
LGR4	• Critical for the development of female reproductive organs. • LGR4 knockout in mice leads to developmental failures, and its presence is essential for normal follicle development and reproductive health. • Involved in mammary gland development and branching morphogenesis. • Regulates epithelial cell proliferation and differentiation.	-	• Overexpression is linked to poor prognosis and aggressive breast cancer phenotypes. • Promotes tumor growth, invasion, and metastasis.
LGR5	• Marker of stem cells in the ovary and fallopian tubes, facilitating tissue regeneration postovulation. • Involved in cellular proliferation in the cervix and endometrium. • Marker of mammary stem cells that contribute to the development and maintenance of the mammary gland. • Critical for the regeneration and remodeling of the mammary epithelium.	• Implicated in ovarian carcinogenesis, where overexpression correlates with increased tumorigenicity and poor prognosis in high-grade serous ovarian cancer (HGSOC).	• Functions as a cancer stem cell marker in breast cancer. • Associated with tumor initiation, progression, and resistance to therapy. • High expression correlates with poor clinical outcomes.
LGR6	• Involved in stem cell regulation within the fallopian tubes. • Marker of a subset of mammary stem cells responsible for the long-term maintenance of the mammary gland. • Plays a role in ductal morphogenesis and alveologenesis during puberty and pregnancy.	• Contributes to chemoresistance in ovarian cancer; can maintain the stemness in fallopian tube stem cells. • Inhibition can decrease chemoresistance and stemness traits in ovarian cancer cells.	• Less studied compared to LGR4 and LGR5 but suggested implications in tumor initiation and progression. • May contribute to the development of chemotherapeutic resistance of breast cancer cells.

Summary of LGR4–6 Expression in Various Tissues and Their Roles in Stem Cell Maintenance, Tissue Regeneration, and Cancer Progression

Table 2 provides an overview of the expression patterns and functional roles of LGR4–6 across different tissues, focusing on their contributions to stem cell maintenance, tissue regeneration, and cancer progression. LGR4 is widely expressed in the gastrointestinal tract, ovary, kidney, and other tissues, supporting stem cell renewal, particularly in the intestine, and playing a role in cancer metastasis and invasion. LGR5, a well-known stem cell marker, is highly expressed in tissues like the intestine, hair follicles, and mammary glands, where it maintains adult stem cells and is essential for epithelial regeneration. It also serves as a CSC marker in colorectal and liver cancers, linking it to tumor initiation and metastasis. LGR6, found in the skin, mammary glands, and lung, supports epithelial stem cell populations and aids in skin regeneration and wound healing, with emerging evidence suggesting a role in the development of certain cancers, such as skin and breast cancer. Together, these receptors play critical roles in both normal tissue homeostasis and cancer pathology.

Table 2.

Summary of LGR4–6 Expression in Various Tissues and Their Roles in Stem Cell Maintenance, Tissue Regeneration, and Cancer Progression.

Receptor	Expression in tissues	Role in stem cell maintenance	Role in tissue regeneration	Role in cancer progression
LGR4	Expressed in the gastrointestinal tract, ovary, kidney, liver, and reproductive tissues	Supports stem cell self-renewal, especially in the intestinal crypts and ovarian stem cells	Promotes regeneration in intestinal and skin tissues by activating Wnt signaling	Linked to tumor growth, invasion, and metastasis in several cancers, including colorectal and breast cancer
LGR5	Found in the intestine, stomach, hair follicles, mammary glands, and fallopian tubes	Marks adult stem cells, particularly in intestinal crypts and hair follicles; critical for their maintenance	Plays a key role in the regeneration of gut and skin epithelia	Identified as a cancer stem cell marker in colorectal, ovarian, and liver cancers; associated with tumor initiation and metastasis
LGR6	Detected in skin, mammary glands, and lung tissues	Maintains a subset of epithelial stem cells, particularly in the skin and mammary glands	Involved in skin regeneration and wound healing processes	Suggested to contribute to tumorigenesis in skin and breast cancers, though less studied compared to LGR4 and LGR5

Future Perspectives

LGR receptors, which include LGR4–6, belong to the family of GPCRs and are found on cell membranes⁹². They are distinct among GPCRs for their unique structure: in addition to the traditional GPCR seven-transmembrane domain, they possess a large extracellular LRR domain⁹³. This domain allows LGR receptors to interact with RSPO ligands, which significantly potentiate Wnt/β-catenin signaling, a pathway central to stem cell maintenance, tissue regeneration, and developmental processes⁹⁴. Because of their roles in Wnt pathway modulation, LGR4–6 are widely studied as potential therapeutic targets for regenerative medicine and cancer treatment^13,49,95. However, this potential is accompanied by certain challenges.

Potential as Therapeutic Targets

Stem cell maintenance and tissue regeneration: LGR4–6 receptors play crucial roles in the maintenance of adult stem cells in tissues such as the intestine, mammary glands, skin, and reproductive organs²⁰. Targeting LGR4–6 receptors may support regenerative therapies by enhancing tissue repair and renewal through activation of resident stem cells¹⁴. For example, in diseases or injuries where stem cell activation is beneficial, RSPO-LGR agonists could be used to stimulate Wnt signaling, supporting healing and regeneration in tissues with limited regenerative capacity^96,97.

Cancer therapy: Overexpression or dysregulation of LGR4–6 is associated with cancer progression, particularly in colorectal, ovarian, liver, and skin cancers^13,49,95. Since LGR5 is a marker of CSCs, it presents an opportunity to develop targeted therapies that selectively eliminate CSCs, potentially reducing tumor recurrence and metastasis⁶⁹. Small molecules or antibodies that inhibit LGR5-mediated signaling could prevent CSC proliferation, providing a novel approach to more effectively target and eradicate tumor-initiating cells⁴⁹.

Potential for modulating Wnt/β-catenin signaling in disease: Because LGR receptors potentiate Wnt signaling, which is implicated in various developmental disorders and degenerative diseases, modulating LGR activity offers a means of fine-tuning Wnt pathway activation⁹⁸. Therapeutics that modulate LGR receptors could help rebalance Wnt signaling in diseases where the pathway is either overactive (e.g., cancer) or underactive (e.g., certain degenerative diseases), offering a broad application spectrum^99,100.

Challenges Associated With Targeting LGR Receptors

Complexity of Wnt signaling regulation: The Wnt/β-catenin pathway is highly context dependent, with effects that vary significantly between tissues and developmental stages¹. As LGR4–6 do not directly activate the G-protein pathway like traditional GPCRs but instead amplify Wnt signaling, targeting these receptors without disrupting normal tissue homeostasis is challenging¹⁰¹. Overactivation could lead to unintended tissue proliferation, increasing cancer risk, while underactivation might impair essential regenerative processes¹⁰².

Off-target effects and specificity: The structural similarity between LGR4, LGR5, and LGR6 raises concerns about specificity when developing therapeutics^27,103. Selective targeting is difficult because these receptors often co-express in tissues, and their signaling pathways overlap¹⁰⁴. Designing drugs that can specifically inhibit or activate individual LGR receptors without affecting others remains a complex task. Off-target effects could disrupt normal tissue function or result in undesirable side effects, especially in rapidly proliferating tissues¹⁰⁵.

Tumor heterogeneity and resistance in cancer therapy: In cancers, particularly those with LGR5 as a CSC marker, tumor heterogeneity complicates therapy¹⁰⁶. CSCs often exhibit resistance to traditional therapies, and selective targeting of LGR5+ CSCs may still allow for tumor recurrence if other pathways compensate¹⁰⁷. In addition, cancers may adapt to targeted therapies over time, potentially leading to therapeutic resistance and limiting the long-term effectiveness of LGR-targeted treatments¹⁰⁸.

The roles of LGR4–6 in female reproductive organs are increasingly being recognized, especially in the context of stem cell biology and cancer. Future research should focus on elucidating the precise molecular mechanisms by which these receptors contribute to normal physiology and pathological conditions, such as cancer. This understanding can facilitate the development of targeted therapies aimed at modulating LGR signaling pathways for the treatment of reproductive cancers and other related disorders.

Footnotes

Authors’ Contributions

Conceptualization: D.C.D.; Methodology: K.H.W.; software, D.-C.D.; validation, D.-C.D., K.-H.W., and K.-C.W., and Y.-H.C.; formal analysis: D. C. D., K.-H.W., and K.-C.W., and Y.-H.C.; interpretation of data: K.-C.W. and D.-C.D.; resources: D.-C.D. and Y.-H.C.; data curation: Y.-H.C. and D.-C.D.; writing: D.-C.D., K.-H.W., and K.-C.W., and Y.-H.C.; original draft preparation: K.-H.W., Y.-H.C., and D.-C.D.; review and editing: D.-C.D.; supervision: D.-C.D. All authors have read and agreed to the published version of this manuscript.

Availability of Data and Material

The datasets generated and/or analyzed in the current study are available from the corresponding author upon reasonable request.

Ethical Approval

This study was approved by our Institutional Review Board.

Statement of Human and Animal Rights

This article does not contain any studies with human or animal subjects.

Statement of Informed Consent

There are no human subjects in this article and informed consent is not applicable.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was funded by the National Science and Technology Council (grant nos: NSTC 111-2314-B-303-019 and NSTC 112-2314-B-303-010-MY3).

ORCID iD

Dah-Ching Ding

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Role of Leucine-Rich Repeat–Containing G-Protein-Coupled Receptors 4–6 (LGR4–6) in the Ovary and Other Female Reproductive Organs: A Literature Review

Abstract

Keywords

Introduction

Mechanism of Interaction Between LGR4–6 and Wnt/β-Catenin Signaling Pathway

LGR4–6 as receptors for RSPO ligands

Stabilization of FZD and LRP5/6 co-receptors

Activation of Wnt/β-catenin signaling

Nuclear translocation and gene activation

Role in stem cell maintenance and regeneration

Implications in cancer progression

The Temporal Expression Patterns of LGR4–6 in Reproductive Tissues and Stem Cells

LGR4 in ovarian and uterine tissues

LGR5 in fallopian tubes and ovarian epithelium

LGR6 in mammary glands and skin stem cells

Distinct temporal roles and implications in cancer progression

LGR4 Function in Ovarian Epithelial Stem Cells

RSPO1 and LGR5 Function in Ovarian Epithelium Stem Cells

The Function of LGR5 in Fallopian Tube Stem Cells

Functions of RSPO1 and LGR6 in FTSCs

LGR5 and LGR6 in High-Grade Serous Ovarian Cancer

Recent Studies on LGR6 and Ovarian Cancer

Recent Studies on LGR5 and Ovarian Cancer

Recent Studies on LGR4–6 in Other Female Reproductive Organs

Cervix

Endometrium

Breast

The Roles of LGR4 in Reproductive Tissues or Cells Beyond the Wnt Pathway

Conclusion and Perspectives

Summary of Key Roles of LGR4–6 in Female Reproductive Organs (Table 1)

Summary of LGR4–6 Expression in Various Tissues and Their Roles in Stem Cell Maintenance, Tissue Regeneration, and Cancer Progression

Future Perspectives

Potential as Therapeutic Targets

Challenges Associated With Targeting LGR Receptors

Footnotes

Authors’ Contributions

Availability of Data and Material

Ethical Approval

Statement of Human and Animal Rights

Statement of Informed Consent

Declaration of Conflicting Interests

Funding

ORCID iD

References