Abstract
According to the American Association of Cancer Research (AACR), a Cancer Stem Cell is a cell within a tumor that possesses the capacity to self-renew and to cause the heterogeneous lineages of cancer cells that constitutes the tumor [1]. Cancer Stem Cells (CSCs) are involved in the metastatic process, in the resistance to therapeutic treatments of many types of human cancers and consequently in the onset of recurrences. Numerous translational studies have been conducted to understand CSC characteristics and evaluate association between CSC-related biomarkers and clinical outcomes. The CSC theory can explain also a tumor relapse after that a tumor has been completely surgically removed (R0 macroscopical zero residual resection) or after an apparently complete response to chemotherapy. CSCs, in fact, showed a marked ability to reduce intracellular accumulation of chemotherapic agents by active drug extrusion, increased chemoresistance and survival, as well as elevated membrane transporter activity. In addition, it is possible that these cancer stem cells may nest in the “secured” (niche) sites of our body, where they may remain undisturbed for a long time, even years, until a stimulus arrives to awaken them, causing the disease to resume. CSCs, in fact, are able to use a variety of cellular pathways to survive to anticancer treatments. More recently CSCs have been described in several solid tumors, expressing specific biomarkers. Another field of research should be focused on the realization of diagnostic instruments to follow up patients after R0 surgical resection or after a complete response for an early detection and management of relapse and metastasis.
Introduction
An outstanding research effort is currently pointing towards the understanding of the molecular mechanisms underlying the proliferation of tumors. Clinical evidence indicates indeed the presence of different responses to same anticancer treatments, thus revealing the existence of mechanisms, at the molecular level, not fully unraveled yet. The caveat to be borne in mind is that any cancer formation carries a distinctive genetic structure, which determines its evolution and the reaction to external stimuli [2].
Genetic diversity within tumors can act as a substrate for natural selection and tumor evolution. In addition, genomic instability processes during tumor development cause heterogeneity among tumors [3].
The analysis of the genetically driven instability of tumors received a major boost with the introduction of stochastic models as well as hierarchical models [4, 5, 6]. In the stochastic modelling perspective, tumors are considered as biologically homogeneous and the evidence of different responses to clinical treatment is traced back to genetic mutations on individual cells which occur according to probabilistic laws [7]. Cancer heterogeneity has been also explained by a Darwinian evolutionary system: genotoxic agents (UV light, tobacco-specific carcinogens, chemotherapeutics) induce mutational processes at the DNA sequences and only the fittest variants survive, subject to a natural selection [8].
On the other hand, the hierarchical model of cancer proposes that a tumor arises from CSCs generated by mutations in either normal embryonic stem cells (ESCs) or progenitor cells, which may be present at birth or accumulated over time resulting in cells possessing the ability for uncontrolled growth and propagation [9, 10].
In this paper, we define the CSC model, the functional features of CSCs, and the clinical implications of CSC-associated biomarkers, usable as target for drugs, able to overcome the resistance of tumors to the common chemotherapy in order to prevent and monitor a relapse or a metastasis.
Cancer stem cells: Models, properties and functional biomarkers
Tumor heterogeneity, progression and therapeutic resistance have been interpreted with the Darwinian principles of evolution [8]. As mentioned above, the stochastic approach to tumor growth modelling the evolution of cancer cells is influenced by random factors in which the intrinsic characteristics play no special role. On the contrary, in the hierarchical model perspective the specific functional ability of a specific class of cells (CSCs) may give rise to tumor growth.
This system is not unidirectional and the two different approaches are not alternative, as recent studies have observed the ability of non-CSCs to “de-differentiate” into CSCs, due to epigenetic or environmental factors, which further increase the complexity of tumor biology and treatment [11].
The CSC theory can explain also a tumor relapse after that a tumor has been completely surgically removed (R0 macroscopical zero residual resection) or after an apparently complete response to chemotherapy. CSCs, in fact, showed a marked ability to reduce intracellular accumulation of chemotherapic agents by active drug extrusion, increased chemoresistance and survival, as well as elevated membrane transporter activity.
In addition, it is possible that these cancer stem cells may nest in the “secured” (niche) sites of our body, where they may remain undisturbed for a long time, even years, until a stimulus arrives to awaken them, causing the disease to resume. CSCs, in fact, are able to use a variety of cellular pathways to survive to anticancer treatments [12].
For example, in the case of blood cancer, it has been hypothesized that leukemic stem cells are going to fall within the bone marrow, just in the same “niche” that holds and protects healthy stem cells.
Another important feature of the CSCs is the expression of different surface markers. At the beginning they have been isolated from acute myeloid leukemia in which the rare CD34
More recently CSCs have been described in several solid tumors, expressing specific biomarkers. Among these markers, both CD44 and CD133 were widely used for isolating CSC from solid tumors. CD44 is also involved in the tumorigenic CSCs of colon cancer, ovarian carcinoma, head and neck squamous cell carcinoma [14] and prostate cancer. CD133 (prominin-1) has been considered to be an important CSC marker in solid malignancies as prostate carcinoma [15], colorectal cancer [16], brain tumor [17], ovarian carcinoma [18], osteosarcoma [14] and lung cancer [19].
In hepatocellular carcinomas (HCC), Zhu et al. [20] demonstrated that cancer stem cells express both CD133 and CD44. HCC expressing stemness-related markers has been reported to show a more aggressive behaviour, compared with conventional HCCs without stemness-related marker expression [21].
In glioblastoma multiforme, CD133 expression correlated with shorter progression-free and overall survival in patients treated with surgery followed by radiotherapy and chemotherapy [22].
High CD133 and CD44v6 expression have been demonstrated to be CSC markers in colorectal cancer, connected with cell migration and generation of distant metastases negative prognostic factor for overall survival [23].
In breast cancer has been observed a correlation between TAZ expression and higher Ki-67 levels and, more importantly, shorter survival outcomes [24] and an association between TAZ expression and pathological complete response rate in HER2-positive breast cancer patients treated with neoadjuvant trastuzumab and chemotherapy [25].
In melanoma CD20 [26], CD133 [27], ABCG2 [27], MDR1 [28], OR CD271 [29] have been described as potential markers of cells with increased tumorigenic potential [1].
Those target tumor stem cells were thought to be extremely rare on the basis of studies showing that only a small fraction of cells in a tumor can seed a tumor in an immunocompromised mouse [30].
Quintana et al. demonstrated that modification in xenotransplantation assays can dramatically increase the detection of cells with tumorigenic potential. Rare tumorigenic cells in NOD/SCID mice may have very common cells with tumorigenic capacity under other conditions. In this study 27% of high aggressive melanoma cells were able to induce tumors in inter- lukin-2 receptor gamma chain deficient NOD/scid mice, which lack T, B and NK cells [31].
Conclusions
In summary, the properties in vitro of tumor cell stem cells are: unlimited proliferative potential, self-renewal and differentiation capacity, expression of embryonic and tissue stem cell markers, high chemoresistance. In vivo, CSCs, instead, show high tumorigenic potential and ability to mimic the histology of the original tumor in murine models.
Stem cells proliferate throughout life and are more likely to accumulate genetic mutations compared to the more differentiated cells with shorter life span [32, 33].
In some instances, studies on synthetic lethality provided useful information that is currently translated into promising clinical trials for selective tumor types. Yet, these studies did not involve the analysis of CSCs, which have a different gene expression pattern than their tumor progeny and may respond differently to targeted therapy [34].
In fact BRAF-mutated Colorectal-CSCs are resistant to BRAF and EGFR targeting agents [35].
CSC targeting drugs should act by interfering with self-renewal, vital anti-apoptotic or metabolic CSCs pathways.
An effective anticancer therapy is expected to target and, eventually, to inhibit the activity of the CSCs. In fact, the latter proved to be rather resistant to current standard treatments. To this aim, a substantial advancement in the anticancer therapy demands an improved method to control of the signaling pathways, known to be responsible for the cancer growth rate.
Another field of research should be focused on the realization of diagnostic instruments to follow up patients after R0 surgical resection or after a complete response for an early detection and management of relapse and metastasis.