Embryonic signature in breast cancers; Pluripotency roots of cancer stem cells

Abstract

Drug discovery programs for preclinical oncology typically select compounds which have a predilection for inducing cytotoxic effects in cancer cell lines and subsequently, for inhibiting the growth of the transplanted cancer cells in vivo (Winquist et al., 2010). Unfortunately, the cytotoxic effect in vitro and inhibition of tumor growth in animal models are not the end story for curing cancer in preclinical models. The reason behind that is the exciting of small sub type of cells that are relatively resistance to therapy and able to repopulate in vivo, called cancer stem cells (CSCs). O leis et al. recently reported that the pluripotency gene Sox2 but not Oct4 or Nanog is expressed in early stage of breast tumor. Furthermore, the authors demonstrated that Sox2 downregulation, inhibited mammosphere formation and delayed tumor formation in xenograft tumor initiation models (Leis et al., 2012). In this review, we will shed the light on the importance of Sox2 in breast and other tissue tumorigenesis and associated aggressiveness.

1. Brief introduction on Sox2 and breast cancer stem cells

The cancer stem cell (CSC) hypothesis postulates that tumors are maintained by a self-renewing CSC population that undergoes asymmetric cell divisions to give rise both self-renewal and cell committed to differentiate to constitute the majority of tumor cells (McDermott and Wicha, 2010). There are now numerous studies which have identified cancer stem cells in leukemia (Bonnet and Dick, 1997), breast, (Al-Hajj et al., 2003) brain (Singh et al., 2003), lung (Ho et al., 2007), colon (Ricci-Vitiani et al., 2007), and others. Tumor recurrence and treatment failure are well known in cancer therapy and more recently a link gets closed to cancer stem cells. CSCs must have resistance to a given therapy to survive primary treatment. A number of factors that may govern this phenomenon, including stem cell quiescence, protected niche environment, up regulated expression of xenobiotic efflux pumps, enhanced anti-apoptotic and DNA repair pathways (Bomken et al., 2010), In breast tumors, the use of neoadjuvant regimens showed that conventional chemotherapy could lead to enrichment in CSCs in treated patients as well as in xenografted mice (Li et al., 2008; Yu et al., 2007). This suggests that many currently available cancer therapies, affected the bulk of tumor cells, but failed to eliminate CSCs, which survive to regenerate new tumors. The first identification of breast cancer stem cells was defined by the combined expression of cell surface markers CD44⁺/CD24^−/low/lin⁻. As few as 200 of these cells generated tumors in NOD/SCID mice whereas 20,000 cells that did not display this phenotype failed to generate tumor (Al-Hajj et al., 2003). Later studies suggested that aldehyde dehydrogenase 1 (ALDH-1), a detoxifying enzyme responsible for oxidation of retinol to retinoic acid, may be a more potent marker of breast CSCs (Charafe-Jauffret et al., 2010; Ginestier et al., 2007; Morimoto et al., 2009). ALDH-1-positive breast CSCs can induce tumor formation with as few as 500 cells. Breast cancer cells that expressed ALDH-1 were more likely to be estrogen receptor (ER) negative, progesterone receptor (PR) negative, and human-epidermal growth factor receptor type 2 (HER-2) positive, and frequently developed distant metastases. ALDH-1-positive cells are resistant to conventional chemotherapy with paclitaxel and epirubicin (Tanei et al., 2009). Previous studies have shown that adult stem cells can be identified by a side population (SP) phenotype. A SP isolated from the breast cancer cell line MCF7 was found to represent small percentage of the total cell line and it contained the tumorigenic fraction, as demonstrated by transplantation experiments in NOD/SCID mice xenografts. This fraction was also able to reconstitute the initial heterogeneity of the cell line (Kondo et al., 2004; Patrawala et al., 2005).

The Sox gene family has been identified through their homology to the high-mobility group (HMG) box of sex-determining region Y, and encodes transcription factors that bind to DNA through a HMG domain and plays critical roles in cell fate determination, differentiation and proliferation (Kamachi et al., 2000; Wegner, 1999). Induction of Sox2 in mouse neural stem cells blocks their differentiation, and down regulation of Sox2 in these cells causes their premature exit from the cell cycle and differentiation into neurons (Graham et al., 2003). Pluripotency-associated transcription factors like Nanog, Sox2 and Oct4 are known as regulators of embryonic stem cell state, more recently have been identified in tumors of various origins. The expression of embryonic transcriptional factors in high degree may be associated with less differentiated cancers (Ben-Porath et al., 2008) Sox2 was detected as an immunogenic antigen in a significant percentage of small cell lung cancer patients (Gure et al., 2000), meningioma patients (Comtesse et al., 2005) and involved in invasion and metastasis of pancreatic intraepithelial neoplasia (Sanada et al., 2006). In the breast, Sox2 expression has been observed in 43% of basal cell-like breast carcinomas and was found to be strongly correlated with CK5/6, EGFR, and vimentin immunoreactivity, suggesting that Sox2 may play a role in conferring a less differentiated phenotype in these tumors (Rodriguez-Pinilla et al., 2007). Furthermore, Yupeng Chen et al. reported that by screened paraffin-embedded mammary tissue sections from 19 normal and 56 breast cancer patients, the expression of Sox2 was strongly positive in breast carcinoma cells but very weak in normal mammary epithelium cells. Moreover, Western blotting analysis of immunoreactive Sox2 in established mammary epithelial cell lines showed that Sox2 expression was higher in cancer but not normal breast cell lines, suggesting a role in tumorigenesis (Chen et al., 2008).

2. Sox2 and Wnt/β-catenin self-renewal pathway

Different signaling pathways such as Wnt, Hedgehog and Notch have been implicated in various cellular processes during development that include differentiation, migration and proliferation. Interestingly, dysregulation of each of these pathways in the mammary gland generates breast cancers in transgenic mice (Huelsken et al., 2000; Kelly et al., 2004; Soriano et al., 2000; Vorechovsky et al., 1999). A critical and most studied Wnt pathway is canonical Wnt signaling, which functions by regulating the amount of the transcriptional co-activator β-catenin that controls the key developmental gene expression programs (MacDonald et al., 2009). In the absence of secreted glycolipoprotein Wnt, cytoplasmic β-catenin protein is constantly degraded by the action of the Axin complex, which is composed of the scaffolding protein Axin, the tumor suppressor adenomatous polyposis coli gene product (APC), casein kinase 1 (CK1), and glycogen synthase kinase 3 (GSK3). CK1 and GSK3 sequentially phosphorylate the amino terminal region of β-catenin, resulting in β-catenin recognition by β-Trcp, an E3 ubiquitin ligase subunit, and subsequent β-catenin ubiquitination and proteasomal degradation (He et al., 2004). Activation of Wnt receptors blocks the GSK3β activity and degradation of β-catenin, which is translocated to the nucleus where it interacts with members of the T cell factor (TCF)/LEF family of HMG-domain transcription factors to activate Wnt target gene transcription (Cadigan and Nusse, 1997; Eastman and Grosschedl, 1999) Fig. 1. As transcription factors, all Sox proteins, including Sox2, do not stably bind to DNA on their own. Rather, Sox proteins exert their transcription regulation function through interacting with partner proteins (Wilson and Koopman, 2002). β-catenin partnered with Sox2 in gene transcription regulation in breast cancer cells (Chen et al., 2008). Furthermore, blocking canonical Wnt signaling within the developing retina inhibits Sox2 expression, reduces cell proliferation, inhibits the onset of proneural gene expression, and biases individual progenitors toward a nonneural fate, without altering the expression of multiple progenitor markers and mimics the same results by blocking Sox2 (Van Raay et al., 2005).

Figure 1.

Schematic illustration of self-renewal pathway: Wnt/β-Catenin.

3. Sox2 associated aggressiveness and chemoresistance

The lethality of cancer is mainly caused by its properties of metastasis, drug resistance, and subsequent recurrence (Adhikari et al., 2011). High rate of mortality from breast cancer persists due to the emergence of metastases in distant organs, commonly the lungs (Minn et al., 2005). Sox2 expression was detected in 28% of invasive breast carcinoma and high expression may promote metastatic potential (Lengerke et al., 2011). Overexpression of Sox2 in MCF-7 breast cancer cells promotes cell proliferation and tumorigenesis by facilitating the G1/S transition of the cell cycle (Chen et al., 2008). The molecular basis of Sox2 interference with breast cancer metastasis is poorly understood. Knockdown of Sox2 in D121 lung carcinoma cells lead to decrease in the migration of the side population cells and upregulated apoptosis. Furthermore, downregulation of Sox2 in D121 cells markedly suppressed their metastatic potential in syngeneic mice (Xiang et al., 2011). Consistent with lung carcinoma cells, Sox2 promotes cell proliferation, DNA synthesis and increased apoptosis-resistant properties in human prostate cancer cell line DU145. Downregulation of Sox2 decreased tumorigenesis and chemoresistance of DU145 cells in vivo (Jia et al., 2011). It is worthy to note that while overexpression of Sox2 and induced tumorigenesis had been demonstrated in many different types of solid tumors, it is not in case of gastric cancer (Otsubo et al., 2008).

4. Conclusion

This work provides a basis for further studies aimed at investigating the possible way to targeting Sox2- driven cancer cells which may be an effective therapeutic strategy for selective killing of breast cancer stem cells.