The Tumor Suppressor p53 Regulates Polarity of Self-Renewing Divisions in Mammary Stem Cells

Abstract

Stem-like cells may be integral to the development and maintenance of human cancers. Direct proof is still lacking, mainly because of our poor understanding of the biological differences between normal and cancer stem cells (SCs). Using the ErbB2 transgenic model of breast cancer, we found that self-renewing divisions of cancer SCs are more frequent than their normal counterparts, unlimited and symmetric, thus contributing to increasing numbers of SCs in tumoral tissues. SCs with targeted mutation of the tumor suppressor p53 possess the same self-renewal properties as cancer SCs, and their number increases progressively in the p53 null premalignant mammary gland. Pharmacological reactivation of p53 correlates with restoration of asymmetric divisions in cancer SCs and tumor growth reduction, without significant effects on additional cancer cells. These data demonstrate that p53 regulates polarity of cell division in mammary SCs and suggest that loss of p53 favors symmetric divisions of cancer SCs, contributing to tumor growth.

The emerging role of p53 in stem cells

“…However, the geometric expansion of the p53 À/À mammospheres and the 5-to 6-fold increase in the number of SCs per sphere together strongly suggest that p53 controls the mode of SC divisions. In addition, we confirmed in vivo that p53 À/À SCs prevalently adopt a symmetric strategy of self-renewing divisions [34]. Notably, the increased numbers of SCs observed in p53 À/À breast tissues and mammospheres might also be due to reprogramming of progenitor or somatic cells, as a consequence of p53 loss; the relative contribution of reprogramming, however, remains to be determined.…”

“…p53 in mammary stem cells Adult mammary SCs can be cultured in vitro in nonadherent conditions as floating aggregates called mammospheres [31,32]. We have recently characterized both human [33] and mouse [34] mammospheres from normal tissues (wild type, WT) as spherical colonies derived from the clonal expansion of individual mammary SCs. Mammospheres contain an average of 1 SC per sphere and can be propagated over several passages of dissociation and reformation.…”

“…Despite showing self-renewal capability, mammosphere serial replating leads to functional exhaustion of WT SCs, thus indicating that they possess limited life span in culture. In addition, direct imaging of the initial cell division and analysis of the partitioning of the cell fate determinant Numb has revealed that WT mammary SCs prevalently divide through asymmetric division [34], a specific division strategy that ensures both SC self-renewal and differentiation. Following asymmetric division in fact, one daughter cell retains SC identity, while the other commits towards differentiation [35].…”

“…Accordingly, p53 À/À mice, at least in the murine genetic background C57/BL6, rarely develop mammary tumors [34,[36][37][38][39]. Nevertheless, several observations suggest that loss of p53 is involved in the etiopathology of both human and mouse mammary tumors: (i) mutations of p53 are found in many breast cancers and women affected by the Li-Fraumeni syndrome (an inherited predisposition to cancer development linked to germline mutations in the p53 gene) often develop breast tumors [38]; (ii) in the BALB/c background, approximately 75% of p53 À/À mice have microscopic lesions in the mammary gland (sarcomas, epithelial hyperplasia and alterations in stromal morphology) [40]; (iii) transplantation of the p53 À/À BALB/c epithelium into fat pads of WT syngeneic mice leads to the development of mammary carcinomas in 60-75% of mice [40,41]; (iv) in a conditional mammary tumor model, approximately 70% of mice that carry tissue-specific inactivation of p53 develop mammary carcinomas [42]; and (v) in ErbB2 transgenic mice, which develop mammary carcinomas with high penetrance and short latency, p53 impairment is responsible for the immortal behavior and the geometric expansion of mammary CSCs in vitro and for carcinoma growth in vivo [34].…”

Gamma-secretase inhibitors target tumor-initiating cells in a mouse model of ERBB2 breast cancer

“…We performed quantitative sphere-and colony-forming assays, which can be used to approximate the frequency of mammary epithelial stem and progenitor cells in heterogeneous cell populations (Reynolds and Weiss, 1992;Dontu et al, 2003). Although the nature of the cells that form spheres is not known, mouse mammosphere-and tumorsphereforming cells co-fractionate with mammary epithelial stem cells (Liao et al, 2007;Cicalese et al, 2009) and mammary TIC (Liu et al, 2007), respectively. Moreover, agents that affect the frequency of sphere-forming cells correspondingly alter the frequency of mammary epithelial stem cells and TIC suggesting that these cells initiate sphere-formation (Dontu et al, 2003(Dontu et al, , 2004Liao et al, 2007;Mani et al, 2008;Cicalese et al, 2009).…”