Notch signaling mediates p63-induced quiescence: a new facet of p63/Notch crosstalk.

Cellular quiescence, a specialized form of growth arrest, is a carefully controlled developmental program that is essential for the maintenance of normal stem cell regenerative potential. Accumulating evidence suggests that the quiescence program may be co-opted by tumor cells, increasing their long-term survival and decreasing their sensitivity to a variety of anticancer therapies. In the mammary gland, pathways involved in stem cell self-renewal include those controlled by p63, a p53-related transcription factor, and Notch, a family of proteins with diverse roles in development and cancer. In the September 15th issue of Cell Cycle, Kent et al. provide new mechanistic insights into the maintenance of mammary cellular quiescence. They demonstrate that expression of ΔNp63a, the predominant isoform of p63 expressed in mammary epithelial cells, promotes cellular quiescence, and they provide direct evidence that activation of Notch3, one of four Notch family members expressed in mammals, is a principle mediator of p63-induced quiescence (Fig. 1A). These findings add a new dimension to the already substantial and complex interplay between p63 and Notch signaling in the epithelium.

Figure 1. Stage-specific regulation and function of p63 and Notch signaling in the mammary gland. (A) Activation of Notch expression by p63 inhibits cell cycle progression, thereby promoting quiescence which is associated with self-renewal of mammary stem cells. (B) In contrast, reciprocal inhibition of p63 and Notch signaling controls cell fate determination of bipotential progenitor cells along basal (p63-associated) or luminal (Notch-associated) epithelial lineages.

An essential role for p63 in early mammary gland development is evidenced by the phenotype of constitutive p63-null mice, which show a complete lack of stratified squamous epithelia and their derivatives, including mammary glands, and die shortly after birth., This dramatic phenotype has been attributed in part to a fundamental role for p63 in maintaining proliferative potential of stem cells in the epithelium, which is consistent with a contribution by p63 to stem cell quiescence. In the adult mammary gland, p63 expression is restricted to the basal rather than luminal epithelial layer, and p63 expression has been shown to contribute to the induction and/or maintenance of basal cell fate. Remarkably similar dual roles for Notch signaling in stem cell preservation and cell fate determination in the mammary gland have been described. Notably, however, whereas both p63 and Notch family members, including Notch3, have been linked to self-renewal of stem-like cells, Notch signaling is thought to drive commitment toward the luminal rather than basal cell fate. Indeed, in vivo models of normal mammary gland development have demonstrated the ability of p63 expression to counter Notch signaling and promote basal cell fate, while Notch activation drives luminal differentiation associated with p63 downregulation (Fig. 1B)., Collectively, the manuscript by Kent et al. and data cited above support a model involving complex, reciprocal regulation of p63 and Notch in the mammary gland.

How do we reconcile experiments demonstrating positive regulation vs. those showing negative reciprocal regulation of p63 and Notch expression and function? One possible explanation is that Notch-stimulating and -repressing activities of p63 may in fact be occurring in different cells. For example, in addition to regulating expression of Notch itself, p63 is also known to regulate expression of Notch ligands, which could control Notch signaling in a non-cell-autonomous manner. Furthermore, selective regulation of different Notch family members by p63 may be involved, as Notch1 and Notch2 are expressed at higher levels in luminal rather than basal cells, whereas Notch3 and Notch4 are comparably or possibly more highly expressed in basal epithelial cells. Analysis of distinct mammary cell populations derived from in vivo studies should help clarify the details of Notch and p63 cross regulation.

Further work will also be required in order to understand the fascinating association of p63 and Notch signaling pathways with quiescence and preservation of early stem/progenitor cells on the one hand, and with proliferation and differentiation of later committed progenitors on the other (Fig. 1).- This dichotomy could again be explained by differential expression and functions for different Notch family members in distinct developmental stages or, in the case of p63, by differential isoform expression. Another possibility involves expression levels, as suggested by a recent study that used an in vitro three-dimensional model to show that high Notch activity inhibits proliferation, whereas low Notch activity induces a hyperproliferative response. Both cellular context and other collaborating signal transduction pathways are also likely to contribute to this stage-specific phenotypic output of Notch and p63 activation. In this regard, it is intriguing that genetic evidence points to context-specific contributions of p63 and Notch to both tumorigenesis and tumor suppression., Unraveling the emerging web of crosstalk between p63 and Notch will be essential if we are to exploit the potential therapeutic benefit of targeting these two pathways in human cancer.