The distinguishing feature of adult stem cells is their extraordinary capacity to divide prior to the onset of senescence. While stratified epithelia such as skin, prostate, and breast are highly regenerative and account disproportionately for human cancers, genes essential for the proliferative capacity of their stem cells remain unknown. Here we analyze p63, a gene whose deletion in mice results in the catastrophic loss of all stratified epithelia. We demonstrate that p63 is strongly expressed in epithelial cells with high clonogenic and proliferative capacity and that stem cells lacking p63 undergo a premature proliferative rundown. Additionally, we show that p63 is dispensable for both the commitment and differentiation of these stem cells during tissue morphogenesis. Together, these data identify p63 as a key, lineage-specific determinant of the proliferative capacity in stem cells of stratified epithelia.

While studies of the adaptor SH3BP2 have implicated a role in receptor-mediated signaling in mast cells and lymphocytes, they have failed to identify its function or explain why SH3BP2 missense mutations cause bone loss and inflammation in patients with cherubism. We demonstrate that Sh3bp2 "cherubism" mice exhibit trabecular bone loss, TNF-alpha-dependent systemic inflammation, and cortical bone erosion. The mutant phenotype is lymphocyte independent and can be transferred to mice carrying wild-type Sh3bp2 alleles through mutant fetal liver cells. Mutant myeloid cells show increased responses to M-CSF and RANKL stimulation, and, through mechanisms of increased ERK 1/2 and SYK phosphorylation/activation, they form macrophages that express high levels of TNF-alpha and osteoclasts that are unusually large. M-CSF and RANKL stimulation of myeloid cells that overexpress wild-type SH3BP2 results in similar large osteoclasts. This indicates that the mutant phenotype reflects gain of SH3BP2 function and suggests that SH3BP2 is a critical regulator of myeloid cell responses to M-CSF and RANKL stimulation.

The recent discoveries of p63 and p73, homologs of the tumor suppressor p53, raised the possibility of a network of these family members governing cell cycle arrest and apoptosis in response to stress. However, mice lacking p73 show no tendency for spontaneous tumors, and mutations in p63 or p73 are rare in human tumors, rendering any obligate role of these genes in cell death and tumor suppression unclear. In an effort to reconcile these incongruent data, we examined the genetic interactions between p53, p63, and p73 in well-established paradigms of p53-dependent and -independent T cell death using primary, genetically defined lymphocytes. Our findings challenge the generality of the notion that p63 and p73 are required for p53 function or for apoptosis in T cells.

SummaryWe have recently identified a second p53-related p73L gene, also referred to as p63/p51/p40/KET gene, which encodes the 2 major isoforms p73L and p51 resulting from different exon usage at their amino terminal regions. Although p73L and p51 are suspected to play oncogenic and tumour suppressive roles in mammalian cells, respectively, no evidence of linkage between the expression of these isoforms and human cancers has been reported so far. In this study, we first investigated the expression profile of p51 and p73L in various human tumour cell lines and found that a novel isoform, termed ∆Np73L, was predominantly expressed in squamous cell carcinomas. The expression profile of ∆Np73L/p73L/p51 in primary human skin cancer specimens showed that the expression of p51 was frequently lost (62%) but was detected in all normal skin samples. In p51-expressing skin cancers, ∆Np73L expression was associated at a high frequency (75%) though it was not detected in normal skin tissues. Transient co-transfection data indicate the possibility that ∆Np73L can inhibit p53-, and more preferentially, p51-mediated transactivation. These data suggest that the loss of expression of p51 and/or the expression of ∆Np73L might contribute to the pathogenesis of human squamous cell carcinomas.

No abstract

Tumor suppressor p53 has been shown to transactivate epidermal growth factor receptor (EGFR) expression through binding to a putative p53 responsive element in the EGFR promoter between nucleotides ؊265 and ؊239 (EGFRp53RE). Isotypes of p63 gene products, recently identified as p53 relatives, have a similar function to transactivate several p53 target gene promoters. However, our results indicate that TAp63␥ has a very low ability to bind to the EGFRp53RE and surprisingly represses both basal EGFR promoter activity and endogenous EGFR expression. Transient transfection assays show that the EGFR promoter region between ؊348 and ؊293, containing two Sp1 sites, is crucial for the repression of the EGFR expression by TAp63␥. Mutations in these Sp1 sites in the reporter constructs result in loss of the TAp63␥ repression effect. We further show that TAp63␥ directly interacts with Sp1 by immunoprecipitation analysis and that TAp63␥ impairs Sp1 binding to the target DNA site in electrophoretic mobility shift assays. These results suggest that TAp63␥ is involved in the regulation of the EGFR gene expression through interactions with basal transcription factors.

Tumor suppressor p53 has been shown to repress expression of vascular endothelial growth factor (VEGF), an endothelial cell-speci®c mitogen and a key mediator of tumor angiogenesis. The p63 gene, recently identi®ed as a p53-relative, encodes multiple isoforms with structural and functional similarities and di erences from p53. In this study, we show the evidence that the two major isoforms of the p63 gene, TAp63g (p51A) and dNp63a (p73L), represses and upregulates VEGF expression, respectively, on transcription and protein levels. Transient transfection assays show that a hypoxia-inducible factor (HIF) 1 binding site within the VEGF promoter region is responsible for both upregulation and repression by dNp63a and by TAp63g, respectively, of the VEGF promoter activity. We also show that TAp63g targets HIF1a for promoting proteasomal degradation but that dNp63a targets HIF1a for stabilization. Mammalian two-hybrid assays show that HIF1a-dependent transcription is repressed by TAp63g as well as by p53, whereas it is upregulated by dNp63a in collaboration with a transcription coactivator p300. Our data also show that dNp63a acts as a dominant-negative reagent toward both p53-and TAp63g-mediated degradation of HIF1a and repression of HIF1a-dependent transcription. These results suggest that p63 is involved in the regulation of the VEGF gene expression and that modulation of VEGF expression by TAp63g and dNp63a is closely correlated with their distinct roles on the regulation of HIF1a stability.

Significance: The skin interfollicular epidermis (IFE) is an organism's first line of defense against a harmful environment and physical damage. During homeostasis and wound repair, the IFE is rejuvenated constantly by IFE stem cells (SCs) that are capable of both proliferation and differentiation. However, the identity and behavior of IFE SCs remain controversial. Recent Advances: Two opposing theories exist regarding homeostasis of the IFE. On the basis of morphological and proliferative characteristics, one posits that the IFE is composed of a discrete epidermal proliferative unit comprised of *10 transit-amplifying (TA) cells and a centrally located SC in the basal layer. The other suggests that homeostasis of the IFE is maintained by a single progenitor population in the basal layer. A recent study has challenged these two apparently distinct models and demonstrated that the basal layer of the IFE contains both SCs and TA cells, which make distinct contributions to tissue homeostasis and repair. Moreover, phosphorylation levels of the transcription factor p63, the master regulator of the proliferative potential of epidermal SCs, can be used to distinguish self-renewing SCs from TA cells with more limited proliferative potential. Critical Issues: As technologies advance, IFE SCs can be identified at a singlecell level. Refinements of their identification and characterization are critical, not only for SC biology but also for the development of novel clinical applications. Future Directions: Understanding the signaling pathways that control selfrenewal and differentiation of IFE SCs will aid in developing novel cell-based therapeutics targeting degenerative epidermal diseases and wound repair. SCOPE AND SIGNIFICANCEIn this review, I discuss some of the major findings that have advanced our understanding of the behavior of epidermal stem cells (SCs) and their immediate progeny, transitamplifying (TA) cells. Particular emphasis is paid to those in the interfollicular epidermis (IFE), in light of their importance in homeostasis and tissue regeneration after injury. I also discuss the recent key findings on the regulation of p63, a transcription factor essential for the maintenance of the proliferative potential of epithelial SCs in both homeostatic and disease conditions of the epidermis, such as chronic wound healing. TRANSLATIONAL RELEVANCEThe role of epidermal SCs in contributing to homeostatic maintenance of the skin and wound repair has been well acknowledged for many years. Over the past decade, characterization of SCs and their differentiating progeny has been successfully refined, owing to the development of nucleotidelabeling and lineage-tracing methodologies. Elucidating the molecular mechanisms controlling the behavior of these cell types will provide novel strategies for the treatment of traumatic and degenerative skin diseases. As p63 is highly expressed in SCs of other epithelial tissues as well as many types of tumors of epithelial origin, these studies will also contribute to our understanding of...