p63 regulates proliferation and differentiation of developmentally mature keratinocytes

Abstract

p63 is a multi-isoform p53 family member required for epidermal development. Contrasting roles for p63 in either the initial commitment to the stratified epithelial cell fate or in stem cell-based self-renewal have been proposed. To investigate p63 function in a post-developmental context, we used siRNAs directed against p63 to down-regulate p63 expression in regenerating human epidermis. Loss of p63 resulted in severe tissue hypoplasia and inhibited both stratification and differentiation in a cell-autonomous manner. Although p63-deficient cells exhibited hypoproliferation, differentiation defects were not due to tissue hypoplasia. Simultaneous p63 and p53 knockdown rescued the cell proliferation defect of p63 knockdown alone but failed to restore differentiation, suggesting that defects in epidermal proliferation and differentiation are mediated via p53-dependent and -independent mechanisms, respectively. Furthermore, ⌬Np63 isoforms are the main mediators of p63 effects, although TAp63 isoforms may contribute to late differentiation. These data indicate that p63 is required for both the proliferative and differentiation potential of developmentally mature keratinocytes.[Keywords: Epidermis; skin; differentiation; proliferation; p63; p53] Supplemental material is available at http://www.genesdev.org.

p63 in epithelial development

“…In fact, it has been shown that the blockade of cell proliferation due to lack of p63, is p53-dependent. Consistently, p53 down regulation rescues the cell proliferation defect due to a p63 knockdown, but not the incapacity of p63-less cells to differentiate [29,30]. An additional signaling pathway playing a crucial role in epidermal stratification and keratinocytes differentiation is through Notch [39][40][41][42][43].…”

“…Simultaneous knockdown of p53 in the context of p63 loss is able to rescue cell proliferation, however the differentiation defect is still present [29]. These data [27,29] are therefore somewhat in contrast with the results from Roop's group [24,31], who showed that, in vivo siRNA for TAp63 produced a dramatic phenotype, with failure to organize pluristratified epithelia but no significant effect was observed on the basal layer. However, in our model [27] we investigated the role in early development using a genetically pure model, with the total absence of one isoform, the elegant in vivo study by Roop using siRNA addressed a slightly different question, representing a model of isoform unbalance.…”

“…The accepted activity was its dominant negative effect exerted towards the TAp63 isoform, as suggested by co-transfection experiments in luciferase reporter assays [3]. However, in addition to the competition between TAp63 and Np63 isoforms, growing evidence indicated that these N-terminal isoforms had different transcriptional specificities a specificity confirmed by functional domain characterization [32][33][34], and microarray analysis showing that both isoforms can induce gene transcription [35,29]. As expected from genes important for developmental and differentiation processes, the more strongly represented classes of target genes are those concerned with cell adhesion, the cytoskeleton, and the cell cycle.…”

“…In the same work the roles of p63 in differentiation and cell proliferation were shown to be separated, indeed down-regulation of p63 causes a cell cycle arrest in keratinocytes that is p53-dependent [29;30]. Simultaneous knockdown of p53 in the context of p63 loss is able to rescue cell proliferation, however the differentiation defect is still present [29]. These data [27,29] are therefore somewhat in contrast with the results from Roop's group [24,31], who showed that, in vivo siRNA for TAp63 produced a dramatic phenotype, with failure to organize pluristratified epithelia but no significant effect was observed on the basal layer.…”

TRAF4 is potently induced by TAp63 isoforms and localised according to differentiation in SCCHN

“…In developmentally mature keratinocytes, p63 is required for both the proliferative and differentiation potential of these cells, and a subset of p63 targets are involved in the major signaling pathways for epithelial differentiation such as the Notch, Wnt and TGFb pathways. 32,10 Though little is known about the biological functions of TRAF4, our results indicate that the localization of the TRAF4 protein is associated with histological differentiation of SCCHN and that p63 cooperating with TRAF4 might be involved in the kerati- nocyte differentiation pathway. Since TRAF4 is also transactivated by p53 and TAp73, and as p53 family members are known to interact physically and functionally, 33,34 the in vivo regulation of TRAF4 expression is highly complex and tissue specific.…”