14-3-3σ regulates keratinocyte proliferation and differentiation by modulating Yap1 cellular localization.

The homozygous repeated epilation (Er/Er) mouse mutant of the gene encoding 14-3-3σ displays an epidermal phenotype characterized by hyperproliferative keratinocytes and undifferentiated epidermis. Heterozygous Er/+ mice develop spontaneous skin tumors and are highly sensitive to tumor-promoting 7,12-dimethylbenzanthracene/12-O-tetradecanoyl-phorbol-13-acetate induction. The molecular mechanisms underlying 14-3-3σ regulation of epidermal proliferation, differentiation, and tumor formation have not been well elucidated. In this study, we found that Er/Er keratinocytes failed to sequester Yap1 in the cytoplasm, leading to its nuclear localization during epidermal development in vivo and under differentiation-inducing culture conditions in vitro. In addition, enhanced Yap1 nuclear localization was also evident in 7,12-dimethylbenzanthracene/12-O-tetradecanoyl-phorbol-13-acetate-induced tumors from Er/+ skin. Furthermore, short hairpin RNA (shRNA) knockdown of Yap1 expression in Er/Er keratinocytes inhibited their proliferation, suggesting that YAP1 functions as a downstream effector of 14-3-3σ controlling epidermal proliferation. We then demonstrated that keratinocytes express all seven 14-3-3 protein isoforms, some of which form heterodimers with 14-3-3σ, either full-length wild type (WT) or the mutant form found in Er/Er mice. However, Er 14-3-3σ does not interact with Yap1, as demonstrated by coimmunoprecipitation. We conclude that Er 14-3-3σ disrupts the interaction between 14-3-3 and Yap1, and thus fails to block Yap1 nuclear transcriptional function, causing continued progenitor expansion and inhibition of differentiation in the Er/Er epidermis.

14-3-3σ and p63 play opposing roles in epidermal tumorigenesis.

14-3-3σ plays a regulatory role in epidermal epithelial differentiation and loss of 14-3-3σ leads to increased proliferation and impaired differentiation. A tumor suppressor function for 14-3-3σ has been proposed based on the fact that some epithelial-derived tumors lose 14-3-3σ expression. p63, a p53 family member, is a master regulator of epidermal epithelial proliferation and differentiation and is necessary for the epidermal development. The function of p63 in tumorigenesis is still controversial and poorly defined as multiple isoforms have been found to play either collaborative or opposing roles. By using 'repeated epilation' heterozygous (Er/+) mice containing a dominant-negative 14-3-3σ mutation, the functional relationship of p63 with 14-3-3σ in epidermal proliferation, differentiation and tumorigenesis was investigated. It was found that p63, particularly the ΔNp63α isoform, was strongly expressed in 14-3-3σ-deficient keratinocytes and knockdown of p63 remarkably inhibited proliferation in these cells. To study the functional roles of 14-3-3σ and p63 in epidermal tumorigenesis, we adopted a 7,12-dimethylbenzanthracene/12-O-tetradecanoyl-phorbol-13-acetate (DMBA/TPA) two-stage tumorigenesis procedure to induce formation of skin papillomas and squamous cell carcinomas in Er/+ mice and identified strong p63 expression in resultant tumors. The loss of one allele of p63 caused by the generation of Er/+/p63(+/-) double compound mice decreased the sensitivity to DMBA-/TPA-induced tumorigenesis as compared with Er/+ mice. This study shows that p63 and 14-3-3σ play opposing roles in the development of skin tumors and that the accumulation of p63 is essential for Ras/14-3-3σ mutation-induced papilloma formation and squamous cell carcinoma carcinogenesis.