p38δ Regulates p53 to Control p21Cip1 Expression in Human Epidermal Keratinocytes

Background: Keratinocytes cease proliferation during differentiation, and the mechanism that mediates these events is not well understood. Results: PKC increases p38 activity, which increases p53 transcription and acts to increase p21(Cip1) promoter activity. Conclusion: PKC drives a MAPK cascade to increase p53 to control keratinocyte proliferation. Significance: This study provides detailed information regarding the mechanisms that control cell proliferation. PKC suppresses keratinocyte proliferation via a mechanism that involves increased expression of p21(Cip1). However, the signaling mechanism that mediates this regulation is not well understood. Our present studies suggest that PKC activates p38 leading to increased p21(Cip1) promoter activity and p21(Cip1) mRNA/protein expression. We further show that exogenously expressed p38 increases p21(Cip1) mRNA and protein and that p38 knockdown or expression of dominant-negative p38 attenuates this increase. Moreover, p53 is an intermediary in this regulation, as p38 expression increases p53 mRNA, protein, and promoter activity, and p53 knockdown attenuates the activation. We demonstrate a direct interaction of p38 with PKC and MEK3 and show that exogenous agents that suppress keratinocyte proliferation activate this pathway. We confirm the importance of this regulation using a stratified epidermal equivalent model, which mimics in vivo-like keratinocyte differentiation. In this model, PKC or p38 knockdown results in reduced p53 and p21(Cip1) levels and enhanced cell proliferation. We propose that PKC activates a MEKK1/MEK3/p38 MAPK cascade to increase p53 levels and p53 drives p21(Cip1) gene expression.