Protein Kinase C (PKC) δ suppresses keratinocyte proliferation by increasing p21 Cip1 level by a KLF4 transcription factor-dependent mechanism

Abstract

PKCδ increases keratinocyte differentiation and suppresses keratinocyte proliferation and survival. However, the mechanism of proliferation suppression is not well understood. The present studies show that PKCδ overexpression increases p21 Cip1 mRNA and protein level and promoter activity and that treatment with dominant-negative PKCδ, PKCδ-siRNA, or rottlerin inhibits promoter activation. Analysis of the p21 Cip1 promoter upstream regulatory region reveals three DNA segments that mediate PKCδ-dependent promoter activation. The PKCδ response element most proximal to the transcription start site encodes six GC-rich DNA elements. Mutation of these sites results in a loss of PKCδ-dependent promoter activation. Gel mobility supershift and chromatin immunoprecipitation reveal that these DNA elements bind the Kruppel-like transcription factor KLF4. PKCδ increases KLF4 mRNA and protein level and KLF4 binding to the GC-rich elements in the p21 Cip1 proximal promoter. In addition, KLF4-siRNA inhibits PKCδ-dependent p21 Cip1 promoter activity. PKCδ increases KLF4 expression leading to enhanced KLF4 interaction with the GC-rich elements in the p21 Cip1 promoter to activate transcription. © 2011 by The American Society for Biochemistry and Molecular Biology, Inc.