Keratin 10 gene expression during differentiation of mouse epidermis requires transcription factors C/EBP and AP-2

The epidermis forms a vital barrier composed of stratified keratinocytes and their differentiated products. One of these products, keratin K10, is critical to epidermal integrity, because mutations in k10 lead to abnormal blistering. For the normal expression of k10, differentiation-associated transcription factors C/EBPalpha, C/EBPbeta, and AP-2 are well positioned to play an important role. Here, regulation of the k10 gene is examined in keratinocytes in the skin of normal mice and in transgenic mice carrying targeted deletions of c/ebpbeta and ap-2alpha. In cultured cells, C/EBPalpha and C/EBPbeta are each capable of activating the k10 promoter via three binding sites, identified by site-directed mutagenesis. In a given epidermal cell in vivo, however, the selection of C/EBPalpha versus C/EBPbeta for k10 regulation is determined via a third transcription factor, AP-2. This novel regulatory scheme involves: (1) unique gradients of expression for each transcription factor, i.e., C/EBPbeta and AP-2 most abundant in the lower epidermis, C/EBPalpha in the upper; (2) C/EBP-binding sites in the ap-2alpha gene promoter, through which C/EBPbeta stimulates ap-2alpha; and (3) AP-2 binding sites in the c/ebpalpha promoter, through which AP-2 represses c/ebpalpha. Promoter-analysis and gene-expression data presented herein support a regulatory model in which C/EBPbeta activates and maintains AP-2 expression in basal keratinocytes, whereas AP-2 represses C/EBPalpha in those cells. In response to differentiation signals, loss of AP-2 expression leads to derepression of the c/ebpalpha promoter and activation of k10 as cells migrate upward.