Novel protein kinase C isoforms regulate human keratinocyte differentiation by activating a p38δ mitogen-activated protein kinase cascade that targets CCAAT/enhancer-binding protein α

The novel protein kinase C (nPKC) isoforms are important regulators of human involucrin (hINV) gene expression during keratinocyte differentiation (Efimova, T., and Eckert, R. L. (2000) J. Biol. Chem. 275, 1601-1607). Although the regulatory mechanism involves mitogenactivated protein kinase (MAPK) activation, the role of individual MAPK isoforms has not been elucidated. We therefore examined the effects of individual nPKCs on MAPK activation. We observe unique changes whereby nPKC expression simultaneously increases p38 activity and decreases ERK1 and ERK2 activity. Although p38alpha, p38beta, and p38delta are expressed in keratinocytes, only a single isoform, p38delta, accounts for the increased p38 activity. Parallel studies indicate that this isoform is also activated by treatment with the keratinocyte regulatory agents, 12-O-tetradecanoylphorbol-13-acetate, calcium, and okadaic acid. These changes in MAPK activity are associated with increased C/EBPalpha transcription factor expression and DNA binding to the hINV promoter and increased hINV gene expression. Expression of PKCdelta, PKCepsilon, or PKCeta causes a 10-fold increase in hINV promoter activity, whereas C/EBPa expression produces a 25-fold increase. However, simultaneous expression of both proteins causes a synergistic 100-fold increase in promoter activity. These responses are eliminated by the dominant-negative C/EBP isoform, GADD153, and are also inhibited by dominant-negative forms of Ras, MEKK1, MEK3, and p38. These results suggest that the nPKC isoforms produce a unique shift in MAPK activity via a Ras, MEKK1, MEK3 pathway, to increase p38delta and inhibit ERK1/2 and ultimately increase C/EBPa binding to the hINV promoter and hINV gene expression.