Signal transduction in the onset of terminal keratinocyte differentiation induced by 1 alpha,25-dihydroxyvitamin D3: role of protein kinase C translocation.

We have investigated the possible involvement of phosphoinositide turnover in the keratinocyte differentiation induced by 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3]. The mass contents of inositol 1,4,5-trisphosphate and 1,2-diacylglycerol and intracellular calcium level were measured in murine keratinocytes stimulated with 1 alpha,25(OH)2D3 or its derivatives. Although production of these second messengers was enhanced, there were no significant differences in time- and dose-dependences between 1 alpha,25(OH)2D3 and its derivatives. These vitamin D3 compounds promoted the translocation from the cytosol to membrane of protein kinase C (PKC). Despite such common profiles in the early signal transduction parameters, only 1 alpha,25(OH)2D3 induced formation of a cornified envelope characteristic of keratinocyte differentiation. Down-regulation of PKC by prolonged pretreatment with PDBu or inhibition of the enzyme with H-7 caused marked suppression of 1 alpha,25(OH)2D3-induced formation of cornified envelopes. These findings imply that PKC is necessary but not sufficient for the onset of terminal differentiation by 1 alpha,25(OH)2D3, and also that another as yet unspecified signal generated specifically by the active vitamin D3 is required for keratinocyte differentiation.