CHOLECALCIFEROL INDUCES PROSTAGLANDIN E-2 BIOSYNTHESIS AND TRANSGLUTAMINASE ACTIVITY IN HUMAN KERATINOCYTES

In this study, we examined the effects of cholecalciferol, a primary k eratinocyte metabolite and precursor of the hydroxylated form of vitam in D-3, 1 alpha,25-dihydroxyvitamin D-3 [1 alpha,25(OH)(2)D-3], on pro staglandin E-2 (PGE(2)) production in human keratinocytes by examining its respective effects on cyclooxygenase-1 (COX-1), cyclooxygenase-2 (COX-2), and cytosolic phospholipase A(2) (cPLA(2)) expression, the ra te-limiting enzymes regulating PGE(2) biosynthesis and differentiation of keratinocytes, Cholecalciferol induced PGE(2) production, whereas 1 alpha,25(OH)(2)D-3 had no effect on PGE(2) production both in normal human epidermal keratinocytes and in the immortalized human keratinoc yte cell line, HaCaT. In HaCaT cells, neither COX-1 mRNA nor protein w as detectable without stimulation and COX-1 expression did not increas e in response to cholecalciferol treatment. Although cPLA(2) mRNA and protein were constitutively expressed in untreated HaCaT cells, expres sion levels did not increase in response to cholecalciferol treatment; however, unlike COX-1 and cPLA(2) expression, COX-2 mRNA and COX-2 pr otein expression increased in response to cholecalciferol treatment, C alphostin C, a potent protein kinase C inhibitor, significantly reduce d cholecalciferol-induced PGE(2) production by inhibiting cholecalcife rol-enhanced COX-2 mRNA and protein expression. These results indicate that (i) 1 alpha,25(OH)(2)D-3 does not induce PGE(2) biosynthesis in keratinocytes, (ii) cholecalciferol-induced PGE(2) production is prima rily COX-2 dependent, and (iii) cholecalciferol enhances both COX-2 mR NA and protein expression, via a protein kinase C-dependent mechanism in human keratinocytes, Furthermore, cholecalciferol increased total c ellular transglutaminase activity dose dependently, suggesting a poten tial role for cholecalciferol in regulating the differentiation of hum an keratinocytes.