Cholecalciferol induces prostaglandin E2 biosynthesis and transglutaminase activity in human keratinocytes

In this study, we examined the effects of cholecalciferol, a primary keratinocyte metabolite and precursor of the hydroxylated form of vitamin D-3, 1 alpha,25-dihydroxyvitamin D-3 [1 alpha,25(OH)(2)D-3], on prostaglandin 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 rate-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 keratinocyte cell line, HaCaT. In HaCaT cells, neither COX-1 mRNA nor protein was detectable without stimulation and COX-1 expression did not increase in response to cholecalciferol treatment. Although cPLA(2) mRNA and protein were constitutively expressed in untreated HaCaT cells, expression levels did not increase in response to cholecalciferol treatment; however, unlike COX-1 and cPLA(2) expression, COX-2 mRNA and COX-2 protein expression increased in response to cholecalciferol treatment, Calphostin C, a potent protein kinase C inhibitor, significantly reduced cholecalciferol-induced PGE(2) production by inhibiting cholecalciferol-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 primarily COX-2 dependent, and (iii) cholecalciferol enhances both COX-2 mRNA and protein expression, via a protein kinase C-dependent mechanism in human keratinocytes, Furthermore, cholecalciferol increased total cellular transglutaminase activity dose dependently, suggesting a potential role for cholecalciferol in regulating the differentiation of human keratinocytes.