Rapid suppression of 7-dehydrocholesterol reductase activity in keratinocytes by vitamin D

7-Dehydrocholesterol (7DHC) serves as the sterol substrate for both cholesterol and vitamin D-3 (cholecalciferol) synthesis. The pivotal enzyme in these two pathways is 7-dehydrocholesterol reductase (DHCR7), which converts 7DHC to cholesterol. Treatment of adult human epidermal keratinocytes (HEKa) with 10 mu M cholecalciferol resulted in a rapid decrease in DHCR7 activity (19% of control activity at 2 h). This loss of activity was observed only in HEKa cells, a primary cell line cultured from normal human skin, and not in an immortalized skin cell line (HaCaT cells) nor in two hepatoma cell lines. The decrease in DHCR7 activity was not due to direct inhibition or to dephosphorylation of the enzyme, and enzyme protein levels were not decreased. 25-Hydroxyvitamin D-3 had a lesser effect on DHCR7 activity, while 1 alpha,25-dihydroxyvitamin D-3 had no effect on DHCR7, indicating that the vitamin D receptor is not involved. Treatment with cholecalciferol did not lead to the accumulation of 7-dehydrocholesterol, and a 50% decrease in lanosterol synthesis in these cells suggests that cholecalciferol down-regulates the entire cholesterolgenic pathway. As vitamin D has been reported to be an inhibitor of hedgehog (Hh) signaling through Smo, we tested the effect of cyclopamine, an established inhibitor of the His pathway, on DHCR7 activity. Cyclopamine (10 mu M) also rapidly decreased DHCR7 activity (50% of control activity at 3 h), suggesting that vitamin D-3 may modulate DHCR7 activity and cholesterol/vitamin D-3 synthesis by inhibiting hedgehog signaling. This article is part of a Special Issue entitled '17th Vitamin D Workshop'. (C) 2014 Elsevier Ltd. All rights reserved.