UVB-induced conversion of 7-dehydrocholesterol to 1α,25-dihydroxyvitamin D3 in an in vitro human skin equivalent model

We have previously shown that keratinocytes in vitro can convert biologically inactive vitamin D-3 to the hormone calcitriol (1 alpha ,25-dihydroxyvitamin D-3). This study was initiated to test whether the ultraviolet-B-induced photolysis of provitamin D-3 (7-dehydrocholesterol), which results in the formation of vitamin D-3, can generate calcitriol in an in vivo-like human skin equivalent model made of fibroblasts in a collagen matrix as the dermal component and keratinocytes as the epidermal component. Cultures were preincubated with increasing concentrations of 7-dehydrocholesterol (0.53-5.94 nmol per cm(2) human skin equivalent) at 37 degreesC and irradiated with monochromatic ultraviolet B at wavelengths ranging from 285 to 315 nm (effective ultraviolet doses 7.5-45 mJ per cm(2)). In our in vitro model irradiation with ultraviolet B resulted in a sequential metabolic process with generation of previtamin D-3 followed by the time-dependent formation of vitamin 133, 25- hydroxyvitamin D-3, and ultimately calcitriol in the femtomolar range. Unirradiated cultures and irradiated cultures without keratinocytes generated no calcitriol. Irradiation of skin equivalents at wavelengths > 315 nm generated no or only trace amounts of calcitriol. The ultraviolet-B-triggered conversion of 7-dehydrocholesterol to calcitriol was strongly inhibited by ketoconazole indicating the involvement of P450 mixed function oxidases. The amount of calcitriol generated was dependent on the 7-dehydrocholesterol concentration, on wavelength, and on ultraviolet B dose. Hence, keratinocytes in the presence of physiologic concentrations of 7-dehydrocholesterol and irradiated with therapeutic doses of ultraviolet B may be a potential source of biologically active calcitriol within the epidermis.