UVB-induced conversion of 7-dehydrocholesterol to 1 alpha,25-dihydroxyvitamin D-3 (calcitriol) in the human keratinocyte line HaCaT

We have previously shown that keratinocytes in vitro can convert biological ly inactive vitamin D-3, to the hormone calcitriol, The present study was i nitiated to test whether ultraviolet B (UVB)-induced photolysis of provitam in D-3, (7-dehydrocholesterol, [7-DHC]) which results in the formation of v itamin D-3, also leads to the generation of calcitriol in keratinocytes. Su bmerged monolayers of Ha-CaT keratinocytes were preincubated with 7-DHC (25 muM at 37 degreesC and irradiated with monochromatic UVB at different wave lengths (effective UV-doses: 7.5-60 mJ/cm(2)) or a narrow-band fluorescent lamp Philips TL-01 (UVB-doses: 125-1500 mJ/cm(2)). Irradiation with both so urces of UVB resulted in the generation of different amounts of previtamin D-3, in our in vitro model followed by time-dependent isomerization to vita min D-3, and consecutive formation of calcitriol in the picomolar range, Un irradiated cultures or cultures exposed to wavelengths >315 mm generated no or only trace amounts of calcitriol, The conversion of vitamin D-3, genera ted after UVB irradiation to calcitriol is inhibited by ketoconazole indica ting the involvement of P450 mixed function oxidases in this chemical react ion. The generation of calcitriol was wavelength- and UVB dose dependent an d reached approximately 18-fold higher levels after irradiation at 297 nm t han at 310 nm (effective UVB dose: 30 mJ/cm(2)). Hence, keratinocytes may b e a potential source of biologically active calcitriol within epidermis, wh en irradiated with therapeutical doses of UVB.