UVB-induced conversion of 7-dehydrocholesterol to 1alpha,25-dihydroxyvitamin D3 in an in vitro human skin equivalent model.
We have previously shown that keratinocytes in vitro can convert biologically inactive vitamin D3 to the hormone calcitriol (1alpha,25-dihydroxyvitamin D3). This study was initiated to test whether the ultraviolet-B-induced photolysis of provitamin D3 (7-dehydrocholesterol), which results in the formation of vitamin D3, 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 cm2 human skin equivalent) at 37 degrees C and irradiated with monochromatic ultraviolet B at wavelengths ranging from 285 to 315 nm (effective ultraviolet doses 7.5-45 mJ per cm2). In our in vitro model irradiation with ultraviolet B resulted in a sequential metabolic process with generation of previtamin D3 followed by the time-dependent formation of vitamin D3, 25-hydroxyvitamin D3, 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.
Lehmann
B
B
Department of Dermatology, Carl Gustav Carus Medical School, Dresden University of Technology, Germany. Bodo.lehmann@mailbox.tu-dresden.de
Genehr
T
T
Knuschke
P
P
Pietzsch
J
J
Meurer
M
M
D002762:Cholecalciferol / Q000096:biosynthesis* / Q000378:metabolism; D003094:Collagen; D003684:Dehydrocholesterols / Q000378:metabolism*; D004307:Dose-Response Relationship, Radiation; D005347:Fibroblasts / Q000378:metabolism / Q000528:radiation effects; D006801:Humans; D006887:Hydroxycholecalciferols / Q000096:biosynthesis*; D015603:Keratinocytes / Q000378:metabolism / Q000528:radiation effects; D017768:Skin, Artificial; D014466:Ultraviolet Rays
D003684:Dehydrocholesterols; D006887:Hydroxycholecalciferols; D002762:Cholecalciferol; D003094:Collagen; C016705:7-dehydrocholesterol; C008088:alfacalcidol