An evaluation of the biologic activity and vitamin D receptor binding affinity of the photoisomers of vitamin D3 and previtamin D3

Skin is the site of previtamin D3 and vitamin D3 synthesis and their isomerization in response to ultraviolet irradiation. At present, little is known about the function of the photoisomers of previtamin D3 and vitamin D3 in skin cells. In this study we investigated the antiproliferative activity of the major photoisomers and their metabolites in cultured human keratinocytes by determining their influence on 3H-thymidine incorporation into DNA. Our results demonstrated that vitamin D3 and 25-hydroxytachysterol3 were effective in inhibiting 3H-thymidine incorporation at both 10−8 and 10−6 M in a dose-dependent manner. Lumisterol, tachysterol3, 5,6-trans-vitamin D3, and 25-hydroxy-5,6-transvitamin D3 only induced significant inhibition at 10−6 M. 25-Hydroxytachysterol3 was approximately 10- to 100-fold more active than tachysterol3. 7-Dehydrocholesterol was not active even at 10−6 M. The dissociation constants of vitamin D receptor (VDR) for 25-hydroxytachysterol3, 25-hydroxy-5,6-trans-vitamin D3, and 5,6-trans-vitamin D3 were 22, 58, and 560 nM, respectively. The dissociation constants for 7-dehydrocholesterol, tachysterol, and lumisterol were greater than 20 μM. In conclusion, vitamin D3, its photoisomers, and the photoisomers of previtamin D3 have antiproliferative activity in cultured human keratinocytes. However, the antiproliferative activity did not correlate with their binding affinity for VDR. The results suggest that some of the photoproducts may be metabolized to their 25-hydroxylated and 1α,25-dihydroxylated counterparts before acting on VDR. Alternatively, a different receptor may recognize these photoproducts or another mechanism may be involved in modulating the antiproliferative activity of the photoisomers examined.