1,25-Dihydroxyvitamin D3 and the vitamin D analogue KH1060 induce hyperproliferation in normal mouse epidermis. A BrdUrd/DNA flow cytometric study.

1,25-dihydroxyvitamin D3 (calcitriol) affects differentiation and proliferation of epidermal keratinocytes in vitro and in vivo. We have studied the topical effects of calcitriol (0.08-2.0 micrograms/ml) and of a new vitamin D analogue, the epi-20-analogue KH1060 (0.4-2.0 micrograms/ml) on epidermal proliferation in normal hairless mice. Epidermis was examined at intervals from 4 h to 8 days after a single-dose application. The mitotic rate was assessed by the stathmokinetic method and hyperplasia was scored in histological sections. Cell cycle parameters were measured by bivariate bromodeoxyuridine (BrdUrd)/DNA flow cytometry on isolated epidermal basal cells after pulse-labelling with BrdUrd. Both calcitriol and KH1060 induced a dose- and time-dependent increase in the mitotic rate and in hyperplasia, the latter drug being the most effective. Calcitriol and KH1060 induced changes in the cell cycle traverse compatible with the regenerative reaction seen after other hyperplasiogens, but with an additionally increased accumulation of cells in the G2 phase. This is similar to that seen after topical application of retinoic acid to mouse skin. Our results are thus in contrast to the anti-proliferative effects of calcitriol observed in vitro and following treatment of the hyperproliferative disease psoriasis with calcitriol as well as other vitamin D analogues.