AbstractAimThis paper compares the effects of MC903 (calcipotriol) and 1,25‐dihydroxyvitamin D3[1,25(OH)2D3; calcitriol] on differentiation and proliferation of normal human keratinocytes cultured in serum‐free medium. In order to understand the inhibitory mechanism of MC903, we examined its effect on cell cycle kinetics and the phosphorylation of retinoblastoma gene product (pRB), a tumor suppressor gene products, in normal human keratinocytes.BackgroundThe hormonally active form of vitamin D3, 1,25‐dihydroxy vitamin D3[1,25(OH)2D3], regulates the differentiation and proliferation of epidermal keratinocytes in vitro. MC903 is a novel vitamin D3analogue which is at least 100 times less potent than 1,25(OH)2D3in its effects on calcium homeostasis.MethodsWe analyzed cell differentiation and cell cycle by flow cytometry using a FACScan, and pRB phosphorylation by Western blotting and densitometer.ResultsMC903 induced growth inhibition and differentiation of human keratinocytes. Cell cycle analysis demonstrated that 10‐6M of MC903 induced cell cycle arrest in both G1/G0 (62.4 ± 0.7% versus 56.5 ± 1.7% in control,p<0.01) and G2 + M (19.2 ± 0.3% versus 14.0 ± 0.9% in control,p<0.01) phase. 10‐6M of MC903 also increased the depnosphorylated pRB from 25% at 0 h to 84% at 48 h, as well as 1,25(OH)2D3.ConclusionsSince pRB phosphorylation is supposed to be essential for the progression from G1 to S phase, the inhibition of pRB phosphorylation could be responsible for the G1/G0 growth arrest induced by MC903 in normal human