Vitamin D-3 analogs (MC 1288, KH 1060, EB 1089, CS 1558, and CB 1093): studies on their mechanism of action

Selected 20-epi and 20-normal vitamin D-3 analogs were studied. First, poin t mutations were introduced into human vitamin D receptor (VDR) to identify residues important for ligand binding. In helices three, four and five, Hi s229, Asp232, Ser237 and Arg274 seem to have an important role in the bindi ng of calcitriol. Surprisingly, the 20-epi analog MC 1288 did not bind to S er237. Second, the effects of analogs on VDR degradation were studied. The transcriptionally active 20-epi analogs protected VDR against degradation m ore efficiently than the 20-normal analogs and calcitriol. With proteasome inhibitor MG-132 formation of Sug-1-RXR beta -VDR-VDRE complex was detected . The 20-epi analogs effectively prevented its formation. Thus, the 20-epi analogs induce a VDR conformation, which prevents binding of factors mediat ing VDR degradation. Third, the analogs were found to be powerful regulator s of cell cycle progression in MG-63 cells. They arrested cell cycle in the G0/G1 phase at lower concentrations and earlier time points than calcitrio l. This was accompanied by hypophosphorylation of Rb followed by strong inh ibition of Cdk2 activity. This correlated with increased levels of p27. Cdk 2 and cyclin E levels were downregulated but those of p21 and cyclin D1 wer e not affected. Thus, a similar sequence of events with calcitriol and the analogs in inhibiting MG-63 cell growth was detected but the analogs had mu ch longer lasting and stronger effects than calcitriol. A unifying scheme f or the varying effects of vitamin D-3 analogs is presented. (C) 2001 Elsevi er Science Inc. All rights reserved.