Effects of novel 26,27-dialkyl analogs of 1 alpha,25-dihydroxyvitamin D3 on differentiation-inducing activity of human promyelocytic leukemia (HL-60) cells in serum-supplemented or serum-free culture.

Monocytic differentiation-inducing activity of steroidal side chain-lengthened 26,27-dialkyl analogs of 1 alpha,25-dihydroxyvitamin D3 was examined in human promyelocytic leukemia (HL-60) cells in serum-supplemented or serum-free culture. The order of in vitro potency for reducing nitroblue tetrazolium was 1 alpha,25-dihydroxy-26,27-dimethylvitamin D3 greater than or equal to 1 alpha,25-dihydroxy-26,27-diethylvitamin D3 much greater than 1 alpha,25-dihydroxy-26,27-dipropylvitamin D3 under serum-free culture conditions. Analysis by sucrose density-gradient centrifugation or polyethylene glycol precipitation technique showed that the potency order for differentiation-inducing activity correlated well with binding affinity of these analogs for vitamin D3 receptor of HL-60 cells. Under serum-supplemented culture conditions, the lack of correlation between biologic activity and analog-binding affinity for receptor was caused by differences in binding affinity of these analogs for serum vitamin D-binding proteins. These results suggest that serum vitamin D-binding proteins apparently modulate monocytic differentiation of HL-60 cells by these analogs under serum-supplemented culture conditions.