The effects of 1α,24(S)-dihydroxyvitamin D₂ analog on cancer cell proliferation and cytokine expression

It is well established that 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃), the active metabolite of vitamin D, plays a role in regulating proliferation and differentiation of cells, in addition to its classic function in mineral homeostasis. Recent studies have also provided evidence for the involvement of 1α,25(OH)₂D₃ in regulating the immune system. However, therapeutic application of 1α,25(OH)₂D₃ to hyperproliferative diseases such as cancer, or for immunologic purposes, is thwarted by its hypercalcemic activity. In order to overcome this obstacle, analogs of 1α,25(OH)₂D₃ have been produced that exhibit decreased hypercalcemic activity while retaining the growth and immunologic regulating properties. In the present study, the efficacy of 1α,24(S)-dihydroxyvitamin D₂ (1α,24(S)(OH)₂D₂), a vitamin D₂ analog, in restraining cell proliferation was compared to that of 1α,25(OH)₂D₃. In parallel studies, cancer cell lines were grown in increased concentrations (10^-10-10^-7 M) of each compound for various incubation periods (1-4 days). Growth was assessed by measuring [³H]thymidine incorporation. The results revealed that 1α,24(S)(OH)₂D₂ significantly inhibits proliferation to an extent similar to that observed for 1α,25(OH)₂D₃. Moreover, incubating the human leukemia cell line, HL-60, with 1α,24(S)(OH)₂D₂ resulted in an induction of differentiation of these promyelomonocyte cells into monocyte-macrophage-like cells, in a manner similar to that observed with 1α,25(OH)₂D₃. Using a Western procedure, it was also shown that 1α,24(S)(OH)₂D₂ like 1α,25(OH)₂D₃ enhances the expression of vitamin D receptors (VDR) in the rat osteosarcoma cell line, ROS 17/2.8. The expression of tumor necrosis factor (TNF) alpha (TNF-α) in human peritoneal macrophages (HPM) obtained from uremic patients treated with continuous ambulatory peritoneal dialysis (CAPD) was found to be regulated by 1α,25(OH)₂D₃ as well as by 1α,24(S)(OH)₂D₂. Incubations of HPM with 1α,25(OH)₂D₃ or 1α,24(S)(OH)₂D₂, have inhibited the expression of TNF-α on both mRNA and protein levels. These results suggest that 1α,25(OH)₂D₃ has a role in controlling the rate of inflammation in the peritoneal cavity of CAPD treated patients. Since 1α,24(S)(OH)₂D₂ does not cause hypercalcemia, the present results encourage the possible use of this vitamin D₂ analog in the treatment of cancer and hyper-inflammatory diseases.