Metabolism of 2-Methyl Analogs of 1α,25-Dihydroxyvitamin D₃ in Rat Osteosarcoma Cells (UMR 106)

Abstract

The secosteroid hormone, 1a,25-dihydroxyvitamin D 3 [1a,25(OH) 2 D 3 ] has a wide range of activities: in addition to its classical role in calcium homeostasis, 1a,25(OH) 2 D 3 is also involved in inhibiting proliferation and inducing differentiation of various normal and cancer cells. 1) To investigate the structure-function relationship and to develop potential therapeutic agents for the treatment of cancer, psoriasis and osteoporosis a large number of analogs of 1a,25(OH) 2 D 3 with structural modifications of both the side chain and the A-ring have been synthesized and biologically evaluated. Biological activity studies of these various synthetic analogs of 1a,25(OH) 2 D 3 have shown that many of them possessed highly potent, selective activities. 1)The activities of 1a,25(OH) 2 D 3 and its potent synthetic analogs are known to be primarily vitamin D receptor (VDR) mediated. The structure-function studies of the vitamin D molecule 2) have shown that the 1a,3b-configuration of hydroxyl groups is crucial for effective binding to the VDR and thus for full expression of the vitamin D activity, and epimerization of each hydroxyl group dramatically decreases the activity by about 100 times for 1b and 10 times for 3a 3,4) due to lack of affinity for VDR. We recently synthesized all eight possible A-ring diastereomers of 2-methyl-1a,25(OH) 2 D 3 (these analogs contain a methyl group on C-2 of the A-ring, and differ from each other in the stereochemistry of the methyl group on C-2 and the hydroxyl groups on C-1 and C-3) in order to investigate the structure-function relationship of the natural hormone and demonstrated that in addition to the C-1 and C-3 hydroxyl groups, the configuration of the 2-methyl group also considerably alters the activity.5-7) The 2-methyl analogs show unique biological properties. For example, the VDR binding affinities exhibited by the 1a-isomers are significantly higher than those exhibited by the 1b-isomers. Furthermore, out of all the 1a-isomers, the 2a-methyl isomers, when compared to the corresponding 2b-methyl isomers, showed much higher potency in inducing differentiation of HL-60 cells (Table 1) indicating that the 2a-methyl group enhances the VDR affinity whereas the 2b-methyl group decreases the VDR affinity. Our further studies have shown that the 2a-methyl isomers bearing 1a,3b-hydroxyl groups which strongly induced cell differentiation, failed to stimulate apoptosis. In contrast, the 2b-methyl isomers bearing 1b and 3a-hydroxyl groups which failed to induce differentiation of HL-60 cells, strongly stimulated apoptosis (Fig. 1). These findings indicate that the structural requirement for inducing apoptosis of HL-60 cells was clearly different from that for inducing differentiation of these cells. 8,9) Thus, all these findings suggest that a simple modification such as addition of a methyl group to the hormone can lead to a potent analog and the potency is highly dependent on the configuration not only of the C-1 and C-3 hydroxy groups but also of the 2-methyl group. At present ...

The Role of Vitamin D and the Vitamin D Receptor in Bone Oncology

“…Similarily, whether the VDR is also expressed in other bone cancers such as chondrosarcoma and Ewing's sarcoma, still needs to be investigated. As to the vitamin D metabolizing enzymes, we can be sure that they are widely expressed in primary bone cancers since a lot of studies on vitamin D metabolites have routinely been performed in osteosarcoma and other cell lines (73)(74)(75)(76). However, almost no studies exist with respect to the biological relevance of the expression of vitamin D metabolising enzymes in primary bone cancer.…”