Inhibitors of 25-hydroxyvitamin D3-1alpha-hydroxylase: thiavitamin D analogs and biological evaluation

Evidence that both 1α,25-dihydroxyvitamin D3 and 24-hydroxylated D3 enhance human osteoblast differentiation and mineralization

Vitamin D plays a major role in the regulation of mineral homeostasis and affects bone metabolism. So far, detailed knowledge on the vitamin D endocrine system in human bone cells is limited. Here we investigated the direct effects of 1alpha,25-(OH)2D3 on osteoblast differentiation and mineralization. Also, we studied the impact of 24-hydroxylation, generally considered as the first step in the degradation pathway of vitamin D, as well as the role of the nuclear and presumed membrane vitamin D receptor (VDR). For this we used a human osteoblast cell line (SV-HFO) that has the potency to differentiate during culture forming a mineralized extracellular matrix in a 3-week period. Transcriptional analyses demonstrated that both 1alpha,25-(OH)2D3 and the 24-hydroxylated metabolites 24R,25-(OH)2D3 and 1alpha,24R,25-(OH)3D3 induced gene transcription. All metabolites dose-dependently increased alkaline phosphatase (ALP) activity and osteocalcin (OC) production (protein and RNA), and directly enhanced mineralization. 1Alpha,24R,25-(OH)3D3 stimulated ALP activity and OC production most potently, while for mineralization it was equipotent to 1alpha,25-(OH)2D3. The nuclear VDR antagonist ZK159222 almost completely blocked the effects of all metabolites. Interestingly, 1beta,25-(OH)2D3, an inhibitor of membrane effects of 1alpha,25-(OH)2D3 in the intestine, induced gene transcription and increased ALP activity, OC expression and mineralization. In conclusion, not only 1alpha,25-(OH)2D3, but also the presumed 24-hydroxylated "degradation" products stimulate differentiation of human osteoblasts. 1Alpha,25-(OH)2D3 as well as the 24-hydroxylated metabolites directly enhance mineralization, with the nuclear VDR playing a central role. The intestinal antagonist 1beta,25-(OH)2D3 acts in bone as an agonist and directly stimulates mineralization in a nuclear VDR-dependent way.

Synthesis and biological activities of 2 beta-chloro-, 2 beta-fluoro-, and 2 beta-methoxy-1 alpha,25-dihydroxyvitamin D3

Using 1 alpha,2 alpha-oxido-cholesta-5,7-diene-3 beta,25-diol (2) as a starting material, the provitamins of calcitriol with an additional 2 beta-chloro-, 2 beta-fluoro-, and 2 beta-methoxy-substituent (3,4,5) are obtained by transdiaxial opening of the oxirane ring with nucleophiles. An efficient irradiation process is described and used for the synthesis of the 2 beta-substituted calcitriols NS2 (2 beta-Cl), NS6 (2 beta-F), and NS7 (2 beta-OCH3). The affinity of these three vitamin D3 derivatives to the vitamin D receptor (VDR) and was determined. These three A-ring derivatives of 1,25(OH)2D3 were further tested for their proliferation-inhibitory and anti-adipogenic activity and gene regulatoric activity in the vitamin D3-sensitive, murine, mesenchymal cell line C3H10T1/2. The VDR-affinity of the 2 beta-chloro derivative, NS2 (2 beta-Cl), was identical to 1,25(OH)2D3 and its vitamin D binding protein (DBP)-affinity was in the range of 1,25(OH)2D3. NS2 inhibited the proliferation of C3H10T1/2(BMP-4)-cells in the presence of fetal calf serum (FCS) 9 times, and, in the absence of FCS, 111 times lower, as compared with 1,25(OH)2D3. The ID50 dose of adipogenesis-inhibition of NS2 was 13 times higher than the ID50 dose of 1,25(OH)2D3. NS6 (2 beta-F) displayed a slightly higher affinity than 1,25(OH)2D3 to the VDR and DBP-affinity. The proliferation-inhibitory activity in the presence of FCS was 90 times higher, as compared with 1,25(OH)2D3. In the FCS-free proliferation assay NS6 displayed an inhibitory activity in the range of 1,25(OH)2D3. NS6 showed an 5 times higher potency to inhibit (pre)adipocyte-differentiation in C3H10T1/2(BMP-4)-cells than 1,25(OH)2D3. NS7 (2 beta-OCH3) showed the lowest VDR-affinity and the highest DBP-affinity of the tested substances, as compared with 1,25(OH)2D3 (11 times lower and 35 times higher respectively). Its proliferation-inhibitory activity in the FCS-free medium was 9 times and in the FCS-containing assay 67 times lower in comparison with 1,25(OH)2D3. A 1250 times higher NS7-dose was needed to reach the anti-adipogenic potency of 1,25(OH)2D3. All tested substances displayed a similar ability to activate a vitamin D responsive element-regulated reporter gene compared to 1,25(OH)2D3 (NS2 and NS6: 1.3 times higher activity; NS7: 1,4 times lower activity).