kh-1060 and Osteosarcoma

We have compared the actions of calcitriol and its three synthetic analogs, 20-epi-22-oxa-24a,26a,27a-trihomo-1 alpha,25-dihydroxyvitamin D3 (KH 1060), 1 alpha,24S-(OH)2-22-ene-26,27-cyclopropyl vitamin D3 (MC 903) and 20-epi-1 alpha,25-dihydroxyvitamin D3 (MC 1288), on the expression of two marker genes of differentiated osteoblasts, namely alkaline phosphatase and osteocalcin, using human MG-63 osteosarcoma cells. Calcitriol and the analogs had qualitatively similar stimulatory effects on target-gene activation. Quantitatively, MC 903 behaved in most experiments essentially as the parent compound calcitriol. In vitamin D receptor/DNA complex formation MC 903, however, was more potent than calcitriol. In contrast, the 20-epi analogs, KH 1060 and MC 1288, were much more potent even at lower concentrations, than calcitriol and MC 903 in stimulating alkaline phosphatase activity, osteocalcin mRNA synthesis and osteocalcin secretion. The stimulation occurred to a greater degree and for a longer period than with calcitriol. This effect was apparently mediated by stronger and longer lasting binding of the vitamin D receptor to the osteocalcin vitamin D responsive element by the 20-epi analogs. After a 6-h treatment and during subsequent culture without hormone, the effects of the 20-epi analogs were also stronger and lasted longer than those with calcitriol or MC 903. Collectively, at comparable and lower concentrations, the 20-epi analogs, KH 1060 and MC 1288, mediate much stronger and longer lasting stimulatory effects than calcitriol or its analog MC 903 on target-gene expression associated with the differentiated phenotype of the MG-63 human osteosarcoma cells.

Topics: Alkaline Phosphatase; Base Sequence; Binding Sites; Calcitriol; Cell Differentiation; Dose-Response Relationship, Drug; Gene Expression Regulation, Neoplastic; Humans; Molecular Sequence Data; Osteocalcin; Osteosarcoma; Promoter Regions, Genetic; Receptors, Calcitriol; RNA, Messenger; Stereoisomerism; Time Factors; Transcriptional Activation; Tumor Cells, Cultured

Although numerous studies have shown potent antiproliferative and differentiation-inducing effects of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) and its analogs on cells not directly related to bone metabolism, only few reports focussed on the effects of these analogs on bone. We compared the action of several recently developed analogs with that of 1,25-(OH)2D3 on human (MG-63) and rat (ROS 17/2.8) osteoblast-like cells and on in vitro bone resorption. In MG-63 cells the analogs EB1089 and KH1060 were about 166,000 and 14,000 times more potent than 1,25-(OH)2D3 in stimulating type I procollagen and 100 and 6,000 times more potent in stimulating osteocalcin production, respectively. Also in ROS 17/2.8 cells EB1089 and KH1060 were most potent in inducing osteocalcin synthesis. In vitro bone resorption was 2.3 and 17.5 times more potently stimulated by EB1089 and KH1060, respectively. In MG-63 cells, 1,25-(OH)2D3 and the analogs inhibited cell proliferation, whereas both 1,25-(OH)2D3 and the analogs stimulated the growth of ROS 17/2.8 cells. Differences in potency could neither be explained by affinity for the vitamin D receptor nor by a differential involvement of protein kinase C in the action of the analogs. Together, these data show that also in bone the analogs EB1089 and KH1060 are more potent than 1,25-(OH)2D3 but that the potency of the analogs compared to 1,25-(OH)2D3 is dependent on the biological response. On the basis of these observations it can be concluded that the reported reduced calcemic effect in vivo is not the result of a decreased responsiveness of bone to these analogs. Lastly, in view of eventual clinical application of 1,25-(OH)2D3-analogs, the observed stimulation of in vitro bone resorption and growth of an osteosarcoma cell line warrant in vivo studies to further examine these effects.

Topics: Animals; Antineoplastic Agents; Binding, Competitive; Bone Resorption; Calcitriol; Cell Division; Cells, Cultured; Cholecalciferol; Enzyme Inhibitors; Glyceryl Ethers; Humans; In Vitro Techniques; Osteoblasts; Osteocalcin; Osteosarcoma; Procollagen; Protein Kinase C; Rats; Receptors, Calcitriol; Structure-Activity Relationship; Tumor Cells, Cultured