Effects of Osteoporosis-Inducing Drugs on Vitamin D-Related Gene Transcription and Mineralization in MG-63 and Saos-2 Cells.

Vitamin D3 is important for calcium and phosphate homeostasis. To exert its effects, vitamin D3 has to be enzymatically activated into 1,25D3 (1,25-dihydroxyvitamin D3 ). Regulation by endogenous vitamin D metabolites of the activation and inactivation of 1,25D3 is important to maintain adequate amounts of active vitamin D3 . Vitamin D deficiency and low bone mineral density have been linked to treatments with antiretroviral drugs and glucocorticoids. However, the causes of drug-induced osteoporosis remain unclear. The antiretroviral drugs efavirenz and ritonavir as well as the glucocorticoid dexamethasone were included in this study. Their effects on transcription of vitamin D-regulating enzymes in MG-63 cells were investigated. Ritonavir and dexamethasone both induced transcription of CYP27B1, the enzyme responsible for the formation of 1,25D3 . Efavirenz, however, suppressed CYP27B1 expression. When administered together with endogenous vitamin D metabolites, dexamethasone and efavirenz counteracted the 1,25D3 -mediated up-regulation of CYP24A1, which inactivates 1,25D3 . This suggests that the drugs may interfere with local regulation of the vitamin D metabolizing system in osteoblasts. Studies on mineralization were performed in MG-63 cells and Saos-2 cells by measuring calcium concentrations accumulated over time. The effects of efavirenz, ritonavir and dexamethasone and/or vitamin D metabolites were examined. 1,25D3 induced mineralization in both cell lines. Efavirenz administered alone did not affect mineralization but suppressed the inducing effects of 1,25D3 on mineralization in both MG-63 cells and Saos-2 cells. In summary, the results suggest that antiretroviral drugs and glucocorticoids may adversely affect bone by interference with the vitamin D system in osteoblasts.