SPECIES-SPECIFIC GLUCOCORTICOID AND 1,25-DIHYDROXYVITAMIN-D RESPONSIVENESS IN MOUSE MC3T3-E1 OSTEOBLASTS - DEXAMETHASONE INHIBITS OSTEOBLAST DIFFERENTIATION AND VITAMIN-D DOWN-REGULATES OSTEOCALCIN GENE-EXPRESSION

The mouse MC3T3-E1 cell line is nontumorigenic and undergoes a typical program of osteoblast differentiation in vitro, producing a bone-like mineralized extracellular matrix. We report responses of these cells to dexamethasone (Dex) and 1,25-(OH)(2)D-3 that are in contrast to fin dings from other osteoblast culture systems. First, chronic exposure o f both early- and late-passaged MC3T3-E1 cells to 10(-7) M Dex, initia ted during the proliferation period, blocked osteoblast differentiatio n, in contrast to the enhanced differentiation observed in cultures of fetal rat calvarial-derived cells. Secondly, 1,25-(OH)(2)D-3 did not up-regulate expression (messenger RNA or protein synthesis) of the end ogenous mouse osteocalcin (OC) gene. Several lines of evidence are pre sented that suggest this response is caused by sequence specific prope rties of the mouse OC vitamin D response element. We also observed bot h qualitative and quantitative differences in expression of cell growt h (histone H2B) and phenotype-related genes (collagen, OC, osteopontin , glucocorticoid receptor, and 1,25-(OH)(2)D-3 receptor), between pre- and postmineralization stage osteoblasts, in response to 24 h steroid hormone treatment. Our findings in MC3T3-E1 cells are consistent with current concepts of selective influences of 1,25-(OH)(2)D-3 and gluco corticoids as a function of osteoblast maturation. However, the inhibi tion of osteoblast differentiation by chronic Dex at 10(-7) M and the down-regulation of OC by 1,25-(OH)(2)D-3 are novel observations releva nt to species-specific responsiveness of mouse bone-expressed genes to steroid hormones during osteoblast differentiation.