Effects of 1 Alpha,25-dihydroxyvitamin D3 and Glucocorticoids on the Growth of Rat and Mouse Osteoblast-like Bone Cells

The effects of 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) and its interaction with glucocorticoids to regulate bone cell growth were studied in osteoblast-like (OH) cell cultures. Owing to our earlier findings that species difference and cell density at the time of treatment modified hormonal responses, comparisons were made between rat and mouse cells and sparse and dense cultures. 1,25(OH)2D3 inhibited cell proliferation in both species regardless of cell density. The magnitude of inhibition was larger in mouse cells, but the sensitivity to 1,25(OH)2D3 was the same for both species. Other metabolites, 25(OH)D3 and 24R,25(OH)2D3, were greater than 100-fold less potent than 1,25(OH)2D3 even in serum-free medium, which is similar to their ratio of affinity for the 1,25(OH)2D3 receptor. Dexamethasone, as previously shown, inhibited sparse and dense mouse cell cultures and sparse rat cell cultures while stimulating dense rat cell cultures to grow. The inhibitory actions of 1,25(OH)2D3 were not additive to the inhibitory dexamethasone effects. However, 1,25(OH)2D3 addition resulted in attenuation of the stimulatory effect of dexamethasone. These responses to 1,25(OH)2D3 and dexamethasone were dependent on cell density and not selective attachment of certain cell types at either plating density. In conclusion, the findings demonstrated that 1,25(OH)2D3 exerts an inhibiting action on both mouse and rat bone cell proliferation. This effect must be reconciled with the in vivo beneficial actions of 1,25(OH)2D3 on bone metabolism. Also, the likelihood of decreased cell number must be considered when biochemical activities are assessed after vitamin D treatment in vitro.