EFFECTS OF RETINOIC ACID AND DEXAMETHASONE ON PROLIFERATION, DIFFERENTIATION, AND GLUCOCORTICOID RECEPTOR EXPRESSION IN CULTURED HUMAN OSTEOSARCOMA CELLS

HOS-8603 is a newly established human osteosarcoma cell line with phen otypic characteristics of osteoblasts. When these cells were grown in monolayer culture in the presence of dexamethasone (Dex) or retinoic a cid (RA), there was a significant inhibition of proliferation in a con centration-dependent manner. The combined effects of Dex and RA depend ed upon the concentrations: at low concentrations (< 10 nM) the effect s of Dex and RA were additive, whereas at high concentrations the effe cts were antagonistic. Anchorage-independent growth studies performed in methylcellulose culture indicated that Dex or RA inhibited colony f ormation by HOS-8603 cells. Treatment of HOS-8603 cells with 100 nM De x induced alkaline phosphatase activity in a time-dependent manner, re aching a maximum of about 6.5-fold over basal levels. All these effect s of Dex on HOS-8603 cells could be reversed by RU 486, a potent antig lucocorticoid. Based upon saturation of specific binding and Scatchard plot analysis, we demonstrated that a saturable, high-affinity glucoc orticoid receptor (GR) existed in HOS-8603 cells, suggesting that the effects of glucocorticoids on HOS-8603 cells are mediated by the speci fic GR. Finally, we further investigated the homologous and heterologo us regulation of GR in HOS-8603 cells. Treatment of these cells with D ex led to a time-dependent decrease in GR concentrations. This homolog ous GR downregulation occurred not only at the level of hormone bindin g but also at the level of GR mRNA. In contrast, RA was capable of inc reasing GR concentrations in a concentration- and time- dependent mann er. Scatchard analysis indicated that the RA-induced increase in GR bi nding capacity was not accompanied by any significant changes in GR af finity. Quantitative dot blot analysis also revealed a time-dependent increase in GR mRNA levels when HOS-8603 cells were treated with RA. T his study shows the potential importance of hormonal interactions in H OS-8603 cell function.