INHIBITION OF MITOGEN-ACTIVATED PROTEIN-KINASE ACTIVITY AND PROLIFERATION OF AN EARLY OSTEOBLAST CELL-LINE (MBA-15.4) BY DEXAMETHASONE - ROLE OF PROTEIN PHOSPHATASES

Chronic glucocorticoid therapy causes rapid bone loss and clinical ost eoporosis. We found that although the glucocorticoid, dexamethasone, s timulated osteoblast maturation, it also inhibited proliferation of a preosteoblastic cell line, MBA-15.4. The dexamethasone-induced decline in preosteoblast proliferation correlated with a 30-40% reduction in protein kinase C/TPA-stimulated mitogen-activated protein kinase (MAPK ) activity. These steroid effects only became evident after 6-24 h tre atment, implying that dexamethasone acts on de novo synthesis of prote ins. Because MAPK is inactivated by dephosphorylation of tyrosine and threonine residues, cells were treated concomitantly for 24 h with dex amethasone and inhibitors of tyrosine (sodium orthovanadate) and/or se rine/threonine phosphatases (sodium fluoride). MAPK activity and cell proliferation were restored when MBA-15.4 cells were treated with vana date, suggesting that dexamethasone up-regulates tyrosine phosphatase activity. Inactivation of serine/threonine phosphatases with sodium fl uoride had no effect. Inhibition of the PKA pathway (which is growth i nhibitory in mature osteoblasts) with H-89 did not reverse the effects of dexamethasone. Pretreatment with dexamethasone inhibited both peak -and extended activation plateau-phases of MAPK activity. Both phases were fully restored by pretreatment with vanadate, implicating more th an one tyrosine phosphatase. Cycloheximide, alone or in combination wi th dexamethasone, prevented drop-off from plateau to basal levels, sug gesting that an inducible dual-specificity phosphatase regulates the p lateau-phase. We conclude that dexamethasone may inhibit preosteoblast growth via a novel tyrosine phosphatase pathway.