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 osteoporosis. We found that although the glucocorticoid, dexamethasone, stimulated 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 treatment, implying that dexamethasone acts on de novo synthesis of proteins. Because MAPK is inactivated by dephosphorylation of tyrosine and threonine residues, cells were treated concomitantly for 24 h with dexamethasone and inhibitors of tyrosine (sodium orthovanadate) and/or serine/threonine phosphatases (sodium fluoride). MAPK activity and cell proliferation were restored when MBA-15.4 cells were treated with vanadate, suggesting that dexamethasone up-regulates tyrosine phosphatase activity. Inactivation of serine/threonine phosphatases with sodium fluoride had no effect. Inhibition of the PKA pathway (which is growth inhibitory 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 than one tyrosine phosphatase. Cycloheximide, alone or in combination with dexamethasone, prevented drop-off from plateau to basal levels, suggesting that an inducible dual-specificity phosphatase regulates the plateau-phase. We conclude that dexamethasone may inhibit preosteoblast growth via a novel tyrosine phosphatase pathway.