Transforming growth factor β enhances parathyroid hormone stimulation of adenylate cyclase in clonal osteoblast‐like cells

Abstract

The effects of transforming growth factor β (TGFβ) on parathyroid hormone (PTH)-responsive adenylate cyclase were examined in clonal rat osteosarcoma cells (UMR-106) with the osteoblast phenotype. Purified TGFβ incubated with UMR-106 cells for 48 hr produced a concentration-dependent increase in PTH stimulation of adenylate cyclase, with maximal increase in PTH response (37%) occurring at 1 ng/ml TGFβ. TGFβ also enhanced receptor-mediated activation of adenylate cyclase by isoproterenol and prostaglandin E₂ (PGE₂) and nonreceptormediated enzyme activation by cholera toxin and forskolin. In cells in which PTH-stimulated adenylate cyclase activity was augmented by treatment with pertussis toxin, the incremental increase in PTH response produced by TGFβ was reduced by 33%. However, TGFβ neither mimicked nor altered the ability of pertussis toxin to catalyze the ADP-ribosylation of a 41, 000-Da protein, presumably the α subunit of the inhibitory guanine nucleotide-binding regulatory component (G_i) of adenylate cyclase, in cholate-extracted UMR-106 cell membranes. TGFβ also had no effect on the levels of α or β subunits of G_i, as assessed by immunotransfer blotting. In time course studies, brief (≥ 30 min) exposure of cells to TGFβ during early culture was sufficient to increase PTH response but only after exposed cells were subsquently allowed to grow for prolonged periods. TGFβ enhancement of PTH and isoproterenol responses was blocked by prior treatment of cells with cycloheximide but not indomethacin. The results suggest that TGFβ enhances PTH response in osteoblast-like cells by action(s) exerted at nonreceptor components of adenylate cyclase. The effect of TGFβ may involve G_i, although in a manner unrelated to either pertussis toxin-catalyzed ADP-ribosylation of the α subunit of G_i or changes in levels of G_i subunits. The regulatory action of TGFβ on adenylate cyclase is likely to be mediated by the rapid generation of cellular signals excluding prostaglandins, followed by a prolonged sequence of events involving protein synthesis. These observations suggest a mechanism by which TGFβ may regulate osteoblast responses to systemic hormones.