Parathyroid hormone and parathyroid hormone-related peptide stimulate insulin-like growth factor-binding protein secretion by rat osteoblast-like cells through a adenosine 3′,5′-monophosphate-dependent mechanism

Specific insulin-like growth factor-binding proteins (IGFBPs) that may enhance or inhibit insulin-like growth factor (IGF) action are produced in various tissues. In the present study we demonstrated that IGFBPs are synthesized and secreted by rat osteoblast-like cells (UMR 106-01). PTH and PTH-related peptide (PTHrP) were potent stimuli for IGFBP production by UMR cells, whereas GH, IGF-I, insulin, epidermal growth factor, and T₃ had little or no effect. A maximal 8- to 30-fold increase in IGFBP production was attained at 10^-7-10^-6 M PTH and PTHrP, with a half-maximal effect at approximately 10^-9 M. By Western blot analysis, PTH and PTHrP markedly and selectively increased the production of 29,000 mol wt (M_r) and, to a lesser extent, 24,000 M_r IGFBPs. Agents that elevate intracellular cAMP by different mechanisms [(Bu)₂CAMP, forskolin, and isobutylmethylxanthine] mimicked the effect of PTH and PTHrP on IGFBP synthesis. In comparison, PTH did not stimulate IGFBP production in fibroblasts and ROS 17/2.8 cells, which secrete IGFBPs of 42,000, 38,000, 34,000, 28,000, and 24,000 M_r, but not of 29,000 M_r. The PTH-responsive IGFBPs from UMR cells were nonglycosylated proteins with preferential affinity for IGF-I over IGF-II. These IGFBPs were not immunoprecipitated with antisera against rat IGFBP-2 or human IGFBP-1. Thus, PTH and PTHrP increase the production in UMR 106-01 cells of discrete IGFBP forms with M_r of 29,000 and 24,000 through a cAMP-mediated mechanism, independent of IGF-I synthesis. Taken with the known effects of PTH on IGF production in bone cells, the data suggest that PTH and PTHrP may modulate local IGF action in bone through the regulation of specific IGFBP availability.