Parathyroid hormone induces RGS-2 expression by a cyclic adenosine 3′,5′-monophosphate-mediated pathway in primary neonatal murine osteoblasts

Abstract

Parathyroid hormone (PTH) is a promising anabolic agent for the treatment of osteoporosis. However, PTH is also potently catabolic. To help delineate the molecular mediators of PTH’s opposing effects on skeletal metabolism, we have examined PTH-induced regulator of G-protein signaling-2 (RGS-2) expression and function in murine osteoblasts. RGS proteins are GTPase-activating proteins (GAPs) that regulate GTP-binding protein-coupled receptor (GPCR) signaling by enhancing the intrinsic GTPase activity of Gα subunits. We found that 10 nmol/L PTH maximally induced RGS-2 mRNA in murine MC3T3-E1 cells, rat Py1a and ROS-17/2.8 cells, primary mouse osteoblasts (MOB cells), and mouse calvariae organ culture at 1–2 h posttreatment. PTH signaling through its receptor, PTHR1, is coupled to cAMP-protein kinase A (PKA), protein kinase C (PKC), and calcium signaling pathways. We examined the effect of selective signaling agonists and antagonists on RGS-2 expression in MOB cells to determine which pathway(s) mediates PTH-induced RGS-2 expression. Although selective activation of all three pathways led to RGS-2 expression, cAMP-PKA activation with 10 nmol/L PTH and 10 μmol/L forskolin elicited the strongest induction. Similarly, RGS-2 mRNA expression was most strongly inhibited by the PKA inhibitor, H89 (10–30 μmol/L). The phorbol ester, PMA (1 μmol/L), which activates the PKC pathway, and ionomycin (1 μmol/L), which activates the calcium pathway, produced small but detectable elevations in RGS-2 mRNA levels. Overnight treatment with 1 μmol/L PMA to deplete PKC did not affect subsequent RGS-2 induction by PTH, but significantly inhibited PMA-induced RGS-2 expression. Treatment with 1–100 nmol/L PTH(3-34), which does not activate cAMP-PKA signaling, did not induce RGS-2 expression. MOB cells pretreated with 3 μg/mL cycloheximide produced sustained RGS-2 mRNA levels 2 h after 10 nmol/L PTH treatment. Actinomycin D (5 μg/mL) completely blocked 10 nmol/L PTH-induced RGS-2 expression. Finally, we tested the effect of RGS-2 overexpression on PTH- and fluprostenol-induced interleukin (IL)-6 promoter activity in MOB cells. PTH induces IL-6 through PKA activation, whereas fluprostenol induces IL-6 through PKC activation. We found that RGS-2 overexpression significantly inhibited IL-6 promoter activity following fluprostenol treatment, but not following PTH treatment. We conclude that RGS-2 is a PTH-induced primary response gene in murine osteoblasts that is induced mainly through the cAMP-PKA pathway and specifically inhibits Gαq-coupled receptors.