Dynamic Regulation of RGS2 in Bone: Potential New Insights into Parathyroid Hormone Signaling Mechanisms

The initial steps involved in mediating the transduction of PTH signal via its G protein-coupled receptors are well understood and occur through the activation of cAMP and phospholipase C pathways. However, the cellular and molecular mechanisms for subsequent receptor desensitization are less well understood. Recently, a new family of GTPase activating proteins known as regulators of G protein signaling (RGS), has been implicated in desensitization of several G protein-coupled ligand-induced processes. At present, it is not known whether any of the RGS proteins play a role in PTH signaling. Using the differential display method, we screened for genes that are selectively expressed after a single sc injection of human PTH (1–38) (8μ g/100 g) in osteoblast-enriched femoral metaphyseal spongiosa of young male rats (3–4 weeks old). We found and cloned one full-length complementary DNA that encodes a 211-amino acid RGS protein and shares 97% sequence identity with mouse and human RGS2. Based on sequence similarity, we have designated this clone as rat RGS2. Northern blot analysis confirmed that the expression of RGS2 messenger RNA (mRNA) is rapidly and transiently increased by human PTH (1–38) in both metaphyseal (4-to 5-fold) and diaphyseal (2- to 3-fold) bone, as well as in cultured osteoblast cultures (2- to 37-fold). In vitro, forskolin and dibutyryl cAMP similarly elevated RGS2 mRNA. In vivo, PTH analog (1–31) [which stimulates intracellular cAMP accumulation, PTHrP (1–34), and prostaglandin E2] induced RGS2 mRNA expression; whereas PTH analogs (3–34) and (7–34), which do not stimulate cAMP production, had no effect on expression. In tissue distribution analysis, RGS2 is widely expressed and was detected in all tissues examined (heart, spleen, liver, skeletal muscle, kidney, and testis), with significant expression in two nonclassical PTH-sensitive tissues: the brain, and the heart. After PTH injection, RGS2 mRNA expression was induced in rat bone but not in any of the other tissues examined. These findings demonstrate that RGS2 is regulated by PTH, prostaglandin E2, and PTHrP and that regulation by PTH in bone occurs via the cAMP pathway. Additionally, these results suggest the exciting possibility that increased RGS2 expression in osteoblasts may be one of the early events influencing PTH signaling.