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 a ctivation of cAMP and phospholipase C pathways. However, the cellular and m olecular mechanisms for subsequent receptor desensitization are less well u nderstood. Recently, a new family of GTPase activating proteins known as re gulators of G protein signaling (RGS), has been implicated in desensitizati on 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. Us ing the differential display method, we screened for genes that are selecti vely expressed after a single sc injection of human PTH (1-38) (8 mu 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 thi s clone as rat RGS2. Northern blot analysis confirmed that the expression o f RGS2 messenger RNA (mRNA) is rapidly and transiently increased by human P TH (1-38) in both metaphyseal (4-to B-fold) and diaphyseal (2- to 3-fold) b one, as well as in cultured osteoblast cultures (2- to 37-fold). In vitro, forskolin and dibutyryl cAMP similarly elevated RGS2 mRNA. In vivo, PTH ana log (1-31) [which stimulates intracellular cAMP accumulation, PTHrP (1-34), and prostaglandin E-2] 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 w as detected in all tissues examined (heart, spleen, liver, skeletal muscle, kidney, and testis), with significant expression in two nonclassical PTH-s ensitive tissues: the brain, and the heart. After PTH injection, RGSS mRNA expression was induced in rat hone but not in any of the other tissues exam ined. These findings demonstrate that RGS2 is regulated by PTH, prostagland in E-2, and PTHrP and that regulation by PTH in bone occurs via the cAMP pa thway. Additionally, these results suggest the exciting possibility that in creased RGS2 expression in osteoblasts may be one of the early events influ encing PTH signaling.