Apoptosis Mediated by Activation of the G Protein-Coupled Receptor for Parathyroid Hormone (PTH)/ PTH-Related Protein (PTHrP)

Abstract

The present studies were carried out to evaluate the mechanisms by which PTH/PTHrP receptor (PTHR) activation influences cell viability. In 293 cells expressing recombinant PTHRs, PTH treatment markedly reduced the number of viable cells. This effect was associated with a marked apoptotic response including DNA fragmentation and the appearance of apoptotic nuclei. Similar effects were evidenced in response to serum withdrawal or to the addition of tumor necrosis factor (TNFalpha). Addition of caspase inhibitors or overexpression of bcl-2 partially abrogated apoptosis induced by serum withdrawal. Caspase inhibitors also protected cells from PTH-induced apoptosis, but overexpression of bcl-2 did not. The effects of PTH on cell number and apoptosis were neither mimicked by activators of the cAMP pathway (forskolin, isoproterenol) nor blocked by an inhibitor (H-89). However, elevation of Ca(i)2+ by addition of thapsigargin induced rapid apoptosis, and suppression of Ca(i)2+ by overexpression of the calcium- binding protein, calbindin D28k, inhibited PTH-induced apoptosis. The protein kinase C inhibitor GF 109203X partially inhibited PTH-induced apoptosis. Regulator of G protein signaling 4 (RGS4) (an inhibitor of the activity of the alpha-subunit of Gq) suppressed apoptotic signaling by the PTHR, whereas the C-terminal fragment of GRK2 (an inhibitor of the activity of the beta(gamma)-subunits of G proteins) was without effect. Chemical mutagenesis allowed selection of a series of 293 cell lines resistant to the apoptotic actions of PTH; a subset of these were also resistant to TNFalpha. These results suggest that 1) apoptosis produced by PTHR and TNF receptor signaling involve converging pathways; and 2) Gq-mediated phospholipase C/Ca2+ signaling, rather than Gs-mediated cAMP signaling, is required for the apoptotic effects of PTHR activation.