Pr. Turner et al., Apoptosis mediated by activation of the G protein-coupled receptor for parathyroid hormone (PTH)/PTH-related protein (PTHrP), MOL ENDOCR, 14(2), 2000, pp. 241-254
The present studies were carried out to evaluate the mechanisms by which PT
H/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 i
ncluding DNA fragmentation and the appearance of apoptotic nuclei. Similar
effects were evidenced in response to serum withdrawal or to the addition o
f tumor necrosis factor (TNF alpha). Addition of caspase inhibitors or over
expression of bcl-2 partially abrogated apoptosis induced by serum withdraw
al. Caspase inhibitors also protected cells from PTH-induced apoptosis, but
overexpression of bcl-2 did not. The effects of PTH on cell number and apo
ptosis 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 D
28k, inhibited PTH-induced apoptosis. The protein kinase C inhibitor GF 109
203X partially inhibited PTH-induced apoptosis. Regulator of G protein sign
aling 4 (RGS4) tan inhibitor of the activity of the alpha-subunit of G(q))
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 prot
eins) was without effect. Chemical mutagenesis allowed selection of a serie
s of 293 cell lines resistant to the apoptotic actions of PTH; a subset of
these were also resistant to TNF alpha. These results suggest that 1) apopt
osis produced by PTHR and TNF receptor signaling involve converging pathway
s; and 2) Gq-mediated phospholipase C/Ca2+ signaling, rather than Gs-mediat
ed cAMP signaling, is required for the apoptotic effects of PTHR activation
.