Abstract
Prolongation of cell survival through prevention of apoptosis is considered to be a significant factor leading to anabolic responses in bone. The current studies were carried out to determine the role of the small GTPase, RhoA, in osteoblast apoptosis, since RhoA has been found to be critical for cell survival in other tissues. We investigated the effects of inhibitors and activators of RhoA signaling on osteoblast apoptosis. In addition, we assessed the relationship of this pathway to parathyroid hormone (PTH) effects on apoptotic signaling and cell survival. RhoA is activated by geranylgeranylation, which promotes its membrane anchoring. In serumβstarved MC3T3βE1 osteoblastic cells, inhibition of geranylgeranylation with geranylgeranyl transferase I inhibitors increased activity of caspaseβ3, a component step in the apoptosis cascade, and increased cell death. Dominant negative RhoA and Y27632, an inhibitor of the RhoA effector Rho kinase, also increased caspaseβ3 activity. A geranylgeranyl group donor, geranylgeraniol, antagonized the effect of the geranylgeranyl tranferase I inhibitor GGTIβ2166, but could not overcome the effect of the Rho kinase inhibitor. PTH 1β34, a potent antiβapoptotic agent, completely antagonized the stimulatory effects of GGTIβ2166, dominant negative RhoA, and Y27632, on caspaseβ3 activity. The results suggest that RhoA signaling is essential for osteoblastic cell survival but that the survival effects of PTH 1β34 are independent of this pathway. J. Cell. Biochem. 106: 896β902, 2009. Β© 2009 WileyβLiss, Inc.