Alterations in molecular chaperones and eIF2α during lung endothelial cell apoptosis

We have previously demonstrated that inhibition of CAAX carboxyl methylation with AGGC caused redistribution and condensation of the ER molecular chaperones, glucose-regulated protein (GRP)-94 and calnexin; an effect that was attenuated by overexpression of dominant active RhoA. We have also shown that AGGC decreased GRP94 protein level; an effect that was dependent on caspase activity. In the present study, we tested the effects of inhibition of posttranslational processing of CAAX proteins on localization and protein levels of molecular chaperones and phosphorylation and protein level of eIF2α. We found that both AGGC, which inhibits CAAX carboxyl methylation, and simvastatin, which inhibits CAAX geranylgeranylation, caused relocalization of GRP94, calnexin, and calreticulin, effects that were not seen during endothelial apoptosis induced by TNF-α or ultraviolet (UV) irradiation. These results suggest that posttranslational processing of CAAX proteins is important in maintaining localization of molecular chaperones normally found in the ER. We also noted that AGGC, but not simvastatin, TNF-α, or UV irradiation, decreased protein levels of most molecular chaperones. Increased eIF2α phosphorylation was observed in the early stages of apoptosis, which was independent of the cause of apoptosis. These results suggest that eIF2α phosphorylation is a common early response to apoptosis-inducing stimuli. Interestingly, eIF2α protein level was decreased in the late stages of apoptosis induced by AGGC, TNF-α, and UV irradiation: an effect that was prevented by caspase inhibition. Thus we speculate that caspase(s)-dependent proteolysis of molecular chaperones and eIF2α may be novel signaling pathways of apoptosis. We also speculate that increased eIF2α phosphorylation is a defensive response against endothelial cell apoptosis.