Distinct endoplasmic reticulum signaling pathways regulate apoptotic and necrotic cell death following iodoacetamide treatment

Environmental stress induces the synthesis of glucose-regulated proteins (G rps) in the endoplasmic reticulum (ER) and heat shock proteins (Hsps) in th e cytoplasm. Iodoacetamide (IDAM), a prototypical alkyating agent, induces both Grp and Hsp synthesis in renal epithelial cells and causes necrosis wh ich is prevented by prior activation of the ER stress response (pre-ER stre ss) [Liu, H., et al. (1997) J. Biol. Chem. 272, 21751-21759]. In this study , we examined the biochemical pathways leading to IDAM-induced apoptosis an d investigated the role of the ER stress response in apoptotic cell death. The antioxidant N,N'-diphenyl-p-phenylenediamine (DPPD) prevented necrosis after IDAM treatment, but the cells went on to die with hallmarks of apopto sis, i.e., cell detachment, caspase-3 activation, cleavage of poly(ADP-ribo se)polymerase (PARP), and DNA-ladder formation, all of which were blocked b y the general caspase inhibitor zVAD. As with IDAM-induced necrosis, dithio threitol protected against apoptosis, but cell permeable calcium chelators did not, suggesting that distinct biochemical pathways mediate these two fo rms of cell death. Pre-ER stress, but not heat shock, prevented IDAM-induce d apoptosis. pkASgrp78 cells are deficient in Grp78 induction due to expres sion of a grp78 antisense RNA and are more sensitive to necrosis. However, these cells were resistant to IDAM-induced apoptosis and had increased basa l levels of Grp94 and a KDEL-containing protein of about 50 kDa. Thus, the expression of grp78 antisense perturbs ER functions and activates expressio n of other ER stress genes accounting for the resistance to apoptosis. Take n together, the data describe functionally distinct signaling pathways thro ugh which the ER regulates apoptosis and necrosis caused by chemical toxica nts.