Accumulation of abnormal proteins and endoplasmic reticulum stress accompany neurodegenerative diseases including Huntington's disease. We show that the expression of mutant huntingtin proteins with extended polyglutamine repeats differentially affected endoplasmic reticulum signaling cascades linked to the inositol-requiring enzyme-1 (IRE1) pathway. Thus, the p38 and c-Jun N-terminal kinase pathways were activated, while the levels of the nuclear factor-kappa B-p65 (NF-kappa B-p65) protein decreased. Downregulation of NF-kappa B signaling was linked to decreased antioxidant levels, increased oxidative stress, and enhanced cell death. Concomitantly, calpain was activated, and treatment with calpain inhibitors restored NF-kappa B-p65 levels and increased cell viability. The calpain regulator, calpastatin, was low in cells expressing mutant huntingtin, and overexpression of calpastatin counteracted the deleterious effects caused by N-terminal mutant huntingtin proteins. These results show that calpastatin and an altered NF-kappa B-p65 signaling are crucial factors involved in oxidative stress and cell death mediated by mutant huntingtin proteins.