Accumulating evidence suggests that endoplasmatic reticulum (ER) stress by mechanisms that include ER Ca²⁺ depletion via nitric oxide (NO)-dependent downregulation of SERCA2b contributes to -cell death in type 1 diabetes. To clarify if the molecular pathways elicited by NO and ER Ca²⁺ depletion differ, we here compare the direct effects of NO, in the form of the NO donor S-nitroso-N-acetyl-D,L-penicillamine (SNAP), with the effects of SERCA2 inhibitor Thapsigargin (TG) on mitogen-activated protein kinases (MAPK), nuclear factor B (NFB), Bcl-2 proteins, ER stress and apoptosis.

Exposure of INS-1E cells to TG or SNAP caused caspase-3 cleavage and apoptosis. Both TG and SNAP induced activation of the pro-apoptotic transcription factor C/EBP homologous protein (CHOP). However, other classical ER stress-induced markers such as up-regulation of ER chaperone Bip and alternative splicing of the transcription factor Xbp-1 were exclusively activated by TG. TG exposure caused NFB activation, as assessed by IB degradation and NFB binding to its binding sites. Inhibition of NFB or the Bcl-2 family member Bax pathways protected -cells against TG- but not SNAP-induced -cell death.

These data suggest that NO generation and direct SERCA2 inhibition cause two quantitative and qualitative different forms of ER stress. In contrast to NO, direct ER stress induced by SERCA-inhibition causes activation of ER stress signalling pathways and elicit proapoptotic signalling via NFB and Bax.