Nitric oxide-induced apoptosis in pancreatic β cells is mediated by the endoplasmic reticulum stress pathway

Abstract

Excessive nitric oxide (NO) production in cytokine-activated ␤ cells has been implicated in ␤ cell disruption in type 1 diabetes. ␤ cells are very vulnerable to NO-induced apoptosis. However, the mechanism underlying this phenomenon is unclear. Low concentrations of NO that lead to apoptosis apparently do not cause severe DNA damage in mouse MIN6 ␤ cells. CHOP, a C͞EBP homologous protein that is induced by endoplasmic reticulum (ER) stress and plays a role in growth arrest and cell death, was induced by a NO donor, S-nitroso-N-acetyl-D,L-penicillamine (SNAP). SNAP increased cytosolic Ca 2؉ , and only agents depleting ER Ca 2؉ induced CHOP expression and led to apoptosis, suggesting that NO depletes ER Ca 2؉ . Overexpression of calreticulin increased the Ca 2؉ content of ER and afforded protection to cells against NO-mediated apoptosis. Furthermore, pancreatic islets from CHOP knockout mice showed resistance to NO. We conclude that NO depletes ER Ca 2؉ , causes ER stress, and leads to apoptosis. Thus, ER Ca 2؉ stores are a new target of NO, and the ER stress pathway is a major mechanism of NO-mediated ␤ cell apoptosis.

Roles of CHOP/GADD153 in endoplasmic reticulum stress

“…NO causes the depletion of ER Ca 2 þ and induces CHOP mRNA in primary neuronal cultures as well as in pancreatic bcells. 56,94 Consistent with this result, pretreatment with an NO synthase inhibitor could restore ER Ca 2 þ after brain ischemia, and mice lacking the inducible NO synthase gene had a decreased susceptibility to ischemia. 95,96 Furthermore, we recently found that CHOP À/À mice have smaller infarcts than wild-type animals subjected to bilateral carotid artery occlusion.…”

“…Although CHOP À/À cells are resistant to ER stress-mediated apoptosis, CHOP À/À mice have a normal development and normal fertility, 56,58 properties similar to those of caspase 12 À/ À and ASK1 À/À cells and mice. Thus CHOP is dispensable for organogenesis and development, probably because of redundancy of the apoptosis pathway.…”

“…31 CHOP À/À mice exhibit reduced apoptosis in response to ER stress. 56,58,59 Thus, CHOP plays an important role in ER stress-induced apoptosis. Concerning the downstream of CHOP in the apoptosis pathway, cells lacking CHOP's major dimerization partner C/EBPb are also resistant to ER stress-induced apoptosis, suggesting that CHOP works as a transcriptional factor that regulates genes involved in either survival or death.…”

“…80,81 We found that NO causes ER stress by depleting ER Ca 2 þ stores and leads to apoptosis in b-cells via CHOP induction (Figure 4a). 56 Maintenance of Ca 2 þ homeostasis in the ER is essential for protein folding. Molecular components Figure 4 (a) Pathways of NO-induced cell death.…”

“…[82][83][84] Overexpression of calreticulin, which is a major Ca 2 þ -binding protein in the ER, prevents b-cells from NO-mediated apoptosis. 56 It should be noted that the expression of SERCA2b and calreticulin is induced by ER stress, which means that increased ER Ca 2 þ stores are likely to be required to adapt ER stress conditions. Furthermore, DNA microarray analysis showed that CHOP is induced by IL-1b plus IFN-g in primary rat b-cells.…”