Xu et al. used human cell lines that expressed inducible nitric oxide synthase (iNOS) under the control of regulated promoters to investigate the effects of inhibiting mitochondrial respiration with nitric oxide (NO), which competes with oxygen for cytochrome c oxidase. NO, acting independently of soluble guanylate kinase activity, stimulated the expression of glucose-regulated protein 78 (Grp78), an endoplasmic reticulum (ER)-resident chaperone protein whose expression is enhanced as part of the ER stress response. The authors used Western analysis to show that NO produced an increase in the amount of the soluble transcription factor p50 ATF6, which has been implicated in the regulation of Grp78 expression. ATF6 is generated through a calcium-dependent process involving regulated intramembranous proteolysis. NO-dependent stimulation of p50 ATF6 production and of Grp78 expression was attenuated in cells depleted of intracellular calcium by exposure to a calcium ionophore in the absence of extracellular calcium. NO protected against cytotoxicity produced by the ER calcium ATPase inhibitor thapsigargin, and this protection (as well as the increase in Grp78 mRNA) was abrogated by silencing two serine proteases involved in the proteolytic generation of p50 ATF6 with siRNA. Both an intracellular calcium chelator and cyclosporin A (which interferes with mitochondrial calcium signaling) reduced NO-dependent ATF6 cleavage and prevented the NO-dependent increase in Grp78 and protection from thapsigargin cytotoxicity. Thus, the authors propose that NO-dependent inhibition of mitochondrial respiration affects calcium signaling between the mitochondria and the ER, thereby stimulating production of p50 ATF6 and thus of genes involved in the ER stress response.
W. Xu, L. Liu, I. G. Charles, S. Moncada, Nitric oxide induces coupling of mitochondrial signalling with the endoplasmic reticulum stress response. Nat. Cell Biol. 6, 1129-1134 (2004). [Online Journal]