Calcium signaling: NO stress for ER

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Endogenously generated nitric oxide (NO) couples calcium flux changes in mitochondria to the ER stress response, resulting in upregulation of the cytoprotective chaperone Grp78.

The signaling molecule nitric oxide (NO) competes with oxygen to reversibly bind to cytochrome c oxidase in the electron transport chain. It has a physiological role in the control of cell respiration, but can also interfere with normal mitochondrial function. Xu et al. now demonstrate that NO can induce the endoplasmic reticulum (ER) stress response by changing the mitochondrial calcium flux.

The authors studied two human cell lines that express inducible NO synthase (iNOS). Differential hybridization showed that increases in NO result in enhanced transcription from the chaperone glucose-regulated protein 78 (Grp78) gene promoter. Induced Grp78 expression is mediated by stress response elements in the promoter, which are known to be responsive to the membrane-associated ER stress-responsive transcription factor p50 ATF6. Indeed, the increase in Grp78 protein went hand in hand with increases in the cleaved, active form of ATF6, which is regulated by intracellular calcium levels.

To further investigate the functional significance of these observations, Xu et al. tested the ability of NO to protect cells against cytotoxicity. They showed that the NO-mediated increase in Grp78 conferred marked cytoprotection against a selective ER calcium-ATPase inhibitor, and that this cytoprotection was abolished when cells were treated with a calcium chelator or cyclosporin A, which disrupts mitochondrial calcium flux.

ATF6 cleavage is regulated by intramembrane proteolysis (RIP), which involves the site-1 (S1P) and site-2 (S2P) proteases. Xu et al. used small inhibitory RNA interference to silence the S1P or S2P promoters, demonstrating that absence of S1P and S2P abolishes both NO-mediated cytoprotection and increased Grp78. They concluded that both S1P and S2P are intermediates in the NO-dependent cleavage of ATF6 and in the increase of Grp78 expression.

Finally, the authors demonstrated that NO-induced Grp78 expression is diminished in cells devoid of mitochondrial DNA (rho ⁰), consistent with their findings that NO-mediated mitochondrial disruption is coupled to the activation of the ER stress response.

Thus, NO-mediated disruption of the mitochondrial respiratory chain appears to lead to changes in calcium flux, which activates the calcium sensitive S1P and S2P proteases in the Golgi that cleave and activate ATF6. The cleaved ATF6 component then translocates to the nucleus where it induces expression of ER stress-responsive genes such as Grp78. It will be interesting to see whether future research will define additional interactions of NO with components of the stress response pathway.

Myrto Raftopoulou
Signaling Gateway

References

Xu, Weiming, Liu, Lizhi, Charles, Ian G, & Moncada, Salvador Nitric oxide induces coupling of mitochondrial signalling with the endoplasmic reticulum stress response. Nature Cell Biology 6, 1129–1134 (2004); 10.1038/ncb1188 | Article | PubMed |