Hydrogen peroxide down-regulates inositol 1,4,5-trisphosphate receptor content through proteasome activation

Hydrogen peroxide (H2O2) is implicated in the regulation of signaling pathways leading to changes in vascular smooth muscle function. Contractile effects produced by H2O2 are due to the phosphorylation of myosin light chain kinase triggered by increases in intracellular calcium (Ca2+) from intracellular stores or influx of extracellular Ca2+. One mechanism for mobilizing such stores involves the phosphoinositide pathway. Inositol 1,4,5-trisphosphate (IP3) mobilizes intracellular Ca2+ by binding to a family of receptors (IP3Rs) on the endoplasmic–sarcoplasmic reticulum that act as ligand-gated Ca2+ channels. IP3Rs can be rapidly ubiquitinated and degraded by the proteasome, causing a decrease in cellular IP3R content. In this study we show that IP3R1 and IP3R3 are down-regulated when vascular smooth muscle cells (VSMC) are stimulated by H2O2, through an increase in proteasome activity. Moreover, we demonstrate that the decrease in IP3R by H2O2 is accompanied by a reduction in calcium efflux induced by IP3 in VSMC. Also, we observed that angiotensin II (ANGII) induces a decrease in IP3R by activation of NADPH oxidase and that preincubation with H2O2 decreases ANGII-mediated calcium efflux and planar cell surface area in VSMC. The decreased IP3 receptor content observed in cells was also found in aortic rings, which exhibited a decreased ANGII-dependent contraction after treatment with H2O2. Altogether, these results suggest that H2O2 mediates IP3R down-regulation via proteasome activity.