Nox4 reprograms cardiac substrate metabolism via protein O-GlcNAcylation to enhance stress adaptation.

Cardiac hypertrophic remodeling during chronic hemodynamic stress is associated with a switch in preferred energy substrate from fatty acids to glucose, usually considered to be energetically favorable. The mechanistic interrelationship between altered energy metabolism, remodeling, and function remains unclear. The ROS-generating NADPH oxidase-4 (Nox4) is upregulated in the overloaded heart, where it ameliorates adverse remodeling. Here, we show that Nox4 redirects glucose metabolism away from oxidation but increases fatty acid oxidation, thereby maintaining cardiac energetics during acute or chronic stresses. The changes in glucose and fatty acid metabolism are interlinked via a Nox4-ATF4-dependent increase in the hexosamine biosynthetic pathway, which mediates the attachment of O-linked N-acetylglucosamine (O-GlcNAcylation) to the fatty acid transporter CD36 and enhances fatty acid utilization. These data uncover a potentially novel redox pathway that regulates protein O-GlcNAcylation and reprograms cardiac substrate metabolism to favorably modify adaptation to chronic stress. Our results also suggest that increased fatty acid oxidation in the chronically stressed heart may be beneficial

Nox4 regulates InsP3 receptor‐dependent Ca2+ release into mitochondria to promote cell survival

Cells subjected to environmental stresses undergo regulated cell death (RCD) when homeostatic programs fail to maintain viability. A major mechanism of RCD is the excessive calcium loading of mitochondria and consequent triggering of the mitochondrial permeability transition (mPT), which is especially important in post-mitotic cells such as cardiomyocytes and neurons. Here, we show that stress-induced upregulation of the ROS-generating protein Nox4 at the ER-mitochondria contact sites (MAMs) is a pro-survival mechanism that inhibits calcium transfer through InsP 3 receptors (InsP 3R). Nox4 mediates redox signaling at the MAM of stressed cells to augment Akt-dependent phosphorylation of InsP 3R, thereby inhibiting calcium flux and mPT-dependent necrosis. In hearts subjected to ischemia–reperfusion, Nox4 limits infarct size through this mechanism. These results uncover a hitherto unrecognized stress pathway, whereby a ROS-generating protein mediates pro-survival effects through spatially confined signaling at the MAM to regulate ER to mitochondria calcium flux and triggering of the mPT. </p

Targeted redox inhibition of protein phosphatase 1 by Nox4 regulates eIF2a-mediated stress signaling

Source: doi: 10.15252/embj.201592394Phosphorylation of translation initiation factor 2α (eIF2α) attenuates global protein synthesis but enhances translation of activating transcription factor 4 (ATF4) and is a crucial evolutionarily conserved adaptive pathway during cellular stresses. The serine–threonine protein phosphatase 1 (PP1) deactivates this pathway whereas prolonging eIF2α phosphorylation enhances cell survival. Here, we show that the reactive oxygen species‐generating NADPH oxidase‐4 (Nox4) is induced downstream of ATF4, binds to a PP1‐targeting subunit GADD34 at the endoplasmic reticulum, and inhibits PP1 activity to increase eIF2α phosphorylation and ATF4 levels. Other PP1 targets distant from the endoplasmic reticulum are unaffected, indicating a spatially confined inhibition of the phosphatase. PP1 inhibition involves metal center oxidation rather than the thiol oxidation that underlies redox inhibition of protein tyrosine phosphatases. We show that this Nox4‐regulated pathway robustly enhances cell survival and has a physiologic role in heart ischemia–reperfusion and acute kidney injury. This work uncovers a novel redox signaling pathway, involving Nox4–GADD34 interaction and a targeted oxidative inactivation of the PP1 metal center, that sustains eIF2α phosphorylation to protect tissues under stress

Improved Cardiac Metabolism Following in Vivo Treatment of Type 2 Diabetic Mice with Fenofibrate Depends on Reduction of Plasma Lipids, as Well as Glucose

This article is part of Ahmed M. Khalid's PhD thesis, which is available in Munin: http://hdl.handle.net/10037/1802The plasma supply of energy substrates plays a key role in determining the cardiac metabolic phenotype. In diabetes, a high plasma supply of fatty acids (FA) leads to a predominant oxidation of FA for energy production, while glucose oxidation is markedly suppressed. The db/db mouse is a well accepted model of type 2 diabetes, showing hyperglycemia, hyperlipidemia, and hyperinsulinemia. Hearts from these mice exhibit altered substrate metabolism, characterized by an over-reliance on FA for energy production and low contribution of glucose. In the present study we tested whether the capacity for glucose utilization could be recovered in isolated working hearts from db/db mice following long-term (4 weeks) treatment with fenofibrate, using two different doses of the compound (0.1% and 0.2%, given as admixture to the diet). Mice treated with K-111 (a PPARα agonist, previously known as BM 17.0744) served as positive controls. In line with previous results, treatment with K-111 resulted in a significant reduction of the plasma concentrations of FA, triacylglycerol (TG) and glucose. Low-dose (0.1 %) fenofibrate treatment resulted in reduced plasma concentration of FA and TG, whereas the concentration of glucose was unaffected. With high-dose (0.2 %) fenofibrate, however, significant reductions of both lipids and glucose were obtained. Hearts from K-111-treated db/db mice showed a 74% decrease in FA oxidation and a near 2-fold increase in glucose oxidation. Treatment with low-dose fenofibrate failed to improve cardiac metabolism, whereas high-dose fenofibrate caused a similar shift in cardiac metabolism as seen with K-111. The alterations in cardiac metabolism were associated with changes in the myocardial and hepatic expression of PPARα-regulated target genes. These results indicate that reduction of plasma lipids alone is not sufficient for improving cardiac metabolism in diabetes, and that reduction of plasma glucose is also required

Improved Cardiac Metabolism Following in Vivo Treatment of Type 2 Diabetic Mice with Fenofibrate Depends on Reduction of Plasma Lipids, as Well as Glucose

The plasma supply of energy substrates plays a key role in determining the cardiac metabolic phenotype. In diabetes, a high plasma supply of fatty acids (FA) leads to a predominant oxidation of FA for energy production, while glucose oxidation is markedly suppressed. The db/db mouse is a well accepted model of type 2 diabetes, showing hyperglycemia, hyperlipidemia, and hyperinsulinemia. Hearts from these mice exhibit altered substrate metabolism, characterized by an over-reliance on FA for energy production and low contribution of glucose. In the present study we tested whether the capacity for glucose utilization could be recovered in isolated working hearts from db/db mice following long-term (4 weeks) treatment with fenofibrate, using two different doses of the compound (0.1% and 0.2%, given as admixture to the diet). Mice treated with K-111 (a PPARα agonist, previously known as BM 17.0744) served as positive controls. In line with previous results, treatment with K-111 resulted in a significant reduction of the plasma concentrations of FA, triacylglycerol (TG) and glucose. Low-dose (0.1 %) fenofibrate treatment resulted in reduced plasma concentration of FA and TG, whereas the concentration of glucose was unaffected. With high-dose (0.2 %) fenofibrate, however, significant reductions of both lipids and glucose were obtained. Hearts from K-111-treated db/db mice showed a 74% decrease in FA oxidation and a near 2-fold increase in glucose oxidation. Treatment with low-dose fenofibrate failed to improve cardiac metabolism, whereas high-dose fenofibrate caused a similar shift in cardiac metabolism as seen with K-111. The alterations in cardiac metabolism were associated with changes in the myocardial and hepatic expression of PPARα-regulated target genes. These results indicate that reduction of plasma lipids alone is not sufficient for improving cardiac metabolism in diabetes, and that reduction of plasma glucose is also required