SUMMARY Remodeling of the tricarboxylic acid (TCA) cycle is a metabolic adaptation accompanying inflammatory macrophage activation. During this process, endogenous metabolites can adopt regulatory roles that govern specific aspects of inflammatory response, as recently shown for succinate, which regulates the pro-inflammatory IL-1β-HIF-1α axis. Itaconate is one of the most highly induced metabolites in activated macrophages, yet its functional significance remains unknown. Here, we show that itaconate modulates macrophage metabolism and effector functions by inhibiting succinate dehydrogenase-mediated oxidation of succinate. Through this action, itaconate exerts anti-inflammatory effects when administered in vitro and in vivo during macrophage activation and ischemia-reperfusion injury. Using newly generated Irg1−/− mice, which lack the ability to produce itaconate, we show that endogenous itaconate regulates succinate levels and function, mitochondrial respiration, and inflammatory cytokine production during macrophage activation. These studies highlight itaconate as a major physiological regulator of the global metabolic rewiring and effector functions of inflammatory macrophages.

We hypothesized that the lipid-activated transcription factor, the peroxisome proliferator-activated receptor ␣ (PPAR␣), plays a pivotal role in the cellular metabolic response to fasting. Short-term starvation caused hepatic steatosis, myocardial lipid accumulation, and hypoglycemia, with an inadequate ketogenic response in adult mice lacking PPAR␣ (PPAR␣ ؊͞؊ ), a phenotype that bears remarkable similarity to that of humans with genetic defects in mitochondrial fatty acid oxidation enzymes. In PPAR␣ ؉͞؉ mice, fasting induced the hepatic and cardiac expression of PPAR␣ target genes encoding key mitochondrial (mediumchain acyl-CoA dehydrogenase, carnitine palmitoyltransferase I) and extramitochondrial (acyl-CoA oxidase, cytochrome P450 4A3) enzymes. In striking contrast, the hepatic and cardiac expression of most PPAR␣ target genes was not induced by fasting in PPAR␣ ؊͞؊ mice. These results define a critical role for PPAR␣ in a transcriptional regulatory response to fasting and identify the PPAR␣ ؊͞؊ mouse as a potentially useful murine model of inborn and acquired abnormalities of human fatty acid utilization.Starvation triggers a complex array of adaptive metabolic responses. A prominent feature of the energy-metabolic response to fasting includes a switch to reliance on fatty acids and ketones for energy production (1-4) and an augmentation in the capacity for mitochondrial fatty acid oxidation (FAO) in tissues with high oxidative energy demands such as heart and liver (5). The importance of the fasting-inducible capacity for cellular lipid utilization is underscored by the dramatic phenotype of human inborn errors in mitochondrial FAO enzymes (6). Children afflicted with genetically determined enzymatic defects in the FAO pathway typically are asymptomatic under normal feeding conditions. However, short-term fasting, such as that associated with an infectious illness, precipitates a dramatic and often fatal clinical picture characterized by hypoketotic hypoglycemia, liver dysfunction, and cardiomyopathy (6-8). Postmortem studies of FAO enzyme-deficient patients have demonstrated marked intracellular accumulation of neutral lipid in liver and heart. The capacity to oxidize fats is also diminished in several common acquired cardiac diseases including cardiac hypertrophy and myocardial ischemia (9-17). The molecular pathogenesis of target organ dysfunction from inherited and acquired alterations in cellular FAO has not been elucidated.A previous study in rodents demonstrated that the hepatic expression of genes encoding mitochondrial FAO enzymes is induced, at the transcriptional level, in response to fasting (5). This transcriptional regulatory response likely plays a key role in the fasting-induced augmentation of FAO capacity in liver and other oxidative tissues. The mechanisms involved in the fasting-induced transcriptional activation of FAO enzyme genes are unknown. However, recent studies have identified a role for a nuclear receptor, the peroxisome proliferatoractivated receptor ␣ (PPAR␣), in the ...

Rationale-Recent advancements have brought to light the origins, complexity, and functions of tissue-resident macrophages. However, in the context of tissue injury or disease, large numbers of monocytes infiltrate the heart and are thought to contribute to adverse remodeling and heart failure pathogenesis. Little is understood about the diversity of monocytes and monocyte-derived macrophages recruited to the heart after myocardial injury, including the mechanisms that regulate monocyte recruitment and fate specification.Objective-We sought to test the hypothesis that distinct subsets of tissue-resident CCR2− (C-C chemokine receptor 2) and CCR2+ macrophages orchestrate monocyte recruitment and fate specification after myocardial injury.Methods and Results-We reveal that in numerous mouse models of cardiomyocyte cell death (permanent myocardial infarction, reperfused myocardial infarction, and diphtheria toxin cardiomyocyte ablation), there is a shift in macrophage ontogeny whereby tissue-resident macrophages are predominately replaced by infiltrating monocytes and monocyte-derived macrophages. Using syngeneic cardiac transplantation to model ischemia-reperfusion injury and distinguish tissue-resident from recruited cell populations in combination with intravital 2-photon microscopy, we demonstrate that monocyte recruitment is differentially orchestrated by distinct subsets of tissue-resident cardiac macrophages. Tissue-resident CCR2+ macrophages promote monocyte recruitment through an MYD88 (myeloid differentiation primary response 88)dependent mechanism that results in release of MCPs (monocyte chemoattractant proteins) and monocyte mobilization. In contrast, tissue-resident CCR2− macrophages inhibit monocyte recruitment. Using CD (cluster of differentiation) 169-DTR (diphtheria toxin receptor) and CCR2-DTR mice, we further show that selective depletion of either tissue-resident CCR2− or CCR2+ macrophages before myocardial infarction results in divergent effects on left ventricular function, Bajpai et al.

Background In myocardial ischemia, induction of autophagy via the AMP-induced protein kinase (AMPK) pathway is protective, whereas reperfusion stimulates autophagy with BECLIN-1 upregulation, and is implicated in causing cell death. We examined flux through the macro-autophagy pathway as a determinant of the discrepant outcomes in cardiomyocyte cell death in this setting Methods and Results Reversible left anterior descending coronary artery ligation was performed in mice with cardiomyocyte-restricted expression of GFP-tagged microtubule associated protein light chain-3 (LC3) to induce ischemia (120 minutes) or ischemia-reperfusion (IR, 30–90 minutes) with saline or chloroquine (CQ) pretreatment (n=4/group). Autophagosome clearance, assessed as the ratio of punctate LC3 abundance in saline to CQ treated samples was markedly impaired with IR as compared with sham controls. Reoxygenation increased cell death in neonatal rat cardiomyocytes (NRCMs) as compared with hypoxia alone; markedly increased autophagosomes but not autolysosomes (assessed as punctate dual fluorescent mCherry-GFP tandem tagged LC3 expression); and impaired clearance of polyglutamine aggregates, indicating impaired autophagic flux. The resultant autophagosome accumulation was associated with increased reactive oxygen species (ROS) and mitochondrial permeabilization leading to cell death, which was attenuated by cyclosporine A pretreatment. Hypoxia-reoxygenation injury was accompanied by ROS-mediated BECLIN-1 upregulation and reduction in Lysosome Associated Membrane Protein-2 (LAMP2), a critical determinant of autophagosome-lysosome fusion. Restoration of LAMP2 levels synergizes with partial BECLIN-1 knockdown to restore autophagosome processing and attenuate cell death following hypoxia-reoxygenation. Conclusions Ischemia-reperfusion injury impairs autophagosome clearance mediated in part by ROS-induced decline in LAMP2 and upregulation of BECLIN-1, contributing to increased cardiomyocyte death.