Abstract Body: Background: TRAF2 mediates mitophagy in cardiomyocytes to restrain inflammation. TRAF2 is also expressed in macrophages and its abundance is increased in human hearts with ischemic cardiomyopathy. Macrophages are the primary source for 25-hydroxycholesterol (25HC) generation via cholesterol 25-hydroxylase (Ch25h), an interferon-inducible gene, and 25HC protects against post-myocardial infarction remodeling. TRAF2 transduces cytokine signaling but its role in cardiac macrophages is unknown.
Hypothesis: TRAF2 mediates macrophage mitophagy and regulates Ch25h expression to preserve cardiac function.
Methods: Mice with two Traf2 floxed and a Cx3CR1Cre allele (TRAF2 MacKO) or with CX3CR1erT2Cre and fed tamoxifen chow for 2 weeks for inducible ablation at 8 weeks of age (TRAF2 Mac-iKO) were compared with floxed controls. Mitophagy and cell death were assessed by flow cytometry for mitoKeima and propidium iodide. TRAF2 was adenovirally knocked down or overexpressed in RAW264.7 macrophages. TRAF2 Mac-iKO and Ch25h null mice were subjected to closed chest cardiac ischemia reperfusion (IR) injury to assess LV remodeling at 4 weeks.
Results: Cardiac macrophages from 20-week-old TRAF2 MacKO mice exhibit impaired mitophagy and increased cell death with reduced resident and monocyte derived populations. Despite no elevation in circulating TNFα, IL-1β, or IL-6, splenomegaly with increased neutrophils and Ly6C^high monocytes was observed. These mice develop cardiac hypertrophy with preserved LV systolic function, reduced oxygen consumption and abnormal mitochondrial ultrastructure. TRAF2 Mac-iKO mice demonstrate normal LV size and systolic function without splenic enlargement 4 weeks after Traf2 ablation, but manifest increased LV end diastolic volume and reduced ejection fraction (EF) after IR. 20-week-old uninjured TRAF2 Mac-iKO mice develop LV systolic dysfunction, increased LV mass and cardiomyocyte cross sectional area, fibrosis, and mitochondrial abnormalities. TRAF2 levels directly correlates with Ch25h expression in RAW264.7 cells. Ch25h null mice exhibit worse post IR LVEF versus controls.
Conclusion: Macrophage TRAF2 is essential for maintaining macrophage mitophagy and Ch25h expression to preserve cardiac homeostasis.