Abstract 103: Cytochrome b5 Reductase 3 Regulates Myoglobin Redox State and Controls Cardiac Function

Oxidative stress contributes to the adverse pathophysiology of cardiac remodeling in heart failure. During oxidative stress, hemoprotein heme-iron can be oxidized, often altering the function of these proteins. We previously reported that cytochrome b5 reductase 3 (Cyb5R3) regulates heme iron redox in the vascular wall. While conducting vasoreactivity studies with a small-molecule Cyb5R3 inhibitor, we unexpectedly observed inhibition resulted in dilated cardiomyopathy (DCM) with reduced ejection fraction. Based on this study, we generated the first cardiomyocyte Cyb5R3 knockout mouse. Heart specific Cyb5R3 knockout resulted in DCM with >50% lethality in 15 days. H&E staining, wheat germ agglutinin immunofluorescence staining, transmission electron microscopy and diffusion tensor MRI shows that loss of Cyb5R3 causes myocardial atrophy and fibrosis. Due to the known heme iron reducing function of Cyb5R3, electron paramagnetic resonance (EPR) was used to assess the abundance of reduced and oxidized myoglobin ex vivo. Cyb5R3 knockout hearts had approximately 30% more oxidation relative to controls. Hypoxyprobe staining, which senses tissue pO2 below 5mmHg, showed Cyb5R3 expression is required for maintaining intra-cardiomyocyte pO 2 , likely through myoglobin heme-iron reduction. Decreased pO 2 was supported by altered mitochondrial dynamics. Complex IV activity was significantly reduced, along with decreased mitochondrial size and total ATP. Heme degradation protein, heme oxygenase-1 (HO-1) was significantly upregulated, suggesting that Cyb5R3 knockout may result in increased free-heme toxicity leading to intracellular damage. To translate these findings to humans, we conducted a retrospective study using a high-frequency T117S variant in African Americans. T117S carriers had significantly accelerated mortality post-first acute cardiac event. A T117S mouse was generated and received trans-aortic constriction (TAC) for 2 weeks. Preliminary results suggest T117S TAC mice have accelerated cardiac remodeling and time to death after pressure overload relative to C57B6/J TAC controls. Together, these data support Cyb5R3 expression may have an important role as a heme-iron reductase in cardiomyocyte function.