The heme oxygenase-1 (Hmox1; HO-1) pathway was tested for defense of mitochondrial quality control in cardiomyocyte-specific Hmox1 KO mice (HO-1[CM]–/–) exposed to oxidative stress (100% O2). After 48 hours of exposure, these mice showed persistent cardiac inflammation and oxidative tissue damage that caused sarcomeric disruption, cardiomyocyte death, left ventricular dysfunction, and cardiomyopathy, while control hearts showed minimal damage. After hyperoxia, HO-1(CM)–/– hearts showed suppression of the Pgc-1α/nuclear respiratory factor-1 (NRF-1) axis, swelling, low electron density mitochondria by electron microscopy (EM), increased cell death, and extensive collagen deposition. The damage mechanism involves structurally deficient autophagy/mitophagy, impaired LC3II processing, and failure to upregulate Pink1- and Park2-mediated mitophagy. The mitophagy pathway was suppressed through loss of NRF-1 binding to proximal promoter sites on both genes. These results indicate that cardiac Hmox1 induction not only prevents heme toxicity, but also regulates the timing and registration of genetic programs for mitochondrial quality control that limit cell death, pathological remodeling, and cardiac fibrosis.
Hagir B. Suliman, Jeffrey E. Keenan, Claude A. Piantadosi
(A) qPCR analysis of cardiac HO-1 in WT/Cre, HO-1/Cre, and HO-1(CM)–/– mice 8d after tamoxifen administration. Results are expressed mean ± SEM; horizontal bars represent mean values.*P < 0.05 for pre- vs. posthyperoxia; n = 6/group). (B) Western blot shows HO-1 protein analysis. (C) HO-1 mRNA expression in WT/Cre and HO-1(CM)–/– after tamoxifen before and after 100% O2 (hyperoxia). Results are mean ± SEM; horizontal bars represent mean values.*P < 0.05 for pre- vs. posthyperoxia; n = 6). (D) Representative Western blot shows HO-1 and HO-2 protein levels in WT/Cre and HO-1(CM)–/– from hearts pre- and posthyperoxia. (E) Images of mid-sagittal cardiac sections from WT/Cre and HO-1(CM)–/– mice after tamoxifen pre- and posthyperoxia, demonstrate thinning of ventricular walls in HO-1(CM)–/– hearts after O2. Right panels are photomicrographs of LV sections stained with H&E of control WT/Cre mice after tamoxifen in air and exposed to hyperoxia for 48h and air for 8d; control HO-1(CM)–/– mice after tamoxifen in air and hyperoxia for 48h and air for 8d. Deletion of HO-1 followed by O2 leads to foci of acute inflammation (long arrow). There is myofibrillar loss, cardiomyocyte swelling, paleness, and cell death (short arrow) compared with cardiomyocytes in WT/Cre. Tamoxifen alone did not perturb cytoarchitecture, but after HO-1 deletion, hyperoxia causes persistent cardiac damage (original magnification, ×250; scale bar = 10 μm). (F) Western analysis for myocardial ANF and BNP in WT/Cre and HO-1(CM)–/– mice. Coomassie staining was used as a loading control. Densitometry for (G) ANF and (H) BNP (mean ± SEM; *P < 0.05 for pre- vs. posthyperoxia; n = 6/group; 2-way ANOVA).