Heart-Specific Overexpression of HMG-CoA Synthase 2 Induces Mitochondrial Stress—Adaptation via the ATF4 Pathway

The liver is the central organ for ketone body production, and HMG-CoA synthase2 (HMGCS2) is a rate-limiting enzyme involved in this process. We reported that fasting markedly increased mRNA expression of HMGCS2 in mouse heart, whereas its protein expression was faint and increased only marginally with fasting. To clarify the pathophysiological role of HMGCS2 in the heart, we generated transgenic (Tg) mice with heart-specific HMGCS2 overexpression and analyzed the effects on metabolic status in the myocardium. Plasma concentrations of β-hydroxybutyrate (βOHB), glucose and free fatty acids were comparable between Tg and wild type (Wt) mice, whereas the cardiac βOHB content in Tg mice was increased 10-fold (fed state) or 2-fold (24h-fasted state) over that of Wt mice. Confocal microscopy demonstrated that the HMGCS2 protein co-localized with a mitochondrial protein, ATP5A, in cardiomyocytes of Tg mice. Transmission electron microscopy showed many swollen mitochondria, some of which contained vacuole-like vesicles and decreased cristae in Tg hearts, and ultrasonography showed impaired systolic function. Microarray gene expression analysis revealed that approximately 800 genes were altered >2-fold in Tg hearts compared with Wt hearts. Among the altered genes, ATF4 and its target genes, including PHGDH, MTHFD2 and ASNS which promote glutathione production, were upregulated in Tg hearts. Consistent with this result, a metabolome analysis performed by capillary electrophoresis-mass spectrometry demonstrated that glutathione was increased in Tg hearts, suggesting an adaptation to mitochondrial stress. In addition, 3 branch-chain amino acids were increased in Tg hearts compared with Wt hearts. These results indicate that myocardial HMGCS2 can produce ketone bodies in mitochondria, leading to mitochondrial stress and cardiac dysfunction. The limited induction of HMGCS2 protein upon fasting might constitute a protective mechanism for cardiomyocytes. Disclosure Y. Zenimaru: None. J. Suzuki: None. T. Nakaya: None. M. Yamada: None. M. Ichikawa: None. S. Sato: None. M. Imagawa: None. F.B. Kraemer: None. T. Konoshita: None. T. Ishizuka: None.