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IgG4-related disease (IgG4-RD) is a fibro-inflammatory disorder characterized by lymphoplasmacytic infiltration of numerous IgG4-positive plasma cells, leading to fibrous thickening in the affected tissue. Typical cardiovascular manifestations of IgG4-RD are periaortitis, coronary arteritis, and pericarditis. Rare cases of myocardial involvement in IgG4-RD have been reported, but surgical resection or open biopsy was required for the diagnosis in those cases. Here, we report a case in which percutaneous transcatheter biopsy under the guidance of intracardiac echocardiography was useful for diagnosis of IgG4-RD manifested as an intracavitary right atrial mass, extending into the superior vena cava. Successful transcatheter diagnosis of myocardial involvement of IgG4-RD led to immediate favorable response to steroid therapy. Including the present case, previous IgG4-RD cases with myocardial involvement are reviewed to delineate its clinical characteristics.

Systemic branched‐chain amino acid (BCAA) metabolism is dysregulated in cardiometabolic diseases. We previously demonstrated that upregulated AMP deaminase 3 (AMPD3) impairs cardiac energetics in a rat model of obese type 2 diabetes, Otsuka Long‐Evans‐Tokushima fatty (OLETF). Here, we hypothesized that the cardiac BCAA levels and the activity of branched‐chain α‐keto acid dehydrogenase (BCKDH), a rate‐limiting enzyme in BCAA metabolism, are altered by type 2 diabetes (T2DM), and that upregulated AMPD3 expression is involved in the alteration. Performing proteomic analysis combined with immunoblotting, we discovered that BCKDH localizes not only to mitochondria but also to the endoplasmic reticulum (ER), where it interacts with AMPD3. Knocking down AMPD3 in neonatal rat cardiomyocytes (NRCMs) increased BCKDH activity, suggesting that AMPD3 negatively regulates BCKDH. Compared with control rats (Long‐Evans Tokushima Otsuka [LETO] rats), OLETF rats exhibited 49% higher cardiac BCAA levels and 49% lower BCKDH activity. In the cardiac ER of the OLETF rats, BCKDH‐E1α subunit expression was downregulated, while AMPD3 expression was upregulated, resulting in an 80% lower AMPD3‐E1α interaction compared to LETO rats. Knocking down E1α expression in NRCMs upregulated AMPD3 expression and recapitulated the imbalanced AMPD3‐BCKDH expressions observed in OLETF rat hearts. E1α knockdown in NRCMs inhibited glucose oxidation in response to insulin, palmitate oxidation, and lipid droplet biogenesis under oleate loading. Collectively, these data revealed previously unrecognized extramitochondrial localization of BCKDH in the heart and its reciprocal regulation with AMPD3 and imbalanced AMPD3‐BCKDH interactions in OLETF. Downregulation of BCKDH in cardiomyocytes induced profound metabolic changes that are observed in OLETF hearts, providing insight into mechanisms contributing to the development of diabetic cardiomyopathy.

Aims/Introduction We previously showed that upregulation of myocardial adenosine monophosphate deaminase (AMPD) is associated with pressure overload‐induced diastolic dysfunction in type 2 diabetes hearts. Here, we examined involvement of AMPD localized in the endoplasmic reticulum–mitochondria interface in mitochondrial Ca 2+ overload and its pathological significance. Materials and Methods We used type 2 diabetes Otsuka Long–Evans Tokushima Fatty rats (OLETF) and non‐diabetes Long–Evans Tokushima Otsuka Fatty rats (LETO) as well as AMPD3‐overexpressing H9c2 cells and human embryonic kidney 293 cells. Results OLETF, but not LETO, showed diastolic dysfunction under the condition of phenylephrine‐induced pressure overload. The levels of 90‐kDa AMPD3 in outer mitochondrial membranes/endoplasmic reticulum and mitochondria‐associated endoplasmic reticulum membrane (MAM) fractions were significantly higher in OLETF than in LETO. The area of the MAM quantified by electron microscopic analysis was 57% larger, mitochondrial Ca 2+ level under the condition of pressure overload was 47% higher and Ca 2+ retention capacity in MAM‐containing crude mitochondria isolated before the pressure overloading was 21% lower in OLETF than in LETO (all P ‐values <0.05). Transfection of FLAG‐AMPD3 in cells resulted in significant enlargement of the MAM area, and impairment in pyruvate/malate‐driven adenosine triphosphate‐stimulated and uncoupler‐stimulated mitochondrial respiration compared with those in control cells. Conclusions The findings suggest that 90‐kDa AMPD3 localized in the endoplasmic reticulum–mitochondria interface promotes formation of the MAM, inducing mitochondrial Ca 2+ overload and dysfunction in type 2 diabetes hearts.

Background A metabolomic study in the human heart suggested a pivotal role of amino acid (AA) metabolism in fatty acid oxidation, which is dysregulated in type 2 diabetes mellitus (T2DM) and heart failure. We previously reported that aberrant up-regulation of AMP deaminase 3 (AMPD3) impairs cardiac energetics in T2DM hearts, and AMPD3 was recently shown to be activated by fasting and to promote AA metabolism and fatty acid oxidation in skeletal muscle. A sodium glucose cotransporter 2 inhibitor (SGLT2i) has been shown to augment systemic AA metabolism, but its effect on cardiac AA metabolism remains unknown. Purpose We hypothesized that AMPD3 has a role in AA and lipid metabolism in cardiomyocytes and that the protective effect of an SGLT2i in diabetic hearts is mediated by modification of AA and lipid metabolism. Methods and results Proteomic analyses of AMPD3 immunoprecipitates in rat hearts revealed that AMPD3 interacted with the E1α and E2 components of the BCKDH complex, a rate-limiting enzyme of branched-chain AA (BCAA) catabolism. Immunoblotting using subcellular fractions revealed that BCKDH localized not only in the mitochondria matrix but also in the cytosol and endoplasmic reticulum (ER) and that AMPD3 interacted with BCKDH in the cytosol and ER. Despite comparable expression of BCKDH components and phosphorylation of E1α at Ser293, significant accumulation of BCAA was observed in T2DM rats (OLETF; 317±30 nmol/g) compared to that in control rats (LETO; 213±16 nmol/g), and the accumulation of BCAA was accompanied by up-regulation of AMPD3 in the cytosol and ER by 98% and 231%, respectively. In cardiomyocytes, disruption of BCAA catabolism by knockdown of BCKDH-E1α resulted in a 5.8-fold increase in AMPD3 at the transcriptional level and blunted lipid droplet biogenesis in response to a long-chain fatty acid challenge. Next, myocardial infarction (MI) was induced in LETO and OLETF pretreated with empagliflozin (10 mg/kg/day, 14 days) or a vehicle. Pathway analysis of cardiac metabolites revealed arginine biosynthesis and BCAA metabolism as the most significantly changed pathways with empagliflozin, with BCAA (791±187 nmol/g), glutamate, glutamine and urea being significantly increased. Empagliflozin restored myocardial ATP and survival after MI in OLETF to levels comparable to those in LETO. Electron microscopy showed a significantly higher prevalence of myocardium lipid droplets in OLETF, which was further increased by empagliflozin. Conclusions The results support the hypotheses that imbalance of extra-mitochondrial AMPD3-BCKDH interaction underlies dysregulated BCAA metabolism in T2DM hearts and that activation of cardiac AA metabolism by an SGLT2i normalizes fatty acid overload through sequestration into intracellular lipid droplets. FUNDunding Acknowledgement Type of funding sources: Foundation. Main funding source(s): Boehringer Ingelheim