Impaired mitochondrial biogenesis causes skeletal muscle damage in diabetes. However, whether and how mitochondrial biogenesis is impaired in the diabetic heart remains largely unknown. Whether adiponectin (APN), a potent cardioprotective molecule, regulates cardiac mitochondrial function has also not been previously investigated. In this study, electron microscopy revealed significant mitochondrial disorders in ob/ob cardiomyocytes, including mitochondrial swelling and cristae disorientation and breakage. Moreover, mitochondrial biogenesis of ob/ob cardiomyocytes is significantly impaired, as evidenced by reduced Ppargc-1a/Nrf-1/Tfam mRNA levels, mitochondrial DNA content, ATP content, citrate synthase activity, complexes I/III/V activity, AMPK phosphorylation, and increased PGC-1α acetylation. Since APN is an upstream activator of AMPK and APN plasma levels are significantly reduced in ob/ob mice, we further tested the hypothesis that reduced APN in ob/ob mice is causatively related to mitochondrial biogenesis impairment. One week of APN treatment of ob/ob mice activated AMPK, reduced PGC-1α acetylation, increased mitochondrial biogenesis, and attenuated mitochondrial disorders. In contrast, knocking out APN inhibited AMPK-PGC-1α signaling and impaired both mitochondrial biogenesis and function. The ob/ob mice exhibited lower survival rates and exacerbated myocardial injury after MI, when compared to controls. APN supplementation improved mitochondrial biogenesis and attenuated MI injury, an effect that was almost completely abrogated by the AMPK inhibitor compound C. In high glucose/high fat treated neonatal rat ventricular myocytes, siRNA-mediated knockdown of PGC-1α blocked gAd-enhanced mitochondrial biogenesis and function and attenuated protection against hypoxia/reoxygenation injury. In conclusion, hypoadiponectinemia impaired AMPK-PGC-1α signaling, resulting in dysfunctional mitochondrial biogenesis that constitutes a novel mechanism for rendering diabetic hearts more vulnerable to enhanced MI injury.

The branched-chain amino acids (BCAA) accumulated in type 2 diabetes are independent contributors to insulin resistance. The activity of branched-chain a-keto acid dehydrogenase (BCKD) complex, rate-limiting enzyme in BCAA catabolism, is reduced in diabetic states, which contributes to elevated BCAA concentrations. However, the mechanisms underlying decreased BCKD activity remain poorly understood. Here, we demonstrate that mitochondrial phosphatase 2C (PP2Cm), a newly identified BCKD phosphatase that increases BCKD activity, was significantly downregulated in ob/ob and type 2 diabetic mice. Interestingly, in adiponectin (APN) knockout (APN 2/2 ) mice fed with a high-fat diet (HD), PP2Cm expression and BCKD activity were significantly decreased, whereas BCKD kinase (BDK), which inhibits BCKD activity, was markedly increased. Concurrently, plasma BCAA and branched-chain a-keto acids (BCKA) were significantly elevated. APN treatment markedly reverted PP2Cm, BDK, BCKD activity, and BCAA and BCKA levels in HD-fed APN 2/2 and diabetic animals.Additionally, increased BCKD activity caused by APN administration was partially but significantly inhibited in PP2Cm knockout mice. Finally, APN-mediated upregulation of PP2Cm expression and BCKD activity were abolished when AMPK was inhibited. Collectively, we have provided the first direct evidence that APN is a novel regulator of PP2Cm and systematic BCAA levels, suggesting that targeting APN may be a pharmacological approach to ameliorating BCAA catabolism in the diabetic state.The branched-chain amino acids (BCAA) are essential amino acids such as leucine, isoleucine, and valine; their homeostasis is determined largely by catabolic activities in a number of organs including liver, muscle and adipose tissue (1-3). The first step of BCAA catabolism generates a set of corresponding branched-chain a-keto acids (BCKA), which are irreversibly decarboxylated by the branched-chain a-keto acid dehydrogenase (BCKD) complex (4). As with most nutrients, maintaining of the physiological level of BCAA is critical for cell metabolism and survival. However, many researchers have described increased BCAA and BCKA levels in diabetes and obesity (3,(5)(6)(7)(8). Furthermore, BCAA and their catabolites are strongly associated with insulin resistance (9-11), and elevated BCAA contributes to the development of insulin resistance (10,12). Mechanistically, elevated BCAA levels activate mTOR/p70S6 kinase, resulting in an increased I insulin receptor substrate-1 phosphorylation, thereby inhibiting phosphatidylinositol 3-kinase. This inhibition of phosphatidylinositol 3-kinase in turn leads to impaired insulin signaling (13,14). It is also reported that BCAA are independent predictors of insulin resistance, diabetes, and cardiovascular events (15-17). Therefore, it is necessary to determine the mechanisms of abnormal BCAA catabolism in order to better understand their association with metabolic-related pathogenesis. The BCKD complex is the rate-limiting enzyme in BCAA catabolism (4,12); regulation o...