Akt2 deficiency alleviates oxidative stress in the heart and liver via up-regulating SIRT6 during high-fat diet-induced obesity.

The present study aims to investigate the role of AKT2 in the pathogenesis of hepatic and cardiac lipotoxicity induced by lipid overload-induced obesity and identify its downstream targets. WT and Akt2 KO mice were fed either normal diet, or high-fat diet (HFD) to induce obesity model in vivo. Human hepatic cell line (L02 cells) and neonatal rat cardiomyocytes (NRCMs) were used as in vitro models. We observed that during HFD-induced obesity, Akt2 loss-of-function mitigated lipid accumulation and oxidative stress in the liver and heart tissue. Mechanistically, down-regulation of Akt2 promotes SIRT6 expression in L02 cells and NRCMs, the latter deacetylates SOD2, which promotes SOD2 activity and therefore alleviates oxidative stress-induced injury of hepatocytes and cardiomyocytes. Furthermore, we also proved that AKT2 inhibitor protects hepatocytes and cardiomyocytes from HFD-induced oxidative stress. Therefore, our work prove that AKT2 plays an important role in the regulation of obesity-induced lipid metabolic disorder in the liver and heart. Our study also indicates AKT2 inhibitor as a potential therapy for obesity-induced hepatic and cardiac injury.

SIRT6 regulates obesity-induced oxidative stress via ENDOG/SOD2 signaling in the heart.

The sirtuin 6 (SIRT6) participates in regulating glucose and lipid homeostasis. However, the function of SIRT6 in the process of cardiac pathogenesis caused by obesity-associated lipotoxicity remains to be unveiled. This study was designed to elucidate the role of SIRT6 in the pathogenesis of cardiac injury due to nutrition overload-induced obesity and explore the downstream signaling pathways affecting oxidative stress in the heart. In this study, we used Sirt6 cardiac-specific knockout murine models treated with a high-fat diet (HFD) feeding to explore the function and mechanism of SIRT6 in the heart tissue during HFD-induced obesity. We also took advantage of neonatal cardiomyocytes to study the role and downstream molecules of SIRT6 during HFD-induced injury in vitro, in which intracellular oxidative stress and mitochondrial content were assessed. We observed that during HFD-induced obesity, Sirt6 loss-of-function aggravated cardiac injury including left ventricular hypertrophy and lipid accumulation. Our results evidenced that upon increased fatty acid uptake, SIRT6 positively regulated the expression of endonuclease G (ENDOG), which is a mitochondrial-resident molecule that plays an important role in mitochondrial biogenesis and redox homeostasis. Our results also showed that SIRT6 positively regulated superoxide dismutase 2 (SOD2) expression post-transcriptionally via ENDOG. Our study gives a new sight into SIRT6 beneficial role in mitochondrial biogenesis of cardiomyocytes. Our data also show that SIRT6 is required to reduce intracellular oxidative stress in the heart triggered by high-fat diet-induced obesity, involving the control of ENDOG/SOD2.

Metformin alleviates HFD-induced oxidative stress in hepatocyte via activating SIRT6/PGC-1α/ENDOG signaling.

Metformin is accepted as a first-line drug for the therapy of Type 2 diabetes (T2D), while its mechanism is still controversial. In the present study, by taking advantage of mouse model of high-fat-diet (HFD)-induced obesity and primary mouse hepatocytes (PMHCs) as well as human hepatocyte L02 cell line, we aimed to investigate the involvement of SIRTs during the application of metformin for the therapy of T2D. Our data evidenced that during HFD-induced obesity, there was elevation of nucleus protein acetylation. Analysis of liver tissue showed that among all SIRT members, SIRT6 expression was significantly down-regulated during HFD feeding, which was sustained to regular level with metformin administration. Our result also showed that SIRT6 suppressed intracellular oxidative stress upon FAs stimulation in PMHCs and L02 cells. Mechanistically, SIRT6, but not SIRT1 promoted PGC-1α expression. We further prove that ENDOG is downstream of PGC-1α. In addition, we evidenced that ENDOG protects hepatocytes from lipid-induced oxidative stress, and down-regulation of Endog blunted the protective role of metformin in defending against FAs-induced oxidative stress. Our study established a novel mechanism of metformin in counteracting lipid-induced hepatic injury via activating SIRT6/PGC-1α/ENDOG signaling, thus providing novel targets of metformin in the therapy of T2D.

IL-6 protects cardiomyocytes from oxidative stress at the early stage of LPS-induced sepsis.

Pro-inflammatory cytokines play important roles in sepsis-induced cardiac injury. Among various cytokines, the function of Interleukin-6 (IL-6) in the regulation of cardiomyocyte injury remains to be elucidated. This study aimed to investigate whether IL-6 plays a key role in the sepsis-induced cardiomyocyte injury and the possible mechanism. Mice deficient for Il-6 exhibited impaired heart rhythm after LPS stimulation. Histological analysis revealed significantly increased oxidative stress after LPS stimulation in the heart with Il-6 knockout. On the contrary, IL-6 supplementation alleviated LPS-induced oxidative stress. Mechanically, IL-6 facilitates Nrf2 expression and its nucleus translocation, which subsequently promotes the expression of antioxidant genes and sustains redox homeostasis in cardiomyocytes, and Nrf2 deletion results in elevated oxidative stress during LPS stimulation and cannot be inverted by IL-6 supplement. Our study presents a new sight for the protective role of IL-6 during the pathological development of LPS-induced cardiac injury, which functions as an anti-oxidant molecule via activating Nrf2 signaling.

AKT2 regulates development and metabolic homeostasis via AMPK-depedent pathway in skeletal muscle.

Skeletal muscle is responsible for the majority of glucose disposal in the body. Insulin resistance in the skeletal muscle accounts for 85-90% of the impairment of total glucose disposal in patients with type 2 diabetes (T2D). However, the mechanism remains controversial. The present study aims to investigate whether AKT2 deficiency causes deficits in skeletal muscle development and metabolism, we analyzed the expression of molecules related to skeletal muscle development, glucose uptake and metabolism in mice of 3- and 8-months old. We found that AMP-activated protein kinase (AMPK) phosphorylation and myocyte enhancer factor 2 (MEF2) A (MEF2A) expression were down-regulated in AKT2 knockout (KO) mice, which can be inverted by AMPK activation. We also observed reduced mitochondrial DNA (mtDNA) abundance and reduced expression of genes involved in mitochondrial biogenesis in the skeletal muscle of AKT2 KO mice, which was prevented by AMPK activation. Moreover, AKT2 KO mice exhibited impaired AMPK signaling in response to insulin stimulation compared with WT mice. Our study establishes a new and important function of AKT2 in regulating skeletal muscle development and glucose metabolism via AMPK-dependent signaling.