Autophagy was involved in the protective effect of metformin on hyperglycemia-induced cardiomyocyte apoptosis and Connexin43 downregulation in H9c2 cells.

Background: Increased cardiomyocyte apoptosis under high glucose condition contributes to diabetic cardiomyopathy. Degradation of cardiac Connexin43 (Cx43) has been associated with cardiac dysfunction in diabetic heart. Clinical and experimental studies suggested that metformin (Met) exhibits cardioprotective properties against diabetes. Aim: The aim of this study was to investigate the effect and underlying signaling mechanisms of metformin on apoptosis and Cx43 expression in H9c2 cells presenting with hyperglycemia conditions. Methods: In the present study, H9c2 cardiac cells were incubated with 5.5 mM glucose, 33.3 mM glucose, 33.3 mM glucose with metformin at two dose (100 µM, 1 mM) for 96 hours, and 1 mM metformin with chloroquine (50 µM) in 33.3 mM glucose medium. Cell viability was determined by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide (MTT) cell survival assay. Cytotoxicity was determined by the release of lactate dehydrogenase (LDH). The expression of Cx43, autophagic maker protein (LAMP-1, Beclin-1, p62 and LC3) and apoptosis maker protein (Bcl-2 and Bax) were determined by western blot. Results: The results showed that high glucose increased apoptosis and decreased Cx43 expression. Interestingly, metformin attenuated hyperglycemia-increased apoptosis and restored Cx43 expression. Moreover, this treatment caused autophagy as well, which indicated by up-regulation of autophagy-related proteins LAMP-1, Beclin-1, p62 and reduction in the ratio of LC3-II/LC3-I. In addition, administration autophagy inhibitor chloroquine (CQ) did not block the effect of metformin on Cx43 expression while increasing Cx43 content, together with an increased apoptosis. Conclusion: Administration metformin can protect the H9c2 cells against hyperglycemia-induced apoptosis and Cx43 down-regulation, in part, mediated through the induction of autophagy pathway.

Non-alcoholic Fatty Liver Disease Is Associated With Cardiovascular Outcomes in Subjects With Prediabetes and Diabetes: A Prospective Community-Based Cohort Study.

Background: There have been no studies of the effect of non-alcoholic fatty liver disease (NAFLD) on cardiovascular events (CVEs) in patients with pre-diabetes (pre-DM), and diabetes mellitus (DM). We performed a community-based cohort study to evaluate the relationship between NAFLD and CVEs in patients with glucose metabolism disorder. Methods: We enrolled 71,852 participants from the Kailuan study who had not experienced CVEs, after excluding alcohol abuse and other liver diseases. NAFLD was assessed using abdominal ultrasonography. Besides, participants were categorized by glucose metabolism status [normal glucose regulation (NGR), pre-DM, and DM]. All subjects were followed up for the occurrence of CVEs. Results: During a median of 13.01 (0.64) years of follow-up, 6,037 CVEs occurred. NAFLD was present in 22,525 (31.3%), and compared with participants without NAFLD, those with NAFLD had a 12.3% [95% confidence interval (CI) 1.059-1.191, P < 0.001] higher risk of CVEs, after adjustment for potential confounders. The hazard ratios for patients with mild, moderate, and severe NAFLD were 1.104 (95% CI 1.035-1.179, P < 0.001), 1.149 (95% CI 1.055-1.251, P < 0.001), and 1.235 (95% CI 1.059-1.441, P < 0.001), respectively. Moreover, participants with pre-DM plus NAFLD and participants with DM plus NAFLD had 1.267-fold (95% CI 1.151-1.395, P < 0.001) and 1.829-fold (95% CI 1.666-2.008, P < 0.001) higher risks of CVEs, respectively, compared with those with NGR and no NAFLD. The addition of the combination of NAFLD and glucose metabolism status to the crude Cox model increased the C-statistic by 0.0066 (0.0053-0.0080, P < 0.001). Conclusions: NAFLD is associated with higher risks of CVEs. Moreover, NAFLD is an independent predictor of CVEs in patients with pre-DM and DM, suggesting that NAFLD may provide greater risk predictive value for patients with glucose metabolism disorder.

Metformin protects high glucose­cultured cardiomyocytes from oxidative stress by promoting NDUFA13 expression and mitochondrial biogenesis via the AMPK signaling pathway.

Tissue damage in diabetes is at least partly due to elevated reactive oxygen species production by the mitochondrial respiratory chain during hyperglycemia. Sustained hyperglycemia results in mitochondrial dysfunction and the abnormal expression of mitochondrial genes, such as NADH: Ubiquinone oxidoreductase subunit A13 (NDUFA13). Metformin, an AMP­activated protein kinase (AMPK) activator, protects cardiomyocytes from oxidative stress by improving mitochondrial function; however, the exact underlying mechanisms are not completely understood. The aim of the present study was to investigated the molecular changes and related regulatory mechanisms in the response of H9C2 cardiomyocytes to metformin under high glucose conditions. H9C2 cells were subjected to CCK­8 assay to assess cell viability. Reactive oxygen species generation was measured with DCFH­DA assay. Western blotting was used to analyze the expression levels of NDUFA13, AMPK, p­AMPK and GAPDH. Reverse transcription­quantitative PCR was used to evaluate the expression levels of mitochondrial genes and transcription factors. It was observed that metformin protected H9C2 cardiomyocytes by suppressing high glucose (HG)­induced elevated oxidative stress. In addition, metformin stimulated mitochondrial biogenesis, as indicated by increased expression levels of mitochondrial genes (NDUFA1, NDUFA2, NDUFA13 and manganese superoxide dismutase) and mitochondrial biogenesis­related transcription factors [peroxisome proliferator­activated receptor­gamma coactivator­1α, nuclear respiratory factor (NRF)­1, and NRF­2] in the metformin + HG group compared with the HG group. Moreover, metformin promoted mitochondrial NDUFA13 protein expression via the AMPK signaling pathway, which was abolished by pretreatment with the AMPK inhibitor, Compound C. The results suggested that metformin protected cardiomyocytes against HG­induced oxidative stress via a mechanism involving AMPK, NDUFA13 and mitochondrial biogenesis.

Upregulation of connexin 43 and apoptosis­associated protein expression by high glucose in H9c2 cells was improved by resveratrol via the autophagy signaling pathway.

The expression of connexin43 (Cx43) protein and the apoptotic rate of cardiomyocytes may be regulated by autophagy and associated with diabetic cardiomyopathy. It is possible that the beneficial effect of resveratrol on diabetic cardiomyocytes occurs via the autophagy pathway. However, it remains to be elucidated whether resveratrol treatment may attenuate the hyperglycemia­induced remodeling of Cx43 and apoptosis through the regulation of autophagy. H9c2 cardiac cells were incubated with 5.5 and 25 mM glucose, 25 mM glucose with chloroquine (50 µM), and 25 mM glucose with or without resveratrol (10, 25 µM) for 24 h. H9c2 cells were also incubated with 25 µM resveratrol in the presence of chloroquine (50 µM). Cell viability was determined using an MTT cell survival assay. Cytotoxicity was determined by quantification of the release of lactate dehydrogenase. The expression of Cx43, autophagic maker proteins [Beclin­1, p62 and microtubule­associated protein 1 light chain 3 (LC3)], apoptosis maker proteins (B­cell lymphoma­2 and Bcl­2 associated X protein), AMP­activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) were determined using western blotting. Resveratrol treatment led to reduced Cx43 expression levels compared with the 25 mM glucose treatment and significantly reduced the expression of apoptosis­associated proteins in H9c2 cells under hyperglycemic conditions. Autophagy was increased as indicated by the upregulation of Beclin­1 and p62 expression and the reduced LC3­II/LC3­I ratio. AMPK expression was increased, whereas mTOR expression was reduced in the resveratrol treatment groups. Treatment with chloroquine reversed effect of resveratrol. In conclusion, administration resveratrol may protect H9c2 cells against hyperglycemia­induced Cx43 upregulation and apoptosis, which may be mediated through the induction of the autophagy signaling pathway.