Suppression of autophagy is protective in high glucose-induced cardiomyocyte injury

Abstract

Abbreviations: DMEM, Dulbecco's modified essential medium; 3-MA, 3-methyladenine; BAF, bafilomycin A 1 ; GFP, green fluorescent protein; AVs, autophagic vacuoles; LC3, microtubule-associated protein 1 light chain 3; mRFP, monomeric red fluorescent protein; mTOR, mammalian target of rapamycin; mTORC1, mTOR complex 1; BECN1, Beclin 1; ULK1, unc-51-like kinase 1; PI, propidium iodide; Rap, rapamycin; AMPK, AMP-activated protein kinase; ACC, acetyl-CoA carboxylase; S6K, p70 ribosomal protein S6 kinase; 4EBP, elongation factor 4E binding protein; PARP, Poly (ADP-ribose) polymerase; shRNA, short hairpin RNA; β-gal, β-galactosidaseHyperglycemia is linked to increased heart failure among diabetic patients. However, the mechanisms that mediate hyperglycemia-induced cardiac damage remain poorly understood. Autophagy is a cellular degradation pathway that plays important roles in cellular homeostasis. Autophagic activity is altered in the diabetic heart, but its functional role has been unclear. In this study, we determined if mimicking hyperglycemia in cultured cardiomyocytes from neonatal rats and adult mice could affect autophagic activity and myocyte viability. High glucose (17 or 30 mM) reduced autophagic flux compared with normal glucose (5.5 mM) as indicated by the difference in protein levels of LC3-II (microtubule-associated protein 1 light chain 3 form II) or the changes of punctate fluorescence patterns of GFP-LC3 and mRFP-LC3 in the absence and presence of the lysosomal inhibitor bafilomycin A 1 . Unexpectedly, the inhibited autophagy turned out to be an adaptive response that functioned to limit high glucose cardiotoxicity. Indeed, suppression of autophagy by 3-methyladenine or short hairpin RNA-mediated silencing of the Becn1 or Atg7 gene attenuated high glucose-induced cardiomyocyte death. Conversely, upregulation of autophagy with rapamycin or overexpression of Becn1 or Atg7 predisposed cardiomyocytes to high glucose toxicity. Mechanistically, the high glucose-induced inhibition of autophagy was mediated at least partly by increased mTOR signaling that likely inactivated ULK1 through phosphorylation at serine 467. Together, these findings demonstrate that high glucose inhibits autophagy, which is a beneficial adaptive response that protects cardiomyocytes against high glucose toxicity. Future studies are warranted to determine if autophagy plays a similar role in diabetic heart in vivo.

Diminished Autophagy Limits Cardiac Injury in Mouse Models of Type 1 Diabetes

“…It was thus hypothesized that insulin deficiency (type 1 diabetes) or insulin resistance (type 2 diabetes) would increase autophagic activity (29). However, insulin deficiency is normally accompanied by increased glucose levels that can directly inhibit autophagy as we showed previously (52). That is probably why autophagy is reduced rather than increased in the diabetic heart, which reflects a net effect of insulin deficiency, hyperglycemia, and/or other changes associated with diabetes.…”

Fibroblast Growth Factor 21 Protects Against High Glucose Induced Cellular Damage and Dysfunction of Endothelial Nitric-Oxide Synthase in Endothelial Cells

“…However, the cause of the increment of serum FGF21 and its impacts on cardiovascular tissues remain to be determined. It is well known that endothelial dysfunction plays a crucial role in the pathogenesis of diabetic vascular complications [34,40,45,46,47,48,49]. High glucose condition increases NADPH oxidase activity in endothelial microparticles that promote vascular inflammation [50].…”

mTOR plays a critical role in p53-induced oxidative kidney cell injury in HIVAN

“…Interestingly, the mTOR pathway has been reported to contribute to the pathogenesis of diabetic and HIV-associated nephropathy (25,26). Moreover, the effector molecules of these entities-glucose and HIVhave also been reported to activate both kidney cell mTOR and p53 pathways (23,35,36,46). However, there are no data on cross talk between mTOR and p53 in the development of these diseases.…”