In myocardial ischemia, induction of autophagy via the AMP-induced protein kinase pathway is protective, whereas reperfusion stimulates autophagy with BECLIN-1 upregulation and is implicated in ...causing cell death. We examined flux through the macroautophagy pathway as a determinant of the discrepant outcomes in cardiomyocyte cell death in this setting.
Reversible left anterior descending coronary artery ligation was performed in mice with cardiomyocyte-restricted expression of green fluorescent protein-tagged microtubule-associated protein light chain-3 to induce ischemia (120 minutes) or ischemia/reperfusion (30-90 minutes) with saline or chloroquine pretreatment (n=4 per group). Autophagosome clearance, assessed as the ratio of punctate light chain-3 abundance in saline to chloroquine-treated samples, was markedly impaired with ischemia/reperfusion compared with sham controls. Reoxygenation increased cell death in neonatal rat cardiomyocytes compared with hypoxia alone, markedly increased autophagosomes but not autolysosomes (assessed as punctate dual fluorescent mCherry-green fluorescent protein tandem-tagged light chain-3 expression), and impaired clearance of polyglutamine aggregates, indicating impaired autophagic flux. The resultant autophagosome accumulation was associated with increased reactive oxygen species and mitochondrial permeabilization, leading to cell death, which was attenuated by cyclosporine A pretreatment. Hypoxia-reoxygenation injury was accompanied by reactive oxygen species-mediated BECLIN-1 upregulation and a reduction in lysosome-associated membrane protein-2, a critical determinant of autophagosome-lysosome fusion. Restoration of lysosome-associated membrane protein-2 levels synergizes with partial BECLIN-1 knockdown to restore autophagosome processing and to attenuate cell death after hypoxia-reoxygenation.
Ischemia/reperfusion injury impairs autophagosome clearance mediated in part by reactive oxygen species-induced decline in lysosome-associated membrane protein-2 and upregulation of BECLIN-1, contributing to increased cardiomyocyte death.
Autophagy, a lysosomal degradative pathway, is potently stimulated in the myocardium by fasting and is essential for maintaining cardiac function during prolonged starvation. We tested the hypothesis ...that intermittent fasting protects against myocardial ischemia-reperfusion injury via transcriptional stimulation of the autophagy-lysosome machinery. Adult C57BL/6 mice subjected to 24-h periods of fasting, every other day, for 6 wk were protected from in-vivo ischemia-reperfusion injury on a fed day, with marked reduction in infarct size in both sexes as compared with nonfasted controls. This protection was lost in mice heterozygous null for Lamp2 (coding for lysosomal-associated membrane protein 2), which demonstrate impaired autophagy in response to fasting with accumulation of autophagosomes and SQSTM1, an autophagy substrate, in the heart. In lamp2 null mice, intermittent fasting provoked progressive left ventricular dilation, systolic dysfunction and hypertrophy; worsening cardiomyocyte autophagosome accumulation and lack of protection to ischemia-reperfusion injury, suggesting that intact autophagy-lysosome machinery is essential for myocardial homeostasis during intermittent fasting and consequent ischemic cardioprotection. Fasting and refeeding cycles resulted in transcriptional induction followed by downregulation of autophagy-lysosome genes in the myocardium. This was coupled with fasting-induced nuclear translocation of TFEB (transcription factor EB), a master regulator of autophagy-lysosome machinery; followed by rapid decline in nuclear TFEB levels with refeeding. Endogenous TFEB was essential for attenuation of hypoxia-reoxygenation-induced cell death by repetitive starvation, in neonatal rat cardiomyocytes, in-vitro. Taken together, these data suggest that TFEB-mediated transcriptional priming of the autophagy-lysosome machinery mediates the beneficial effects of fasting-induced autophagy in myocardial ischemia-reperfusion injury.
Hypoxia-inducible pro-death protein BNIP3 (BCL-2/adenovirus E1B 19-kDa interacting protein 3), provokes mitochondrial permeabilization causing cardiomyocyte death in ischemia-reperfusion injury. ...Inhibition of autophagy accelerates BNIP3-induced cell death, by preventing removal of damaged mitochondria. We tested the hypothesis that stimulating autophagy will attenuate BNIP3-induced cardiomyocyte death. Neonatal rat cardiac myocytes (NRCMs) were adenovirally transduced with BNIP3 (or LacZ as control; at multiplicity of infection = 100); and autophagy was stimulated with rapamycin (100 nM). Cell death was assessed at 48 h. BNIP3 expression increased autophagosome abundance 8-fold and caused a 3.6-fold increase in cardiomyocyte death as compared with control. Rapamycin treatment of BNIP3-expressing cells led to further increase in autophagosome number without affecting cell death. BNIP3 expression led to accumulation of autophagosome-bound LC3-II and p62, and an increase in autophagosomes, but not autolysosomes (assessed with dual fluorescent mCherry-GFP-LC3 expression). BNIP3, but not the transmembrane deletion variant, interacted with LC3 and colocalized with mitochondria and lysosomes. However, BNIP3 did not target to lysosomes by subcellular fractionation, provoke lysosome permeabilization or alter lysosome pH. Rather, BNIP3-induced autophagy caused a decline in lysosome numbers with decreased expression of the lysosomal protein LAMP-1, indicating lysosome consumption and consequent autophagosome accumulation. Forced expression of transcription factor EB (TFEB) in BNIP3-expressing cells increased lysosome numbers, decreased autophagosomes and increased autolysosomes, prevented p62 accumulation, removed depolarized mitochondria and attenuated BNIP3-induced death. We conclude that BNIP3 expression induced autophagosome accumulation with lysosome consumption in cardiomyocytes. Forced expression of TFEB, a lysosomal biogenesis factor, restored autophagosome processing and attenuated BNIP3-induced cell death.
Accumulating evidence attests to a prosurvival role for autophagy under stress, by facilitating removal of damaged proteins and organelles and recycling basic building blocks, which can be utilized ...for energy generation and targeted macromolecular synthesis to shore up cellular defenses. These observations are difficult to reconcile with the dichotomous prosurvival and death-inducing roles ascribed to macroautophagy in cardiac ischemia and reperfusion injury, respectively. A careful reexamination of 'flux' through the macroautophagy pathway reveals that autophagosome clearance is markedly impaired with reperfusion (reoxygenation) in cardiomyocytes following an ischemic (hypoxic) insult, resulting from reactive oxygen species (ROS)-mediated decline in LAMP2 and increase in BECN1 abundance. This results in impaired autophagy that is 'ineffective' in protecting against cell death with ischemia-reperfusion injury. Restoration of autophagosome clearance and by inference, 'adequate' autophagy, attenuates reoxygenation-induced cell death.
Obesity-induced diabetes is characterized by hyperglycemia, insulin resistance, and progressive beta cell failure. In islets of mice with obesity-induced diabetes, we observe increased beta cell ...death and impaired autophagic flux. We hypothesized that intermittent fasting, a clinically sustainable therapeutic strategy, stimulates autophagic flux to ameliorate obesity-induced diabetes. Our data show that despite continued high-fat intake, intermittent fasting restores autophagic flux in islets and improves glucose tolerance by enhancing glucose-stimulated insulin secretion, beta cell survival, and nuclear expression of NEUROG3, a marker of pancreatic regeneration. In contrast, intermittent fasting does not rescue beta-cell death or induce NEUROG3 expression in obese mice with lysosomal dysfunction secondary to deficiency of the lysosomal membrane protein, LAMP2 or haplo-insufficiency of BECN1/Beclin 1, a protein critical for autophagosome formation. Moreover, intermittent fasting is sufficient to provoke beta cell death in nonobese lamp2 null mice, attesting to a critical role for lysosome function in beta cell homeostasis under fasting conditions. Beta cells in intermittently-fasted LAMP2- or BECN1-deficient mice exhibit markers of autophagic failure with accumulation of damaged mitochondria and upregulation of oxidative stress. Thus, intermittent fasting preserves organelle quality via the autophagy-lysosome pathway to enhance beta cell survival and stimulates markers of regeneration in obesity-induced diabetes.
In cardiac ischemia-reperfusion injury, reactive oxygen species (ROS) generation and upregulation of the hypoxia-inducible protein BNIP3 result in mitochondrial permeabilization, but impairment in ...autophagic removal of damaged mitochondria provokes programmed cardiomyocyte death. BNIP3 expression and ROS generation result in upregulation of beclin-1, a protein associated with transcriptional suppression of autophagy-lysosome proteins and reduced activation of transcription factor EB (TFEB), a master regulator of the autophagy-lysosome machinery. Partial beclin-1 knockdown transcriptionally stimulates lysosome biogenesis and autophagy via mTOR inhibition and activation of TFEB, enhancing removal of depolarized mitochondria. TFEB activation concomitantly stimulates mitochondrial biogenesis via PGC1α induction to restore normally polarized mitochondria and attenuate BNIP3- and hypoxia-reoxygenation-induced cell death. Conversely, overexpression of beclin-1 activates mTOR to inhibit TFEB, resulting in declines in lysosome numbers and suppression of PGC1α transcription. Importantly, knockdown of endogenous TFEB or PGC1α results in a complete or partial loss, respectively, of the cytoprotective effects of partial beclin-1 knockdown, indicating a critical role for both mitochondrial autophagy and biogenesis in ensuring cellular viability. These studies uncover a transcriptional feedback loop for beclin-1-mediated regulation of TFEB activation and implicate a central role for TFEB in coordinating mitochondrial autophagy with biogenesis to restore normally polarized mitochondria and prevent ischemia-reperfusion-induced cardiomyocyte death.
An acute injection of brain-derived neurotrophic factor (BDNF) in the hypothalamic paraventricular nucleus (PVN) reduces body weight by decreasing feeding and increasing energy expenditure (EE), in ...animals on standard laboratory chow. Animals have divergent responses to a high-fat diet (HFD) exposure, with some developing obesity and others remaining lean. In the current study, we tested two hypotheses: 1) BDNF in the PVN reverses HFD-induced obesity, and 2) animals with higher body fat have a greater physiological response to BDNF than those with less body fat. Eighty-four 10-wk old rats were allowed HFD ad libitum for 9 wk and then prepared with bilateral PVN cannulas. Animals were then divided into tertiles based on their body fat rank: high, intermediate, and low (H, I, and L). Each group was further divided into 2 subgroups and then PVN injected with BDNF or control (artificial cerebrospinal fluid, aCSF) every other day for 3 wk. Energy intake (EI), body weight, and body composition were measured. At study's end, rats were killed to allow measurement of other metabolic indices. In parallel, another 12 rats were fed control diet (CD), PVN-cannulated and injected with aCSF. HFD exposure induced obesity, particularly in the H body fat group, with a significant increase in EI, body weight, fat mass, liver size, and serum glucose, triglycerides, insulin, and leptin. BDNF significantly reduced EI, body weight, body fat, lean mass, and serum metabolic indices. These BDNF effects were greatest in the H body fat group. These data indicate that BDNF reduced HFD-induced obesity and metabolic syndrome-like measures, and the animals with the most body fat had the most significant response to BDNF.
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