Discordant signaling and autophagy response to fasting in hearts of obese mice: Implications for ischemia tolerance

Abstract

Autophagy is regulated by nutrient and energy status and plays an adaptive role during nutrient deprivation and ischemic stress. Metabolic syndrome (MetS) is a hypernutritive state characterized by obesity, dyslipidemia, elevated fasting blood glucose levels, and insulin resistance. It has also been associated with impaired autophagic flux and larger-sized infarcts. We hypothesized that diet-induced obesity (DIO) affects nutrient sensing, explaining the observed cardiac impaired autophagy. We subjected male friend virus B NIH (FVBN) mice to a high-fat diet, which resulted in increased weight gain, fat deposition, hyperglycemia, insulin resistance, and larger infarcts after myocardial ischemia-reperfusion. Autophagic flux was impaired after 4 wk on a high-fat diet. To interrogate nutrient-sensing pathways, DIO mice were subjected to overnight fasting, and hearts were processed for biochemical and proteomic analysis. Obese mice failed to upregulate LC3-II or to clear p62/SQSTM1 after fasting, although mRNA for LC3B and p62/SQSTM1 were appropriately upregulated in both groups, demonstrating an intact transcriptional response to fasting. Energy- and nutrient-sensing signal transduction pathways [AMPK and mammalian target of rapamycin (mTOR)] also responded appropriately to fasting, although mTOR was more profoundly suppressed in obese mice. Proteomic quantitative analysis of the hearts under fed and fasted conditions revealed broad changes in protein networks involved in oxidative phosphorylation, autophagy, oxidative stress, protein homeostasis, and contractile machinery. In many instances, the fasting response was quite discordant between lean and DIO mice. Network analysis implicated the peroxisome proliferator-activated receptor and mTOR regulatory nodes. Hearts of obese mice exhibited impaired autophagy, altered proteome, and discordant response to nutrient deprivation.

Activation of IGF-1 receptors and Akt signaling by systemic hyperinsulinemia contributes to cardiac hypertrophy but does not regulate cardiac autophagy in obese diabetic mice

“…This is different from the diet-induced obese (DIO) mouse model, where the defect in cardiac autophagy appears to be caused by an impairment in lysosomal clearance [37, 38, 40]. The basis for this difference is not entirely known but may involve the induction of mTOR signaling in ( ob/ob ) hearts versus no change or a reduction in this pathway in DIO mice [38, 40]. However, other DIO studies with longer durations and higher fat content in the diet have reported higher Akt and mTOR signaling in the heart [16, 37], similar to what we observed in ( ob/ob ) hearts.…”

“…Using ( ob/ob ) mice we showed that autophagic flux is impaired in the heart and this is due to alterations in autophagosome formation rather than a defect in their clearance by lysosomes. This is different from the diet-induced obese (DIO) mouse model, where the defect in cardiac autophagy appears to be caused by an impairment in lysosomal clearance [37, 38, 40]. The basis for this difference is not entirely known but may involve the induction of mTOR signaling in ( ob/ob ) hearts versus no change or a reduction in this pathway in DIO mice [38, 40].…”

“…Given the inhibitory effect of insulin signaling on myocardial autophagy, it was hypothesized that autophagy would be enhanced in insulin resistant hearts. However, accumulating evidence suggest that auophagy is rather decreased in the heart with obesity, insulin resistance and T2DM [37–40]. The upstream and downstream signals involved in cardiac autophagy suppression in obesity and T2DM are not well understood.…”

Myocardial stress and autophagy: mechanisms and potential therapies

“…124 In contrast, a similar type 2 diabetic patient cohort exhibits decreased cardiac Atg5 protein expression, and no change in LC3. 131 The autophagic response to fasting-and ischemia-induced metabolic stress is blunted in obese insulin resistant mice, 59, 128 which may underlie exacerbated ischemic injury evident in diabetes. Glycophagy may play an important role in diabetic cardiomyocytes, coincident with disturbances in glycogen metabolism.…”

New and revisited approaches to preserving the reperfused myocardium

“…Similarly, time-restricted feeding (8 h per day) results in a period of fasting and can protect mice against the ill effects of a high-fat diet 252 . A high-fat diet has been shown to impair autophagy 253,254 and reduce ischaemic tolerance 255 , underscoring the importance of intact autophagic flux in cardioprotection. Like caloric restriction and intermittent fasting, exercise upregulates autophagy and is associated with cardioprotection 256,257 .…”