Development of the Metabolic Syndrome Leads to Increased Ischemic Injury and is Associated with Dysregulated Cardiac Fasting Response and Attenuation of Autophagy in Mice
Authors: Andres Allen , Kooren Joel , Tucker Kyle , Parker Sarah , Crowgey Erin , Ravindran Nandini , Ito Bruce , Gottlieb Roberta , Van Eyk Jennifer , Mentzer Robert 
Published: November 2023

Background

Metabolic syndrome (MetS) is characterized by obesity, dyslipidemia, elevated blood glucose levels and insulin resistance. In preclinical studies this hypernutritive state is associated with an impairment of cardiac autophagy and an increase in the heart's susceptibility to ischemia/reperfusion injury. Whether this represents a cause and effect relationship is unknown. Given that cardiac autophagy is strongly upregulated in response to nutrient depletion, we examined the cardiac fasting response of diet‐induced obese (DIO) mice that have developed features of MetS. We hypothesized that development of MetS would impair cardiac autophagy induction in response to stress. The aims of this study were to (i) examine the cardiac autophagy response to a stress challenge (fasting) in mice with features of MetS; (ii) demonstrate the increased vulnerability to ischemia in hearts of mice subjected to diet‐induced obesity (DIO); and (iii) use proteomics to gain mechanistic insights into the effects of high‐fat diet on cardiac autophagy and the response to stress.

Methods

FVBN mice were fed chow or a high‐fat diet (HFD) for up to 20wks. Autophagy was monitored by western blot for LC3, Beclin‐1, and p62. Autophagic flux was assessed by comparing LC3‐II in hearts of paired animals at baseline or 2h after chloroquine administration. The AMPK and mTOR signaling pathways, important in energy and nutrient sensing and regulating autophagy, were examined. Proteomic analysis of left ventricle lysates was also performed.

Results

DIO mice developed obesity, hyperinsulinemia, hyperleptinemia, and insulin resistance, clear features of MetS. LC3 lipidation and other markers of autophagy were induced by fasting in hearts of control mice but not in DIO mice, revealing loss of coordination between nutrient signaling and autophagy. Impaired autophagic flux was evidenced by failure to increase LC3‐II after lysosomal blockade with chloroquine. Fasting reduced phosphorylation of S6 (a downstream marker of mTOR activity), increased AMPK activation, and increased mRNA levels of autophagy genes LC3B and p62 both in chow‐fed and DIO mice, indicating intact signaling in response to nutrient deprivation. Protein profiles were analyzed using custom scripts and cytoscape which revealed dysregulation of metabolism favoring carbohydrate oxidation in response to fasting in DIO mice. Moreover, DIO mice displayed a decrease in several proteins important for handling oxidative stress. Infarct size and mortality were greater in the HFD cohort of mice subjected to 60min ischemia and 2h reperfusion.

Conclusion

In this model of MetS, cardiac autophagy and autophagic flux were impaired. Although the AMPK and mTOR nutrient sensing pathways remained intact, autophagy induction was lost in the MetS mice. Proteomic analysis revealed a global dysregulation of the response to fasting challenge in DIO mice, including a shift away from fatty acid oxidation and attenuated induction of redox regulators. Failure to oxidize fatty acids despite high levels of lipids may favor cardiac deposition of triglycerides which can impair autophagy by attenuating lysosomal function. These findings offer insights into the exacerbation of ischemic injury in MetS.

Support or Funding Information

NIH P01 HL112730