Cardiac Remodelling and Vulnerability to Ischaemia/Reperfusion Injury with Ageing and High-Fat Diet

Abstract

Background: Ageing and high-fat diet (HFD) are both associated with metabolic, cellular, structural, and functional remodelling of the heart. There are conflicting reports regarding the vulnerability of the ageing heart to cardiac insults including ischaemia and reperfusion (I/R), and similarly, the vulnerability of the heart after HFD. Very few studies have been done on the effects of HFD in the ageing heart. In this work, we first investigated the effects of ageing on cardiac remodelling and vulnerability to I/R compared to adults. We then investigated the effects of HFD in the ageing heart compared to normal diet (ND) aged matched controls. We hypothesized that both ageing and HFD increase the vulnerability of the heart to I/R injury. Methods: C57BL/6 adult (8 weeks) and ageing (80 weeks) male mice were used in this work. Ageing mice were fed a standard ND or HFD for a period of 24 weeks. Animal body weights, heart weights, and epidydimal fat pads were all measured. Plasma glucose and lipid metabolites were also measured using nuclear magnetic resonance (NMR). Hearts were extracted and processed for measuring cardiac energy metabolites and amino acids (AA) using high performance liquid chromatography (HPLC). Additionally, they were used for protein and phospho-protein quantification using liquid chromatography tandem mass spectrometry (LC-MS/MS). Histological examination of cardiac tissue included using light microscopy and electron microscopy for ultrastructural examination. Finally, vulnerability of the heart to I/R injury was tested using the Langendorff perfusion system. Results: Ageing animals had significant increase in body weight, epidydimal fat pads, cardiac hypertrophy, with decreased plasma glucose levels. Remodelling of the ageing heart included metabolic changes such as changes in cardiac energy metabolites with decreased phosphorylation potential, and a decrease in total protein amino acid pool as well as taurine. Additionally, cellular remodelling was observed as cardiac protein groups were mainly upregulated during aging including mitochondrial proteins, ionic channel proteins, antioxidant enzymes, apoptosis-related proteins, structural (collagen) proteins, and lipid and carbohydrate metabolism. Phospho-proteins (ionic transport related proteins, cardiac signalling and apoptosis, metabolism and energy production, and structural proteins) were also mainly upregulated with ageing. Finally, ageing was associated with structural change including increased lipid deposits in the heart and an increase in capillary/myocyte ratio. There was an increase in total size and lumen size of coronary arteries with decrease in arterial wall/total artery and lumen size ratios, and there were changes in mitochondrial morphometry (mitochondria became smaller in size and elongated) but not mitochondrial distribution. Feeding aging mice HFD was associated with marked obesity, increased body fat, and significantly higher plasma VLDL and LDL, but lower glucose and lactate levels. HFD was also associated with atherosclerosis, smaller coronary arteries with smaller lumen sizes, and a higher artery wall/artery size and lumen size ratios. Additionally, HFD reduced capillary/myocyte ratio and increased fibrosis in ageing hearts. Metabolically, HFD reduced total energy rich phosphates and this was associated with altered mitochondrial morphometry as they became larger in size and more rounded, with more densely distributed PN and SSL mitochondria. Finally, majority of proteins that changed after HFD in ageing were downregulated. These included mitochondrial proteins and carbohydrates metabolism. Proteins that increased with HFD included structural (collagen) proteins, apoptotic, and lipid metabolism. Antioxidant enzymes and ionic transport related proteins changed in both directions (up and downregul