A study on IRE1α controlling mitochondrial function and lipid metabolism in HFpEF

Heart failure is a very common disease with a very high prevalence. Rather than being a diagnosis with a defined pathomechanism, heart failure describes a clinical syndrome. The cardinal symptoms of HF include dyspnea, ankle swelling and fatigue. A distinction is made between preserved (HFpEF), moderately reduced (HFmrEF) or reduced ejection fraction (HFrEF). The treatment of HFpEF has so far often been frustrating. HFpEF has been associated with obesity. Obesity can lead to disruption of cardiomyocyte homeostasis and metabolism, resulting in lipid accumulation and thus lipotoxicity. It has been shown that this lipotoxicity triggers stress in the endoplasmic reticulum (ER stress) and activates the unfolded protein response (UPR) system. The UPR system is a signaling pathway present in cardiomyocytes that consists of three pathways: ATF6, PERK and IRE1a/ XBP1s. The IRE1a/XBP1s axis is thought to play an important role in the pathophysiology of HFpEF. In this dissertation, the interaction between IRE1a and lipid accumulation in cardiomyocytes was investigated. The study focused on the effects of lipid accumulation on IRE1a concentration and activation and on the influence of IRE1a concentration on lipid accumulation in cardiomyocytes. For this purpose, left ventricular cardiomyocytes from neonatal rats were isolated. Palmitic acid, a saturated fatty acid, and oleic acid, an unsaturated fatty acid, were used in the experiments to achieve lipid accumulation and to determine any difference in the IRE1a response between saturated and unsaturated fatty acids. Western blots and RT-PCR were used to detect the amount and activation of IRE1a protein in the cells. The Seahorse assay and the oil red staining method were used to investigate mitochondrial activity and lipid accumulation in the cell. It was shown that the assay of IRE1a was more appropriate in NRVMs than in H9C2 cells. Subsequently, a stronger IRE1a expression was shown after palmitate treatment than after oleate. Furthermore, lipid accumulation activated IRE1a in cardiomyocytes. However, the concentration of IRE1a was not affected by the concentration of lipids. Furthermore, IRE1a had no effect on mitochondrial activity and thus on fatty acid oxidation. Taken together, these results suggest that IRE1a is activated by lipotoxicity but does not directly interact with mitochondrial function. However, further studies are needed to evaluate the function of IRE1a and its role in the pathophysiology of HFpEF. Targeting IRE1a and the UPR system could help to develop an effective treatment for HFpEF.