Deficiency of Prostaglandin E2 Receptor Subtype 4 Exacerbates Left Ventricular Remodeling and Myocardial Fibrosis in Diet-induced Mice

Abstract

Left ventricular (LV) remodeling is closely associated with the development of congestive heart failure. Obese populations have a high prevalence to promote LV remodeling, however the mechanism remains unexplored. Although prostaglandin E2 receptor subtype 4 (EP4) affects cardiac function, no available information focuses on its role in LV remodeling. The aim of this study was to investigate the role of EP4 in LV remodeling under high-fat condition. Six-week old male EP4 wildtype and knockout mice were fed with standard chow or high-fat diet (HFD, 20% protein, 60% fat and 20% carbohydrate) for eight weeks. The effects of EP4 deficiency on cardiac function, cardiomyocytes remodeling and myocardial fibrosis in mice were studied. The possible regulatory mechanisms were further investigated. Echocardiography showed that there is no differ in cardiac function in EP4 wildtype and knockout mice under normal diet feeding. Heart from HFD-fed EP4 knockout mice had a 54% increase in relative wall thickness and a 38.1% reduction in LV diastolic volume, but preserved LV mass as compared to that from wild type littermates, suggesting EP4 knockout mice were having worse LV concentric remodeling under high-fat feeding. HFD fed-EP4 knockout mice also developed cardiac hypertrophy, as showed by an increased ratio of heart-to-body weight and an enlarged cardiomyocytes size. Such conclusion was further reinforced by studies where EP4 deficiency elevated mRNA expression of cardiac hypertrophy markers, atrial natriuretic peptide (ANP) (by 68.8%) and brain natriuretic peptide (BNP) (by 241.7%). The absence of EP4 also induced severe myocardial fibrosis, as showed by elevated mRNA expression of collagen synthetic genes (collagen I, collagen III, transforming growth factor beta 1) and suppressed degradation of matrix collagen genes (matrix metalloproteinase-2, decorin). Such results were confirmed by staining collagen in cardiac tissue and there was an increased collagen content in EP4 knockout mice. The phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), a protein that promotes concentric remodeling and hypertrophy, was increased in cardiac tissue of high-fat treated EP4 knockout mice. Lastly, in high-glucose treated rat origin cardiomyocytes (H9C2), treatment with EP4 antagonist (L161,982) increased gene level of ANP, BNP, collagen I and collagen III, and protein presence of ERK1/2, while such increase was reversed with ERK inhibitors U0126 and PD98059 pretreatment. In summary, EP4 deficiency induced LV concentric remodeling, cardiomyocyte hypertrophy and myocardial fibrosis, which may be through activated ERK1/2 signaling. These findings collectively showed that EP4 plays an essential role in regulation of LV remodeling and myocardial fibrosis through ERK1/2 signaling axis under high-fat condition.