Endothelial dysfunction is an early defect in obesity, which is a major contributor to increased cardiovascular morbidity and mortality such as arterial stiffness, atherosclerosis, and hypertension. Recent studies have demonstrated a critical role of acid sphingomyelinase (ASM)-ceramide signaling in instigation of Nlrp3 inflammasomes and endothelial dysfunction during obesity and diabetes. The present proposal seeks to explore a novel mechanism mediating transcriptional control of ASM gene expression in ECs and determine how dysregulated ASM expression and activity promote vascular injury in obesity. Enhancer of zeste homolog 2 (Ezh2) is a histone methyltransferase that normally suppresses methylated genes, serving as a crucial epigenetic regulatory mechanism in gene expression. In preliminary studies, we found that loss of Ezh2 function increased ASM expression and ceramide levels in the intima leading to neointimal lesions in the carotid arteries of mice fed high fat diet (HFD). Such Ezh2-mediated suppression of ASM gene expression and ceramide signaling were also confirmed in cultured ECs. Based on these observations, we propose a hypothesis that loss of endothelial Ezh2 function upregulates ASM gene expression and augments ceramide production under hyperlipidemic conditions, which trigger Nlrp3 inflammasome activation and produce endothelial injury resulting in subsequent neointimal lesions on the carotid arterial wall. To test this hypothesis, the following Specific Aims are proposed.
Specific Aim 1 will determine whether endothelial ASM activation due to loss of Ezh2 function contributes to endothelial dysfunction or injury at the early stage of obesity using endothelium-specific Ezh2 knockout mice (Ezh2ecKO) and their wild type littermates.
Specific Aim 2 attempts to test how Ezh2-regulated ASM activation leads to endothelial dysfunction or injury by studying the role of ceramide and ceramide-enriched membrane rafts, Nlrp3 inflammasome activation, pyroptosis, endothelium- dependent vasodilation, inter-endothelial junction disruption, and adaptive endothelial progenitor cell landing or differentiation.
In Specific Aim 3, we will explore the molecular mechanisms by which loss of Ezh2 function activates ASM with a main focus on the roles of histone and DNA methylation in cultured ECs from Ezh2ecKO mice and their wild type littermates. The findings will provide new insights into the pathogenesis of endothelial dysfunction and identify Ezh2-ASM pathway as therapeutic target for prevention or treatment of vaculopathy associated with obesity.
Endothelial dysfunction refers to the loss of normal protective function of endothelial cells that locate within the inner layer of arteries. This grant application for the first time explores a tonic regulation of sphingolipid metabolism in endothelial cells by an enzyme that usually modifies the chromosomal DNA and how such regulation contributes to obesity-induced endothelial dysfunction and thereby provokes vascular injury and artery hardening. Our findings will answer an important question of how vascular injury and artery hardening start within cells at the very early stage of patients with cardiovascular risk factor such as hyperlipidemia and obesity and will help develop new strategies for treatment and prevention of vascular injury and artery hardening in these patients.
