Endothelial dysfunction in adipose triglyceride lipase deficiency

Adipose triglyceride lipase (ATGL) catalyzes the initial, rate-limiting step of triglyceride hydrolysis and is predominantly expressed in adipose tissue but also found to a lesser extent in cardiac muscle, skeletal muscle, testis, and other tissues. Systemic deletion of the ATGL gene in mice results in massive accumulation of neutral lipids in many tissues; humans with ATGL gene mutations develop neutral lipid storage disease with fat accumulation in multiple tissues. In cardiac muscle, an age-dependent increase of myocyte lipid droplets is associated with progresively developing cardiac dysfunction, leading to premature death of ATGL-deficient mice. Thus, ATGL knockout appears to be a promising genetic mouse model of lipotoxic cardiovascular diseases associated with neutral lipid accumulation and obesity. Characterization of the cardiovascular phenotype of ATGL knockout mice revealed that these animals suffer form severe endothelial dysfunction apparent as almost complete loss of relaxation to endothelium-dependent agonists despite normal vascular smooth muscle function in terms of contractility and endothelium-independent relaxation to nitric oxide (NO). Our preliminary findings are consistent with the hypothesis that ATGL deficiency interferes with the biosynthesis and/or release of endothelium-derived NO. This was very surprising because a link between lipolysis and NO signaling has not been reported. Correcting cardiac dysfunction by feeding ATGL knockouts a PPARa agonist or by selective overexpression of ATGL in cardiac muscle resulted in about 50 % improvement of endothelial function, suggesting the involvement of at least two mechanisms, one of which is a consequence of reduced cardiac output. It is the aim of the current project to clarify the mechanisms underlying the development of endothelial dysfunction in ATGL deficiency. This will be done by combination of biochemical and functional studies using aortas isolated from genetically altered mice. To clarify the relative contribution of cardiac and endothelial ATGL to the phenotype of ATGL-/- mice, we will use mouse lines with exclusive overexpression or conditional knockout of ATGL in cardiac muscle and endothelial cells, respectively. The genetic approach will be complemented by feeding the mice with an established PPARa agonist. In addition, we will study whether endothelial function is affected by perivascular adipose tissue, which is about 15-fold increased in ATGL deficiency. The results are expected to reveal hitherto unrecognized signaling pathways that are essentially involved in vascular homeostasis and dysregulated in lipotoxic cardiovascular diseases.

Due to modern life style (overnutrition, physical inactivity, stress etc.) the prevalence of obesity and obesity-mediated diseases has been escalating world-wide within the last decades and imposes an enormous economic and health-political burden on society. Facing the devastating medical consequences of obesity, the development of novel therapeutic strategies and appropriate animal models is pivotal.In 2004 adipose triglyceride lipase was identified as key enzyme of mammalian triglyceride catabolism. With the design of mice lacking adipose triglyceride lipase (designated as AKO mice) the rodent correlate to human neutral lipid storage disease with myopathy became available. AKO mice are characterized by massive accumulation of neutral lipids in multiple adipose and non-adipose tissues, especially in the heart, where it leads to the progressive development of lethal cardiomyopathy. Due to a unique metabolic profile AKO mice represent an excellent tool to study obesity-mediated health consequences such as cardiovascular disease.The aim of this project was to identify the cellular and molecular mechanism(s) underlying endothelial dysfunction observed previously in AKO mice. In particular, we aimed to characterize the cardiac phenotype in greater detail and to identify potential heart-independent mechanisms that might cause the vascular defect. Biochemical analysis showed that oxidative inflammatory stress as well as disturbances in the ubiquitin-proteasome system might contribute to impaired cardiac performance. To identify potential heart-derived mediators linking cardiac and endothelial dysfunction, we measured plasma levels of different candidates known to be involved in promoting cardiovascular diseases. Despite promising preliminary data the identity of the mediator(s) is still unclear. Since vessels of AKO mice are coated with large amounts of perivascular adipose tissue we were interested in the potential contribution of this tissue to the vascular defect. Biochemical analysis revealed a state of oxidative stress and chronic inflammation in perivascular adipose tissue of AKO mice, a condition that seems to occur independently of the heart. Elucidation of the subcellular mechanisms operative in cardiovascular dysfunction of AKO mice might pave the way for development of novel therapeutic strategies to rescue obesity-mediated cardiovascular pathologies in humans.