Lipases in monoacylglycerol and retinyl ester catabolism

The catabolism of monoacylglycerols (MG) and retinyl ester (RE), the storage form of Vitamin A, is incompletely understood. RE and MG contain a single carbonyl ester bond which is esterified with long-chain fatty acids. The mobilization of retinol from RE stores as well as the degradation of MG requires the activity of enzymes capable of hydrolyzing these bonds. In preliminary studies, we identified three enzymes which hydrolyzed MG in vitro. Notably, these enzymes also hydrolyzed RE, indicating a role in RE and MG catabolism. A fourth enzyme hydrolyzed RE and had no activity against MG. The aim of the proposed work is to define the in vivo role of these four enzymes in the degradation of MG and in the mobilization of cellular RE stores. Studies will be performed in mutant mouse lines and cell culture models. MG represent short-lived intermediary products of triacylglycerol and phospholipid metabolism. To date, monoglyceride lipase is considered as the rate-limiting enzyme in the degradation of MG, although in vivo evidence is still lacking. The monoacylglycerol "2-arachidonoyl glycerol" (2-AG) belongs to a family of compounds designated as endocannabinoids. These compounds are mimicked by 9-tetrahydrocannabinol (THC), the major psychoactive component of marijuana, and are involved in the control of multiple biologic processes including those which control learning and memory processes, emotional behavior, pain, appetite regulation, energy metabolism, blood pressure, and bone growth. The multiple potentials of endocannabinoids make them a subject of great interest in pharmacological research. However, the use of these substances as drugs is limited by their rapid cellular degradation, which prevents their effective medical use. Thus, endocannabinoid degrading enzymes, like monoglyceride lipase, are considered as pharmacological targets to modulate endocannabinoid levels. Vitamin A is an essential micronutrient and has numerous important functions including a role in vision, reproduction, immunity, and the development and maintenance of differentiated tissues. Retinol represents the transport form of vitamin A and can be converted into bioactive metabolits, whereas RE represent the inactive storage form. Excess retinol is esterified with long-chain fatty acids and stored predominantly in stellate cells of the liver. The storage of RE may counteract vitamin A toxicity and also represents a depot which can provide the body with sufficient amounts of vitamin A for several weeks. The controlled mobilization of retinol requires the activity of lipases capable of hydrolyzing retinyl ester stores. Currently, the rate-limiting RE hydrolyzing enzymes are unknown. Despite the eminent importance of MG and RE in biologic processes, in vivo evidence for a role of a specific enzyme in the hydrolysis of these compounds is lacking. The identification of enzymes involved in MG or RE hydrolysis and the characterization of mice deficient for these lipases will provide novel insights into the catabolism of acylglycerides and endocannabinoids, as well as into the regulation of vitamin A homeostasis.

Retinoids and monoglycerides are important signaling lipids involved in the regulation of numerous physiological processes. The availability of these compounds for cellular processes is strongly dependent on lipases catalyzing either the mobilization of signaling molecules or their degradation. The first aim of this project was to define the role of specific lipases in retinylester catabolism. Retinoids, also known as Vitamin A, are a group of unsaturated lipids including retinol, retinal, and retinoic acid. These compounds are essential for development, growth, maintenance of the immune system and vision. Retinoids are stored in the form of retinylester primarily in the liver and released according to the demand of the body. This process is catalyzed by lipases and is essential for retinoid homeostasis. Up to date, the identity of retinylester hydrolases is unknown. In this project, we could identify a lipase which hydrolyzes retinylester into retinol and fatty acids. Our observations indicate that this lipase affects retinoid homeostasis and consequently retinoid-dependent cellular processes. The second goal of the project was to define the specific role of lipases in monoglyceride metabolism. The monoacylglycerol species 2-arachidonoyl glycerol (2-AG) belongs to a family of compounds designated as endocannabinoids. These compounds are mimicked by ?9-tetrahydrocannabinol (THC), the major psychoactive component of marijuana, and are involved in the control of multiple biologic processes including those which control learning and memory processes, emotional behavior, pain, appetite regulation, energy metabolism, blood pressure, and bone growth. We generated a mouse model lacking the major monoglyceride lipase (MGL) resulting in the accumulation of 2-AG and other monoglyceride species in brain and other tissues. Interestingly, these mice exhibited desensitization of the endocannabinoid system similar as observed after chronical exposure to THC. Furthermore, we could show that deletion of MGL attenuates the development of insulin resistance, a major risk factor for type 2 diabetes. These observations indicate an important role of MGL in endocannabinoid signaling and in the pathogenesis of metabolic disease.