Lipolysis - a highly regulated multi-enzyme complex mediates the catabolism of cellular fat stores

Retinyl ester hydrolases and their roles in vitamin A homeostasis

In mammals, dietary vitamin A intake is essential for the maintenance of adequate retinoid (vitamin A and metabolites) supply of tissues and organs. Retinoids are taken up from animal or plant sources and subsequently stored in form of hydrophobic, biologically inactive retinyl esters (REs). Accessibility of these REs in the intestine, the circulation, and their mobilization from intracellular lipid droplets depends on the hydrolytic action of RE hydrolases (REHs). In particular, the mobilization of hepatic RE stores requires REHs to maintain steady plasma retinol levels thereby assuring constant vitamin A supply in times of food deprivation or inadequate vitamin A intake. In this review, we focus on the roles of extracellular and intracellular REHs in vitamin A metabolism. Furthermore, we will discuss the tissue-specific function of REHs and highlight major gaps in the understanding of RE catabolism. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Fat in the skin: Triacylglycerol metabolism in keratinocytes and its role in the development of neutral lipid storage disease

Keratinocyte differentiation is essential for skin development and the formation of the skin permeability barrier. This process involves an orchestrated remodeling of lipids. The cleavage of precursor lipids from lamellar bodies by β-glucocerebrosidase, sphingomyelinase, phospholipases and sterol sulfatase generates ceramides, non-esterified fatty acids and cholesterol for the lipid-containing extracellular matrix, the lamellar membranes in the stratum corneum. The importance of triacylglycerol (TAG) hydrolysis for the formation of a functional permeability barrier was only recently appreciated. Mice with defects in TAG synthesis (acyl-CoA:diacylglycerol acyltransferase-2-knock-out) or TAG catabolism (comparative gene identification-58, -CGI-58-knock-out) develop severe permeability barrier defects and die soon after birth because of desiccation. In humans, mutations in the CGI-58 gene also cause (non-lethal) neutral lipid storage disease with ichthyosis. As a result of defective TAG synthesis or catabolism, humans and mice lack ω-(O)-acylceramides, which are essential lipid precursors for the formation of the corneocyte lipid envelope. This structure plays an important role in linking the lipid-enriched lamellar membranes to highly cross-linked corneocyte proteins. This review focuses on the current knowledge of biochemical mechanisms that are essential for epidermal neutral lipid metabolism and the formation of a functional skin permeability barrier.

The resurgence of Hormone-Sensitive Lipase (HSL) in mammalian lipolysis

The ability to store energy in the form of energy-dense triacylglycerol and to mobilize these stores rapidly during periods of low carbohydrate availability or throughout the strong metabolic demand is a highly conserved process, absolutely essential for survival. In the industrialized world the regulation of this pathway is viewed as an important therapeutic target for disease prevention. Adipose tissue lipolysis is a catabolic process leading to the breakdown of triacylglycerols stored in fat cells, and release of fatty acids and glycerol. Mobilization of adipose tissue fat is mediated by the MGL, HSL and ATGL, similarly functioning enzymes. ATGL initiates lipolysis followed by the actions of HSL on diacylglycerol, and MGL on monoacylglycerol. HSL is regulated by reversible phosphorylation on five critical residues. Phosphorylation alone, however, is not enough to activate HSL. Probably, conformational alterations and a translocation from the cytoplasm to lipid droplets are also involved. In accordance, Perilipin functions as a master regulator of lipolysis, protecting or exposing the triacylglycerol core of a lipid droplet to lipases. The prototype processes of hormonal lipolytic control are the β-adrenergic stimulation and suppression by insulin, both of which affect cytoplasmic cyclic AMP levels. Lipolysis in adipocytes is an important process in the management of body energy reserves. Its deregulation may contribute to the symptoms of type 2 diabetes mellitus and other pathological situations. We, herein, discuss the metabolic regulation and function of lipases mediating mammalian lipolysis with a focus on HSL, quoting newly identified members of the lipolytic proteome.

β-Adrenoceptor Signaling Networks in Adipocytes for Recruiting Stored Fat and Energy Expenditure

THE ADIPOCYTE IS LIKE A BANK: a place to store excess (caloric) cash in times of plenty, and from which one can withdraw savings during "lean times." The β-adrenoceptors (βAR) are the gateways to this mobilization of fat to be consumed in other tissues. This review discusses the βAR signaling pathway(s) in white and brown adipocytes. Studies in rodent models show that brown adipocytes nestled with white fat depots correlate with and are considered a key enabling factor in resistance to diet-induced obesity. Since it is now recognized that adult humans have brown adipocytes, knowing the steps in these signaling pathways may provide the opportunity to manipulate adipocytes to be net consumers of energy.