Lipolysis and lipases in white adipose tissue – An update

Lipolysis is defined as the sequential hydrolysis of triacylglycerol (TAG) stored in cell lipid droplets. For many years, it was believed that hormone-sensitive lipase (HSL) and monoacylglycerol lipase (MGL) were the main enzymes catalyzing lipolysis in the white adipose tissue. Since the discovery of adipose triglyceride lipase (ATGL) in 2004, many studies were performed to investigate and characterize the actions of this lipase, as well as of other proteins and possible regulatory mechanisms involved, which reformulated the concept of lipolysis. Novel findings from these studies include the identification of lipolytic products as signaling molecules regulating important metabolic processes in many non-adipose tissues, unveiling a previously underestimated aspect of lipolysis. Thus, we present here an updated review of concepts and regulation of white adipocyte lipolysis with a special emphasis in its role in metabolism homeostasis and as a source of important signaling molecules.

New concepts in white adipose tissue physiology

Numerous studies address the physiology of adipose tissue (AT). The interest surrounding the physiology of AT is primarily the result of the epidemic outburst of obesity in various contemporary societies. Briefly, the two primary metabolic activities of white AT include lipogenesis and lipolysis. Throughout the last two decades, a new model of AT physiology has emerged. Although AT was considered to be primarily an abundant energy source, it is currently considered to be a prolific producer of biologically active substances, and, consequently, is now recognized as an endocrine organ. In addition to leptin, other biologically active substances secreted by AT, generally classified as cytokines, include adiponectin, interleukin-6, tumor necrosis factor-alpha, resistin, vaspin, visfatin, and many others now collectively referred to as adipokines. The secretion of such biologically active substances by AT indicates its importance as a metabolic regulator. Cell turnover of AT has also recently been investigated in terms of its biological role in adipogenesis. Consequently, the objective of this review is to provide a comprehensive critical review of the current literature concerning the metabolic (lipolysis, lipogenesis) and endocrine actions of AT.

Macrófagos, inflamación, tejido adiposo, obesidad y resistencia a la insulina / Macrophages, inflammation, adipose tissue, obesity and insulin resistance

La obesidad se asocia con un estado inflamatorio implicado en el desarrollo de aterosclerosis y resistencia a la insulina. Los macrófagos son claves en la génesis de estos procesos. La obesidad induce la acumulación de macrófagos en el tejido adiposo. Los macrófagos producen muchas de las moléculas inflamatorias secretadas por el tejido adiposo. Las proteínas quimioatrayentes de monocitos (MCP) y sus receptores son fundamentales en la respuesta inflamatoria y en el reclutamiento de células inmunes en sitios de inflamación. La expresión en el tejido adiposo de una MCP, la quimiocina del ligando 2 del motif C-C (CCL2 o MCP1), está incrementada en proporción a la adiposidad. El receptor 2 de quimiocina del motif C-C (CCR2) regula el reclutamiento y quimiotaxis de monocitos y macrófagos, es necesario para las respuestas inflamatorias dependientes de macrófagos y para el desarrollo de aterosclerosis. Ya que el receptor CCR2 regula las respuestas inflamatorias locales, se ha postulado que las MCP, actuando a través de su receptor CCR2, podrían regular la inflamación inducida por la obesidad en el tejido adiposo. Este documento se enfoca en dilucidar los mecanismos moleculares y genéticos que permiten reclutar y retener macrófagos en el tejido adiposo.

Obesity is associated with a complex systemic inflammatory reaction that has been associated with the development of atherosclerosis and insulin resistance. Obesity also induces macrophage accumulation in adipose tissue. Macrophages produce many of the pro inflammatory molecules released by adipose tissue and have been implicated in the development of obesity-induced adipose tissue inflammation. Monocyte chemoattractant proteins (MCPs) and their receptors play key roles in the development of inflammatory responses and are crucial for the recruitment of immune cells towards inflammation sites. Adipose tissue expression of at least 1 MCP, C-C motif chemokine ligand-2 (CCL2 or MCP1), increases in proportion to adiposity. The C-C motif chemokine receptor-2 (CCR2) regulates monocyte and macrophage recruitment and is necessary for macrophage-dependent inflammatory responses and the development of atherosclerosis. Because CCR2 regulates monocyte and macrophage chemotaxis and local inflammatory responses, it has been hypothesized that monocyte chemoattractant molecules acting through CCR2 might regulate obesity-induced inflammation in adipose tissue. Our study focuses on the molecular and genetic mechanisms that recruit and retain macrophages in adipose tissue.