Glucose and white adipose tissue metabolism. Effects of site and sex on the fate of glucose

Abstract

The current theories about endothelial inflammation in the adipose tissue in metabolic syndrome point to hypoxia as one of its main causes. Nevertheless, our group has found that both 3T3L1 adipocytes, under normoxic conditions, and WAT, in vivo, consume large amount of glucose, with high production of lactate and glycerol irrespective of oxygen availability. This thesis is based on the hypothesis that adipocytes act essentially as glycolytic cells impervious to hypoxia, and their metabolism may help reduce the blood glucose levels. We assumed that hypoxia, could affect the cells of the stromal fraction, eliciting an inflammatory response. To this purpose, we have studied ex vivo the glycolytic and anaerobic capacity of adult adipocytes and stromal vascular cells of both sex and obtained from different WAT sites, developing the methodology needed for a quantitative comparative analysis of data obtained from the same cultured cells well. We found that adipocytes, despite being the cells present in WAT in lower numbers, occupied almost the whole volume of the tissue, a consequence of their huge size due to their inert fat vacuole. The overall “live cell" volume represented only about 1.5% of the tissue, thus showing a very high metabolic activity of WAT in relative terms. Adipocytes ex vivo incubated with glucose, also took large amounts of the sugar, irrespective of its concentration, releasing instead 3C metabolites, such as lactate and glycerol to the medium. Lactate was fully derived from glucose and was produced at a steady pace, irrespective the presence of oxygen, to form the ATP needed for cell functions. Glycerol efflux increased over time and its origin shifted from glycolitic to glycolytic-lipolytic: new-formed glycerol was incorporated into TAG, by esterification with acyl-CoA, derived from the same TAG lipolisis. The coexistence of these processes appears as a “futile cycle”, with the probable function further to waste excess energy. Lipogenesis was limited because the size of cells limits the access to oxidative mitochondrial pathways. The release of 3C metabolites seems to be a mechanism to lower glycemia, defend WAT against excess of substrate, and provide 3C fragments as more accessible substrates for other tissues. Mesenteric WAT adipocytes presented the highest metabolic activity, probably to help the hepatic handling of NEFA and reduce the flow of intestinal glucose to the liver. While in almost all WAT sites, excess mitochondrial pyruvate is returned to the cytoplasm to keep forming lactate; in female mesenteric adipocytes it is in part oxidized to acetyl-CoA to fuel lipogenesis. Stromal vascular cells also released lactate, even more than adipocytes per unit of tissue weight, but not glycerol nor NEFA. Red blood cells produced lactate, but its contribution was quantitatively minimal. Thus, stromal cells, acted in consonance with adipocytes, in all sites and sexes examined, wasting glucose in an anaerobic way, producing high amounts of 3C units. Thus, reinforcing the idea that WAT may be an active protagonist both in energy handling and in the body control of glycemia.

Las teorías actuales sobre la inflamación endotelial en el tejido adiposo en el síndrome metabólico apuntan a la hipoxia como una de sus causas. Nuestro grupo ha encontrado que adipocitos 3T3L1, bajo condiciones normóxicas, consumen grandes cantidades de glucosa, con elevada producción de lactato y glicerol. La hipótesis de esta tesis es que los adipocitos actuan como células glucolíticas y su metabolismo ayuda a reducir la glucemia, mientre la hipoxia podría afectar las células estromales. Hemos estudiado ex vivo la capacidad glucolítica de adipocitos adultos y células estromales de ambos sexos y de diferentes ubicaciones del TAB, desarrollando la metodología para un análisis cuantitativo. Hallamos que los adipocitos eran las células menos númerosas del tejido y la parte "viva" representaba solo el 1,5% del total, mostrando así el TAB una actividad metabólica muy elevada. Los adipocitos ex vivo captaban grandes cantidades de glucosa, independientemente de su concentración; liberando metabolitos de 3C. El lactato, liberado a ritmo constante y independientemente de la presencia de oxígeno, procedía totalmente de la glucosa. El eflujo de glicerol aumentó con el tiempo y su origen cambió de glucolítico a glucolítico-lipolítico. El glicerol neoformado se incorporaba a los TAG, reciclado acil-CoA de la misma lipolisis. Este"ciclo fútil", probablemente sirve para desperdiciar e energía, disminuir la glucemia, y proporcionar sustratos energéticos para otros tejidos. Los adipocitos del TAB presentaban la más alta actividad metabólica; probablemente para ayudar al manejo hepático de ácidos grasos y reducir el flujo de glucosa intestinal. Mientras que en todas las localizaciones, el exceso de piruvato mitocondrial regresaba al citoplasma y seguía formando lactato; en los adipocitos de las hembras de mesentérico, parte se oxidaba a acetil-CoA para sostener la lipogénesis. Las células estromales, por unidad de tejido, liberaban más lactato que los adipocitos, pero no glicerol ni ácidos grasos. Los eritrocitos produjeron mínimas cantidades de lactato.Así las células estromales, actúan en consonancia con los adipocitos, en todas las localizaciones estudiadas y en ambos sexos. Estos resultados refuerzan la idea de que el TAB puede ser un protagonista activo en el manejo de la energía y en el control de la glucemia.