Análisis en animales transgénicos del papel del incremento en la gliceroneogénesis o en la captación de glucosa por el tejido adiposo
Obesity and type 2 diabetes are the two most important health problems in Western societies. The mechanisms underlying the development of obesity, insulin resistance and type 2 diabetes are not fully understood. Obesity results from an imbalance between energy intake and expenditure, leading to increased energy storage in the form of triglycerides in white adipose tissue (WAT). Adipocytes synthesize triglyceride by esterification of free fatty acids (FFA) with glycerol-3-phosphate and net lipid deposition occurs when esterification is higher than lipolysis. Although glucose is traditionally viewed as the main precursor of the glycerol backbone for triglyceride synthesis, it may also be synthesized from pyruvate or lactate through glyceroneogenesis when glucose supply is limited, as fasting conditions.
A regulatory step of glyceroneogenesis is catalized by phosphoenolpyruvate carboxykinase (PEPCK). Thus, in the first part of this study, increase in glyceroneogenesis was analyzed in transgenic mice overexpressing PEPCK in adipose tissue. PEPCK overexpression led to increased FFA re-esterification, higher adipocyte size, fat mass, body weight and circulating adiponectin and decreased circulating FFA. Moreover, despite obesity, glucose tolerance and whole-body insulin sensitivity were preserved. This suggests that obesity without increased circulating FFA does not lead to insulin resistance or type 2 diabetes. Surprisingly, when fed a high-fat diet for a short period (6 weeks), transgenic mice developed severe obesity and were more hyperinsulinemic, glucose intolerance and insulin resistance than controls. In these animals, high-fat feeding in the presence of increased FFA reesterification leads to triglyceride accumulation in white and brown adipose tissue and to fat storage saturation. This impairs the role of white adipose tissue in buffering the flux of circulating lipids, leading to fat deposition in liver, hypertriglyceridemia and insulin resistance. Furthermore, fat accumulation in brown adipose tissue probably may reduce diet-induced thermogenesis, contributing to severe obesity and insulin resistance. All these results suggest that the regulation of the lipid storage capacity of white adipose tissue is crucial to the maintenance of insulin sensitivity.
On the other hand, alterations in nutrient partitioning and particularly in glucose metabolism may contribute to obesity and type 2 diabetes. In an attempt to increase glucose uptake and triglyceride esterification, transgenic mice were engineered to increase glucose phosphorylation in adipose tissue. To this end, and to avoid the feedback inhibition of hexokinase activity by glucose-6-phosphate, the liver enzyme glucokinase (GK) was expressed under the control of aP2 promoter. Transgenic mice showed high levels of GK expression in white adipose tissue which led to increased adipose tissue basal glucose uptake. Consistent with this, transgenic mice presented increased glucose tolerance and higher whole-body insulin sensitivity than controls. Surprisingly, despite increased glucose uptake, body fat content and glycerol-3 phosphate synthesis were not altered in transgenic mice compared with controls. In vitro as well as in vivo studies using nuclear magnetic resonance showed that adipose tissue from transgenic mice released higher amounts of lactate than that from controls. This suggests that the major product from increased glucose uptake is lactate rather than glycerol-3 phosphate, leading to higher circulating lactate levels within physiological range.
All these results reinforce the idea that glyceroneogenesis and PEPCK may be crucial to regulate triglyceride synthesis in adipose tissue. However, the increased glucose uptake in adipocytes is not sufficient to promote glycerol-3-phosphate synthesis and lipid deposition.
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Advisor:Bosch i Tubert, Fàtima; Franckhauser, Sylvie
School:Universitat Autónoma de Barcelona
Source Type:Master's Thesis
Keywords:406 departament de bioquimica i biologia molecular
Date of Publication:05/29/2006