The role of free fatty acids in the pathogenesis of insulin resistance and diabetes mellitus -13C/31P - NMR studies

Free fatty acids (FFA) play a pivotal role in the pathogenesis of insulin resistance since elevation of plasma FFA concentrations is not only associated with human obesity but may also preceed the manifestation of type II diabetes mellitus independently of body weight and hyperlipidemia. Despite the classical in vitro studies of Randle and co- workers, the interaction between FFA and glucose metabolism and particularly the mechanism by which FFA induce insulin resistance at level of the liver and skeletal muscle is still under debate. Prior to the advent of in vivo NMR spectroscopy, studies on human glucose metabolism were limited to tracer experiments and/or determination of glycogen and glucose metabolites in biopsies. By using non-invasive in vivo 13C and 31P NMR spectroscopy we have recently reported that - in contrast to the Randle hypothesis FFA decrease insulin-dependent glucose disposal which is associated with a blunted increase of intramyocellular glucose-6-phosphate and with reduction in rates of glucose oxidation and glycogen synthesis in human calf muscle. This proposal aims at further localization along the metabolic pathway in vivo of free fatty acid interaction with glucose metabolism both at the level of glucose transport, glucose phosphorylation, and glycogen synthesis. To this end, intracellular steps of metabolic pathways will be traced in liver as well as skeletal muscle employing NMR spectroscopy in man. In detail, the foreseen experiments will examine: (I) the primary (early) action of free fatty acids on the time course and concentration-dependent changes of intramyocellular g6p concentrations, (II) the effect of free fatty acids on intramyocellular free glucose concentration and glycogen synthesis, and intramyocellular triglyceride concentration, and (III) the effects of free fatty acid elevation on hepatic glucose production and gluconeogenesis in vivo in healthy subjects, offspring of and patients with overt type II diabetes mellitus. Preliminary results of Protocol 1 demonstrate a lack of an initial peak along with a rapid decline of muscle glucose-6-phosphate concentrations even at fasting plasma FFA levels, suggesting that FFA primarily cause a defect of glucose transport/phosphorylation under physiological conditions in man.

Background: Obesity and fat-rich diets increase the blood concentrations of free fatty acids. Free fatty acids are considered to impair insulin action, i.e. to induce insulin resistance, which relates to the development of the frequent type of diabetes mellitus (type 2 diabetes) and to premature vascular defects (myocardial infarction, stroke). Aims: How free fatty acids affect insulin action and in particular glucose metabolism in cells of the human body is still unclear. Thus, the studies of this grant examined the mechanism of action and the consequences of increased blood concentrations of free fatty acids on glucose metabolism in muscle and liver as well as blood flow of certain organs of the human body. Methods: The experimental protocols were performed by combining infusion of lipids with the application of new developed methods such as in vivo NMR spectroscopy, which makes it possible to exactly determine glucose uptake and disposal as glycogen directly without discomfort in cells of the human body. Results: The studies on skeletal muscle of the calf showed that even small increases in free fatty acid concentrations inhibit both the insulin stimulated as well as insulin independent, glucose uptake. These data support the novel hypothesis that free fatty acids do not need to be burned (oxidized) within cells before they affect glucose metabolism. Free fatty acids rather block glucose transport directly and rapidly and therefore lead to insulin resistance. The studies on the liver found that free fatty acids can induce an increase in blood glucose concentration in healthy humans, as long as the release of insulin by the pancreas is prevented. The increase in blood glucose levels is due to augmented synthesis of new glucose within the liver, but even more results from the effects of free fatty acids on muscle. Free fatty acids play also a role in the insulin resistance of patients with diabetes and/or impaired insulin release (partial agenesis of the pancreas). The studies on blood flow demonstrated that free fatty acids modulate blood flow of the eye and in the skin and that they impair blood vessel function. These changes are similar to that observed in patients with insulin resistance and /or type 2 diabetes. Conclusions: Taken together the studies provided evidence that free fatty acids play an important role in the development of insulin resistance and thereby in the pathogenesis of non-insulin dependent diabetes. Thus, the action of free fatty acids represents a target for novel concept for the treatment of diabetes.