Resumen de The overexpression of carnitine palmitoyl transferase1 and its influence on the lipid metabolism of rat hepatocytes
Whilst current definitions may still define insulin resistance in terms of insulin effects on glucose metabolism, the last decade has seen a shift from the traditional 'glucocentric' view of the syndrome associated with insulin resistance to an increasingly acknowledged 'lipocentric' viewpoint. The notion that lipids may act as signaling factors that regulate metabolic functions in target tissues was first suggested more than 40 years ago, when Randle et al. hypothesized that obesity-associated insulin resistance could be explained by substrate competition between increased circulating NEFA and glucose for oxidative metabolism in insulin-responsive cells. The importance of NEFA and lipid metabolism was also outlined by McGarry, who suggested that insulin resistance and concomitant hyperglycemia could be viewed in the context of underlying abnormalities of lipid metabolism. More recently, glucose uptake, rather than intracellular glucose metabolism, has been implicated as a rate-limiting step for NEFA-induced insulin resistance. In this model, NEFA and some of their metabolites, including acyl-CoA, ceramides and diacyglycerol, have been demonstrated to serve as signaling molecules that activate protein kinases such as PKC, JNK and IKK. These kinases can impair insulin signaling by increasing inhibitory serine phosphorylation of insulin receptor substrates (IRS), the key mediators of insulin signaling, and activating an inflammatory response.
In this study, we tried to overexpress the enzyme, carnitine palmitoyl trasferase 1(CPT1) and CPT1AM, a mutant but insensitive form of CPT1 which is insensitive to its physiological inhibitor malonyl-CoA on primary rat hepatocytes. CPT1 is a key regulatory enzyme which involves in the entry of long chain acyl coA in to the mitochindria for ß-oxidation. Overexpression of CPT1A and CPT1AM by recombinant adenovirus gave a 2-fold increase in the activity of the enzyme and protein levels. This activity was accompanied by a 1.7-fold increase in the mitochondrial ß-oxidation. Increasing the ß-oxidation meant reduction in the accumulation of intracellular NEFA. Thus we found that by efectively increasing the fatty acid oxidation it is possible to reduce the accumulation of fatty acids and triglyceride content which was reduced to 50% on our system. Moreover, accumulation of lipid metabolites are known to cause insulin resistance via blocking the insulin signaling pathway. We showed that both at basal conditions and insulin resistant state, caused by incubating with pathophysiological concentratios of palmitate, we could succussfully restore insulin sensitivity in hepatocytes.
On the other hand, increase in the fatty acid oxidation in hepatocytes induced oxidative stress and this stress lead to the increase in the expression of inflammatory interleukins expression. Hence we conclude that overexpression of CPT1A and CPT1AM protected hepatocytes from fatty acid induced insulin resistance and reduced TG content at the expense of inducing oxidative stress and inflammation. This side effect although seen seems not to interfere with the lipid clearing capacity of CPT1 by enhanced ß-oxidation.
