Signalling pathways and combinatory effects of insulin and amino acids in isolated rat hepatocytes

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Abstract
Liver metabolism is influenced by hormones and nutrients. Amino acids such as glutamine or leucine induce an anabolic response, which resembles that of insulin in muscle and adipose tissue. In this work, the signalling pathways and the effects of insulin were compared to those of glutamine and leucine in isolated hepatocytes from normal and streptozotocin‐diabetic rats. Glutamine increased cell volume and induced an anabolic response characterized by an activation of acetyl‐CoA carboxylase (ACC), glycogen synthase (GS) and p70 ribosomal S6 kinase (p70S6K), the key enzymes in fatty acid, glycogen and protein synthesis, respectively. The effects of glutamine were independent of insulin and did not share its signalling components. Leucine, which is poorly metabolized by the liver and does not modify cell volume, activated ACC and p70S6K, and exerted a synergistic effect on the glutamine‐induced activation of ACC and p70S6K. These amino acids did not affect insulin signalling. Insulin alone had no anabolic effect in hepatocytes, despite the activation of protein kinase B. Nevertheless, it enhanced the activation of ACC and p70S6K induced by leucine. However, insulin injected intravenously activated rat liver p70S6K. In hepatocytes from streptozotocin‐diabetic animals, the metabolic responses to the amino acids and insulin were similar to those in normal hepatocytes. We conclude that glutamine, insulin and leucine exert different effects that are mediated by different signalling pathways, although their effects are combinatory. The anabolic effect of insulin in hepatocytes was strictly dependent on the permissive action of leucine.