Phosphorylation of protein kinase B, the key enzyme in insulin-signaling cascade, is enhanced in linoleic and arachidonic acid-treated HT29 and HepG2 cells

Abstract

Objectives: Defects in the insulin-signaling pathway have been implicated in the pathogenesis of impaired glucose uptake, insulin resistance, and type 2 diabetes. However, the specific defects that precipitate these abnormalities are yet to be fully elucidated. After binding to insulin, the plasma membrane-embedded insulin receptor transmembrane protein initiates a cascade of phosphorylation that leads to the activation of protein kinase B (AKT) and subsequently to the initiation of some metabolic actions of insulin. The activities of this receptor, insulin binding, and tyrosine kinase activation is dependent on its plasma lipid environment. Published data on the influence of omega-3 and -6 polyunsaturated fatty acids on insulin response are scarce. Moreover, the findings of the published investigations, most of which used omega-3 and -6, polyunsaturated fatty-acid blends, have been inconclusive. Hence, further, well thought out research is needed. The aim of the current study was to elucidate the effect of treatments with linoleic acid (LNA), arachidonic acid (ARA), alpha-linolenic acid (ALA), docoshexaenoic acid (DHA), and eicosapentaenoic acid (EPA) on cell membrane composition and consequently on the insulin-signaling pathway and specifically AKT phosphorylation.

Methods: Human colon adenocarcinoma (HT29) and liver hepatocellular (HepG2) cells were treated with or without 40 µM of LNA, ARA, ALA, EPA, or DHA for 48 h, the fatty-acid composition of phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) from the treated cells by capillary gas liquid chromatograph. Cells were incubated for 30 min with or without human insulin (50 ng/mL), and the phosphorylation of AKT was assessed with the use of Western blotting.

Results: The fatty acids were incorporated in the PtdCho and PtdEtn of both cell lines, but the level of incorporation was higher in HT29. Phosphorylation of AKT increased when HT29 was treated with LNA (P < 0.05) and ARA (P < 0.01) but not with ALA, EPA, or DHA. A similar but non-significant increase in AKT phosphorylation was observed in LNA- and ARA- treated HepG2 cells.

Conclusions: The finding of this investigation demonstrates that plasma membrane lipid bilayer enrichment with LNA or ARA treatment enhances insulin action by AKT activation.

Keywords: AKT phosphorylation; Membrane phospholipids; Omega 3 fatty acids; Omega 6 fatty acids.