Skeletal muscle is the principal site for insulin-stimulated glucose disposal but its ability to utilise glucose is diminished by insulin resistance associated with obesity and consequent elevations of free fatty acids (FFAs). Chronic sub-clinical inflammation, characterized by increased macrophage infiltration into obese adipose tissue is a hallmark of the insulin resistance syndrome. However, little is known about the potential role of muscle macrophages in mediating local effects of elevated fatty acid levels. Here we aimed to determine how macrophage-derived factors might directly impact skeletal muscle glucose utilisation. Activated J774 macrophages were incubated with or without saturated (palmitic acid) or unsaturated (palmitoleic acid) FFAs or lipopolysaccharide (LPS) for eight hours. Differentiated C2C12 myotubes were then incubated with macrophage-conditioned medium for 6 or 16 hours and the effects on glycogen synthesis and intracellular signalling assessed. Results are derived from three or four independent experiments. Incubation of myotubes with conditioned medium from palmitic acid-treated J774s for 16 hours caused a 53% reduction in insulin-stimulated glycogen synthesis, accompanied by a reduction in phosphorylation of PI3-kinase pathway intermediates IRS-1 (Tyr612), Akt (Ser473), AS160 (Thr642) and GSK3β (Ser9). Palmitic acid-conditioned medium also activated MAP kinases (JNK and p38) and reduced IκBα protein, implying activation of inflammatory pathways in myotubes. These changes were mirrored by the effects of LPS-conditioned medium. In contrast, palmitoleic acid-conditioned medium improved glycogen synthesis by 36% after 16 hours treatment and increased phosphorylation of IRS-1, AS160 and GSK3β. Furthermore, palmitoleic acid-conditioned medium negated the effect of palmitic acid on MAP kinase activation. These findings demonstrate that macrophage-derived factors may play a direct role in mediating the effects of elevated saturated plasma FFAs on muscle insulin sensitivity but that these effects can be ameliorated by increasing availability of unsaturated FFAs.
Declaration of interest: There is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported.
Funding: No specific grant from any funding agency in the public, commercial or not-for-profit sector.