Modulating serine palmitoyl transferase (SPT) expression and activity unveils a crucial role in lipid-induced insulin resistance in rat skeletal muscle cells

International audienceSaturated fatty acids, such as palmitate, promote accumulation of ceramide which impairs activation and signaling of protein kinase B (PKB/Akt) to important end-points such as glucose transport. Serine Palmitoyl Transferase (SPT) is a key enzyme regulating ceramide synthesis from palmitate and represents a potential molecular target in curbing lipid-induced insulin resistance. Here we explore effects of palmitate upon insulin action in L6 muscle cells in which SPT expression/activity have been reduced by shRNA or sustained incubation with myriocin, a SPT inhibitor. Incubation of L6 myotubes with palmitate (16h) increases intramyocellular ceramide and reduces insulin-stimulated PKB activation and glucose uptake. PKB inhibition was not associated with impaired IRS signaling and was ameliorated by short-term treatment with myriocin. Silencing SPT expression (~90%) by shRNA or chronic cell incubation with myriocin (7 days) markedly suppressed SPT activity and palmitate-driven ceramide synthesis. However, challenging these muscle cells with palmitate still inhibited the hormonal activation of PKB. This inhibition was associated with reduced IRS1/p85-PI3-kinase coupling that arises from diverting palmitate towards greater DAG synthesis, which elevates IRS1 serine phosphorylation via activation of DAG-sensitive PKCs. Treatment of SPT-shRNA cells or those treated chronically with myriocin with PKC inhibitors antagonised palmitate-induced loss in insulin signaling. The findings indicate that SPT plays a crucial role in desensitizing muscle cells to insulin in response to incubation with palmitate. Whilst short-term inhibition of SPT ameliorates palmitate/ceramide-induced insulin resistance, sustained loss/reduction in SPT expression/activity promotes greater partitioning of palmitate towards DAG synthesis, which impacts negatively upon IRS1-directed insulin signaling