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

• Published in: Biochemical journal Vol. 417; no. 3; p. 791
• Main Authors: Watson, Maria L, Coghlan, Matthew, Hundal, Harinder S
• Format: Journal Article
• Language: English
• Published: England 01-02-2009
• Subjects: Animals; Biophysical Phenomena; Cell Membrane - metabolism; Diglycerides - biosynthesis; Insulin - metabolism; Insulin - pharmacology; Insulin Receptor Substrate Proteins - metabolism; Insulin Resistance - physiology; Muscle, Skeletal - enzymology; Muscle, Skeletal - metabolism; Palmitates - metabolism; Palmitates - pharmacology; Phosphorylation; Rats; Serine C-Palmitoyltransferase - genetics; Serine C-Palmitoyltransferase - metabolism; Serine C-Palmitoyltransferase - physiology; Signal Transduction
• ISSN: 1470-8728
• DOI: 10.1042/bj20081149

Saturated fatty acids, such as palmitate, promote accumulation of ceramide, which impairs activation and signalling of PKB (protein kinase B; also known as Akt) to important end points such as glucose transport. SPT (serine palmitoyl transferase) is a key enzyme regulating ceramide synthesis from palmitate and represents a potential molecular target in curbing lipid-induced insulin resistance. In the present study we explore the effects of palmitate upon insulin action in L6 muscle cells in which SPT expression/activity has been decreased by shRNA (small-hairpin RNA) or sustained incubation with myriocin, an SPT inhibitor. Incubation of L6 myotubes with palmitate (for 16 h) increases intramyocellular ceramide and reduces insulin-stimulated PKB activation and glucose uptake. PKB inhibition was not associated with impaired IRS (insulin receptor substrate) signalling and was ameliorated by short-term treatment with myriocin. Silencing SPT expression (approximately 90%) by shRNA or chronic cell incubation with myriocin (for 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-PI3K (phosphoinositide 3-kinase) coupling that arises from diverting palmitate towards greater DAG (diacylglycerol) synthesis, which elevates IRS1 serine phosphorylation via activation of DAG-sensitive PKCs (protein kinase Cs). Treatment of SPT-shRNA cells or those treated chronically with myriocin with PKC inhibitors antagonized palmitate-induced loss in insulin signalling. The findings of the present study indicate that SPT plays a crucial role in desensitizing muscle cells to insulin in response to incubation with palmitate. While 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 signalling.