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

Maria L. Watson, Matthew Coghlan, Harinder S. Hundal (Lead / Corresponding author)

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    64 Citations (Scopus)

    Abstract

    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 (similar to 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.

    Original languageEnglish
    Pages (from-to)791-801
    Number of pages11
    JournalBiochemical Journal
    Volume417
    Issue number1
    DOIs
    Publication statusPublished - 1 Feb 2009

    Keywords

    • ceramide
    • diacylglycerol
    • insulin receptor substrate 1 (IRS1)
    • myriocin
    • palmitate
    • protein kinase B (PKB)
    • protein kinase C (PKC)
    • PROTEIN-KINASE-C
    • PLECKSTRIN HOMOLOGY DOMAIN
    • FATTY-ACIDS
    • RECEPTOR SUBSTRATE-1
    • DEPENDENT MECHANISM
    • CERAMIDE SYNTHESIS
    • 3T3-L1 ADIPOCYTES
    • KAPPA-B
    • ACTIVATION
    • INHIBITION

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