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 (∼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.
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Research Article|
January 16 2009
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;
Maria L. Watson
*Division of Molecular Physiology, Sir James Black Centre, College of Life Sciences, University of Dundee, Dundee, DD1 5EH, Scotland, U.K.
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Matthew Coghlan;
Matthew Coghlan
†AstraZeneca, CVGI, Alderley Park, Macclesfield, Cheshire SK10 4TG, U.K.
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Harinder S. Hundal
Harinder S. Hundal
1
*Division of Molecular Physiology, Sir James Black Centre, College of Life Sciences, University of Dundee, Dundee, DD1 5EH, Scotland, U.K.
1To whom correspondence should be addressed (email h.s.hundal@dundee.ac.uk).
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Publisher: Portland Press Ltd
Received:
June 05 2008
Revision Received:
September 30 2008
Accepted:
October 16 2008
Accepted Manuscript online:
October 16 2008
Online ISSN: 1470-8728
Print ISSN: 0264-6021
© The Authors Journal compilation © 2009 Biochemical Society
2009
Biochem J (2009) 417 (3): 791–801.
Article history
Received:
June 05 2008
Revision Received:
September 30 2008
Accepted:
October 16 2008
Accepted Manuscript online:
October 16 2008
Citation
Maria L. Watson, Matthew Coghlan, Harinder S. Hundal; Modulating serine palmitoyl transferase (SPT) expression and activity unveils a crucial role in lipid-induced insulin resistance in rat skeletal muscle cells. Biochem J 1 February 2009; 417 (3): 791–801. doi: https://doi.org/10.1042/BJ20081149
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