The effects of ceramide synthases and protein kinase c epsilon on glucose homeostasis and lipid metabolism

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Copyright: Diakanastasis, Barbara
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Abstract
Insulin resistance contributes strongly to Type 2 Diabetes, which is a global epidemic. A strong link exists between dietary lipid excess and the development of insulin resistance. Two key bioactive lipid metabolites implicated in the development of insulin resistance are the sphingolipid ceramide and diacylglycerol, an activator of protein kinase c epsilon (PKCe). The mechanisms linking the actions of these metabolites to insulin resistance are unclear. The aims of this thesis were to (i) assess the effect of ceramide synthase (CerS) overexpression on key elements of skeletal muscle sphingolipid metabolism and insulin action and (ii) examine the effect of PKCe deletion solely in adipose tissue on glucose and lipid homeostasis. CerS isoforms were overexpressed in lipid-treated L6 skeletal myotubes. This did not cause compensatory changes in the mRNA expression of sphingolipid metabolism proteins. The effect of CerS overexpression on flux through ceramide synthesis pathways was assessed. Increased flux via the salvage pathway was seen after CerS overexpression. In addition, GLUT4 translocation to the plasma membrane, a key aspect of skeletal muscle insulin action, was increased or unaltered after CerS overexpression. Glucose tolerance was tested in fat-fed mice with PKCe deletion only in adipose tissue. Improved glucose tolerance was seen in these mice during short and long term fat feeding. This was linked to a greater decrease in plasma fatty acids and without compensatory rises in insulin secretion, suggesting this deletion enhanced whole body insulin sensitivity. This was confirmed by euglycaemic-hyperinsulinaemic clamp. Improved hepatic insulin sensitivity, potentially via decreased plasma fatty acids, was shown to drive this improvement. PKCe deletion in adipose tissue was also linked to smaller adipocyte size, an indicator of improved insulin sensitivity. Gene expression analysis showed that these protective effects were not via altered mRNA expression of lipid metabolism or inflammatory genes. Increased protein expression of lipid esterification enzymes however, was observed. This research has highlighted a novel protective role for ceramide during skeletal muscle insulin resistance upon specific CerS isoform overexpression. We have also shown for the first time that (i) exclusive PKCe deletion in adipose tissue improves glucose and lipid homeostasis during insulin resistance (ii) hepatic insulin sensitivity is modulated indirectly by PKCe.
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Author(s)
Diakanastasis, Barbara
Supervisor(s)
Schmitz-Peiffer, Carsten
Biden, Trevor
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Publication Year
2015
Resource Type
Thesis
Degree Type
Masters Thesis
UNSW Faculty
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