Lipid Rafts in Pancreatic Beta- and Alpha-cell Stimulus-secretion Coupling

Type 2 diabetes is hallmarked by insufficient beta-cell insulin secretion and inappropriate alpha-cell glucagon secretion concomitant to peripheral insulin resistance. However, the mechanisms underlying dysregulation of pancreatic beta- and alpha-cells in type 2 diabetes require further investigation. Whereas triglycerides and saturated free fatty acids have been well recognized to cause beta-cell dysfunction, the physiological and/or pathological role of cholesterol on beta- and alpha-cells is less well examined. Cholesterol is the major component of membrane microdomains, termed lipid rafts. Numerous signaling and transport proteins have been found to be targeted to lipid raft microdomains, where the function of the associated membrane proteins could be distinctly regulated. I have identified the expression of lipid raft constituent proteins, caveolin-1/2 in pancreatic beta-cells; and caveolin-2 in alpha-cells. A variety of membrane proteins (ion channels and SNARE proteins) critical for beta- and alpha-cell stimulus-secretion coupling were found to be associated with cholesterol-rich lipid raft microdomians, and the properties of those ion channels (Kv2.1, Kv4.1/4.3, and Cav1.2 channels) and SNARE proteins were closely regulated by cholesterol-rich lipid rafts. Acute depletion of cholesterol from the plasma membrane with methyl-beta-cyclodextrin caused an elevated basal hormone secretion from both beta- and alpha-cells and a loss of glucose-stimulated insulin secretion, implicating that cholesterol-rich lipid rafts play an important role in regulating exocytosis of these two types of islet cells. Chronic pharmacological inhibition of beta-cell endogenous cholesterol biosynthesis with squalene epoxidase inhibitor caused an impairment of both Cav channels and SNARE protein exocytotic machinery, indicating that intracellular cholesterol and its homeostasis are critical for maintaining normal beta-cell function. The work presented in this thesis provided clear evidence that cholesterol-rich lipid rafts play a critical role in maintaining the normal function of pancreatic ion channels and SNARE proteins to regulate pancreatic beta- and alpha-cells stimulus-secretion coupling. Manipulation of cholesterol level of beta- and alpha-cells could be a potential target for a therapeutic intervention in the treatment of type 2 diabetes.Ph