Abstract 34: Apolipoprotein A-I Increases Insulin Secretion from β-cells via a Protein Kinase A Dependent Mechanism

Abstract

Introduction: The progression to hyperglycaemia in type 2 diabetes is marked by β-cell insulin secretory dysfunction and cell loss. We have previously demonstrated that apolipoprotein (apo) A-I, the major protein constituent of high density lipoproteins (HDL) increases insulin expression and secretion from β-cells. Clinical data also suggests that pharmacological elevation of HDL levels is associated with improved glycemic control in patients with type 2 diabetes. With the current interest in HDL raising therapeutics, defining the mechanism by which apoA-I acts on insulin secretion is of importance. Objective: To elucidate the cell signalling events responsible for increasing insulin secretion from pancreatic β-cells treated with lipid-free apoA-I. Methods: Ins-1E (rat insulinoma) cells were pre-treated for 30 min with the Protein kinase A (PKA) specific inhibitor H89 (20 μM), soluble and transmembrane adenyl cyclase specific inhibitors (KH7, 30 μM and 2’5’ dideoxyadenosine, 50 μM, respectively) or vehicle control, then incubated for 1 h with lipid-free apoA-I (final concentration 1 mg/mL) under both basal (2.8 mM) and high (25 mM) glucose conditions. The insulin concentration in the culture supernatants was determined by radioimmunoassay and the cells were either lysed for protein analysis by western blotting or treated with 0.1 M HCl for determining cAMP by enzyme immunoassay. Results: Incubation of Ins-1E cells with apoA-I increased insulin secretion up to 3-fold. This increase was no longer apparent when the cells were pre-treated with H89. Incubation with apoA-I increased cAMP accumulation in Ins-1E cells 2.5-fold. This increase was totally inhibited when the cells were pre-incubated with 2’5’ dideoxyadenosine but not by KH7, indicating that transmembrane adenyl cyclase(s) are responsible for this response. ApoA-I also activated the small GTPase Cdc42, which may link cell surface apoA-I receptors with transmembrane adenyl cyclases. Conclusion: ApoA-I increases insulin secretion from pancreatic β-cells via a PKA-dependent mechanism involving transmembrane, but not soluble, adenyl cyclases and possibly Cdc42. This provides a possible explanation of the clinical observations that increased HDL may be beneficial in type 2 diabetes.