Description:

Pancreatic beta cells store large amounts of triglyceride (TG) distributed throughout the cytosol and have recently been shown to contain hormone-sensitive lipase. Using the lipase inhibitor orlistat, we have recently provided evidence that GLP-1 enhances insulin release from beta cells by stimulating lipolysis. In addition, islets from hormone-sensitive lipase-deficient mice have been shown to exhibit impaired glucose-induced insulin secretion. Thus, fatty acids (FA) may be mobilized from stored TG as a result of raising cAMP and activating protein kinase A. Moderate levels of TG could potentiate insulin release by providing FA for required lipid signals. Accumulation of excess lipid droplets is thought to be associated with aberrant islet function and impaired insulin release.

Progress:

Using cultured cells and islets we are trying to determine the effects of increased intracellular lipid droplets on beta cell function. Accumulation of both oleate and BODIPY-FA results in lipid droplet formation in single cells. Clonal pancreatic beta cells (HIT) incubated with oleate or BFA for 15-40 hrs showed an increase in intracellular lipid droplets as observed by both scanning electron microscopy (SEM) and fluorescence confocal microscopy. The number of intracellular lipid droplets increases as time of incubation with 0.1 mM oleate:BSA (2:1) increased from 15 to 40 hrs compared to control cells not treated with FA. Accumulation of intracellular lipid droplets occurs more rapidly at higher FA:BSA ratios (7:1 compared to 2:1). Lipid droplet like structures (averaging 1 µm in diameter) were identified in a cross-section of a normal mouse islet using BODIPY-FA and confocal microscopy. Preliminary indications suggest a peripheral location of droplets in the cell, which could impact membrane functions such as Ca²⁺ entry and exocytosis. The effect of TG loading on insulin secretion from the beta cell is presently under investigation.