Endogenous Cholesterol Is Essential for Insulin Secretion.

Insulin secretion from pancreatic β-cells is mediated by the opening of Ca[sub v] channels and exocytosis is facilitated by the secretory SNARE protein machinery. We have observed these factors are sensitive to the acute removal of membrane cholesterol. However, less is known about the chronic changes in endogenous cholesterol and its biosynthesis in regulating β-cell stimulus-secretion coupling. We have examined the effects of inhibiting endogenous β-cell cholesterol biosynthesis by using a specific squalene epoxidase inhibitor, NB598. We detected the expression of squalene epoxidase in primary human, rat and mouse islet cells and clonal MIN6 and INS-1 cells. A cholesterol reduction of 40% - 52% from pancreatic islets from mouse and human was observed after being cultured with 10/µM NB598 for 48 hr. We found NB598 significantly inhibited both basal and glucose-stimulated insulin secretion from mouse and human pancreatic islets. Voltage-dependent Ca[sup 2+] (Ca[sub v]) channels were significantly reduced by NB598, an effect which may contribute to the observed impairment of insulin secretion. Rapid photolytic release of intracellular caged Ca[sup 2+]and simultaneous measurements of the changes in membrane capacitance revealed that NB598 also inhibited exocytosis primarily through reducing insulin granule mobilization. These results implicate the direct perturbation of β-cell exocytotic secretory proteins. Furthermore, NB598 accelerated the steady-state voltage dependence of voltage-gated K[sup +] (K[sub v]) channel inactivation, but did not affect the current density or channel activation. These data implicate endogenous cholesterol in pancreatic β-cells plays a critical role in regulating insulin secretion. Two distinct roles of cholesterol are involved in maintaining normal β-cell function. The first is through the functional regulation of Ca[sub v] and K[sub v] channels. The second is cholesterol is critical for the mobilization and fusion of insulin secretory granules with plasma membranes, in part, involving the formation of cholesterol-rich domain within the secretory vesicles and on the plasma membrane. [ABSTRACT FROM AUTHOR]