Regulation of glucose-dependent insulin secretion by insulin: Possible role of AMP-activated protein kinase

Abstract

Aims

Extracellular insulin affects insulin secretion from pancreatic β-cells in an autocrine fashion, but the role of glucose in this signaling pathway remains unclear. This study was conducted to evaluate the glucose dependency of extracellular insulin-mediated regulation of insulin secretion and the potential underlying mechanism.

Main methods

Pancreatic β-cells from male Sprague–Dawley rats and INS-1, a rat insulinoma cell line, were used. The mechanism of extracellular insulin-mediated, glucose-dependent insulin secretion was explored by analyzing the activity of ATP-sensitive K+ (KATP) channels, changes in cell membrane potential, and cytosolic free Ca2+ concentration ([Ca2+]c), as well as phosphorylation of the insulin signaling pathway and the metabolic sensor AMP-activated protein kinase (AMPK).

Key findings

Treatment of native β-cells with 100 nM insulin under basal glucose conditions (≤ 5 mM) reduced subsequent high glucose-induced insulin secretory responses, demonstrating less inhibition of KATP channels and decreased elevation of [Ca2+]c. In contrast, insulin treatment under high glucose conditions potentiated the insulin secretory responses of β-cells. While insulin treatment attenuated phosphorylation on the Thr172 of AMPK and the Ser789 of insulin receptor substrate (IRS)-1, which was increased by lowering glucose concentration, it enhanced phosphorylation of AMPK and IRS-1, which was decreased by elevating glucose concentration. This glucose-dependent regulation of insulin even occurred in the presence of LY294002, a phosphoinositide-3 kinase inhibitor.

Significance

Considering that the phosphorylated AMPK could inhibit KATP currents in β-cells, which triggers glucose-stimulated insulin secretion, extracellular insulin may regulate the phosphorylation status of AMPK through IRS-1 to modulate insulin secretion in a glucose-dependent way.

Keywords AMP-activated protein kinase; ATP-sensitive K+ channel; β-cell insulin receptor; Insulin secretion regulation; Glucose-dependent insulin secretion