ER Calcium Levels Help Regulate K(ATP) Channel Trafficking to the Plasma Membrane of Pancreatic Beta Cells

Abstract

The closing of beta cell K(ATP) channels acutely triggers glucose-induced insulin secretion. However, the trafficking of K(ATP) channels to the beta cell plasma membrane has recently been shown to regulate beta cell secretory function over a longer time course, and changes in the number of K(ATP) channels can help set islet glucose sensitivity. To test whether endoplasmic reticulum free calcium level helps control K(ATP) trafficking to the cell surface, insulin-secreting INS-1 cells were treated for 16-18 h. with either palmitate (PALM; 250 µM), the SERCA inhibitor thapsigargin (TG; 200 nM) or tunicamycin (TM; 10 µg/ml); these treatments alter ER Ca. INS-1 membrane fractions were immunoblotted for SUR1, a K(ATP) channel subunit, Glucose Transporter 2 (GLUT2) or Kv2.1 channels. Isolated islets were transfected with the ER Ca sensor D4ER to monitor ER Ca, and basal ER Ca levels were expressed as FRET ratios. TM-treated cells had low levels of glycosylated SUR1 and GLUT2 on their surface membranes whereas the trafficking of Kv2.1 was unaffected. PALM or TG treatment also reduced surface K(ATP), albeit more modestly. To test if ER Ca was reduced due to Ca egress through ER IP3 receptors, we treated cells with xestospongin C (XeC) to inhibit IP3Rs. XeC prevented ER Ca reduction due to TG treatment and increased K(ATP) trafficking. Furthermore, TM or TG treated islets were more glucose sensitive. These data suggest that ER Ca contributes to the control of SUR1 trafficking to the cell surface and/or ER protein processing. Dynamic regulation of surface K(ATP) by ER Ca may provide a novel negative feedback mechanism to regulate beta cell Ca2+ influx.