Novel Roles for Geranylgeranyl Transferase-III (GGTase-III) in Insulin Secretion

Post-translational prenylation of G proteins is implicated in physiological insulin secretion. It has been reported recently that GGTase-III participates in the functional regulation of Ykt6, a synaptobrevin homolog, via geranylgeranylation. However, potential localization and putative regulatory roles of GGTase-III in insulin secretion remains unknown. The current study is aimed at determining the expression and contributory roles of GGTase-III in glucose- and KCl-induced insulin secretion from pancreatic β-cells.

Mouse islets were isolated by the collagenase digestion method. Human islets were from Prodo Laboratories. INS-1 832/13 cells were transfected with either control (scrambled) or siRNA-PTAR1 (the α-subunit of GGTase-III) using lipofectamine RNAiMax. Insulin released into the medium was quantified using a commercially available Insulin ELISA kit. Expression of GGTase-III subunits and ykt6 was determined by Western blotting and quantified by densitometry.

Western blotting revealed that both subunits of GGTase-III (PTAR1 and RabGGTB) are expressed in human islets, mouse islets and INS-1 832/13 cells. Transfection of INS-1 832/13 cells with siRNA-PTAR1 resulted in significant reduction (~50%) in the expression of PTAR1. siRNA-mediated knockdown of PTAR1 significantly attenuated (~60%) glucose-stimulated insulin secretion (GSIS) in INS-1 832/13 cells. Furthermore, insulin secretion elicited via KCl-induced membrane depolarization was markedly reduced (~69%) in INS-1 832/13 cells following PTAR1 depletion. Lastly, immunoblotting data suggested expression of Ykt6, a known substrate for GGTase-III, in human islets, rodent islets, and INS-1 832/13 cells.

GGTase-III-dependent signaling step is necessary for glucose- and KCl-induced insulin secretion.