The TIAM1-RAC1-NOX2 signalling axis is activated in the initial stages of diabetes to increase intracellular ROS leading to mitochondrial damage and accelerated capillary cell apoptosis. Strategies targeting TIAM1-RAC1 signalling could have the potential to halt the progression of diabetic retinopathy in the early stages of the disease.

Extant studies have implicated the Rho subfamily of guanosine triphosphate-binding proteins (G-proteins; e.g., Rac1) in physiological insulin secretion from isolated ␤-cells. However, very little is known with regard to potential regulation by G-protein regulatory factors (e.g., the guanosine diphosphate-dissociation inhibitor [GDI]) of insulin secretion from the islet ␤-cell. To this end, using Triton X-114 phase partition, co-immunoprecipitation, and sucrose density gradient centrifugation approaches, we report coexistence of GDI with Rac1 in insulin-secreting ␤-cells (INS cells). Overexpression of wild-type GDI significantly inhibited glucose-induced, but not KCl-or mastoparan-induced, insulin secretion from INS cells. Furthermore, glucose-stimulated insulin secretion (GSIS) was significantly increased in INS cells in which expression

The majority of small G-proteins undergo posttranslational modifications (e.g., isoprenylation) at their C-terminal cysteine residues. Such modifications increase their hydrophobicity, culminating in translocation of the modified proteins to their relevant membranous sites for interaction with their respective effectors. Previously, we reported glucose-dependent activation and membrane association of

OBJECTIVEPosttranslational prenylation (e.g., farnesylation) of small G-proteins is felt to be requisite for cytoskeletal remodeling and fusion of secretory vesicles with the plasma membrane. Here, we investigated roles of protein farnesylation in the signaling steps involved in Raf-1/extracellular signal–related kinase (ERK1/2) signaling pathway in glucose-induced Rac1 activation and insulin secretion in the pancreatic β-cell.RESEARCH DESIGN AND METHODSThese studies were carried out in INS 832/13 cells and normal rat islets. Molecular biological (e.g., overexpression or small interfering RNA [siRNA]–mediated knockdown) and pharmacologic approaches were used to determine roles for farnesylation in glucose-mediated activation of ERK1/2, Rac1, and insulin secretion. Activation of ERK1/2 was determined by Western blotting. Rac1 activation (i.e., Rac1.GTP) was quantitated by p21-activated kinase pull-down assay. Insulin release was quantitated by enzyme-linked immunosorbent assay.RESULTSCoprovision of structure-specific inhibitors of farnesyl transferase (FTase; e.g., FTI-277 or FTI-2628) or siRNA-mediated knockdown of FTase β-subunit resulted in a significant inhibition of glucose-stimulated ERK1/2 and Rac1 activation and insulin secretion. Pharmacologic inhibition of Raf-1 kinase using GW-5074 markedly reduced the stimulatory effects of glucose on ERK1/2 phosphorylation, Rac1 activation, and insulin secretion, suggesting that Raf-1 kinase activation may be upstream to ERK1/2 and Rac1 activation leading to glucose-induced insulin release. Lastly, siRNA-mediated silencing of endogenous expression of ERK1/2 markedly attenuated glucose-induced Rac1 activation and insulin secretion.CONCLUSIONSTogether, our findings provide the first evidence of a role for protein farnesylation in glucose-mediated regulation of the Raf/ERK signaling pathway culminating in the activation of Rac1, which has been shown to be necessary for cytoskeletal reorganization and exocytotic secretion of insulin.