Tiam1/Vav2-Rac1 axis: A tug-of-war between islet function and dysfunction

Abstract

Glucose-stimulated insulin secretion [GSIS] from the islet β-cell involves a well-orchestrated interplay between metabolic and cationic events. It is well established that intracellular generation of adenine and guanine nucleotide triphosphates [e.g., ATP and GTP] represents one of the requisite signaling steps in GSIS. The small molecular mass GTP-binding proteins [G-proteins; e.g., Rac1 and Cdc42] have been shown to regulate islet β-cell function including actin cytoskeletal remodeling and fusion of insulin granules with the plasma membrane for GSIS to occur. In this context, several regulatory factors for these G-proteins have been identified in the pancreatic β-cell; these include guanine nucleotide exchange factors [GEFs] and guanine nucleotide dissociation inhibitors [GDI]. Recent pharmacological and molecular biological evidence identified Tiam1 and Vav2 as GEFs for Rac1 in promoting physiological insulin secretion. Paradoxically, emerging evidence in multiple cell types, including the islet β-cell, suggests key roles for Rac1 in the onset of cellular dysfunction under conditions of metabolic stress and diabetes. Furthermore, functional inactivation of either Tiam1 or Vav2 appears to attenuate sustained activation of Rac1 and its downstream signaling events [activation of stress kinases] under conditions of metabolic stress. Together, these findings suggest both "friendly" and "non-friendly" roles for Tiam1/Vav2-Rac1 signaling axis in islet β-cell in health and diabetes. Our current understanding of the field and the knowledge gaps that exist in this area of islet biology are heighted herein. Furthermore, potential caveats in the specificity and selectivity of pharmacological inhibitors that are available currently are discussed in this Commentary.

Keywords: G proteins; Glucotoxicity; Islet β-cell; Rac1; Tiam1; Vav2.

Figure 1. Schematic representation of activation-deactivation cycle of Rac1: Roles of Tiam1 and Vav2 as GEFs for Rac1 activation

It is well established that G proteins undergo activation [GTP-bound conformation] and deactivation [GDP-bound conformation]. Shown here is the activation-deactivation [GTP hydrolytic] cycle of Rac1. GDP-bound Rac1 [inactive] is converted to its active [GTP-bound] form by at least two GEFs, namely Tiam1 and Vav2. Published evidence implicates that both Tiam1 and Vav2 are expressed in a variety of insulin-secreting cells including clonal INS-1 832/13 cells, rodent islets and human islets. NSC23766 and Ehop-016 inhibit Tiam1 and Vav2, respectively [see text for additional details]. GTP-bound Rac1, in turn, regulates functions for multiple effector proteins, including Nox2. Following transmission of requisite signals, the active form of Rac1 is converted back to its inactive conformation mediated by the GTPase activity that is intrinsic to the G protein. It has been shown that additional regulatory proteins/factors, namely the GTPase-activating protein [GAP], further facilitate the conversion of the active G protein to its inactive form. The identity of GAP for Rac1 is an understudied area in the islet β-cell.

Figure 2. Schematic representation of assembly and activation of NoX2

Nox2 holoenzyme consists of cytosolic and membrane-associated components. The cytosolic core includes p67^phox, p47^phox, p40^phox and the small G-protein, Rac1. The membrane core is comprised of gp91^phox p22^phox, and the small G protein, Rap1. Upon activation, Rac1.GDP [inactive] is converted to Rac1.GTP [active], which binds to p67^phox, and the complex translocates to the membrane. Existing evidence in other cell types suggests that phosphorylation of p47^phox also triggers its translocation to the membrane to form the Nox2 holoenzyme complex that culminates in the enzyme activation and associated increase in ROS. As depicted, Rap1, another small G protein is also associated with the membrane fraction. However, its regulatory role in Nox2 activation still remains an open area of investigation. It is likely that Rap1 plays key roles in the localization of Tiam1 and Vav2 in appropriate locations within the membrane for optimal activation of Rac1-mediated regulation of Nox2 assembly and activation [see text for additional discussion].

Figure 3. Our proposed model for dual regulatory roles for Tiam1/Vav2-Rac1 signaling axis in islet function and dysfunction

We propose that under physiological conditions, glucose promotes activation of Rac1 [~15 minutes], which is mediated by Tiam1 and/or Vav2 as evidenced by inhibition of GSIS by specific inhibitors of Tiam1-Rac1 [NSC23766] and Vav2-Rac1 [Ehop-016]. Data from multiple laboratories have demonstrated activation of Nox2 under these conditions resulting in transient increase in intracellular ROS generation and associated increase in ROS generation under these conditions. It has also been suggested that tonic increase in ROS may promote cytoskeletal remodeling, which is requisite for translocation of insulin-laden secretory granules to the plasma membrane for fusion and exocytotic secretion of insulin. On the other hand, chronic exposure of pancreatic β-cells to glucotoxic conditions results in sustained activation of Rac1, also mediated by Tiam1 and/or Vav2 leading to increased oxidative stress due to sustained activation of Nox2. Evidence is also emerging to support activation of stress kinases [p38 MAPK, p53 and JNK1/2] under these conditions leading to mitochondrial dysfunction and caspase activation. Furthermore, recent studies have reported significant alterations in the nuclear compartment [increased caspase-mediated degradation of lamins] under these conditions leading to cell demise [see text for additional details]. Please note that while the description provided in this figure highlights critical regulatory roles of Tiam1/Vav2-Rac1 in islet β-cell dysfunction under the duress of glucotoxicity, it is also evident that such a pathway is relevant for models of lipotoxicity and inflammation [e.g., exposure to proinflammatory cytokines].