Rho Family GTPases and Rho GEFs in Glucose Homeostasis

Abstract

Dysregulation of glucose homeostasis leading to metabolic syndrome and type 2 diabetes is the cause of an increasing world health crisis. New intriguing roles have emerged for Rho family GTPases and their Rho guanine nucleotide exchange factor (GEF) activators in the regulation of glucose homeostasis. This review summates the current knowledge, focusing in particular on the roles of Rho GEFs in the processes of glucose-stimulated insulin secretion by pancreatic β cells and insulin-stimulated glucose uptake into skeletal muscle and adipose tissues. We discuss the ten Rho GEFs that are known so far to regulate glucose homeostasis, nine of which are in mammals, and one is in yeast. Among the mammalian Rho GEFs, P-Rex1, Vav2, Vav3, Tiam1, Kalirin and Plekhg4 were shown to mediate the insulin-stimulated translocation of the glucose transporter GLUT4 to the plasma membrane and/or insulin-stimulated glucose uptake in skeletal muscle or adipose tissue. The Rho GEFs P-Rex1, Vav2, Tiam1 and β-PIX were found to control the glucose-stimulated release of insulin by pancreatic β cells. In vivo studies demonstrated the involvement of the Rho GEFs P-Rex2, Vav2, Vav3 and PDZ-RhoGEF in glucose tolerance and/or insulin sensitivity, with deletion of these GEFs either contributing to the development of metabolic syndrome or protecting from it. This research is in its infancy. Considering that over 80 Rho GEFs exist, it is likely that future research will identify more roles for Rho GEFs in glucose homeostasis.

Keywords: GLUT4 glucose transporter; Rho GEF; Rho GTPase; glucose homeostasis; glucose-stimulated insulin secretion; guanine nucleotide exchange factor; insulin-stimulated glucose uptake; metabolic syndrome; small G protein; type 2 diabetes.

Figure 4

P-Rex family Rac GEFs in insulin signaling. Insulin activates the insulin receptor (IR), leading to receptor conformational changes which promote transphosphorylation and subsequent activation of the receptor tyrosine kinase activity. Insulin receptor substrate-1 (IRS1) is phosphorylated and recruits PI3K to the plasma membrane. Class 1 PI3Ks phosphorylate PtdIns(4,5)P₂ to generate PIP₃ [84]. PIP₃ promotes the translocation of PDK1 and Akt to the membrane through interaction with the PH domains, and phosphorylation of Akt by PDK1 and mTORC2 (PDK2) leads to the full activation of Akt. P-Rex1 and P-Rex2 are direct binding partners of mTORC1 and mTORC2 [132], both of which are important effectors of the insulin-signaling pathway. Both P-Rex proteins activate Rac1, and P-Rex1 has been implicated in the insulin-dependent translocation of GLUT4 to the plasma membrane in adipocytes [137]. P-Rex2 has been identified to inhibit PTEN through a mechanism independent of the GEF activity, and thus regulates the PIP₃ signal [129,130]. Inversely, PTEN also inhibits P-Rex2 [131]. Figure made with BioRender.

Figure 1

Regulation of Small GTPase activity. The activity of small GTPases, including the Rho family, is regulated by guanine nucleotide exchange factors (GEFs), GTPase-activating proteins (GAPs), and guanine nucleotide dissociation inhibitors (GDIs). GEFs activate Rho GTPases by promoting GTP loading. The active, GTP-bound form of the Rho GTPase adopts a conformation that allows it to interact with effector proteins. GAPs inactivate Rho GTPases by stimulating GTP hydrolysis. GDIs prevent the activation of Rho GTPases by promoting cytosolic localization. Figure made with BioRender.

Figure 2

Domain structure of Rho GEFs involved in glucose homeostasis. The Rho GEFs are classified into two families, 70 Dbl-type and 11 DOCK-type proteins in mammals. This figure depicts the domain structures of the nine mammalian Dbl-type GEFs and of yeast Dck1, a homologue of mammalian DOCK1, which have been implicated to date in the regulation of glucose homeostasis. Dbl-type Rho GEFs are characterized by a catalytic Dbl homology (DH) domain and a tandem membrane-targeting pleckstrin homology (PH) domain. DOCK-type Rho GEFs have a membrane-targeting DHR-1 domain and a catalytic DHR-2 domain. The structures of the DH and DHR-2 catalytic domains differ, but the guanine nucleotide exchange reaction they catalyze to activate Rho GTPases is the same. Most Rho GEFs harbor additional domains that aid in their regulation. Rho GEFs adopt an auto-inhibitory conformation that is relieved by the binding of signals to their regulatory domains. DH, Dbl homology. PH, pleckstrin homology. DEP, disheveled, EGL-10 and pleckstrin. PDZ, PSD−95, DLG, ZO-1 protein–protein interactions. PEST, motif rich in proline (P), glutamic acid (E), serine (S), and threonine (T). CC, coiled coil. Ex, conserved sequence in Tiam1. RBD, Ras-binding domain. CH, calponin homology. C1, zinc finger cysteine-rich domain. SH3/2, SRC Homology 3/2. LH, Lsc homology. Sec14, lipid-binding domain. Spectrin, three-helix bundle structures. Ig, Immunoglobulin-like. FibIII, fibronectin III binding. Kinase, serine/threonine protein kinase. SMART EMBL software was used to determine domain structure.

Figure 3

Metabolic tissue distribution of mammalian Rho GEFs that are known to be involved in the regulation of glucose homeostasis. Nine mammalian Rho GEFs are currently known to regulate glucose homeostasis in vitro or in vivo. Their distribution in human metabolic tissues is shown here, as extracted from public database BioGPS (http://biogps.org, accessed on 10 April 2021) [74]. Data are mean mRNA expression values determined by Affymetrix microarray. Units are z-scores of mean fluorescence intensity, determined using multiple probes for each transcript and processed using gcrma algorithms. Genes with z-scores above 5, indicated by the stippled black line, are considered to be expressed in that tissue. The graph was drawn using GraphPad Prism 8.

Figure 5

The roles of Rho family GEFs in pancreatic β cell glucose homeostasis. (A,B) depict normal and un-physiologically high glucose conditions, respectively, separated by the dashed line in the figure. (A) Under conditions of normal glucose homeostasis in the pancreatic β cell, glucose entry mediates insulin secretion. Vav2 and Tiam1-Rac1 have been implicated in remodeling the F-actin cytoskeleton to enable this secretory process [42,98]. P-Rex1 was implicated in glucose-induced insulin secretion and shown to aid in Rac1 localization at the plasma membrane (PM) under high-glucose conditions [77]. β-PIX competes with caveolin-1 (Cav1) for binding to Cdc42 and activates Cdc42 as part of the secondary sustained phase of insulin secretion. (B) Stress-related levels of glucose (or fatty acids) lead to the apoptotic death of pancreatic β cells and a diabetic phenotype, and this is possibly linked to hyper-activation of the Tiam1–Rac1 pathway. Figure made with BioRender.