Insulin-stimulated Interaction between Insulin Receptor Substrate 1 and p85α and Activation of Protein Kinase B/Akt Require Rab5

Rab proteins in endocytosis and Glut4 trafficking

The intracellular trafficking of numerous proteins requires a tight control to fulfil their physiological functions. It is the case of the adipocyte and muscle glucose transporter Glut4 that is retained intracellularly until insulin induces its recruitment to the plasma membrane. Rabs are evolutionarily conserved small GTPases that control intracellular traffic events from yeast to mammalian cells. In the past few decades, considerable progresses have been made in identifying the route of Glut4, the Rabs involved in controlling it, and more recently the connection between insulin signalling and Glut4 trafficking through Rab activity control.

Small G proteins in insulin action: Rab and Rho families at the crossroads of signal transduction and GLUT4 vesicle traffic

Insulin stimulates glucose uptake into muscle and adipose tissues through glucose transporter 4 (GLUT4). GLUT4 cycles between the intracellular compartments and the plasma membrane. GLUT4 traffic-regulating insulin signals are largely within the insulin receptor-insulin receptor substrate-phosphatidylinositol 3-kinase (IR-IRS-PI3K) axis. In muscle cells, insulin signal bifurcates downstream of the PI3K into one arm leading to the activation of the Ser/Thr kinases Akt and atypical protein kinase C, and another leading to the activation of Rho family protein Rac1 leading to actin remodelling. Activated Akt inactivates AS160, a GTPase-activating protein for Rab family small G proteins. Here we review the roles of Rab and Rho proteins, particularly Rab substrates of AS160 and Rac1, in insulin-stimulated GLUT4 traffic. We discuss: (1) how distinct steps in GLUT4 traffic may be regulated by discrete Rab proteins, and (2) the importance of Rac1 activation in insulin-induced actin remodelling in muscle cells, a key element for the net gain in surface GLUT4.

The interaction of Akt with APPL1 is required for insulin-stimulated Glut4 translocation

APPL1 (adaptor protein containing PH domain, PTB domain, and leucine zipper motif 1) is an Akt/protein kinase B-binding protein involved in signal transduction and membrane trafficking pathways for various receptors, including receptor tyrosine kinases. Here, we establish a role for APPL1 in insulin signaling in which we demonstrate its interaction with Akt2 by co-immunoprecipitation and pulldown assays. In primary rat adipocytes and skeletal muscle, APPL1 and Akt2 formed a complex that was dissociated upon insulin stimulation in both tissues. To investigate possible APPL1 function in adipocytes, we analyzed Akt phosphorylation, 2-deoxyglucose uptake, and Glut4 translocation by immunofluorescence following APPL1 knockdown by small interfering and short hairpin RNAs. We show that APPL1 knockdown suppressed Akt phosphorylation, glucose uptake, and Glut4 translocation. We also tested the effect in 3T3-L1 adipocytes of expressing full-length APPL1 or an N- or a C-terminal APPL1 construct. Interestingly, expression of full-length APPL1 and its N terminus suppressed insulin-stimulated 2-deoxyglucose uptake and Glut4 translocation to roughly the same extent (40-60%). We confirmed by cellular fractionation that Glut4 translocation was substantially blocked in 3T3-L1 adipocytes transfected with full-length APPL1. By cellular fractionation, APPL1 was localized mainly in the cytosol, and it showed a small degree of re-localization to the light microsomes and nucleus in response to insulin. By immunofluorescence, we also show that APPL1 partially co-localized with Glut4. These data suggest that APPL1 plays an important role in insulin-stimulated Glut4 translocation in muscle and adipose tissues and that its N-terminal portion may be critical for APPL1 function.

GAPex-5 mediates ubiquitination, trafficking, and degradation of epidermal growth factor receptor

Upon ligand stimulation, epidermal growth factor receptor (EGFR) is rapidly ubiquitinated, internalized, and sorted to lysosomes for degradation. Rab5 has been shown to play an important role in the early stages of EGFR trafficking. GAPex-5 is a newly described Rab5 exchange factor. Herein, we investigate the role of GAPex-5 on EGFR trafficking and degradation. Down-regulation of GAPex-5 by RNA interference decreases epidermal growth factor-stimulated EGFR degradation. Moreover, ubiquitination of EGFR is impaired by depletion of GAPex-5. This inhibitory effect is due to a decrease in the interaction between the adapter protein c-Cbl and EGFR, but not the phosphorylation state of EGFR. Consistently, when examined by immunofluorescence microscopy in cells depleted of GAPex-5, ligand-bound EGFR appeared trapped in early endosomes and the trafficking of internalized receptor from early to late endosomes was impaired. In agreement with the depletion studies, EGFR degradation is enhanced by overexpressing GAPex-5 wild type, but not GAPex-5DeltaGAP, a mutant lacking the Ras GTPase-activating protein (GAP) domain. This is consistent with the finding that c-Cbl binds specifically to the Ras GAP domain. Finally, overexpression of dominant negative Rab5a or depletion of all three isoforms of Rab5 does not inhibit ubiquitination of EGFR, which suggests that GAPex-5-mediated EGFR ubiquitination is independent of Rab5 activation. Collectively, the results suggest a novel mechanism by which EGF-stimulated receptor ubiquitination and trafficking are mediated via GAPex-5.

The Rab5 Activator ALS2/alsin Acts as a Novel Rac1 Effector through Rac1-activated Endocytosis

Mutations in the ALS2 gene cause a number of recessive motor neuron diseases, indicating that the ALS2 protein (ALS2/alsin) is vital for motor neurons. ALS2 acts as a guanine nucleotide exchange factor (GEF) for Rab5 (Rab5GEF) and is involved in endosome dynamics. However, the spatiotemporal regulation of the ALS2-mediated Rab5 activation is unclear. Here we identified an upstream activator for ALS2 and showed a functional significance of the ALS2 activation in endosome dynamics. ALS2 preferentially interacts with activated Rac1. In the cells activated Rac1 recruits cytoplasmic ALS2 to membrane ruffles and subsequently to nascent macropinosomes via Rac1-activated macropinocytosis. At later endocytic stages macropinosomal ALS2 augments fusion of the ALS2-localized macropinosomes with the transferrin-positive endosomes, depending on the ALS2-associated Rab5GEF activity. These results indicate that Rac1 promotes the ALS2 membranous localization, thereby rendering ALS2 active via Rac1-activated endocytosis. Thus, ALS2 is a novel Rac1 effector and is involved in Rac1-activated macropinocytosis. All together, loss of ALS2 may perturb macropinocytosis and/or the following membrane trafficking, which gives rise to neuronal dysfunction in the ALS2-linked motor neuron diseases.

Rin1 interacts with signal-transducing adaptor molecule (STAM) and mediates epidermal growth factor receptor trafficking and degradation

Rin1, the prototype of a new family of multidomain Rab5 exchange factors, has been shown to play an important role in the endocytosis of the epidermal growth factor receptor (EGFR). Herein, we examined the role of Rin1 in the down-regulation of EGFR following EGF stimulation. We observed that overexpression of Rin1 accelerates EGFR degradation in EGF-stimulated cells. In concordance, depletion of endogenous Rin1 by RNA interference resulted in a substantial reduction of EGFR degradation. We showed that Rin1 interacts with signal-transducing adaptor molecule 2 (STAM2), a protein that associates with hepatocyte growth factor-regulated substrate and plays a key role in the endosomal sorting machinery. Green fluorescent protein (GFP)-Rin1 co-localizes with hemagglutinin (HA)-STAM2 and with endogenous hepatocyte growth factor-regulated substrate. Furthermore, wild type STAM2, but not a deletion mutant lacking the SH3 domain, co-immunoprecipitates with endogenous Rin1. This interaction is dependent on the proline-rich domain (PRD) of Rin1 as Rin1DeltaPRD, a mutant lacking the PRD, does not interact with STAM2. Moreover, EGFR degradation was not accelerated by expression of the Rin1DeltaPRD mutant. Together these results suggest that Rin1 regulates EGFR degradation in cooperation with STAM, defining a novel role for Rin1 in regulating endosomal trafficking.