Insulin stimulates guanine nucleotide exchange on Rab4 via a wortmannin-sensitive signaling pathway in rat adipocytes

Insulin signaling accelerates the anterograde movement of Rab4 vesicles in axons through Klp98A/KIF16B recruitment via Vps34-PI3Kinase

Rab4 GTPase organizes endosomal sorting essential for maintaining the balance between recycling and degradative pathways. Rab4 localizes to many cargos whose transport in neurons is critical for regulating neurotransmission and neuronal health. Furthermore, elevated Rab4 levels in the CNS are associated with synaptic atrophy and neurodegeneration in Drosophila and humans, respectively. However, how the transport of Rab4-associated vesicles is regulated in neurons remains unknown. Using in vivo time-lapse imaging of Drosophila larvae, we show that activation of insulin signaling via Dilp2 and dInR increases the anterograde velocity, run length, and flux of Rab4 vesicles in the axons. Molecularly, we show that activation of neuronal insulin signaling further activates Vps34, elevates the levels of PI(3)P on Rab4-associated vesicles, recruits Klp98A (a PI(3)P-binding kinesin-3 motor) and activates their anterograde transport. Together, these observations delineate the role of insulin signaling in regulating axonal transport and synaptic homeostasis.

Drosophila transmembrane protein 214 (dTMEM214) regulates midgut glucose uptake and systemic glucose homeostasis

The availability of glucose transporter in the small intestine critically determines the capacity for glucose uptake and consequently systemic glucose homeostasis. Hence a better understanding of the physiological regulation of intestinal glucose transporter is pertinent. However, the molecular mechanisms that regulate sodium-glucose linked transporter 1 (SGLT1), the primary glucose transporter in the small intestine, remain incompletely understood. Recently, the Drosophila SLC5A5 (dSLC5A5) has been found to exhibit properties consistent with a dietary glucose transporter in the Drosophila midgut, the equivalence of the mammalian small intestine. Hence, the fly midgut could serve as a suitable model system for the study of the in vivo molecular underpinnings of SGLT1 function. Here, we report the identification, through a genetic screen, of Drosophila transmembrane protein 214 (dTMEM214) that acts in the midgut enterocytes to regulate systemic glucose homeostasis and glucose uptake. We show that dTMEM214 resides in the apical membrane and cytoplasm of the midgut enterocytes, and that the proper subcellular distribution of dTMEM214 in the enterocytes is regulated by the Rab4 GTPase. As a corollary, Rab4 loss-of-function phenocopies dTMEM214 loss-of-function in the midgut as shown by a decrease in enterocyte glucose uptake and an alteration in systemic glucose homeostasis. We further show that dTMEM214 regulates the apical membrane localization of dSLC5A5 in the enterocytes, thereby revealing dTMEM214 as a molecular regulator of glucose transporter in the midgut.

GLUT4 On the move

Insulin rapidly stimulates GLUT4 translocation and glucose transport in fat and muscle cells. Signals from the occupied insulin receptor are translated into downstream signalling changes in serine/threonine kinases within timescales of seconds, and this is followed by delivery and accumulation of the glucose transporter GLUT4 at the plasma membrane. Kinetic studies have led to realisation that there are distinct phases of this stimulation by insulin. There is a rapid initial burst of GLUT4 delivered to the cell surface from a subcellular reservoir compartment and this is followed by a steady-state level of continuing stimulation in which GLUT4 recycles through a large itinerary of subcellular locations. Here, we provide an overview of the phases of insulin stimulation of GLUT4 translocation and the molecules that are currently considered to activate these trafficking steps. Furthermore, we suggest how use of new experimental approaches together with phospho-proteomic data may help to further identify mechanisms for activation of these trafficking processes.

Serotonin improves glucose metabolism by Serotonylation of the small GTPase Rab4 in L6 skeletal muscle cells

BACKGROUND

Serotonin (5-HT) improves insulin sensitivity and glucose metabolism, however, the underlying molecular mechanism has remained elusive. Previous studies suggest that 5-HT can activate intracellular small GTPases directly by covalent binding, a process termed serotonylation. Activated small GTPases have been associated with increased GLUT4 translocation to the cell membrane. Therefore, we investigated whether serotonylation of small GTPases may be involved in improving Insulin sensitivity and glucose metabolism.

METHODS

Using fully differentiated L6 rat skeletal muscle cells, we studied the effect of 5-HT in the absence or presence of insulin on glycogen synthesis, glucose uptake and GLUT4 translocation. To prove our L6 model we additionally performed preliminary experiments in C2C12 murine skeletal muscle cells.

RESULTS

Incubation with 5-HT led to an increase in deoxyglucose uptake in a concentration-dependent fashion. Accordingly, GLUT4 translocation to the cell membrane and glycogen content were increased. These effects of 5-HT on Glucose metabolism could be augmented by co-incubation with insulin and blunted by co incubation of 5-HT with monodansylcadaverine, an inhibitor of protein serotonylation. In accordance with this observation, incubation with 5-HT resulted in serotonylation of a protein with a molecular weight of approximately 25 kDa. We identified this protein as the small GTPase Rab4, the activity of which has been shown to be stimulated by both insulin signalling and serotonylation.

CONCLUSION

Our data suggest that 5-HT elicits its beneficial effects on Glucose metabolism through serotonylation of Rab4, which likely represents the converging point between the insulin and the 5-HT signalling cascades.

Testis-Specific GTPase (TSG): An oligomeric protein

BACKGROUND

Ras-related proteins in brain (Rab)-family proteins are key members of the membrane trafficking pathway in cells. In addition, these proteins have been identified to have diverse functions such as cross-talking with different kinases and playing a role in cellular signaling. However, only a few Rab proteins have been found to have a role in male germ cell development. The most notable functions of this process are performed by numerous testis-specific and/or germ cell-specific genes. Here, we describe a new Rab protein that is specifically expressed in male germ cells, having GTPase activity.

RESULTS

Testis-specific GTPase (TSG) is a male-specific protein that is highly expressed in the testis. It has an ORF of 1593 base pairs encoding a protein of 530 amino acids. This protein appears in testicular cells approximately 24 days postpartum and is maintained thereafter. Immunohistochemistry of testicular sections indicates localized expression in germ cells, particularly elongating spermatids. TSG has a bipartite nuclear localization signal that targets the protein to the nucleus. The C-terminal region of TSG contains the characteristic domain of small Rab GTPases, which imparts GTPase activity. At the N-terminal region, it has a coiled-coil motif that confers self-interaction properties to the protein and allows it to appear as an oligomer in the testis.

CONCLUSION

TSG, being expressed in the male gonad in a developmental stage-specific manner, may have a role in male germ cell development. Further investigation of TSG function in vivo may provide new clues for uncovering the secrets of spermatogenesis.

Rab18 dynamics in adipocytes in relation to lipogenesis, lipolysis and obesity

Lipid droplets (LDs) are organelles that coordinate lipid storage and mobilization, both processes being especially important in cells specialized in managing fat, the adipocytes. Proteomic analyses of LDs have consistently identified the small GTPase Rab18 as a component of the LD coat. However, the specific contribution of Rab18 to adipocyte function remains to be elucidated. Herein, we have analyzed Rab18 expression, intracellular localization and function in relation to the metabolic status of adipocytes. We show that Rab18 production increases during adipogenic differentiation of 3T3-L1 cells. In addition, our data show that insulin induces, via phosphatidylinositol 3-kinase (PI3K), the recruitment of Rab18 to the surface of LDs. Furthermore, Rab18 overexpression increased basal lipogenesis and Rab18 silencing impaired the lipogenic response to insulin, thereby suggesting that this GTPase promotes fat accumulation in adipocytes. On the other hand, studies of the β-adrenergic receptor agonist isoproterenol confirmed and extended previous evidence for the participation of Rab18 in lipolysis. Together, our data support the view that Rab18 is a common mediator of lipolysis and lipogenesis and suggests that the endoplasmic reticulum (ER) is the link that enables Rab18 action on these two processes. Finally, we describe, for the first time, the presence of Rab18 in human adipose tissue, wherein the expression of this GTPase exhibits sex- and depot-specific differences and is correlated to obesity. Taken together, these findings indicate that Rab18 is involved in insulin-mediated lipogenesis, as well as in β-adrenergic-induced lipolysis, likely facilitating interaction of LDs with ER membranes and the exchange of lipids between these compartments. A role for Rab18 in the regulation of adipocyte biology under both normal and pathological conditions is proposed.

Subcellular trafficking of the substrate transporters GLUT4 and CD36 in cardiomyocytes

Cardiomyocytes use glucose as well as fatty acids for ATP production. These substrates are transported into the cell by glucose transporter 4 (GLUT4) and the fatty acid transporter CD36. Besides being located at the sarcolemma, GLUT4 and CD36 are stored in intracellular compartments. Raised plasma insulin concentrations and increased cardiac work will stimulate GLUT4 as well as CD36 to translocate to the sarcolemma. As so far studied, signaling pathways that regulate GLUT4 translocation similarly affect CD36 translocation. During the development of insulin resistance and type 2 diabetes, CD36 becomes permanently localized at the sarcolemma, whereas GLUT4 internalizes. This juxtaposed positioning of GLUT4 and CD36 is important for aberrant substrate uptake in the diabetic heart: chronically increased fatty acid uptake at the expense of glucose. To explain the differences in subcellular localization of GLUT4 and CD36 in type 2 diabetes, recent research has focused on the role of proteins involved in trafficking of cargo between subcellular compartments. Several of these proteins appear to be similarly involved in both GLUT4 and CD36 translocation. Others, however, have different roles in either GLUT4 or CD36 translocation. These trafficking components, which are differently involved in GLUT4 or CD36 translocation, may be considered novel targets for the development of therapies to restore the imbalanced substrate utilization that occurs in obesity, insulin resistance and diabetic cardiomyopathy.

Endocytosis, recycling, and regulated exocytosis of glucose transporter 4

Glucose transporter 4 (GLUT4) is responsible for the uptake of glucose into muscle and adipose tissues. Under resting conditions, GLUT4 is dynamically retained through idle cycling among selective intracellular compartments, from whence it undergoes slow recycling to the plasma membrane (PM). This dynamic retention can be released by command from intracellular signals elicited by insulin and other stimuli, which result in 2-10-fold increases in the surface level of GLUT4. Insulin-derived signals promote translocation of GLUT4 to the PM from a specialized compartment termed GLUT4 storage vesicles (GSV). Much effort has been devoted to the characterization of the intracellular compartments and dynamics of GLUT4 cycling and to the signals by which GLUT4 is sorted into, and recruited from, GSV. This review summarizes our understanding of intracellular GLUT4 traffic during its internalization from the membrane, its slow, constitutive recycling, and its regulated exocytosis in response to insulin. In spite of specific differences in GLUT4 dynamic behavior in adipose and muscle cells, the generalities of its endocytic and exocytic itineraries are consistent and an array of regulatory proteins that regulate each vesicular traffic event emerges from these cell systems.

Metformin induces Rab4 through AMPK and modulates GLUT4 translocation in skeletal muscle cells

Metformin is a major oral anti-diabetic drug and is known as an insulin sensitizer. However, the mechanism by which metformin acts is unclear. In this study, we found that AICAR, an AMPK activator, and metformin increased the expression of Rab4 mRNA and protein levels in skeletal muscle C2C12 cells. The promoter activity of Rab4 was increased by metformin in an AMPK-dependent manner. Metformin stimulated the phosphorylation of AS160, Akt substrate, and Rab GTPase activating protein (GAP), and also increased the phosphorylation of PKC-zeta, which is a critical molecule for glucose uptake. Knockdown of AMPK blocked the metformin-induced phosphorylation of AS160/PKC-zeta. In addition, a colorimetric absorbance assay showed that insulin-induced translocation of GLUT4 was suppressed in Rab4 knockdown cells. Moreover, Rab4 interacted with PKC-zeta but not with GLUT4. The C-terminal-deleted Rab4 mutant, Rab4ΔCT, showed diffuse sub-cellular localization, while wild-type Rab4 localized exclusively to the perinuclear membrane. Unlike Rab4ΔCT, wild-type Rab4 co-localized with PKC-zeta. Together, these results demonstrate that metformin induces Rab4 expression via AMPK-AS160-PKC-zeta and modulates insulin-mediated GLUT4 translocation.

Pulmonary surfactant protein A enhances endolysosomal trafficking in alveolar macrophages through regulation of Rab7

Surfactant protein A (SP-A), the most abundant pulmonary soluble collectin, modulates innate and adaptive immunity of the lung, partially via its direct effects on alveolar macrophages (AM), the most predominant intra-alveolar cells under physiological conditions. Enhanced phagocytosis and endocytosis are key functional consequences of AM/SP-A interaction, suggesting a SP-A-mediated modulation of small Rab (Ras related in brain) GTPases that are pivotal membrane organizers in both processes. In this article, we show that SP-A specifically and transiently enhances the protein expression of endogenous Rab7 and Rab7b, but not Rab5 and Rab11, in primary AM from rats and mice. SP-A-enhanced GTPases are functionally active as determined by increased interaction of Rab7 with its downstream effector Rab7 interacting lysosomal protein (RILP) and enhanced maturation of cathepsin-D, a function of Rab7b. In AM and RAW264.7 macrophages, the SP-A-enhanced lysosomal delivery of GFP-Escherichia coli is abolished by the inhibition of Rab7 and Rab7 small interfering RNA transfection, respectively. The constitutive expression of Rab7 in AM from SP-A(-/-) mice is significantly reduced compared with SP-A(+/+) mice and is restored by SP-A. Rab7 blocking peptides antagonize SP-A-rescued lysosomal delivery of GFP-E. coli in AM from SP-A(-/-) mice. Activation of Rab7, but not Rab7b, by SP-A depends on the PI3K/Akt/protein kinase Cζ (PKCζ) signal transduction pathway in AM and RAW264.7 macrophages. SP-A induces a Rab7/PKCζ interaction in these cells, and the disruption of PKCζ by small interfering RNA knockdown abolishes the effect of SP-A on Rab7. The data demonstrate a novel role for SP-A in modulating endolysosomal trafficking via Rab7 in primary AM and define biochemical pathways involved.

Rab4 facilitates cyclic adenosine monophosphate-stimulated bile acid uptake and Na+-taurocholate cotransporting polypeptide translocation

Cyclic adenosine monophosphate (cAMP) stimulates hepatic bile acid uptake by translocating sodium-taurocholate (TC) cotransporting polypeptide (Ntcp) from an endosomal compartment to the plasma membrane. Rab4 is associated with early endosomes and involved in vesicular trafficking. This study was designed to determine the role of Rab4 in cAMP-induced TC uptake and Ntcp translocation. HuH-Ntcp cells transiently transfected with empty vector, guanosine triphosphate (GTP) locked dominant active Rab4 (Rab4(GTP)), or guanosine diphosphate (GDP) locked dominant inactive Rab4 (Rab4(GDP)) were used to study the role of Rab4. Neither Rab4(GTP) nor Rab4(GDP) affected either basal TC uptake or plasma membrane Ntcp level. However, cAMP-induced increases in TC uptake and Ntcp translocation were enhanced by Rab4(GTP) and inhibited by Rab4(GDP). In addition, cAMP increased GTP binding to endogenous Rab4 in a time-dependent, but phosphoinositide-3-kinase-independent manner.

CONCLUSION

Taken together, these results suggest that cAMP-mediated phosphoinositide-3-kinase-independent activation of Rab4 facilitates Ntcp translocation in HuH-Ntcp cells.

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.

Effects of Emdogain on osteoblast gene expression

OBJECTIVE

Emdogain (EMD) is a protein extract purified from porcine enamel and has been introduced in clinical practice to obtain periodontal regeneration. EMD is composed mainly of amelogenins (90%), while the remaining 10% is composed of non-amelogenin enamel matrix proteins such as enamelins, tuftelin, amelin and ameloblastin. Enamel matrix proteins seem to be involved in root formation. EMD has been reported to promote proliferation, migration, adhesion and differentiation of cells associated with healing periodontal tissues in vivo.

DESIGN

How this protein acts on osteoblasts is poorly understood. We therefore attempted to address this question by using a microarray technique to identify genes that are differently regulated in osteoblasts exposed to enamel matrix proteins.

RESULTS

By using DNA microarrays containing 20,000 genes, we identified several upregulated and downregulated genes in the osteoblast-like cell line (MG-63) cultured with enamel matrix proteins (Emd). The differentially expressed genes cover a broad range of functional activities: (i) signaling transduction, (ii) transcription, (iii) translation, (iv) cell cycle regulation, proliferation and apoptosis, (v) immune system, (vi) vesicular transport and lysosome activity, and (vii) cytoskeleton, cell adhesion and extracellular matrix production.

CONCLUSIONS

The data reported are the first genome-wide scan of the effect of enamel matrix proteins on osteoblast-like cells. These results can contribute to our understanding of the molecular mechanisms of bone regeneration and as a model for comparing other materials with similar clinical effects.

Signal transduction gRABs attention

Rab proteins are small GTPases involved in the regulation of vesicular membrane traffic. Research done in the past years has demonstrated that some of these proteins are under the control of signal transduction pathways. Still, several recent papers point out to a new unexpected role for this family of Ras-related proteins, as potential regulators of intracellular signaling pathways. In particular, several evidence indicate that members of the Rab family of small GTPases, through their effectors, are key molecules participating to the regulation of numerous signal transduction pathways profoundly influencing cell proliferation, cell nutrition, innate immune response, fragmentation of compartments during mitosis and apoptosis. Even more surprisingly, direct involvement of Rab proteins in signaling to the nucleus has been demonstrated. This review will focus on aspects of Rab proteins function connected to signal transduction and, in particular, connections between membrane traffic and other cell pathways will be examined.

Akt Activation Is Required at a Late Stage of Insulin-Induced GLUT4 Translocation to the Plasma Membrane

Insulin stimulates the translocation of glucose transporter GLUT4 from intracellular vesicles to the plasma membrane (PM). This involves multiple steps as well as multiple intracellular compartments. The Ser/Thr kinase Akt has been implicated in this process, but its precise role is ill defined. To begin to dissect the role of Akt in these different steps, we employed a low-temperature block. Upon incubation of 3T3-L1 adipocytes at 19 C, GLUT4 accumulated in small peripheral vesicles with a slight increase in PM labeling concomitant with reduced trans-Golgi network labeling. Although insulin-dependent translocation of GLUT4 to the PM was impaired at 19 C, we still observed movement of vesicles toward the surface. Strikingly, insulin-stimulated Akt activity, but not phosphatidylinositol 3 kinase activity, was blocked at 19 C. Consistent with a multistep process in GLUT4 trafficking, insulin-stimulated GLUT4 translocation could be primed by treating cells with insulin at 19 C, whereas this was not the case for Akt activation. These data implicate two insulin-regulated steps in GLUT4 translocation: 1) redistribution of GLUT4 vesicles toward the cell cortex—this process is Akt-independent and is not blocked at 19 C; and 2) docking and/or fusion of GLUT4 vesicles with the PM—this process may be the major Akt-dependent step in the insulin regulation of glucose transport.

Regulated membrane trafficking of the insulin-responsive glucose transporter 4 in adipocytes

Since the discovery of insulin roughly 80 yr ago, much has been learned about how target cells receive, interpret, and respond to this peptide hormone. For example, we now know that insulin activates the tyrosine kinase activity of its cell surface receptor, thereby triggering intracellular signaling cascades that regulate many cellular processes. With respect to glucose homeostasis, these include the function of insulin to suppress hepatic glucose production and to increase glucose uptake in muscle and adipose tissues, the latter resulting from the translocation of the glucose transporter 4 (GLUT4) to the cell surface membrane. Although simple in broad outline, elucidating the molecular intricacies of these receptor-signaling pathways and membrane-trafficking processes continues to challenge the creative ingenuity of scientists, and many questions remain unresolved, or even perhaps unasked. The identification and functional characterization of specific molecules required for both insulin signaling and GLUT4 vesicle trafficking remain key issues in our pursuit of developing specific therapeutic agents to treat and/or prevent this debilitating disease process. To this end, the combined efforts of numerous research groups employing a range of experimental approaches has led to a clearer molecular picture of how insulin regulates the membrane trafficking of GLUT4.

Protein kinase B/Akt acts via glycogen synthase kinase 3 to regulate recycling of alpha v beta 3 and alpha 5 beta 1 integrins

Protein kinase B (PKB)/Akt is known to promote cell migration, and this may contribute to the enhanced invasiveness of malignant cells. To elucidate potential mechanisms by which PKB/Akt promotes the migration phenotype, we have investigated its role in the endosomal transport and recycling of integrins. Whereas the internalization of alpha v beta 3 and alpha 5 beta 1 integrins and their transport to the recycling compartment were independent of PKB/Akt, the return of these integrins (but not internalized transferrin) to the plasma membrane was regulated by phosphatidylinositol 3-kinases and PKB/Akt. The blockade of integrin recycling and cell spreading on integrin ligands effected by inhibition of PKB/Akt was reversed by inhibition of glycogen synthase kinase 3 (GSK-3). Moreover, expression of nonphosphorylatable active GSK-3 beta mutant GSK-3 beta-A9 suppressed recycling of alpha 5 beta 1 and alpha v beta 3 and reduced cell spreading on ligands for these integrins, indicating that PKB/Akt promotes integrin recycling by phosphorylating and inactivating GSK-3. We propose that the ability of PKB/Akt to act via GSK-3 to promote the recycling of matrix receptors represents a key mechanism whereby integrin function and cell migration can be regulated by growth factors.

Stretch activation of GTP-binding proteins in C2C12 myoblasts

Mechanical stimulation has been proposed as a fundamental determinant of muscle physiology. The mechanotransduction of strain and strain rate in C2C12 myoblasts were investigated utilizing a radiolabeled GTP analogue to detect stretch-induced GTP-binding protein activation. Cyclic uniaxial strains of 10% and 20% at a strain rate of 20% s(-1) rapidly (within 1 min) activated a 25-kDa GTPase (183 +/- 17% and 186 +/- 19%, respectively), while 2% strain failed to elicit a response (109 +/- 11%) relative to controls. One, five, and sixty cycles of 10% strain elicited 187 +/- 20%, 183 +/- 17%, and 276 +/- 38% increases in activation. A single 10% stretch at 20% s(-1), but not 0.3% s(-1), resulted in activation. Insulin activated the same 25-kDa band in a dose-dependent manner. Western blot analysis revealed a panel of GTP-binding proteins in C2C12 myoblasts, and tentatively identified the 25-kDa GTPase as rab5. In separate experiments, a 40-kDa protein tentatively identified as Galpha(i) was activated (240 +/- 16%) by 10% strain at 1 Hz for 15 min. These results demonstrate the rapid activation of GTP-binding proteins by mechanical strain in myoblasts in both a strain magnitude- and strain rate-dependent manner.

Insulin-induced GLUT4 translocation involves protein kinase C-lambda-mediated functional coupling between Rab4 and the motor protein kinesin

Insulin stimulates glucose transport by promoting translocation of GLUT4 proteins from the perinuclear compartment to the cell surface. It has been previously suggested that the microtubule-associated motor protein kinesin, which transports cargo toward the plus end of microtubules, plays a role in translocating GLUT4 vesicles to the cell surface. In this study, we investigated the role of Rab4, a small GTPase-binding protein, and the motor protein KIF3 (kinesin II in mice) in insulin-induced GLUT4 exocytosis in 3T3-L1 adipocytes. Photoaffinity labeling of Rab4 with [gamma-(32)P]GTP-azidoanilide showed that insulin stimulated Rab4 GTP loading and that this insulin effect was inhibited by pretreatment with the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor LY294002 or expression of dominant-negative protein kinase C-lambda (PKC-lambda). Consistent with previous reports, expression of dominant-negative Rab4 (N121I) decreased insulin-induced GLUT4 translocation by 45%. Microinjection of an anti-KIF3 antibody into 3T3-L1 adipocytes decreased insulin-induced GLUT4 exocytosis by 65% but had no effect on endocytosis. Coimmunoprecipitation experiments showed that Rab4, but not Rab5, physically associated with KIF3, and this was confirmed by showing in vitro association using glutathione S-transferase-Rab4. A microtubule capture assay demonstrated that insulin stimulation increased the activity for the binding of KIF3 to microtubules and that this activation was inhibited by pretreatment with the PI3-kinase inhibitor LY294002 or expression of dominant-negative PKC-lambda. Taken together, these data indicate that (i) insulin signaling stimulates Rab4 activity, the association of Rab4 with kinesin, and the interaction of KIF3 with microtubules and (ii) this process is mediated by insulin-induced PI3-kinase-dependent PKC-lambda activation and participates in GLUT4 exocytosis in 3T3-L1 adipocytes.

Regulated transport of the glucose transporter GLUT4

In muscle and fat cells, insulin stimulates the delivery of the glucose transporter GLUT4 from an intracellular location to the cell surface, where it facilitates the reduction of plasma glucose levels. Understanding the molecular mechanisms that mediate this translocation event involves integrating our knowledge of two fundamental processes--the signal transduction pathways that are triggered when insulin binds to its receptor and the membrane transport events that need to be modified to divert GLUT4 from intracellular storage to an active plasma membrane shuttle service.

PDGF-regulated rab4-dependent recycling of alphavbeta3 integrin from early endosomes is necessary for cell adhesion and spreading

BACKGROUND

It has been postulated that the regulation of integrin vesicular traffic facilitates cell migration by internalizing integrins at the rear of the cell and transporting them forward within vesicles for exocytosis at the leading edge to form new contacts with the extracellular matrix. The rab family of GTPases control key targeting events in the endo/exocytic pathway; therefore, these GTPases may be involved in the regulation of cell-matrix contact assembly.

RESULTS

The endo/exocytic cycle of alphavbeta3 and alpha5beta1 integrins was studied using mouse 3T3 fibroblast cell lines. In serum-starved cells, internalized integrins were transported through rab4-positive, early endosomes and arrived at the rab11-positive, perinuclear recycling compartment approximately 30 min after endocytosis. From the recycling compartment, integrins were recycled to the plasma membrane in a rab11-dependent fashion. Following treatment with PDGF, alphavbeta3 integrin, but not alpha5beta1, was rapidly recycled directly back to the plasma membrane from the early endosomes via a rab4-dependent mechanism without the involvement of rab11. This rapid recycling pathway directed alphavbeta3 to numerous small puncta distributed evenly across the dorsal surface of the cell, and the integrin only became localized into focal complexes at later times following PDGF addition. Interestingly, inhibition of PDGF-stimulated alphavbeta3 recycling using dominant-negative rab4 mutants compromised cell adhesion and spreading on vitronectin (a ligand for alphavbeta3), but adhesion to fibronectin (a ligand for alpha5beta1 and alphavbeta3) was unchanged.

CONCLUSIONS

We propose that growth factor-regulated, rab4-dependent recycling of alphavbeta3 integrin from early endosomes to the plasma membrane is a critical upstream event in the assembly of cell-matrix contacts.

The role of small G-proteins in the regulation of glucose transport (review)

Insulin increases the rate of glucose transport into fat and muscle cells by stimulating the translocation of intracellular Glut 4-containing vesicles to the plasma membrane. This results in a marked increase in the amount of the facilitative glucose transporter Glut 4 at the cell surface, allowing for an enhanced glucose uptake. This process requires a continuous cycling through the early endosomes, a Glut 4 specific storage compartment and the plasma membrane. The main effect of insulin is to increase the rate of Glut 4 trafficking from its specific storage compartment to the plasma membrane. The whole phenomenon involves signal transduction from the insulin receptor, vesicle trafficking (sorting and fusion processes) and actin cytoskeleton modifications, which are all supposed to require small GTPases. This review describes the potential role of the various members of the Ras, Rad, Rho, Arf and Rab families in the traffic of the Glut 4-containing vesicles.

Expression of a prenylation-deficient Rab4 inhibits the GLUT4 translocation induced by active phosphatidylinositol 3-kinase and protein kinase B

The small GTPase Rab4 has been shown to participate in the subcellular distribution of GLUT4 under both basal and insulin-stimulated conditions in adipocytes. In the present work, we have characterized the effect of Rab4 DeltaCT, a prenylation-deficient and thus cytosolic form of Rab4, in this process. We show that the expression of Rab4 DeltaCT in freshly isolated adipocytes inhibits insulin-induced GLUT4 translocation, but only when this protein is in its GTP-bound active form. Further, it not only blocks the effect of insulin, but also that of a hyperosmotic shock, but does not interfere with the effect of zinc ions on GLUT4 translocation. Rab4 DeltaCT was then shown to prevent GLUT4 translocation induced by the expression of an active form of phosphatidylinositol 3-kinase or of protein kinase B, without altering the activities of the enzymes. Our results are consistent with a role of Rab4 DeltaCT acting as a dominant negative protein towards Rab4, possibly by binding to Rab4 effectors.

Spatial compartmentalization of signal transduction in insulin action

Insulin resistance is thought to be the primary defect in the pathophysiology of type 2 diabetes. Thus, understanding the cellular mechanisms of insulin action may contribute significantly to developing new treatments for this disease. Although the effects of insulin on glucose and lipid metabolism are well documented, gaps remain in our understanding of the precise molecular mechanisms of signal transduction for the hormone. One potential clue to understanding the unique cellular effects of insulin may lie in the compartmentalization of signaling molecules and metabolic enzymes. We review this evidence, and speculate on how PI-3 kinase-independent and -dependent signaling pathways both diverge from the insulin receptor and converge at discrete targets to insure the specificity of insulin action.

Direct interaction of Rab4 with syntaxin 4

In the present study, we examined the possible interaction between Rab4 and syntaxin 4, both having been implicated in insulin-induced GLUT4 translocation. Rab4 and syntaxin 4 were coimmunoprecipitated from the lysates of electrically permeabilized rat adipocytes. The interaction between the two proteins was reduced by insulin treatment and increased by the addition of guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). An in vitro binding assay revealed that the bacterially expressed Rab4 was bound to a glutathione S-transferase fusion protein containing the cytoplasmic domain of syntaxin 4 (GST-syntaxin 4-(1-273)) but not to syntaxin 1A or vesicle-associated membrane protein-2. The interaction between Rab4 and syntaxin 4 seemed to be regulated by the guanine nucleotide status of Rab4, because 1) GTPgammaS treatment of the cells significantly increased, but guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS) treatment decreased the amount of Rab4 pulled down with GST-syntaxin 4-(1-273) from the cell lysates; 2) GTPgammaS loading on Rab4 caused a marked increase in the affinity of Rab4 to syntaxin 4 whereas GDPbetaS loading had little effect; and 3) a GTPase-deficient mutant of Rab4 (Rab4(Q67L)), but not a GTP-binding-defective mutant (Rab4(S22N)), was bound to GST-syntaxin 4-(1-273). Although insulin stimulated [gamma-(32)P]GTP binding to Rab4 in a time-dependent fashion, its effect on the Rab4 interaction with syntaxin 4 was apparently biphasic; an initial increase in Rab4 associated with syntaxin 4 was followed by a gradual dissociation of the GTPase from syntaxin 4. Finally, the binding of Rab4(Q67L) to GST-syntaxin 4-(1-273) was inhibited by munc-18c in a dose-dependent manner, indicating that GTP-loaded Rab4 binds to syntaxin 4 in the open conformation. These results suggest that 1) Rab4 interacts with syntaxin 4 in a direct and specific manner, and 2) the interaction is regulated by the guanine nucleotide status of Rab4 as well as by the conformational status of syntaxin 4.

Mechanism and regulation of GLUT-4 vesicle fusion in muscle and fat cells

Twenty years ago it was shown that recruitment of glucose transporters from an internal membrane compartment to the plasma membrane led to increased glucose uptake into fat and muscle cells stimulated by insulin. The final step of this process is the fusion of glucose transporter 4 (GLUT-4)-containing vesicles with the plasma membrane. The identification of a neuronal soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex as a requirement for synaptic vesicle-plasma membrane fusion led to the search for homologous complexes outside the nervous system. Indeed, isoforms of the neuronal SNAREs were identified in muscle and fat cells and were shown to be required for GLUT-4 incorporation into the cell membrane. In addition, proteins that bind to nonneuronal SNAREs were cloned and proposed to regulate vesicle fusion. We have summarized the molecular mechanisms leading to membrane fusion in nonneuronal systems, focusing on the role of SNAREs and accessory proteins (Munc18c, synip, Rab4, and VAP-33) in incorporation of GLUT-4 into the plasma membrane. Potential modes of regulation of this process are discussed, including SNARE phosphorylation and interaction with the cytoskeleton.

Bidirectional transcytosis of IgG by the rat neonatal Fc receptor expressed in a rat kidney cell line: a system to study protein transport across epithelia

The neonatal Fc receptor, FcRn, transports immunoglobulin G (IgG) across cellular barriers between mother and offspring. FcRn also protects circulating IgG from catabolism, probably during transport across the capillary endothelium. Only one cell culture model of transcytosis has been used extensively, the transport of IgA from the basolateral to the apical surface of Madin-Darby canine kidney cells by the polymeric immunoglobulin receptor (pIgR). We report that rat inner medullary collecting duct (IMCD) cells transfected with DNA encoding the (alpha) subunit of rat FcRn specifically and saturably transport Fc when grown as polarized monolayers. Using this system, we have found that transcytosis by FcRn, like transcytosis by the pIgR, depends upon an intact microtubule system. FcRn differs most strikingly from the pIgR in its ability to transport its ligand in both the apical to basolateral and basolateral to apical directions. The phosphatidylinositol 3-kinase inhibitors wortmannin and LY294002 inhibited basolateral to apical transport by FcRn more than apical to basolateral transport, suggesting that there are differences in the mechanisms of transport in the two directions. Lastly, we found that transcytosis by FcRn depends upon vesicular acidification. We anticipate that the IMCD cell culture model will allow further elucidation of the mechanism of IgG transport by FcRn.

Expression of a prenylation-deficient Rab4 interferes with propagation of insulin signaling through insulin receptor substrate-1

Rab proteins are small GTP-binding proteins of the Ras superfamily that function in the regulation of vesicle transport processes. The Rab4 isoform has been implicated in insulin action. For instance, overexpression of a prenylation-deficient form of Rab4 has been shown to inhibit insulin-dependent GLUT4 translocation. Other steps affected by Rab4 in the cascade of events resulting from insulin receptor activation have not been elucidated. In the present studies, we measured effects on insulin-signaling proteins in 3T3-L1 adipocytes transiently expressing cytoplasmic forms of Rab4 and Rab5. Expression of a mutant Rab4 lacking a prenylation site resulted in reduced insulin-dependent phosphorylation ofcytoplasmic and internal membrane-associated insulin receptor substrate-1, leading to decreased insulin receptor substrate-1-associated phosphatidylinositol 3'-OH kinase activation and decreased Akt activation. These effects were not observed upon introduction of a similar mutant form of Rab5. These data indicate that Rab4 or a Rab4-associated protein is involved at one or more steps in propagating the insulin signal, in addition to any role it may play in the regulation of GLUT4 vesicle translocation. Our results support models of insulin signaling in which regulation of internal membrane trafficking plays a role in transduction of the insulin signal.

Role of the actin cytoskeleton in insulin action

Insulin has diverse effects on cells, including stimulation of glucose transport, gene expression, and alterations of cell morphology. The hormone mediates these effects by activation of signaling pathways which utilize, 1) adaptor molecules such as the insulin receptor substrates (IRS), the Src and collagen homologs (Shc), and the growth factor receptor binding protein 2 (Grb2); 2) lipid kinases such as phosphatidylinositol 3-kinase (PI 3-Kinase); 3) small G proteins; and 4) serine, threonine, and tyrosine kinases. The activation of such signaling molecules by insulin is now well established, but we do not yet fully understand the mechanisms integrating these seemingly diverse pathways. Here, we discuss the involvement of the actin cytoskeleton in the propagation and regulation of insulin signals. In muscle cells in culture, insulin induces a rapid actin filament reorganization that coincides with plasma membrane ruffling and intense accumulation of pinocytotic vesicles. Initiation of these effects of insulin requires an intact actin cytoskeleton and activation of PI 3-kinase. We observed recruitment PI 3-kinase subunits and glucose transporter proteins to regions of reorganized actin. In both muscle and adipose cells, actin disassembly inhibited early insulin-induced events such as recruitment of glucose transporters to the cell surface and enhanced glucose transport. Additionally, actin disassembly inhibited more prolonged effects of insulin, including DNA synthesis and expression of immediate early genes such as c-fos. Intact actin filaments appear to be essential for mediation of early events such as association of Shc with Grb2 in response to insulin, which leads to stimulation of gene expression. Preliminary observations support a role for focal adhesion signaling complexes in insulin action. These observations suggest that the actin cytoskeleton facilitates propagation of the morphological, metabolic, and nuclear effects of insulin by regulating proper subcellular distribution of signaling molecules that participate in the insulin signaling pathway.

Involvement of Rab4 in regulated exocytosis of rat pancreatic acini

BACKGROUND & AIMS

Rab4, a Ras-related small guanosine triphosphate (GTP)-binding protein, has been suggested to participate in exocytosis. The function of Rab4 in regulated exocytosis of pancreatic acini was examined in this study.

METHODS

Subcellular localization of Rab4 was determined by Western blotting and immunohistochemistry. The Rab4 function in regulated exocytosis was examined by introducing Rab4 hypervariable carboxy-terminal domain peptide (Rab4 peptide) and anti-Rab4 antibody into streptolysin O-permeabilized acini. The regulation of Rab4 by cholecystokinin (CCK) and 12-O-tetradecanoyl-phorbol 13-acetate (TPA) was investigated by examining their effects on [32P]GTP binding rate into the Rab4 immunoprecipitates. The participation of protein kinase C in the Rab4 regulation by CCK was confirmed by calphostin C pretreatment of acini.

RESULTS

Rab4 was localized on zymogen granule membranes. Both Rab4 peptide and anti-Rab4 antibody enhanced calcium-stimulated amylase release from streptolysin O-permeabilized acini, suggesting the inhibitory role of Rab4 in exocytosis. CCK and TPA increased GTP binding to Rab4. Calphostin C attenuated the stimulatory effect of CCK on GTP binding to Rab4.

CONCLUSIONS

Rab4 negatively modulates regulated exocytosis of pancreatic acini and is controlled by CCK through a protein kinase C pathway.

A requirement for phosphatidylinositol 3-kinase in pseudopod extension

Phagocytosis requires actin assembly and pseudopod extension, two cellular events that coincide spatially and temporally. The signal transduction events underlying both processes may be distinct. We tested whether phagocytic signaling resembles that of growth factor receptors, which induce actin polymerization via activation of phosphatidylinositol 3-kinase (PI 3-kinase). Fcgamma receptor-mediated phagocytosis was accompanied by a rapid increase in the accumulation of phosphatidylinositol 3,4,5-trisphosphate in vivo, and addition of wortmannin (WM) or LY294002, two inhibitors of PI 3-kinase(s), inhibited phagocytosis but not Fcgamma receptor-directed actin polymerization. However, both compounds prevented maximal pseudopod extension, suggesting that PI 3-kinase inhibition produced a limitation in membrane required for pseudopod extension. Availability of plasma membrane was not limiting for phagocytosis, because blockade of ingestion in the presence of WM was not overcome by reducing the number of particles adhering to macrophages. However, decreasing bead size, and hence the magnitude of pseudopod extension required for particle engulfment, relieved the inhibition of phagocytosis in the presence of WM or LY294002 by up to 80%. The block in phagocytosis of large particles occurred before phagosomal closure, because both compounds inhibited spreading of macrophages on substrate-bound IgG. Macrophage spreading on IgG was accompanied by exocytic insertion of membrane from an intracellular source, as measured by the dye FM1-43. These results indicate that one or more isoforms of PI 3 kinase are required for maximal pseudopod extension but not phagocytosis per se. We suggest that PI 3-kinase is required for coordinating exocytic membrane insertion and pseudopod extension.

From insulin receptor signalling to Glut 4 translocation abnormalities in obesity and insulin resistance

Insulin resistance is commonly associated with obesity in rodents. Using mice made obese with goldthioglucose (GTG-obese mice), we have shown that insulin resistance results from defects at the level of the receptor and from intracellular alterations in insulin signalling pathway, without major alteration in the number of the Glut 4 glucose transporter. Activation of phosphatidylinositol 3-kinase (PI 3-kinase) was found to be profoundly affected in response to insulin. This defect appears very early in the development of obesity, together with a marked decrease in IRS 1 tyrosine phosphorylation. In order to better understand the abnormalities in glucose transport in insulin resistance, we have studied the pathway leading from the insulin receptor kinase stimulation to the translocation of the Glut 4 containing vesicles. This stimulation involves the activation of PI 3-kinase, which in turns activates protein kinase B. We have then focussed at the mechanism of vesicle exocytosis, and more specifically at the role of the small GTPase Rab4 in this process. We have shown that Rab4 participates, first in the intracellular retention of the Glut 4 containing vesicles, second in the insulin signalling pathway leading to glucose transporter translocation.

Molecular aspects of the endocytic pathway

Observation of the flow of material along the endocytic pathway has lead to the description of the basic architecture of the pathway and provided insight into the relationship between compartments. Significant advances have been made in the study of endocytic transport steps at the molecular level, of which studies of cargo selection, vesicle budding and membrane fusion events comprise the major part. Progress in this area has been driven by two approaches, yeast genetics and in vitro or cell-free assays, which reconstitute particular transport steps and allow biochemical manipulation. The complex protein machineries that control vesicle budding and fusion are significantly conserved between the secretory and endocytic pathways such that proteins that regulate particular steps are often part of a larger family of proteins which exercise a conserved function at other locations within the cell. Well characterized examples include vesicle coat proteins, rabs (small GTPases) and soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein (SNAP) receptors (SNAREs). Intracompartmental pH, lipid composition and cytoskeletal organization have also been identified as important determinants of the orderly flow of material within the endocytic pathway.

Complexity of trypanosomatid endocytosis pathways revealed by Rab4 and Rab5 isoforms in Trypanosoma brucei

Small G proteins of the Rab family are responsible for vesicle fusion and control flux during intracellular transport. Rab5 is important in endosome maturation and Rab4 in recycling of endocytic material. Three Rab5 isoforms identified so far in mammals and three in the yeast genome suggest that conservation of multiple Rab5 isoforms is required for sophisticated regulation of endocytosis. Trypanosoma brucei homologues of Rab5 and Rab4 (TbRab5A and TbRab4) have been identified. Here we report cloning of a second Rab5 homologue, TbRab5Bp. The TbRAB5A and -5B genes are not linked in the genome, and phylogenetic reconstruction indicates that multiple Rab5 isoforms in yeast, mammals, and trypanosomes evolved independently. Northern blots demonstrate that TbRab5A, -5B, and TbRab4 messages are expressed in bloodstream form (BSF) and procyclic forms of the parasite even though endocytosis is not very active in the latter form. mRNA levels of TbRab5A and -4 are constitutive. Multiple-sized TbRab5B messages at very low abundance are detected, with greater expression in BSF. Also, the TbRab5B mRNA has a large 3'-untranslated region suggestive of potentially complex regulation, and therefore TbRab5Bp may be an important regulator of differential endocytosis levels between BSF and procyclic stage parasites. Affinity purified antibodies raised to C-terminal peptide sequences of all three TbRab proteins recognized small vesicular cytoplasmic structures, which for TbRab5Ap and -5Bp are predominantly near the flagellar pocket. TbRab5Bp colocalizes with invariant surface glycoprotein 100 (ISG100), a protein entering the endocytotic pathway in BSF parasites, whereas in procyclic cells populations of vesicles stained with both TbRab5Ap and -5Bp substantially overlap; TbRab5 proteins are therefore components of the endocytotic pathway. TbRab4p localizes to vesicular structures throughout the cytoplasm, with some overlap with TbRab5Bp, but the majority occupying a different compartment to the TbRab5s. Therefore the trypanosome endosomal system has been functionally dissected for the first time; these reagents provide a unique opportunity for manipulation of the protozoan endosomal system to further our understanding of drug uptake mechanisms and virulence.

Estrogen stimulates intracellular traffic in the liver of Rana esculenta complex by modifying Rab protein content

During vitellogenesis in oviparous animals, estrogens induce the synthesis of the yolk precursor vitellogenin, a lipophosphoprotein rich in cholesterol. Estrogens also induce the activity of 3-hydroxy-3-methylglutaryl CoA reductase, that is necessary for the lipidation of vitellogenin. This increased enzyme activity could also be important for the production of isoprenoid groups that post-translationally modify proteins such as the Rab proteins, which are small G proteins involved in intracellular traffic. The effect of estrogens on the production of prenylated proteins and on the levels of Rab proteins in the liver of Rana esculenta complex has been studied. An increase of the Rabs specifically involved in the exocytic pathway was observed and is probably related to the need for export of massive amounts of newly synthesized vitellogenin.

Phosphoinositide 3-kinase: the key switch mechanism in insulin signalling

Insulin plays a key role in regulating a wide range of cellular processes. However, until recently little was known about the signalling pathways that are involved in linking the insulin receptor with downstream responses. It is now apparent that the activation of class 1a phosphoinositide 3-kinase (PI 3-kinase) is necessary and in some cases sufficient to elicit many of insulin's effects on glucose and lipid metabolism. The lipid products of PI 3-kinase act as both membrane anchors and allosteric regulators, serving to localize and activate downstream enzymes and their protein substrates. One of the major ways these lipid products of PI 3-kinase act in insulin signalling is by binding to pleckstrin homology (PH) domains of phosphoinositide-dependent protein kinase (PDK) and protein kinase B (PKB) and in the process regulating the phosphorylation of PKB by PDK. Using mechanisms such as this, PI 3-kinase is able to act as a molecular switch to regulate the activity of serine/threonine-specific kinase cascades important in mediating insulin's effects on endpoint responses.

Phosphatidylinositol 3-kinase is required for the regulation of hepatitis B surface antigen production and mitogen-activated protein kinase activation by insulin but not by TPA

Insulin suppresses hepatitis B surface antigen (HBsAg) gene expression and stimulates cell proliferation in human hepatoma Hep3B cells. 12-O-tetradecanoyl phorbol-13-acetate, TPA, has been demonstrated to mimic insulin actions in these cells. We examined the role of phosphatidylinositol 3-kinase (PI 3-kinase) in the signaling pathways of insulin and TPA towards these two biological phenomena in Hep3B cells. The pre-treatment of 5 microM of wortmannin diminished insulin suppressed HBsAg production and completely abolished insulin stimulated cell proliferation. However, wortmannin had no effect on TPA actions in both HBsAg suppression and cell growth stimulation. We further investigated the effect of wortmannin in mitogen-activated protein kinases (MAPKs) activation induced by insulin or TPA. After the pretreatment of wortmannin, insulin activated MAPKs was completely blocked, but TPA was still capable to activate MAPKs. These results suggest that PI 3-kinase is involved in insulin actions but not in TPA effects, and allow us to dissociate the signaling pathways of insulin and TPA in human hepatoma Hep3B cells.

Inhibition of endosome fusion by wortmannin persists in the presence of activated Rab5

Rab5-dependent endosome fusion is sensitive to the phosphoinositide 3-kinase inhibitor, wortmannin. It has been proposed that phosphoinositide 3-kinase activity may be required for activation of rab5 by influencing its nucleotide cycle such as to promote its active GTP state. In this report we demonstrate that endosome fusion remains sensitive to wortmannin despite preloading of endosomes with stimulatory levels of a GTPase-defective mutant rab5(Q79L) or of a xanthosine triphosphate-binding mutant, rab5(D136N), in the presence of the nonhydrolysable analogue XTPgammaS. These results suggest that activation of rab5 cannot be the principal function of the wortmannin-sensitive factor on the endosome fusion pathway. This result is extrapolated to all GTPases by demonstrating that endosome fusion remains wortmannin sensitive despite prior incubation with the nonhydrolysable nucleotide analogue GTPgammaS. Consistent with these results, direct measurement of clathrin-coated vesicle-stimulated nucleotide dissociation from exogenous rab5 was insensitive to the presence of wortmannin. A large excess of rab5(Q79L), beyond levels required for maximal stimulation of the fusion assay, afforded protection against wortmannin inhibition, and partial protection was also observed with an excess of wild-type rab5 independent of GTPgammaS.

The small guanosine triphosphate-binding protein Rab4 is involved in insulin-induced GLUT4 translocation and actin filament rearrangement in 3T3-L1 cells

Insulin's stimulation of glucose transport involves the translocation of vesicles containing the glucose transporter GLUT4 to the plasma membrane. Small GTP-binding proteins have been implicated in the regulation of vesicular traffic. We studied the effects of microinjection of wild-type Rab4 glutathione S-transferase fusion protein (WT Rab4), a GTP-binding defective mutant (Rab4 N121I), a guanosine triphosphatase-defective mutant (Rab4 Q67L), and a Rab4 antibody on insulin-induced GLUT4 translocation in 3T3-L1 adipocytes. Microinjection of Rab4 N121I and Rab4 antibodies had no effect on basal GLUT4 staining, but inhibited insulin-induced GLUT4 translocation by 50% compared with that in control IgG-injected cells. WT Rab4 and Rab4 Q67L microinjection had no effect on either basal or insulin-induced GLUT4 translocation. Premixing and coinjection of the Rab4 antibody with WT Rab4 almost completely abolished its inhibitory effect on insulin-induced GLUT4 translocation. In contrast, microinjection of an antibody directed against the highly conserved region of Rab3 proteins had no effect on insulin-induced GLUT4. These results point to a direct role of Rab4 in insulin-induced GLUT4 translocation, and that this effect is dependent on nucleotide binding to the protein. We also studied the effect of microinjection of the same proteins on insulin-induced actin filament rearrangement (membrane ruffling) in the same cell line. Microinjection of Rab4 N121I and Rab4 antibodies inhibited insulin-induced membrane ruffling by 40%, whereas WT Rab4 or a Rab3 antibody injection had no effect on cytoskeletal rearrangement. In summary, 1) Rab4 is a necessary component of the insulin/GLUT4 translocation signaling pathway; 2) the function of Rab4 in this pathway requires GTP binding; 3) Rab4 also participates in the process of insulin-induced membrane ruffling; and 4) Rab3 proteins do not seem to be involved in these processes.