Insulin Signaling to Glucose Transport

A major, if not the major, endpoint of insulin signaling is the stimulation of glucose uptake (transport) in muscle and fat, the focus of the remainder of this chapter. Currently, at least two distinct pathways (Fig. 3) have been implicated in this process. Both pathways may be required for translocation of the insulin-regulated glucose transporter, GLUT4, a 12-transmembrane-spanning protein, from a vesicular storage compartment (GSV) within the cell, to the plasma membrane. Its insertion makes it competent to transport glucose into the cell (for reviews, see references [10] to [12]). The most extensively characterized of these pathways begins with the IRS family of proteins (see Chapter 71). Tyrosine phosphorylation of these proteins by the insulin receptor provides recognition sites for Src homology domain 2 (SH2)-containing proteins. Of significance for several pathways in insulin action, including those leading to glucose transport and glycogen synthesis, is the binding and activation of the type 1A phosphatidylinositol 3 (PI3) kinase (for reviews, see references [10], [12], and [13]). This enzyme phosphorylates inositol phospholipids in the plasma membrane, increasing PI3,4,5-trisphosphate (PI345P3)

levels and enabling the recruitment and activation of a serine/threonine kinase, phosphoinositide-dependent protein kinase (PDK1), via association of its pleckstrin homology (PH) domain with this phospholipid. PDK1 can then phos-phorylate and activate two families of proteins implicated in stimulating glucose transport, the atypical protein kinases C ZA, and protein kinase B (PKB) isoforms (for reviews, see references [12] and [13]). Unfortunately, downstream targets of these kinases relevant for stimulation of glucose transport have remained elusive despite intensive investigation.

Another putative pathway implicated in insulin stimulation of glucose transport is PI3-kinase independent and involves tyrosine phosphorylation of the protooncogene Cbl by the insulin receptor tyrosine kinase, possibly via association with the Cbl-associated protein (CAP) to the receptor within caveolin-rich lipid rafts (for reviews, see references

[10] and [11]; Fig. 3). This enables the recruitment of the adaptor protein Crkll and its associated guanine exchange factor, C3G, resulting in activation of the Rho family GTPase, TC10. How this pathway intersects the PI3-kinase-dependent pathway to stimulate GLUT4 translocation is unclear. At least one of the pathways must act on the vesicular compartments storing GLUT4 to move them to the cell surface.

Recent data suggest involvement of cytoskeleton proteins such as vimentin, a-tubulin, dynein, and/or cortical actin in moving GLUT4 to the cell surface [14-16]. Indeed, one recent study provided a possible link between the TC10 pathway and mobilization of cortical F-actin, demonstrating in adipocytes that insulin causes cortical localization of the regulatory Wiscott-Aldrich syndrome protein (WASP) and actin-related protein 3 (Arp3), as well as actin polymerization [15]. Thus, it seems likely that the cytoskeleton is involved in trafficking GLUT4, although the detail remains to be determined.

Following translocation of GLUT4 vesicles to regions below the cell surface, the final stages of trafficking and fusion of the GLUT4 vesicles have been shown to involve soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins (for reviews, see references

[11] and [16]). These were first described as proteins mediating the fusion of neurotransmitter vesicles at synaptic terminals, and much of the molecular detail of SNARE protein interactions has been elucidated for this system [18]. In the case of plasma membrane trafficking of GLUT4, the functional vesicle-associated membrane protein (VAMP; also known as synaptobrevin) isoform is VAMP2. This protein binds two plasma membrane SNARE proteins, syntaxin 4 (STX4) and SNAP23, to form a so-called SNARE complex involving interaction between coiled-coil regions of the three proteins. Although the formation of this complex is possibly sufficient to enable fusion of the GLUT4 vesicle with the plasma membrane, thus placing GLUT4 proteins on the cell surface to transport glucose into the cell, effective fusion also requires N-ethylmaleimide-sensitive factor (NSF) and soluble NSF attachment protein (a-SNAP)-driven hydrolysis of adenosine triphosphate (ATP). The precise role of NSF and a-SNAP in fusion, however, is unclear.

Additional accessory proteins have been demonstrated to regulate this fusion process, although the roles of these accessory factors are likewise unclear (for reviews, see references [11] and [16]). Munc18c is a STX4-binding protein that may have a role as a molecular chaperone for STX4 and in fusion. Other proteins implicated in regulation of fusion events include synip (STX4-interacting protein), synapto-tagmin, VAP-33, and Rab4. How these proteins interact with the fusion proteins to regulate this important process remains to be established. Further regulation is provided by the endosomal trafficking of the transporters via clathrin-coated pits for recycling via constitutive or insulin-regulated compartments (for reviews, see references [12] and [17]). Although there is general agreement that the majority of GLUT4 is segregated into a unique insulin-sensitive endosomal compartment (GSV) in the basal state, the nature of this compartment and its regulation remain unclear despite the identification of targeting motifs within the C- and N-terminal regions of GLUT4. These areas offer fruitful avenues for investigation over the coming years.

Acknowledgment

The authors regret that due to space restrictions there was not space to cite the significant contributions of many investigators to the field; we have instead cited representative reviews that cover the respective areas in greater detail. The authors thank Dr. Tom Garrett for permission to use Fig. 2.

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