Shian-Huey Chiang

The exocyst complex is required for targeting of Glut4 to the plasma membrane by insulin

Insulin stimulates glucose transport by promoting exocytosis of the glucose transporter Glut4 (refs 1, 2). The dynamic processes involved in the trafficking of Glut4-containing vesicles, and in their targeting, docking and fusion at the plasma membrane, as well as the signalling processes that govern these events, are not well understood. We recently described tyrosine-phosphorylation events restricted to subdomains of the plasma membrane that result in activation of the G protein TC10 (refs 3, 4). Here we show that TC10 interacts with one of the components of the exocyst complex, Exo70. Exo70 translocates to the plasma membrane in response to insulin through the activation of TC10, where it assembles a multiprotein complex that includes Sec6 and Sec8. Overexpression of an Exo70 mutant blocked insulin-stimulated glucose uptake, but not the trafficking of Glut4 to the plasma membrane. However, this mutant did block the extracellular exposure of the Glut4 protein. So, the exocyst might have a crucial role in the targeting of the Glut4 vesicle to the plasma membrane, perhaps directing the vesicle to the precise site of fusion.

TCGAP, a multidomain Rho GTPase‐activating protein involved in insulin‐stimulated glucose transport

Insulin stimulates glucose uptake in fat and muscle cells via the translocation of the GLUT4 glucose transporter from intracellular storage vesicles to the cell surface. The signaling pathways linking the insulin receptor to GLUT4 translocation in adipocytes involve activation of the Rho family GTPases TC10α and β. We report here the identification of TCGAP, a potential effector for Rho family GTPases. TCGAP consists of N‐terminal PX and SH3 domains, a central Rho GAP domain and multiple proline‐rich regions in the C‐terminus. TCGAP specifically interacts with cdc42 and TC10β through its GAP domain. Although it has GAP activity in vitro, TCGAP is not active as a GAP in intact cells. TCGAP translocates to the plasma membrane in response to insulin in adipocytes. The N‐terminal PX domain interacts specifically with phos phatidylinositol‐(4,5)‐bisphosphate. Overexpression of the full‐length and C‐terminal fragments of TCGAP inhibits insulin‐stimulated glucose uptake and GLUT4 translocation. Thus, TCGAP may act as a downstream effector of TC10 in the regulation of insulin‐stimulated glucose transport.

TC10 and insulin-stimulated glucose transport

Insulin stimulates glucose uptake in insulin-responsive tissues by means of the translocation of the glucose transporter GLUT4 from intracellular sites to the plasma membrane. Two pathways are required, one involving activation of a phosphatidylinositol 3-kinase (PI 3-kinase) and downstream protein kinases, and one involving activation of the Rho-family GTPase TC10. TC10 activation by insulin is catalyzed by the exchange factor C3G, which is translocated to lipid rafts along with its binding partner CrkII as a consequence of Cbl tyrosine phosphorylation by the insulin receptor. This activation of TC10 is dependent on localization of TC10 in the lipid raft subdomains of the plasma membrane. We describe experimental approaches using the insulin-responsive cell line 3T3-L1 adipocytes to study the role of TC10 in insulin-stimulated glucose transport.

Vinexin family (SORBS) proteins play different roles in stiffness-sensing and contractile force generation

ABSTRACT Vinexin, c-Cbl associated protein (CAP) and Arg-binding protein 2 (ArgBP2) constitute an adaptor protein family called the vinexin (SORBS) family that is targeted to focal adhesions (FAs). Although numerous studies have focused on each of the SORBS proteins and partially elucidated their involvement in mechanotransduction, a comparative analysis of their function has not been well addressed. Here, we established mouse embryonic fibroblasts that individually expressed SORBS proteins and analysed their functions in an identical cell context. Both vinexin-α and CAP co-localized with vinculin at FAs and promoted the appearance of vinculin-rich FAs, whereas ArgBP2 co-localized with α-actinin at the proximal end of FAs and punctate structures on actin stress fibers (SFs), and induced paxillin-rich FAs. Furthermore, both vinexin-α and CAP contributed to extracellular matrix stiffness-dependent vinculin behaviors, while ArgBP2 stabilized α-actinin on SFs and enhanced intracellular contractile forces. These results demonstrate the differential roles of SORBS proteins in mechanotransduction. Summary: Comparing the functions of vinexin family (SORBS) proteins reveals that vinexin-α and CAP regulate vinculin behavior depending on the stiffness of the extracellular matrix, while ArgBP2 plays a role in generating greater contractile forces.

An inhibitor of the protein kinases TBK1 and IKK-[epsiv] improves obesity-related metabolic dysfunctions in mice

Emerging evidence suggests that inflammation provides a link between obesity and insulin resistance. The noncanonical IκB kinases IKK-ɛ and TANK-binding kinase 1 (TBK1) are induced in liver and fat by NF-κB activation upon high-fat diet feeding and in turn initiate a program of counterinflammation that preserves energy storage. Here we report that amlexanox, an approved small-molecule therapeutic presently used in the clinic to treat aphthous ulcers and asthma, is an inhibitor of these kinases. Treatment of obese mice with amlexanox elevates energy expenditure through increased thermogenesis, producing weight loss, improved insulin sensitivity and decreased steatosis. Because of its record of safety in patients, amlexanox may be an interesting candidate for clinical evaluation in the treatment of obesity and related disorders.

An inhibitor of the protein kinases TBK1/IKKε improves obesity-related metabolic dysfunctions

Emerging evidence suggests that inflammation provides a link between obesity and insulin resistance. The noncanonical IκB kinases IKK-ɛ and TANK-binding kinase 1 (TBK1) are induced in liver and fat by NF-κB activation upon high-fat diet feeding and in turn initiate a program of counterinflammation that preserves energy storage. Here we report that amlexanox, an approved small-molecule therapeutic presently used in the clinic to treat aphthous ulcers and asthma, is an inhibitor of these kinases. Treatment of obese mice with amlexanox elevates energy expenditure through increased thermogenesis, producing weight loss, improved insulin sensitivity and decreased steatosis. Because of its record of safety in patients, amlexanox may be an interesting candidate for clinical evaluation in the treatment of obesity and related disorders.