Gq-coupled Receptors Transmit the Signal for GLUT4 Translocation via an Insulin-independent Pathway

Abstract

Guanosine 5-O-(3-thiotriphosphate) (GTP␥S) induces the translocation of glucose transporter type 4 (GLUT4) from an intracellular pool to the cell surface and increases glucose uptake in adipocytes. The GTP-binding protein(s) responsible for the translocation has remained to be identified. Using a sensitive and quantitative method to assess the translocation of c-MYC epitope-tagged GLUT4, we obtained evidence that the activation of receptor-coupled G q (neither G i nor G s ) triggered GLUT4 translocation in cells, independently of insulin signaling pathway(s). Platelet-activating factor (PAF) induced GLUT4 translocation in the cells expressing the G i -and G q -coupled PAF receptor, but the translocation was induced even after pretreatment with wortmannin, an islet-activating protein and phorbol 12,13-dibutyrate. Norepinephrine triggered GLUT4 translocation in cells expressing the G q -coupled ␣ 1 -adrenergic receptor, but prostaglandin E 2 did not cause GLUT4 translocation in cells expressing the G s -coupled EP4 receptor or the G i -coupled EP3␣ receptor. The norepinephrine-stimulated GLUT4 translocation and glucose uptake via G q may possibly contribute to the fuel supply required for thermogenesis in brown adipocytes and for the enhanced contractility in cardiomyocytes, both of which have an abundant endogenous GLUT4. Glucose transporter type 4 (GLUT4)1 is expressed exclusively in adipocytes and myocytes (1, 2). Translocation of GLUT4 from an intracellular pool to the plasma membrane is thought to be a major mechanism of glucose uptake in response to insulin in these tissues (1-4). 3T3-L1 adipocytes have been recognized as an ideal model to investigate the mechanism of GLUT4 translocation and glucose uptake, because these cells have a large amount of endogenous GLUT4 and insulin receptors (5).To examine molecular mechanisms of GLUT4 translocation in 3T3-L1 adipocytes and Chinese hamster ovary (CHO) cells, we developed a highly sensitive and quantitative method to measure directly c-MYC epitope-tagged GLUT4 (GLUT4myc) on the cell surface (6). Using this system, we have found that phosphatidylinositol 3-kinase (PI 3-kinase, p85/p110 heterodimer type) is involved in signal transductions of GLUT4 translocation not only by insulin but also by platelet-derived growth factor and epidermal growth factor (7-9). Other investigators reported that PI 3-kinase plays a pivotal role in the insulin-stimulated GLUT1 and GLUT4 translocations in CHO cells and adipocytes, respectively (10 -12). Therefore, the CHO cell system as well as 3T3-L1 adipocytes are useful for studying molecular mechanisms of GLUT4 translocation (see "Discussion").On the other hand, GLUT4 translocation is also induced in permeabilized adipocytes by treatment with guanosine 5Ј-O-(3-thiotriphosphate) (GTP␥S) of nonhydrolyzable GTP analogs, as well as by insulin (13,14). The GTP␥S-induced GLUT4 translocation was also observed in 3T3-L1 adipocytes and CHO cells stably expressing the GLUT4myc, using this method of detection (6). However, the GTP-binding prote...

Increased Insulin Sensitivity and Hypoinsulinemia in APS Knockout Mice

“…4A). We previously developed a sensitive and quantitative method to measure GLUT4 translocation by detecting C-Myc epitope-tagged GLUT4 (GLUT4myc), which is exogenously expressed on the cell surface (23,(33)(34)(35)(36). Using this system, we found that insulin-stimulated GLUT4 translocation was not altered by transient expression of APS wild-type or Y618F mutant in 3T3L1-G4myc-CAR⌬1 adipocytes (Fig.…”

Molecular Basis of Insulin-stimulated GLUT4 Vesicle Trafficking

“…These treatments can result in increased AMP levels, and it has been suggested that activation of the AMPdependent protein kinase stimulates glucose uptake (29). G protein-coupled receptors linked to G q have also been observed to induce GLUT4 translocation (30). Future studies will be necessary to ascertain the specific roles and relationships between protein kinase B, protein kinase C and , and/or other PI 3-kinase-dependent as well as independent signaling processes in GLUT4 translocation.…”

The Rab GTPase-Activating Protein AS160 as a Common Regulator of Insulin- and G.ALPHA.q-Mediated Intracellular GLUT4 Vesicle Distribution

“…in a previous study we reported that activation of gαq protein-coupled receptors (gαqPcrs), such as the α1b adrenergic receptor (α1bar) or b2 bradykinin receptor (bK 2 r), also triggered gLuT4 translocation and stimulated glucose uptake in cultured cells [30,31]. gLuT4 translocation and glucose uptake induced by gαqPcr activation is physiologically important in muscles and adipocytes [32][33][34][35].…”

“…chO-gLuT4myc cells stably expressing the human α1b-adrenergic receptors and the human bradykinin B 2 receptors, were called chO-gLuT4myc-α1bar cells and chO-gLuT4myc-bK 2 r cells respectively and were as previously described [30,31]. L6-gLuT4myc cells were L6 cells stably expressing gLuT4myc and L6-gLuT4myc-bK 2 r cells were L6-gLuT4myc cells stably expressing the mouse bradykinin b 2 receptors, as previously described [31].…”

“…at day 4 or 5, L6 myotubes were used for experiments. 3T3L1 fibroblasts were cultured to the stage of adipocytes as previously described [30].…”