Glut-2 is a low-affinity transporter present in the plasma membrane of pancreatic beta-cells, hepatocytes and intestine and kidney absorptive epithelial cells of mice. In beta-cells, Glut-2 has been proposed to be active in the control of glucose-stimulated insulin secretion (GSIS; ref. 2), and its expression is strongly reduced in glucose-unresponsive islets from different animal models of diabetes. However, recent investigations have yielded conflicting data on the possible role of Glut-2 in GSIS. Whereas some reports have supported a specific role for Glut-2 (refs 5,6), others have suggested that GSIS could proceed normally even in the presence of low or almost undetectable levels of this transporter. Here we show that homozygous, but not heterozygous, mice deficient in Glut-2 are hyperglycaemic and relatively hypo-insulinaemic and have elevated plasma levels of glucagon, free fatty acids and beta-hydroxybutyrate. In vivo, their glucose tolerance is abnormal. In vitro, beta-cells display loss of control of insulin gene expression by glucose and impaired GSIS with a loss of first phase but preserved second phase of secretion, while the secretory response to non-glucidic nutrients or to D-glyceraldehyde is normal. This is accompanied by alterations in the postnatal development of pancreatic islets, evidenced by an inversion of the alpha- to beta-cell ratio. Glut-2 is thus required to maintain normal glucose homeostasis and normal function and development of the endocrine pancreas. Its absence leads to symptoms characteristic of non-insulin-dependent diabetes mellitus.

The high metabolic requirements of the mammalian central nervous system require specialized structures for the facilitated transport of nutrients across the blood-brain barrier. Stereospecific high-capacity carriers, including those that recognize glucose, are key components of this barrier, which also protects the brain against noxious substances. Facilitated glucose transport in vertebrates is catalyzed by a family of carriers consisting of at least five functional isoforms with distinct tissue distributions, subcellular localizations and transport kinetics. Several of these transporters are expressed in the mammalian brain. GLUT-1, whose sequence was originally deduced from cDNAs cloned from human hepatoma and rat brain, is present at high levels in primate erythrocytes and brain endothelial cells. GLUT1 has been cloned and positionally mapped to the short arm of chromosome 1 (1p35-p31.3; refs 6-8). Despite substantial metabolic requirements of the central nervous system, no genetic disease caused by dysfunctional blood-brain barrier transport has been identified. Several years ago, we described two patients with infantile seizures, delayed development and acquired microcephaly who have normal circulating blood glucose, low-to-normal cerebrospinal fluid (CSF) lactate, but persistent hypoglycorrachia (low CSF glucose) and diminished transport of hexose into isolated red blood cells (RBC). These symptoms suggested the existence of a defect in glucose transport across the blood brain barrier. We now report two distinct classes of mutations as the molecular basis for the functional defect of glucose transport: hemizygosity of GLUT1 and nonsense mutations resulting in truncation of the GLUT-1 protein.

Background-Perivascular adipose tissue (PVAT)-derived relaxing factor (PVATRF) significantly regulates vascular tone. Its chemical nature remains unknown. We determined whether palmitic acid methyl ester (PAME) was the PVATRF and whether its release and/or vasorelaxing activity decreased in hypertension. Methods and Results-Using superfusion bioassay cascade technique, tissue bath myography, and gas chromatography/mass spectrometry, we determined PVATRF and PAME release from aortic PVAT preparations of Wistar Kyoto rats and spontaneously hypertensive rats. The PVAT of Wistar Kyoto rats spontaneously and calcium dependently released PVATRF and PAME. Both induced aortic vasorelaxations, which were inhibited by 4-aminopyridine (2 mmol/L) and tetraethylammonium 5 and 10 mmol/L but were not affected by tetraethylammonium 1 or 3 mmol/L, glibenclamide (3 mol/L), or iberiotoxin (100 nmol/L). Aortic vasorelaxations induced by PVATRF-and PAME-containing Krebs solutions were not affected after heating at 70°C but were equally attenuated after hexane extractions. Culture mediums of differentiated adipocytes, but not those of fibroblasts, contained significant PAME and caused aortic vasorelaxation. The PVAT of spontaneously hypertensive rats released significantly less PVATRF and PAME with an increased release of angiotensin II. In addition, PAME-induced relaxation of spontaneously hypertensive rats aortic smooth muscle diminished drastically, which was ameliorated significantly by losartan. Conclusions-We found that PAME is the PVATRF, causing vasorelaxation by opening voltage-dependent K ϩ channels on smooth muscle cells. Diminished PAME release and its vasorelaxing activity and increased release of angiotensin II in the PVAT suggest a noble role of PVAT in pathogenesis of hypertension. The antihypertensive effect of losartan is attributed partly to its reversing diminished PAME-induced vasorelaxation. (Circulation. 2011;124:1160-1171.) Key Words: angiotensin II Ⅲ fatty acids Ⅲ hypertension Ⅲ potassium ion channels Ⅲ vasomotor tone Ⅲ vasorelaxation Ⅲ losartan T he systemic blood vessels are surrounded by various amounts of perivascular adipose tissue (PVAT). Since the first report by Soltis and Cassis in 1991 that PVAT attenuated contraction of aortic rings to norepinephrine, 1 it has been well accepted that the anticontractile effect of PVAT is due to release of relaxing factor(s) from these adipocytes. 2 The vasodilation induced by PVAT-derived relaxing factor (PVATRF) is independent of the endothelium, cyclooxygenase, or cytochrome P450 pathway. 2,3 The general consensus is that PVATRFinduced vasodilation is due to opening of potassium channels on the smooth muscle cells. [2][3][4][5] The chemical identity of the PVATRF, however, remains unknown. Clinical Perspective on p 1171Our recent studies using superfusion bioassay cascade technique have demonstrated release of an endogenous potent vasodilator, palmitic acid methyl ester (PAME), from the superior cervical ganglion and retina of the rat. 6,7 Because PAME is hydro...

Both the anabolic hormone insulin and contractile activity stimulate the uptake of glucose into mammalian skeletal muscle. In this study, we examined the role of phosphatidylinositol 3-kinase (PI 3-kinase), a putative mediator of insulin actions, in the stimulation of hexose uptake in response to hormone and contraction. Phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-triphosphate accumulate in skeletal muscle exposed to insulin but not hypoxia, which mimics stimulation of the contractile-dependent pathway of hexose transport activation. The fungal metabolite wortmannin, an inhibitor of PI 3-kinase, completely blocks the appearance of 3'-phospholipids in response to insulin. Moreover, wortmannin entirely prevented the increase in hexose uptake in muscle exposed to insulin but was without effect on muscle stimulated by repetitive contraction or hypoxia. These results support the view that PI 3-kinase is involved in the signaling pathways mediating insulin-responsive glucose transport in skeletal muscle but is not required for stimulation by hypoxia or contraction. Furthermore, these data indicate that there exist at least two signaling pathways leading to activation of glucose transport in skeletal muscle with differential sensitivities to wortmannin.

Vitamin D receptor gene polymorphisms were associated with type 1 diabetes in a Taiwanese population. However, functional studies are needed to establish the role of the vitamin D receptor in the pathogenesis of type 1 diabetes mellitus.

Insulin regulates hexose uptake by the redistribution of glucose transport proteins from intracellular compartments to the cell surface. We have submitted the trafficking of GLUT1, GLUT4, and GLUT1/GLUT4 chimeras to a mathematical analysis in the context of different models. Our data suggest that a model with one intracellular and one cell surface compartment can describe the glucose transporter-trafficking kinetics in fibroblasts. Moreover, the difference in cellular distribution between GLUT1 and GLUT4 overexpressed in fibroblasts is best explained by a slower rate of movement of GLUT4 to the plasma membrane. In 3T3-L1 adipocytes, glucose transporter-trafficking kinetics is adequately described by a three-pool model which includes flow of transporters from the endosomal compartment to cell surface. The kinetic roles of previously identified motifs in GLUT4 trafficking were defined in proposed fibroblast and adipocyte glucose transporter-trafficking models. The C-terminus is important in reducing the exocytosis rate from the endosomal compartment to the cell surface in both fibroblasts and adipocytes, and the N-terminus behaves similarly in adipocytes. The C-terminus has an additional signal(s) that allows GLUT4 to be sequestered more efficiently into the insulin responsive vesicle compartment. Mutation of the dileucine motif in the C-terminus significantly reduces the endocytosis of GLUT4 in both fibroblasts and adipocytes, but these amino acids appear not to be primarily responsible for the different kinetics of wild-type GLUT1 and GLUT4.

Abstract. In adipose and muscle cells, insulin stimulates a rapid and dramatic increase in glucose uptake, primarily by promoting the redistribution of the GLUT4 glucose transporter from its intracellular storage site to the plasma membrane. In contrast, the more ubiquitously expressed isoform GLUT1 is localized at the cell surface in the basal state, and shows a less dramatic translocation in response to insulin. To identify sequences involved in the differential subcellular localization and hormone-responsiveness of these isoforms, chimeric GLUT1/GLUT4 transporters were stably expressed in mouse 3T3-L1 adipocytes. The NH 2 terminus of GLUT4 contains sequences capable of sequestering the transporter inside the cell, although not in an insulin-sensitive pool. In contrast, the COOH-terminal 30 amino acids of GLUT4 are sufficient for its correct localization to an intraceUular storage pool which translocates to the cell surface in response to insulin. The dileucine motif within this domain, which is required for intracellular sequestration of chimeric transporters in fibroblasts, is not critical for targeting to the hormone-responsive compartment in adipocytes. Analysis of rates of internalization of chimeric transporter after the removal of insulin from cells, as well as the subcellular distribution of transporters in cells unexposed to or treated with insulin, leads to a three-pool model which can account for the data. LUCOSE uptake into mammalian cells is accom-plished by a family of proteins, the facilitative glucose transporters, which differ in their tissue distribution and physiological roles. Two of the isoforms identified to date, GLUT1 and GLUT4, are expressed in adipose and muscle, those tissues which exhibit a marked increase in glucose uptake in response to insulin (5). The differential localization of these isoforms within the cell may contribute to their distinct functions. GLUT1, which is expressed in virtually all tissues, is distributed to both the plasma membrane and the interior of the cell in the basal state. Insulin causes a two-to fivefold increase in the amount of GLUT1 present at the cell surface, a recruitment similar to other recycling membrane proteins such as the insulin-like growth factor II and transferrin receptors (39). GLUT4, which is expressed exclusively in insulinresponsive cell types, is excluded from the plasma membrane and instead localizes to an intracellular storage pool in the basal state. Insulin stimulates the specific recruitment of these vesicles to the cell surface, increasing by 10-to 40-fold the amount of GLUT4 present at the cell surface, and thereby dramatically increasing the hexose uptake capacity of the cell (8,9,21,24,34,(36)(37)(38)49 The mechanisms responsible for the insulin-regulated movement of GLUT4 in adipose and muscle may be similar to those utilized by other cell types for regulated secretion. This is supported by the ability of GLUT4 to segregate to large dense core vesicles when expressed in the neuroendocrine cell line PC12 (23). The involvement ...

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