Overexpression of glucose transporter protein 5 in sciatic nerve of streptozotocin-induced diabetic rats

The localization and the expression level of glucose transporter 5 (GLUT 5), detected by immunohistochemical and Western blot analyses, and motor nerve conduction velocity (MNCV) in normal and streptozotocin (STZ)-induced diabetic rats were compared. The effects of insulin and recombinant human insulin-like growth factor-I (rhIGF-I) were also investigated. GLUT 5 was localized in Schwann cells and axons. GLUT 5 was overexpressed in both sites 5 weeks after STZ injection and MNCV was decreased significantly in STZ-induced diabetic rats as compared with normal rats. These deviations returned to normal rat level after 2 weeks medication with insulin or rhIGF-I, started 3 weeks after STZ injection. These results suggest that overexpression of GLUT 5 may be a trigger for diabetic neuropathy.

Attachment of PC12 cells to adhesion substratum induces the accumulation of glucose transporters (GLUTs) and stimulates glucose metabolism

The levels of glucose transporters (GLUTs), specifically GLUT3 and GLUT1, increased dramatically in PC12 cells that were cultured on suitable adhesion substrata (poly-1-lysine [PLL]) and induced to differentiate with nerve growth factor (NGF). Closer examination of this response revealed that: (1) cellular attachment to PLL was sufficient to stimulate the increase in GLUT immunoreactivity, and (2) NGF alone was not effective unless the cells were cultured on PLL-treated surfaces. The response to PLL was detected as early as 4 hr after plating the cells and peaked within 24-48 hr. Other adhesion substrata, such as collagen and poly-1-ornithine, evoked a similar response, although the latter polymer was far less effective. The increase in GLUTs appeared to result from an accumulation of existing transporters because this response was not blocked by inhibiting protein synthesis. Cellular adhesion to PLL was also accompanied by a rapid activation of glucose metabolism. Thus, specific recognition of the adhesion substratum not only provides a context for cell attachment, but also elicits important functional changes in GLUT activity.

Modulation of GDP-dissociation inhibitor protein membrane retention by the cellular redox state in adipocytes

Small GTPases of the Rab family play a key role in the regulation of vesicular transport in eukaryotic cells. As they cycle on and off membranes, Rab proteins rely on the escort services of the GDP-dissociation inhibitor (GDI) proteins. While specific recognition of Rab-GDI complexes by membrane targets is suggested, the mechanisms underlying the subsequent GDI release into the cytosol remain unknown. In this study, we demonstrate that modulations of the cellular redox status in intact rat fat cells, 3T3-L1 adipocytes in culture, and other cultured cell types result in rapid, effective, dose-dependent, and selective membrane dynamics of GDI-1 and -2, membrane retention under reduced redox state, or dissociation under oxidized conditions. GDI retention on adipocyte membranes is associated with a complete arrest of insulin-induced translocation of GLUT4 glucose transporters onto plasma membrane. Together, these data suggest, first, that following Rab delivery to membranes, GDI release is promoted by a shift in the redox state and, second, that arrest of GDIs on membranes inhibits intracellular membrane trafficking events.

Loss of glucose transporter-2 precedes insulin loss in the nonobese diabetic and the low-dose streptozotocin mouse models: a comparative immunohistochemical study by light and confocal microscopy

Glucose transporter-2 (glut2) is underexpressed in beta cells of several rodent models of non-insulin-dependent diabetes mellitus (NIDDM). This may also be true for rodent models of insulin-dependent diabetes mellitus (IDDM). The present study examines two murine models of autoimmune IDDM, the nonobese diabetic (NOD) and the low-dose streptozotocin (stz) murine models for changes in the expression of glut2 by double-label light and confocal microscopy during various stages of the disease. The spatial distribution of glut2 cells was also examined in relation to insulin immunoreactive cells and the islet inflammatory cells during these stages. In both the female NOD mouse and the female Swiss mouse without stz treatment, glut2 colocalized with insulin in virtually all the beta cells. In the NOD mouse, islets with moderate to advanced insulitis showed either an absence or considerably reduce expression of glut2 in insulin-containing beta cells. Cells with reduced glut2 expression were usually located adjacent to the region of insulitis. At onset of diabetes, glut2 immunolabeling was reduced despite the preservation of weak insulin immunoreactivity. In Swiss mice treated repeatedly with stz, glut2 labeling began to decline in select Beta cells after the fourth injection in approximately 50% of the islets, despite the lack of insulitis. At this stage expression of glut2 fell in a small number of islets with evidence of early macrophage infiltration. Loss of glut2 became more pronounced in nondiabetic Swiss mice after the fifth injection. At this stage glut2 labeling in the plasma membrane appeared diffuse and variable. At onset of stz-induced diabetes, glut2 expression significantly fell, despite weak immunoreactivity for insulin. This loss was associated with an enhanced influx of both macrophages and T lymphocytes within the islets of diabetes mice. In both the NOD and the low-dose stz mouse models, loss of glut2 thus occurs from an early stage and precedes hyperglycaemia. This loss may be mediated by immune and nonimmune mechanisms.

GLUT1 glucose transporter expression in colorectal carcinoma: a marker for poor prognosis

BACKGROUND

Malignant cells exhibit increased glycolytic metabolism, and in many cases increased glucose transporter gene expression. The authors hypothesized that GLUT1 glucose transporter expression is increased in colorectal carcinoma, and that the degree of expression might have prognostic significance.

METHODS

GLUT1 glucose transporter immunostaining was studied in normal colon and benign colon adenomas and in 112 colorectal carcinomas from patients for whom long term clinical outcome was known.

RESULTS

GLUT1 immunostaining was absent in normal colorectal epithelium and tubular adenomas, and absent or only weakly apparent in tubulovillous adenomas. The majority of carcinomas (101 of 112; 90%) had GLUT1 immunostaining. Tumors from 92 patients had low GLUT1 expression (< 50% of cells were GLUT1 positive) and 19 of these patients (21%) died of disease during follow-up. In contrast, tumors from 20 patients had high GLUT1 expression (> 50% of cells were GLUT1 positive) and 9 of these patients (45%) died of disease during follow-up. Disease specific mortality was greater in patients with high GLUT1 tumors (relative risk of 2.4; P=0.02). In a multivariate analysis to assess whether high GLUT1 staining correlated with increased mortality independently of Dukes stage, the risk of death from colon carcinoma in the group with high GLUT1 staining was 2.3 times that in the group with low GLUT1 staining, a difference that approached statistical significance (P=0.07).

CONCLUSIONS

GLUT1 glucose transporter expression is associated strongly with neoplastic progression in the colon, and assessment of the extent of GLUT1 immunostaining in colorectal carcinoma identifies patients with a poorer prognosis.

GLUT-4 Deficiency and severe peripheral resistance to insulin in the teleost fish tilapia

Teleost fish, in general, are glucose intolerant; this trait has been attributed to piscine islets secreting insulin primary in response to amino acid secretogogues rather than glucose. However, pancreatic islet from the teleost fish tilapia, when transplanted into diabetic nude mice, were glucose responsive even though tilapia were severely glucose intolerant. This suggested a strong peripheral resistance to the glucostatic effects of insulin. Using Western blotting with polyclonal antibodies as well as Northern analysis for mRNA, tilapia tissues were found to be devoid of GLUT-4, the insulin-sensitive glucose transporter responsible for the hypoglycemic effect of insulin in mammals. The absence of GLUT-4 in peripheral tissues may explain why tilapia, and possibly other teleost fish, are severely glucose intolerant. This suggests that tilapia islets have evolved along mammalian lines to be glucose sensitive while tilapia peripheral tissue have diverged widely. Using the same methods, tilapia were found to have a very limited tissue distribution of the insulin-independent glucose transporter, GLUT-1, which is responsible for basal glucose transport in mammalian cells. It is suggested that tilapia provide a naturally occurring GLUT-4 knockout model.

A single glucose transporter configuration in normal primate brain endothelium: comparison with resected human brain

Cellular distribution of the Glut1 glucose transporter in normal primate brains was analyzed by immunogold electron microscopy. Two configurations of endothelial Glut1 glucose transporter (high and low density capillaries) have been found in resections of traumatically injured and epileptogenic human brain; the objective of the present study was to ascertain whether these same 2 capillary populations, expressing high and low glucose transporter densities, were the common configuration in normal brain. The relative numbers of Glut1 glucose transporter-associated gold particles on luminal and abluminal endothelial cell membranes were determined within the cerebral cortex of several normal, nonhuman primates. Low Glut1 densities were seen in brain endothelia of both the rhesus and squirrel monkey cortex, with slightly greater quantities of Glut1 in vervet monkey cortices. The Glut1 transporter was most highly expressed in the baboon cortex, approaching the concentrations seen in human brains. In the rhesus, squirrel, and vervet monkeys, Glut1 concentrations were greater on the abluminal than luminal capillary membranes. In contrast, mean luminal membrane Glut1 concentrations were greater in baboons, resembling the distribution seen in the human brain. Brain regional differences in transporter concentration were seen in comparing membrane densities in the baboon cortex (approximately 15 Glut1-gold particles per micrometer), hippocampus (approximately 12 Glut1 gold particles per micrometer), cerebellum (approximately 6 Glut1-gold particles per micrometer), and retinal microvasculature (approximately 20 Glut1-gold particles per micrometer). We conclude that a single, uniform Glut1 distribution characterizes brain capillaries of normal nonhuman primates, and hypothesize that the presence of high and low density glucose transporter endothelial cells (seen in human traumatic injury and seizure resections) represents a pathologic response to brain insult.

GLUT4 glucose transporter expression in rodent brain: effect of diabetes

This study describes the regional and cellular expression of the insulin-sensitive glucose transporter, GLUT4, in rodent brain. A combination of in situ hybridization, immunohistochemistry and immunoblot techniques was employed to localize GLUT4 mRNA and protein to the granule cells of the olfactory bulb, dentate gyrus of the hippocampus and the cerebellum, with the greatest level of expression being in the cerebellum. Estimates of the concentration of GLUT4 in cerebellar membranes indicate that this transporter isoform is present in significant amounts, relative to the other isoforms, GLUT1 and GLUT3. Cerebellar GLUT4 expression was increased in the genetically diabetic, hyperinsulinemic, db/db mouse relative to the non-diabetic control, and even higher levels were observed in db/db female than db/db male mice. Levels of expression of GLUT4 protein in cerebellum appear to respond to the level of circulating insulin, and are reduced in the hypoinsulinemic streptozotocin-diabetic rat. Exercise training also results in reduced insulin levels and comparably reduced levels of GLUT4 in the cerebellum. These studies demonstrate a chronic insulin-sensitive regulation of GLUT4 in rodent brain and raise the possibility of acute modulations of glucose uptake in these GLUT4 expressing cells.

The 45 kDa form of glucose transporter 1 (GLUT1) is localized in oligodendrocyte and astrocyte but not in microglia in the rat brain

We and others have previously reported that glucose transporter 1 (GLUT1)-like 45 kDa protein is localized to parenchymal cells in the brain. However, the precise cellular localization has remained unclear. In the present study, we examined the cellular localization of GLUT1 in the rat brain by double immunostaining methods and immunoelectron microscopic analysis using a rabbit antiserum specific to GLUT1. Western blot analysis of the rat brain revealed that the antiserum detected a strong band with a molecular weight of 45 kDa and a weak band of about 55 kDa, which corresponded respectively to the known molecular weights of the GLUT1 proteins in the brain parenchymal cells and the brain microvessels. Immunohistochemical staining revealed a large number of GLUT1-immunoreactive glial cells and microvessels in almost every region of the brain. Double immunofluorescence analysis demonstrated that the GLUT1-like 45 kDa protein occurred in many galactocerebroside-positive oligodendrocytes and in some glial fibrillary acidic protein (GFAP)-positive astrocytes. No GLUT1-immunoreactivity was observed in OX42-positive microglia. Immunoelectron microscopic examination confirmed that the GLUT1-immunoreactivity was mainly localized in the cytoplasm of the oligodendrocytes and astrocytes. The results indicate that the 45 kDa form of GLUT1 protein exists in the glial cells including astrocytes and oligodendrocytes.

beta-cell-specific inactivation of the mouse Ipf1/Pdx1 gene results in loss of the beta-cell phenotype and maturity onset diabetes

To study the late beta-cell-specific function of the homeodomain protein IPF1/PDX1 we have generated mice in which the Ipf1/Pdx1 gene has been disrupted specifically in beta cells. These mice develop diabetes with age, and we show that IPF1/PDX1 is required for maintaining the beta cell identity by positively regulating insulin and islet amyloid polypeptide expression and by repressing glucagon expression. We also provide evidence that IPF1/PDX1 regulates the expression of Glut2 in a dosage-dependent manner suggesting that lowered IPF1/PDX1 activity may contribute to the development of type II diabetes by causing impaired expression of both Glut2 and insulin.

Pre-embedding staining of single muscle fibers for light and electron microscopy studies of subcellular organization

Skeletal muscle fibers are large, multinucleated cells which pose a challenge to the morphologist. In the course of studies of the distribution of the glucose transporter GLUT4, in muscle, we have compared different preparative procedures, for both light (LM) and electron microscopy (EM) immunocytochemistry. Here we show that pre-embedding staining of single teased fibers, or of single enzymatically dissociated fibers, has several advantages over the use of sections for observing discrete patterns that extend over long distances in the cells. We report on an optimization study carried out to establish fixation and permeabilization conditions for EM immunogold labeling of the fibers. We find that a simple fixation with depolymerized paraformaldehyde alone, followed by permeabilization with 0.01% saponin, offers the best compromise between the conflicting demands of unhindered tissue penetration and morphology preservation.

GLUT4 translocation by insulin in intact muscle cells: detection by a fast and quantitative assay

We report a rapid and sensitive colorimetric approach to quantitate the amount of glucose transporters exposed at the surface of intact cells, using L6 muscle cells expressing GLUT4 containing an exofacial myc epitope. Unstimulated cells exposed to the surface 5 fmol GLUT4myc per mg protein. This value increased to 10 fmol/mg protein in response to insulin as 2-deoxyglucose (10 microM) uptake doubled. The results are substantiated by immunofluorescent detection of GLUT4myc in unpermeabilized cells and by subcellular fractionation. We further show that wortmannin and the cytoskeleton disruptors cytochalasin D and latrunculin B completely blocked these insulin effects. The rapid quantitative assay described here could be of high value to study insulin signals and to screen for potential anti-diabetic drugs.

Structural organization of the perivascular astrocyte endfeet and their relationship with the endothelial glucose transporter: a confocal microscopy study

Despite the increasing evidence for a prominent role played by the perivascular endfeet of astrocytes in the functional metabolic coupling between astrocytes and neurons, a clear picture of their spatial organization is still lacking. To examine the three-dimensional structure of the astrocyte endfeet and their relationships with the endothelial cells, coronal rat brain sections immunolabeled for the two astroglial markers [glial fibrillary acidic protein (GFAP)/S-100beta] and the endothelial glucose transporter (GLUT1) were analyzed under the confocal microscope. Double immunolabeling of GFAP and S-100beta showed numerous well-defined astrocytes sending one or more endfeet to the vasculature. Examination of GFAP immunolabeling at higher magnification showed that these endfeet consist of well-defined rosette-like structures lying on the vessel wall. Double immunostaining of GFAP and GLUT1 showed that the endothelial cells were the main targets of these repeated geometrical units formed by the astrocyte endfeet. When three-dimensional images were reconstructed, obvious privileged anatomical relationships were observed between endfeet and individual endothelial cells. These anatomical data provide strong support for the involvement of astrocytes in cerebral metabolic coupling. The finger-like appearance of astrocyte endfeet could allow direct metabolic exchanges between intracerebral vessels and non-glial elements such as nerve terminals.

GLUT1 glucose transporter: a highly sensitive marker of malignancy in body cavity effusions

Malignant cells exhibit increased rates of glycolysis and glucose uptake, and several types of cancer have been reported to overexpress the GLUT1 glucose transporter. The diagnosis of malignancy in body cavity effusions remains a dilemma in certain cases, despite recent progress in diagnostic immunocytochemistry. We used immunostaining to detect the facilitative glucose transporter, GLUT1, in cytologic preparations of body cavity effusions and washes. With the use of standard avidin-biotin immunostaining for GLUT1, we examined cell blocks of body cavity effusions or washings from 31 carcinomas, 1 lymphoma, and 25 benign effusions or washes. GLUT1 staining occurred in the malignant cell population in 29 (93.5%) of 31 carcinomatous effusions or washes. The characteristic staining pattern consisted of dense, linear staining of the plasma membrane, with accentuation at cell-cell borders, with or without cytoplasmic staining. Erythrocytes showed positive GLUT1 membrane staining, consistent with previous reports. Of 25 benign effusions, 20 were nonstaining (excepting erythrocytes), and 5 contained rare single mesothelial cells, with equivocal to very weak membrane staining. Staining of these cells was readily distinguishable from the characteristic strong staining of malignant cells, and these cells were easily distinguished from tumor cells by their benign morphologic characteristics. At least three of these latter five specimens were from patients with cirrhosis. In all of the other cases, mesothelial cells, histiocytes, and other inflammatory cells did not stain. These findings suggest that GLUT1 immunostaining could be useful in diagnostic cytopathology. The findings also suggest that enhanced glycolysis, which requires increased glucose transport, might be a survival adaptation for tumor cells in effusions, a significant number of which are hypoxic.

Mechanisms of insulin-resistant glucose utilization in rat skeletal muscle

Defects in glucose uptake are among the primary defects associated with peripheral insulin resistance, but fundamental mechanisms leading to this state are poorly understood. In order to elucidate mechanisms leading toward defects in glucose transport, we have used a partially pancreatectomized infusion (PxI) animal model with infusions of saline, glucose, or insulin to examine individual and combined effects of hyperglycemia and hyperinsulinemia on skeletal muscle glucose utilization. Moderate hyperglycemia induced by pancreatectomy reduced basal hindlimb muscle glucose utilization by 57% without affecting maximal insulin-stimulated glucose utilization; insulin administered in an amount sufficient to correct this hyperglycemia did not alter basal glucose utilization, but maximal insulin-stimulated glucose utilization was sharply diminished (75%); hyperglycemia with hyperinsulinemia similarly reduced basal and maximal insulin-stimulated glucose utilization. In order to establish the role of the glucose transporter protein in these insulin-resistant states, we quantified GLUT 4 content by immunoblotting and GLUT 4 mRNA by solution hybridization/RNAse protection assays. Hyperglycemia (2 weeks) reduced total muscle GLUT 4 protein content (53%) and mRNA (46%), while subsequent hyperinsulinemia (72 h) with either normo- or hyperglycemia partially restored both total GLUT 4 protein and mRNA levels. As insulin-stimulated GLUT 4 content in plasma membranes was not diminished by combined hyperglycemia/hyperinsulinemia, these results indicate functional GLUT 4 translocation in this model and suggest suppression of GLUT 4 transporter activity.

Molecular characterization of the glucose-sensing mechanism in the clonal insulin-secreting BRIN-BD11 cell line

BRIN-BD11 cells represent a novel insulin-secreting cell line generated by electrofusion. Molecular characterization of these cells demonstrated the presence of mRNA and protein for the two key elements of the beta cell glucose-sensing system, GLUT2 and glucokinase. While levels of GLUT2 expression and 3-O-methyl-D-glucose equilibration were similar for both the BRIN-BD11 cell line and the parental control RINm5F cells, glucokinase expression was substantially higher in BRIN-BD11 cells. Expression of the two-component KATP channel complex, KIR6.2 and SUR1, was similar in both cells. However, while control RINm5F cells were completely unresponsive to glucose, BRIN-BD11 cells responded to physiological millimolar concentrations of this hexose sugar. These studies strongly suggest that the glucose-sensing ability of insulin-secreting cells is largely dictated by the level of glucokinase, as opposed to GLUT2, expression. Thus, BRIN-BD11 cells expressing the key attributes of the normal beta cell provide an interesting model for elucidation of regulatory principles of beta cell function.

Selective impairment in GLUT4 translocation to transverse tubules in skeletal muscle of streptozotocin-induced diabetic rats

We previously reported that insulin induces the translocation of GLUT4 to both the plasma membrane and the transverse tubules (T-tubules) in rat skeletal muscle (Am J Physiol 270:E667-E676, 1996). The aim of the present study was to investigate whether the insulin-resistant glucose utilization of skeletal muscle from streptozotocin (STZ)-induced diabetic rats is linked to an impaired translocation of GLUT4 to the plasma membrane, the T-tubules, or both surface compartments. Whole-body insulin-mediated glucose disposal, assessed during a hyperinsulinemic-euglycemic clamp, was reduced by 48% (P < 0.01) in diabetic rats as compared with controls. Subcellular membrane fractions enriched with plasma membranes, T-tubules, or GLUT4-enriched intracellular membranes were isolated from hindlimb muscles of control and insulin-stimulated rats, and GLUT4 content was measured by Western blot analysis. In the absence of insulin (unstimulated), GLUT4 content in muscle of diabetic rats was markedly lower (by approximately 40%) in both the T-tubules and the intracellular membrane fraction as compared with controls. In contrast, the transporter protein levels were similar in the plasma membrane fraction. In skeletal muscle of control animals, the hyperinsulinemic clamp induced GLUT4 translocation from the intracellular membrane pool to both the plasma membrane and the T-tubule-enriched fractions (approximately 2.2-fold to approximately 2.5-fold). Surprisingly, insulin increased plasma membrane GLUT4 content to comparable levels in control and diabetic rat skeletal muscle. However, insulin-mediated GLUT4 translocation to the T-tubules was significantly reduced in the same muscle. Whole-body insulin action was significantly correlated with GLUT4 protein levels in the T-tubules, but not with the transporter content in either plasma membranes or intracellular membranes. These results strongly suggest that peripheral resistance to insulin action on glucose disposal in STZ-induced diabetic rats is caused by a selective impairment of GLUT4 translocation to skeletal muscle T-tubules.

GLUT2 in pancreatic islets: crucial target molecule in diabetes induced with multiple low doses of streptozotocin in mice

In mice, diabetes can be induced by multiple low doses of streptozotocin (MLD-STZ), i.e., 40 mg/kg body wt on each of 5 consecutive days. In this model, diabetes develops only when STZ induces both beta-cell toxicity and T-cell-dependent immune reactions. The target molecule(s) of MLD-STZ-induced beta-cell toxicity are not known, however. In this study, we report that GLUT2 is a target molecule for MLD-STZ toxicity. Ex vivo, a gradual decrement of both GLUT2 protein and mRNA expression was found in pancreatic islets isolated from MLD-STZ-treated C57BL/6 male mice, whereas mRNA expression of beta-actin, glucokinase, and proinsulin remained unaffected. Significant reduction of both GLUT2 protein and mRNA expression was first noted 1 day after the third STZ injection, clearly preceding the onset of hyperglycemia. The extent of reduction increased with the number of STZ injections administered and increased over time, after the last, i.e., fifth, STZ injection. The STZ-induced reduction of GLUT2 protein and mRNA was not due to an essential loss of beta-cells, because ex vivo, not only the total RNA yield and protein content in isolated islets, but also proinsulin mRNA expression, failed to differ significantly in the differently treated groups. Furthermore, islets isolated from MLD-STZ-treated donors responded to the nonglucose secretagogue arginine in a pattern similar to that of solvent-treated donors. Interestingly, the MLD-STZ-induced reduction of both GLUT2 protein and mRNA was prevented by preinjecting mice with 5-thio-D-glucose before each STZ injection. Apparently, GLUT2 is a crucial target molecule of MLD-STZ toxicity, and this toxicity seems to precede the immune reactions against beta-cells.

Effects of cell-permeable ceramides and tumor necrosis factor-alpha on insulin signaling and glucose uptake in 3T3-L1 adipocytes

Incubation of 3T3-L1 adipocytes with C2- and C6-ceramides (N-acetyl- and N-hexanoylsphingosines) but not dihydro-C2-ceramide increased 2-deoxyglucose uptake in the absence of insulin. This effect was inhibited by PD 98059, LY 294002, and rapamycin, which block the activation of mitogen-activated protein kinase, phosphatidylinositol (PI) 3-kinase, and ribosomal S6 kinase, respectively. Long-term increases in PI 3-kinase activity associated with insulin receptor substrate 1 (IRS-1) increased GLUT1 and GLUT4 concentrations in plasma membranes. This together with increased GLUT1 (but not GLUT4) synthesis explains the increase in non-insulin-dependent glucose uptake. C2-ceramide inhibited insulin-stimulated glucose uptake after 2 h by decreasing insulin-induced translocation of GLUT1 and GLUT4 to plasma membranes. This occurred when there was no increase in basal glucose uptake or decrease in activation of IRS-1 or PI 3-kinase. Incubation for 24 h with tumor necrosis factor-alpha (TNF-alpha) but not C2-ceramide decreased the concentration and insulin-induced tyrosine phosphorylation of IRS-1 in this experimental system. Cell-permeable ceramides mimic some effects of TNF-alpha, especially in stimulating basal glucose uptake. We identified a site for inhibiting insulin-stimulated glucose uptake that is downstream of PI 3-kinase. Our work provides further mechanisms for the effects of TNF-alpha and ceramides in increasing non-insulin-dependent glucose uptake and decreasing insulin-stimulated uptake in vivo.

Distinct localization of renin and GLUT-4 in juxtaglomerular cells of mouse kidney

The insulin-responsive glucose transporter, GLUT-4, is found primarily in adipocytes and skeletal muscle cells, where it is sequestered in a specialized recycling compartment, from which it can be recruited to the cell surface following insulin stimulation. Lower levels of GLUT-4 are also expressed in other tissues, including the kidney, where it is present particularly in cells of the afferent arteriole and juxtaglomerular apparatus (JGA). The exact nature of GLUT-4-containing compartments and their relationship to other regulated trafficking pathways in different cells are not yet well defined. The trafficking of GLUT-4 has been studied in different cells with regulated secretory pathways, and a recent study shows that, in cardiomyocytes, GLUT-4 is sorted and packaged into multiple regulated pathways (J. W. Slot, G. Garruti, S. Martin, V. Oorschot, G. Pshuma, E. W. Kraegen, R. Laybutt, G. Thibault, and D. E. James. J. Cell Biol. 137: 1243-1254, 1997). In the kidney, cells of the JGA synthesize and secrete their major product, renin, via a well-established, regulated, secretory pathway. These cells also express GLUT-4 and thus offer the potential to directly compare the localization and trafficking of GLUT-4 and renin in a unique cell type. The present study was undertaken to investigate the intracellular distribution of GLUT-4 in mouse kidney cortex and to determine whether GLUT-4 and renin are trafficked in the same or in separate regulated pathways. Ultrathin cryosections of mouse kidney were labeled by the immunogold technique and viewed by electron microscopy, demonstrating the distribution of GLUT-4 in cells of the JGA, afferent arteriole, and distal tubule. In granular cells of the JGA, renin was localized in secretory granules of the regulated secretory pathway, whereas GLUT-4 labeling in the same cells was found in a distinct tubulovesicular compartment located adjacent to the trans-Golgi network. We show that granular cells have separate, morphologically distinct compartments for the sequestration of renin and GLUT-4, providing evidence that there may be distinct pathways for the sorting and trafficking of these two proteins.

Depot-specific differences in adipose tissue gene expression in lean and obese subjects

Intra-abdominal and subcutaneous adipose tissue display important metabolic differences that underlie the association of visceral, but not subcutaneous, fat with obesity-related cardiovascular and metabolic problems. Because the molecular mechanisms contributing to these differences are not yet defined, we compared by reverse transcription-polymerase chain reaction the expression of 15 mRNAs that encode proteins of known importance in adipocyte function in paired omental and subcutaneous abdominal biopsies. No difference in mRNA expression between omental and subcutaneous adipose tissue was observed for hormone sensitive lipase, lipoprotein lipase, 6-phosphofructo-1-kinase, insulin receptor substrate 1, p85alpha regulatory subunit of phosphatidylinositol-3-kinase, and Rad. Total amount of insulin receptor expression was significantly higher in omental adipose tissue. Most of this increase was accounted for by expression of the differentially spliced insulin receptor lacking exon 11, which is considered to transmit the insulin signal less efficiently than the insulin receptor with exon 11. Perhaps consistent with a less efficient insulin signaling, a twofold reduction in GLUT4, glycogen synthase, and leptin mRNA expression was observed in omental adipose tissue. Finally peroxisome proliferator activated receptor-gamma (PPAR-gamma) mRNA levels were significantly lower in visceral adipose tissue in subjects with a BMI <30 kg/m2, but not in obese subjects, indicating that relative PPAR-gamma expression is increased in omental fat in obesity. This suggests that altered expression of PPAR-gamma might play a role in adipose tissue distribution and expansion.

Gliclazide treatment of streptozotocin diabetic rats restores GLUT4 protein content and basal glucose uptake in skeletal muscle

This study examined whether the treatment of streptozotocin (STZ)-diabetic rats with gliclazide (5 mg/kg body weight twice daily orally) increases muscle glucose uptake. Rats were treated (group G, n = 10) or untreated (group D, n = 11) for 12 days. Normal rats served as controls (group C, n = 11). At the end of the treatment, both basal and insulin-stimulated glucose uptake by the perfused hindquarters were measured. In gastrocnemious muscles, the protein content of GLUT4 and the insulin binding and tyrosine kinase activity of partially purified solubilized insulin receptors were measured. Group G had a lower mean glycemic value during the treatment period than group D (mean +/- SEM, 17 +/- 0.6 v 19.7 +/- 0.5 mmol/L, P < .05), without differences in serum insulin levels. Basal glucose uptake by the hindquarters was significantly higher in group G versus group D (2.8 +/- 0.3 v 1.3 +/- 0.2 mumol/g/h, P < .05), and was not different versus group C (3.6 +/- 0.2 mumol/g/h). Insulin-stimulated glucose uptake was higher (P < .05) in group C compared with the two groups of diabetic rats. Glucose uptake at 10(-7) mol/L insulin was higher in group G than in group D (9.2 +/- 0.6 v 7.0 +/- 0.6 mumol/g/h, P < .05). Both insulin binding and tyrosine kinase activity were similar in muscle insulin receptors from both groups of diabetic rats. The GLUT4 protein content was higher in group G than in group D (95 +/- 10 v 57 +/- 7 arbitrary units [AU]/microgram protein, P < .05) and similar to that of group C (113 +/- 13 AU/microgram protein). In conclusion, gliclazide has a glucose-lowering effect in STZ-diabetic rats that could be attributed to an increase in muscle glucose clearance by a post-insulin receptor mechanism, probably related to a normalization of GLUT4 content.

Influence of highly purified eicosapentaenoic acid ethyl ester on insulin resistance in the Otsuka Long-Evans Tokushima Fatty rat, a model of spontaneous non-insulin-dependent diabetes mellitus

We investigated the effect of long-term administration of highly purified eicosapentaenoic acid ethyl ester (EPA-E), an n-3 polyunsaturated fatty acid derived from fish oil, in comparison to the effects of lard, olive oil, safflower oil, or distilled water as the control on the development of insulin resistance in Otsuka Long-Evans Tokushima Fatty (OLETF) rats, a model of spontaneous non-insulin-dependent diabetes mellitus (NIDDM) with obesity. After 17 or 18 weeks of treatment, the glucose infusion rate (GIR) in the euglycemic insulin-glucose clamp test only showed a significant increase in EPA-E-treated rats compared with control rats given distilled water alone as the vehicle. The GIR in EPA-E-treated animals was approximately three times greater than in the controls. This is the first report to display the influence of various fatty acids on the development of insulin resistance in OLETF rats. We demonstrated that EPA-E prevents the onset of insulin resistance, whereas olive oil and safflower oil have no effect and lard exacerbates insulin resistance. Fatty acid analysis of phospholipids in skeletal muscle showed a significant increase of the C18:2, C20:5, and C22:5 components in EPA-E-treated rats and, conversely, a significant decrease in C20:4. In addition, EPA-E-treated rats showed a significant increase in GLUT4 mRNA in skeletal muscle when compared with control rats. Our results indicate that the beneficial effect of EPA-E on insulin resistance in OLETF rats is likely to be dependent on modification of the phospholipid components of the skeletal muscle membrane. These findings suggest that dietary fatty acids may play a key role in the development of insulin resistance in patients with NIDDM.

Glucose transporter isoform (GLUT) 2 expression in beta-cells of long-term syngeneic islet grafts

Syngeneic islets were transplanted into the liver of streptozotocin (STZ)-induced diabetic LEW.1W rats, and the expression of the glucose transporter isoform GLUT 2, an essential component of the glucose-sensing mechanism of the pancreatic beta-cell, was determined in the grafted islet tissue. Graft-bearing liver was obtained 12, 36, and 60 weeks after transplantation, and tissue sections were immunoperoxidase stained for GLUT 2 and major islet peptides. Islet cell aggregates of different sizes were found in the portal tract and in juxtaposition to the hepatocytes. At all time points, beta-cells in the grafts displayed GLUT 2 expression comparable to that of islets in nondiabetic rats. Islet cells containing immunoreactive insulin and islet amyloid polypeptide were plentiful, while those staining positive for glucagon and somatostatin were scarce in these grafts. The results show that beta-cells in islets engrafted in the liver, although initially exposed to chronic hyperglycemia, have the capability of stably expressing GLUT 2 over long-term periods.

Evaluation of fluorodeoxyglucose-positron emission tomographic scanning and its association with glucose transporter expression in medullary thyroid carcinoma and pheochromocytoma: a clinical and molecular study

BACKGROUND

Imaging of metastatic sites of medullary thyroid carcinoma (MTC) is successful in less than 60% of cases of residual or recurrent disease. Positron emission tomography (PET) with [18F]fluoro-2-deoxy-D-glucose (FDG) takes advantage of the fact that malignant tumors are capable of increased uptake and use of glucose, which is mediated by the members of the glucose transporter family of proteins (GLUT 1 through GLUT 5).

METHODS

FDG-PET images of 10 patients with recurrent or persistent MTC after primary operation were compared with images by computed tomography or magnetic resonance imaging. Identified metastatic lesions were assessed by intraoperative findings and pathology reports. Expression of GLUT 1 through GLUT 5 was examined by Western blot analysis of tumor tissue from eight of the patients evaluated and an additional panel of 10 MTCs and seven pheochromocytomas.

RESULTS

FDG-PET identified 31 foci of FDG accumulation in 10 patients, and 16 of these metastatic sites were resected and confirmed by histologic analysis. Only 11 foci were demonstrated by computed tomographic or magnetic resonance imaging. None of the glucose transporters examined displayed significant expression. Two pheochromocytomas were successfully imaged by FDG-PET.

CONCLUSIONS

FDG-PET imaging can be useful in the localization of cervicomediastinal MTC metastases and pheochromocytoma. The increased glucose uptake in these tumors, as evidenced by FDG-PET, does not appear to be attributable to the expression of the glucose transporter proteins GLUT 1 through GLUT 5.