Angiotensin II stimulates glucose transport activity in cultured vascular smooth muscle cells

Differential Effects of 25-Hydroxyvitamin D3 versus 1α 25-Dihydroxyvitamin D3 on Adipose Tissue Browning in CKD-Associated Cachexia

Patients with chronic kidney disease (CKD) often have low serum concentrations of 25(OH)D3 and 1,25(OH)2D3. We investigated the differential effects of 25(OH)D3 versus 1,25(OH)2D3 repletion in mice with surgically induced CKD. Intraperitoneal supplementation of 25(OH)D3 (75 μg/kg/day) or 1,25(OH)2D3 (60 ng/kg/day) for 6 weeks normalized serum 25(OH)D3 or 1,25(OH)2D3 concentrations in CKD mice, respectively. Repletion of 25(OH)D3 normalized appetite, significantly improved weight gain, increased fat and lean mass content and in vivo muscle function, as well as attenuated elevated resting metabolic rate relative to repletion of 1,25(OH)2D3 in CKD mice. Repletion of 25(OH)D3 in CKD mice attenuated adipose tissue browning as well as ameliorated perturbations of energy homeostasis in adipose tissue and skeletal muscle, whereas repletion of 1,25(OH)2D3 did not. Significant improvement of muscle fiber size and normalization of fat infiltration of gastrocnemius was apparent with repletion of 25(OH)D3 but not with 1,25(OH)2D3 in CKD mice. This was accompanied by attenuation of the aberrant gene expression of muscle mass regulatory signaling, molecular pathways related to muscle fibrosis as well as muscle expression profile associated with skeletal muscle wasting in CKD mice. Our findings provide evidence that repletion of 25(OH)D3 exerts metabolic advantages over repletion of 1,25(OH)2D3 by attenuating adipose tissue browning and muscle wasting in CKD mice.

The Ang III/AT2R Pathway Enhances Glucose Uptake by Improving GLUT1 Expression in 3T3-L1 Adipocytes

Angiotensin III (Ang III) is a heptapeptide derived from Ang II that has been confirmed as the preferred agonist of angiotensin II type 2 receptor (AT2R). Recent studies have revealed AT2R mainly exerts anti-inflammation effects. However, the effects of the Ang III/AT2R pathway on adipocytes remain unknown. Here, the effects of Ang III on glucose uptake were examined. The results showed that AT2R expression was upregulated during adipogenesis in 3T3-L1 preadipocytes, whereas AT1R expression was diminished. Also, Ang III (10 nM) significantly increased glucose uptake by 3T3-L1 adipocytes, which was blocked by PD123319, an AT2R blocker, but not by irbesartan, an AT1R blocker. Ang III also induced the expression of glucose transporter type 1 (GLUT1). These stimulatory effects were inhibited by pretreatment with PD123319, but not with irbesartan. Together, these results indicate that Ang III enhances glucose uptake by upregulating GLUT1 expression via AT2R.

Expression of conventional and novel glucose transporters, GLUT1, -9, -10, and -12, in vascular smooth muscle cells

Intimal hyperplasia is characterized by exaggerated proliferation of vascular smooth muscle cells (VSMCs). Enhanced VSMC growth is dependent on increased glucose uptake and metabolism. Facilitative glucose transporters (GLUTs) are comprised of conventional GLUT isoforms (GLUT1-5) and novel GLUT isoforms (GLUT6-14). Previous studies demonstrate that GLUT1 overexpression or GLUT10 downregulation contribute to phenotypic changes in VSMCs. To date, the expression profile of all 14 GLUT isoforms has not been fully examined in VSMCs. Using the proliferative and differentiated phenotypes of human aortic VSMCs, the present study has determined the relative abundance of GLUT1-14 mRNAs by quantitative real-time PCR analysis. Twelve GLUT mRNAs excluding GLUT7 and GLUT14 were detectable in VSMCs. In the proliferative phenotype, the relative abundance of key GLUT mRNAs was GLUT1 (∼43%)>GLUT10 (∼26%)>GLUT9 (∼13%)>GLUT12 (∼4%), whereas in the differentiated phenotype the relative abundance was GLUT10 (∼28%)>GLUT1 (∼25%)>GLUT12 (∼20%)>GLUT9 (∼14%), together constituting 86-87% of total GLUT transcripts. To confirm the expression of key GLUT proteins, immunoblot and immunocytochemical analyses were performed using GLUT isoform-specific primary antibodies. The protein bands characteristic of GLUT1, -9, -10, and -12 were detected in VSMCs in parallel with respective positive controls. In particular, GLUT1 protein expression showed different molecular forms representative of altered glycosylation. While GLUT1 protein displayed a predominant distribution in the plasma membrane, GLUT9, -10, and -12 proteins were mostly distributed in the intracellular compartments. The present study provides the first direct evidence for GLUT9 and GLUT12 expression in VSMCs in conjunction with the previously identified GLUT1 and GLUT10.

Hypoxia but not inflammation augments glucose uptake in human macrophages: Implications for imaging atherosclerosis with 18fluorine-labeled 2-deoxy-D-glucose positron emission tomography

OBJECTIVES

This study investigated the regulation of glucose uptake in cells that participate in atherogenesis by stimuli relevant to this process, to gain mechanistic insight into the origin of the (18)fluorine-labeled 2-deoxy-D-glucose (FdG) uptake signals observed clinically.

BACKGROUND

Patient studies suggest that positron emission tomography (PET) using FdG can detect "active" atherosclerotic plaques, yet the mechanism giving rise to FdG signals remains unknown.

METHODS

We exposed cells to conditions thought to operate in atheroma and determined rates of glucose uptake.

RESULTS

Hypoxia, but not pro-inflammatory cytokines, potently stimulated glucose uptake in human macrophages and foam cells. Statins attenuated this process in vitro, suggesting that these agents have a direct effect on human macrophages. Immunohistochemical study of human plaques revealed abundant expression of proteins regulating glucose utilization, predominantly in macrophage-rich regions of the plaques-regions previously proved hypoxic. Smooth-muscle cells and endothelial cells markedly increased rates of glucose uptake when exposed to pro-inflammatory cytokines.

CONCLUSIONS

Glucose uptake and, probably, FdG uptake signals in atheroma may reflect hypoxia-stimulated macrophages rather than mere inflammatory burden. Cytokine-activated smooth-muscle cells also may contribute to the FdG signal.

Regulation of glucose uptake in mesangial cells stimulated by high glucose: role of angiotensin II and insulin

Mesangial cells (MCs) play a central role in the pathogenesis of diabetic nephropathy (DN). MC dysfunction arises from excessive glucose uptake through insulin-independent glucose transporter (GLUT1). The role of the insulin-dependent transporter (GLUT4) remains unknown. This study evaluated the effect of high glucose on GLUT1, GLUT4, and fibronectin expression levels. Glucose uptake was determined in the absence and presence of insulin. Angiotensin II has been implicated as a mediator of MC abnormalities in DN, and its effects on the GLUTs expression were evaluated in the presence of losartan. MCs were exposed to normal (NG, 10 mM) or high (HG, 30 mM) glucose for 1, 4, 12, 24, and 72 hrs. Glucose uptake was elevated from 1 hr up to 24 hrs of HG, but returned to NG levels after 72 hrs. HG induced an early (1-, 4-, and 12-hrs) rise in GLUT1 expression, returning to NG levels after 72 hrs, whereas GLUT4 was overexpressed at later timepoints (24 and 72 hrs). HG during 4 hrs induced a 40% rise in glucose uptake, which was unaffected by insulin. In contrast, after 72 hrs, glucose uptake was increased by 50%, only under insulin stimulus. Losartan blunted the effects of HG on GLUT1, GLUT4, and fibronectin expression and on glucose uptake. Results suggest that MCs can be highly susceptible to the HG environment since they uptake glucose in both an insulin-independent and insulin-dependent manner. The beneficial effects of angiotensin II inhibition in DN may also involve a decrease in the rate of glucose uptake by MCs.

Mechanism of high glucose induced angiotensin II production in rat vascular smooth muscle cells

Angiotensin II (Ang II), a circulating hormone that can be synthesized locally in the vasculature, has been implicated in diabetes-associated vascular complications. This study was conducted to determine whether high glucose (HG) (approximately 23.1 mmol/L), a diabetic-like condition, stimulates Ang II generation and the underlying mechanism of its production in rat vascular smooth muscle cells. The contribution of various enzymes involved in Ang II generation was investigated by silencing their expression with small interfering RNA in cells exposed to normal glucose (approximately 4.1 mmol/L) and HG. Angiotensin I (Ang I) was generated from angiotensinogen by cathepsin D in the presence of normal glucose or HG. Although HG did not affect the rate of angiotensinogen conversion, it decreased expression of angiotensin-converting enzyme (ACE), downregulated ACE-dependent Ang II generation, and upregulated rat vascular chymase-dependent Ang II generation. The ACE inhibitor captopril reduced Ang II levels in the media by 90% in the presence of normal glucose and 19% in HG, whereas rat vascular chymase silencing reduced Ang II production in cells exposed to HG but not normal glucose. The glucose transporter inhibitor cytochalasin B, the aldose reductase inhibitor alrestatin, and the advanced glycation end product formation inhibitor aminoguanidine attenuated HG-induced Ang II generation. HG caused a transient increase in extracellular signal-regulated kinase (ERK)1/2 phosphorylation, and ERK1/2 inhibitors reduced Ang II accumulation by HG. These data suggest that polyol pathway metabolites and AGE can stimulate rat vascular chymase activity via ERK1/2 activation and increase Ang II production. In addition, decreased Ang II degradation, which, in part, could be attributable to a decrease in angiotensin-converting enzyme 2 expression observed in HG, contributes to increased accumulation of Ang II in vascular smooth muscle cells by HG.

The link between Glut-1 and hypertension in diabetic nephropathy

Nephropathy is a major diabetic microvascular complication; both metabolic and hemodynamic perturbations play critical roles in its occurrence and progression toward end-stage renal disease. Improvements in metabolic and blood pressure control have been shown to confer protection from this diabetic complication. In this article, we review the facilitative glucose transporter Glut-1, its regulation, and its potential role in linking metabolic and hemodynamic perturbations in the pathophysiologic processes that lead to kidney injury in diabetes. We propose that an auto-maintaining mechanism of hemodynamic perturbations and increased tissue angiotensin II may be involved in the initiation and maintenance of a loop in which transforming growth factor beta1 and Glut-1 upregulation play important roles in the pathophysiology of diabetic-induced kidney lesions. The understanding of the molecular mechanisms that link glomerular hypertension and excessive glucose metabolism may provide insight into new therapeutic strategies for the treatment of diabetic renal disease.

Thrombin-induced glucose transport via Src-p38 MAPK pathway in vascular smooth muscle cells

Thrombin is a mitogen for vascular smooth muscle cells (VSMC) and has been implicated in the development in atherosclerosis. However, little is known about the role of thrombin in glucose transport in VSMC. In this study, we examined the effect of thrombin on glucose uptake in rat A10 VSMC. We found that thrombin induced glucose uptake in a dose-dependent manner while hirudin, a potent thrombin inhibitor, prevented glucose uptake in the cells. PP2, a selective inhibitor of Src, prevented the thrombin-induced glucose uptake, but did not affect insulin-induced uptake. We also examined whether mitogen-activated protein kinase (MAPK) inhibitors influenced thrombin-induced glucose uptake. The p38 MAPK inhibitor (SB203580) inhibited thrombin-induced glucose uptake, but the MEK inhibitor (PD98059) did not. In contrast to thrombin, SB203580 did not affect insulin-induced glucose uptake. Furthermore, thrombin failed to translocate the insulin-sensitive glucose transporter GLUT4. These findings suggest that thrombin stimulates glucose transport via Src and subsequent p38 MAPK activation in VSMC.

ANG II increases 2-deoxyglucose uptake in mouse embryonic stem cells

It is now suggested that all components of the renin-angiotensin system are present in many tissues, including the embryo and may play a major role in embryo development and differentiation. However, little is known regarding whether ANG II regulates glucose transport in mouse embryonic stem (ES) cells. Thus, the effects of ANG II on [3H]-2-deoxyglucose (2-DG) uptake and its related signal pathways were examined in mouse ES cells. ANG II significantly increased cell proliferation and 2-DG uptake in concentration- and time-dependent manner (>18 h, >10(-8) M) and increased mRNA and protein level of GLUT1 by 31+/-7% and 22+/-5% compared to control, respectively. Actinomycin D and cycloheximide completely blocked the effect of ANG II on 2-DG uptake. ANG II-induced increase of 2-DG uptake was blocked by losartan, an ANG II type 1 (AT1) receptor blocker, but not by PD 123319, an ANG II type 2 (AT2) receptor blocker. In addition, ANG II-induced stimulation of 2-DG uptake was attenuated by phospholipase C (PLC) inhibitors, neomycin and U 73122 and ANG II increased inositol phosphates (IPs) formation by 37+/-8% of control. Protein kinase C (PKC) inhibitors, staurosporine, bisindolylmaleimide I, and H-7 also blocked ANG II-induced stimulation of 2-DG uptake. Indeed, ANG II activated a PKC translocation from the cytosolic to membrane fraction, suggesting a role of PKC. A 23187 (Ca2+ ionophore) increased 2-DG uptake and nifedifine (L-type Ca2+ channel blocker) blocked it. In conclusion, ANG II increased 2-DG uptake by PKC activation via AT1 receptor in mouse ES cells.

Agents in development for the treatment of diabetic nephropathy

Diabetic nephropathy is a leading cause of end-stage renal disease, and accounts for significant morbidity and mortality in patients with diabetes. Diabetic nephropathy seems to occur as a result of an interaction between metabolic and haemodynamic factors, which activate common pathways that lead to renal damage. In the past, the treatment of diabetic nephropathy has focused on the control of hyperglycaemia. Newer targets, some of which are linked to glucose-dependent pathways, appear to be a major focus of new treatments directed against the development and progression of renal damage as a result of diabetes. It is anticipated that additional therapeutic approaches that inhibit both metabolic and haemodynamic pathways will include strategies that target growth factors, cytokines and intracellular second messengers. Such an approach is expected to lead to improved therapies for the treatment of diabetic nephropathy.

Neurobarrier coupling in the brain: a partner of neurovascular and neurometabolic coupling?

Neurovascular and neurometabolic coupling help the brain to maintain an appropriate energy flow to the neural tissue under conditions of increased neuronal activity. Both coupling phenomena provide us, in addition, with two macroscopically measurable parameters, blood flow and intermediate metabolite fluxes, that are used to dynamically image the functioning brain. The main energy substrate for the brain is glucose, which is metabolized by glycolysis and oxidative breakdown in both astrocytes and neurons. Neuronal activation triggers increased glucose consumption and glucose demand, with new glucose being brought in by stimulated blood flow and glucose transport over the blood-brain barrier. Glucose is shuttled over the barrier by the GLUT-1 transporter, which, like all transporter proteins, has a ceiling above which no further stimulation of the transport is possible. Blood-brain barrier glucose transport is generally accepted as a nonrate-limiting step but to prevent it from becoming rate-limiting under conditions of neuronal activation, it might be necessary for the transport parameters to be adapted to the increased glucose demand. It is proposed that the blood-brain barrier glucose transport parameters are dynamically adapted to the increased glucose needs of the neural tissue after activation according to a neurobarrier coupling scheme. This review presents neurobarrier coupling within the current knowledge on neurovascular and neurometabolic coupling, and considers arguments and evidence in support of this hypothesis.

RNA interference-mediated reduction in GLUT1 inhibits serum-induced glucose transport in primary human skeletal muscle cells

Using RNA interference (RNAi), we specifically down-regulate protein expression in differentiated human skeletal myotube cultures. Serum stimulation of myotubes increases glucose uptake. Using a sensitive photolabeling technique, we demonstrate that this increase in glucose uptake is accompanied by increased cell-surface content of glucose transporter (GLUT) 1. Using RNAi, we specifically reduce GLUT1 mRNA and protein expression, leading to inhibition of serum-mediated increase in glucose transport. Thus, we demonstrate the utility of RNAi in a primary human differentiated cell system, and apply this methodology to demonstrate that serum-mediated increase in glucose transport in human skeletal muscle cells is dependent on GLUT1.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

The role of the angiotensin system in cardiac glucose homeostasis: therapeutic implications

Resistance to the metabolic actions of insulin is thought to play a determining role in the aetiology of a great variety of disorders, including essential hypertension, accelerated atherosclerosis and cardiomyopathies. ACE inhibitors are recognised as being highly effective therapy for hypertension and cardiac insufficiency, and have a more beneficial effect on survival rate than expected on the basis of known mechanisms of action. The mechanism responsible for these extremely positive effects are just beginning to be understood and appear to be linked to the effects these drugs have on metabolism. The relationship between the insulin and angiotensin II (Ang II) signalling pathways needs to be fully clarified in order to prevent or correct the target organ damage resulting from changes in the cross-talk of these two hormonal systems. In recent years, Ang II has been shown to play a central role in cardiovascular and neuroendocrine physiology as well as in cellular cycle control. Moreover, the fact that Ang II utilises the insulin-receptor substrate (IRS)-1 to relay signals towards their intracellular destination, provides the biochemical explanation of how these two systems interact in a healthy organism and in a diseased one. Since it is overactivity of the renin-angiotensin system that seems to impair the intracellular response to insulin signalling, cardiovascular drugs that modulate the cellular transmission of Ang II have attracted particular interest. As well as the already widely-used ACE inhibitors, selective blockers of the Ang II type 1 receptor (AT(1)) have been shown to be clinically effective in the control of haemodynamic parameters, but with perhaps a less striking effect on glucose homeostasis. Many trials have investigated the effect of Ang II blockade on systemic glucose homeostasis. The inhibition of Ang II by ACE-inhibitors frequently showed a positive effect on glycaemia and insulin sensitivity, while information on the effects of AT(1) receptor antagonists on glucose homeostasis is more limited and controversial. An important limitation of these studies has been the short treatment and follow-up periods, even for the 'so called' long-term studies which were only 6 months. Several investigators have focused on the effects of the nuclear factors involved in gene transcriptions, especially with respect to the agonists/antagonists of peroxisome proliferator-activated receptors (PPARs) and their intriguing interconnections with the insulin and Ang II subcellular pathways. In fact, in vitro and in vivo experimental studies have shown that thiazolidinediones (selective PPAR-gamma ligands) are not only powerful insulin sensitisers, but also have anti-hypertensive and anti-atherosclerotic properties. In addition to conventional pharmacological approaches, attempts have been made to use genetic transfer in the treatment of cardiovascular and metabolic disorders. The development of powerful viral vectors carrying target genes has allowed us to restore the expression/function of specific proteins involved in the cellular mechanism of insulin resistance, and research now needs to move beyond animal models. Although a clearer picture is now emerging of the pathophysiological interaction between insulin and Ang II, especially from pre-clinical studies, there is much to be done before experimental findings can be used in daily clinical practice.

Semicarbazide-sensitive amine oxidase in vascular smooth muscle cells: differentiation-dependent expression and role in glucose uptake

Cultured vascular smooth muscle cells (VSMCs) derived from rat aortic media were used to examine semicarbazide-sensitive amine oxidase (SSAO) expression during their differentiation process. In a defined serum-free medium permissive for in vitro VSMC differentiation, there was a large increase in SSAO mRNA and protein levels and in the related enzyme activity during the course of cell culture. This pattern of expression was concomitant with that of some smooth muscle-specific mRNA markers of differentiation. mRNAs in differentiated cultured VSMCs were comparable to those detected in total aorta and media. Pharmacological properties of SSAO present in VSMCs were similar to enzyme activities previously described in the aortic wall. In this model, we also demonstrated that methylamine, a physiological substrate of SSAO, activated 2-deoxyglucose transport in a time- and dose-dependent manner. This methylamine effect was reproduced by other SSAO substrates and was prevented by the SSAO inhibitor semicarbazide. It was antagonized in the presence of catalase, suggesting that SSAO-activated glucose transport was mediated through H(2)O(2) production. In addition, methylamine promoted glucose transporter 1 accumulation at the cell surface. Thus, we demonstrate for the first time the differentiation-dependent expression of SSAO in VSMCs and its role in the regulation of VSMC glucose uptake.

Interleukin-1 beta and tumor necrosis factor-alpha stimulate the cat-2 gene of the L-arginine transporter in cultured vascular smooth muscle cells

The production of nitric oxide (NO) from L-arginine by nitric oxide synthase (NOS) in cytokine-stimulated vascular smooth muscle cells (VSMC) is thought to play an important role in the pathophysiology of several vascular disease states including septic shock. This study examines the relationship between cytokine-stimulated NO production and L-arginine transport in cultured VSMC. Cultured VSMC from rat aorta were stimulated with interleukin-1 beta, tumor necrosis factor-alpha, and/or angiotensin II (Ang II); and the accumulation of nitrite, a stable product of NO metabolism, in the culture media and the rates of net L-arginine uptake were measured. Interleukin-1 beta and tumor necrosis factor-alpha, alone or in combination, stimulated both the uptake of L-arginine and the accumulation of nitrite in the culture media in a dose-dependent manner. Inhibition of NOS activity by substituted analogues of L-arginine had no effect on cytokine-stimulated L-arginine transport. Ang II in the presence of cytokines up-regulated L-arginine transport while inhibiting nitrite accumulation. Two forms of the L-arginine transporter, cat-1b and cat-2, are expressed in VSMC. Northern analysis revealed that the cytokine-stimulated increase in L-arginine transport coincided with increased levels of cat-2 mRNA. In contrast, cat-1b does not appear to be regulated by cytokines at the mRNA level, although significant increases in response to Ang II were observed. These results show that, while cytokines can stimulate both NOS activity and L-arginine uptake, NO production is not required to signal the increase in L-arginine transport. Furthermore, Ang II and cytokine stimulation of L-arginine uptake involves the differential regulation of the cationic amino acid transporter (cat) genes.

Down-regulation of glucose transport by elevated extracellular glucose concentrations in cultured rat aortic smooth muscle cells does not normalize intracellular glucose concentrations

Vascular disease is a prominent complication of diabetes mellitus, and hyperglycemia has been implicated as a risk factor for the development of these vascular complications. It has previously been suggested that down-regulation of glucose transport in response to hyperglycemia might serve a protective role by decreasing intracellular glucose concentrations. In the present study, regulation of glucose transport by extracellular glucose concentrations was investigated in cultured rat vascular smooth muscle cells (VSMCs). Confluent quiescent VSMCs were exposed to medium containing either normal (5 mmol/L) or elevated (20 mmol/L) extracellular glucose concentrations for 24 hours. VSMCs exposed to elevated extracellular glucose concentrations (with or without serum) for 24 hours exhibited significant decreases in 2-deoxyglucose (2-DG) and D-glucose uptake rates. This decreased glucose transport was associated with a decrease in the Vmax of D-glucose transport without a change in KM. In the absence of serum, a decrease in the quantity of GLUT-1 transport protein at the plasma membrane was noted in cells exposed to elevated extracellular glucose concentrations for 24 hours. Intracellular glucose concentrations were estimated by using two methods, and the results revealed significantly higher intracellular glucose concentrations in the cells exposed to elevated extracellular glucose concentrations for 24 hours. These results suggest that down-regulation of glucose transport in cultured VSMCs exposed to elevated extracellular glucose concentrations for 24 hours does not occur to an extent that normalizes intracellular glucose concentrations. This prolonged increase in intracellular glucose concentrations and the potential associated toxicity may explain the increased incidence of vascular complications in patients with diabetes mellitus.

Angiotensin II stimulates system y+ and cationic amino acid transporter gene expression in cultured vascular smooth muscle cells

The effect of angiotensin II (Ang II) on the transport of cationic amino acids has been examined in vascular smooth muscle cells (VSMC) isolated from rat aortae. Ang II stimulated the uptake rates of radiolabeled arginine and lysine in a time- and concentration-dependent manner. The stimulated arginine uptake could be blocked by pretreatments with cycloheximide and actinomycin D or co-treatment with valsartan, an antagonist specific for Ang II receptor subtype-1. The modulation by Ang II was bidirectional as the efflux of arginine was also stimulated, 5-fold over basal. Using reverse transcription-coupled polymerase chain reaction methodology, a partial cDNA with 94% sequence identity to that of cationic amino acid transporter subtype-1 (CAT-1) of mouse fibroblasts was obtained from VSMC. This sequence also exhibited 14 base changes compared with the sequence of ecotropic retrovirus receptor (ERR)/CAT-1 from rat hepatoma. Northern analyses with this partial CAT-1 cDNA and CAT-2 cDNA of mouse T-lymphocytes showed that Ang II rapidly stimulated the expression of both CAT-1 and CAT-2 in VSMC. Both signals peaked at 2 h after exposure to Ang II. The CAT-1 signal decayed over the next 6 h to levels 3-fold above basal, which are maintained up until 24 h. The induced CAT-2 mRNA concentration also decayed rapidly but increased again between 16 and 24 h to levels comparable with those observed at 2 h.

Angiotensin II increases glucose uptake and glucose transporter-1 mRNA levels in astroglia

In this study, we have investigated the effects of angiotensin II (ANG II) on glucose uptake into astroglia cultured from adult rat hypothalamus and brain stem. ANG II (30 min to 4 h; 10(-9) to 10(-6) M) stimulated time- and concentration-dependent increases in the uptake of 2-deoxy-D-[3H]glucose into cultured astroglia. This effect of ANG II (10(-7) M) is via AT1 receptors and protein kinase C (PKC), since it was inhibited by losartan (10(-6) M) and staurosporine (10(-6) M), respectively. Furthermore, this ANG II action was inhibited by both cycloheximide (1 microgram/ml) and actinomycin D (10(-6) M), indicating that synthesis of new glucose transporters is involved. This was confirmed by the finding that ANG II (30 min to 4 h; 10(-9) to 10(-5) M) stimulated time- and concentration-dependent increases in the steady-state levels of glucose transporter-1 (GLUT-1) mRNA in these cultures. In addition, the increase in steady-state levels of GLUT-1 mRNA elicited by ANG II was mediated by AT1 receptors and PKC. These data suggest that ANG II stimulates glucose uptake into cultured astroglia via a pathway that involves AT1 receptors, PKC, and increased steady-state levels of GLUT-1 mRNA.

Endothelin mobilizes calcium and enhances glucose uptake in cultured human skeletal myoblasts and L6 myotubes

In this study we used endothelin as a paradigm to explore the concept that some vasoactive agents, acting through mobilization of Ca2+ and stimulation of protein kinase C, can interact with human skeletal muscle and modify its glucose transport. Cultured human skeletal myoblasts from the vastus lateralis demonstrated two subclasses of high-affinity endothelin receptors and a robust increase in cytosolic free Ca2+ upon exposure to endothelin. The endothelin-evoked rise in cytosolic free Ca2+ primarily resulted from Ca2+ mobilization from intracellular organelles. Both endothelin and insulin enhanced [3H]deoxy-D-glucose uptake in human myoblasts, but their effects were not additive. These findings also were observed in differentiated myotubes of L6 skeletal muscle cells. Moreover, [3H]deoxy-D-glucose uptake in human myoblasts was enhanced by treatment with phorbol 12-myristate 13-acetate. The endothelin- and insulin-mediated increases in [3H]deoxy-D-glucose were totally ablated by treatment with calphostin C. Such observations suggest that endothelin can enhance glucose uptake in human skeletal muscle. This is mediated through mechanisms that are at least partially protein kinase C dependent. Thus, increased levels of endothelin in vascular beds may contribute to altered glucose metabolism in essential hypertension.

Regulation of cell proliferation and growth by angiotensin II

The peptide hormone angiotensin II (AngII) has clearly defined physiologic roles as a regulator of vasomotor tone and fluid homeostasis. In addition AngII has trophic or mitogenic effects on a variety of target tissues, including vascular smooth muscle and adrenal cells. More recent data indicate that AngII exhibits many characteristics of the 'classical' peptide growth factors such as EGF/TGF alpha, PDGF and IGF-1. These include the capacity for local generation ('autocrine or paracrine' action) and the ability to stimulate tyrosine phosphorylation, to activate MAP kinases and to increase expression of nuclear proto-oncogenes. The type 1 AngII receptor, which is responsible for all known physiologic actions of AngII, has been cloned. Activation of this receptor leads to elevated phosphoinositide hydrolysis, mobilization of intracellular Ca2+ and diacylglycerol, and activation of Ca2+/calmodulin and Ca2+/phospholipid-dependent Ser/Thr kinases, as well as Ca2+ regulated tyrosine kinases. The existence of other AngII receptor subtypes has been postulated, but the function(s) of these sites remains unclear. In vascular smooth muscle, AngII can promote cellular hypertrophy and/or hyperplasia, depending in part on the patterns of induction of secondary factors that are known to stimulate (PDGF, IGF-1, basic FGF) or inhibit (TGF-beta) mitosis. Together, these findings have suggested that AngII plays important roles in both the normal development and pathophysiology of vascular, cardiac, renal and central nervous system tissues.

Growth factors, mitogens, oncogenes and the regulation of glucose transport

The erythrocyte (or HepG2/brain) type glucose transporter (GLUT 1) was the first of the family of facilitative glucose transporter proteins to be cloned [M. Mueckler et al., Science 229, 941-945, 1985]. GLUT 1 is expressed in most tissue types, all cell lines, transformed cells and tumour cells. It is thought to be responsible for "housekeeping" levels of glucose transport, i.e. the uptake of glucose required for oxidative phosphorylation. The rate of glucose transport via GLUT 1 can be regulated under conditions in which the metabolic rate must be adjusted such as cell division (mitosis and meiosis), differentiation, transformation and nutrient starvation. Here we review the recent literature on the control of glucose transport of mitogens, growth factors and oncogenes, and discuss some of the implications for the integration of cellular signalling pathways and cell growth.

Characterization of system L and system y+ amino acid transport activity in cultured vascular smooth muscle cells

The uptake of L-leucine and L-lysine into vascular smooth muscle cells cultured from the aortas of rats has been investigated. Both amino acids are taken up by saturable systems that are independent of the presence of a Na+ gradient and can be stimulated in trans by neutral bulky amino acids for leucine and cationic amino acids for lysine. Leucine uptake is inhibited competitively in cis by several neutral amino acids, whereas lysine uptake is inhibited strongly by other cationic amino acids but also significantly by neutral amino acids such as leucine. The leucine inhibition is noncompetitive. Cells preloaded with leucine and lysine could also export these amino acids and the rate of efflux was stimulated by the presence of appropriate amino acids in trans. These data are all consistent with leucine being transported largely if not entirely by System L and lysine by the System y+ transporter.