Prolonged treatment with the beta3-adrenergic agonist CL 316243 induces adipose tissue remodeling in rat but not in guinea pig: 2) modulation of glucose uptake and monoamine oxidase activity

Beta3-adrenergic agonists are well-recognited to promote lipid mobilisation and adipose tissue remodeling in rodents, leading to multilocular fat cells enriched in mitochondria. However, effects of beta3-adrenergic agonists on glucose transport are still controversial. In this work, we studied in white adipose tissue (WAT) the influence of sustained beta3-adrenergic stimulation on the glucose transport and on the mitochondrial monoamine oxidase (MAO) activity. As one-week administration of CL 316243 (CL, 1 mg/kg/d) induces beta-adrenergic desensitization in rat but not in guinea pig adipocytes, attention was paid to compare these models. When expressing glucose uptake as nmoles of 2-deoxyglucose/100 mg cell lipids, maximally stimulated uptake was increased in adipocytes of WAT from treated rats but not from treated guinea pigs. However, basal hexose uptake was also increased in CL-treated rats and, as a consequence, the dose-dependent curves for insulin stimulation were similar in control and CL-treated rats when expressed as fold increase over basal. Insulin-induced lipogenesis was unchanged in rat or guinea pig adipocytes after CL-treatment. The glucose carriers GLUT4 and corresponding mRNA were increased in subcutaneous WAT or in brown adipose tissue (BAT) but not in visceral WAT or muscles of CL-treated rats. There was an increase of MAO activity in WAT and BAT, but not in liver, of CL-treated rats while no change was detected in guinea pigs. These findings show that only rat adipocytes, which are beta3-adrenergic-responsive, respond to chronic beta3-AR agonist by an increase of GLUT4 content and MAO activity, despite a desensitization of all beta-adrenoceptor subtypes.

The release of soluble VAP-1/SSAO by 3T3-L1 adipocytes is stimulated by isoproterenol and low concentrations of TNFalpha

Plasma level of the protein VAP-1/SSAO (Vascular Adhesion Protein-1/Semicarbazide-Sensitive Amine Oxidase) is increased in diabetes and/or obesity and may be related to vascular complications associated to these pathologies. The aim of this work was to complete a preceding study where we described the role played by some hormones or metabolites, implicated in diabetes and/or obesity, in the regulation of the release of VAP-1/SSAO by 3T3-L1 adipocytes. Here we focused on the previously observed effect produced by TNFalpha in the release of VAP-1/SSAO and studied the effect of a beta-adrenergic compound, isoproterenol. Both compounds stimulated the release of VAP-1/SSAO to the culture medium but had a different effect on the VAP-1/SSAO membrane form. While TNFalpha produced a decrease on VAP-1/SSAO membrane form content, isoproterenol did not modify it. We thus observed two different ways of regulation of the release of VAP-1/SSAO by 3T3-L1 adipocytes by metabolites implicated in diabetes and adipose tissue physiopathology. Our work permits a better understanding of this increased plasma VAP-1/SSAO levels observed in diabetes.

Tyramine and benzylamine partially but selectively mimic insulin action on adipose differentiation in 3T3-L1 cells

Biogenic amines like tyramine, methylamine and the non-naturally occuring amine, benzylamine, have been described to promote adipose conversion of murine 3T3 preadipocytes. To further investigate these novel effects of amines, we studied whether they selectively mimic the long-term adipogenic action of insulin. To this aim, we decided to use the 3T3-L1 cell line since this model needs a complex combination of inducers to trigger the differentiation programme: insulin, isobutylmethylxanthine (IBMX, an activator of cAMP-signal transduction pathway) and the synthetic glucocorticoid, dexamethasone. A cell culture protocol was designed, by which each component of the differentiation cocktail was replaced with either benzylamine or tyramine, in order to determine whether these amine oxidase substrates could substitute any of the differentiation inducers in 3T3-L1 cells. The incomplete lipid accumulation found in cells grown under IBMX- or dexamethasone-free conditions was not improved by the daily addition of amines to the culture medium. Insulin was the only component of adipose differentiation cocktail of 3T3-L1 that could be replaced, although partially, by tyramine or benzylamine. When used at 0.5 mM, these amines resulted in a significant increase of triacylglycerol accumulated eight days after confluence, when compared to cells kept without insulin. This partial insulin replacement was totally abolished by SSAO-inhibitors, while MAO-blockade did not reduce lipid accumulation. As previously reported for other insulin-sensitive processes, such as stimulation of glucose transport or lipolysis inhibition in mature adipocytes, the stimulation of adipogenesis by tyramine and benzylamine was an SSAO-dependent mechanism that apparently shared common signaling pathways with insulin.

Stimulation of glucose transport by semicarbazide-sensitive amine oxidase activity in adipocytes from diabetic rats

Semicarbazide-sensitive amine oxidase (SSAO) is highly expressed in adipose cells, and substrates of SSAO such as benzylamine in combination with low concentrations of vanadate strongly stimulate glucose transport and GLUT4 recruitment in mouse 3T3-L1 adipocytes and in isolated rat adipocytes. Here we examined whether this combination of molecules also stimulates glucose transport in adipocytes from streptozotocin-induced diabetic rats and from Goto-Kakizaki diabetic rats. As previously reported, adipocytes obtained from streptozotocin-induced diabetic rats, showed a reduced stimulation of glucose transport in response to insulin. Under these conditions, the combination of benzylamine and vanadate caused a marked stimulation of glucose transport that was similar to the stimulation detected in control adipocytes. Adipocytes isolated from Goto-Kakizaki diabetic rats also showed a defective response to insulin; however, acute incubation in the presence of benzylamine and vanadate stimulated glucose transport in these cells to the same extent than in adipocytes from non-diabetic rats. These data indicate that adipocytes obtained from two different models of animal diabetes do not show resistance to the activation of glucose transport by SSAO activity, which is in contrast to the well reported resistance to insulin action. It seems to suggest that SSAO activity in combination with vanadate triggers a glucose transport-activating intracellular pathway that remains intact in the diabetic state. Further, our data support the view that the combination of benzylamine and vanadate could be an effective therapy in diabetes.

Combined treatment with benzylamine and low dosages of vanadate enhances glucose tolerance and reduces hyperglycemia in streptozotocin-induced diabetic rats

Semicarbazide-sensitive amine oxidase (SSAO) is highly expressed in adipose cells, and substrates of SSAO, such as benzylamine, in combination with low concentrations of vanadate strongly stimulate glucose transport and GLUT4 recruitment in 3T3-L1 and rat adipocytes. Here we examined whether acute and chronic administration of benzylamine and vanadate in vivo enhances glucose tolerance and reduces hyperglycemia in diabetic rats. Acute intravenous administration of these drugs enhanced glucose tolerance in nondiabetic rats and in streptozotocin (STZ)-induced diabetic rats. This occurred in the absence of changes in plasma insulin concentrations. However, the administration of benzylamine or vanadate alone did not improve glucose tolerance. The improvement caused by benzylamine plus vanadate was abolished when rats were pretreated with the SSAO-inhibitor semicarbazide. Chronic administration of benzylamine and vanadate exerted potent antidiabetic effects in STZ-induced diabetic rats. Although daily administration of vanadate alone (50 and 25 micromol x kg(-1) x day(-1) i.p.) for 2 weeks had little or no effect on glycemia, vanadate plus benzylamine reduced hyperglycemia in diabetic rats, enhanced basal and insulin-stimulated glucose transport, and upregulated GLUT4 expression in isolated adipocytes. In all, our results substantiated that acute and chronic administration of benzylamine with low dosages of vanadate have potent antidiabetic effects in rats.

Amine oxidase substrates mimic several of the insulin effects on adipocyte differentiation in 3T3 F442A cells

We have previously reported that substrates of monoamine oxidase (MAO) and semicarbazide-sensitive amine oxidase (SSAO) exert short-term insulin-like effects in rat adipocytes, such as stimulation of glucose transport. In the present work, we studied whether these substrates could also mimic long-term actions of insulin. Adipose differentiation of 3T3 F442A cells, which is highly insulin-dependent, served as a model to test the effects of sustained administration of amine oxidase substrates. Daily treatment of confluent cells with 0.75 mM tyramine (a substrate of MAO and SSAO) or benzylamine (a substrate of SSAO) over 1 week caused the acquisition of typical adipocyte morphology. The stimulation of protein synthesis and triacylglycerol accumulation caused by tyramine or benzylamine reached one half of that promoted by insulin. This effect was insensitive to pargyline (an MAO inhibitor), but was inhibited by semicarbazide (an SSAO inhibitor) and by N-acetylcysteine (an antioxidant agent), suggesting the involvement of the H(2)O(2) generated during SSAO-dependent amine oxidation. Chronic administration of amine oxidase substrates also induced the emergence of adipose conversion markers, such as aP2, glycerol-3-phosphate dehydrogenase, the glucose transporter GLUT4, and SSAO itself. Moreover, cells treated with amines acquired the same insulin sensitivity regarding glucose transport as adipocytes classically differentiated with insulin. In all, most of the adipogenic effects of amines were additive to insulin. Our data reveal that amine oxidase substrates partially mimic the adipogenic effect of insulin in cultured preadipocytes. Furthermore, they suggest that SSAO not only represents a novel late marker of adipogenesis, but could also be directly involved in the triggering of terminal adipocyte differentiation.

Substrates of semicarbazide-sensitive amine oxidase co-operate with vanadate to stimulate tyrosine phosphorylation of insulin-receptor-substrate proteins, phosphoinositide 3-kinase activity and GLUT4 translocation in adipose cells

It has been shown that the combination of benzylamine or tyramine and low concentrations of vanadate markedly stimulates glucose transport in rat adipocytes by a mechanism that requires semicarbazide-sensitive amine oxidase (SSAO) activity and H(2)O(2) formation. Here we have further analysed the insulin-like effects of the combination of SSAO substrates and vanadate and we have studied the signal-transduction pathway activated in rat adipocytes. We found that several SSAO substrates (benzylamine, tyramine, methylamine, n-decylamine, histamine, tryptamine or beta-phenylethylamine), in combination with low concentrations of vanadate, stimulate glucose transport in isolated rat adipocytes. Furthermore, SSAO substrates together with vanadate stimulated the recruitment of GLUT4 to the cell surface in isolated rat adipocytes. Benzylamine plus vanadate also stimulated glucose transport and GLUT4 translocation in 3T3-L1 adipocytes. Benzylamine or tyramine in combination with vanadate potently stimulated the tyrosine phosphorylation of both insulin receptor substrate (IRS)-1 and IRS-3. In contrast, benzylamine and vanadate caused only a weak stimulation of insulin receptor kinase. Benzylamine or tyramine in combination with vanadate also stimulated phosphoinositide 3-kinase activity; wortmannin abolished the stimulatory effect of benzylamine and vanadate on glucose transport in adipose cells. Furthermore, the administration of benzylamine and vanadate in vivo caused a rapid lowering of plasma glucose levels, which took place in the absence of alterations in plasma insulin. On the basis of these results we propose that SSAO activity regulates glucose transport in adipocytes. SSAO oxidative activity stimulates glucose transport via the translocation of GLUT4 carriers to the cell surface, resulting from a potent tyrosine phosphorylation of IRS-1 and IRS-3 and phosphoinositide 3-kinase activation. Our results also indicate that substrates of SSAO might regulate glucose disposal in vivo.

Factors involved in GLUT-1 glucose transporter gene transcription in cardiac muscle

Glucose constitutes a major fuel for the heart, and high glucose uptake during fetal development is coincident with the highest level of expression of the glucose transporter GLUT-1 during life. We have previously reported that GLUT-1 is repressed perinatally in rat heart, and GLUT-4, which shows a low level of expression in the fetal stage, becomes the main glucose transporter in the adult. Here, we show that the perinatal expression of GLUT-1 and GLUT-4 glucose transporters in heart is controlled directly at the level of gene transcription. Transient transfection assays show that the -99/-33 fragment of the GLUT-1 gene is sufficient to drive transcriptional activity in rat neonatal cardiomyocytes. Electrophoretic mobility shift assays demonstrate that the transcription factor Sp1, a trans-activator of GLUT-1 promoter, binds to the -102/-82 region of GLUT-1 promoter during the fetal state but not during adulthood. Mutation of the Sp1 site in this region demonstrates that Sp1 is essential for maintaining a high transcriptional activity in cardiac myocytes. Sp1 is markedly down-regulated both in heart and in skeletal muscle during neonatal life, suggesting an active role for Sp1 in the regulation of GLUT-1 transcription. In all, these results indicate that the expression of GLUT-1 and GLUT-4 in heart during perinatal development is largely controlled at a transcriptional level by mechanisms that might be related to hyperplasia and that are independent from the signals that trigger cell hypertrophy in the developing heart. Furthermore, our results provide the first functional insight into the mechanisms regulating muscle GLUT-1 gene expression in a live animal.

Insulin-like growth factor-II, phosphatidylinositol 3-kinase, nuclear factor-kappaB and inducible nitric-oxide synthase define a common myogenic signaling pathway

Insulin-like growth factors (IGFs) are potent inducers of skeletal muscle differentiation and phosphatidylinositol (PI) 3-kinase activity is essential for this process. Here we show that IGF-II induces nuclear factor-kappaB (NF-kappaB) and nitric-oxide synthase (NOS) activities downstream from PI 3-kinase and that these events are critical for myogenesis. Differentiation of rat L6E9 myoblasts with IGF-II transiently induced NF-kappaB DNA binding activity, inducible nitric-oxide synthase (iNOS) expression, and nitric oxide (NO) production. IGF-II-induced iNOS expression and NO production were blocked by NF-kappaB inhibition. Both NF-kappaB and NOS activities were essential for IGF-II-induced terminal differentiation (myotube formation and expression of skeletal muscle proteins: myosin heavy chain, GLUT 4, and caveolin 3), which was totally blocked by NF-kappaB or NOS inhibitors in rat and human myoblasts. Moreover, the NOS substrate L-Arg induced myogenesis in the absence of IGFs in both rat and human myoblasts, and this effect was blocked by NOS inhibition. Regarding the mechanisms involved in IGF-II activation of NF-kappaB, PI 3-kinase inhibition prevented NF-kappaB activation, iNOS expression, and NO production. Moreover, IGF-II induced, through a PI 3-kinase-dependent pathway, a decrease in IkappaB-alpha protein content that correlated with a decrease in the amount of IkappaB-alpha associated with p65 NF-kappaB.

High glucose concentrations inhibit glucose phosphorylation, but not glucose transport, in human endothelial cells

Glucose uptake is autoregulated in a variety of cell types and it is thought that glucose transport is the major step that is subjected to control by sugar availability. Here, we examined the effect of high glucose concentrations on the rate of glucose uptake by human ECV-304 umbilical vein-derived endothelial cells. A rise in the glucose concentration in the medium led a dose-dependent decrease in the rate of 2-deoxyglucose uptake. The effect of high glucose was independent of protein synthesis and the time-course analysis indicated that it was relatively slow. The effect was not due to inhibition of glucose transport since neither the expression nor the subcellular distribution of the major glucose transporter GLUT1, nor the rate of 3-O-methylglucose uptake was affected. The total in vitro assayed hexokinase activity and the expression of hexokinase-I were similar in cells treated or not with high concentrations of glucose. In contrast, exposure of cells to a high glucose concentration caused a marked decrease in phosphorylated 2-deoxyglucose/free 2-deoxyglucose ratio. This suggests the existence of alterations in the rate of in vivo glucose phosphorylation in response to high glucose. In summary, we conclude that ECV304 human endothelial cells reduce glucose utilization in response to enhanced levels of glucose in the medium by inhibiting the rate of glucose phosphorylation, rather than by blocking glucose transport. This suggests a novel metabolic effect of high glucose on cellular glucose utilization.

Benfluorex improves muscle insulin responsiveness in middle-aged rats previously subjected to long-term high-fat feeding

It has been reported that benfluorex ameliorates the insulin resistance induced by high-fat feeding when its administration is initiated at the same time as the change in diet. Here we have examined whether benfluorex reverses insulin resistance when this is established in middle-aged rats chronically maintained on a high-fat diet. Untreated 12-month-old rats that had been subjected to a high-fat diet for the last 6 months showed markedly lower insulin-induced stimulation of 2-deoxyglucose uptake by strips of soleus muscle and a reduced expression of GLUT4 glucose carriers in skeletal muscle. However, animals subjected to the same protocol but treated with benfluorex during the last month of high-fat feeding showed marked improvement in insulin-stimulated glucose transport by soleus muscle. Benfluorex treatment caused a substantial increase in the content of GLUT4 protein in white muscle; however, GLUT4 levels in red muscle remained low. Our results indicate: (i) that benfluorex treatment in middle-aged rats reverses the insulin resistance induced by high-fat feeding in soleus muscle; (ii) benfluorex is active even when it is administered once the insulin-resistant state is already established; (iii) reversion of muscle insulin resistance by benfluorex can occur independently of modifications in GLUT4 protein expression.

p70 S6 kinase activation is not required for insulin-like growth factor-induced differentiation of rat, mouse, or human skeletal muscle cells

Insulin-like growth factors (IGFs) are potent stimulators of muscle differentiation, and phosphatidylinositol 3-kinase (PI 3-kinase) is an essential second messenger in this process. Little is known about the downstream effectors of the IGF/PI 3-kinase myogenic cascade, and contradictory observations have been reported concerning the involvement of p70 S6 kinase. In an attempt to clarify the role of p70 S6 kinase in myogenesis, here we have studied the effect of rapamycin on rat, mouse, and human skeletal muscle cell differentiation. Both insulin and IGF-II activated p70 S6 kinase in rat L6E9 and mouse Sol8 myoblasts, which was markedly inhibited at 1 ng/ml rapamycin concentrations. Consistent with previous observations in a variety of cell lines, rapamycin exerted a potent inhibitory effect on L6E9 and Sol8 serum-induced myoblast proliferation. In contrast, even at high concentrations (20 ng/ml), rapamycin had no effect on IGF-II-induced proliferation or differentiation. Indeed, neither the morphological differentiation, as assessed by myotube formation, nor the expression of muscle-specific markers such as myogenin, myosin heavy chain, or GLUT4 (glucose transporter-4) glucose carriers was altered by rapamycin. Moreover, here we extended our studies on IGF-II-induced myogenesis to human myoblasts derived from skeletal muscle biopsies. We show that, as observed for rat and mouse muscle cells, human myoblasts can be induced to form multinucleated myotubes in the presence of exogenous IGF-II. Moreover, IGF-II-induced human myotube formation was totally blocked by LY294002, a specific PI 3-kinase inhibitor, but remained unaffected in the presence of rapamycin.

The amino acid transport system y+L/4F2hc is a heteromultimeric complex

4F2hc is an almost ubiquitous transmembrane protein in mammalian cells; upon expression in Xenopus laevis oocytes, it induces amino acid transport with characteristics of system y+L. Indirect evidence fostered speculation that function requires the association of 4F2hc with another protein endogenous to oocytes and native tissues. We show that expression of system y+L-like amino acid transport activity by 4F2hc in oocytes is limited by an endogenous factor and that direct covalent modification of external cysteine residue(s) of an oocyte membrane protein blocks system y+L/4F2hc transport activity, based on the following. 1) Induction of system y+L-like activity saturates at very low doses of human 4F2hc cRNA (0.1 ng/oocyte). This saturation occurs with very low expression of 4F2hc at the oocyte surface, and further increased expression of the protein at the cell surface does not result in higher induction of system y+L-like activity. 2) Human 4F2hc contains only two cysteine residues (C109 and C330). We mutated these residues, singly and in combination, to serine (C109S; CS1, C330S; CS2 and C109S-C330S, Cys-less). Mutation CS2 had no effect on the expressed system y+L-like transport activity, whereas C109S-containing mutants (CS1 and Cys-less) retained only partial y+L-like transport activity (30 to 50% of wild type). 3) Hg2+, the organic mercury compounds pCMB, and the membrane-impermeant pCMBS almost completely inactivated system y+L-like induced by human 4F2hc wild type and all the mutants studied. This was reversed by ss-mercaptoethanol, indicating that external cysteine residue(s) are the target of this inactivation. 4) Sensitivity to Hg2+ inactivation is increased by pretreatment of oocytes with ss-mercaptoethanol or in the C109S-containing mutants (CS1 and Cys-less). The increased Hg2+ reactivity of C109S-containing mutants supports the possibility that C109 may be linked by a disulfide bond to the Hg2+-targeted cysteine residue of the associated protein. These results indicate that 4F2hc is intimately associated with a membrane oocyte protein for the expression of system y+L amino acid transport activity. To our knowledge, this is the first direct evidence for a heteromultimeric protein structure of an organic solute carrier in mammals.

Role of semicarbazide-sensitive amine oxidase on glucose transport and GLUT4 recruitment to the cell surface in adipose cells

The previous characterization of an abundant population of non-adrenergic imidazoline-I2 binding sites in adipocytes and the recent demonstration of the interplay between these binding sites and amine oxidases led us to analyze the amine oxidase activity in membranes from isolated rat adipocytes. Adipocyte membranes had substantial levels of semicarbazide-sensitive amine oxidase (SSAO). SSAO activity and immunoreactive SSAO protein were maximal in plasma membranes, and they were also detectable in intracellular membranes. Vesicle immunoisolation analysis indicated that GLUT4-containing vesicles from rat adipocytes contain substantial levels of SSAO activity and immunoreactive SSAO protein. Immunotitration of intracellular GLUT4 vesicles indicated that GLUT4 and SSAO colocalize in an endosomal compartment in rat adipocytes. SSAO activity was also found in GLUT4 vesicles from 3T3-L1 adipocytes and rat skeletal muscle. Benzylamine, a substrate of SSAO activity, caused a marked stimulation of glucose transport in isolated rat adipocytes in the presence of very low vanadate concentrations that by themselves were ineffective in exerting insulin-like effects. This synergistic effect of benzylamine and vanadate on glucose transport was totally abolished in the presence of semicarbazide, a specific inhibitor of SSAO. Subcellular membrane fractionation revealed that the combination of benzylamine and vanadate caused a recruitment of GLUT4 to the plasma membrane of adipose cells. The stimulatory effects of benzylamine and vanadate on glucose transport were blocked by catalase, suggesting that hydrogen peroxide production coupled to SSAO activity plays a crucial regulatory role. Based on these results we propose that SSAO activity might contribute through hydrogen peroxide production to the in vivo regulation of GLUT4 trafficking in adipose cells.

Tyramine and vanadate synergistically stimulate glucose transport in rat adipocytes by amine oxidase-dependent generation of hydrogen peroxide

Nonadrenergic imidazoline I2-binding sites colocalize with monoamine oxidase (MAO) in various tissues. As white adipocytes from various species have been reported to be very rich in I2-sites, the authors consider whether these cells show a substantial MAO activity and explore its functional role. Oxidation of [14C]tyramine by rat adipocyte membranes was dependent on both MAO and semicarbazide-sensitive amine oxidase (SSAO). Tyramine oxidation was identical in membranes and in intact adipocytes (Vmax: 11-12 nmol/min/mg protein). A similar effect of MAO and SSAO inhibitors was obtained in both the intact cells and the membranes: half of the activity was sensitive to semicarbazide and the other half more easily inhibited by MAO-A than by MAO-B inhibitors. As the reaction catalyzed by amine oxidases generates H2O2, which mimicks certain insulin effects in adipocytes, we tested whether tyramine oxidation influences glucose transport in adipocytes. One mM tyramine weakly stimulated glucose transport. A clear potentiation of tyramine effect occurred in the presence of 0.1 mM vanadate, ineffective by itself, reaching half-maximal insulin stimulation. This stimulation was sensitive to MAO and SSAO inhibitors and to catalase. The 5-fold activation of glucose transport was accompanied by translocation of GLUT4 transporters to the plasma membrane. This shows that tyramine is readily oxidized by adipocytes and potentiates the effects of vanadium on glucose transport through release of hydrogen peroxide. The role of the amine oxidases, which are highly expressed in adipocytes, allows them to be considered as more than mere scavengers of circulating amines.

Insulin-like growth factors require phosphatidylinositol 3-kinase to signal myogenesis: dominant negative p85 expression blocks differentiation of L6E9 muscle cells

Phosphatidylinositol 3 (PI 3)-kinases are potently inhibited by two structurally unrelated membrane-permeant reagents: wortmannin and LY294002. By using these two inhibitors we first suggested the involvement of a PI 3-kinase activity in muscle cell differentiation. However, several reports have described that these compounds are not as selective for PI 3-kinase activity as assumed. Here we show that LY294002 blocks the myogenic pathway elicited by insulin-like growth factors (IGFs), and we confirm the specific involvement of PI 3-kinase in IGF-induced myogenesis by overexpressing in L6E9 myoblasts a dominant negative p85 PI 3-kinase-regulatory subunit (L6E9-delta p85). IGF-I, des(1-3)IGF-I, or IGF-II induced L6E9 skeletal muscle cell differentiation as measured by myotube formation, myogenin gene expression, and GLUT4 glucose carrier induction. The addition of LY294002 to the differentiation medium totally inhibited these IGF-induced myogenic events without altering the expression of a non-muscle-specific protein, beta1-integrin. Independent clones of L6E9 myoblasts expressing a dominant negative mutant of the p85-regulatory subunit (delta p85) showed markedly impaired glucose transport activity and formation of p85/p110 complexes in response to insulin, consistent with the inhibition of PI 3-kinase activity. IGF-induced myogenic parameters in L6E9-delta p85 cells, ie. cell fusion and myogenin gene and GLUT4 expression, were severely impaired compared with parental cells or L6E9 cells expressing wild-type p85. In all, data presented here indicate that PI 3-kinase is essential for IGF-induced muscle differentiation and that the specific PI 3-kinase subclass involved in myogenesis is the heterodimeric p85-p110 enzyme.

Regulation of glucose transport, and glucose transporters expression and trafficking in the heart: studies in cardiac myocytes

Cardiac muscle is characterized by a high rate of glucose consumption. In the absence of insulin, glucose transport into cardiomyocytes limits the rate of glucose utilization and therefore it is important to understand the regulation of glucose transporters. Cardiac muscle cells express 2 distinct glucose transporters, GLUT4 and GLUT1; although GLUT4 is quantitatively the more important glucose transporter expressed in heart, GLUT1 is also expressed at a substantial level. In isolated rat cardiomyocytes, insulin acutely stimulates glucose transport and translocates both GLUT4 and GLUT1 from an intracellular site to the cell surface. Recent evidence indicates the existence of at least 2 distinct intracellular membrane populations enriched in GLUT4 with a different protein composition. Elucidation of the intracellular location of these 2 GLUT4 vesicle pools in cardiac myocytes, their role in GLUT4 trafficking, and their relation to insulin-induced GLUT4 translocation needs to be addressed.

Characterization of two distinct intracellular GLUT4 membrane populations in muscle fiber. Differential protein composition and sensitivity to insulin

A major objective for the understanding of muscle glucose disposal is the elucidation of the intracellular trafficking pathway of GLUT4 glucose carriers in the muscle fiber. In this report, we provide functional and biochemical characterization of two distinct intracellular GLUT4 vesicle pools obtained from rat skeletal muscle. The two pools showed a differential response to insulin; thus, one showed a marked decrease in GLUT4 levels but the other did not. They also showed a markedly different protein composition as detected by quantitative vesicle immunoisolation analysis. The GLUT4 pool showing no response to insulin contained SCAMP proteins and the vSNARE proteins VAMP2 and cellubrevin, whereas only VAMP2 was found in the insulin-recruitable GLUT4 pool. SDS-PAGE and further silver staining of the immunoprecipitates revealed discrete polypeptide bands associated to the insulin-sensitive pool, and all these polypeptide bands were found in the insulin-insensitive population. Furthermore, some polypeptide bands were exclusive to the insulin-insensitive population. The presence of cellubrevin and SCAMP proteins, endosomal markers, suggest that the insulin-insensitive GLUT4 membrane population belongs to an endosomal compartment. In addition, we favor the view that the insulin-sensitive GLUT4 membrane pool is segregated from the endosomal GLUT4 population and is undergoes exocytosis to the cell surface in response to insulin. Intracellular GLUT4 membranes obtained from skeletal muscle contain cellubrevin, and VAMP2 and GLUT4-vesicles from cardiomyocytes also contain cellubrevin. This suggests that vSNARE proteins are key constituents of GLUT4 vesicles. The presence of the tSNARE protein SNAP25 in skeletal muscle membranes and SNAP25 and syntaxin 1A and syntaxin 1B in cardiomyocyte plasma membranes further suggest a role of the SNAREs in GLUT4 trafficking in muscle.

Chronic high-fat feeding and middle-aging reduce in an additive fashion Glut4 expression in skeletal muscle and adipose tissue

The interaction of middle-aging and chronic high-fat feeding on glucose transport in skeletal muscle and adipose tissue was examined. To this end, we studied the effects of 6 month treatment with a high-fat diet in 12-month old rats. Chronic high-fat feeding led to a substantial reduction in GLUT4 glucose transporter expression both in adipose tissue and in skeletal muscle, which was additive to the reduction in GLUT4 protein content detected in aged rats. In parallel, the high-fat diet led to a reduced insulin-stimulated glucose transport in the incubated soleus muscle and isolated adipocytes; insulin resistance induced by high-fat feeding was superimposed to the decreased insulin response detected in aged rats. Different mechanisms were responsible for GLUT4 repression in response to high-fat feeding or aging in skeletal muscles and adipose tissue.

Cyclic adenosine 3',5'-monophosphate regulates GLUT4 and GLUT1 glucose transporter expression and stimulates transcriptional activity of the GLUT1 promoter in muscle cells

We have previously reported that innervation-dependent basal contractile activity regulates in an inverse manner the expression of GLUT1 and GLUT4 glucose transporters in skeletal muscle. Based on the facts that muscle innervation decreases and muscle denervation increases cAMP levels, we investigated whether cAMP might mediate the effects of innervation/denervation on glucose transporter expression. Treatment of L6E9 myotubes with 8-bromo-cAMP, forskolin, or monobutyryl-8-bromo-cAMP led to a marked decrease in GLUT4 protein levels; 8-bromo-cAMP also diminished GLUT4 messenger RNA (mRNA), suggesting pretranslational repression. In contrast, L6E9 myoblasts and myotubes responded to 8-bromo-cAMP or forskolin by increasing the cell content of GLUT1 protein. Induction of GLUT1 protein was a consequence of the activation of different mechanisms in myoblast and myotube cells; whereas 8-bromo-cAMP treatment caused a substantial increase in GLUT1 mRNA in myoblasts, no change in GLUT1 mRNA was detected in myotubes. The increase in GLUT1 mRNA in L6E9 myoblasts induced by 8-bromo-cAMP was the result of transcriptional activation, as concluded from transfection analysis of 2.1 kilobases of the rat GLUT1 gene promoter fused to the bacterial chloramphenicol acetyltransferase gene. Furthermore, the stimulatory effect of 8-bromo-cAMP on the transcriptional activity of the GLUT1 promoter required a 33-bp sequence lying 5' upstream of the transcription start site. In all, cAMP inversely regulates GLUT4 and GLUT1 glucose transporter expression in muscle cells. Furthermore, our results suggest that down-regulation of GLUT4 expression and up-regulation of GLUT1 expression in muscle associated with denervation are partly attributable to cAMP.

Effect of cations on the tyrosine kinase activity of the insulin receptor: inhibition by fluoride is magnesium dependent

We have recently reported that fluoride interacts directly with the insulin receptor, which causes inhibition of its phosphotransferase activity. The inhibitory effect of fluoride on phosphotransferase activity is not due to the formation of complexes with aluminium and occurs in the absence of alterations to the binding of ATP or insulin. In this report we substantiate that the tyrosine kinase activity of insulin receptors partially purified from rat skeletal muscle shows a strict requirement of Mg2+ ions (Ka near 11 mM). This effect of Mg2+ was inhibited in a competitive manner by Mn2+, which is compatible with competition of both divalent ions for binding sites. The inhibition of tyrosine kinase activity caused by fluoride was dependent on the concentration of Mg2+ in the medium and no inhibitory effect was detected at low concentrations of Mg2+. Moreover, the addition of increasing concentrations of Mn2+ in the presence of a constant high concentration of Mg2+, led to a gradual decrease in the inhibitory effect of fluoride. These results indicate that the Mg-insulin receptor complex is the major fluoride-susceptible form. Based on the characteristics of the inhibition of tyrosine kinase shown by fluoride it might be proposed that its action is exerted by the formation of multi-ionic MgF complexes analogous to Pi, which bind to the insulin receptor kinase.

Myogenesis and MyoD down-regulate Sp1. A mechanism for the repression of GLUT1 during muscle cell differentiation

Muscle cell differentiation caused a reduction of glucose transport, GLUT1 glucose transporter expression, and GLUT1 mRNA levels. A fragment of 2.1 kilobases of the rat GLUT1 gene linked to chloramphenicol acetyltransferase drove transcriptional activity in myoblasts, and differentiation caused a decrease in transcription. Transient transfection of 5' and 3' deletion constructs showed that the fragment -99/-33 of the GLUT1 gene drives transcriptional activity of the GLUT1 gene and participates in the reduced transcription after muscle differentiation. Electrophoretic mobility shift assays showed the binding of Sp1 protein to the fragment -102/-37 in the myoblast state but not in myotubes, and Sp1 was found to transactivate the GLUT1 promoter. Western blot analysis indicated that Sp1 was drastically down-regulated during myogenesis. Furthermore, the forced over-expression of MyoD in C3H10T1/2 cells mimicked the effects observed during myogenesis, Sp1 down-regulation and reduced transcriptional activity of the GLUT1 gene promoter. In all, these data suggest a regulatory model in which MyoD activation during myogenesis causes the down-regulation of Sp1, which contributes to the repression of GLUT1 gene transcription and, therefore, leads to the reduction in GLUT1 expression and glucose transport.

An intracellular trafficking defect in type I cystinuria rBAT mutants M467T and M467K

The human rBAT protein elicits sodium-independent, high affinity obligatory exchange of cystine, dibasic amino acids, and some neutral amino acids in Xenopus oocytes (Chillarón, J., Estévez, R., Mora, C., Wagner, C. A., Suessbrich, H., Lang, F., Gelpí, J. L., Testar, X., Busch, A. E., Zorzano, A., and Palacín, M. (1996) J. Biol. Chem. 271, 17761-17770). Mutations in rBAT have been found to cause cystinuria (Calonge, M. J., Gasparini, P., Chillarón, J., Chillón, M., Galluci, M., Rousaud, F., Zelante, L., Testar, X., Dallapiccola, B., Di Silverio, F., Barceló, P., Estivill, X., Zorzano, A., Nunes, V., and Palacín, M. (1994) Nat. Genet. 6, 420-426). We have performed functional studies with the most common point mutation, M467T, and its relative, M467K, using the oocyte system. The Km and the voltage dependence for transport of the different substrates were the same in both M467T and wild type-injected oocytes. However, the time course of transport was delayed in the M467T mutant: maximal activity was accomplished 3-4 days later than in the wild type. This delay was cRNA dose-dependent: at cRNA levels below 0.5 ng the M467T failed to achieve the wild type transport level. The M467K mutant displayed a normal Km, but the Vmax was between 5 and 35% of the wild type. The amount of rBAT protein was similar in normal and mutant-injected oocytes. In contrast to the wild type, the mutant proteins remained endoglycosidase H-sensitive, suggesting a longer residence time in the endoplasmic reticulum. We quantified the amount of rBAT protein in the plasma membrane by surface labeling with biotin 2 and 6 days after injection. Most of the M467T and M467K protein was located in an intracellular compartment. The converse situation was found in the wild type. Despite the low amount of M467T protein reaching the plasma membrane, the transport activity at 6 days was the same as in the wild type-injected oocytes. The increase in plasma membrane rBAT protein between 2 and 6 days was completely dissociated from the rise in transport activity. These data indicate impaired maturation and transport to the plasma membrane of the M467T and M467K mutant, and suggest that rBAT alone is unable to support the transport function.

Insulin-induced recruitment of glucose transporter 4 (GLUT4) and GLUT1 in isolated rat cardiac myocytes. Evidence of the existence of different intracellular GLUT4 vesicle populations

Using isolated rat cardiomyocytes we have examined: 1) the effect of insulin on the cellular distribution of glucose transporter 4 (GLUT4) and GLUT1, 2) the total amount of these transporters, and 3) the co-localization of GLUT4, GLUT1, and secretory carrier membrane proteins (SCAMPs) in intracellular membranes. Insulin induced 5.7- and 2.7-fold increases in GLUT4 and GLUT1 at the cell surface, respectively, as determined by the nonpermeant photoaffinity label [3H]2-N-[4(1-azi-2,2,2-trifluoroethyl)benzoyl]-1, 3-bis-(D-mannos-4-yloxy)propyl-2-amine. The total amount of GLUT1, as determined by quantitative Western blot analysis of cell homogenates, was found to represent a substantial fraction ( approximately 30%) of the total glucose transporter content. Intracellular GLUT4-containing vesicles were immunoisolated from low density microsomes by using monoclonal anti-GLUT4 (1F8) or anti-SCAMP antibodies (3F8) coupled to either agarose or acrylamide. With these different immunoisolation conditions two GLUT4 membrane pools were found in nonstimulated cells: one pool with a high proportion of GLUT4 and a low content in GLUT1 and SCAMP 39 (pool 1) and a second GLUT4 pool with a high content of GLUT1 and SCAMP 39 (pool 2). The existence of pool 1 was confirmed by immunotitration of intracellular GLUT4 membranes with 1F8-acrylamide. Acute insulin treatment caused the depletion of GLUT4 in both pools and of GLUT1 and SCAMP 39 in pool 2.

IN CONCLUSION

1) GLUT4 is the major glucose transporter to be recruited to the surface of cardiomyocytes in response to insulin; 2) these cells express a high level of GLUT1; and 3) intracellular GLUT4-containing vesicles consist of at least two populations, which is compatible with recently proposed models of GLUT4 trafficking in adipocytes.

Localization, by linkage analysis, of the cystinuria type III gene to chromosome 19q13.1

Cystinuria is an autosomal recessive aminoaciduria in which three urinary phenotypes (I, II, and III) have been described. An amino acid transporter gene, SLC3A1 (formerly rBAT), was found to be responsible for this disorder. Mutational and linkage analysis demonstrated the presence of genetic heterogeneity in which the SLC3A1 gene is responsible for type I cystinuria but not for type II or type III. In this study, we report the identification of the cystinuria type III locus on the long arm of chromosome 19 (19q13.1), obtained after a genomewide search. Pairwise linkage analysis in a series of type III or type II families previously excluded from linkage to the cystinuria type I locus (SLC3A1 gene) revealed a significant maximum LOD score (zeta max) of 13.11 at a maximum recombination fraction (theta max) of .00, with marker D19S225. Multipoint linkage analysis performed with the use of additional markers from the region placed the cystinuria type III locus between D19S414 and D19S220. Preliminary data on type II families also seem to place the disease locus for this rare type of cystinuria at 19q13.1 (significant zeta max = 3.11 at theta max of .00, with marker D19S225).

Evidence for posttranscriptional regulation of GLUT4 expression in muscle and adipose tissue from streptozotocin-induced diabetic and benfluorex-treated rats

In this study we explored the expression of GLUT4 glucose carriers in muscle and adipose tissues from streptozotocin-induced diabetic and benfluorex-treated rats. In nondiabetic rats, benfluorex treatment decreased GLUT4 protein content in muscle and brown adipose tissue, with no change in GLUT4 mRNA. This effect occurred in the presence of normal circulating levels of insulin and glucose. Seventeen days after streptozotocin injection, diabetic rats showed a decreased GLUT4 protein content in adipose tissues and in both red and white skeletal muscle. Diabetic rats showed decreased GLUT4 mRNA levels in white and brown adipose tissue, whereas messenger concentrations remained unaltered in red and white fibers of skeletal muscle. The interaction of benfluorex and diabetes on GLUT4 protein expression showed a tissue-specific pattern. Benfluorex treatment to some extent prevented the decrease in GLUT4 protein in white and brown adipose tissue and in white muscle associated with diabetes. In contrast, diabetes and benfluorex caused an additive decrease in GLUT4 expression in red skeletal muscle. The effects of benfluorex on GLUT4 content in tissues from diabetic rats occurred in the absence of alterations in GLUT4 mRNA levels, suggesting a modification of translational or posttranslational steps. Benfluorex did not ameliorate the hyperglycemia of diabetic rats. Our results indicate that red and white skeletal muscle respond to diabetes and benfluorex in a heterogeneous manner, which suggests the existence of differences in the mechanisms that regulate GLUT4 expression. Furthermore, our data indicate that GLUT4 expression in muscle and adipose tissue can be regulated by modification of translational or posttranslational steps.

Genomic structure and organization of the human rBAT gene (SLC3A1)

Cystinuria is an autosomal recessive disorder of amino acid transport, manifesting as three phenotypes (I, II, and III). An amino acid transport gene, rBAT, is responsible for cystinuria. Mutation and linkage analyses have demonstrated the disease to be heterogeneous, with rBAT being the defective gene in type I cystinuria. The genomic structure of the human rBAT gene (HGMW-approved symbol SLC 3A1) has been established via two strategies: (i) construction of two different genomic libraries by subcloning the Mega-YAC921B6 (CEPH), containing rBAT, in Lambda ZAP and screening using rBAT cDNA and different PCR products; and (ii) generation and sequencing of genomic fragments by long PCR using rBAT cDNA-derived primers. The rBAT gene spans approximately 45 kb and consists of 10 exons. The introns range from 500 to 13,000 bp. All splice sites conform to the GT/AG rule. The promoter region has been further analyzed, and a predicted TATA box 98 bp upstream of the first coding ATG was identified. In addition an Alu repeat has been detected 72 bp upstream of the predicted TATA box.

Molecular analysis of the cystinuria disease gene: identification of four new mutations, one large deletion, and one polymorphism

A cystinuria disease gene (rBAT) has recently been identified, but evidence strongly suggests that only Type-I cystinuria is due to mutations in this gene. Sixteen point mutations and a large deletion causing the disease have so far been described in the rBAT gene sequence. To identify new mutated alleles, genomic DNA was analyzed, after the determination of the entire genomic structure of the rBAT gene, by RNA-single strand conformation polymorphism analysis, an accurate and sensitive method able to detect nucleotide changes. Four new point mutations, a large deletion, and a common intragenic polymorphism were detected. These new mutations increase to 22 the number of mutated alleles so far characterized in rBAT. In addition, the frequency of 21 mutations was assessed in a sample of accurately defined Type-I cystinuria chromosomes. They account for about 58% of all Type-I chromosomes, mutation M467T being the most common (0.26).

Phosphatidylinositol 3-kinase inhibitors block differentiation of skeletal muscle cells

Skeletal muscle differentiation involves myoblast alignment, elongation, and fusion into multinucleate myotubes, together with the induction of regulatory and structural muscle-specific genes. Here we show that two phosphatidylinositol 3-kinase inhibitors, LY294002 and wortmannin, blocked an essential step in the differentiation of two skeletal muscle cell models. Both inhibitors abolished the capacity of L6E9 myoblasts to form myotubes, without affecting myoblast proliferation, elongation, or alignment. Myogenic events like the induction of myogenin and of glucose carrier GLUT4 were also blocked and myoblasts could not exit the cell cycle, as measured by the lack of mRNA induction of p21 cyclin-dependent kinase inhibitor. Overexpresssion of MyoD in 10T1/2 cells was not sufficient to bypass the myogenic differentiation blockade by LY294002. Upon serum withdrawal, 10T1/2-MyoD cells formed myotubes and showed increased levels of myogenin and p21. In contrast, LY294002-treated cells exhibited none of these myogenic characteristics and maintained high levels of Id, a negative regulator of myogenesis. These data indicate that whereas phosphatidylinositol 3-kinase is not indispensable for cell proliferation or in the initial events of myoblast differentiation, i.e. elongation and alignment, it appears to be essential for terminal differentiation of muscle cells.

Obligatory amino acid exchange via systems bo,+-like and y+L-like. A tertiary active transport mechanism for renal reabsorption of cystine and dibasic amino acids

Mutations in the rBAT gene cause type I cystinuria, a common inherited aminoaciduria of cystine and dibasic amino acids due to their defective renal and intestinal reabsorption (Calonge, M. J., Gasparini, P., Chillarón, J., Chillón, M., Gallucci, M., Rousaud, F., Zelante, L., Testar, X., Dallapiccola, B., Di Silverio, F., Barceló, P., Estivill, X., Zorzano, A., Nunes, V., and Palacín, M. (1994) Nat. Genet. 6, 420-426; Calonge, M. J., Volipini, V., Bisceglia, L., Rousaud, F., De Sanctis, L., Beccia, E., Zelante, L., Testar, X., Zorzano, A., Estivill, X., Gasparini, P., Nunes, V., and Palacín, M.(1995) Proc. Natl. Acad. Sci. U. S. A. 92, 9667-9671). One important question that remains to be clarified is how the apparently non-concentrative system bo,+-like, associated with rBAT expression, participates in the active renal reabsorption of these amino acids. Several studies have demonstrated exchange of amino acids induced by rBAT in Xenopus oocytes. Here we offer evidence that system bo,+-like is an obligatory amino acid exchanger in oocytes and in the "renal proximal tubular" cell line OK. System bo, +-like showed a 1:1 stoichiometry of exchange, and the hetero-exchange dibasic (inward) with neutral (outward) amino acids were favored in oocytes. Obligatory exchange of amino acids via system bo,+-like fully explained the amino acid-induced current in rBAT-injected oocytes. Exchange via system bo,+-like is coupled enough to ensure a specific accumulation of substrates until the complete replacement of the internal oocyte substrates. Due to structural and functional analogies of the cell surface antigen 4F2hc to rBAT, we tested for amino acid exchange via system y+L-like. 4F2hc-injected oocytes accumulated substrates to a level higher than CAT1-injected oocytes (i.e. oocytes expressing system y+) and showed exchange of amino acids with the substrate specificity of system y+L and L-leucine-induced outward currents in the absence of extracellular sodium. In contrast to L-arginine, system y+L-like did not mediate measurable L-leucine efflux from the oocyte. We propose a role of systems bo,+-like and y+L-like in the renal reabsorption of cystine and dibasic amino acids that is based on their active tertiary transport mechanism and on the apical and basolateral localization of rBAT and 4F2hc, respectively, in the epithelial cells of the proximal tubule of the nephron.

The molecular basis of cystinuria: the role of the rBAT gene

The cDNAs of mammalian amino acid transporters already identified could be grouped into four families. One of these protein families is composed of the protein rBAT and the heavy chain of the cell surface antigen 4F2 (4F2hc). The cRNAs of rBAT and 4F2hc induce amino acid transport activity via systems b(0,+) -like and y(+)L -like inXenopus oocytes respectively. Surprisingly, neither rBAT nor 4F2hc is very hydrophobic, and they seem to be unable to form a pore in the plasma membrane. This prompted the hypothesis that rBAT and 4F2hc are subunits or modulators of the corresponding amino acid transporters. The association of rBAT with a light subunit of ~40kDa has been suggested, and such an association has been demonstrated for 4F2hc.The b(0,+)-like system expressed in oocytes by rBAT cRNA transports L-cystine, L-dibasic and L-neutral amino acids with high-affinity. This transport system shows exchange of amino acids through the plasma membrane ofXenopus oocytes, suggesting a tertiary active transport mechanism. The rBAT gene is mainly expressed in the outer stripe of the outer medulla of the kidney and in the mucosa of the small intestine. The protein localizes to the microvilli of the proximal straight tubules (S3 segment) of the nephron and the mucosa of the small intestine. All this suggested the participation of rBAT in a high-affinity reabsorption system of cystine and dibasic amino acids in kidney and intestine, and indicated rBAT (named SLC3A1 in Gene Data Bank) as a good candidate gene for cystinuria. This is an inherited aminoaciduria due to defective renal and intestinal reabsorption of cystine and dibasic amino acids. The poor solubility of cystine causes the formation of renal cystine calculi. Mutational analysis of the rBAT gene of patients with cystinuria is revealing a growing number (~20) of cystinuria-specific mutations, including missense, nonsense, deletions and insertions. Mutations M467T (substitution of methionine 467 residue for threonine) and R270X (stop codon at arginine residue 270) represent approximately half of the cystinuric chromosomes where mutations have been found. Mutation M467T reduces transport activity of rBAT in oocytes. All this demonstrates that mutations in the rBAT gene cause cystinuria.Three types of cystinuria (types, I, II and III) have been described on the basis of the genetic, biochemical and clinical manifestations of the disease. Type I cystinuria has a complete recessive inheritance; type I heterozygotes are totally silent. In contrast, type II and III heterozygotes show, respectively, high or moderate hyperaminoaciduria of cystine and dibasic amino acids. Type III homozygotes show moderate, if any, alteration of intestinal absorption of cystine and dibasic amino acids; type II homozygotes clearly show defective intestinal absorption of these amino acids. To date, all the rBAT cystinuria-specific mutations we have found are associated with type I cystinuria (~70% of the chromosomes studied) but not to types II or III. This strongly suggests genetic heterogeneity for cystinuria. Genetic linkage analysis with markers of the genomic region of rBAT in chromosome 2 (G band 2p16.3) and intragenic markers of rBAT have demonstrated genetic heterogeneity for cystinuria; the rBAT gene is linked to type I cystinuria, but not to type III. Biochemical, genetic and clinical studies are needed to identify the additional cystinuria genes; a low-affinity cystine reabsortion system and the putative light subunit of rBAT are additional candidate genes for cystinuria.

The rBAT gene is responsible for L-cystine uptake via the b0,(+)-like amino acid transport system in a "renal proximal tubular" cell line (OK cells)

Several studies have shown that the cRNA of human, rabbit, or rat rBAT induces in Xenopus oocytes sodium-independent, high affinity uptake of L-cystine via a system b0,(+)-like amino acid exchanger. We have shown that mutations in rBAT cause type I cystinuria (Calonge, M. J., Gasparini, P., Chillarón, J., Chillón, M., Gallucci, M., Rousaud, F., Zelante, L., Testar, X., Dallapiccola, B., Di Silverio, F., Barceló, P., Estivill, X., Zorzano, A., Nunes, V., and Palacín, M. (1994) Nat. Genet. 6, 420-425; Calonge, M. J., Volipini, V., Bisceglia, L., Rousaud, F., De Sanctis, L., Beccia, E., Zelante, L., Testar, X., Zorzano, A., Estivill, X., Gasparini, P., Nunes, V., and Palacín, M. (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 9667-9671). Apart from oocytes, no other expression system has been used for transfection of functional rBAT activity. Furthermore, the b0,(+)-like transport activity has not been clearly described in the kidney or intestine. Here, we report that a "proximal tubular-like" cell line derived from opossum kidney (OK cells) expresses an rBAT transcript. Poly(A)+ RNA from OK cells induced by system b0,(+)-like transport activity in oocytes. This was hybrid-depleted by human rBAT antisense oligonucleotides. A polymerase chain reaction-amplified cDNA fragment (approximately 700 base pairs) from OK cell RNA corresponds to an rBAT protein fragment 65-69% identical to those from human, rabbit and rat kidneys. We have also examined transport of l-cystine in OK cells and found characteristics very similar to the amino acid exchanger activity induced by rBAT cRNA in oocytes. Uptake of L-cystine was of high affinity, sodium-independent and shared with L-arginine and L-leucine. It was trans-stimulated by amino acids with the same specificity as rBAT-induced transport activity in oocytes. Furthermore, it was localized to the apical pole of confluent OK cells. To demonstrate that the rBAT protein is functionally related to this transport activity, we have transfected OK cells with human rBAT antisense and sense sequences. Transfection with rBAT antisense, but not with rBAT sense, resulted in the specific reduction of rBAT mRNA expression and b0,(+)-like transport activity. These results demonstrate that rBAT is functionally related to the L-cystine uptake via system b0,(+)-like in the apical pole of the renal OK cell line.

Expression and insulin-regulated distribution of caveolin in skeletal muscle. Caveolin does not colocalize with GLUT4 in intracellular membranes

Caveolin is believed to play an important role in sorting processes, vesicular trafficking, transmembrane signaling, and molecular transport across membranes. In this study we have evaluated the expression and distribution of caveolin in skeletal muscle and its interaction with GLUT4 glucose carriers. Caveolin was expressed to substantial levels in muscle and its expression was regulated in muscle; aging and high fat diet enhanced caveolin expression in skeletal muscle and inversely, myogenesis down-regulated caveolin in L6E9 cells. Under fasting conditions, most of caveolin was found in intracellular membranes and the caveolin present in the cell surface was found in both sarcolemma and T-tubules. Insulin administration led to a redistribution of caveolin from intracellular high density membrane fractions to intracellular lighter density fractions and to the cell surface; this pattern of insulin-induced redistribution was different to what was shown by GLUT4. These results suggests that caveolin is a component of an insulin-regulated machinery of vesicular transport in muscle. Quantitative immunoisolation of GLUT4 vesicles obtained from different intracellular GLUT4 populations revealed the absence of caveolin which substantiates the lack of colocalization of intracellular GLUT4 and caveolin. This indicates that caveolin is not involved in intracellular GLUT4 trafficking in skeletal muscle.

Insulin-induced redistribution of GLUT4 glucose carriers in the muscle fiber. In search of GLUT4 trafficking pathways

Insulin rapidly stimulates glucose transport in muscle fiber. This process controls the utilization of glucose in skeletal muscle, and it is deficient in various insulin-resistant states, such as non-insulin-dependent diabetes mellitus. The effect of insulin on muscle glucose transport is mainly due to the recruitment of GLUT4 glucose carriers to the cell surface of the muscle fiber. There is increasing evidence that the recruitment of GLUT4 carriers triggered by insulin affects selective domains of sarcolemma and transverse tubules. In contrast, GLUT1 is located mainly in sarcolemma and is absent in transverse tubules, and insulin does not alter its cellular distribution in muscle fiber. The differential distribution of GLUT1 and GLUT4 in the cell surface raises new questions regarding the precise endocytic and exocytic pathways that are functional in the muscle fiber. The current view of insulin-induced GLUT4 translocation is based mainly on studies performed in adipocytes. These studies have proposed the existence of intracellular compartments of GLUT4 that respond to insulin in a highly homogeneous manner. However, studies performed in skeletal muscle have identified insulin-sensitive as well as insulin-insensitive intracellular GLUT4-containing membranes. These data open a new perspective on the dynamics of intracellular GLUT4 compartments in insulin-sensitive cells.

Disruption of GLUT1 glucose carrier trafficking in L6E9 and Sol8 myoblasts by the phosphatidylinositol 3-kinase inhibitor wortmannin

In this study we have used wortmannin, a highly specific inhibitor of phosphatidylinositol (PI) 3-kinase, to assess the role of this enzyme on GLUT1 glucose carrier distribution and glucose transport activity in myoblasts from two skeletal-muscle cell lines, L6E9 and Sol8. As detected in L6E9 cells, myoblasts exhibited basal and insulin-stimulated PI 3-kinase activities. Incubation of intact myoblasts with wortmannin resulted in a marked inhibition of both basal and insulin-stimulated PI 3-kinase activities. L6E9 and Sol8 myoblasts showed basal and insulin-stimulated glucose transport activities, both of them inhibited by wortmannin in a dose-dependent manner (IC50 approximately 10-20 nM). Concomitantly, immunofluorescence analysis revealed that 1 h treatment with wortmannin led to a dramatic intracellular accumulation of GLUT1 carriers (the main glucose transporter expressed in L6E9 and Sol8 myoblasts) in both cell systems. The effect of wortmannin on GLUT1 cellular redistribution was independent of the presence of insulin. The cellular distribution of two structural plasma-membrane components such as beta 1-integrin or the alpha 1 subunit of the Na(+)-K(+)-ATPase were unaffected by wortmannin in both the absence and the presence of insulin. As a whole, our results indicate that PI 3-kinase is necessary to basal and insulin-stimulated glucose transport in L6E9 and Sol8 myoblasts. Moreover, immunofluorescence assays suggest that in both cellular models there is a constitutive GLUT 1 trafficking pathway (independent of insulin) that involves PI 3-kinase and which, when blocked, locks GLUT1 in a perinuclear compartment.

The T-tubule is a cell-surface target for insulin-regulated recycling of membrane proteins in skeletal muscle

(1) In this study we have determined the distribution of various membrane proteins involved in insulin-activated glucose transport in T-tubules and in sarcolemma from rat skeletal muscle. Two independent experimental approaches were used to determine the presence of membrane proteins in T-tubules: (i) the purification of T-tubules free from sarcolemmal membranes by lectin agglutination, and (ii) T-tubule vesicle immunoadsorption. These methods confirmed that T-tubules from rat skeletal muscle were enriched with dihydropyridine receptors and tt28 protein and did not contain the sarcolemmal markers dystrophin or beta 1-integrin. Both types of experiments revealed an abundant content of GLUT4 glucose carriers, insulin receptors and SCAMPs (secretory carrier membrane proteins) in T-tubule membranes. (2) Acute administration in vivo of insulin caused an increased abundance of GLUT4 in T-tubules and sarcolemma. On the contrary, insulin led to a 50% reduction in insulin receptors present in T-tubules and in sarcolemma, demonstrating that insulin-induced insulin receptor internalization affects T-tubules in the muscle fibre. The alteration in the content of GLUT4 and insulin receptors in T-tubules was a consequence of insulin-induced redistribution of these proteins. SCAMPs also redistributed in muscle membranes in response to insulin. They were recruited by insulin from intracellular high-density fractions to intracellular lighter-density fractions and to the cell surface, showing a pattern of insulin-induced cellular redistribution distinct from those of GLUT4 and the insulin receptor. (3) In conclusion, the T-tubule is a cell-surface target for membrane proteins involved in recycling such as SCAMPs or for membrane proteins that acutely redistribute in response to insulin such as GLUT4 or insulin receptors.

Genetic heterogeneity in cystinuria: the SLC3A1 gene is linked to type I but not to type III cystinuria

Cystinuria is an autosomal recessive amino-aciduria where three urinary phenotypes have been described (I, II, and III). An amino acid transporter gene, SLC3A1 (formerly rBAT), was found to be responsible for this disorder. To assess whether mutations in SLC3A1 are involved in different cystinuria phenotypes, linkage with this gene and its nearest marker (D2S119) was analyzed in 22 families with type I and/or type III cystinuria. Linkage with heterogeneity was proved (alpha = 0.45; P < 0.008). Type I/I families showed homogeneous linkage to SLC3A1 (Zmax > 3.0 at theta = 0.00; alpha = 1), whereas types I/III and III/III were not linked. Our data suggest that type I cystinuria is due to mutations in the SLC3A1 gene, whereas another locus is responsible for type III. This result establishes genetic heterogeneity for cystinuria, classically considered as a multiallelic monogenic disease.

Molecular genetics of cystinuria: identification of four new mutations and seven polymorphisms, and evidence for genetic heterogeneity

A cystinuria disease gene (rBAT) has been recently identified, and some mutations causing the disease have been described. The frequency of these mutations has been investigated in a large sample of 51 Italian and Spanish cystinuric patients. In addition, to identify new mutated alleles, genomic DNA has been analyzed by an accurate and sensitive method able to detect nucleotide changes. Because of the lack of information available on the genomic structure of rBAT gene, the study was carried out using the sequence data so far obtained by us. More than 70% of the entire coding sequence and 8 intron-exon boundaries have been analyzed. Four new mutations and seven intragenic polymorphisms have been detected. All mutations so far identified in rBAT belong only to cystinuria type I alleles, accounting for approximately 44% of all type I cystinuric chromosomes. Mutation M467T is the most common mutated allele in the Italian and Spanish populations. After analysis of 70% of the rBAT coding region, we have detected normal sequences in cystinuria type II and type III chromosomes. The presence of rBAT mutated alleles only in type I chromosomes of homozygous (type I/I) and heterozygous (type I/III) patients provides evidence for genetic heterogeneity where rBAT would be responsible only for type I cystinuria and suggests a complementation mechanism to explain the intermediate type I/type III phenotype.

Insulin and insulin-like growth factor I (IGF-I) stimulate GLUT4 glucose transporter translocation in Xenopus oocytes

1. The heterologous expression of glucose transporters GLUT4 and GLUT1 in Xenopus oocytes has been shown to cause a differential targeting of these glucose-carrier isoforms to cellular membranes and a distinct induction of glucose transport activity. In this study we have evaluated the effect of insulin and insulin-like growth factor I (IGF-I) on glucose uptake and glucose transporter distribution in Xenopus oocytes expressing mammalian GLUT4 and GLUT1 glucose carriers. 2. Insulin and IGF-I stimulated 2-deoxyglucose uptake in GLUT4-expressing oocytes, but not in GLUT1-expressing oocytes or in water-injected oocytes. The stimulatory effect of insulin and IGF-I on 2-deoxyglucose uptake in GLUT4-expressing oocytes occurred via activation of the IGF-I receptor. 3. Subcellular-fractionation studies indicated that insulin and IGF-I stimulated translocation of GLUT4 to the cell surface of the oocyte. 4. Incubation of intact oocytes with insulin stimulated phosphatidylinositol 3-kinase activity, an effect that was blocked by the additional presence of wortmannin. Furthermore, wortmannin totally abolished the insulin-induced stimulation of 2-deoxyglucose uptake in GLUT4-expressing oocytes. 5. In this study, both the insulin-induced GLUT4 carrier translocation and GLUT4-dependent insulin-stimulated glucose transport have been reconstituted in the Xenopus oocyte. These observations, together with the fact that wortmannin, as found in adipocytes, inhibits insulin-stimulated glucose transport in oocytes, suggest that the heterologous expression of GLUT4 in oocytes is a useful experimental model by which to study the cell biology of insulin-induced GLUT4 translocation.

Regulation of GLUT5 gene expression in rat intestinal mucosa: regional distribution, circadian rhythm, perinatal development and effect of diabetes

1. GLUT5 gene expression was studied in small intestine under a variety of conditions characterized by altered intestinal absorption of monosaccharides. 2. RNA-blotting studies showed that GLUT5 mRNA was abundantly expressed in rat and rabbit intestine and kidney, but it was not detected in heart or brown adipose tissue. GLUT5 mRNA levels were higher in the upper segments of the small intestine (duodenum and proximal jejunum) than in the lower segments (distal jejunum and ileum). 3. The intestinal expression of GLUT5 mRNA in rat proximal jejunum showed circadian rhythm. A 12-fold increase in GLUT5 mRNA levels was detected at the end of the light cycle and at the beginning of the dark cycle when compared with the early light period. In keeping with this, GLUT5 protein content in brush-border membranes was also increased at the beginning of the dark cycle compared with that in the light period. 4. In streptozotocin-induced diabetes an 80% increase in GLUT5 mRNA levels in mucosa from the proximal jejunum was detected under conditions in which enhanced intestinal absorption of monosaccharides has been reported. 5. The intestinal expression of GLUT5 mRNA showed regulation during perinatal development. Levels of GLUT5 mRNA were low during fetal life, increased progressively during the postnatal period and reached levels comparable with the adult state after weaning. Weaning on to a high-fat diet partially prevented the induction of GLUT5 gene expression. 6. Our results indicate that GLUT5 gene expression is tightly regulated in small intestine. Regulation involves maximal expression in the upper part of the small intestine, circadian rhythm, developmental regulation dependent on the fat and carbohydrate content in the diet at weaning and enhanced expression in streptozotocin-induced diabetes. Furthermore, changes observed in intestinal GLUT5 expression correlate with reported alterations in intestinal absorption of fructose. This suggests a regulatory role for GLUT5 in fructose uptake by absorptive enterocytes.

Assignment of the gene responsible for cystinuria (rBAT) and of markers D2S119 and D2S177 to 2p16 by fluorescence in situ hybridization

We have established rBAT (named as SLC3A1 in the Genome Data Base) as a gene responsible for cystinuria, a heritable disorder of amino acid transport. The cystinuria locus has been mapped by linkage between microsatellite markers D2S119 and D2S177. Fluorescence in situ hybridization (FISH) either with Alu-polymerase-chain-reaction (PCR)-amplified sequences of a yeast artificial chromosome (YAC) containing the rBAT gene or with rBAT-specific PCR-amplified genomic fragments, and chromosome G-banding have cytogenetically mapped rBAT to 2p16.3. In order to correlate the physical and genetic information on cystinuria, we have performed FISH with combinations of Alu-PCR-amplified sequences from YACs containing rBAT or the D2S119 and D2S177 loci. In all cases, a fused signal is obtained that demonstrates their close physical location; this allows the assignment of rBAT, cystinuria and their linked markers, D2S119 and D2S177, to 2p16.

Isolation and characterization of distinct domains of sarcolemma and T-tubules from rat skeletal muscle

1. Several cell-surface domains of sarcolemma and T-tubule from skeletal-muscle fibre were isolated and characterized. 2. A protocol of subcellular fractionation was set up that involved the sequential low- and high-speed homogenization of rat skeletal muscle followed by KCl washing, Ca2+ loading and sucrose-density-gradient centrifugation. This protocol led to the separation of cell-surface membranes from membranes enriched in sarcoplasmic reticulum and intracellular GLUT4-containing vesicles. 3. Agglutination of cell-surface membranes using wheat-germ agglutinin allowed the isolation of three distinct cell-surface membrane domains: sarcolemmal fraction 1 (SM1), sarcolemmal fraction 2 (SM2) and a T-tubule fraction enriched in protein tt28 and the alpha 2-component of dihydropyridine receptor. 4. Fractions SM1 and SM2 represented distinct sarcolemmal subcompartments based on different compositions of biochemical markers: SM2 was characterized by high levels of beta 1-integrin and dystrophin, and SM1 was enriched in beta 1-integrin but lacked dystrophin. 5. The caveolae-associated molecule caveolin was very abundant in SM1, SM2 and T-tubules, suggesting the presence of caveolae or caveolin-rich domains in these cell-surface membrane domains. In contrast, clathrin heavy chain was abundant in SM1 and T-tubules, but only trace levels were detected in SM2. 6. Immunoadsorption of T-tubule vesicles with antibodies against protein tt28 and against GLUT4 revealed the presence of GLUT4 in T-tubules under basal conditions and it also allowed the identification of two distinct pools of T-tubules showing different contents of tt28 and dihydropyridine receptors. 7. Our data on distribution of clathrin and dystrophin reveal the existence of subcompartments in sarcolemma from muscle fibre, featuring selective mutually exclusive components. T-tubules contain caveolin and clathrin suggesting that they contain caveolin- and clathrin-rich domains. Furthermore, evidence for the heterogeneous distribution of membrane proteins in T-tubules is also presented.

Cystinuria caused by mutations in rBAT, a gene involved in the transport of cystine

Cystinuria is a classic heritable aminoaciduria that involves the defective transepithelial transport of cystine and dibasic amino acids in the kidney and intestine. Six missense mutations in the human rBAT gene, which is involved in high-affinity transport of cystine and dibasic amino acids in kidney and intestine, segregate with cystinuria. These mutations account for 30% of the cystinuria chromosomes studied. Homozygosity for the most common mutation (M467T) was detected in three cystinuric siblings. Mutation M467T nearly abolished the amino acid transport activity induced by rBAT in Xenopus oocytes. These results establish rBAT as a cystinuria gene.

Perinatal hypothyroidism impairs the normal transition of GLUT4 and GLUT1 glucose transporters from fetal to neonatal levels in heart and brown adipose tissue. Evidence for tissue-specific regulation of GLUT4 expression by thyroid hormone

GLUT1 and GLUT4 glucose transporter expression is highly regulated in muscle and adipose tissue during perinatal life. Here we have investigated the role of thyroid hormones in the regulation of GLUT4 induction and GLUT1 repression associated to neonatal development. Perinatal hypothyroidism markedly impaired GLUT4 protein induction in heart. This effect was heart specific, and a greater expression of GLUT4 was detected in brown adipose tissue from neonatal hypothyroid rats compared with controls. These changes in GLUT4 protein expression were not detected in brown adipose tissue or heart when hypothyroidism was induced in adult rats. These results indicate that GLUT4 induction during perinatal life is highly sensitive to thyroid hormones in both heart and adipose tissue. Perinatal hypothyroidism was characterized by decreased cardiac GLUT4 mRNA concentrations. T3 injection caused a marked increase in cardiac levels of GLUT4 mRNA in hypothyroid neonates. Thus, in 13-day-old hypothyroid rats, GLUT4 mRNA levels increased 3-fold 1 h after T3 injection. Under these conditions, retinoic acid also caused a rapid increase in cardiac GLUT4 mRNA levels from hypothyroid neonates. In addition, cardiac levels of GLUT4 protein markedly increased in fetuses and in neonates 24 h after T3 injection. These findings suggest that a direct effect of thyroid hormones is the promotion of cardiac GLUT4 gene expression. GLUT1 protein expression was markedly enhanced in brown adipose tissue and heart during neonatal hypothyroidism as well as in hypothyroidism induced in adult rats. This was concomitant to greater levels of GLUT1 mRNA in hearts from hypothyroid neonates. Immunofluorescence analysis indicated that cardiomyocytes from hypothyroid pups contained an enhanced level of GLUT1 protein. Furthermore, T3 injection caused a decrease in cardiac levels of GLUT1 mRNA in hypothyroid neonates. These results indicate that thyroid hormone manipulation leads to inverse regulation of GLUT1 and GLUT4 glucose transporter gene expression in the neonatal heart. We conclude that thyroid hormones play a pivotal role controlling the transition of glucose transporter carriers from fetal to neonatal levels in heart and brown adipose tissue.

High and polarized expression of GLUT1 glucose transporters in epithelial cells from mammary gland: acute down-regulation of GLUT1 carriers by weaning

During lactation, the mammary gland shows high metabolic activity, which is dependent at least in part on the availability of glucose. We have studied the regulation of glucose transporter expression in different types of cell in rat mammary gland during the reproductive cycle. Glucose transporter expression varied markedly in the mammary gland during ontogeny. Thus, GLUT1 protein expression increased progressively during pregnancy, reaching the highest levels during lactation. A peak of lactation, GLUT1 protein content, expressed per g issue, was greater in mammary gland than in GLUT1-enriched tissues such as rat brain. In contrast, GLUT4 showed a marked decrease during pregnancy and practically disappeared during lactation. Regardless of the developmental stage, GLUT4 was expressed in adipocytes but not in epithelial cells from mammary gland. On the other hand, GLUT1 was expressed in both cell types. The overall pattern of GLUT1 and GLUT4 expression during the reproductive cycle reflects differences in the cell composition of the mammary gland. Thus, whereas adipocytes predominate before pregnancy, epithelial cells are the main cell type in the mammary gland during late pregnancy and lactation. Immunofluorescence and immunoelectron microscopy indicated that GLUT1 was essentially localized to the basolateral domain of epithelial cells in the mammary gland at peak of lactation, and hardly any labeling was found in the luminal membrane of epithelial cells. GLUT1 expression was acutely regulated in epithelial cells from mammary gland. Thus, GLUT1 protein markedly decreased in lactating rats 24 h after abrupt weaning in the presence of a moderate decrease in GLUT1 mRNA levels. The effect of weaning on GLUT1 protein content was not due to the fall in the plasma concentration of PRL. Thus, bromocriptine treatment for 24 h decreased GLUT1 mRNA levels in the mammary gland, but did not alter the content of GLUT1 protein. Our results demonstrate 1) the existence of a high and polarized expression of GLUT1 glucose transporters in epithelial cells from mammary gland, and 2) that GLUT1 expression is acutely regulated at a posttranslational level by weaning; this is not mimicked by bromocriptine treatment, which rules out PRL as the regulatory factor involved in the effect.

rBAT, related to L-cysteine transport, is localized to the microvilli of proximal straight tubules, and its expression is regulated in kidney by development

We have recently isolated a renal cDNA clone (rBAT) that induces amino acid transport in oocytes either as a component or as a specific activator of a system bo,(+)-like transporter. (Bertran, J., Werner, A., Moore, M. L., Stange, G., Markovich, D., Biber, J., Testar, X., Zorzano, A., Palacín, M., and Murer, H. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 5601-5605. In order to obtain additional information on the rBAT protein, we have investigated the cellular localization of rBAT and its expression during development in rat kidney. A polyclonal antibody raised against rBAT recognizes a specific protein band of approximately 90 kDa, which is highly enriched in rat renal brush border membranes. Immunofluorescence and immunoelectron microscopy studies demonstrated that rBAT is expressed in the microvilli domain of S3 epithelial cells (i.e. straight tubules). The onset of rBAT mRNA expression in kidney was detected during late fetal life. In keeping with this, induction in oocytes of L-cystine uptake due to fetal rBAT-related mRNA was approximately 10% of the induction obtained with rBAT-related mRNA from adult kidneys. rBAT mRNA levels were low in early postnatal life, and only at the end of lactation did they increase steeply, attaining approximately 50% of adult values after weaning. rBAT protein was undetectable in total membrane preparations of kidneys from fetuses and early neonates, weakly detected during lactation, and represented < 15% of adult values after weaning. The postnatal expression of rBAT and its specific location in the microvilli of epithelial cells from the S3 segment of the proximal tubule coincide with postnatal maturation of cystine resorption and with the site of high affinity resorption of cysteine in kidney. This is consistent with the involvement of rBAT in a b(o,+)-like high-affinity resorption system for cysteine in the proximal straight tubule of the nephron.

Beta 3-adrenergic receptors are responsible for the adrenergic inhibition of insulin-stimulated glucose transport in rat adipocytes

The inhibition of insulin-stimulated glucose transport by isoprenaline, a mixed beta-adrenergic-receptor (AR) agonist, is well documented in rat adipocytes. Since it has been described that rat adipocytes possess not only beta 1- and beta 2- but also beta 3-ARs, the influence of various subtype-selective beta-AR agonists and antagonists on 2-deoxyglucose (2-DG) transport was assessed in order to characterize the beta-AR subtype involved in the adrenergic counter-regulation of the insulin effect. The stimulation of 2-DG transport by insulin was counteracted, in a dose-dependent manner, by all the beta-AR agonists tested, and the magnitude of the inhibition followed the rank order: BRL 37344 > isoprenaline = noradrenaline >> dobutamine = procaterol. The same rank order of potency was obtained for lipolysis activation. This is not in accordance with the pharmacological definition of a beta 1- or a beta 2-adrenergic effect, but agrees with the pharmacological pattern of a beta 3-adrenergic effect. The inhibitory effect of the beta 3-agonist BRL 37344 on insulin-stimulated 2-DG transport was not reversed by either the selective beta 1-antagonist ICI 89406 or the beta 2-antagonist ICI 118551. In addition, neither of these beta-antagonists was able to block the isoprenaline and noradrenaline effects, supporting major beta 3-adrenoceptor-subtype involvement in the adrenergic inhibition of insulin-stimulated 2-DG transport. Like isoprenaline, BRL 37344 inhibited (60% inhibition) insulin-stimulated glucose transport only when adenosine deaminase was present in the assay. Furthermore, the maximal inhibitory effects of isoprenaline and BRL 37344 were not additive, and were both dependent on albumin concentration in the incubation medium: they increased when the albumin concentration decreased in the medium from 3.5 to 1%. To conclude, the similarities between isoprenaline and BRL 37344 action on insulin-stimulated 2-DG transport, the poor efficacy of the beta 1-/beta 2-agonists and the lack of effect of selective beta 1- and beta 2-antagonists are compelling arguments to support the important role of beta 3-adrenoceptors in the adrenergic inhibition of glucose transport in rat adipocytes.

GLUT-4 and GLUT-1 glucose transporter expression is differentially regulated by contractile activity in skeletal muscle

Mammalian skeletal muscle expresses GLUT-4 and GLUT-1 glucose transporters. Here, we have investigated whether GLUT-1 and GLUT-4 expression is regulated in muscle by contractile activity. GLUT-1 mRNA levels were high in skeletal muscle at days 16 and 17 of fetal life and decreased markedly by days 19 and 21. In contrast, GLUT-4 mRNA levels were clearly detectable at day 21 of fetal life, and they increased progressively during postnatal life. The timing data for GLUT-4 induction and GLUT-1 repression suggest that these processes are related to skeletal muscle innervation. GLUT-4 mRNA decreased markedly in adult rat and rabbit tibialis anterior muscle after severage of peroneal nerve. In contrast, GLUT-1 mRNA levels showed a 9-fold increase in rat muscle 3 days after denervation. Direct stimulation of rabbit tibialis anterior muscle with extracellular electrodes protected GLUT-4 mRNA levels against the effect of denervation. This indicates that the repression of GLUT-4 mRNA associated with denervation is due, at least in part, to electrical activity. Increased contractile activity induced for 24 h by indirect electrical stimulation at low frequency caused a marked and specific increase in GLUT-1 mRNA levels in rabbit tibialis anterior muscle. Our results indicate that (a) innervation-dependent basal contractile activity regulates in an inverse manner the expression of GLUT-1 and GLUT-4 in skeletal muscle, and (b) enhanced contractile activity stimulates GLUT-1 expression in the absence of modifications to GLUT-4 expression. This suggests the existence of different factors which depend on contractile activity and which control GLUT-1 and GLUT-4 glucose transporter expression in skeletal muscle.

Expression cloning of a human renal cDNA that induces high affinity transport of L-cystine shared with dibasic amino acids in Xenopus oocytes

A renal cDNA clone (rBAT) that induces system bo,+-like amino acid transport activity in Xenopus oocytes has recently been isolated (Bertran, J., Werner, A., Moore, M. L., Strange, G., Markovich, D., Biber, J., Testar, X., Zorzano, A., Palacín, and Murer, H. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 5601-5605). Here we show the isolation of a cDNA clone by screening a human kidney cortex cDNA library for expression of sodium-independent transport of L-[3H]arginine in Xenopus oocytes. The cRNA of this clone induces in oocytes, in addition to the uptake of L-arginine, that of L-[35S]cystine and L-[3H]leucine. Expressed uptake of these amino acids is mutually cis-inhibitable by the other 2 amino acids. Expressed uptake of L-cystine is saturable and shows an apparent Km in the micromolar range. All these characteristics resemble induction of system bo,+ related to rBAT in the oocytes. Human rBAT mRNA (approximately 2.5 kilobases) is found in kidney, small intestine (i.e., jejunum), pancreas, and liver. Human kidney poly(A)+ RNA (mRNA) induces sodium-independent uptake of L-cystine, L-arginine, and L-leucine in Xenopus oocytes. Hybrid depletion with an antisense oligonucleotide of the isolated clone greatly prevents (80-97%) human kidney mRNA-dependent induction of the uptake of these amino acids (i.e., L-cystine, L-arginine, and L-leucine). The isolated clone (2304 base pairs in length) contains a poly(A) tail and encodes a predicted 78.8-kDa protein which is 85 and 80% identical to the rabbit and rat rBAT, respectively. This predicted protein corresponds to a membrane glycoprotein, and contains six potential N-glycosylation sites which might be functional in the oocyte: [35S] methionine labeling of oocytes shows a specific band of 94 kDa in crude membranes of these human cRNA-injected oocytes; treatment of these oocytes with tunicamycin shifts the cRNA-specific translation product to approximately 72 kDa. We conclude that we have isolated a functional cDNA corresponding to human rBAT. The isolation of this human cDNA would lead to the study of the possible involvement of rBAT in human hyperaminoacidurias.