Regulation of the human Na⁺-dependent glucose cotransporter hSGLT2

The human Na(+)-glucose cotransporter SGLT2 is expressed mainly in the kidney proximal convoluted tubule where it is considered to be responsible for the bulk of glucose reabsorption. Phosphorylation profiling has revealed that SGLT2 exists in a phosphorylated state in the rat renal proximal tubule cortex, so we decided to investigate the regulation of human SGLT2 (hSGLT2) by protein kinases. hSGLT2 was expressed in human embryonic kidney (HEK) 293T cells, and the activity of the protein was measured using radiotracer and whole cell patch-clamp electrophysiology assays before and after activation of protein kinases. 8-Bromo-adenosine cAMP (8-Br-cAMP) was used to activate protein kinase A, and sn-1,2-dioctanoylglycerol (DOG) was used to activate protein kinase C (PKC). 8-Br-cAMP stimulated D-[α-methyl-(14)C]glucopyranoside ([(14)C]α-MDG) uptake and Na(+)-glucose currents by 200% and DOG increased [(14)C]α-MDG uptake and Na(+)-glucose currents by 50%. In both cases the increase in SGLT2 activity was marked by an increase in the maximum rate of transport with no change in glucose affinity. These effects were completely negated by mutation of serine 624 to alanine. Insulin induced a 250% increase in Na(+)-glucose transport by wild-type but not S624A SGLT2. Parallel studies confirmed that the activity of hSGLT1 was regulated by PKA and PKC due to changes in the number of transporters in the cell membrane. hSGLT1 was relatively insensitive to insulin. We conclude that hSGLT1 and hSGLT2 are regulated by different mechanisms and suggest that insulin is an SGLT2 agonist in vivo.     

Human cardiomyocytes express high level of Na+/glucose cotransporter 1 (SGLT1)

We have quantitatively measured gene expression for the sodium-dependent glucose cotransporters 1 and 2 (SGLT1 and SGLT2) in 23 human tissues using the method of real time PCR. As predicted, our results revealed that the expression of SGLT1 was very high in the small intestine (1.2E + 6 molecules/microg total RNA) relative to that in the kidney (3E + 4 molecules/microg total RNA). Surprisingly, we observed that the expression of SGLT1 in human heart was unexpectedly high (3.4E + 5 molecules/microg total RNA), approximately 10-fold higher than that observed in kidney tissue. DNA sequencing confirmed that the PCR amplified fragment was indeed the human SGLT1 gene. Moreover, in situ hybridization studies using a digoxigenin (DIG)-labeled antisense cRNA probe corresponding to human SGLT1 cDNA confirm that human cardiomyocytes express SGLT1 mRNA. In contrast, the expression of SGLT2 in human tissues appears to be ubiquitous, with levels ranging from 6.7E + 4 molecules/microg total RNA (in skeletal muscle) to 3.2E + 6 molecules/microg total RNA (in kidney), levels 10-100-fold higher than the expression of SGLT1 in the same tissues. Our finding that human cardiomyocytes express high levels of SGLT1 RNA suggests that SGLT1 may have a functional role in cardiac glucose transport. Since several SGLT inhibitors are currently in development as potential anti-diabetic agents, it may be important to assess the functional consequences of inhibition of SGLT1 in the heart.     

抗糖尿病药物SGLT抑制剂最新研究进展

钠-葡萄糖协转运蛋白（SGLT）是一类在小肠（SGLT-1）和肾脏近曲小管（SGLT-1、SGLT-2）发现的蛋白基因家族,负责吸收 和重吸收葡萄糖。在肾脏处,SGLT 蛋白,特别是SGLT-2 蛋白将肾小球滤过液中的绝大部分葡萄糖重新转运进入血液,从而维持 体内血糖的稳定与平衡。SGLT 蛋白抑制剂通过阻断该蛋白的转运机制,使葡萄糖随尿液排出从而降低血糖,为糖尿病的治疗提 供了创造性的思路。本文重点阐述了SGLT 蛋白和SGLT 抑制剂的作用机制,以及近年来SGLT-2 抑制剂的研发上市情况。     

Aminoglycoside antibiotics reduce glucose reabsorption in kidney through down-regulation of SGLT1

Nephrotoxicity is known to be a major clinical side effect of aminoglycoside antibiotics. Aminoglycosides cause damage to proximal tubular cells in kidney, however the mechanism of toxicity is still unclear. In order to elucidate the mechanism of nephrotoxicity, we studied the effect of aminoglycoside antibiotics on glucose transport systems in vitro and in vivo. As a result, we found that the aminoglycosides significantly reduced Na(+)/glucose cotransporter (SGLT1)-dependent glucose transport and also down-regulated mRNA and protein levels of the SGLT1 in pig proximal tubular LLC-PK(1) cells. To obtain evidence about SGLT1 down-regulation in vivo, we studied the mRNA expression of SGLT1 using gentamicin C-treated murine kidney and found that gentamicin C down-regulated SGLT1 in vivo as well as in vitro. Furthermore, the gentamicin C-treated mice showed significant rise in urinary glucose excretion. These results indicate that one of the mechanisms of aminoglycoside nephrotoxicity is the down-regulation of SGLT1, which causes reduction in glucose reabsorption in kidney.     

Changes in Sodium or Glucose Filtration Rate Modulate Expression of Glucose Transporters in Renal Proximal Tubular Cells of Rat

Renal glucose reabsorption is mediated by luminal sodium-glucose cotransporters (SGLTs) and basolateral facilitative glucose transporters (GLUTs). The modulators of these transporters are not known, and their substrates glucose and Na⁺ are potential candidates. In this study we examined the role of glucose and Na⁺ filtration rate on gene expression of glucose transporters in renal proximal tubule. SGLT1, SGLT2, GLUT1 and GLUT2 mRNAs were assessed by Northern blotting; and GLUT1 and GLUT2 proteins were assessed by Western blotting. Renal cortex and medulla samples from control rats (C), diabetic rats (D) with glycosuria, and insulin-resistant 15-month old rats (I) without glycosuria; and from normal (NS), low (LS), and high (HS) Na⁺-diet fed rats were studied. Compared to C and I rats, D rats increased (P < 0.05) gene expression of SGLT2 by ∼36%, SGLT1 by ∼20%, and GLUT2 by ∼100%, and reduced (P < 0.05) gene expression of GLUT1 by more than 50%. Compared to NS rats, HS rats increased (P < 0.05) SGLT2, GLUT2, and GLUT1 expression by ∼100%, with no change in SGLT1 mRNA expression, and LS rats increased (P < 0.05) GLUT1 gene expression by ∼150%, with no changes in other transporters. In summary, the results showed that changes in glucose or Na⁺ filtrated rate modulate the glucose transporters gene expression in epithelial cells of the renal proximal tubule. Received: 14 July 2000/Revised: 8 March 2001     

Is active glucose transport present in bovine ciliary body epithelium?

Hyperglycemia is a major risk factor for diabetic cataract formation. Effective regulation of glucose transport by the ciliary body epithelium (CBE) is pivotal to normal glycemic control in the anterior eye, which in turn affects the glucose level of the crystalline lens. The present study aimed to characterize the glucose transport mechanisms across the bovine blood-aqueous barrier (BAB) represented by the CBE. With an Ussing-type chamber, the glucose transport kinetics were measured and characterized in the presence and absence of various glucose transporter inhibitors. The saturation characteristics of the CBE to glucose were estimated from an Eadie-Hofstee plot. The mRNA expression of glucose transporters in specific regions of the bovine CBE was assessed using RT-PCR. The trans-CBE glucose flux was found to be sensitive to the glucose transporter inhibitors cytochalasin B, phloretin, and phlorizin. The transport system had a kinetic constant of 5.3 mM and a maximum velocity of 349.5 nmol.h(-1).cm(-2). Gene expression for GLUT1, GLUT3, GLUT4, GLUT5, and SGLT2 was observed in both the pars plana and pars plicata regions of the bovine CBE. This study demonstrates that glucose transport across the bovine CBE is primarily passive in nature. However, the novel findings of 1) the presence of a phlorizin-sensitive glucose flux and 2) gene expression for SGLT2 mean that a potential role for active glucose transport cannot be ruled out. The elucidation of the exact function of SGLT2 in the bovine CBE may shed important light on the glucose transport and physiology of the BAB and inform future studies of glycemic control in relation to diabetic cataract formation.     

Expression of mRNA for glucose transport proteins in jejunum, liver, kidney and skeletal muscle of pigs

Although pigs are adapted to starch-rich diets and have high turnover rates of glucose, very scarce information is available on the molecular basis of glucose transport. Therefore, the present study attempted a systematic screening for the presence of mRNA of glucose transport proteins in main organs of glucose absorption, production and conservation. From the members of the solute carrier family SLC5A (sodium glucose cotransporter), the porcine jejunum was positive for SGLT1 and SGLT3, but also contained detectable levels of SGLT5. Liver contained SGLT1, SGLT5, traces of SGLT3 and, in one of five pigs, SGLT2. Kidney contained SGLT1, SGLT2, SGLT3, SGLT5 and hardly detectable levels of SGLT4. Skeletal muscle showed weak signals for SGLT3 and SGLT5. Screening for members of the SLC2A family (facilitated glucose transporter) in intestine revealed the presence of mRNA for GLUT1, GLUT2, GLUT5, GLUT7 and GLUT8, while GLUT3, GLUT4, GLUT10 and GLUT11 were also detectable. The liver contained GLUT1, GLUT2 and GLUT8 mRNA, while GLUT3, GLUT4, GLUT5, GLUT10 and GLUT11 were poorly detectable. The kidney was positive for GLUT1, GLUT2, GLUT5, GLUT8 and GLUT11, but traces of GLUT3, GLUT4 and GLUT10 could also be detected. Skeletal muscle had the strongest signal for GLUT4, while GLUT1, GLUT3, GLUT5, GLUT8, GLUT10 and GLUT11 showed weak signals. A total of 12 unique partial cDNA sequences were submitted to GenBank. In conclusion, this study provides molecular insight into the organ-specific expression of glucose transporters in pigs and thus sheds light on the way of glucose handling in this omnivorous species.     

Functional characterisation of human SGLT-5 as a novel kidney-specific sodium-dependent sugar transporter

Sodium glucose cotransporters (SGLT) actively catalyse carbohydrate transport across cellular membranes. Six of the 12 known SGLT family members have the capacity to bind and/or transport monosaccharides (SGLT-1 to 6); of these, all but SGLT-5 have been characterised. Here we demonstrate that human SGLT-5 is exclusively expressed in the kidney. Four splice variants were detected and the most abundant SGLT-5-mRNA was functionally characterised. SGLT-5 mediates sodium-dependent [(14)C]-α-methyl-D-glucose (AMG) transport that can be inhibited by mannose, fructose, glucose, and galactose. Uptake studies using demonstrated high capacity transport for mannose and fructose and, to a lesser extent, glucose, AMG, and galactose. SGLT-5 mediated mannose, fructose and AMG transport was weakly (μM potency) inhibited by SGLT-2 inhibitors. In summary, we have characterised SGLT-5 as a kidney mannose transporter. Further studies are warranted to explore the physiological role of SGLT-5.     

How Drugs Interact with Transporters: SGLT1 as a Model

Drugs are transported by cotransporters with widely different turnover rates. We have examined the underlying mechanism using, as a model system, glucose and indican (indoxyl-beta-D: -glucopyranoside) transport by human Na(+)/glucose cotransporter (hSGLT1). Indican is transported by hSGLT1 at 10% of the rate for glucose but with a fivefold higher apparent affinity. We expressed wild-type hSGLT1 and mutant G507C in Xenopus oocytes and used electrical and optical methods to measure the kinetics of glucose (using nonmetabolized glucose analogue alpha-methyl-D: -glucopyranoside, alphaMDG) and indican transport, alone and together. Indican behaved as a competitive inhibitor of alphaMDG transport. To examine protein conformations, we recorded SGLT1 capacitive currents (charge movements) and fluorescence changes in response to step jumps in membrane voltage, in the presence and absence of indican and/or alphaMDG. In the absence of sugar, voltage jumps elicited capacitive SGLT currents that decayed to steady state with time constants (tau) of 3-20 ms. These transient currents were abolished in saturating alphaMDG but only slightly reduced (10%) in saturating indican. SGLT1 G507C rhodamine fluorescence intensity increased with depolarizing and decreased with hyperpolarizing voltages. Maximal fluorescence increased approximately 150% in saturating indican but decreased approximately 50% in saturating alphaMDG. Modeling indicated that the rate-limiting step for indican transport is sugar translocation, whereas for alphaMDG it is dissociation of Na(+) from the internal binding sites. The inhibitory effects of indican on alphaMDG transport are due to its higher affinity and a 100-fold lower translocation rate. Our results indicate that competition between substrates and drugs should be taken into consideration when targeting transporters as drug delivery systems.     

Cytostar-T scintillating microplate assay for measurement of sodium-dependent bile acid uptake in transfected HEK-293 cells

Real-time measurements of bile acid uptake into HEK-293 cell monolayers expressing the human sodium/bile acid cotransporters have been demonstrated using Cytostar-T microplates with an integral scintillating base. In these 96-well microplates, which permits culturing and observation of adherent cell monolayers, uptake of (14)C-labeled glycocholate and taurocholate into transfected HEK-293 cells was time-dependent, sodium-stimulated, and saturable. The sodium-activated uptake of 30 microM [(14)C]glycocholate (GC) via the ileal (IBAT) and liver (LBAT) transporters was 30-40 times higher than GC uptake in a sodium-free background. In addition, ouabain inhibition of the plasma membrane Na(+), K(+)-ATPase, causing the sodium gradient to collapse, resulted in total loss of glycocholate transport. Induction of gene expression by sodium butyrate showed that the amount of labeled bile acid accumulated in the cell monolayers at steady state was a function of the total amount of transporter expressed. Uptake of labeled bile acids was inhibited both by the specific IBAT inhibitor, 2164U90, and by various bile acids. No major difference was observed between IBAT and LBAT in their specificity for the bile acids tested while the dihydroxy bile acids had the highest affinity for both the transporters studied. The Cytostar-T proximity assay has been demonstrated to be an accurate and reproducible method for monitoring specific bile acid transport in transfected mammalian cells and the results are similar to those obtained by traditional methods. We conclude that the technique is an attractive approach to the cellular study of membrane transport of radiolabeled solutes in general and suggest a role in screening and characterization of novel transport inhibitors.     

Regulation of jejunal glucose transporter expression by forskolin

We have investigated the effects of forskolin on enterocyte membrane expression of the glucose transporters, SGLT1 and GLUT2, which are thought to be the main entry and efflux pathways for glucose, respectively. Forskolin treatment increased SGLT1 but decreased GLUT2 expression in mid and lower villus enterocytes. No change in transporter expression was noted in upper villus cells. Likewise, cyclic AMP levels were raised in mid and lower but not upper villus cells. The implications of these data for glucose transport are discussed.     

Expression of the Na+-d-Glucose Cotransporter SGLT1 in Neurons

Abstract: In brains of the rabbit, pig, and human, expression of the high-affinity Na⁺-d -glucose cotransporter SGLT1 and of the protein RS1, which alters the activity of SGLT1, was demonstrated. In situ hybridization showed that SGLT1 and RS1 are transcribed in pyramidal cells of brain cortex and hippocampus and in Purkinje cells of cerebellum. In neurons of pig brain SGLT1 protein was demonstrated by western blotting with synaptosomal membranes and by immunohistochemistry, which showed SGLT1 in pyramidal and Purkinje cells. To test whether SGLT1 in neurons may be activated during increased d -glucose consumption, an epileptic seizure was induced in rat brain, and the uptake of specific nonmetabolized substrates of SGLT1 {[¹⁴C]methyl-α-d -glucopyranoside ([¹⁴C]AMG)} and of Na⁺-independent transporters {2-deoxy-d -[¹⁴C]glucose([¹⁴C]2-DG)} was analyzed by autoradiography. During the seizure the uptake of AMG and 2-DG was increased in the focus. Within two hours after the seizure 2-DG uptake in the focus returned to normal. In contrast, the AMG uptake in the focus area was still increased 1 day later. The data show that the high-affinity Na⁺-d -glucose cotransporter SGLT1 is expressed in neurons and can be up-regulated.     

Phosphatidylinositol 3-kinase mediates inhibitory effect of angiotensin II on sodium/glucose cotransporter in renal epithelial cells

Effects of angiotensin II (ANGII) on regulation of sodium/glucose cotransporter (SGLT1) activity were investigated in LLC-PK(1) cells, renal proximal epithelial cell line. ANGII inhibited alpha-[14C] methyl-D-glucopyranoside (AMG) uptake into LLC-PK(1) cells in a dose-dependent manner. This inhibition was based on a decrease in maximal transport rate (Vmax) of AMG from 2.20 nmol/mg protein/15 min to 1.19 nmol/mg protein/15 min, although apparent affinity constant (Km) did not alter. In western blot analysis, protein level of SGLT1 in brush border membrane (BBM) was decreased by ANGII, although total SGLT1 was not altered. In the aspect of intracellular signal transduction, ANGII blocked the formation of cAMP. Pertussis toxin, an inactivator of Gi protein that control intracellular cAMP level, completely prevented the decrease of AMG uptake caused by ANGII. 8-Br-cAMP, a cell membrane permeable cAMP analogue, increased AMG uptake and protein level of SGLT1 in BBM. Both wortmannin and LY294002 that are phosphatidylinositol (PI) 3-kinase inhibitors, inhibited the SGLT1 activity, and also attenuated the effect of 8-Br-cAMP on SGLT1 activity. Those inhibitors prevented the 8-Br-cAMP-induced expression of SGLT1 in plasma membrane. We conclude that ANGII plays an important role in post-translational regulation in SGLT1. Inhibition of SGLT1 translocation is suggested to be caused by inactivation of protein kinase A and decrease of PI 3-kinase activity.     

Structural selectivity of human SGLT inhibitors

Human Na(+)-D-glucose cotransporter (hSGLT) inhibitors constitute the newest class of diabetes drugs, blocking up to 50% of renal glucose reabsorption in vivo. These drugs have potential for widespread use in the diabetes epidemic, but how they work at a molecular level is poorly understood. Here, we use electrophysiological methods to assess how they block Na(+)-D-glucose cotransporter SGLT1 and SGLT2 expressed in human embryonic kidney 293T (HEK-293T) cells and compared them to the classic SGLT inhibitor phlorizin. Dapagliflozin [(1S)-1,5,5-anhydro-1-C-{4-chloro-3-[(4-ethoxyphenyl)methyl]phenyl}-D-glucitol], two structural analogs, and the aglycones of phlorizin and dapagliflozin were investigated in detail. Dapagliflozin and fluoro-dapagliflozin [(1S)-1,5-anhydro-1-C-{4-chloro-3-[(4-ethoxyphenyl)methyl]phenyl}-4-F-4-deoxy-D-glucitol] blocked glucose transport and glucose-coupled currents with ≈100-fold specificity for hSGLT2 (K(i) = 6 nM) over hSGLT1 (K(i) = 400 nM). As galactose is a poor substrate for SGLT2, it was surprising that galacto-dapagliflozin [(1S)-1,5-anhydro-1-C-{4-chloro-3-[(4-ethoxyphenyl)methyl]phenyl}-D-galactitol] was a selective inhibitor of hSGLT2, but was less potent than dapagliflozin for both transporters (hSGLT2 K(i) = 25 nM, hSGLT1 K(i) = 25,000 nM). Phlorizin and galacto-dapagliflozin rapidly dissociated from SGLT2 [half-time off rate (t(1/2,Off)) ≈ 20-30 s], while dapagliflozin and fluoro-dapagliflozin dissociated from hSGLT2 at a rate 10-fold slower (t(1/2,Off) ≥ 180 s). Phlorizin was unable to exchange with dapagliflozin bound to hSGLT2. In contrast, dapagliflozin, fluoro-dapagliflozin, and galacto-dapagliflozin dissociated quickly from hSGLT1 (t(1/2,Off) = 1-2 s), and phlorizin readily exchanged with dapagliflozin bound to hSGLT1. The aglycones of phlorizin and dapagliflozin were poor inhibitors of both hSGLT2 and hSGLT1 with K(i) values > 100 μM. These results show that inhibitor binding to SGLTs is composed of two synergistic forces: sugar binding to the glucose site, which is not rigid, and so different sugars will change the orientation of the aglycone in the access vestibule; and the binding of the aglycone affects the binding affinity of the entire inhibitor. Therefore, the pharmacophore must include variations in both the structure of the sugar and the aglycone.     

Development of a scintillation proximity assay for analysis of Na+/Cl- -dependent neurotransmitter transporter activity

Human placental choriocarcinoma (JAR) cells endogenously expressing glycine transporter type 1a (GlyT1a) have been cultured in 96-well scintillating microplates to develop a homogenous screening assay for the detection of GlyT1 antagonists. In these microplates uptake of [14C]glycine was time dependent and saturable with a Michaelis-Menten constant (Km) of 27+/-3 microM. The GlyT1 transport inhibitors sarcosine, ALX-5407, and Org-24598 were tested and shown to block [14C]glycine uptake with expected IC50 values of 37.5+/-4.6 microM, 2.8+/-0.6 nM, and 6.9+/-0.9 nM, respectively. The [14C]glycine uptake process was sensitive to membrane Na+ gradient as blockade of membrane Na+/K+-ATPase by ouabain or Na+ exchanger by benzamil-disrupted glycine accumulation in JAR cells. Glycine influx was not affected by concentration of dimethyl sulfoxide up to 2%. The versatility of this technological approach was further confirmed by the characterization of a saturable [14C]taurine uptake in JAR cells. Taurine transport was of high affinity with a Km of 10.2+/-1.7 microM and fully inhibited by ALX-5407 (IC50=522 +/-83 nM). The developed assay is homogenous, rapid, versatile and amenable to automation for the discovery of new neurotransmitter transporter inhibitors.     

Sugar binding residue affects apparent Na+ affinity and transport stoichiometry in mouse sodium/glucose cotransporter type 3B

SGLT1 is a sodium/glucose cotransporter that moves two Na(+) ions with each glucose molecule per cycle. SGLT3 proteins belong to the same family and are described as glucose sensors rather than glucose transporters. Thus, human SGLT3 (hSGLT3) does not transport sugar, but extracellular glucose depolarizes the cell in which it is expressed. Mouse SGLT3b (mSGLT3b), although it transports sugar, has low apparent sugar affinity and partially uncoupled stoichiometry compared with SGLT1, suggesting that mSGLT3b is also a sugar sensor. The crystal structure of the Vibrio parahaemolyticus SGLT showed that residue Gln(428) interacts directly with the sugar. The corresponding amino acid in mammalian proteins, 457, is conserved in all SGLT1 proteins as glutamine. In SGLT3 proteins, glutamate is the most common residue at this position, although it is a glycine in mSGLT3b and a serine in rat SGLT3b. To test the contribution of this residue to the function of SGLT3 proteins, we constructed SGLT3b mutants that recapitulate residue 457 in SGLT1 and hSGLT3, glutamine and glutamate, respectively. The presence of glutamine at residue 457 increased the apparent Na(+) and sugar affinities, whereas glutamate decreased the apparent Na(+) affinity. Moreover, glutamate transported more cations per sugar molecule than the wild type protein. We propose a model where cations are released intracellularly without the release of sugar from an intermediate state. This model explains the uncoupled charge:sugar transport phenotype observed in wild type and G457E-mSGLT3b compared with SGLT1 and the sugar-activated cation transport without sugar transport that occurs in hSGLT3.     

Effects of SGLT2 Inhibition in Human Kidney Proximal Tubular Cells—Renoprotection in Diabetic Nephropathy?

Sodium/glucose cotransporter 2 (SGLT2) inhibitors are oral hypoglycemic agents used to treat patients with diabetes mellitus. SGLT2 inhibitors block reabsorption of filtered glucose by inhibiting SGLT2, the primary glucose transporter in the proximal tubular cell (PTC), leading to glycosuria and lowering of serum glucose. We examined the renoprotective effects of the SGLT2 inhibitor empagliflozin to determine whether blocking glucose entry into the kidney PTCs reduced the inflammatory and fibrotic responses of the cell to high glucose. We used an in vitro model of human PTCs. HK2 cells (human kidney PTC line) were exposed to control 5 mM, high glucose (HG) 30 mM or the profibrotic cytokine transforming growth factor beta (TGFβ1; 0.5 ng/ml) in the presence and absence of empagliflozin for up to 72 h. SGLT1 and 2 expression and various inflammatory/fibrotic markers were assessed. A chromatin immunoprecipitation assay was used to determine the binding of phosphorylated smad3 to the promoter region of the SGLT2 gene. Our data showed that TGFβ1 but not HG increased SGLT2 expression and this occurred via phosphorylated smad3. HG induced expression of Toll-like receptor-4, increased nuclear deoxyribonucleic acid binding for nuclear factor kappa B (NF-κB) and activator protein 1, induced collagen IV expression as well as interleukin-6 secretion all of which were attenuated with empagliflozin. Empagliflozin did not reduce high mobility group box protein 1 induced NF-κB suggesting that its effect is specifically related to a reduction in glycotoxicity. SGLT1 and GLUT2 expression was not significantly altered with HG or empagliflozin. In conclusion, empagliflozin reduces HG induced inflammatory and fibrotic markers by blocking glucose transport and did not induce a compensatory increase in SGLT1/GLUT2 expression. Although HG itself does not regulate SGLT2 expression in our model, TGFβ increases SGLT2 expression through phosphorylated smad3.     

The role of facilitated diffusion in glucose transport across the apical membrane of enterocytes

In chronic experiments on Wistar rats, glucose and galactose absorption in the isolated loop of the small intestine considerably decreased in presence of both phloridzine am phloritine (inhibitors of the glucose transporters SGLT1 and GLUT2). The load of the isolated loop with glucose or galactose solutions scarcely influenced the absorption of 2-deoxi-D-glucose (substrate for GLUT2). According to the immunocytochemical analysis by means of confocal microscopy, after the load of the isolated loop with glucose (75 mM) the labels to GLUT2 and proteinkinase C (PKC betalI) were concentrated mainly in the apical part of the enterocytes, whereas after the load with the Ringer solution--in the basal part of the enterocytes. It was shown on the mathematical model that the part of the facilitated diffusion in the total glucose absorption was considerably lesser in comparison with the active transport mediated by SGLT1. Thus the findings support the hypothesis about a recruitment of the transporter GLUT2 into the apical membrane of the enterocytes and its involvement in glucose transfer across this membrane. However, under natural conditions, the active transport is the main mechanism of glucose absorption, whereas the facilitated diffusion plays a certain role only at high carbohydrate loads.