Evidence for expression of the facilitated glucose transporter in rat hepatocytes

GLUT3 is present in Clone 9 liver cells and translocates to the plasma membrane in response to insulin

Clone 9 cells have been reported to express only the GLUT1 facilitative glucose transporter; however, previous studies have not examined Clone 9 cells for GLUT3 content. The current study sought to profile the presence of glucose transporters in Clone 9 cells, H4IIE hepatoma cells, and L6 myoblasts and myotubes. While the other cell types contained the expected complement of transporters, Clone 9 cells had GLUT3 which was previously not reported. Interestingly, both GLUT3 mRNA and protein were detected in Clone 9 cells, but only mRNA for GLUT1 was detected. Glucose transport in Clone 9 cells was insulin-sensitive in a concentration-dependent manner, concomitant with the presence of GLUT3 in the plasma membrane after insulin treatment. Although basal glucose uptake was unaffected, insulin-stimulated glucose uptake was abolished with siRNA-mediated GLUT3 knockdown. These results contradict previous reports that Clone 9 cells exclusively express GLUT1 and suggest GLUT3 is a key insulin-sensitive glucose transporter required for insulin-stimulated glucose uptake by Clone 9 cells.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Co-transfection of GK and mhPINS genes into HepG2 cells confers glucose-stimulated insulin secretion

BACKGROUND

The purpose of this study was to construct an 'artificial beta cell' that can exhibit physiologic glucose-stimulated insulin secretion for the treatment of type 1 diabetes.

METHODS

Retroviral vector containing the glucokinase (GK) gene and mutated human proinsulin (mhPINS) gene was constructed. HepG2 cells were first infected with recombinant retrovirus carrying the GK and mhPINS genes, then selectively cultured with G418 to obtain the positive clones. GK and mhPINS gene transcription and expression were identified by radioimmunity, Western blot and RT-PCR techniques. Finally, the dose-response effect of glucose on insulin secretion from those HepG2 cells that expressed both GK and mhPINS genes was tested with HepG2 cells that only expressed the mhPINS gene as a control.

RESULTS

HepG2 cells with transferred GK and mhPINS genes were selectively cultured with G418 and the positive clones were obtained in 3 weeks. Four clones with GK and mhPINS gene expression were selected from 20 positive clones by radioimmunity and Western blot. We picked up one clone with a strong GK and mhPINS gene expression and named it clone Beta. In clone Beta, differences in insulin secretion at 0.5 and 0.75 mmol/L glucose concentrations were not significant (P>0.05) and differences in insulin secretion at 2.0, 3.0, 4.0, 5.0 and 6.0 mmol/L glucose concentrations were not significant (P>0.05), while there were significant differences in insulin secretion at other glucose concentrations(P<0.05). The artificial beta cell, clone Beta, obtained a glucose-stimulated insulin secretion with maximal insulin secretion at 1.75-2.00 mmol/L glucose concentrations.

DISCUSSION

An artificial beta cell that exhibits glucose-stimulated insulin secretion can be constructed successfully.

D-Glucose uptake in fish hepatocytes: mediated by transporter in rainbow trout (Oncorhynchus mykiss), but only by diffusion in prespawning lamprey (Lampetra fluviatilis) and in RTH-149 cell line

The transport of D-glucose into rainbow trout (Oncorhynchus mykiss) and river lamprey (Lampetra fluviatilis) hepatocytes, as well as into rainbow trout hepatoblastoma cell line RTH-149 was studied using tracer methods. The half-time for D-glucose equilibration was 15 s for rainbow trout. The half-times for the non-metabolizable D-glucose analog, 3-O-methyl-D-glucose equilibration were 8 s, 37 s and 38 s for rainbow trout, lamprey and RTH-149 cells, respectively. The 3-O-methyl-D-glucose was taken up by rainbow trout hepatocytes by facilitated diffusion in addition to simple diffusion. The uptake showed saturation kinetics with the K(m) of 37 mM and V(max) of 62 mmol kg(-1) cells min(-1). The uptake was sensitive to phloretin and cytochalasin B, but not affected by ouabain. The 3-O-methyl-D-glucose uptake by lamprey hepatocytes and RTH-149 cells showed no indication of saturation up to 160 mM, and was not affected by phloretin, cytochalasin B or ouabain, which suggests the mode of transport to be by passive diffusion. However, immunocytochemical stainings revealed the existence of mammalian type GLUT1 and GLUT2 transporters in all cells studied. The lack of a functioning carrier-mediated glucose uptake in lamprey hepatocytes might be due to its physiological state (prespawning starvation). The minor 3-O-methyl-D-glucose uptake into RTH-149 cells compared to freshly isolated rainbow trout hepatocytes might reflect low metabolic activity of the cell lines. Under the conditions applied the RTH-149 cell line is no suitable in vitro model for glucose transport in fish cells.

Two histone deacetylase inhibitors, trichostatin A and sodium butyrate, suppress differentiation into osteoclasts but not into macrophages

Histone deacetylase (HDAC) inhibitors are emerging as a new class of anticancer therapeutic agents and have been demonstrated to induce differentiation in some myeloid leukemia cell lines. In this study, we show that HDAC inhibitors have a novel action on osteoclast differentiation. The effect of 2 HDAC inhibitors, trichostatin A (TSA) and sodium butyrate (NaB), on osteoclastogenesis was investigated using rat and mouse bone marrow cultures and a murine macrophage cell line RAW264. Both TSA and NaB inhibited the formation of preosteoclast-like cells (POCs) and multinucleated osteoclast-like cells (MNCs) in rat bone marrow culture. By reverse transcription-polymerase chain reaction analysis, TSA reduced osteoclast-specific mRNA expression of cathepsin K and calcitonin receptor (CTR). In contrast, TSA and NaB did not affect the formation of bone marrow macrophages (BMMs) induced by macrophage colony-stimulating factor as examined by nonspecific esterase staining. Fluorescence-activated cell sorting analysis showed that TSA did not affect the surface expression of macrophage markers for CD11b and F4/80 of BMMs. TSA and NaB also inhibited osteoclast formation and osteoclast-specific mRNA expression in RAW264 cells stimulated with receptor activator of nuclear factor-kappa B (NF-kappa B) ligand (RANKL). Transient transfection assay revealed that TSA and NaB dose dependently reduced the sRANKL-stimulated or tumor necrosis factor alpha (TNF-alpha)-stimulated transactivation of NF-kappa B-dependent reporter genes. The treatment of RAW264 cells with TSA and NaB inhibited TNF-alpha-induced nuclear translocation of NF-kappa B and sRANKL-induced activation of p38 mitogen-activated protein kinase (MAPK) signals. These data suggest that both TSA and NaB exert their inhibitory effects by modulating osteoclast-specific signals and that HDAC activity regulates the process of osteoclastogenesis.

Constitutive and regulated expression of processed insulin following in vivo hepatic gene transfer

To test whether hepatocytes engineered in vivo can serve as surrogate beta cells by similarly secreting mature insulin in a glucose-sensitive manner, we prepared adenoviral vectors encoding wild-type proinsulin (hIns-wt), a modified proinsulin cleavable by the ubiquitously expressed protease furin (hIns-M3), or each of the two beta cell specific pro-insulin convertases PC2 and PC3. Following a detailed in vitro characterization of the proteins produced by our vectors, we infected the liver and, for comparison, the muscle of a chemically induced murine model of type I diabetes. Insulin expression from the transduced tissues was extensively characterized and showed to be constitutive rather than regulated. To obtain regulated expression, we placed expression of hIns-M3 under the control of the dimerizer-inducible transcription system. Hormone secretion from mouse liver was negligible in the absence of the dimerizer drug rapamycin, was inducible in a dose-dependent manner upon its administration, and reversible following drug withdrawal. These data confirm liver as a promising target for in vivo expression of processed insulin. While suggesting that hepatocytes cannot provide authentic glucose-responsive regulation, these results demonstrate that pharmacological regulation is a promising alternative route to the controlled delivery of insulin following hepatic gene transfer.

A tumor-associated glycosylation change in the glucose transporter GLUT1 controlled by tumor suppressor function in human cell hybrids

Studies of human cell hybrids have provided evidence that the tumorigenicity of a cervical carcinoma (HeLa) is under the control of a putative tumor suppressor on chromosome 11. Using these human cell hybrids, we found a tumor-associated glycosylation change in the glucose transporter GLUT1, which is an N-linked glycoprotein at the plasma membrane. The non-tumorigenic HeLa × fibroblast cell hybrid CGL1 and the normal diploid fibroblast WI38 expressed the 50–55 kDa GLUT1, whereas in a tumorigenic segregant hybrid, CGL4, as well as in parental HeLa cells, GLUT1 glycosylation was altered and its molecular mass was about 70 kDa. However, the altered GLUT1 glycosylation was not observed in SV40-transformed WI38 cells, suggesting a correlation between this glycosylation change and a putative tumor suppressor function. Further investigations using glycosidases, glycosylation inhibitors and lectin-affinity chromatography demonstrated that the tumor-associated glycosylation change in GLUT1 was mainly due to the increase in N-acetyl-lactosamine repeats in the N-linked oligosaccharides. In accordance with the altered glycosylation, affinity for 2-deoxyglucose in the tumorigenic CGL4 cells increased 2-fold, but there was little change in the Vmax. These results suggest there may be a functional role for the modulation by glycosylation of GLUT1 in the tumorigenic behavior of CGL4 and HeLa cells.

GLUT-1 and GLUT-2 mRNA, protein, and glucose transporter activity in cultured fetal and adult hepatocytes

To understand glycogenesis in the fetal hepatocyte, we examined glucose transport in cultured fetal and adult male rat hepatocytes. GLUT-1 mRNA was detected in fetal hepatocytes at isolation but in adult hepatocytes only after culture. GLUT-1 mRNA was more abundant in fetal than in adult hepatocytes (P < 0.005). GLUT-1 protein paralleled its message. GLUT-2 mRNA was more abundant in adult than in fetal hepatocytes (P < 0.05), and abundance did not change during culture, but GLUT-2 protein was discordantly regulated. There was more GLUT-2 protein in fetal hepatocytes at 45 h (P < 0.025). An Eadie-Hofstee plot of 3-O-methylglucose transport appeared to have two linear components. One component was presumed to be GLUT-1 [variable Michaelis constant (Km) approximating 6-8 mM, maximal uptake rate (Vmax) for fetal vs. adult hepatocytes 106 vs. 35 nmol.min-1.mg protein-1], and a second was presumed to be GLUT-2 (Km of 23 mM, Vmax for fetal vs. adult hepatocytes 198 vs. 92 nmol.min-1.mg protein-1). Early phosphorylation of 2-deoxyglucose was greater in fetal than in adult hepatocytes, but transport was always greater than phosphorylation. Increased expression of both GLUT-1 and GLUT-2 by fetal hepatocytes permits greater glucose uptake and positions the fetal rat hepatocyte for efficient glycogenesis at low plasma glucose concentration.

Differential regulation of glucose transporter 1 and 2 mRNA expression by epidermal growth factor and transforming growth factor-beta in rat hepatocytes

We have examined by Northern blot analysis the expression of two members of the glucose transporter family of genes (GLUT-1 and GLUT-2) in regenerating liver and in hepatocytes cultured under various conditions. GLUT-1, although thought to be a growth-associated gene, is not expressed in normal or regenerating liver, whereas GLUT-2, a liver-specific gene, is abundant in normal liver and gradually up-regulated during liver regeneration. Conversely, in hepatocytes cultured conventionally on dried rat tail collagen (RTC) in the presence of EGF and insulin, which potentiate proliferation, GLUT-1 mRNA is rapidly and abundantly expressed, whereas GLUT-2 is depressed. To investigate the causes of this "switch" in glucose transporter expression seen when hepatocytes are removed from the liver and cultured under the conventional proliferative conditions, we examined the effects of specific growth factors and extracellular matrices on cultured hepatocytes. EGF, a potent liver mitogen, although causing a threefold induction of GLUT-1, was found to have no effect on GLUT-2 expression, suggesting that the increase in GLUT-2 seen in regenerating liver is not due to EGF. Inhibition of protein synthesis by cycloheximide in cultured hepatocytes does not prevent the induction of GLUT-1 mRNA. In addition, treatment of cells with cycloheximide appears to stabilize the GLUT-2 mRNA, preventing the usual down-regulation of this gene in cultured hepatocytes. The expression of the two glucose transporter mRNAs also differed when the hepatocytes were adherent to particular cell matrices. Culture of hepatocytes on a reconstituted basement membrane gel matrix (EHS) is known to restrain their growth and mediate high levels of differentiated hepatocytic functions that are lost under conventional culture conditions. Unlike cells on RTC, hepatocytes on EHS expressed low levels of GLUT-1 mRNA, and decreased GLUT-2 mRNA. TGF-beta, an attenuator of DNA synthesis, when added to cultures on RTC, substantially down-regulated GLUT-2 but had no effect on GLUT-1. We propose that the effectors, EGF, TGF-beta and basement membrane components, play a significant role in the regulation of expression of GLUT-1 and GLUT-2 in hepatocytes.

Expression of glucose transporters (GLUT 1 and GLUT 4) in primary cultured rat adipocytes: differential evolution with time and chronic insulin effect

We previously reported that in cultured adipose cell lines insulin increased selectively the expression of Glut 1, in contrast to in vivo regulation where variations in insulinemia have been shown to affect only GLUT 4. We have addressed here the question of the long-term regulation of GLUT 1 and GLUT 4 in fat cells by using primary cultures of rat adipocytes. Epididymal fat cells were isolated by collagenase and cultured 4 days in DMEM supplemented with BSA 1%, FCS 1%, and glucose 10 mM. GLUT 1 and GLUT 4 proteins were assessed in total cellular membranes by Western blotting, using specific antibodies against their respective C-terminal peptides. GLUT 1 steadily increased over culture time to reach at day 3, a level 3-fold higher than the initial value. In contrast, GLUT 4 decreased sharply and stabilized at day 3, at 30% of the initial value. The changes in GLUT 1 and GLUT 4 mRNAs with culture time were parallel to changes in the corresponding proteins, suggesting a pre-translational level of regulation. The expression of the lipogenic enzyme, fatty acid synthetase (FAS), highly expressed in fat cell, decreased over time following a pattern closely parallel to that of GLUT 4. Chronic exposure to insulin added at day 2 had no effect on GLUT 4 expression but increased the expression of GLUT 1 and FAS by 70% and 36%, respectively. Glucose consumption was stable over 4 days of culture, while lactate production increased from 24 to 36% of glucose utilization, in agreement with the loss in FAS. Glucose consumption increased only slightly with insulin (+160%), in good keeping with the low levels of expression of both GLUT 4 and FAS in these cultured cells. These data indicate that culture alters oppositely the expression of GLUT 1 and GLUT 4 in rat adipocytes and suggest that factor(s) other than insulin predominate in their regulation in vivo.

Glucose transporter isotypes switch in T-antigen-transformed pancreatic beta cells growing in culture and in mice

High-level expression of the low-Km glucose transporter isoform GLUT-1 is characteristic of many cultured tumor and oncogene-transformed cells. In this study, we tested whether induction of GLUT-1 occurs in tumors in vivo. Normal mouse beta islet cells express the high-Km (approximately 20 mM) glucose transporter isoform GLUT-2 but not the low-Km (1 to 3 mM) GLUT-1. In contrast, a beta cell line derived from an insulinoma arising in a transgenic mouse harboring an insulin-promoted simian virus 40 T-antigen oncogene (beta TC3) expressed very low levels of GLUT-2 but high levels of GLUT-1. GLUT-1 protein was not detectable on the plasma membrane of islets or tumors of the transgenic mice but was induced in high amounts when the tumor-derived beta TC3 cells were grown in tissue culture. GLUT-1 expression in secondary tumors formed after injection of beta TC3 cells into mice was reduced. Thus, high-level expression of GLUT-1 in these tumor cells is characteristic of culture conditions and is not induced by the oncogenic transformation; indeed, overnight culture of normal pancreatic islets causes induction of GLUT-1. We also investigated the relationship between expression of the different glucose transporter isoforms by islet and tumor cells and induction of insulin secretion by glucose. Prehyperplastic transgenic islet cells that expressed normal levels of GLUT-2 and no detectable GLUT-1 exhibited an increased sensitivity to glucose, as evidenced by maximal insulin secretion at lower glucose concentrations, compared with that exhibited by normal islets. Further, hyperplastic islets and primary and secondary tumors expressed low levels of GLUT-2 and no detectable GLUT-1 on the plasma membrane; these cells exhibited high basal insulin secretion and responded poorly to an increase in extracellular glucose. Thus, abnormal glucose-induced secretion of insulin in prehyperplastic islets in mice was independent of changes in GLUT-2 expression and did not require induction of GLUT-1 expression.

Glucose transporter isotypes switch in T-antigen-transformed pancreatic beta cells growing in culture and in mice

High-level expression of the low-Km glucose transporter isoform GLUT-1 is characteristic of many cultured tumor and oncogene-transformed cells. In this study, we tested whether induction of GLUT-1 occurs in tumors in vivo. Normal mouse beta islet cells express the high-Km (approximately 20 mM) glucose transporter isoform GLUT-2 but not the low-Km (1 to 3 mM) GLUT-1. In contrast, a beta cell line derived from an insulinoma arising in a transgenic mouse harboring an insulin-promoted simian virus 40 T-antigen oncogene (beta TC3) expressed very low levels of GLUT-2 but high levels of GLUT-1. GLUT-1 protein was not detectable on the plasma membrane of islets or tumors of the transgenic mice but was induced in high amounts when the tumor-derived beta TC3 cells were grown in tissue culture. GLUT-1 expression in secondary tumors formed after injection of beta TC3 cells into mice was reduced. Thus, high-level expression of GLUT-1 in these tumor cells is characteristic of culture conditions and is not induced by the oncogenic transformation; indeed, overnight culture of normal pancreatic islets causes induction of GLUT-1. We also investigated the relationship between expression of the different glucose transporter isoforms by islet and tumor cells and induction of insulin secretion by glucose. Prehyperplastic transgenic islet cells that expressed normal levels of GLUT-2 and no detectable GLUT-1 exhibited an increased sensitivity to glucose, as evidenced by maximal insulin secretion at lower glucose concentrations, compared with that exhibited by normal islets. Further, hyperplastic islets and primary and secondary tumors expressed low levels of GLUT-2 and no detectable GLUT-1 on the plasma membrane; these cells exhibited high basal insulin secretion and responded poorly to an increase in extracellular glucose. Thus, abnormal glucose-induced secretion of insulin in prehyperplastic islets in mice was independent of changes in GLUT-2 expression and did not require induction of GLUT-1 expression.

The ubiquitous glucose transporter GLUT-1 belongs to the glucose-regulated protein family of stress-inducible proteins

In mammals, glucose transport is mediated by five structurally related glucose transporters that show a characteristic cell-specific expression. However, the rat brain/HepG2/erythrocyte-type glucose transporter GLUT-1 is expressed at low levels in most cells. The reason for this coexpression is not clear. GLUT-1 is negatively regulated by glucose. Another family of proteins, glucose-regulated proteins (GRPs), is also ubiquitously expressed and stimulated by glucose deprivation and other cellular stresses. We therefore hypothesized that GLUT-1 may be a glucose-regulated stress protein. This was tested by subjecting L8 myocytes and NIH 3T3 fibroblasts to glucose starvation or exposure to the calcium ionophore A23187, 2-mercaptoethanol, or tunicamycin, all known to increase GRP levels. The mRNA for GLUT-1 was augmented by 50-300% in a time-dependent manner, similarly to the changes in GRP-78 mRNA. Ex vivo incubation of rat soleus muscles induced a marked and concomitant rise in the mRNA levels of GLUT-1 and GRP-78. Finally, calcium ionophore A23187 and 2-mercaptoethanol induced a 2- to 3-fold increase in the levels of the GLUT-1 protein and hexose uptake. In all instances in which GRP-78 and GLUT-1 responded to stress, the transcription of the cell-specific muscle/adipocyte-type insulin-responsive glucose transporter (GLUT-4) did not change. Thus, despite the lack of structural similarity, GLUT-1 and GRP-78 expression is regulated similarly, whereas the regulation of GLUT-4, which is structurally related to GLUT-1, is different. We propose that GLUT-1 belongs to the GRP family of stress proteins and that its ubiquitous expression may serve a specific purpose during cellular stress.

Diabetic hyperglycemia: link to impaired glucose transport in pancreatic beta cells

Glucose uptake into pancreatic beta cells by means of the glucose transporter GLUT-2, which has a high Michaelis constant, is essential for the normal insulin secretory response to hyperglycemia. In both autoimmune and nonautoimmune diabetes, this glucose transport is reduced as a consequence of down-regulation of the normal beta-cell transporter. In autoimmune diabetes, circulating immunoglobulins can further impair this glucose transport by inhibiting functionally intact transporters. Insights into mechanisms of the unresponsiveness of beta cells to hyperglycemia may improve the management and prevention of diabetes.

Elevation of facilitated glucose-transporter messenger RNA in human hepatocellular carcinoma

Complementary DNA of a rat brain glucose transporter gene was used to examine the expression of glucose-transporter messenger RNA in paired human hepatocellular carcinomas and adjacent nontumorous liver tissues, as well as in human hepatoma cell lines and human fetal liver samples. High expression of a major 2.8-kilobase glucose-transporter transcript was seen in all hepatoma cell lines and fetal liver samples examined, whereas a much lower level of expression was observed in liver tissues. When pairs of liver tissues were examined, elevation of glucose-transporter RNA levels was observed in most of the hepatocellular carcinoma tissues examined.