Acute regulation of the facilitative glucose transporter GLUT1 in a hematopoietic cell line

This brief review is focused on the short-term regulation of the facilitative glucose transporter GLUT1 in megakaryocytic cells M07e. The effects of cytokines such as TPO, GM-CSF and SCF and of a low dose of H202 on the transport activity and its kinetic parameters are compared. The possible mechanisms and the signalling pathways involved in the glucose uptake activation are discussed. A role for the cellular redox status in glucose uptake control, possibly related to the status of redox-sensitive enzymes such as tyrosine phosphatases, is suggested.     

Evidence for two different broad-specificity oligopeptide transporters in intestinal cell line Caco-2 and colonic cell line CCD841

Recently the existence of two different Na(+)-coupled oligopeptide transport systems has been described in mammalian cells. These transport systems are distinct from the previously known H(+)/peptide cotransporters PEPT1 and PEPT2, which transport only dipeptides and tripeptides. To date, the only peptide transport system known to exist in the intestine is PEPT1. Here we investigated the expression of the Na(+)-coupled oligopeptide transporters in intestinal cell lines, using the hydrolysis-resistant synthetic oligopeptides deltorphin II and [d-Ala(2),d-Leu(5)]enkephalin (DADLE) as model substrates. Caco-2 cells and CCD841 cells, both representing epithelial cells from human intestinal tract, were able to take up these oligopeptides. Uptake of deltorphin II was mostly Na(+) dependent, with more than 2 Na(+) involved in the uptake process. In contrast, DADLE uptake was only partially Na(+) dependent. The uptake of both peptides was also influenced by H(+) and Cl(-), although to a varying degree. The processes responsible for the uptake of deltorphin II and DADLE could be differentiated not only by their Na(+) dependence but also by their modulation by small peptides. Several dipeptides and tripeptides stimulated deltorphin II uptake but inhibited DADLE uptake. These modulating small peptides were, however, not transportable substrates for the transport systems that mediate deltorphin II or DADLE uptake. These two oligopeptide transport systems were also able to take up several nonopioid oligopeptides, consisting of 9-17 amino acids. This represents the first report on the existence of transport systems in intestinal cells that are distinct from PEPT1 and capable of transporting oligopeptides consisting of five or more amino acids.     

Differential regulation of three glucose transporter genes in a renal epithelial cell line

Three hexose transporter genes, the Na(+)/glucose cotransporters SGLT1 and SGLT3 (formerly SAAT1/pSGLT2) and the facilitative transporter GLUT1, are expressed in a renal epithelial cell line with proximal tubule characteristics. A number of studies have demonstrated that SGLT1 expression is coupled to the cellular differentiation state and is also negatively regulated by its substrate glucose. In the present study, we demonstrate that SGLT3 mRNA expression is relatively unaffected by conditions promoting dedifferentiation (reseeding to a subconfluent density, activation of protein kinase C) or differentiation (confluent cell density, activation of protein kinase A) nor was expression sensitive to hyperglycemic glucose levels in the medium. We further demonstrate that protein kinase A and protein kinase C exert opposing effects on GLUT1 and SGLT1 mRNA levels in polarized cell monolayers, indicating that GLUT1 mRNA is also highly regulated in polarized epithelial cells by agents affecting cell differentiation. The relatively constitutive expression of SGLT3 mRNA suggests a novel role for this low-affinity Na(+)/glucose cotransporter, to provide concentrative glucose uptake under hyperglycemic conditions where expression of high-affinity glucose cotransporter SGLT1 mRNA is significantly downregulated.     

Hormonal regulation of chicken intestinal NHE and SGLT-1 activities

The effects of aldosterone and arginine vasotocin (AVT) on intestinal Na(+)/H(+) exchange (NHE) and Na(+)-sugar cotransport (SGLT-1) activities have been investigated using brush-border membrane vesicles isolated from Hubbard chicken small and large intestines, and they were compared with those induced by either Na(+) depletion or dehydration. Na(+) depletion was induced by feeding the chickens with either a low- or a high-Na(+) diet for either 0.5, 1, 2, 4, or 8 days. Ileal and colonic NHE2 activity increased with the duration of the Na(+) depletion, whereas that of intestinal SGLT-1 decreased, reaching a plateau after 2 days of treatment. Three-hour incubation of the intestine with aldosterone produced the same effects on NHE activity as does Na(+) depletion, without altering SGLT-1 activity. However, 3-h incubation of the intestine with AVT increased intestinal SGLT-1 activity, without affecting intestinal NHE activity. It is concluded that aldosterone regulates apical ileal and colonic NHE2 activity, whereas that of SGLT-1 is regulated by AVT.     

sigma Receptor ligand-induced up-regulation of the H(+)/peptide transporter PEPT1 in the human intestinal cell line Caco-2.

We determined the effects of (+)pentazocine, a selective sigma(1) ligand, on the uptake of glycylsarcosine (Gly-Sar) in the human intestinal cell line Caco-2 which expresses the low affinity/high capacity peptide transporter PEPT1. Confluent Caco-2 cells were treated with various concentrations of (+)pentazocine for desired time (mostly 24 hr). The activity of PEPT1 was assessed by measuring the uptake of [(14)C]Gly-Sar in the presence of a H(+) gradient. (+)Pentazocine increased the uptake of [(14)C]Gly-Sar mediated by PEPT1 in a concentration- and time-dependent manner. Kinetic analyses have indicated that (+)pentazocine increased the maximal velocity (V(max)) for Gly-Sar uptake in Caco-2 cells without affecting the Michaelis-Menten constant (K(t)). In addition, semi-quantitative RT-PCR revealed that treatment of (+)pentazocine increased PEPT1 mRNA in Caco-2 cells in a concentration-dependent manner. These data suggest that sigma(1) receptor ligand (+)pentazocine up-regulates PEPT1 in Caco-2 cells at the level of increased mRNA, causing an increase in the density of the transporter protein in the cell membrane.     

Zinc transporter mRNA expression in the RWPE-1 human prostate epithelial cell line

The human prostate gland undergoes a prominent alteration in Zn+2 homeostasis during the development of prostate cancer. The goal of the present study was to determine if the immortalized human prostate cell line (RWPE-1) could serve as a model system to study the role of zinc in prostate cancer. The study examined the expression of mRNA for 19 members of the zinc transporter gene family in normal prostate tissue, the prostate RWPE-1 cell line, and the LNCaP, DU-145 and PC-3 prostate cancer cell lines. The study demonstrated that the expression of the 19 zinc transporters was similar between the RWPE-1 cell line and the in situ prostate gland. Of the 19 zinc transporters, only 5 had levels that were different between the RWPE-1 cells and the tissue samples; all five being increased (ZnT-6, Zip-1, Zip-3A, Zip-10, and Zip-14). The response of the 19 transporters was also determined when the cell lines were exposed to 75 microM Zn+2 for 24 h. It was shown for the RWPE-1 cells that only 5 transporters responded to Zn+2 with mRNA for ZnT-1 and ZnT-2 being increased while mRNA for ZnT-7, Zip-7 and Zip-10 transporters were decreased. It was shown for the LNCaP, DU-145 and PC-3 cells that Zn+2 had no effect on the mRNA levels of all 19 transporters except for an induction of ZnT-1 in PC-3 cells. Overall, the study suggests that the RWPE-1 cells could be a valuable model for the study of the zinc transporter gene family in the prostate.     

Sequence,tissue distribution and developmental changes in rat intestinal oligopeptide transporter

Complementary DNA clones encoding the rat PepT1 small-intestinal oligopeptide transporter were isolated from a jejunal library by cross-hybridization with a rabbit PepT1 cDNA probe. The cDNA sequence indicates that rat PepT1 is composed of 710 amino acids and shows 77% and 83% amino acid sequence identity with rabbit and human PepT1, respectively. Northern blot analysis detected rat PepT1 mRNA in the small intestine and kidney. Intestinal PepT1 mRNA levels were highest in 4-day-old rats, and then decreased reaching the adult level by day 28 after birth. These results indicate that the expressions of PepT1 gene change markedly during development.     

Down-regulation of ABCB1 transporter by atorvastatin in a human hepatoma cell line and in human peripheral blood mononuclear cells

PURPOSE: The effect of atorvastatin, an HMG-CoA reductase inhibitor, on expression and activity of the drug transporter ABCB1 in HepG2 cells and peripheral blood mononuclear cells (PBMCs) was examined. METHODS: Localization and expression of ABCB1 in hepatocytes was examined by indirect immunofluorescence. Expression of ABCB1 mRNA and ABCB1 activity were examined in atorvastatin-treated and control cells and PBMCs using real-time PCR and Rhodamine 123 efflux assay. RESULTS: Immunohistochemical analysis revealed that ABCB1 is located at the apical membrane of the bile canaliculi. Atorvastatin at 10 and 20 microM up-regulated ABCB1 expression resulting in a significant 1.4-fold increase of the protein levels. Treatment of HepG2 cells with 20 microM atorvastatin caused a 60% reduction on mRNA expression (p<0.05) and a 41% decrease in ABCB1-mediated efflux of Rhodamine123 (p<0.01) by flow cytometry. Correlation was found between ABCB1 mRNA levels and creatine kinase (r=0.30; p=0.014) and total cholesterol (r=-0.31; p=0.010). CONCLUSIONS. Atorvastatin leads to decreased ABCB1 function and modulates ABCB1 synthesis in HepG2 cells and in PBMCs. ABCB1 plays a role in cellular protection as well as in secretion and/or disposition, therefore, inhibition of ABCB1 synthesis may increase the atorvastatin efficacy, leading to a more pronounced reduction of plasma cholesterol.     

Hormonal regulation of benzo[a]pyrene metabolism in human adrenocortical cell cultures

In cultured fetal human adrenocortical cells, metabolism of the carcinogen benzo[a]pyrene was found to be unresponsive to the xenobiotic inducers 3-methylcholanthrene, benz[a]anthracene and 2,3,7,8-tetrachlorodibenzo-p-dioxin. However, exposure of cultures to the hormone adrenocorticotropin (ACTH) for 48 hours stimulated benzo[a]pyrene metabolism 3-fold. The major metabolite was the 7,8-diol. Other compounds which stimulate the production of adrenocortical cell cyclic AMP (forskolin and cholera toxin) as well as monobutyryl cyclic AMP also increased benzo[a]pyrene metabolism. Human adrenocortical cells thus provide an unusual example of hormonal regulation of the metabolism of a carcinogen.     

ATP regulation of the human red cell sugar transporter

Purified human red blood cell sugar transport protein intrinsic tryptophan fluorescence is quenched by D-glucose and 4,6-ethylidene glucose (sugars that bind to the transport), phloretin and cytochalasin B (transport inhibitors), and ATP. Cytochalasin B-induced quenching is a simple saturable phenomenon with Kd app of 0.15 microM and maximum capacity of 0.85 cytochalasin B binding sites per transporter. Sugar-induced quenching consists of two saturable components characterized by low and high Kd app binding parameters. These binding sites appear to correspond to influx and efflux transport sites, respectively, and coexist within the transporter molecule. ATP-induced quenching is also a simple saturable process with Kd app of 50 microM. Indirect estimates suggest that the ratio of ATP-binding sites per transporter is 0.87:1. ATP reduces the low Kd app and increases the high Kd app for sugar-induced fluorescence quenching. This effect is half-maximal at 45 microM ATP. ATP produces a 4-fold reduction in Km and 2.4-fold reduction in Vmax for cytochalasin B-inhibitable D-glucose efflux from inside-out red cell membrane vesicles (IOVs). This effect on transport is half-maximal at 45 microM ATP. AMP, ADP, alpha, beta-methyleneadenosine 5'-triphosphate, and beta, gamma-methyleneadenosine 5'-triphosphate at 1 mM are without effect on efflux of D-glucose from IOVs. ATP modulation of Km for D-glucose efflux from IOVs is immediate in onset and recovery. ATP inhibition of Vmax for D-glucose exit is complete within 5-15 min and is only partly reversed following 30-min incubation in ATP-free medium. These findings suggest that the human red cell sugar transport protein contains a nucleotide-binding site(s) through which ATP modifies the catalytic properties of the transporter.     

Characterization of a human plasma membrane heme transporter in intestinal and hepatocyte cell lines

Heme is the most bioavailable form of dietary iron and a component of many cellular proteins. Controversy exists as to whether heme uptake occurs via specific transport mechanisms or passive diffusion. The aims of this study were to quantify cellular heme uptake with a fluorescent heme analog and to determine whether heme uptake is mediated by a heme transporter in intestinal and hepatic cell lines. A zinc-substituted porphyrin, zinc mesoporphyrin (ZnMP), was validated as a heme homolog in uptake studies of intestinal (Caco-2, I-407) and hepatic (HepG2) cell lines. Uptake experiments to determine time dependence, heme inhibition, concentration dependence, temperature dependence, and response to the heme synthesis inhibitor succinylacetone were performed. Fluorescence microscope images were used to quantify uptake and determine the cellular localization of ZnMP; ZnMP uptake was seen in intestinal and hepatic cell lines, with cytoplasmic uptake and nuclear sparing. Uptake was dose- and temperature dependent, inhibited by heme competition, and saturated over time. Preincubation with succinylacetone augmented uptake, with an increased initial uptake rate. These findings establish a new method for quantifying heme uptake in individual cells and provide strong evidence that this uptake is a regulated, carrier-mediated process.     

Expression and cyclic AMP-dependent regulation of a high affinity serotonin transporter in the human placental choriocarcinoma cell line (JAR).

The JAR human placental choriocarcinoma cell line transports serotonin, accumulating the monoamine inside the cell against a concentration gradient. The transport is energized by an NaCl gradient. Tricyclic (imipramine and desipramine) and non-tricyclic (paroxetine and fluoxetine) antidepressants inhibit the transporter markedly, but reserpine and 5-hydroxytryptophan do not. Ouabain, gramicidin, and nigericin, which reduce or abolish the transmembrane Na+ gradient, and phloridzin, which interferes with glucose transport into the cells, inhibit the transport. Preincubation of the cells with glucose-free medium also causes similar inhibition. The activity of the serotonin transporter in this cell line is stimulated in response to overnight (16-h) incubation with increasing concentrations of cholera toxin (0.1-1,000 ng/ml). Under these conditions the stimulation is maximal at 10 ng/ml cholera toxin (3.1 +/- 0.2-fold). Cholera toxin increases the cAMP content of these cells by several hundredfold within 2 h. Isobutylmethylxanthine (100 microM), dibutyryl cAMP (100 microM), and forskolin (100 microM) mimic the action of cholera toxin, eliciting a 1.6-2.5-fold stimulation of the serotonin transporter activity. The stimulatory effect of cholera toxin is antagonized significantly by simultaneous incubation of the cells with 50 microM N-(2-aminoethyl)-5-isoquinolinesulfonamide, a protein kinase inhibitor. The effect of cholera toxin on serotonin transport is specific because, under similar conditions, cholera toxin inhibits 3-O-methyl-D-glucose transport and does not influence taurine transport in this cell line. There is also no significant change in the protein content of the cells after cholera toxin treatment. Kinetic analysis reveals that cholera toxin causes an increase in the maximal velocity (7.89 +/- 0.67 to 17.55 +/- 1.06 pmol/mg of protein/5 min) and a decrease in the Michaelis-Menten constant (0.52 +/- 0.09 to 0.29 +/- 0.04 microM). These data show that the JAR human placental choriocarcinoma cell line expresses a high affinity serotonin transporter that is sensitive to inhibition by antidepressants and that the activity of the transporter is under cAMP-dependent regulation.