Presence and differential expression of SGLT1, GLUT1, GLUT2, GLUT3 and GLUT5 hexose-transporter mRNAs in Caco-2 cell clones in relation to cell growth and glucose consumption

Prostaglandins protect human intestinal cells against ethanol injury by stabilizing microtubules: role of protein kinase C and enhanced calcium efflux

Prostaglandins (PG) protect gastrointestinal cells against damage induced by ethanol (EtOH) and other noxious agents, a process termed cytoprotection. The present study investigated the relationships between microtubule (MT) stability, protein kinase C (PKC) activation, and calcium efflux as a possible mechanism of PG's protective action using a human colonic cell line (Caco-2) exposed to known damaging concentrations of EtOH (7.5% and 10%). Preincubation of Caco-2 cells with 16,16-dimethyl-PGE2 (PG, 2.6 microM) significantly increased PKC activity in these cells. Pretreatment of Caco-2 cells with 50 microM OAG (a synthetic diacylglycerol and PKC activator) or 30 nM TPA (a direct PKC activator) prior to exposure to 7.5% or 10% EtOH for 5 min significantly reduced cell injury, as determined by trypan blue exclusion, and increased MT stability, as confirmed by confocal microscopy. Pretreatment of Caco-2 cells with 4 alpha-PDD (an inactive phorbol ester, 20 nM) failed to prevent cell injury and disruption of the MT cytoskeleton. Preincubation with staurosporine (a PKC inhibitor, 3 nM) abolished the protective effects of PG in cells exposed to 7.5% and 10% EtOH. Incubation of Caco-2 cells with A23187 (a Ca2+ ionophore), similar to 10% EtOH, caused a significant reduction in cell viability and MT stability. Preincubation with A23187 in combination with PG or OAG prior to subsequent exposure to EtOH significantly abolished the protective effects of PG or OAG pretreatment. Finally, pretreatment with OAG, TPA, or PG resulted in significant increases in calcium-45 efflux, which correlated with increased stability of the MT cytoskeleton. These data suggest that PG possesses direct protective effects against EtOH injury in Caco-2 cells and may act by stabilizing MT through the PKC signal transduction pathway and/or stimulation of calcium efflux from the cells.

Illegitimate expression of apolipoprotein A-II in Caco-2 cells is due to chromatin organization

Transcriptional activity of the human apolipoprotein (apo) A-II promoter has been reported in transiently transfected Caco-2 cells, but not in the intestine in vivo. In the present study we established that the transcription of a stably transfected reporter gene under the control of the -911/+29 human apo A-II, decreases with the onset of the differentiation process. This decrease paralleled that of the expression of the endogenous apo A-II gene. The decrease in apo A-II expression is also followed by a marked increase in the expression of the intestine-specific apo A-IV gene, analyzed here as a marker of enterocytic differentiation. Using clonal glucose metabolic variants of Caco-2 cells we have also observed that the lowest levels of apo A-II mRNA are associated with the lowest rates of glucose consumption. The illegitimate apo A-II transcriptional activity observed in Caco-2 cells is linked to the presence of DNase-I hypersensitive sites within the enhancer. This reflects a chromatin organization which allows, in Caco-2 cells as in the liver, the communication between the apo A-II enhancer and the proximal promoter, unlike what is observed in intestinal epithelial cells.

Development of the human gastrointestinal tract: twenty years of progress

A combination of approaches has begun to elucidate the mechanisms of gastrointestinal development. This review describes progress over the last 20 years in understanding human gastrointestinal development, including data from both human and experimental animal studies that address molecular mechanisms. Rapid progress is being made in the identification of genes regulating gastrointestinal development. Genes directing initial formation of the endoderm as well as organ-specific patterning are beginning to be identified. Signaling pathways regulating the overall right-left asymmetry of the gastrointestinal tract and epithelial-mesenchymal interactions are being clarified. In searching for extrinsic developmental regulators, numerous candidate trophic factors have been proposed, but compelling evidence remains elusive. A critical gene that initiates pancreas development has been identified, as well as a number of genes regulating liver, stomach, and intestinal development. Mutations in genes affecting neural crest cell migration have been shown to give rise to Hirschsprung's disease. Considerable progress has been achieved in understanding specific phenomena, such as the transcription factors regulating expression of sucrase-isomaltase and fatty acid-binding protein. The challenge for the future is to integrate these data into a more complete understanding of the physiology of gastrointestinal development.

Decreased expression of gamma-glutamyltranspeptidase in the intestinal cell line Caco-2 by inducers of cytochrome P450 1A1

Our purpose was to investigate whether inducers of cytochrome P450 1A1 (CYP1A1), which cause a decreased expression in Caco-2 cells, at both the mRNA and protein levels, of membrane proteins associated with the uptake and transport of hexoses, would also affect the expression of gamma-glutamyltranspeptidase (gammaGT) (EC 2.3.2.2). In Caco-2 clonal TC7 cells grown under standard conditions (25 mM glucose), exposure to beta-naphthoflavone (beta-NF), 2,3,7,8-tetrachlorodibenzo-p-dioxin, and 3-methylcholanthrene resulted in increased glucose consumption and decreased gammaGT activity in cells grown to confluence, i.e. when the differentiation is optimum. GammaGT activity was further analyzed during the time course of differentiation of TC7 cells treated or not with beta-naphthoflavone: while gammaGT activity in untreated cells showed a 10-fold increase from the exponential phase of growth until late postconfluence, gammaGT activity in beta-NF-treated cells, although increasing by 4-fold, remained at a much lower level (<25%). This decreased activity of gammaGT was associated with a decreased level of gammaGT mRNA. This inhibiting effect was not dependent on the CYP1A1 activity, as it also occurred in the presence of CYP1A1 inhibitors such as alpha-naphthoflavone, 8-methoxypsoralen or ellipticin. It was however dependent on glucose supply as it was not observed when the cells were cultured in low glucose (1 mM). These results raise the question of whether, in Caco-2 cells, CYP1A1 inducers or the signal transduction system which controls CYP1A1 are involved in the regulation of the expression of gammaGT through a mechanism involving glucose metabolism.

Thyroid hormone regulation of the Na+/glucose cotransporter SGLT1 in Caco-2 cells

The expression of the Na+/glucose cotransporter (SGLT1) in response to thyroid hormone [3,5,3'-tri-iodo-l-thyronine (T3)] was investigated in the enterocytic model cell line Caco-2/TC7. In differentiated cells, T3 treatment induces an average 10-fold increase in glucose consumption as well as a T3 dose-dependent increase in SGLT1 mRNA abundance. Only cells grown on glucose-containing media, but not on the non-metabolizable glucose analogue alpha-methylglucose (AMG), could respond to T3-treatment. The Vmax parameter of AMG transport was enhanced 6-fold by T3 treatment, whereas the protein abundance of SGLT1 was unchanged. The role of Na+ recycling in the T3-related activation of SGLT1 activity was suggested by both the large increase in Na+/K+ATPase protein abundance and the inhibition, down to control levels, of AMG uptake in ouabain-treated cells. Further investigations aimed at identifying the presence of a second cotransporter that could be expressed erroneously in the colon cancer cell line were unsuccessful: T3-treatment did not modify the sugar-specificity profile of AMG transport and did not induce the expression of SGLT2 as assessed by reverse transcription-PCR. Our results show that T3 can stimulate the SGLT1 cotransport activity in Caco-2 cells. Both transcriptional and translational levels of regulation are involved. Finally, glucose metabolism is required for SGLT1 expression, a result that contrasts with the in vivo situation and may be related to the fetal phenotype of the cells.

Interrelationship between the Na+/glucose cotransporter and CFTR in Caco-2 cells: relevance to cystic fibrosis

Both the Na+-dependent glucose cotransporter (SGLT1) and the cystic fibrosis transmembrane conductance regulator (CFTR) modulate Na+ and fluid movement, although in opposite directions. Yet few studies have investigated a possible interrelationship between these two transporters. By using the Caco-2 human colon carcinoma cell line, we confirmed that the activities of these transporters increased with spontaneous differentiation to the enterocytic phenotype. We showed that SGLT1 was positively regulated by Cl- and that optimal activity of CFTR was dependent on the presence of glucose. We also demonstrated that inhibition of CFTR by glibenclamide or diphenylamine-2-carboxylate did not modify the activity of SGLT1 and inhibition of SGLT1 by phlorizin did not modify the activity of CFTR, although it resulted in inhibition of glycoconjugate synthesis. These results point to positive substrate-cross regulation of SGLT1 and CFTR and suggest that NaCl and glucose are important for not only Na+ absorption and fluid movement, but also for cAMP-dependent Cl- efflux, and glycoconjugate synthesis, functions that are known to be anomalous in cystic fibrosis.

Hypoxia and CYP1A1 induction-dependent regulation of proteins involved in glucose utilization in Caco-2 cells

Although induction of cytochrome P-450 1A1 (CYP1A1) in the Caco-2 clone TC7 alters glucose utilization and modifies the expression of sucrase-isomaltase (SI) and hexose transporters, nothing is known of the events that control these effects. In this study, we analyzed the effects of beta-naphthoflavone (beta-NF) and hypoxia on these parameters and expression of key enzymes of glucose metabolism. Both beta-NF and hypoxia induce similar changes: 1) induction of CYP1A1 mRNA; 2) increased glucose consumption and lactic acid production and lower glycogen content; 3) downregulation of SI and upregulation of GLUT1 mRNAs; 4) downregulation of fructose-1,6-bisphosphatase and pyruvate kinase mRNAs and upregulation of phosphoenolpyruvate carboxykinase, pyruvate dehydrogenase, lactate dehydrogenase, and phosphofructokinase mRNAs; and 5) upregulation of c-fos and c-jun mRNAs. Although addition of inhibitors of CYP1A1 catalytic activity to beta-NF-treated cells totally inhibits the enzyme activity, it does not modify CYP1A1 mRNA response and associated effects, thus excluding a direct role for the enzyme per se. These results point to a possible physiological implication of the signal-transduction pathway responsible for CYP1A1 induction.

Transport of quercetin and its glucosides across human intestinal epithelial Caco-2 cells

There is mounting evidence from human epidemiological, animal in vivo, and in vitro studies to suggest beneficial effects related to the consumption of quercetin and its glucosides. However, there is limited knowledge on the oral bioavailability of these natural products. This study examined the intestinal epithelial membrane transport of quercetin, quercetin 4'-glucoside, and quercetin 3,4'-diglucoside, using the Caco-2 human colonic cell line, a model of human intestinal absorption. The apparent permeability (Papp) of each agent was measured in both apical to basal and basal to apical directions. The apical to basolateral flux of quercetin, Papp 5.8 +/- 1.1 x 10(-6) cm x sec(-1) (mean +/- SEM), was more than 10-fold higher than for the paracellular transport marker mannitol, 0.48 +/- 0.09 x 10(-6) cm x sec(-1) (P < 0.01). Under identical conditions, the Papp for the transcellular marker propranolol was about 5-fold higher than for quercetin (P < 0.001). Interestingly, the reverse, basolateral to apical, flux of quercetin (Papp 11.1 +/- 1.2 x 10(-6) cm x sec(-1)) was almost 2-fold higher than the apical to basolateral flux (P < 0.001). In similar experiments, quercetin 4'-glucoside demonstrated no absorption, Papp < 0.02 x 10(-6) cm x sec(-1) in the apical to basal direction, but did demonstrate basal to apical flux, Papp 1.6 +/- 0.2 x 10(-6) cm x sec(-1). Quercetin 3,4'-diglucoside showed a low apical to basolateral transport (Papp 0.09 +/- 0.03 x 10(-6) cm x sec(-1)); its reverse, basolateral to apical, transport was, however, 4-fold higher (P < 0.05). In these cells, glucose was actively transported with an apical to basolateral Papp of 36.8 +/- 1.1 x 10(-6) cm x sec(-1). These observations suggest facile absorption of quercetin through the human intestinal epithelium, but contrary to a previous proposal, they do not support an active transport process for quercetin glucosides.

Correlation between oral drug absorption in humans, and apparent drug permeability in TC-7 cells, a human epithelial intestinal cell line: comparison with the parental Caco-2 cell line

PURPOSE

To determine and compare the relationship between in vivo oral absorption in humans and the apparent permeability coefficients (Papp) obtained in vitro on two human intestinal epithelial cell lines, the parental Caco-2 and the TC-7 clone.

METHODS

Both cell lines were grown for 5-35 days on tissue culture-treated inserts. Cell monolayers were analysed for their morphology by transmission electron micrography, and for their integrity with respect to transepithelial electrical resistance, mannitol and PEG-4000 transport, and cyclosporin efflux. Papp were determined for 20 compounds exhibiting large differences in chemical structure, molecular weight, transport mechanisms, and percentage of absorption in humans.

RESULTS

The TC-7 clone exhibits morphological characteristics similar to those of the parental Caco-2 cell line, concerning apical brush border, microvilli, tight junctions and polarisation of the cell line. The TC-7 clone however appeared more homogenous in terms of cell size. Both cell lines achieved a similar monolayer integrity towards mannitol and PEG-4000. Monolayer integrity was achieved earlier for the TC-7 clone, mainly due to its shorter doubling time, i.e. 26 versus 30 hours for parental Caco-2 cells. When using cyclosporin A as a P-glycoprotein substrate, active efflux was lower in the TC-7 clone than in the parental Caco-2 cells. The Papp and mechanisms of transport (paracellular or transcellular routes, passive diffusion and active transport) were determined for 20 drugs. A relationship was established between the in vivo oral absorption in humans and Papp values, allowing to determine a threshold value for Papp of 2 10(-6) cm/sec, above for which a 100% oral absorption could be expected in humans. Both correlation curves obtained with the two cell types, were almost completely superimposable. These studies also confirmed that the dipeptide transporter is underexpressed in both cell lines.

CONCLUSIONS

On the basis of morphological parameters, biochemical activity and drug transport characteristics, the TC-7 clone appeared to be a valuable alternative to the use of parental Caco-2 cells for drug absorption studies.

Selecting agent hygromycin B alters expression of glucose-regulated genes in transfected Caco-2 cells

Incorporation into plasmids of genes conferring resistance to aminoglycoside antibiotics such as hygromycin B is currently utilized for selection in experiments involving gene transfer in eukaryotic cells. Using a subclone of Caco-2 cells stably transfected with an episomal plasmid containing the hygromycin resistance gene, we observed that transformed cells subcultured in the presence of hygromycin B exhibit, compared with the same cells subcultured in antibiotic-free medium, a sixfold increase in the rates of glucose consumption and lactic acid production and dramatic changes, at mRNA and protein level, of the expressions of sucrase-isomaltase and hexose transporter GLUT-2, which are downregulated, contrasting with an upregulation of hexose transporter GLUT-1. This occurs without significant modifications of the differentiation status of the cells, as demonstrated by the normal expression of villin, ZO-1, dipeptidyl peptidase IV, or Na(+)-K(+)-ATPase. The plasmid copy number is, however, the same, whether or not the cells are cultured in the presence of hygromycin B. These results draw attention to the need to consider antibiotic-dependent alterations of metabolism and gene expression in transfection experiments.

Expression of glucose transporters in human peritoneal mesothelial cells

Glucose containing solutions, the basis of peritoneal dialysis fluids, affect the proliferation and regeneration of peritoneal mesothelial cells (MsC). The aim of this study was to examine mechanisms of glucose transport into MsC, that is, the expression of facilitative glucose transporters (GLUT) and the Na(+)-dependent glucose transporter (SGLT1) in human primary MsC and a transfected MsC line. Since expression of both transporters is differentiation dependent, we investigated the effects of cell differentiation induced by culturing MsC on membranes or by addition of hexamethylene bisacetamide (HMBA; 6 mM), which enhances SGLT1 expression in LLC-PK1 cells. Levels of mRNA for GLUT1 through GLUT4 and SGLT1 were evaluated by reverse transcriptase-polymerase chain reaction (RT-PCR). The presence of the corresponding proteins was examined by Western blotting and localized by immunofluorescence. Active, Na(+)-dependent glucose transport was assessed by alpha-methyl-D-[14C]glucopyranoside (AMG) with and without the SGLT1-specific inhibitor phlorizin and by patch clamp experiments in NaCl or choline-chloride, For Na(+) dependent glucose uptake choline chloride instead of NaCl served as negative control. Facilitative transport was assessed using 2-fluoro-2-deoxy-[14C]-D-glucose (FDG) with and without the inhibitors cytochalasin B or phloretin. Primary and transfected MsC express GLUT1 and GLUT3 mRNA while no transcripts were found for GLUT2 and GLUT4. No SGLT1 transcript was detectable in subconfluent cells. Semiquantitative RT-PCR analysis documented that the addition of the differentiation inducer HMBA to confluent cultures or growth of MsC on membranes for seven days produced a down-regulation of mRNA for GLUT1, no change for GLUT3, and a substantial increase for SGLT1 mRNA. Under these conditions MsC express SGLT1 protein and possess a Na(+)-dependent glucose uptake as assessed by AMG. Phlorizin (1 mM) inhibits AMG uptake by 30 to 40%. In patch clamp experiments the addition of extracellular glucose depolarized the membrane potential only in the presence of sodium. These results indicate that differentiated MsC express GLUT1, GLUT3, and SGLT1. Further characterization of these transport mechanisms and their regulation may help to understand the cellular effects of glucose on MsC in peritoneal dialysis.

Protection against ethanol injury by prostaglandin in a human intestinal cell line: role of microtubules

Prostaglandins have been shown to protect the gastrointestinal (GI) epithelium from injury induced by various luminal insults independent of their known acid-inhibitory effects, a process termed "cytoprotection." The mechanism of this protective action remains unknown. The present investigation determined the role of microtubules (a major cytoskeletal component) in GI injury induced by ethanol (EtOH) and its prevention by 16,16-dimethylprostaglandin E2 (dmPGE2) using cells from a human colonic cell line known as Caco-2 cells. These cells were preincubated in Eagle's minimum essential medium with and without dmPGE2 (2.6 microM) for 15 min and subsequently incubated in media containing 1, 2.5, 5, 7.5, and 10% EtOH. The effects on cell viability and tubulin (the major protein backbone of microtubules) were then determined. EtOH concentrations > or = 2.5% extensively disrupted the microtubules as demonstrated by fragmentation, kinking, and perturbation of the microtubule organizer center. EtOH treatment also led to a significant decrease in the S2 (polymerized) fraction and an increase in the S1 (monomeric) pool of tubulin. Concomitant with these effects were marked decreases in cellular viability. DmPGE2 pretreatment abolished the disruption of microtubules, significantly increased the S2 fraction of tubulin, and increased cellular viability in cultures exposed to EtOH. Furthermore, pretreatment with colchicine, an inhibitor of microtubule assembly, prevented the cytoprotective action of dmPGE2. Taxol, a microtubule stabilizing agent, mimicked the effects of dmPGE2 by also enhancing microtubule integrity and increasing cellular viability in cells exposed to EtOH. Our data indicate that organization and stabilization of microtubules may play an essential role in the mechanism of prostaglandin-induced protection.

Upregulation of SGLT-1 transport activity in rat jejunum induced by GLP-2 infusion in vivo

The effect of in vivo infusion of the peptide hormone glucagon-like peptide 2 (GLP-2) on glucose transport across the rat jejunal brush-border membrane (BBM) was assessed using isolated membrane vesicles. A 2-h infusion of GLP-2 produced a marked acceleration of sodium-dependent glucose uptake into BBM vesicles with a significant overshoot. There was no change in vesicle space or permeability resulting from the hormone infusion. Kinetic analysis showed this stimulation to be the result of a three-fold increase in the maximal rate of transport, with no consistent change in the affinity constant (Km). The time course of this response showed that the effect was observable, but smaller, after only 30 min of hormone infusion and was maximal after 1 h. Sodium-dependent phloridzin binding to the membrane vesicles showed a parallel increase in maximal binding after 1 and 2 h of hormone infusion. Western blotting showed a similar increase in sodium-dependent glucose transporter 1 (SGLT-1) abundance. The effect of GLP-2 could be blocked by luminal brefeldin A or wortmannin. These results indicate that GLP-2 is able to induce trafficking of SGLT-1 from an intracellular pool into the BBM within 60 min and that phosphoinositol 3-kinase may well be involved in the intracellular signaling pathway in this response.

Evaluation of fluorodeoxyglucose-positron emission tomographic scanning and its association with glucose transporter expression in medullary thyroid carcinoma and pheochromocytoma: a clinical and molecular study

BACKGROUND

Imaging of metastatic sites of medullary thyroid carcinoma (MTC) is successful in less than 60% of cases of residual or recurrent disease. Positron emission tomography (PET) with [18F]fluoro-2-deoxy-D-glucose (FDG) takes advantage of the fact that malignant tumors are capable of increased uptake and use of glucose, which is mediated by the members of the glucose transporter family of proteins (GLUT 1 through GLUT 5).

METHODS

FDG-PET images of 10 patients with recurrent or persistent MTC after primary operation were compared with images by computed tomography or magnetic resonance imaging. Identified metastatic lesions were assessed by intraoperative findings and pathology reports. Expression of GLUT 1 through GLUT 5 was examined by Western blot analysis of tumor tissue from eight of the patients evaluated and an additional panel of 10 MTCs and seven pheochromocytomas.

RESULTS

FDG-PET identified 31 foci of FDG accumulation in 10 patients, and 16 of these metastatic sites were resected and confirmed by histologic analysis. Only 11 foci were demonstrated by computed tomographic or magnetic resonance imaging. None of the glucose transporters examined displayed significant expression. Two pheochromocytomas were successfully imaged by FDG-PET.

CONCLUSIONS

FDG-PET imaging can be useful in the localization of cervicomediastinal MTC metastases and pheochromocytoma. The increased glucose uptake in these tumors, as evidenced by FDG-PET, does not appear to be attributable to the expression of the glucose transporter proteins GLUT 1 through GLUT 5.

Glucose transporter Glut 5 expression in microglial cells

Glut 5 is a member of a family of facilitative glucose transporter proteins that are involved in the transportation of glucose and/or fructose across plasma membranes. Glut 5 is one of three isoforms expressed in brain. Using immunocytochemical and immunoblotting techniques in formalin fixed tissue, we have shown that Glut 5 is exclusively expressed in microglial cells of the human and rat brain. Our data demonstrate that unlike other microglial markers, Glut 5 can be used as a microglial marker in formalin fixed tissue.

Sexual and asexual development of Cryptosporidium parvum in five oocyst- or sporozoite-infected human enterocytic cell lines

The human enterocytic cell lines Caco-2, HT29, HCT8 and the Caco-2 clones TC7 and PF11 were studied for their ability to support Cryptosporidium parvum development. Following the addition in cultures of either oocysts or excysted sporozoites, immunofluorescent and transmission electron microscopy revealed the presence of all stages of the parasite life cycle by both procedures, and no difference in the ratio of infected cells was found among cell lines. More oocysts were seen in cell monolayers infected with oocysts than with sporozoites (p < 0.0001). The number of meronts observed was the same after either oocysts or sporozoites inoculation. Data suggest that the two methods yield a same cell infection rate.

Glucose up-regulates expression of the differentiation-associated brush border binding site for enterotoxigenic Escherichia coli colonization factor antigen I in cultured human enterocyte-like cells

The association of enterotoxigenic Escherichia coli expressing colonization factor antigen I (CFA/I) with the cultured human colon adenocarcinoma cell, a model of the mature enterocyte of the small intestine, is dependent on the binding of CFA/I to a brush border-associated component. Binding of the purified radiolabeled [125I]CFA/I- and 14C-labeled CFA/I-positive bacteria could be displaced by an increasing concentration of unlabeled CFA/I. Moreover, we showed that expression of the specific CFA/I binding developed as a function of cell differentiation in Caco-2 cells, whereas expression of the nonspecific binding did not. Expression of the brush border differentiation-associated component acting as a binding site for CFA/I was up-regulated by glucose. Indeed, the enterocyte-like HT-29 glc- cell subpopulation not expressing the CFA/I binding site when cultured in dialyzed serum and hexose-free medium regained the ability to bind CFA/I when the cells were returned to culture medium containing glucose. Furthermore, expression of the brush border-associated CFA/I binding site in the enterocyte-like Caco-2 cells was repressed when the cells were cultured in hexose-free conditions.

Mechanisms and kinetics of alpha-linolenic acid uptake in Caco-2 clone TC7

The uptake kinetics of alpha-linolenic acid (18:3(n - 3)), an essential fatty acid, were investigated in the human intestinal cell line Caco-2. Four clones (PD10, PF11, PD7 and TC7) from the heterogeneous parental Caco-2 cells population were used. After a screening step using isolated cells, the TC7 clone was selected for the study of alpha-linolenic acid uptake. [1-(14)C]linolenic acid dissolved in 10 mM taurocholate was presented to the microvillus plasma membrane (apical side) of TC7 differentiated cells, grown on a semi-permeable polycarbonate membrane. The results show that the initial rate of uptake is not a linear function of the 18:3(n- 3) monomer concentration in the incubation medium. In the monomer concentration range studied (0.2 to 36 microM) apical uptake was saturable and followed Michaelis-Menten kinetics (V(max) = 15.4 +/- 0.6 nmol/mg protein per min, K(m) = 14.3 +/- 1.3 microM). In addition, it was temperature- and energy-dependent but was apparently unaffected by the sodium gradient and intracellular metabolic fate of 18:3(n - 3). Excess of unlabeled saturated or unsaturated long chain fatty acids (C16 to C22) led to a 27-68% reduction of [1-(14)C]linolenic acid uptake. Likewise basolateral uptake was saturable (V(max) = 4.9 +/- 0.7 nmol/mg protein per min, K(m) = 8.7 +/- 2.9 microM). These facts argue in favour of the existence in these human intestinal cells of a carrier-mediated transport system for alpha-linolenic acid and probably other long chain fatty acids as well.

Glucose-dependent transcriptional regulation of the human sucrase-isomaltase (SI) gene

We have previously shown that the transcription of the human sucrase-isomaltase (SI) gene was negatively regulated by glucose. Using two clonal metabolic variants of the human colon adenocarcinoma cell line Caco-2 we demonstrate here that: 1) although similar growth-related variations of phosphoenolpyruvate carboxykinase (PEPCK), frutose 1,6-diphosphatase (F1, 6-dPase), pyruvate kinase (PK) and SI mRNA levels are observed, only F1,6-dPase, PK and SI mRNA levels vary in the same way in response to modifications of glucose utilization; and 2) regulatory elements responsible for the glucose-dependent transcription of the SI gene are located within the -370/+30 region of the promoter.

Differential targeting of facilitative glucose transporters in polarized epithelial cells

We have examined the intracellular localization of five facilitative glucose transporter proteins, one endogenous (GLUT-1) and four exogenous (GLUT-2, -3, -4, and -5), in polarized epithelial cells. GLUT-2, -3, -4, and -5 were stably transfected into Madin-Darby canine kidney (MDCK) cells, and peptide-specific antibodies were used to establish their distribution by immunofluorescence and immunoelectron-microscopic techniques. GLUT-1 and -2 were predominantly targeted to the basolateral domain of the cell, whereas GLUT-3 and -5 were targeted to the apical plasma membrane. The insulin-regulatable glucose transporter GLUT-4 was found in intracellular tubulovesicular structures beneath the surface of the cell. Vectorial 2-deoxy-D-glucose uptake measurements revealed that approximately 95% of glucose entry into wild-type MDCK cells occurs via the basolateral membranes. In GLUT-3-transfected cells, however, apical glucose uptake increased to approximately 55%; this was not observed in cells expressing the other GLUT isoforms. The discrete and differential intracellular localizations of the various GLUTs, in addition to the high level of sequence homology and predicted secondary structure similarity, render the GLUT family ideal for the study of intrinsic targeting motifs involved in the establishment and maintenance of cellular polarity.

Sugar-dependent expression of the fructose transporter GLUT5 in Caco-2 cells

The effect of glucose and fructose and fetal bovine serum on the expression of the fructose transporter GLUT5 was studied in clone PD7 of the human colon cancer cell line Caco-2, which has been characterized previously [Chantret, Rodoloswe, Barbat et al. (1994) J. Cell Sci. 107, 213-225; Mahraoui, Rodolosse, Barbat et al. (1994) Biochem. J. 298, 629-633]. Culture of the cells in dialysed serum and hexose-free media, down-regulated the expression of GLUT5, which was below detection within 3-4 days. This effect was reversed by fructose and glucose feeding of the cells. Fructose feeding yielded a 3-fold higher abundance of GLUT5 protein and mRNA as compared with that expressed in glucose-fed cells. Cells fed normal serum exhibited an inverse hierarchy of expression, with glucose being a better inducer than fructose for the expression of GLUT5. The GLUT5 mRNA and protein abundances obtained in fructose-fed cells did not depend on the type of serum. A linear relationship between cyclic AMP (cAMP) levels and GLUT5 mRNA abundance was found in cells fed dialysed serum, whereas in cells fed normal serum, mRNA abundances were not correlated to cAMP levels. These results indicate that glucose and fructose, together with serum-related factors and cAMP, have combined effects on the expression of GLUT5 in Caco-2 cells.

The human intestinal epithelial cell line Caco-2; pharmacological and pharmacokinetic applications

The gastrointestinal tract remains the most popular and acceptable route of administration for drugs. It offers the great advantage of convenience and many compounds are well absorbed and thereby provide acceptable plasma concentration-time profiles. Currently there is considerable interest from the pharmaceutical industry in development of cell culture systems that would mimic the intestinal mucosa in order to evaluate strategies for investigating and/or enhancing drug absorption. The intestinal epithelial cells of primary interest, from the standpoint of drug absorption and metabolism, are the villus cells, which are fully differentiated cells. An in vitro cell culture system consisting of a monolayer of viable, polarized and fully differentiated villus cells, similar to that found in the small intestine, would be a valuable tool in the study of drug and nutrient transport and metabolism. The Caco-2 cell line, which exhibits a well-differentiated brush border on the apical surface and tight junctions, and expresses typical small-intestinal microvillus hydrolases and nutrient transporters, has proven to be the most popular in vitro model (a) to rapidly assess the cellular permeability of potential drug candidates, (b) to elucidate pathways of drug transport (e.g., passive versus carrier mediated), (c) to assess formulation strategies designed to enhance membrane permeability, (d) to determine the optimal physicochemical characteristics for passive diffusion of drugs, and (e) to assess potential toxic effects of drug candidates or formulation components on this biological barrier. Since differentiated Caco-2 cells express various cytochrome P450 isoforms and phase II enzymes such as UDP-glucuronosyltransferases, sulfotransferases and glutathione-S-transferases, this model could also allow the study of presystemic drug metabolism.

Mechanism of dextran transport across rabbit intestinal tissue and a human colon cell-line (CACO-2)

The in vitro permeabilities of 14C labeled dextrans (10, 40, and 70 kD) were calculated from mass transport across Peyer's patches and non-patch tissues derived from rabbit jejunum, and a human colon cell line (Caco-2) grown as a monolayer on polycarbonate filters. Size distribution of dextrans did not change upon transport as judged from size exclusion chromatography. Permeabilities decreased in a size-dependent manner. Ranking of permeabilities for dextran 10 and 40 kD were: Caco-2 > non-patch tissue > Peyer's patches; while dextran 70 kD demonstrated no difference among the barriers. Tissue resistance, expressed as 1/(permeability.tissue thickness) was virtually the same in Peyer's patches and non-patch tissue, suggesting that tissue thickness and not interaction determines the difference in permeability. ATP depletion with ouabain, Na(+)-azide and 2-deoxy-D-glucose, and low temperature (4 degrees C) did not result in reduced permeabilities suggesting passive transport. The results suggest that the investigated intestinal barriers transport dextrans in a similar fashion independent of their source. However, comparison of the ratios dextran 10 kD/mannitol and PEG 900/mannitol between rabbit tissue and Caco-2 monolayers suggests Caco-2 monolayers may serve as a model to study absorption potential of potentially harmful compounds in coeliac disease, gastroenteritis, and colon carcinoma.

Expression of cytochrome P-450 3A in HT29-MTX cells and Caco-2 clone TC7

HT-29 sublines and Caco-2 clones were analyzed for the expression of cytochrome P-450 3A. The enzyme was found to be expressed in differentiated HT-29 cells selected by resistance to methotrexate and in one of seven Caco-2 clones, TC7. Its expression parallels the differentiation process, with highest levels being observed at late confluency. P-450 3A mRNA and protein patterns, as well as subcellular distribution, are intermediate between those observed in human adult intestine and fetal liver.