Relationship between structure and permeability of tryptophan derivatives across human intestinal epithelial (Caco-2) cells

L-Trp and its derivatives were used as model compounds to clarify structural factors which influence the intestinal epithelial permeation and metabolism of amino-acid derivatives. Permeability of model compounds through Caco-2 cells was used as an in vitro absorption model for human intestinal epithelial cells. The influence of compound concentration, the effects of various transporter substrates on permeability coefficients, and pH dependency of permeability coefficients were investigated. The transcellular permeability of Trp and Trp-NH2 in the direction from the apical side to the basolateral side, in which nutrients and drugs were ordinarily absorbed, declined with increasing concentration and saturated at more than 1 and 0.4 mM, respectively. The permeability coefficients for N-terminal protected Trp derivatives and Ac-Trp-NH2 showed similar and constant values in both from the apical-to-basolateral and basolateral-to-apical directions. In addition, significant inhibition of the apical-to-basolateral permeation of Trp by Leu and Phe was observed. The permeability coefficient ratio at pH 6.3 to that at pH 7.3 was explained by the ratio of the ionic form to the neutral form of the compounds. Based upon these results and the partition coefficients in the 1-octanol/water system, possible absorption mechanism of Trp and its derivatives across Caco-2 cells was proposed.     

Vectorial release of poliovirus from polarized human intestinal epithelial cells.

Polarized epithelial cells represent the primary barrier to virus infection of the host, which must also be traversed prior to virus dissemination from the infected organism. Although there is considerable information available concerning the release of enveloped viruses from such cells, relatively little is known about the processes involved in the dissemination of nonenveloped viruses. We have used two polarized epithelial cell lines, Vero C1008 (African green monkey kidney epithelial cells) and Caco-2 (human intestinal epithelial cells), infected with poliovirus and investigated the process of virus release. Release of poliovirus was observed to occur almost exclusively from the apical cell surface in Caco-2 cells, whereas infected Vero C1008 cells exhibited nondirectional release. Structures consistent with the vectorial transport of virus contained within vesicles or viral aggregates were observed by electron microscopy. Treatment with monensin or ammonium chloride partially inhibited virus release from Caco-2 cells. No significant cell lysis was observed at the times postinfection when extracellular virus was initially detected, and transepithelial resistance and vital dye uptake measurements showed only a moderate decrease. Brefeldin A was found to significantly and specifically inhibit poliovirus biosynthetic processes by an as yet uncharacterized mechanism. The vectorial release of poliovirus from the apical (or luminal) surface of human intestinal epithelial cells has significant implications for viral pathogenesis in the human gut.     

Iron-induced oxidative stress up-regulates calreticulin levels in intestinal epithelial (Caco-2) cells

Calreticulin, a molecular chaperone involved in the folding of endoplasmic reticulum synthesized proteins, is also a shock protein induced by heat, food deprivation, and chemical stress. Mobilferrin, a cytosolic isoform of calreticulin, has been proposed to be an iron carrier for iron recently incoming into intestinal cells. To test the hypothesis that iron could affect calreticulin expression, we investigated the possible associations of calreticulin with iron metabolism. To that end, using Caco-2 cells as a model of intestinal epithelium, the mass and mRNA levels of calreticulin were evaluated as a function of the iron concentration in the culture media. Increasing the iron content in the culture from 1 to 20 microM produced an increase in calreticulin mRNA and a two-fold increase in calreticulin. Increasing iron also induced oxidative damage to proteins, as assessed by the formation of 4-hydroxy-2-nonenal adducts. Co-culture of cells with the antioxidants quercetin, dimethyltiourea and N-acetyl cysteine abolished both the iron-induced oxidative damage and the iron-induced increase in calreticulin. We postulate that the iron-induced expression of calreticulin is part of the cellular response to oxidative stress generated by iron.     

Proline uptake by monolayers of human intestinal absorptive (Caco-2) cells in vitro.

Monolayers of the Caco-2 human intestinal cell line exhibit active and passive uptake systems for the imino acid L-proline. The active transport component is saturable and it is responsible for about two thirds of the observed flux over the nanomolar concentration range, at 37 degrees C and pH 7.4. In contrast to L-phenylalanine, specific L-proline uptake has a high degree of sodium dependency and the efficiency of the carrier system is significantly reduced when protein synthesis (cycloheximide), Na+/K(+)-ATPase (ouabain) or cellular metabolism (sodium azide) are inhibited. The expression of the L-proline carrier by Caco-2 cells was under some degree of nutritional control. Glucose deficiency, over the time scale of the experiment, had no effect. The temperature-dependence of the specific uptake process followed the Arrhenius model with an apparent activation energy of 93.5 kJ nmol-1. This pathway also displayed Michaelis-Menten concentration-dependence with a Ksdm of 5.28 mM and a maximal transport flux (Jsdmax) of 835 pmol min-1 (10(6) cells)-1. Although the passive component was unchanged, the pH of the donor phase exerted a profound effect on the active carrier component. Within the physiological pH range a local maximum efficiency was found at pH 7.4 but dramatic increases were noted as pH 5.0 was approached. In competition studies, with 100-fold excess of a second amino acid, strong inhibition of uptake was found with alpha-aminoisobutyric acid, L-alanine and L-serine whereas moderate inhibition was observed with glycine, D-proline and gamma-aminoisobutyric acid. Aromatic and branched amino acids showed weak (L-valine) or no interaction (L-phenylalanine, L-leucine) with the carrier system. These data indicate that the carrier system for the uptake of L-proline has many features in common with the A system for amino acid transport.     

Correlation between oral drug absorption in humans and apparent drug permeability coefficients in human intestinal epithelial (Caco-2) cells.

Monolayers of a well differentiated human intestinal epithelial cell line, Caco-2, were used as a model to study passive drug absorption across the intestinal epithelium. Absorption rate constants (expressed as apparent permeability coefficients) were determined for 20 drugs and peptides with different structural properties. The permeability coefficients ranged from approximately 5 x 10(-8) to 5 x 10(-5) cm/s. A good correlation was obtained between data on oral absorption in humans and the results in the Caco-2 model. Drugs that are completely absorbed in humans had permeability coefficients greater than 1 x 10(-6) cm/s. Drugs that are absorbed to greater than 1% but less than 100% had permeability coefficients of 0.1-1.0 x 10(-6) cm/s while drugs and peptides that are absorbed to less than 1% had permeability coefficients of less than or equal to 1 x 10(-7) cm/s. The results indicate that Caco-2 monolayers can be used as a model for studies on intestinal drug absorption.     

Sorting of endogenous plasma membrane proteins occurs from two sites in cultured human intestinal epithelial cells (Caco-2)

We studied the postsynthetic sorting of endogenous plasma membrane proteins in a polarized epithelial cell line, Caco-2. Pulse-chase radiolabeling was combined with domain-specific cell surface assays to monitor the arrival of three apical and one basolateral protein at the apical and basolateral cell surface. Apical proteins were inserted simultaneously into both membrane domains. The fraction targeted to the basolateral domain was different for the three apical proteins and was subsequently sorted to the apical domain by transcytosis at different rates. In contrast, a basolateral protein was found in the basolateral membrane only. Thus, sorting of plasma membrane proteins occurred from two sites: the Golgi apparatus and the basolateral membrane. These data explain apparently conflicting results of earlier studies.     

Coronavirus infection of polarized epithelial cells

Epithelial cells are the first host cells to be infected by incoming coronaviruses. Recent observations in vitro show that coronaviruses are released from a specific side of these polarized cells, and this polarized release might be important for the spread of the infection in vivo. Mechanisms for the directional sorting of coronaviruses might be similar to those governing the polar release of secretory proteins.     

Characterization of carnosine uptake and its physiological function in human intestinal epithelial Caco-2 cells

Carnosine (beta-Ala-L-His) is known to have the physiological functions of an antioxidant. Although dietary carnosine is thought to be absorbed across intestinal epithelial cells, the mechanism for this absorption is not yet well understood and its function in the intestinal tract is also obscure. The intestinal transport of carnosine was characterized in the present study by using human intestinal Caco-2 cells, and its physiological function in these cells was further examined. The carnosine uptake was proton-dependent, being activated by lowering the apical pH value. Its uptake was significantly inhibited by other dipeptides, whereas it was not inhibited by other amino acids. These characteristics of the carnosine uptake strongly suggest its transport into the cells via peptide transporter 1 (PepT1). Since carnosine has antioxidative activity, we studied its effect on the H2O2-induced secretion of inflammatory cytokines in Caco-2 cells. The H2O2 induced increase in IL-8 secretion was inhibited by a pretreatment with carnosine for 3 h, this inhibition being presented in a dose-dependent manner. These results suggest that carnosine had a protective effect against oxidative stress in intestinal epithelial cells.     

Endocytosis, recycling, and lysosomal delivery of brush border hydrolases in cultured human intestinal epithelial cells (Caco-2)

Lysosomes of intestinal epithelial cells in vivo and in culture display strong immunoreactivity with monoclonal antibodies against various brush border enzymes as visualized by immunoelectron microscopy. Novel subcellular fractionation procedures were developed to study, by the pulse-chase technique and by internalization assays, the pathway along which two microvillar hydrolases, sucrase-isomaltase and dipeptidylpeptidase IV, are transported to lysosomes in the differentiated colon adenocarcinoma cell line Caco-2. 7-9% of metabolically labeled sucrase-isomaltase of dipeptidylpeptidase IV were present in lysosomes after 7-8 h of chase as intact complex-glycosylated molecules. Appearance of these enzymes in lysosomes was biphasic. Endocytosis studies with radioiodinated antienzyme monoclonal antibodies (monovalent antigen-binding fragments) and by means of cell surface iodination revealed only slow transport of the enzymes to lysosomes at a low level. However, both enzymes were internalized with different efficiencies and recycled to the cell surface via endosomes. These results suggest that in Caco-2 cells a significant amount of newly synthesized sucrase-isomaltase and dipeptidylpeptidase IV is directly imported into lysosomes bypassing the brush border membrane.     

Collagen IV-dependent ERK activation in human Caco-2 intestinal epithelial cells requires focal adhesion kinase

Integrin-initiated extracellular signal-regulated kinase (ERK) activation by matrix adhesion may require focal adhesion kinase (FAK) or be FAK-independent via caveolin and Shc. This remains controversial for fibroblast and endothelial cell adhesion to fibronectin and is less understood for other matrix proteins and cells. We investigated Caco-2 intestinal epithelial cell ERK activation by collagen I and IV, laminin, and fibronectin. Collagens or laminin, but not fibronectin, stimulated tyrosine phosphorylation of FAK, paxillin, and p130(cas) and activated ERK1/2. Shc, tyrosine-phosphorylated by matrix adhesion in many cells, was not phosphorylated in Caco-2 cells in response to any matrix. Caveolin expression did not affect Caco-2 Shc phosphorylation in response to fibronectin. FAK, ERK, and p130(cas) tyrosine phosphorylation were activated after 10-min adhesion to collagen IV. FAK activity increased for 45 min after collagen IV adhesion and persisted for 2 h, while p130(cas) phosphorylation increased only slightly after 10 min. ERK activity peaked at 10 min, declined after 30 min, and returned to base line after 1 h. Transfection with FAK-related nonkinase, but not substrate domain deleted p130(cas), strongly inhibited ERK2 activation in response to collagen IV, indicating Caco-2 ERK activation is at least partly regulated by FAK.     

Suppressive effect of nobiletin and epicatechin gallate on fructose uptake in human intestinal epithelial Caco-2 cells

Abstract

Inhibition of excessive fructose intake in the small intestine could alleviate fructose-induced diseases such as hypertension and non-alcoholic fatty liver disease. We examined the effect of phytochemicals on fructose uptake using human intestinal epithelial-like Caco-2 cells which express the fructose transporter, GLUT5. Among 35 phytochemicals tested, five, including nobiletin and epicatechin gallate (ECg), markedly inhibited fructose uptake. Nobiletin and ECg also inhibited the uptake of glucose but not of L-leucine or Gly-Sar, suggesting an inhibitory effect specific to monosaccharide transporters. Kinetic analysis further suggested that this reduction in fructose uptake was associated with a decrease in the apparent number of cell-surface GLUT5 molecules, and not with a change in the affinity of GLUT5 for fructose. Lastly, nobiletin and ECg suppressed the permeation of fructose across Caco-2 cell monolayers. These findings suggest that nobiletin and ECg are good candidates for preventing diseases caused by excessive fructose intake.     

mRNA localization in polarized intestinal epithelial cells

An important feature of enterocyte maturation is the asymmetrical distribution of cellular functions including protein localization. mRNA sorting is one mechanism for establishment and maintenance of this process in other systems, and we have previously demonstrated differential localization of mRNAs in human enterocytes. To study regulation of mRNA sorting, we established a model in polarized Caco-2 cells. Proxy cDNA constructs containing beta-galactosidase (beta-gal)/green fluorescence protein (GFP) and the 3'-untranslated region (3'-UTR) of either human sucrase-isomaltase or villin were transfected transiently or stably. A control construct contained poly-A sequence in place of 3'-UTR. Expression of GFP was observed by confocal microscopy; intracellular location of the construct mRNA was imaged by in situ hybridization. The sucrase-isomaltase mRNA proxy localized to an apical position in Caco-2 cells as in native enterocytes; the villin mRNA proxy did not show significant localization. The control construct was not localized and was found diffusely throughout the cell. Proxy GFP proteins tended to localize with their mRNA proxies, but with less precision. This study establishes a valuable model for the investigation of mRNA localization in intestinal epithelial cells. Mechanisms controlling asymmetrical distribution of intestinal mRNAs can be now be elucidated.     

Trafficking of Shigella lipopolysaccharide in polarized intestinal epithelial cells.

Bacterial lipopolysaccharide (LPS) at the apical surface of polarized intestinal epithelial cells was previously shown to be transported from the apical to the basolateral pole of the epithelium (Beatty, W.L., and P.J. Sansonetti. 1997. Infect. Immun. 65:4395-4404). The present study was designed to elucidate the transcytotic pathway of LPS and to characterize the endocytic compartments involved in this process. Confocal and electron microscopic analyses revealed that LPS internalized at the apical surface became rapidly distributed within endosomal compartments accessible to basolaterally internalized transferrin. This compartment largely excluded fluid-phase markers added at either pole. Access to the basolateral side of the epithelium subsequent to trafficking to basolateral endosomes occurred via exocytosis into the paracellular space beneath the intercellular tight junctions. LPS appeared to exploit other endocytic routes with much of the internalized LPS recycled to the original apical membrane. In addition, analysis of LPS in association with markers of the endocytic network revealed that some LPS was sent to late endosomal and lysosomal compartments.     

Esterification of xanthophylls by human intestinal Caco-2 cells

We recently found that peridinin, which is uniquely present in dinoflagellates, reduced cell viability by inducing apoptosis in human colon cancer cells. Peridinin is also found in edible clams and oysters because the major food sources of those shellfish are phytoplanktons such as dinoflagellates. Little is known, however, about the fate of dietary peridinin and its biological activities in mammals. The aim of the present study was to investigate the enzymatic esterification of xanthophylls, especially peridinin which is uniquely present in dinoflagellates, using differentiated cultures of Caco-2 human intestinal cells. We found that peridinin is converted to peridininol and its fatty acid esters in differentiated Caco-2 cells treated with 5 μmol/L peridinin solubilized with mixed micelles. The cell homogenate was also able to deacetylate peridinin and to esterify peridininol. Other xanthophylls, such as fucoxanthin, astaxanthin and zeaxanthin, were also esterified, but at relatively lower rates than peridinin. In this study, we found the enzymatic esterification of xanthophylls in mammalian intestinal cells for the first time. Our results suggest that the esterification of xanthophylls in intestinal cells is dependent on their polarity.     

Repetitive deformation activates Src-independent FAK-dependent ERK motogenic signals in human Caco-2 intestinal epithelial cells

Repetitive deformation due to villous motility or peristalsis may support the intestinal mucosa, stimulating intestinal epithelial proliferation under normal circumstances and restitution in injured and inflamed mucosa rich in tissue fibronectin. Cyclic strain enhances Caco-2 and IEC-6 intestinal epithelial cell migration across fibronectin via ERK. However, the upstream mediators of ERK activation are unknown. We investigated whether Src and FAK mediate strain-induced ERK phosphorylation and migration in human Caco-2 intestinal epithelial cells on fibronectin. Monolayers on tissue fibronectin-precoated membranes were subjected to an average 10% repetitive deformation at 10 cycles/min. Phosphorylation of Src-Tyr 418, FAK-Tyr 397-Tyr 576-Tyr 925, and ERK were significantly increased by deformation. The stimulation of wound closure by strain was prevented by Src blockade with PP2 (10 micromol/l) or specific short interfering (si)RNA. Src inhibition also prevented strain-induced FAK phosphorylation at Tyr 397 and Tyr 576 but not FAK-Tyr 925 or ERK phosphorylation. Reducing FAK by siRNA inhibited strain-induced ERK phosphorylation. Transfection of NH2-terminal tyrosine phosphorylation-deficient FAK mutants Y397F, Y576F-Y577F, and Y397F-Y576F-Y577F did not prevent the activation of ERK2 by cyclic strain, but a FAK mutant at the COOH terminal (Y925F) prevented the strain-induced activation of ERK2. Although the Y397F-Y576F-Y577F FAK construct exhibited less basal FAK-Tyr 925 phosphorylation under static conditions, it nevertheless exhibited increased FAK-Tyr 925 phosphorylation in response to strain. These results suggest that repetitive deformation stimulates intestinal epithelial motility across fibronectin in a manner that requires both Src activation and a novel Src-independent FAK-Tyr 925-dependent pathway that activates ERK. This pathway may be an important target for interventions to promote mucosal healing in settings of intestinal ileus or fasting.