Human intestinal cell line Caco-2: a useful model for studying cellular and molecular regulation of biotin uptake

Biotin conjugates in targeted drug delivery: is it mediated by a biotin transporter, a yet to be identified receptor, or (an)other unknown mechanism(s)?

Conjugation of drugs with biotin is a widely studied strategy for targeted drug delivery. The structure-activity relationship (SAR) studies through H3-biotin competition experiments conclude with the presence of a free carboxylic acid being essential for its uptake via the sodium-dependent multivitamin transporter (SMVT, the major biotin transporter). However, biotin conjugation with a payload requires modification of the carboxylic acid to an amide or ester group. Then, there is the question as to how/whether the uptake of biotin conjugates goes through the SMVT. If not, then what is the mechanism? Herein, we present known uptake mechanisms of biotin and its applications reported in the literature. We also critically analyse possible uptake mechanism(s) of biotin conjugates to address the disconnect between the results from SMVT-based SAR and "biotin-facilitated" targeted drug delivery. We believe understanding the uptake mechanism of biotin conjugates is critical for their future applications and further development.

Imaging Biotin Trafficking In Vivo with Positron Emission Tomography

The water-soluble vitamin biotin is essential for cellular growth, development, and well-being, but its absorption, distribution, metabolism, and excretion are poorly understood. This paper describes the radiolabeling of biotin with the positron emission tomography (PET) radionuclide carbon-11 ([¹¹C]biotin) to enable the quantitative study of biotin trafficking in vivo. We show that intravenously administered [¹¹C]biotin is quickly distributed to the liver, kidneys, retina, heart, and brain in rodents-consistent with the known expression of the biotin transporter-and there is a surprising accumulation in the brown adipose tissue (BAT). Orally administered [¹¹C]biotin was rapidly absorbed in the small intestine and swiftly distributed to the same organs. Preadministration of nonradioactive biotin inhibited organ uptake and increased excretion. [¹¹C]Biotin PET imaging therefore provides a dynamic in vivo map of transporter-mediated biotin trafficking in healthy rodents. This technique will enable the exploration of biotin trafficking in humans and its use as a research tool for diagnostic imaging of obesity/diabetes, bacterial infection, and cancer.

Ionic gelated β-cyclodextrin-biotin-carboxymethyl chitosan nanoparticles prepared as carrier for oral delivery of protein drugs

In this paper, the β-cyclodextrin (β-CD) and biotin (Bi) were successfully grafted onto carboxymethyl chitosan (CMCS). And then the β-CD-Bi-CMCS nanoparticles (NPs) were prepared as oral nano-delivery carrier of protein drugs by ionic gelation method. The morphological feature of fabricated drug carrier was determined by dynamic light scattering and transmission electron microscopy. The result showed that the prepared NPs presented spherical structure with an average diameter of 138 nm. Bovine serum albumin (BSA) was selected as model protein drug that was entrapped in prepared drug carrier with satisfactory entrapment efficiency (79.18%) and loading content (3.96%). The drug release profiles of BSA/β-CD-Bi-CMCS NPs were studied at different pH environment for simulated gastric fluid (SGF), simulated intestinal fluid (SIF) and simulated colonic fluid (SCF). It was found that the BSA/β-CD-Bi-CMCS NPs displayed a pH dependent drug release profiles. After 72 h, the cumulative release amount of BSA in SGF, SIF, and SCF was about 20.57, 74.46, and 91%, respectively. Furthermore, the enzymatic degradation and cytotoxicity studies showed the synthesized β-CD-Bi-CMCS NPs had high chemical stability and biocompatibility. This work indicated that the β-CD-Bi-CMCS NPs had the potentiality as promising nanocarriers for oral delivery of protein drugs.

Role of nanoparticle size, shape and surface chemistry in oral drug delivery

Nanoparticles find intriguing applications in oral drug delivery since they present a large surface area for interactions with the gastrointestinal tract and can be modified in various ways to address the barriers associated with oral delivery. The size, shape and surface chemistry of nanoparticles can greatly impact cellular uptake and efficacy of the treatment. However, the interplay between particle size, shape and surface chemistry has not been well investigated especially for oral drug delivery. To this end, we prepared sphere-, rod- and disc-shaped nanoparticles and conjugated them with targeting ligands to study the influence of size, shape and surface chemistry on their uptake and transport across intestinal cells. A triple co-culture model of intestinal cells was utilized to more closely mimic the intestinal epithelium. Results demonstrated higher cellular uptake of rod-shaped nanoparticles in the co-culture compared to spheres regardless of the presence of active targeting moieties. Transport of nanorods across the intestinal co-culture was also significantly higher than spheres. The findings indicate that nanoparticle-mediated oral drug delivery can be potentially improved with departure from spherical shape which has been traditionally utilized for the design of nanoparticles. We believe that understanding the role of nanoparticle geometry in intestinal uptake and transport will bring forth a paradigm shift in nanoparticle engineering for oral delivery and non-spherical nanoparticles should be further investigated and considered for oral delivery of therapeutic drugs and diagnostic materials.

The sodium/multivitamin transporter: a multipotent system with therapeutic implications

The Na(+)/multivitamin transporter (SMVT) is a member of the solute:sodium symporter family that catalyzes the Na(+)-dependent uptake of the structurally diverse water-soluble vitamins pantothenic acid (vitamin B5) and biotin (vitamin H), α-lipoic acid-a vitamin-like substance with strong antioxidant properties-and iodide. The organic substrates of SMVT play central roles in the cellular metabolism and are, therefore, essential for normal human health and development. For example, biotin deficiency leads to growth retardation, dermatological disorders, and neurological disorders. Animal studies have shown that biotin deficiency during pregnancy is directly correlated to embryonic growth retardation, congenital malformation, and death of the embryo. This chapter focuses on the structural and functional features of the human isoform of SMVT (hSMVT); the discovery of which was greatly facilitated by the cloning and expression of hSMVT in tractable expression systems. Special emphasis will be given to mechanistic implications of the transport process of hSMVT that will inform our understanding of the molecular determinants of hSMVT-mediated transport in dynamic context to alleviate the development and optimization of hSMVT as a multipotent platform for drug delivery.

Functional and molecular aspects of biotin uptake via SMVT in human corneal epithelial (HCEC) and retinal pigment epithelial (D407) cells

Sodium-dependent multivitamin transporter (SMVT) is a vital transmembrane protein responsible for translocating biotin and other essential cofactors such as pantothenate and lipoate. Unlike primary cultures of corneal and retinal pigment epithelial (RPE) cells, immortalized cells can be subcultured many times, yet maintain their physiological properties. Hence, the purpose of this study was to delineate the functional and molecular aspects of biotin uptake via SMVT on immortalized human corneal epithelial (HCEC) and RPE (D407) cells. Functional aspects of [(3)H] biotin uptake were studied in the presence of different concentrations of unlabeled biotin, pH, temperature, metabolic inhibitors, ions, substrates, structural analogs and biotinylated prodrug (Biotin-Acyclovir (B-ACV)). Molecular identity of SMVT was examined with reverse transcription-polymerase chain reaction. Biotin uptake was found to be saturable in HCEC and D407 cells with K (m) of 296.2 ± 25.9 and 863.8 ± 66.9 μM and V (max) of 77.2 ± 2.2 and 308.3 ± 10.7 pmol/mg protein/min, respectively. Uptake was found to be pH, temperature, energy, and sodium-dependent. Inhibition of biotin uptake was observed in the presence of structural analogs and specific substrates. Further, uptake was lowered in the presence of B-ACV indicating the translocation of biotinylated prodrug by SMVT. A distinct band at 774 bp confirmed the molecular existence of SMVT in both the cells. This study shows for the first time the functional and molecular presence of SMVT in HCEC and D407 cells. Therefore, these cell lines may be utilized as in vitro models to study the cellular translocation of biotin-conjugated prodrugs.

Targeted lipid based drug conjugates: a novel strategy for drug delivery

A majority of studies involving prodrugs are directed to overcome low bioavailability of the parent drug. The aim of this study is to increase the bioavailability of acyclovir (ACV) by designing a novel prodrug delivery system which is more lipophilic, and at the same time site specific. In this study, a lipid raft has been conjugated to the parent drug molecule to impart lipophilicity. Simultaneously a targeting moiety that can be recognized by a specific transporter/receptor in the cell membrane has also been tethered to the other terminal of lipid raft. Targeted lipid prodrugs i.e., biotin-ricinoleicacid-acyclovir (B-R-ACV) and biotin-12hydroxystearicacid-acyclovir (B-12HS-ACV) were synthesized with ricinoleicacid and 12hydroxystearicacid as the lipophilic rafts and biotin as the targeting moiety. Biotin-ACV (B-ACV), ricinoleicacid-ACV (R-ACV) and 12hydroxystearicacid-ACV (12HS-ACV) were also synthesized to delineate the individual effects of the targeting and the lipid moieties. Cellular accumulation studies were performed in confluent MDCK-MDR1 and Caco-2 cells. The targeted lipid prodrugs B-R-ACV and B-12HS-ACV exhibited much higher cellular accumulation than B-ACV, R-ACV and 12HS-ACV in both cell lines. This result indicates that both the targeting and the lipid moiety act synergistically toward cellular uptake. The biotin conjugated prodrugs caused a decrease in the uptake of [(3)H] biotin suggesting the role of sodium dependent multivitamin transporter (SMVT) in uptake. The affinity of these targeted lipid prodrugs toward SMVT was studied in MDCK-MDR1 cells. Both the targeted lipid prodrugs B-R-ACV (20.25 ± 1.74 μM) and B-12HS-ACV (23.99 ± 3.20 μM) demonstrated higher affinity towards SMVT than B-ACV (30.90 ± 4.19 μM). Further, dose dependent studies revealed a concentration dependent inhibitory effect on [(3)H] biotin uptake in the presence of biotinylated prodrugs. Transepithelial transport studies showed lowering of [(3)H] biotin permeability in the presence of biotin and biotinylated prodrugs, further indicating a carrier mediated translocation by SMVT. Overall, results from these studies clearly suggest that these biotinylated lipid prodrugs of ACV possess enhanced affinity towards SMVT. These prodrugs appear to be potential candidates for the treatment of oral and ocular herpes virus infections, because of higher expression of SMVT on intestinal and corneal epithelial cells. In conclusion we hypothesize that our novel prodrug design strategy may help in higher absorption of hydrophilic parent drug. Moreover, this novel prodrug design can result in higher cell permeability of hydrophilic therapeutics such as genes, siRNA, antisense RNA, DNA, oligonucleotides, peptides and proteins.

Role of the putative N-glycosylation and PKC-phosphorylation sites of the human sodium-dependent multivitamin transporter (hSMVT) in function and regulation

The sodium-dependent multivitamin transporter (SMVT) is a major biotin transporter in a variety of tissues including the small intestine. The human SMVT (hSMVT) polypeptide is predicted to have four N-glycosylation sites and two putative PKC phosphorylation sites but their role in the function and regulation of the protein is not known and was examined in this investigation. Our results showed that the hSMVT protein is glycosylated and that this glycosylation is important for its function. Studies utilizing site-directed mutagenesis revealed that the N-glycosylation sites at positions Asn(138) and Asn(489) are important for the function of hSMVT and that mutating these sites significantly reduces the V(max) of the biotin uptake process. Mutating the putative PKC phosphorylation site Thr(286) of hSMVT led to a significant decrease in the PMA-induced inhibition in biotin uptake. The latter effect was not mediated via changes in the level of expression of the hSMVT protein and mRNA or in its level of expression at the cell membrane. These findings demonstrate that the hSMVT protein is glycosylated, and that glycosylation is important for its function. Furthermore, the study shows a role for the putative PKC-phosphorylation site Thr(286) of hSMVT in the PKC-mediated regulation of biotin uptake.

Inhibition of intestinal biotin absorption by chronic alcohol feeding: cellular and molecular mechanisms

The water-soluble vitamin biotin is essential for normal cellular functions and its deficiency leads to a variety of clinical abnormalities. Mammals obtain biotin from exogenous sources via intestinal absorption, a process mediated by the sodium-dependent multivitamin transporter (SMVT). Chronic alcohol use in humans is associated with a significant reduction in plasma biotin levels, and animal studies have shown inhibition in intestinal biotin absorption by chronic alcohol feeding. Little, however, is known about the cellular and molecular mechanisms involved in the inhibition in intestinal biotin transport by chronic alcohol use. These mechanisms were investigated in this study by using rats and transgenic mice carrying the human full-length SLC5A6 5'-regulatory region chronically fed alcohol liquid diets; human intestinal epithelial Caco-2 cells chronically exposed to alcohol were also used as models. The results showed chronic alcohol feeding of rats to lead to a significant inhibition in carrier-mediated biotin transport events across jejunal brush border and basolateral membrane domains. This inhibition was associated with a significant reduction in level of expression of the SMVT protein, mRNA, and heterogenous nuclear RNA. Chronic alcohol feeding also inhibited carrier-mediated biotin uptake in rat colon. Studies with transgenic mice confirmed the above findings and further showed chronic alcohol feeding significantly inhibited the activity of SLC5A6 5'-regulatory region. Finally, chronic exposure of Caco-2 cells to alcohol led to a significant decrease in the activity of both promoters P1 and P2 of the human SLC5A6 gene. These studies identify for the first time the cellular and molecular parameters of the intestinal biotin absorptive processes that are affected by chronic alcohol feeding.

Role of intestinal transporters in neonatal nutrition: carbohydrates, proteins, lipids, minerals, and vitamins

To support rapid growth and a high metabolic rate, infants require enormous amounts of nutrients. The small intestine must have the complete array of transporters that absorb the nutrients released from digested food. Failure of intestinal transporters to function properly often presents symptoms as "failure to thrive" because nutrients are not absorbed and as diarrhea because unabsorbed nutrients upset luminal osmolality or become substrates of intestinal bacteria. We enumerate the nutrients that constitute human milk and various infant milk formulas, explain their importance in neonatal nutrition, then describe for each nutrient the transporter(s) that absorbs it from the intestinal lumen into the enterocyte cytosol and from the cytosol to the portal blood. More than 100 membrane and cytosolic transporters are now thought to facilitate absorption of minerals and vitamins as well as products of digestion of the macronutrients carbohydrates, proteins, and lipids. We highlight research areas that should yield information needed to better understand the important role of these transporters during normal development.

Vitreal pharmacokinetics of biotinylated ganciclovir: role of sodium-dependent multivitamin transporter expressed on retina

PURPOSE

The objective of this study was to investigate the role of sodium-dependent multiple vitamin transporter (SMVT) on Biotin-Ganciclovir (biotin-GCV) uptake on both human retinal pigmented epithelium cell line (ARPE-19) and rabbit retina. Study also aims to delineate the vitreal pharmacokinetics of biotin-GCV.

METHOD

ARPE-19 was employed to study the in vitro uptake experiments. New Zealand white albino rabbits were used to study in vivo retinal uptake and vitreal pharmacokinetics following intravitreal administration of biotin-GCV. In vitro uptake kinetics of [3H] biotin was determined at various initial concentrations. Competitive inhibition studies were conducted in the presence of unlabelled biotin, desthiobiotin, pantothenic acid, and lipoic acid. Various other uptake studies were performed to functionally characterize the transporter. To provide the molecular evidence of this transporter, Reverse Transcription-Polymerase Chain Reaction (RT-PCR) studies were also conducted. In vivo retinal/choroidal uptake studies were carried out with New Zealand albino rabbits. Unconscious animal ocular microdialysis studies were performed in order to evaluate intravitreal pharmacokinetics of GCV and Biotin-GCV.

RESULTS

Uptake of [3H] biotin into ARPE-19 was linear over 7 min, and found to be saturable with K(m) of 138.25 muM and Vmax of 38.85 pmol/min/mg protein. Both pantothenic acid and lipoic acid decreased significantly in uptake of biotin in the concentration-dependent manner. Uptake of biotin into ARPE-19 was found to be temperature, energy, and Na+ dependent but Cl(-)independent. Further, RT-PCR studies identified a band exhibiting presence of hSMVT on ARPE-19. Biotin-GCV is recognized by SMVT system present on the ARPE-19 and rabbit retina. Vitreal Pharmacokinetics profile reveals that most of the parameters were not significantly different for GCV and Biotin-GCV. However, use of Biotin-GCV may result in sustain levels of regenerated GCV in vitreous.

CONCLUSIONS

SMVT was identified and functionally characterized on ARPE-19 cells. Further, Biotin-GCV shares this transport system. Vitreal pharmacokinetics of the conjugate was determined in unconscious rabbit model.

Cell and molecular aspects of human intestinal biotin absorption

Humans cannot synthesize biotin and thus must obtain this vitamin from exogenous sources. The intestine is exposed to 2 sources of biotin: a dietary source and a bacterial source, which is normal microflora of the large intestine. Dietary protein-bound biotin is converted to free biotin prior to absorption. Absorption of free biotin in the small and large intestine involves a saturable and Na(+)-dependent carrier-mediated process that is shared with pantothenic acid and lipoate. For this reason, the involved transport system is referred to as the sodium-dependent multivitamin transporter (SMVT); in humans, it is designated as hSMVT. The hSMVT system has been cloned, demonstrated to be exclusively expressed at the apical membrane of enterocytes, and shown, by means of gene-specific short interfering RNA, to be the main biotin uptake system that operates in human intestinal epithelial cells. The 5'-regulatory region of the hSMVT gene has also been cloned and characterized both in vitro and in vivo. Further, the human intestinal biotin uptake process was adaptively up-regulated in biotin deficiency via a transcriptionally mediated mechanism(s) that involves Kruppel-like factor 4 sites. Studies on cell biology of hSMVT have shown a region in the cytoplasmic C-terminal domain of the polypeptide to be essential for its targeting to the apical membrane domain of epithelial cells. Intracellular trafficking of the hSMVT protein appears to involve distinct trafficking vesicles that require an intact microtubules network and the motor protein dynein for their mobility.

Expression of drug-metabolizing enzymes, nuclear transcription factors and ABC transporters in Caco-2 cells

1. Caco-2 cells are frequently used in intestinal drug absorption and metabolism studies, but little is known about the effects of drugs on the simultaneous expression of genes coding for drug-metabolizing enzymes (DMEs), nuclear transcription factors and ABC transporters. 2. The gene expression and enzyme activities of control and Aroclor 1254-treated cultures were therefore explored, the latter being a powerful inducer of DMEs. Fourteen- and 80-fold induction of CYP1A1 and CYP1A2 mRNA were shown, whereas expression of other DMEs was either increased (CYP2C8-2C19, 10-fold; CYP3A5, twofold; FMO1, 2 and 5, twofold; epoxide hydrolase, threefold) or repressed (CYP2D6 and CYP2E1 to 75% of control values). 3. Notably, gene copies of CYP3A4 and CYP2B6/7 were below the limit of detection, but a three- and 10-fold induction of HNF 1alpha + beta, HNF-4alpha4 and a similar 10-fold increase in STAT 3 and 4 was observed. 4. Similarly, c/EBP transcripts were only detected in treated cell cultures, but MRP1, its isoforms 3-5 as well as MDR-1 were increased threefold after dosing with Aroclor 1254. 5. Overall, CYP gene expression correlated well with the cognate enzyme activity using testosterone as a marker substrate.

Molecular mechanisms involved in the adaptive regulation of human intestinal biotin uptake: A study of the hSMVT system

Biotin, a water-soluble micronutrient, is vital for cellular functions, including growth and development. The human intestine utilizes the human sodium-dependent multivitamin transporter (hSMVT) for biotin uptake. Evidence exists showing that the intestinal biotin uptake process is adaptively regulated during biotin deficiency. Nothing, however, is known about molecular mechanism(s) involved during this adaptive regulation. This study compared two human-derived intestinal epithelial cell lines (HuTu-80 and Caco-2) during biotin-deficient or biotin-sufficient states and with an approach that assessed carrier-mediated biotin uptake, hSMVT protein and RNA levels, RNA stability, and hSMVT promoter activity. The results showed that during biotin deficiency, a significant and specific upregulation in carrier-mediated biotin uptake occurred in both human intestinal epithelial cell lines and that this increase was associated with an induction in protein and mRNA levels of hSMVT. The increase in mRNA levels was not due to an increase in RNA stability but was associated with an increase in activity of the hSMVT promoter in transfected human intestinal cells. Using promoter deletion constructs and mutational analysis in transiently transfected HuTu-80 and Caco-2 cells, a biotin deficiency-responsive region was mapped to a 103-bp area within the hSMVT promoter that contains gut-enriched Kruppel-like factor (GKLF) sites that confer the response to biotin deficiency. These results confirm that human intestinal biotin uptake is adaptively regulated and provide novel evidence demonstrating that the upregulation is not mediated via changes in hSMVT RNA stability but rather is due to transcriptional regulatory mechanism(s) that likely involve GKLF sites in the hSMVT promoter.

Biotin uptake by rabbit corneal epithelial cells: role of sodium-dependent multivitamin transporter (SMVT)

PURPOSE

The objective of this research was to investigate the presence of sodium-dependent multivitamin transporter (SMVT) on rabbit corneal epithelial cells.

METHODS

Primary cultured rabbit corneal epithelial cells (rPCECs)and freshly excised rabbit corneas were used for characterization of biotin uptake and transport, respectively. Reverse transcription-polymerase chain reaction (RT-PCR) was performed to confirm the molecular identity of SMVT. Liquid chromatography/tandem mass spectrometry (LC-MS/MS) analysis was performed to examine the presence of biotin in rabbit tears.

RESULTS

Uptake of biotin by rPCECs was found to be time and concentration dependent with Km of 32.52 microM and Vmax of 10.43 pmol min- 1 mg protein- 1. Biotin was significantly inhibited in the presence of pantothenic acid and lipoic acid. Biotin uptake was found to be energy and Na+ dependent but H+ and Cl- independent. The uptake was inhibited by valeric acid in a concentration-dependent manner but not much affected in the presence of biotin methyl ester and biocytin with no free carboxyl group. Modulators of both PKC- and PKA-mediated pathways had no effect on biotin uptake, but calcium-calmodulin inhibitor significantly inhibited its uptake. Sodium-dependent multivitamin transporter was identified by RT-PCR in rPCECs. Transport experiments across the rabbit corneas revealed the functional localization of SMVT on the apical side of the cornea, and thereby corroborating with in vitro results with cultured corneal cells. Finally, LC-MS/MS analysis showed the presence of biotin in rabbit tears.

CONCLUSIONS

Results obtained from both in vitro and exvivo studies suggest the possible role of SMVT expressed on corneal epithelial cells for the uptake of biotin, which co-transports pantothenic acid and lipoic acid. Further, the presence of biotin in tears suggests the physiological significance of this transporter in rabbit corneal epithelium.

Prion protein 90-231 contains a streptavidin-binding motif

The biological function of prion protein (PrP) and the physiological relevance of its truncated subtypes and glycoforms is still enigmatic. In this paper, we adduce evidence that recombinant murine PrP fragment 90-231 (mPrP90-231) contains a biotin-mimicking sequence motif that causes binding of the bacterial protein streptavidin to mPrP90-231. As indicated by epitope mapping and proven by analysis of a deletion mutant (mPrP101-231), streptavidin binding is primarily mediated by the amino-terminus of mPrP90-231 with the core-binding sequence represented by residues 94-100. Competition with biotin significantly reduces the interaction pointing to an involvement of streptavidin's biotin-binding site (BBS). Since the BBS of streptavidin shares similarities with the active sites of proteins involved in biotin metabolism we speculate that biotin mimicry by truncated PrP-species may have an impact in vivo.

The blood-brain barrier sodium-dependent multivitamin transporter: a molecular functional in vitro-in situ correlation

The molecular mechanism of biotin brain uptake was investigated using an in vitro bovine blood-brain barrier (BBB) cell model and an in situ mouse brain perfusion technique. A functional uptake/transport correlation of the in vitro and in situ characteristics of biotin uptake was investigated. Morphological and immunochemical characteristics (e.g., factor VIII expression) of the primary culture of brain microvessel endothelial cells (BMECs) were confirmed. Gene expression of the multidrug resistance (Mdr1) and sodium-dependent multivitamin (SMVT) transporters was also determined in BMECs. Biotin transport was saturable and Na(+)-dependent at the luminal side of the BBB. The estimated half-saturation concentrations (K(m)) of biotin uptake in vitro and in situ were 49.1 and 35.5 microM, respectively, supporting the presence of a carrier-mediated biotin transport system. Inhibition studies using various biotin derivatives and structural analogs demonstrated the structural requirements for biotin-SMVT interaction. Desthiobiotin and pantothenic acid significantly inhibited the uptake of biotin, whereas 2-iminobiotin and diaminobiotin were very weak inhibitors. Based on our results, there was a good correlation between the in vitro and in situ BBB models, suggesting that when a single membrane transporter is involved in substrate uptake, flexibility in choosing the experimental model can be afforded. The current results are also consistent with the suggestion that the properties of the BBB are likely to be organ-specific rather than species-specific. Further mechanistic and comparative studies are needed to validate these results. In conclusion, the in vitro transporter-based mechanism studies produced valuable molecular functional transport results that correlated well with in situ results.

Biotin biochemistry and human requirements

Human biotin turnover and requirements can be estimated on the basis of (1) concentrations of biotin and metabolites in body fluids, (2) activities of biotin-dependent carboxylases, and (3) the urinary excretion of organic acids that are formed at increased rates if carboxylase activities are reduced. Recent studies suggest that the urinary excretions of biotin and its metabolite bisnorbiotin, activities of propionyl-CoA carboxylase and beta-methylcrotonyl-CoA carboxylase in lymphocytes, and urinary excretion of 3-hydroxyisovaleric acid are good indicators of marginal biotin deficiency. On the basis of studies using these indicators of biotin deficiency, an adequate intake of 30 microg (123 nmoles) of biotin per day is currently recommended for adults. The dietary biotin intake in Western populations has been estimated to be 35 to 70 microg/d (143-287 nmol/d). Recent studies suggest that humans absorb biotin nearly completely. Conditions that may increase biotin requirements in humans include pregnancy, lactation, and therapy with anticonvulsants or lipoic acid.

Biotin uptake by human intestinal and liver epithelial cells: role of the SMVT system

It has been well established that human intestinal and liver epithelial cells transport biotin via an Na+-dependent carrier-mediated mechanism. The sodium-dependent multivitamin transport (SMVT), a biotin transporter, is expressed in both cell types. However, the relative contribution of SMVT toward total carrier-mediated uptake of physiological (nanomolar) concentrations of biotin by these cells is not clear. Addressing this issue is important, especially in light of the recent identification of a second human high-affinity biotin uptake mechanism that operates at the nanomolar range. Hence, we employed a physiological approach of characterizing biotin uptake by human-derived intestinal Caco-2 and HepG2 cells at the nanomolar concentration range. We also employed a molecular biology approach of selectively silencing the endogenous SMVT of these cells with specific small interfering RNAs (siRNAs), then examining carrier-mediated biotin uptake. The results showed that in both Caco-2 and HepG2 cells, the initial rate of biotin uptake as a function of concentration over the range of 0.1 to 50 nM to be linear. Furthermore, we found that the addition of 100 nM unlabeled biotin, desthiobiotin, or pantothenic acid to the incubation medium had no effect on the uptake of 2.6 nM [3H]biotin. Pretreatment of Caco-2 and HepG2 cells with SMVT specific siRNAs substantially reduced SMVT mRNA and protein levels. In addition, carrier-mediated [3H]biotin (2.6 nM) uptake by Caco-2 and HepG2 cells was severely (P 0.01) inhibited by the siRNAs pretreatment. These results demonstrate that the recently described human high-affinity biotin uptake system is not functional in intestinal and liver epithelial cells. In addition, the results provide strong evidence that SMVT is the major (if not the only) biotin uptake system that operates in these cells.

Glutamine supports recovery from loss of transepithelial resistance and increase of permeability induced by media change in Caco-2 cells

Recent evidence suggests that the conditionally essential amino acid glutamine is important for intestinal barrier function. However, the mechanism remains undefined. To determine the effects of glutamine on permeability of intestinal epithelial cell monolayers, Caco-2 cells were grown on membrane filters and exposed to 4 mmol/L sodium butyrate in order to rapidly achieve high levels of alkaline phosphatase and high transepithelial resistance as seen in functionally mature enterocytes. A standard method of medium exchange consisting of removal and replacement resulted in a catastrophic loss of transepithelial resistance and increase of mannitol and dextran fluxes that required 2-4 hrs and protein synthesis to recover. The effect was attributed to exposure of the upper monolayer surface to atmosphere and could be avoided by refeeding by incremental perfusion. Spontaneously-differentiated Caco-2 monolayers were resistant to this stress. This novel stress test was employed as a sensitive assay for the requirement of glutamine for monolayer transepithelial resistance and mannitol permeability. Pre-stress glutamine availability was more important than Gln-availability during the recovery phase. Thus the transepithelial resistance and permeability of butyrate-induced monolayers is dynamically-regulated in response to atmospheric exposure, by a mechanism that depends on threshold levels of glutamine availability.

Biotin in metabolism and molecular biology

Biotin is a water-soluble vitamin required by all organisms by virtue of its essential role in carboxylation reactions. Although the metabolism and role of biotin in intermediary metabolism are well established, biotin remains one of the most poorly understood water-soluble vitamins in terms of nutritional requirements and responsiveness to physiological and pharmacological states. Significant advances in the understanding of biotin nutriture have been recently accomplished through the description of the kinetics and regulation of biotin transport and improved methods for biotin status assessment. Additionally, the potential role of biotin in the regulation of gene expression has been strengthened through description of altered gene expression during biotin deficiency and through newly described enzymatic activities of the enzyme biotinidase. Given mounting evidence of suboptimum biotin status, a more complete understanding of these aspects of biotin should lead to a greater appreciation of the ways in which biotin aids in the maintenance of health.

Targeted PEG-based bioconjugates enhance the cellular uptake and transport of a HIV-1 TAT nonapeptide

We previously described the enhanced cell uptake and transport of R.I-K(biotin)-Tat9, a large ( approximately 1500 Da) peptidic inhibitor of HIV-1 Tat protein, via SMVT, the intestinal biotin transporter. The aim of the present study was to investigate the feasibility of targeting biotinylated PEG-based conjugates to SMVT in order to enhance cell uptake and transport of Tat9. The 29 kDa peptide-loaded bioconjugate (PEG:(R.I-Cys-K(biotin)-Tat9)8) used in these studies contained eight copies of R.I-K(biotin)-Tat9 appended to PEG by means of a cysteine linkage. The absorptive transport of biotin-PEG-3400 (0.6-100 microM) and the bioconjugate (0.1-30 microM) was studied using Caco-2 cell monolayers. Inhibition of biotin-PEG-3400 by positive controls (biotin, biocytin, and desthiobiotin) was also determined. Uptake of these two compounds was also determined in CHO cells transfected with human SMVT (CHO/hSMVT) and control cells (CHO/pSPORT) over the concentration ranges of 0.05-12.5 microM and 0.003-30 microM, respectively. Nonbiotinylated forms of these two compounds, PEG-3350 and PEG:(R.I-Cys-K-Tat9)8, were used in the control studies. Biotin-PEG-3400 transport was found to be concentration-dependent and saturable in Caco-2 cells (K(m)=6.61 microM) and CHO/hSMVT cells (K(m)=1.26 microM). Transport/uptake was significantly inhibited by positive control substrates of SMVT. PEG:(R.I-Cys-K(biotin)Tat9)8 also showed saturable transport kinetics in Caco-2 cells (K(m)=6.13 microM) and CHO/hSMVT cells (K(m)=8.19 microM). Maximal uptake in molar equivalents of R.I-Cys-K(biotin)Tat9 was 5.7 times greater using the conjugate versus the biotinylated peptide alone. Transport of the nonbiotinylated forms was significantly lower (P<0.001) in all cases. The present results demonstrate that biotin-PEG-3400 and PEG:(R.I-Cys-K(biotin)Tat9)8 interact with human SMVT to enhance the cellular uptake and transport of these larger molecules and that targeted bioconjugates may have potential for enhancing the cellular uptake and transport of small peptide therapeutic agents.

Targeting the sodium-dependent multivitamin transporter (SMVT) for improving the oral absorption properties of a retro-inverso Tat nonapeptide

PURPOSE

To investigate the potential for delivering large peptides orally by altering their absorptive transport pathways and improving intestinal permeability. The absorptive transport of retro-inverso (R.I.-) K-Tat9 and R.I.-K(biotin)-Tat9, novel peptidic inhibitors of the Tat protein of HIV-1, and their interactions with human SMVT (hSMVT), a high affinity, low capacity transporter, were investigated using Caco-2 and transfected CHO cells.

METHODS

Following synthesis on a PAL resin using Fmoc chemistry, the transport of R.I.-K-Tat9 (0.01-25 microM) and R.I.-K(biotin)-Tat9 (0.1-25 microM) was evaluated across Caco-2 cells. The transport and kinetics of biotin, biocytin and desthiobiotin (positive controls for SMVT) were also determined. Uptake of R.I.-K-Tat9 and R.I.K(biotin)-Tat9 (both 0.1-10 microM) was determined in CHO/hSMVT and CHO/pSPORT (control) cells.

RESULTS

The absorptive transport of R.I.-K-Tat9 was passive, low (Pm approximately 1 x 10(-6) cm/sec) and not concentration dependent. R.I.K(biotin)-Tat9 permeability was 3.2-fold higher than R.I.-K-Tat9 demonstrating active (Ea = 9.1 kcal/mole), concentration dependent and saturable transport (Km = 3.3 microM). R.I.-K(biotin)-Tat9 uptake in CHO/hSMVT cells (Km = 1.0 microM) was - 500-fold greater than R.I.-K-Tat9 (at 10 microM). R.I.-K(biotin)-Tat9 transport in Caco-2 and CHO/hSMVT cells was significantly inhibited by known substrates of SMVT including biotin, biocytin, and desthiobiotin. Passive uptake of R.I.-K(biotin)-Tat9 was significantly greater than R.I.-K-Tat9 uptake in CHO/pSPORT cells.

CONCLUSIONS

These results demonstrate that the structural modification of R.I.-K-Tat9 to R.I.-K(biotin)-Tat9 altered its intestinal transport pathway resulting in a significant improvement in its absorptive permeability by enhancing nonspecific passive and carrier-mediated uptake by means of SMVT. The specific interactions between R.I.-K(biotin)-Tat9 and SMVT suggest that targeting approaches utilizing transporters such as SMVT may substantially improve the oral delivery of large peptides.

Cytochalasin B modulation of Caco-2 tight junction barrier: role of myosin light chain kinase

The intracellular mechanisms that mediate cytochalasin-induced increase in intestinal epithelial tight junction (TJ) permeability are unclear. In this study, we examined the involvement of myosin light chain kinase (MLCK) in this process, using the filter-grown Caco-2 intestinal epithelial monolayers. Cytochalasin B (Cyto B) (5 microg/ml) produced an increase in Caco-2 MLCK activity, which correlated with the increase in Caco-2 TJ permeability. The inhibition of Cyto B-induced MLCK activation prevented the increase in Caco-2 TJ permeability. Additionally, myosin-Mg(2+)-ATPase inhibitor and metabolic inhibitors (which inhibit MLCK induced actin-myosin contraction) also prevented the Cyto B-induced increase in Caco-2 TJ permeability. Cyto B caused a late-phase (15-30 min) aggregation of actin fragments into large actin clumps, which was also inhibited by MLCK inhibitors. Cyto B produced a morphological disturbance of the ZO-1 TJ proteins, visually correlating with the functional increase in Caco-2 TJ permeability. The MLCK and myosin-Mg(2+)-ATPase inhibitors prevented both the functional increase in TJ permeability and disruption of ZO-1 proteins. These findings suggested that Cyto B-induced increase in Caco-2 TJ permeability is regulated by MLCK activation.

Mechanism of extracellular calcium regulation of intestinal epithelial tight junction permeability: role of cytoskeletal involvement

Recent studies suggest that an abnormal increase in intestinal tight junction (TJ) permeability may be an important etiologic factor in number of diseases including Crohn's disease, NSAID-associated enteritis, and various infectious diarrheal syndromes. The intracellular processes involved in regulation of intestinal epithelial TJ permeability, however, remain poorly understood. In this study, we used cultured Caco-2 intestinal epithelial cells to examine the intracellular processes involved in extracellular Ca(++) modulation of intestinal epithelial monolayer TJ barrier. Incubation of the filter-grown Caco-2 intestinal monolayers in Ca(++)-free solution (CFS), consisting of modified Krebs-buffer solution containing 0 mM Ca(++) and 1 mM EGTA, resulted in a rapid drop in Caco-2 epithelial resistance and increase in epithelial permeability to paracellular markers mannitol and inulin, indicating an increase in TJ permeability. The increase in Caco-2 TJ permeability was rapidly reversed by the re-introduction of Ca(++) (1.8 mM) into the incubation medium. The CFS-induced increase in Caco-2 TJ permeability was associated with separation of the cytoplasmic and transmembrane TJ proteins, ZO-1 and occludin, and formation of large intercellular openings between the adjoining cells. The CFS-induced modulation of TJ barrier was associated with activation of myosin light chain kinase (MLCK) activity and centripetal retraction of peri-junctional actin and myosin filaments. The inhibition of CFS-induced activation of Caco-2 MLCK with MLCK inhibitor (ML-7) prevented the CFS-induced retraction of actin and myosin filaments and the subsequent alteration of TJ barrier function and structure. Our results suggested that the CFS-induced alteration of TJ proteins and functional increase in TJ permeability was mediated by Caco-2 MLCK activation and the resultant contraction of the peri-junctionally located actin-myosin filaments. Consistent with the role of MLCK in this process, selected inhibitors of Mg(++)-myosin ATPase and metabolic energy, but not protein synthesis inhibitors, also prevented the CFS-induced retraction of actin and myosin filaments and the subsequent increase in TJ permeability. In conclusion, our results indicate that extracellular Ca(++) is crucial for the maintenance of intestinal epithelial TJ barrier function. The removal of extracellular Ca(++) from the incubation medium causes activation of Caco-2 MLCK, which in turn leads to an increase in intestinal monolayer TJ permeability.

Transport of thiamine in human intestine: mechanism and regulation in intestinal epithelial cell model Caco-2

The present study examined the intestinal uptake of thiamine (vitamin B(1)) using the human-derived intestinal epithelial cells Caco-2 as an in vitro model system. Thiamine uptake was found to be 1) temperature and energy dependent and occurred with minimal metabolic alteration; 2) pH sensitive; 3) Na(+) independent; 4) saturable as a function of concentration with an apparent Michaelis-Menten constant of 3.18 +/- 0.56 microM and maximal velocity of 13.37 +/- 0.94 pmol. mg protein(-1). 3 min(-1); 5) inhibited by the thiamine structural analogs amprolium and oxythiamine, but not by unrelated organic cations tetraethylammonium, N-methylnicotinamide, and choline; and 6) inhibited in a competitive manner by amiloride with an inhibition constant of 0.2 mM. The role of specific protein kinase-mediated pathways in the regulation of thiamine uptake by Caco-2 cells was also examined using specific modulators of these pathways. The results showed possible involvement of a Ca(2+)/calmodulin (CaM)-mediated pathway in the regulation of thiamine uptake. No role for protein kinase C- and protein tyrosine kinase-mediated pathways in the regulation of thiamine uptake was evident. These results demonstrate the involvement of a carrier-mediated system for thiamine uptake by Caco-2 intestinal epithelial cells. This system is Na(+) independent and is different from the transport systems of organic cations. Furthermore, a CaM-mediated pathway appears to play a role in regulating thiamine uptake in these cells.

Molecular mechanism of the intestinal biotin transport process

Previous studies have characterized different aspects of the cellular/membrane mechanism and regulation of the intestinal uptake process of the water-soluble vitamin biotin. Little, however, is known about the molecular mechanisms of the uptake process. In this study, we have identified a cDNA from rat small intestine that appears to be involved in biotin transport. The open reading frame of this cloned cDNA consisted of 1,905 bases and was identical to that identified for the vitamin transporter in placental tissue. Significant heterogeneity, however, was found in the 5' untranslated region of this clone, with three distinct variants (II, III, IV) being identified in the small intestine; the placental variant (variant I), however, was not present in the small gut. Variant II was found to be the predominant form expressed in the rat small and large intestines. Functional identity of the cloned intestinal cDNA was confirmed by stable expression in COS-7 cells, which showed a four- to fivefold increase in biotin uptake in transfected COS-7 cells compared with controls. The induced biotin uptake in transfected COS-7 cells was found to be 1) Na(+) dependent, 2) saturable as a function of concentration with an apparent K(m) of 8. 77 microM and a V(max) of 779.7 pmol. mg protein(-1). 3 min(-1), and 3) inhibited by unlabeled biotin and pantothenic acid and their structural analogs. The distribution of complementary mRNA transcripts of the cloned cDNA along the vertical and longitudinal axes of the intestinal tract was also determined. Results of this study describe the molecular characteristics of the intestinal biotin absorption process and report the identification of a cDNA that encodes a Na(+)-dependent biotin uptake carrier that appears to exist in the form of multiple variants.

Molecular and functional characterization of the intestinal Na+-dependent multivitamin transporter

We have cloned a Na+-dependent multivitamin transporter from rabbit intestine (riSMVT). The cDNA codes for a protein of 636 amino acids with 12 putative transmembrane domains. When expressed in mammalian cells, the cDNA induces Na+-dependent uptake of the vitamins pantothenate and biotin. Lipoate is also a substrate for the cDNA-induced uptake process. The affinity constant for the cDNA-specific transport of pantothenate and biotin is approximately 2 and approximately 8 microM, respectively. The Na+:vitamin stoichiometry is greater than 1, indicating that the transport process is electrogenic. The SMVT-specific transcripts of 3.2 kbp are equally distributed throughout the small intestine. We have also cloned SMVT from the human intestinal cell line Caco-2. The Caco-2 SMVT cDNA codes for a protein of 635 amino acids which is homologous to riSMVT and is identical to the SMVT expressed in the human choriocarcinoma cell line JAR. Caco-2 SMVT also catalyzes Na+-dependent uptake of pantothenate, biotin, and lipoate. In oocytes expressing Caco-2 SMVT, all three vitamins evoke inward currents, confirming the electrogenicity of the transport process.

Ethanol modulation of intestinal epithelial tight junction barrier

Previous studies have shown that high concentrations of ethanol (>/=40%) cause functional damage of the gastrointestinal epithelial barrier by direct cytotoxic effect on the epithelial cells. The effects of lower noncytotoxic doses of ethanol on epithelial barrier function are unknown. A major function of gastrointestinal epithelial cells is to provide a barrier against the hostile substances in the gastrointestinal lumen. The apicolaterally located tight junctions (TJs) form a paracellular seal between the lateral membranes of adjacent cells and act as a paracellular barrier. In this study, we investigated the effects of lower doses of ethanol on intestinal epithelial TJ barrier function using filter-grown Caco-2 intestinal epithelial monolayers. The Caco-2 TJ barrier function was assessed by measuring epithelial resistance or paracellular permeability of the filter-grown monolayers. Ethanol (0, 1, 2.5, 5, 7.5, and 10%) produced a dose-related drop in Caco-2 epithelial resistance and increase in paracellular permeability. Ethanol also produced a progressive disruption of TJ protein (ZO-1) with separation of ZO-1 proteins from the cellular junctions and formation of large gaps between the adjacent cells. Ethanol, at the doses used (</=10%), did not cause cytotoxicity (lactate dehydrogenase release) to the Caco-2 cells. Ethanol produced a disassembly and displacement of perijunctional actin and myosin filaments from the perijunctional areas. On ethanol removal, actin and myosin filaments rapidly reassembled at the cellular borders. Ethanol stimulated the Caco-2 myosin light chain kinase (MLCK) activity but did not affect the MLCK protein levels. Specific MLCK inhibitor ML-7 inhibited both ethanol increases in MLCK activity and TJ permeability without affecting the MLCK protein levels. Consistent with these findings, metabolic inhibitors sodium azide and 2,4-dinitrophenol significantly prevented ethanol-induced increase in Caco-2 TJ permeability, whereas cycloheximide or actinomycin D had no effect. The results of this study indicate that ethanol at low noncytotoxic doses causes a functional and structural opening of the Caco-2 intestinal epithelial TJ barrier by activating MLCK.

Intestinal absorption of vitamins

This article provides an overview of advances in understanding the cellular and molecular mechanisms and regulation of intestinal absorption processes of vitamins. The vitamins covered are the water-soluble vitamins folic acid, cobalamin (vitamin B12), biotin, pantothenic acid, and thiamine (vitamin B1) and the lipid-soluble vitamin A. For folate, significant advances have been made in regard to i) digestion of dietary folate polyglutamates to folate monoglutamates by the cloning of the responsible enzyme; ii) identification of the cDNA responsible for the intestinal folate transporter; iii) delineation of intracellular mechanisms that regulate small intestinal folate uptake; and iv) identification and characterization of a specific, pH-dependent, carrier-mediated system for folate uptake at the luminal (apical) membrane of human colonocytes. Studies on cobalamine have focused on cellular and molecular characterization of the intrinsic factor and its receptor. Studies on biotin transport in the small intestine have shown that the uptake process is shared by another water-soluble vitamin, pantothenic acid. Furthermore, a Na-dependent, carrier-mediated biotin uptake system that is also shared with pantothenic acid has been identified at the apical membrane of human colonocytes. This carrier is believed to be responsible for the absorption of the bacterially synthesized biotin and pantothenic acid in the large intestine. Also, preliminary studies have reported the cloning of a biotin transporter from the small intestine. As for thiamine intestinal transport, a study has shown thiamine uptake by small intestinal biopsy specimens to be via a carrier-mediated, Na-independent mechanism, which appears to be up-regulated in thiamine deficiency. Studies on vitamin A intestinal absorption have shown the existence of a receptor-mediated mechanism for the uptake of retinol bound to retinol-binding protein in the small intestine of suckling rats. Another study has shown that retinoic acid increases the mRNA level of the cellular retinol binding protein II and the rate of retinol uptake by Caco-2 intestinal epithelial cells. The study suggested that retinoids may play a role in the regulation of vitamin A intestinal absorption.

Cellular uptake of biotin: mechanisms and regulation

This review describes our knowledge of biotin transport in the small intestine of humans and other mammals and presents recent findings in the area. Previous studies have shown that biotin transport across the brush border membrane of the small intestinal absorptive cells occurs via a carrier-mediated, Na+ gradient-dependent, electroneutral mechanism. Exit of biotin out of the enterocyte, i.e., transport across the basolateral membrane, also occurs via a carrier-mediated process, but the process is Na+ independent and electrogenic. Recent studies from our laboratory have shown that the uptake process of biotin in Caco-2 cells, a human-derived cultured intestinal epithelial cell line, are under the cellular regulation of both a protein kinase C- and a Ca/calmodulin-mediated pathway. In addition, the uptake process is shared by another water-soluble vitamin, pantothenic acid. For the first time, other recent studies have detected the existence of a Na+-dependent, carrier-mediated mechanism for biotin uptake at the apical membrane of colonocytes, which could theoretically mediate absorption of the biotin synthesized by colonic microflora. This system was again found to be shared by pantothenic acid, which is also synthesized by the normal microflora of the large intestine.

Biotinylation of proteins via amino groups can induce binding to U937 cells, HL-60 cells, monocytes and granulocytes

The use of biotinylated ligands for the flow cytometric detection of cell surface receptors has become a popular alternative to radioreceptor assays. Although the biotinylation of a protein is a relatively mild chemical reaction several reports have mentioned the fact that the number and location of biotin moieties coupled to amino groups of a protein can alter its physicochemical properties and impair biological activity. In the present study we show for a variety of biotinylated functionally unaltered ligands that biotinylation by N-hydroxysuccinimide (NHS) esters of biotin can induce a binding to cell surfaces, which is not specific for the respective unlabelled ligand. C1q, C1 inhibitor (C1-INH), alpha 1-antitrypsin (AT), ovalbumin (OV), transferrin and soybean trypsin inhibitor (STI) were labelled with S-NHS-LC-biotin and activated C1s (C1s) with NHS-biotin. Biotinylation of C1q, C1s and C1-INH exerted negligible effects on biological function, antigenicity or electrophoretic mobility but when labelled and unlabelled proteins were assayed for binding to monocytic U937 cells, promyelocytic HL-60 cells, monocytes and granulocytes, a remarkable binding was observed for biotinylated C1q, C1-INH and C1s. In contrast, no binding was observed when we used unlabelled C1q, C1s and C1-INH and employed specific antibodies, alpha-mouse-FITC or alpha-rabbit-FITC for detection. Increasing molar ratios of biotin-to-protein (B : P) for biotinylated AT, OV and STI evoked increased fluorescence intensities of the cells. Most importantly the unlabelled ligands did not compete for cell binding with their biotinylated derivatives, with the exception of transferrin. Preincubation of the cells with an excess of free d-biotin did not reduce binding of biotinylated proteins, thus excluding a potential involvement of biotin receptors. Hydrophobic interaction chromatography revealed a remarkable increase in hydrophobicity of the biotinylated proteins compared to their unlabelled counterparts, suggesting that the biotinylation-induced binding is due to increased hydrophobicity. Our findings indicate that biotinylation by the common amino acid esterification method may be critical for proteins if they are to be used as ligands for receptor binding studies.

Transport of the antibacterial agent oxazolidin-2-one and derivatives across intestinal (Caco-2) and renal (MDCK) epithelial cell lines

The transepithelial passage of the orally bioavailable antibacterial agent oxazolidin-2-one (OXa) and 10 derivatives has been studied with human intestinal (Caco-2) and canine renal (MDCK) cell lines grown on polycarbonate filters. The transepithelial passage was assayed in the apical-to-basolateral (AP-to-BL) direction and in the opposite direction (BL to AP) in both cell lines. The observed passage rates of OXa were similar in both directions in the two cell lines, suggesting passive diffusion. This was further confirmed by the fact that transport kinetics were linear as a function of initial concentration. The rates of AP-to-BL passage of OXa and seven of the derivatives in both cell lines were linearly related to lipophilicity, whether expressed as high-passage liquid chromatography retention time or as the logarithm of the n-octanol-water partition coefficient (log P). These data suggest that the lipophilicity of OXa is important for its observed bioavailability after oral administration. Interestingly, three of the derivatives exhibited a higher passage rate than predicted by lipophilicity. Further studies indicated that this transport was saturable, similar in the two directions, and not affected by energy depletion, suggesting the presence of an additional carrier-mediated facilitated-transport mechanism.

Cytoskeletal regulation of Caco-2 intestinal monolayer paracellular permeability

An abnormal increase in intestinal paracellular permeability may be an important pathogenic factor in various intestinal diseases. The intracellular factors and processes that regulate and cause alteration of intestinal paracellular permeability are not well understood. The purpose of this study was to examine some of the intracellular processes involved in cytoskeletal regulation of intestinal epithelial paracellular permeability using the filter-grown Caco-2 intestinal epithelial monolayers. Cytochalasin-b and colchicine were used to disrupt the cytoskeletal elements, actin microfilaments, and microtubules. Cytochalasin-b (5 micrograms/ml) and colchicine (2 x 10(-5) M) at the doses used caused marked depolymerization and disruption of actin microfilaments and microtubules, respectively. Cytochalasin-b-induced disruption of actin microfilaments resulted in perturbation of tight junctions and desmosomes and an increase in Caco-2 monolayer paracellular permeability. The cytochalasin-b-induced disruption of actin microfilaments and subsequent changes in intercellular junctional complexes and paracellular permeability were not affected by inhibitors of protein synthesis (actinomycin-D or cycloheximide) or microtubule function (colchicine), but were inhibited by metabolic energy inhibitors (2,4-dinitrophenol or sodium azide). The cytochalasin-b-induced disturbance in Caco-2 actin microfilaments and intercellular junctional complexes and increase in paracellular permeability were rapidly reversed. The paracellular pathway "re-tightening" following cytochalasin-b removal was not affected by actinomycin-D, cycloheximide, or colchicine, but was inhibited by 2,4-dinitrophenol and sodium azide. The colchicine-induced disruption of microtubules did not have significant effect on actin microfilaments, intercellular junctions, or paracellular permeability. These findings suggest that cytochalasin-b-induced increase in Caco-2 monolayer paracellular permeability was due to actin microfilament mediated perturbation of intercellular junctional complexes. The re-tightening of paracellular pathways (following removal of cytochalasin-b) resulted from energy-mediated re-assembly of pre-existing actin microfilaments and intercellular junctional complexes. This re-closure process did not require protein synthesis or microtubule-mediated shuttling process.

Radiolabeling of methylphosphonate and phosphorothioate oligonucleotides and evaluation of their transport in everted rat jejunum sacs

PURPOSE

The therapeutic use of antisense oligonucleotides will likely involve their administration over protracted periods of time. The oral route of drug dosing offers many advantages over other possible routes when chronic drug administration is necessary. However, little is known about the potential for oligonucleotide uptake from the gastrointestinal tract. This issue is addressed in the current work.

METHODS

We have developed a simple procedure for radiolabeling oligonucleotides by reductive alkylation with 14C-formaldehyde. We have utilized this approach, as well as 5' addition of fluorophores, to prepare labeled methylphosphonate and phosphorothioate oligonucleotides for use in intestinal transport studies. An everted rat gut sac model was employed to compare the transport of oligonucleotides to that of model compounds whose permeation properties are better understood.

RESULTS

We demonstrate that both methylphosphonate and phosphorothioate oligonucleotides are passively transported across the intestinal epithelium, probably by a paracellular route. The rates of transport for both types of oligonucleotides were similar, and were significantly greater than that of the very high MW polymer blue dextran, but were lower than the transport rate of valproic acid, a low MW compound known to have high oral availability.

CONCLUSIONS

A significant degree of permeation of oligonucleotides across the gastrointestinal epithelium does occur, but it is still unclear whether this is sufficient to permit effective oral administration of oligonucleotides as drugs.