Production of bovine beta-lactoglobulin and hen egg ovalbumin by Trichoderma reesei using precision fermentation technology and testing of their techno-functional properties

The food protein ingredient market is dominated by dairy and egg proteins. Both milk whey and egg proteins are challenging proteins to replace, e.g. with plant proteins, due to the unique structural features of the animal proteins that render them highly functional. Thus, to provide a non-animal source of these important proteins the fungal host Trichoderma reesei was utilized for the biotechnical production of recombinant hen ovalbumin (TrOVA) and bovine beta lactoglobulin (TrBLG). These food proteins were investigated using two different promoter systems to test the concept of effectively expressing them in a fungal host. Both proteins were successfully produced in 24 well plate and bioreactor scale. The production level of TrBLG and TrOVA were 1 g/L and 2 g/L, respectively. Both proteins were further purified and characterized, and their functional properties were tested. TrBLG and TrOVA secondary structures determined by circular dichroism corresponded to the proteins of bovine and hen. The T. reesei produced proteins were found to be N-glycosylated, mostly with Man 5. TrBLG had emulsification properties matching to corresponding bovine protein. TrOVA showed excellent foaming characteristics and heat-induced gelation, although the strength of the gel was somewhat lower than with hen ovalbumin, possibly due to the partial degradation of TrOVA or presence of other host proteins. Biotechnical production of whey and egg proteins using precision fermentation technology offers an innovative way to increase the sustainability of the conventional food industry, without further reliance on animal farming. Industrial relevance: The food protein ingredient market is dominated by dairy (largely whey proteins) and egg proteins. Whey proteins are valuable and versatile food ingredients due to their functional and nutritional quality. They are largely used in meat and milk products, low fat products, bakery, confectionary, infant formulas and sports nutrition. Similarly, egg white protein ovalbumin is a highly functional protein ingredient that facilitates structure formation and high nutritional quality in most food products. Together they comprise 40-70% of the revenue in the animal protein ingredients market. Both whey and egg proteins are extremely challenging proteins to replace, e.g., by plant proteins due to their unique structural features that render them with high functionality. Biotechnical production of whey and egg proteins using precision fermentation technology offers an innovative way to increase the sustainability of the conventional food industry, without further reliance on animal farming.

Ovalbumin production using Trichoderma reesei culture and low-carbon energy could mitigate the environmental impacts of chicken-egg-derived ovalbumin

Ovalbumin (OVA) produced using the fungus Trichoderma reesei (Tr-OVA) could become a sustainable replacement for chicken egg white protein powder-a widely used ingredient in the food industry. Although the approach can generate OVA at pilot scale, the environmental impacts of industrial-scale production have not been explored. Here, we conducted an anticipatory life cycle assessment using data from a pilot study to compare the impacts of Tr-OVA production with an equivalent functional unit of dried chicken egg white protein produced in Finland, Germany and Poland. Tr-OVA production reduced most agriculture-associated impacts, such as global warming and land use. Increased impacts were mostly related to industrial inputs, such as electricity production, but were also associated with glucose consumption. Switching to low-carbon energy sources could further reduce environmental impact, demonstrating the potential benefits of cellular agriculture over livestock agriculture for OVA production.

Functional characterization of a highly specific L-arabinose transporter from Trichoderma reesei

Background

Lignocellulose biomass has been investigated as a feedstock for second generation biofuels and other value-added products. Some of the processes for biofuel production utilize cellulases and hemicellulases to convert the lignocellulosic biomass into a range of soluble sugars before fermentation with microorganisms such as yeast Saccharomyces cerevisiae. One of these sugars is l-arabinose, which cannot be utilized naturally by yeast. The first step in l-arabinose catabolism is its transport into the cells, and yeast lacks a specific transporter, which could perform this task.

Results

We identified Trire2_104072 of Trichoderma reesei as a potential l-arabinose transporter based on its expression profile. This transporter was described already in 2007 as d-xylose transporter XLT1. Electrophysiology experiments with Xenopus laevis oocytes and heterologous expression in yeast revealed that Trire2_104072 is a high-affinity l-arabinose symporter with a K_m value in the range of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document}∼ 0.1–0.2 mM. It can also transport d-xylose but with low affinity (K_m\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document}∼ 9 mM). In yeast, l-arabinose transport was inhibited slightly by d-xylose but not by d-glucose in an assay with fivefold excess of the inhibiting sugar. Comparison with known l-arabinose transporters revealed that the expression of Trire2_104072 enabled yeast to uptake l-arabinose at the highest rate in conditions with low extracellular l-arabinose concentration. Despite the high specificity of Trire2_104072 for l-arabinose, the growth of its T. reesei deletion mutant was only affected at low l-arabinose concentrations.

Conclusions

Due to its high affinity for l-arabinose and low inhibition by d-glucose or d-xylose, Trire2_104072 could serve as a good candidate for improving the existing pentose-utilizing yeast strains. The discovery of a highly specific l-arabinose transporter also adds to our knowledge of the primary metabolism of T. reesei. The phenotype of the deletion strain suggests the involvement of other transporters in l-arabinose transport in this species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01666-4.

Electrophysiological characterization of a diverse group of sugar transporters from Trichoderma reesei

Trichoderma reesei is an ascomycete fungus known for its capability to secrete high amounts of extracellular cellulose- and hemicellulose-degrading enzymes. These enzymes are utilized in the production of second-generation biofuels and T. reesei is a well-established host for their production. Although this species has gained considerable interest in the scientific literature, the sugar transportome of T. reesei remains poorly characterized. Better understanding of the proteins involved in the transport of different sugars could be utilized for engineering better enzyme production strains. In this study we aimed to shed light on this matter by characterizing multiple T. reesei transporters capable of transporting various types of sugars. We used phylogenetics to select transporters for expression in Xenopus laevis oocytes to screen for transport activities. Of the 18 tested transporters, 8 were found to be functional in oocytes. 10 transporters in total were investigated in oocytes and in yeast, and for 3 of them no transport function had been described in literature. This comprehensive analysis provides a large body of new knowledge about T. reesei sugar transporters, and further establishes X. laevis oocytes as a valuable tool for studying fungal sugar transporters.

Studies on sugar transporter CRT1 reveal new characteristics that are critical for cellulase induction in Trichoderma reesei

BACKGROUND

Trichoderma reesei is an ascomycete fungus that has a tremendous capability of secreting extracellular proteins, mostly lignocellulose-degrading enzymes. Although many aspects of the biology of this organism have been unfolded, the roles of the many sugar transporters coded in its genome are still a mystery with a few exceptions. One of the most interesting sugar transporters that has thus far been discovered is the cellulose response transporter 1 (CRT1), which has been suggested to be either a sugar transporter or a sensor due to its seemingly important role in cellulase induction.

RESULTS

Here we show that CRT1 is a high-affinity cellobiose transporter, whose function can be complemented by the expression of other known cellobiose transporters. Expression of two sequence variants of the crt1 gene in Saccharomyces cerevisiae revealed that only the variant listed in the RUT-C30 genome annotation has the capability to transport cellobiose and lactose. When expressed in the Δ crt1 strain, the variant listed in the QM6a genome annotation offers partial complementation of the cellulase induction, while the expression of the RUT-C30 variant or cellobiose transporters from two other fungal species fully restore the cellulase induction.

CONCLUSIONS

These results add to our knowledge about the fungal metabolism of cellulose-derived oligosaccharides, which have the capability of inducing the cellulase production in many species. They also help us to deepen our understanding of the T. reesei lactose metabolism, which can have important consequences as this sugar is used as the inducer of protein secretion in many industrial processes which employ this species.

Biomimetic composites with enhanced toughening using silk-inspired triblock proteins and aligned nanocellulose reinforcements

Silk and cellulose are biopolymers that show strong potential as future sustainable materials. They also have complementary properties, suitable for combination in composite materials where cellulose would form the reinforcing component and silk the tough matrix. A major challenge concerns balancing structure and functional properties in the assembly process. We used recombinant proteins with triblock architecture, combining structurally modified spider silk with terminal cellulose affinity modules. Flow alignment of cellulose nanofibrils and triblock protein allowed continuous fiber production. Protein assembly involved phase separation into concentrated coacervates, with subsequent conformational switching from disordered structures into β sheets. This process gave the matrix a tough adhesiveness, forming a new composite material with high strength and stiffness combined with increased toughness. We show that versatile design possibilities in protein engineering enable new fully biological materials and emphasize the key role of controlled assembly at multiple length scales for realization.

A universal gene expression system for fungi

Biotechnological production of fuels, chemicals and proteins is dependent on efficient production systems, typically genetically engineered microorganisms. New genome editing methods are making it increasingly easy to introduce new genes and functionalities in a broad range of organisms. However, engineering of all these organisms is hampered by the lack of suitable gene expression tools. Here, we describe a synthetic expression system (SES) that is functional in a broad spectrum of fungal species without the need for host-dependent optimization. The SES consists of two expression cassettes, the first providing a weak, but constitutive level of a synthetic transcription factor (sTF), and the second enabling strong, at will tunable expression of the target gene via an sTF-dependent promoter. We validated the SES functionality in six yeast and two filamentous fungi species in which high (levels beyond organism-specific promoters) as well as adjustable expression levels of heterologous and native genes was demonstrated. The SES is an unprecedentedly broadly functional gene expression regulation method that enables significantly improved engineering of fungi. Importantly, the SES system makes it possible to take in use novel eukaryotic microbes for basic research and various biotechnological applications.

Coacervation of resilin fusion proteins containing terminal functionalities

Liquid-liquid phase transition known as coacervation of resilin-like-peptide fusion proteins containing different terminal domains were investigated. Two different modular proteins were designed and produced and their behavior were compared to a resilin-like-peptide without terminal domains. The size of the particle-like coacervates was modulated by the protein concentration, pH and temperature. The morphology and three-dimensional (3D) structural details of the coacervate particles were investigated by cryogenic transmission electron microscopy (cryo-TEM) and tomography (cryo-ET) reconstruction. Selective adhesion of the coacervates on cellulose and graphene surfaces was demonstrated.

Phase transitions as intermediate steps in the formation of molecularly engineered protein fibers

A central concept in molecular bioscience is how structure formation at different length scales is achieved. Here we use spider silk protein as a model to design new recombinant proteins that assemble into fibers. We made proteins with a three-block architecture with folded globular domains at each terminus of a truncated repetitive silk sequence. Aqueous solutions of these engineered proteins undergo liquid-liquid phase separation as an essential pre-assembly step before fibers can form by drawing in air. We show that two different forms of phase separation occur depending on solution conditions, but only one form leads to fiber assembly. Structural variants with one-block or two-block architectures do not lead to fibers. Fibers show strong adhesion to surfaces and self-fusing properties when placed into contact with each other. Our results show a link between protein architecture and phase separation behavior suggesting a general approach for understanding protein assembly from dilute solutions into functional structures.

Single-Molecule Force Spectroscopy Study on Modular Resilin Fusion Protein

The adhesive and mechanical properties of a modular fusion protein consisting of two different types of binding units linked together via a flexible resilin-like-polypeptide domain are quantified. The adhesive domains have been constructed from fungal cellulose-binding modules (CBMs) and an amphiphilic hydrophobin HFBI. This study is carried out by single-molecule force spectroscopy, which enables stretching of single molecules. The fusion proteins are designed to self-assemble on the cellulose surface, leading into the submonolayer of proteins having the HFBI pointing away from the surface. A hydrophobic atomic force microscopy (AFM) tip can be employed for contacting and lifting the single fusion protein from the HFBI-functionalized terminus by the hydrophobic interaction between the tip surface and the hydrophobic patch of the HFBI. The work of rupture, contour length at rupture and the adhesion forces of the amphiphilic end domains are evaluated under aqueous environment at different pHs.

Elastic and pH-Responsive Hybrid Interfaces Created with Engineered Resilin and Nanocellulose

We investigated how a genetically engineered resilin fusion protein modifies cellulose surfaces. We characterized the pH-responsive behavior of a resilin-like polypeptide (RLP) having terminal cellulose binding modules (CBM) and showed its binding to cellulose nanofibrils (CNF). Characterization of the resilin fusion protein at different pHs revealed substantial conformational changes of the protein, which were observed as swelling and contraction of the protein layer bound to the nanocellulose surface. In addition, we showed that employment of the modified resilin in cellulose hydrogel and nanopaper increased their modulus of stiffness through a cross-linking effect.

Enabling low cost biopharmaceuticals: high level interferon alpha-2b production in Trichoderma reesei

BACKGROUND

The filamentous fungus Trichoderma reesei has tremendous capability to secrete over 100 g/L of proteins and therefore it would make an excellent host system for production of high levels of therapeutic proteins at low cost. We have developed T. reesei strains suitable for production of therapeutic proteins by reducing the secreted protease activity. Protease activity has been the major hindrance to achieving high production levels. We have constructed a series of interferon alpha-2b (IFNα-2b) production strains with 9 protease deletions to gain knowledge for further strain development.

RESULTS

We have identified two protease deletions that dramatically improved the production levels. Deletion of the subtilisin protease slp7 and the metalloprotease amp2 has enabled production levels of IFNα-2b up to 2.1 and 2.4 g/L, respectively. With addition of soybean trypsin protease inhibitor the level of production improved to 4.5 g/L, with an additional 1.8 g/L still bound to the secretion carrier protein.

CONCLUSIONS

High levels of IFNα-2b were produced using T. reesei strains with reduced protease secretion. Further strain development can be done to improve the production system by reducing protease activity and improving carrier protein cleavage.

Enabling Low Cost Biopharmaceuticals: A Systematic Approach to Delete Proteases from a Well-Known Protein Production Host Trichoderma reesei

The filamentous fungus Trichoderma reesei has tremendous capability to secrete proteins. Therefore, it would be an excellent host for producing high levels of therapeutic proteins at low cost. Developing a filamentous fungus to produce sensitive therapeutic proteins requires that protease secretion is drastically reduced. We have identified 13 major secreted proteases that are related to degradation of therapeutic antibodies, interferon alpha 2b, and insulin like growth factor. The major proteases observed were aspartic, glutamic, subtilisin-like, and trypsin-like proteases. The seven most problematic proteases were sequentially removed from a strain to develop it for producing therapeutic proteins. After this the protease activity in the supernatant was dramatically reduced down to 4% of the original level based upon a casein substrate. When antibody was incubated in the six protease deletion strain supernatant, the heavy chain remained fully intact and no degradation products were observed. Interferon alpha 2b and insulin like growth factor were less stable in the same supernatant, but full length proteins remained when incubated overnight, in contrast to the original strain. As additional benefits, the multiple protease deletions have led to faster strain growth and higher levels of total protein in the culture supernatant.

Dipeptidyl peptidase IV as a potential target for selective prodrug activation and chemotherapeutic action in cancers

The efficacy of chemotherapeutic drugs is often offset by severe side effects attributable to poor selectivity and toxicity to normal cells. Recently, the enzyme dipeptidyl peptidase IV (DPPIV) was considered as a potential target for the delivery of chemotherapeutic drugs. The purpose of this study was to investigate the feasibility of targeting chemotherapeutic drugs to DPPIV as a strategy to enhance their specificity. The expression profile of DPPIV was obtained for seven cancer cell lines using DNA microarray data from the DTP database, and was validated by RT-PCR. A prodrug was then synthesized by linking the cytotoxic drug melphalan to a proline-glycine dipeptide moiety, followed by hydrolysis studies in the seven cell lines with a standard substrate, as well as the glycyl-prolyl-melphalan (GP-Mel). Lastly, cell proliferation studies were carried out to demonstrate enzyme-dependent activation of the candidate prodrug. The relative RT-PCR expression levels of DPPIV in the cancer cell lines exhibited linear correlation with U95Av2 Affymetrix data (r(2) = 0.94), and with specific activity of a standard substrate, glycine-proline-p-nitroanilide (r(2) = 0.96). The significantly higher antiproliferative activity of GP-Mel in Caco-2 cells (GI₅₀ = 261 μM) compared to that in SK-MEL-5 cells (GI₅₀ = 807 μM) was consistent with the 9-fold higher specific activity of the prodrug in Caco-2 cells (5.14 pmol/min/μg protein) compared to SK-MEL-5 cells (0.68 pmol/min/μg protein) and with DPPIV expression levels in these cells. Our results demonstrate the great potential to exploit DPPIV as a prodrug activating enzyme for efficient chemotherapeutic drug targeting.

Tamoxifen inhibits TRPV6 activity via estrogen receptor-independent pathways in TRPV6-expressing MCF-7 breast cancer cells

The epithelial calcium channel TRPV6 is upregulated in breast carcinoma compared with normal mammary gland tissue. The selective estrogen receptor modulator tamoxifen is widely used in breast cancer therapy. Previously, we showed that tamoxifen inhibits calcium uptake in TRPV6-transfected Xenopus oocytes. In this study, we examined the effect of tamoxifen on TRPV6 function and intracellular calcium homeostasis in MCF-7 breast cancer cells transiently transfected with EYFP-C1-TRPV6. TRPV6 activity was measured with fluorescence microscopy using Fura-2. The basal calcium level was higher in transfected cells compared with nontransfected cells in calcium-containing solution but not in nominally calcium-free buffer. Basal influxes of calcium and barium were also increased. In transfected cells, 10 mumol/L tamoxifen reduced the basal intracellular calcium concentration to the basal calcium level of nontransfected cells. Tamoxifen decreased the transport rates of calcium and barium in transfected cells by 50%. This inhibitory effect was not blocked by the estrogen receptor antagonist, ICI 182,720. Similarly, a tamoxifen-induced inhibitory effect was also observed in MDA-MB-231 estrogen receptor-negative cells. The effect of tamoxifen was completely blocked by activation of protein kinase C. Inhibiting protein kinase C with calphostin C decreased TRPV6 activity but did not alter the effect of tamoxifen. These findings illustrate how tamoxifen might be effective in estrogen receptor-negative breast carcinomas and suggest that the therapeutic effect of tamoxifen and protein kinase C inhibitors used in breast cancer therapy might involve TRPV6-mediated calcium entry. This study highlights a possible role of TRPV6 as therapeutic target in breast cancer therapy.

Interferon-gamma increases expression of the di/tri-peptide transporter, h-PEPT1, and dipeptide transport in cultured human intestinal monolayers

The di/tri-peptide transporter h-PEPT1 plays an important role in the oral absorption of di/tri-peptides and numerous drugs. Inflammatory conditions may influence intestinal xenobiotic transporter function; however, the effects of inflammation on h-PEPT1 have not been well described. This study was conducted to determine the effects of the inflammatory cytokine interferon-gamma (IFN-gamma) on h-PEPT1 mediated dipeptide absorption. Caco-2 monolayers were grown on permeable supports. The effective apical-to-basolateral permeability (P(eff)) of glycylsarcosine (Gly-Sar) was measured following incubation with IFN-gamma or control media. Additional experiments were conducted at 4 degrees C, and with escalating concentrations of Gly-Sar. h-PEPT1 expression was determined using semiquantitative RT-PCR. IFN-gamma 50 ng/ml increased Gly-Sar P(eff) 28.6% compared to controls (p=0.03). In experiments conducted at 4 degrees C, Gly-Sar P(eff) decreased 39.6% in IFN-gamma treated cells (p=0.003) and 28.4% in controls (p=0.006). In controls and IFN-gamma treated cells, concentration dependent transport was seen with escalating concentrations of Gly-Sar. Compared to controls, IFN-gamma 50 and 100 ng/ml increased h-PEPT1 mRNA expression by 14.2% and 11.5%, respectively (p=0.019). In summary, IFN-gamma increases h-PEPT1 expression and permeation of the dipeptide Gly-Sar in Caco-2 monolayers. These findings imply that intestinal absorption of peptides and peptidomimetic drugs may be increased in certain inflammatory conditions.

Enhanced absorption and growth inhibition with amino acid monoester prodrugs of floxuridine by targeting hPEPT1 transporters

A series of amino acid monoester prodrugs of floxuridine was synthesized and evaluated for the improvement of oral bioavailability and the feasibility of target drug delivery via oligopeptide transporters. All floxuridine 5'-amino acid monoester prodrugs exhibited PEPT1 affinity, with inhibition coefficients of Gly-Sar uptake (IC50) ranging from 0.7 - 2.3 mM in Caco-2 and 2.0 - 4.8 mM in AsPC-1 cells, while that of floxuridine was 7.3 mM and 6.3 mM, respectively. Caco-2 membrane permeabilities of floxuridine prodrugs (1.01 - 5.31 x 10(-6 )cm/sec) and floxuridine (0.48 x 10(-6 )cm/sec) were much higher than that of 5-FU (0.038 x 10(-6) cm/sec). MDCK cells stably transfected with the human oligopeptide transporter PEPT1 (MDCK/hPEPT1) exhibited enhanced cell growth inhibition in the presence of the prodrugs. This prodrug strategy offers great potential, not only for increased drug absorption but also for improved tumor selectivity and drug efficacy.

Mechanisms and regulation of epithelial Ca2+ absorption in health and disease

Ca2+ is essential for numerous physiological functions in our bodies. Therefore, its homeostasis is finely maintained through the coordination of intestinal absorption, renal reabsorption, and bone resorption. The Ca2+-selective epithelial channels TRPV5 and TRPV6 have been identified, and their physiological roles have been revealed: TRPV5 is important in final renal Ca2+ reabsorption, and TRPV6 has a key role in intestinal Ca2+ absorption. The TRPV5 knockout mice exhibit renal leak hypercalciuria and accordingly upregulate their intestinal TRPV6 expression to compensate for their negative Ca2+ balance. In contrast, despite their severe negative Ca2+ balance, TRPV6-null mice do not display any compensatory mechanism, thus resulting in secondary hyperparathyroidism. These results indicate that the genes for TRPV5 and TRPV6 are differentially regulated in human diseases associated with disturbed Ca2+ balance such as hypercalciuria, osteoporosis, and vitamin D-resistant rickets.

Proline prodrug of melphalan, prophalan-l, demonstrates high therapeutic index in a murine melanoma model

The therapeutic efficacy of prophalan-L, the L-proline prodrug of melphalan that demonstrated prolidase-dependent bioactivation to melphalan, was examined in vivo in a mouse melanoma model. Prophalan-L exhibited 2- to 2.5-fold higher hydrolytic and cytotoxic activity than prophalan-D, the D-analog, in B16-F10 murine melanoma cells in vitro. Prophalan-L cytotoxicity in B16-F10 cells was lower (GI50=221 microM) than that of melphalan (GI50=173 microM). The tumor growth profiles in C57BL/6J mice injected with B16-F10 cells and treated with melphalan (5.5 microg/g i.p.) and equimolar concentrations of the prodrugs demonstrated significant difference between the control (buffered saline) and melphalan or prophalan-L but no significant difference between control and prophalan-D or between melphalan and prophalan-L. Prophalan-L was significantly less toxic than melphalan, while no significant difference was observed in toxicity, measured as percent weight loss, between the prodrugs and saline control. Tumor reduction efficacy at high doses (12 microg/g i.p.) was similar for melphalan and prophalan-L; however, fatal toxicity was associated with melphalan while prophalan-L exhibited significantly lower systemic toxicity. An excellent correlation between GI50 and tumor reduction efficacy was observed for the tested drugs (r2=0.95). Prophalan-L thus demonstrates higher therapeutic index than melphalan in the murine melanoma model.

Comparative gene expression profiles of intestinal transporters in mice, rats and humans

We have studied gene expression profiles of intestinal transporters in model animals and humans. Total RNA was isolated from duodenum and the mRNA expression was measured using Affymetrix GeneChip oligonucleotide arrays. Detected genes from the intestine of mice, rats, and humans were about 60% of 22,690 sequences, 40% of 8739, and 47% of 12,559, respectively. A total of 86 genes involving transporters expressed in mice, 50 genes in rats, and 61 genes in humans were detected. Mice exhibited abundant mRNA expressions for peptide transporter HPT1, amino acid transporters CSNU3, CT1 and ASC1, nucleoside transporter CNT2, organic cation transporter SFXN1, organic anion transporter NBC3, glucose transporter SGLT1, and fatty acid transporters FABP1 and FABP2. Rats showed high expression profiles of peptide transporter PEPT1, amino acid transporters CSNU1 and 4F2HC, nucleoside transporter CNT2, organic cation transporter OCT5, organic anion transporter SDCT1, glucose transporter GLUT2 and GLUT5, and folate carrier FOLT. In humans, the highly expressed genes were peptide transporter HPT1, amino acid transporters LAT3, 4F2HC and PROT, nucleoside transporter CNT2, organic cation transporter OCTN2, organic anion transporters NADC1, NBC1 and SBC2, glucose transporters SGLT1 and GLUT5, multidrug resistance-associated protein RHO12, fatty acid transporters FABP1 and FABP2, and phosphate carrier PHC. Overall these data reveal diverse transcriptomic profiles for intestinal transporters among these species. Therefore, this transcriptional data may lead to more effective use of the laboratory animals as a model for oral drug development.

Why is it Challenging to Predict Intestinal Drug Absorption and Oral Bioavailability in Human Using Rat Model

PURPOSE

To study the correlation of intestinal absorption for drugs with various absorption routes between human and rat, and to explore the underlying molecular mechanisms for the similarity in drug intestinal absorption and the differences in oral bioavailability between human and rat.

MATERIALS AND METHODS

The intestinal permeabilities of 14 drugs and three drug-like compounds with different absorption mechanisms in rat and human jejunum were determined by in situ intestinal perfusion. A total of 48 drugs were selected for oral bioavailability comparison. Expression profiles of transporters and metabolizing enzymes in both rat and human intestines (duodenum and colon) were measured using GeneChip analysis.

RESULTS

No correlation (r(2) = 0.29) was found in oral drug bioavailability between rat and human, while a correlation (r(2) = 0.8) was observed for drug intestinal permeability with both carrier-mediated absorption and passive diffusion mechanisms between human and rat small intestine. Moderate correlation (with r(2) > 0.56) was also found for the expression levels of transporters in the duodenum of human and rat, which provides the molecular mechanisms for the similarity and correlation of drug absorption between two species. In contrast, no correlation was found for the expressions of metabolizing enzymes between rat and human intestine, which indicates the difference in drug metabolism and oral bioavailability in two species. Detailed analysis indicates that many transporters (such as PepT1, SGLT-1, GLUT5, MRP2, NT2, and high affinity glutamate transporter) share similar expression levels in both human and rat with regional dependent expression patterns, which have high expression in the small intestine and low expression in the colon. However, discrepancy was also observed for several other transporters (such as MDR1, MRP3, GLUT1, and GLUT3) in both the duodenum and colon of human and rat. In addition, the expressions of metabolizing enzymes (CYP3A4/CYP3A9 and UDPG) showed 12 to 193-fold difference between human and rat intestine with distinct regional dependent expression patterns.

CONCLUSIONS

The data indicate that rat and human show similar drug intestinal absorption profiles and similar transporter expression patterns in the small intestine, while the two species exhibit distinct expression levels and patterns for metabolizing enzymes in the intestine. Therefore, a rat model can be used to predict oral drug absorption in the small intestine of human, but not to predict drug metabolism or oral bioavailability in human.

N-methylpurine DNA glycosylase and 8-oxoguanine dna glycosylase metabolize the antiviral nucleoside 2-bromo-5,6-dichloro-1-(beta-D-ribofuranosyl)benzimidazole

The rapid in vivo degradation of the potent human cytomegalovirus inhibitor 2-bromo-5,6-dichloro-1-(beta-D-ribofuranosyl)benzimidazole (BDCRB) compared with a structural L-analog, maribavir (5,6-dichloro-2-(isopropylamino)-1-beta-L-ribofuranosyl-1H-benzimidazole), has been attributed to selective glycosidic bond cleavage. An enzyme responsible for this selective BDCRB degradation, however, has not been identified. Here, we report the identification of two enzymes, 8-oxoguanine DNA glycosylase (OGG1) and N-methylpurine DNA glycosylase (MPG), that catalyze N-glycosidic bond cleavage of BDCRB and its 2-chloro homolog, 2,5,6-trichloro-1-(beta-D-ribofuranosyl)benzimidazole, but not maribavir. To our knowledge, this is the first demonstration that free nucleosides are substrates of OGG1 and MPG. To understand how these enzymes might process BDCRB, docking and molecular dynamics simulations were performed with the native human OGG1 crystal coordinates. These studies showed that OGG1 was not able to bind a negative control, guanosine, yet BDCRB and maribavir were stabilized through interactions with various binding site residues, including Phe319, His270, Ser320, and Asn149. Only BDCRB, however, achieved orientations whereby its anomeric carbon, C1', could undergo nucleophilic attack by the putative catalytic residue, Lys249. Thus, in silico observations were in perfect agreement with experimental observations. These findings implicate DNA glycosylases in drug metabolism.

Nucleoside Ester Prodrug Substrate Specificity of Liver Carboxylesterase

Carboxylesterases are among the best characterized prodrug-hydrolyzing enzymes involved in the activation of several therapeutic carbamate and ester prodrugs. The broad specificity of these enzymes makes them amenable for designing prodrugs. Porcine liver carboxylesterase 1 specificity for amino acid esters of three nucleoside analogs [floxuridine, gemcitabine, and 2-bromo-5,6-dichloro-1-(beta-D-ribofuranosyl) benzimidazole] was evaluated to assess optimal structural preferences for prodrug design. The amino acid promoiety and the esterification site influenced carboxylesterase hydrolysis rates up to 1164-fold and the binding affinity up to 26-fold. Carboxylesterase (CES) 1 exhibited high-catalytic efficiency hydrolyzing prodrugs containing a phenylalanyl moiety but was over 100-fold less efficient with valyl or isoleucyl prodrugs, regardless of the nucleoside or esterification site. CES1 catalytic efficiency was 2-fold higher with 5' phenylalanyl monoesters than the corresponding 3' esters of floxuridine. This preference was reversed with phenylalanyl gemcitabine prodrugs, evident from a 2-fold preference for 3' monoesters over 5' esters. The newly characterized esterase valacyclovirase was several hundred-fold more efficient (up to 19,000-fold) than carboxylesterase in hydrolyzing amino acid esters but similar in apparent binding affinity. The specific activities of the two enzymes with several amino acid ester prodrugs clearly suggest that initial hydrolysis rates are relatively low for prodrugs with isoleucyl, aspartyl, and lysyl promoieties for both enzymes compared with those with phenylalanyl, valyl, prolyl, and leucyl progroups. The low relative hydrolysis rates of isoleucyl, aspartyl, and lysyl prodrugs may facilitate prolonged systemic disposition of the nucleoside analogs for improved therapeutic action.

Targeted delivery to PEPT1-overexpressing cells: acidic, basic, and secondary floxuridine amino acid ester prodrugs

Floxuridine is a clinically proven anticancer agent in the treatment of metastatic colon carcinomas and hepatic metastases. However, prodrug strategies may be necessary to improve its physiochemical properties and selectivity and to reduce undesirable toxicity effects. Previous studies with amino acid ester prodrugs of nucleoside drugs targeted to the PEPT1 transporter coupled with recent findings of the functional expression of the PEPT1 oligopeptide transporter in pancreatic adenocarcinoma cell lines suggest the potential of PEPT1 as therapeutic targets for cancer treatment. In this report, we show the feasibility of achieving enhanced transport and selective antiproliferative action of amino acid ester prodrugs of floxuridine in cell systems overexpressing PEPT1. All prodrugs exhibited affinity for PEPT1 (IC50, 1.1-2.3 mmol/L). However, only the prolyl and lysyl prodrugs exhibited enhanced uptake (2- to 8-fold) with HeLa/PEPT1 cells compared with HeLa cells, suggesting that the aspartyl prodrugs are PEPT1 inhibitors. The selective growth inhibition of Madine-Darby canine kidney (MDCK)/PEPT1 cells over MDCK cells by the prodrugs was consistent with the extent of their PEPT1-mediated transport. All ester prodrugs hydrolyzed to floxuridine fastest in Caco-2 cell and MDCK homogenates and slower in human plasma and were most chemically stable in pH 6.0 buffer. Prolyl and lysyl prodrugs were relatively less stable compared with aspartyl prodrugs in buffers and in cell homogenates. The results suggest that optimal design for targeted delivery would be possible by combining both stability and transport characteristics afforded by the promoiety.

Floxuridine Amino Acid Ester Prodrugs: Enhancing Caco-2 Permeability and Resistance to Glycosidic Bond Metabolism

PURPOSE

The aim of this study was to synthesize amino acid ester prodrugs of 5-fluoro-2'-deoxyuridine (floxuridine) to enhance intestinal absorption and resistance to glycosidic bond metabolism.

METHODS

Amino acid ester prodrugs were synthesized and examined for their hydrolytic stability in human plasma, in Caco-2 cell homogenates, and in the presence of thymidine phosphorylase. Glycyl-L: -sarcosine uptake inhibition and direct uptake studies with HeLa/PEPT1 cells [HeLa cells overexpressing oligopeptide transporter (PEPT1)] were conducted to determine PEPT1-mediated transport and compared with permeability of the prodrugs across Caco-2 monolayers.

RESULTS

Isoleucyl prodrugs exhibited the highest chemical and enzymatic stability. The prodrugs enhanced the stability of the glycosidic bond of floxuridine. Thymidine phosphorylase rapidly cleaved floxuridine to 5-fluorouracil, whereas with the prodrugs no detectable glycosidic bond cleavage was observed. The 5'-L: -isoleucyl and 5'-L: -valyl monoester prodrugs exhibited 8- and 19-fold PEPT1-mediated uptake enhancement in HeLa/PEPT1 cells, respectively. Uptake enhancement in HeLa/PEPT1 cells correlated highly with Caco-2 permeability for all prodrugs tested. Caco-2 permeability of 5'-L: -isoleucyl and 5'-L: -valyl prodrugs was 8- to 11-fold greater compared with floxuridine.

CONCLUSIONS

Amino acid ester prodrugs such as isoleucyl floxuridine that exhibit enhanced Caco-2 transport and slower rate of enzymatic activation to parent, and that are highly resistant to metabolism by thymidine phosphorylase may improve oral delivery and therapeutic index of floxuridine.

Amino acid ester prodrugs of 2-bromo-5,6-dichloro-1-(beta-D-ribofuranosyl)benzimidazole enhance metabolic stability in vitro and in vivo

2-Bromo-5,6-dichloro-1-(beta-d-ribofuranosyl)benzimidazole (BDCRB) is a potent and selective inhibitor of human cytomegalovirus (HCMV), but it lacks clinical utility due to rapid in vivo metabolism. We hypothesized that amino acid ester prodrugs of BDCRB may enhance both in vitro potency and systemic exposure of BDCRB through evasion of BDCRB-metabolizing enzymes. To this end, eight different amino acid prodrugs of BDCRB were tested for N-glycosidic bond stability, ester bond stability, Caco-2 cell uptake, antiviral activity, and cytotoxicity. The prodrugs were resistant to metabolism by BDCRB-metabolizing enzymes, and ester bond cleavage was rate-limiting in metabolite formation from prodrug. Thus, BDCRB metabolism could be controlled by the selection of promoiety. In HCMV plaque-formation assays, l-Asp-BDCRB exhibited 3-fold greater selectivity than BDCRB for inhibition of HCMV replication. This potent and selective antiviral activity in addition to favorable stability profile made l-Asp-BDCRB an excellent candidate for in vivo assessment and pharmacokinetic comparison with BDCRB. In addition to rapid absorption and sufficient prodrug activation after oral administration to mice, l-Asp-BDCRB exhibited a 5-fold greater half-life than BDCRB. Furthermore, the sum of area under the concentration-time profile (AUC)(BDCRB) and AUC(prodrug) after l-Asp-BDCRB administration was roughly 3-fold greater than AUC(BDCRB) after BDCRB administration, suggesting that a reservoir of prodrug was delivered in addition to parent drug. Overall, these findings demonstrate that amino acid prodrugs of BDCRB exhibit evasion of metabolizing enzymes (i.e., bioevasion) in vitro and provide a modular approach for translating this in vitro stability into enhanced in vivo delivery of BDCRB.

Permeability Dominates in Vivo Intestinal Absorption of P-gp Substrate with High Solubility and High Permeability

Three purposes are presented in this study: (1) to study the in vivo regional dependent intestinal absorption of a P-gp substrate with high solubility and high permeability, (2) to study the gene expression difference in the various regions of the intestine, and (3) to study the contributions of P-gp or any other transporters for the absorption of a P-gp substrate. The in vivo permeability of verapamil and propranolol were determined by single-pass in situ intestinal perfusion in rat. The gene expression profiles were measured using Affymetrix GeneChip. Correlation analysis between drug in vivo permeability and expression of 3500 genes was performed with nonparametric bootstrap and ANOVA analysis. The permeability of verapamil and propranolol did not demonstrate regional dependency even though significant differences in gene expression were observed in various regions of the intestine. Verapamil permeability significantly correlates with propranolol permeability in both jejunum and ileum, but did not correlate with the permeability of other hydrophilic compounds (valacyclovir, acyclovir, and phenylalanine). Four different regions (duodenum, jejunum, ileum, and colon) showed distinct gene expression patterns with more than 70-499 genes showing at least 5-fold expression differences. Interestingly, P-gp expression is gradually increased by 6-fold from the duodenum to colon. Despite the distinct gene expression patterns in the various regions of the intestine, verapamil permeability did not correlate with any gene expression from 3500 expressed genes in the intestine. A 2-6-fold P-gp expression difference did not seem to associate verapamil permeability in the various intestinal regions in vivo. These data suggest that P-gp plays a minimal role in the in vivo intestinal absorption process of verapamil with high water solubility and high membrane permeability. The intestinal absorption of verapamil in vivo is primarily dominated by its high permeability. However, it is important to note that the findings in this paper do not undermine the importance of P-gp in oral drug bioavailability, drug disposition from the liver, drug efflux from the blood-brain barrier, and drug-drug interaction.

Amino Acid Ester Prodrugs of the Anticancer Agent Gemcitabine: Synthesis, Bioconversion, Metabolic Bioevasion, and hPEPT1-Mediated Transport

Gemcitabine, a clinically effective nucleoside anticancer agent, is a polar drug with low membrane permeability and is administered intravenously. Further, extensive degradation of gemcitabine by cytidine deaminase to an inactive metabolite in the liver affects its activity adversely. Thus, strategies that provide both enhanced transport and high metabolic bioevasion would potentially lead to oral alternatives that may be clinically useful. The objective of this study was to evaluate whether amino acid ester prodrugs of gemcitabine would (a) facilitate transport across intestinal membranes or across cells that express hPEPT1 and (b) provide resistance to deamination by cytidine deaminase. 3'-Monoester, 5'-monoester, and 3',5'-diester prodrugs of gemcitabine utilizing aliphatic (L-valine, D-valine, and L-isoleucine) and aromatic (L-phenylalanine and D-phenylalanine) amino acids as promoieties were synthesized and evaluated for their affinity and direct hPEPT1-mediated transport in HeLa/hPEPT1 cells. All prodrugs exhibited enhanced affinity (IC(50): 0.14-0.16 mM) for the transporter. However, only the 5'-L-valyl and 5'-L-isoleucyl monoester prodrugs exhibited (a) increased uptake (11.25- and 5.64-fold, respectively) in HeLa/hPEPT1 cells compared to HeLa cells and (b) chemical stability in buffers, that were comparable to valacyclovir, a commercially marketed oral amino acid ester prodrug. The widely disparate enzymatic bioconversion profiles of the 5'-L-valyl and 5'-L-isoleucyl prodrugs in Caco-2 cell homogenates along with their significant resistance to deamination by cytidine deaminase suggest that the disposition of gemcitabine following oral administration would be controlled by the rate of bioconversion following transport across the intestinal epithelial membrane. The combined results also suggest that it may be possible to modulate these characteristics by the choice of the amino acid promoiety.

Prolidase, a Potential Enzyme Target for Melanoma: Design of Proline-Containing Dipeptide-like Prodrugs

Bioinformatics tools such as Perl, Visual Basic, Cluster, and TreeView were used to analyze public gene expression databases in order to identify potential enzyme targets for prodrug strategies. The analyses indicated that prolidase might be a desirable enzyme target based on its differential expression in melanoma cancer cell lines and its high substrate specificity for dipeptides containing proline at the carboxy terminus. RT-PCR expression of prolidase and hydrolytic activity against N-glycyl-l-proline (GLY-PRO), a standard substrate of prolidase, determined in tumor cell lines, exhibited a high correlation (r(2) = 0.95). These results suggest the possibility of targeting prolidase with prodrugs of anticancer agents for enhanced selectivity. The feasibility of such a scenario was tested by (a) synthesizing prodrugs of melphalan that comprised linkage of the carboxy terminus of the l-phenylalanine moiety of melphalan to the N-terminus of l and d stereoisomers of proline and (b) determining their bioconversion and antiproliferative activities in SK-MEL-5 cells, a melanoma cancer cell line with high expression levels of prolidase. The results of hydrolysis studies of the l- and d-proline prodrugs of melphalan, designated as prophalan-l and prophalan-d, respectively, indicated a approximately 7-fold higher rate of activation of prophalan-l compared to prophalan-d in SK-MEL-5 cell homogenates. Prophalan-l exhibited cytotoxicity (GI(50) = 74.8 microM) comparable to that of melphalan (GI(50) = 57.0 microM) in SK-MEL-5 cells while prophalan-d was ineffective, suggesting that prolidase-specific activation to the parent drug may be essential for cytotoxic action. Thus, melphalan prodrugs such as prophalan-l that are cleavable by prolidase offer the potential for enhanced selectivity by facilitating cytotoxic activity only in cells overexpressing prolidase.

Transporter and ion channel gene expression after Caco-2 cell differentiation using 2 different microarray technologies

mRNA expression profiles had previously been measured in Caco-2 cells (human colonic carcinoma cells) using either custom-designed spotted oligonucleotide arrays or Affymetrix GeneChip oligonucleotide arrays. The Caco-2 cells used were from different clones and were examined under slightly different culture conditions commonly encountered when Caco-2 cells are used as a model tissue for studying intestinal transport and metabolism in different laboratories. In this study, we compared gene expression profiles of Caco-2 cells generated with different arrays to assess the validity of conclusions derived from the 2 independent studies, with a focus on changes in transporter and ion channel mRNA expression levels on Caco-2 cell differentiation. Significant changes in expression levels upon differentiation were observed with 78 genes, with probes common to both arrays. Of these, 18 genes were upregulated and 36 genes were downregulated. The 2 arrays yielded discrepant results for 24 genes, showing significant changes upon differentiation. The results from the 2 arrays correlated well for genes expressed above average levels (r = 0.75, P < 0.01, n = 25) and poorly for genes expressed at low levels (r = 0.08, P > 0.05, n = 25). Overall correlation across the 2 platforms was r = 0.45 (P < 0.01) for the 78 genes, with similar results from both arrays. Despite differences in experimental conditions and array technology, similar results were obtained for most genes.

Amino acid ester prodrugs of floxuridine: synthesis and effects of structure, stereochemistry, and site of esterification on the rate of hydrolysis

PURPOSE

To synthesize amino acid ester prodrugs of floxuridine (FUdR) and to investigate the effects of structure, stereochemistry, and site of esterification of promoiety on the rates of hydrolysis of these prodrugs in Caco-2 cell homogenates.

METHODS

Amino acid ester prodrugs of FUdR were synthesized using established procedures. The kinetics of hydrolysis of prodrugs was evaluated in human adenocarcinoma cell line (Caco-2) homogenates and pH 7.4 phosphate buffer.

RESULTS

3'-Monoester, 5'-monoester, and 3',5'-diester prodrugs of FUdR utilizing proline, L-valine, D-valine, L-phenylalanine, and D-phenylalanine as promoieties were synthesized and characterized. In Caco-2 cell homogenates, the L-amino acid ester prodrugs hydrolyzed 10 to 75 times faster than the corresponding D-amino acid ester prodrugs. Pro and Phe ester prodrugs hydrolyzed much faster (3- to 30-fold) than the corresponding Val ester prodrugs. Further, the 5'-monoester prodrugs hydrolyzed significantly faster (3-fold) than the 3',5'-diester prodrugs.

CONCLUSIONS

Novel amino acid ester prodrugs of FUdR were successfully synthesized. The results presented here clearly demonstrate that the rate of FUdR prodrug activation in Caco-2 cell homogenates is affected by the structure, stereochemistry, and site of esterification of the promoiety. Finally, the 5'-Val and 5'-Phe monoesters exhibited desirable characteristics such as good solution stability and relatively fast enzymatic conversion rates.

A Fluorescent hPept1 Transporter Substrate for Uptake Screening

PURPOSE

To synthesize fluorescent analogues of hPept1 substrates, FITC-Val-OCH3, Lys-FITC-OH, and Lys-FITC-OCH3, and to characterize their hPept1 transporter-mediated uptake.

METHODS

FITC analogues of amino acids were synthesized using established synthetic procedures, and the extent of their [3H]Gly-Sar uptake inhibition in HeLa/hPept1 cells was determined. The uptake of Lys-FITC-OCH3 was evaluated in HeLa, HeLa/hPept1, and Caco-2 cells in the presence and absence of Gly-Sar using a fluorescence microscopy-based assay. The uptake and transport of the Lys-FITC analogues were also determined in Caco-2 cells using HPLC assays.

RESULTS

In HeLa/hPept1 cells, [3H]Gly-Sar uptake was significantly inhibited by Lys-FITC-OCH3 (74%) but not by FITC-Val-OCH3 (22%). The uptake of Lys-FITC-OCH3 (100 microM) was approximately 10-fold higher in HeLa/hPept1 cells. Also, Lys-FITC-OCH3 (100 microM) uptake in HeLa/hPept1 and Caco-2 cells was reduced by 77% and 80%, respectively, in the presence of 1 mM Gly-Sar. Dipeptides and cephalexin significantly reduced Lys-FITC-OCH3 uptake in Caco-2 cells. The apical permeability of Lys-FITC-OCH3 (1.5 x 10(6) cm/s) in Caco-2 cells was significantly lowered in the presence of Gly-Sar. Fluorescence micrographs revealed that this analogue was localized in the cytoplasm and in the nucleus.

CONCLUSIONS

The combined results indicate that Lys-FITC-OCH3 is recognized and transported by hPept1 in HeLa/hPept1 and by peptide transporters in Caco-2 cells. The results also suggest that Lys-FITC-OCH3 might be a useful fluorescent substrate for rapid assessment of peptide transporter activity in cells of interest.

A Novel High-Throughput PepT1 Transporter Assay Differentiates between Substrates and Antagonists

PepT1 is a transporter of proven pharmaceutical utility for enhancing oral absorption. A high-throughput, robust functional assay, capable of distinguishing PepT1 binders from substrates, allowing identification and/or prediction of drug candidate activation was developed. An MDCK epithelial cell line was transfected with rPepT1. The high level of stable rPepT1 expression that was achieved enabled development of a miniaturized PepT1 assay in a 96-well format, which could be scaled to 384 wells. The assay is based on measurement of membrane depolarization resulting from the cotransport of protons and PepT1 substrates. Membrane potential changes are tracked with a voltage-sensitive fluorescent indicator. Control (mock-transfected) cells are used to determine nonspecific membrane potential changes. A variety of fluorescent dyes were tested during initial assay design, including intracellular pH and membrane potential indicators. A membrane potential indicator was chosen because of its superior performance. Upon PepT1 activation with glycylsarcosine, dose-dependent membrane depolarization was observed with an EC50 of 0.49 mM. Maximum depolarization was dependent on the level of PepT1 expression. Testing of 38 known PepT1 substrates, binders, and nonbinders demonstrated that this assay accurately distinguished substrates from binders and from nonbinders. Initial validation of this novel assay indicates that it is sensitive and robust, and can distinguish between transporter substrates and antagonists. This important distinction has been previously achieved only with lower-throughput assays. This assay might also be used to determine substrate potency and establish a high-quality data set for PepT1 SAR modeling.

Functional expression and characterization of a sodium-dependent nucleoside transporter hCNT2 cloned from human duodenum

We have cloned and functionally expressed a sodium-dependent human nucleoside transporter, hCNT2, from a CNS cancer cell line U251. Our cDNA clone of hCNT2 had the same predicted amino acid sequence as the previously cloned hCNT2 transporter. Of the several cell lines studied, the best hCNT2 transport function was obtained when transiently expressed in U251 cells. Na(+)-dependent uptake of [3H]inosine in U251 cells transiently expressing hCNT2 was 50-fold greater than that in non-transfected cells, and uptake in Na(+)-containing medium was approximately 30-fold higher than that at Na(+)-free condition. The hCNT2 displayed saturable uptake of [3H]inosine with K(m) of 12.8 microM and V(max) of 6.66 pmol/mg protein/5 min. Uptake of [3H]inosine was significantly inhibited by the purine nucleoside drugs dideoxyinosine and cladribine, but not by acyclic nucleosides including acyclovir, ganciclovir, and their prodrugs valacyclovir and valganciclovir. This indicates that the closed ribose ring is important for binding of nucleoside drugs to hCNT2. Among several pyrimidine nucleosides, hCNT2 favorably interacted with the uridine analogue floxuridine. Interestingly, we found that benzimidazole analogues, including maribavir, 5,6-dichloro-2-bromo-1-beta-D-ribofuranosylbenzimidazole (BDCRB), and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), were strong inhibitors of inosine transport, even though they have a significantly different heterocycle structure compared to a typical purine ring. As measured by GeneChip arrays, mRNA expression of hCNT2 in human duodenum was 15-fold greater than that of hCNT1 or hENT2. Further, the rCNT2 expression in rat duodenum was 20-fold higher than rCNT1, rENT1 or rENT2. This suggests that hCNT2 (and rCNT2) may have a significant role in uptake of nucleoside drugs from the intestine and is a potential transporter target for the development of nucleoside and nucleoside-mimetic drugs.

Gene expression in the human intestine and correlation with oral valacyclovir pharmacokinetic parameters

The transport of valacyclovir, the l-valyl ester of acyclovir, has been suggested to be mediated by several carrier-mediated pathways in cell culture and animal models. The role and importance of these transporters in modulating valacyclovir absorption in humans has not been determined, however. Recent advances in genomic technology have facilitated the rapid and simultaneous determination of global mRNA expression profiles for thousands of genes in tissue biopsies directly associated with the absorption process, thereby dramatically increasing the value of studies in humans. In this article, we describe correlations of pharmacokinetic parameters following oral valacyclovir or acyclovir administration with expression levels of intestinal genes in humans. Highly positive and significant correlations were observed with 4F2hc, an activator of cation-preferring amino acid transport systems, and human oligopeptide transporter (HPT1), an oligopeptide transporter expressed at higher levels in the human intestine compared with oligopeptide transporter (PEPT1). The validation of HPT1 microarray data with reverse transcription-polymerase chain reaction and the enhanced valacyclovir uptake in HeLa/HPT1 cells suggest that the role of HPT1 in transport of peptides and peptidomimetics drugs needs to be examined in more detail. The interrelation of 4F2hc and HPT1 in transport may be of interest. No significant correlations of valacyclovir pharmacokinetic parameters with PEPT1 and with organic cation or anion transporter expression levels were observed. The highly negative correlations observed with known efflux pumps such as MDR1 (P-glycoprotein) and MRP2 (cMOAT), as well as with the CYP450 IIIA subfamily may indicate that these proteins may regulate the cellular accumulation and metabolism of acyclovir.

Comparison of human duodenum and Caco-2 gene expression profiles for 12,000 gene sequences tags and correlation with permeability of 26 drugs

PURPOSE

To compare gene expression profiles and drug permeability differences in Caco-2 cell culture and human duodenum.

METHODS

Gene expression profiles in Caco-2 cells and human duodenum were determined by GeneChip analysis. In vivo drug permeability measurements were obtained through single-pass intestinal perfusion in human subjects, and correlated with in vitro Caco-2 transport permeability.

RESULTS

GeneChip analysis determined that 37, 47, and 44 percent of the 12,559 gene sequences were expressed in 4-day andl6-day Caco-2 cells and human duodenum, respectively. Comparing human duodenum with Caco-2 cells, more than 1,000 sequences were determined to have at least a 5-fold difference in expression. There were 26, 38, and 44 percent of the 443 transporters, channels, and metabolizing enzymes detected in 4-day, 16-day Caco-2 cells, and human duodenum, respectively. More than 70 transporters and metabolizing enzymes exhibited at least a 3-fold difference. The overall coefficient of variability of the 10 human duodenal samples for all expressed sequences was 31% (range 3% to 294%) while that of the expressed transporters and metabolizing enzymes was 33% (range 3% to 87%). The in vivo / in vitro drug permeability measurements correlated well for passively absorbed drugs (R2 = 85%). The permeability correlation for carrier-mediated drugs showed 3- 35-fold higher in human above the correlation of passively absorbed drugs. The 2- 595-fold differences in gene expression levels between the Caco-2 cells and human duodenum correlated with the observed 3- 35-fold difference in permeability correlation between carrier-mediated drugs and passively absorbed drugs. CONCLUSIONS; Significant differences in gene expression levels in Caco-2 cells and human duodenum were observed. The observed differences of gene expression levels were consistent with observed differences in carrier mediated drug permeabilities. Gene expression profiling is a valuable new tool for investigating in vitro and in vivo permeability correlation.

Combinatorial use of antibodies to secreted mycobacterial proteins in a host immune system-independent test for tuberculosis

Laboratory diagnosis of tuberculosis is often difficult. Immunodetection of circulating Mycobacterium tuberculosis proteins shed during active infection would not depend on an intact host immune response and could take advantage of the speed and low costs afforded by antibody-based assays. We previously showed that patients with active tuberculosis had increased levels of circulating antigen 85 (Ag85) proteins independent of their tuberculin skin test status (S. I. Bentley-Hibbert, X. Quan, T. Newman, K. Huygen, and H. P. Godfrey, Infect. Immun. 67:581-588, 1999). To extend these observations to a Mycobacterium bovis BCG-vaccinated population and to another secreted mycobacterial protein, Ag85 and PstS-1 (protein antigen B, p38 antigen) were quantified in sera from 97 Chilean tuberculosis patients and healthy controls (many of whom had received BCG as children) using dot immunobinding, mouse monoclonal anti-BCG Ag85 complex antibody, and chicken antipeptide antibodies reactive with M. tuberculosis Ag85B and PstS-1. The latter antibodies had been raised to peptide-derived immunogens expressed on a novel proprietary protein carrier in Escherichia coli. Median serum Ag85 levels measured by using either anti-Ag85 antibody were significantly higher in patients with active tuberculosis than in healthy controls (P, <0.001 to 0.01); the median serum PstS-1 levels were similar in patients and controls. The sensitivity of significantly elevated circulating Ag85 levels in patients with pulmonary tuberculosis measured by anti-Ag85 complex or anti-Ag85B antibodies was 60 and 55%, respectively, but increased to 77% when results obtained with both anti-Ag85 antibodies were considered jointly (P < 0.02). The corresponding specificities for individual and joint consideration were 95, 85, and 80%, respectively. These results indicate that elevated Ag85 levels can be detected in patients with active tuberculosis even after BCG vaccination and suggest that combinatorial use of antibodies directed at different epitopes of this protein could provide a viable strategy for developing new host immune response-independent diagnostic tests for tuberculosis.

Drug inhibition of Gly-Sar uptake and hPepT1 localization using hPepT1-GFP fusion protein

An hPepT1-GFP fusion construct was made to study drug inhibition of dipeptide uptake and apical, basolateral, or subcellular hPepT1 localization. The hPepT1 stop codon was mutated by polymerase chain reaction and was subsequently cloned into the pEGFP-N1 vector. The hPepT1-GFP fusion construct was then transfected into Caco-2 and HeLa cells, and drug inhibition was studied by inhibiting 3H-Gly-Sar uptake. Western blot analysis was used to determine hPepT1-GFP expression levels and confocal microscopy was used to examine the localization. Both anti-hPepT1 antibody and anti-GFP antibody recognized a 120-kd hPepT1-GFP fusion protein in the transfected cells. The 3H-Gly-Sar uptake in transfected HeLa cells was enhanced more than 20 times compared with the control. Valacyclovir (5 mmol/L) was able to completely inhibit 3H-Gly-Sar uptake in these transfected cells. Confocal microscopy showed that the hPepT1-GFP mainly localized in the Caco-2 cell apical membrane, but was present throughout the entire HeLa cell membranes. The hPepT1-GFP fusion protein was not found in either early endosome or lysosome of Caco-2 cells under normal conditions; however, it was detected in some subsets of lysosomes and early endosomes in phorbol 12-myristate 13-acetate (PMA)-treated Caco-2 cells.