Glycerol uptake in HCT-15 human colon cancer cell line by Na(+)-dependent carrier-mediated transport

cAMP Receptor Protein Positively Regulates the Expression of Genes Involved in the Biosynthesis of Klebsiella oxytoca Tilivalline Cytotoxin

Klebsiella oxytoca is a resident of the human gut. However, certain K. oxytoca toxigenic strains exist that secrete the nonribosomal peptide tilivalline (TV) cytotoxin. TV is a pyrrolobenzodiazepine that causes antibiotic-associated hemorrhagic colitis (AAHC). The biosynthesis of TV is driven by enzymes encoded by the aroX and NRPS operons. In this study, we determined the effect of environmental signals such as carbon sources, osmolarity, and divalent cations on the transcription of both TV biosynthetic operons. Gene expression was enhanced when bacteria were cultivated in tryptone lactose broth. Glucose, high osmolarity, and depletion of calcium and magnesium diminished gene expression, whereas glycerol increased transcription of both TV biosynthetic operons. The cAMP receptor protein (CRP) is a major transcriptional regulator in bacteria that plays a key role in metabolic regulation. To investigate the role of CRP on the cytotoxicity of K. oxytoca, we compared levels of expression of TV biosynthetic operons and synthesis of TV in wild-type strain MIT 09-7231 and a Δcrp isogenic mutant. In summary, we found that CRP directly activates the transcription of the aroX and NRPS operons and that the absence of CRP reduced cytotoxicity of K. oxytoca on HeLa cells, due to a significant reduction in TV production. This study highlights the importance of the CRP protein in the regulation of virulence genes in enteric bacteria and broadens our knowledge on the regulatory mechanisms of the TV cytotoxin.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

pH-dependent pyridoxine transport by SLC19A2 and SLC19A3: Implications for absorption in acidic microclimates

SLC19A2 and SLC19A3, also known as thiamine transporters (THTR) 1 and 2, respectively, transport the positively charged thiamine (vitamin B1) into cells to enable its efficient utilization. SLC19A2 and SLC19A3 are also known to transport structurally unrelated cationic drugs, such as metformin, but whether this charge selectivity extends to other molecules, such as pyridoxine (vitamin B6), is unknown. We tested this possibility using Madin-Darby canine kidney II (MDCKII) cells and human embryonic kidney 293 (HEK293) cells for transfection experiments, and also using Caco-2 cells as human intestinal epithelial model cells. The stable expression of SLC19A2 and SLC19A3 in MDCKII cells (as well as their transient expression in HEK293 cells) led to a significant induction in pyridoxine uptake at pH 5.5 compared with control cells. The induced uptake was pH-dependent, favoring acidic conditions over neutral to basic conditions, and protonophore-sensitive. It was saturable as a function of pyridoxine concentration, with an apparent Km of 37.8 and 18.5 μm, for SLC19A2 and SLC19A3, respectively, and inhibited by the pyridoxine analogs pyridoxal and pyridoxamine as well as thiamine. We also found that silencing the endogenous SLC19A3, but not SLC19A2, of Caco-2 cells with gene-specific siRNAs lead to a significant reduction in carrier-mediated pyridoxine uptake. These results show that SLC19A2 and SLC19A3 are capable of recognizing/transporting pyridoxine, favoring acidic conditions for operation, and suggest a possible role for these transporters in pyridoxine transport mainly in tissues with an acidic environment like the small intestine, which has an acidic surface microclimate.

The Coli Surface Antigen CS3 of Enterotoxigenic Escherichia coli Is Differentially Regulated by H-NS, CRP, and CpxRA Global Regulators

Enterotoxigenic Escherichia coli produces a myriad of adhesive structures collectively named colonization factors (CFs). CS3 is a CF, which is assembled into fine wiry fibrillae encoded by the cstA-H gene cluster. In this work we evaluated the influence of environmental cues such as temperature, osmolarity, pH, and carbon source on the expression of CS3 genes. The transcription of cstH major pilin gene was stimulated by growth of the bacteria in colonization factor broth at 37°C; the presence of glycerol enhanced cstH transcription, while glucose at high concentration, high osmolarity, and the depletion of divalent cations such as calcium and magnesium repressed cstH expression. In addition, we studied the role of H-NS, CpxRA, and CRP global regulators in CS3 gene expression. H-NS and CpxRA acted as repressors and CRP as an activator of cstH expression. Under high osmolarity, H-NS, and CpxRA were required for cstH repression. CS3 was required for both, bacterial adherence to epithelial cells and biofilm formation. Our data strengthens the existence of a multi-factorial regulatory network that controls transcription of CS3 genes in which global regulators, under the influence of environmental signals, control the production of this important intestinal colonization factor.

Fecal volatile organic compounds for early detection of colorectal cancer: where are we now?

INTRODUCTION

The fecal volatolome, which is composed of fecal volatile organic compounds (VOCs), seems to hold potential as non-invasive biomarker for the detection of colorectal cancer (CRC) and its precursor lesions advanced adenomas (AA). The potential of the fecal volatolome has been subject of various studies using either chemical analytical or pattern-recognition techniques. The available literature on the potential of the fecal volatolome as CRC and AA biomarker was reviewed.

METHODS

A systematic literature search was conducted in PubMed, Embase, the Cochrane Library, Google Scholar and ResearchGate using the following keywords: Colorectal Cancer, Advanced Adenoma, Volatile Organic Compound, Metabolome, Gas Chromatrography-Mass Spectrometry, Selected-Ion Flow-Tube Mass Spectrometry, eNose, and Fecal Biomarkers.

RESULTS

Eighty-eight titles or abstracts were identified from the search, of which 11 papers describing the potential of the fecal volatolome for CRC detection were selected. In these studies, different techniques were used for the headspace analyses of fecal VOCs, limiting the possibility to compare outcomes. Increased levels of amino acids and short chain fatty acids, and decreased levels of bile acids and polyol alcohols in the gas phase of feces were observed repeatedly. All selected papers reported high diagnostic value for the detection of both CRC and AA based on fecal VOCs.

CONCLUSION

Based on the included studies, fecal VOC analyses seem promising for future screening of CRC and AA, with potentially improved test performances allowing for earlier detection of AA and CRC and consequently earlier initiation of treatment, possibly reducing morbidity and mortality rates next to lower rates of (unnecessary) colonoscopies.

Gut Microbial Transformation of the Dietary Imidazoquinoxaline Mutagen MelQx Reduces Its Cytotoxic and Mutagenic Potency

The diverse community of microbes present in the human gut has emerged as an important factor for cancer risk, potentially by altering exposure to chemical carcinogens. In the present study, human gut bacteria were tested for their capacity to transform the carcinogenic heterocyclic amine 2-Amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MelQx). Eubacterium hallii, Lactobacillus reuteri, and Lactobacillus rossiae were able to convert MelQx to a new microbial metabolite characterized on the basis of high-resolution mass spectrometry and NMR as 9-hydroxyl-2,7-dimethyl-7,9,10,11-tetrahydropyrimido[2′,1′:2,3]imidazo[4,5-f]quinoxaline (MelQx-M1), resulting from conjugation with activated glycerol. Acrolein derived from the decomposition of 3-hydroxypropionaldehyde, which is the product of bacterial glycerol/diol dehydratase activity, was identified as the active compound responsible for the formation of MelQx-M1. A complex human gut microbial community obtained from invitro continuous intestinal fermentation was found to also transform MelQx to MelQx-M1. MelQx-M1 had slightly reduced cytotoxic potency toward human colon epithelial cells invitro, and diminished mutagenic potential toward bacteria after metabolic activation. As bacterially derived acrolein also transformed 2 other HCAs, namely 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine and 2-amino-3-methylimidazo[4,5-f]quinoline, these results generalize the capacity of gut microbiota to detoxify HCAs in the gut, potentially modulating cancer risk.

The promiscuous binding of pharmaceutical drugs and their transporter-mediated uptake into cells: what we (need to) know and how we can do so

A recent paper in this journal sought to counter evidence for the role of transport proteins in effecting drug uptake into cells, and questions that transporters can recognize drug molecules in addition to their endogenous substrates. However, there is abundant evidence that both drugs and proteins are highly promiscuous. Most proteins bind to many drugs and most drugs bind to multiple proteins (on average more than six), including transporters (mutations in these can determine resistance); most drugs are known to recognise at least one transporter. In this response, we alert readers to the relevant evidence that exists or is required. This needs to be acquired in cells that contain the relevant proteins, and we highlight an experimental system for simultaneous genome-wide assessment of carrier-mediated uptake in a eukaryotic cell (yeast).

Dual functional characteristic of human aquaporin 10 for solute transport

BACKGROUND/AIMS

Although aquaglyceroporins have been generally believed to operate in a channel mode, which is of nonsaturable nature, for glycerol as well as for water, we recently found that human aquaporin 9 (hAQP9) operates in a carrier-mediated mode, which is of saturable nature, for glycerol. Based on the finding, we assumed that such a characteristic might be shared by the other aquaglyceroporins and examined the functional characteristics of hAQP10, which is an intestine-specific aquaglyceroporin.

METHODS

Transport assays were conducted using Xenopus laevis oocytes expressing hAQP10 derived from the microinjected cRNA.

RESULTS

The transport of glycerol by hAQP10 was found to be highly saturable with a Michaelis constant of 10.4 μM and specifically inhibited by several glycerol analogs such as monoacetin. Furthermore, when glycerol was preloaded in hAQP10-expressing oocytes, its efflux was trans-stimulated by extracellular glycerol. These results indicate the involvement of a carrier-mediated mechanism in glycerol transport by hAQP10. Interestingly, a channel mechanism was also found to be involved in part in hAQP10-mediated glycerol transport.

CONCLUSION

The present study unveiled the uniquely dual functional characteristic of hAQP10 as a carrier/channel for solute transport, providing a novel insight into its operation mechanism, which would help further elucidate its physiological role.

Functional characterization of three aquaglyceroporins from Trypanosoma brucei in osmoregulation and glycerol transport

Previous studies using bloodstream form Trypanosoma brucei have shown that glycerol transport in this parasite occurs via specific membrane proteins, namely a glycerol transporter and glycerol channels [1]. Later, we cloned, expressed and characterized the transport properties of all three aquaglyceroporins (AQP1-3) [2], which were found permeable for water, glycerol and other small uncharged solutes like dihydroxyacetone [3]. Here, we report on the cellular localization of TbAQP1 and TbAQP3 in bloodstream form trypanosomes. Indirect immunofluorescence analysis showed that TbAQP1 is exclusively localized in the flagellar membrane, whereas TbAQP3 was found in the plasma membrane.In addition, we analyzed the functions of all 3 AQPs, using an inducible inheritable double-stranded RNA interference methodology. All AQP knockdown cell lines were still able to survive hypo-osmotic stress conditions, except AQP2 knockdown parasites. Depleted TbAQP2 negatively impacted cell growth and the regulatory volume recovery, whereas AQP1 und 3 knockdown trypanosomes displayed phenotypes consistent with their localization in external membranes. A simultaneous knockdown of all 3 AQPs showed that the cells were able to substitute the missing glycerol uptake capability through a putative glycerol transporter.

Human faecal microbiota display variable patterns of glycerol metabolism

Significant amounts of glycerol reach the colon microbiota daily through the diet and/or by in situ microbial production or release from desquamated epithelial cells. Some gut microorganisms may anaerobically reduce glycerol to 1,3-propanediol (1,3-PDO), with 3-hydroxypropanal as an intermediate. Accumulation of the latter intermediate may result in the formation of reuterin, which is known for its biological activity (e.g. antimicrobial properties). To date, glycerol metabolism in mixed cultures from the human colon has received little attention. Using in vitro batch incubations of faeces from 10 human individuals, we demonstrated that glycerol addition (140 mM) significantly affects the metabolism and composition of the microbial community. About a third of the samples exhibited rapid glycerol conversion, yielding proportionally higher levels of acetate and 1,3-PDO. In contrast, a slower glycerol metabolism resulted in higher levels of propionate. Furthermore, rapid glycerol metabolism correlated with significant shifts in the Lactobacillus-Enterococcus community, which were not observed in slower glycerol-metabolizing samples. As the conversion of glycerol to 1,3-PDO is a highly reducing process, we infer that the glycerol metabolism may act as an effective hydrogen sink. Given the importance of hydrogen-consuming processes in the gut, this work suggests that glycerol may have potential as a tool for modulating fermentation kinetics and profiles in the gastrointestinal tract.

Carrier-mediated cellular uptake of pharmaceutical drugs: an exception or the rule?

It is generally thought that many drug molecules are transported across biological membranes via passive diffusion at a rate related to their lipophilicity. However, the types of biophysical forces involved in the interaction of drugs with lipid membranes are no different from those involved in their interaction with proteins, and so arguments based on lipophilicity could also be applied to drug uptake by membrane transporters or carriers. In this article, we discuss the evidence supporting the idea that rather than being an exception, carrier-mediated and active uptake of drugs may be more common than is usually assumed - including a summary of specific cases in which drugs are known to be taken up into cells via defined carriers - and consider the implications for drug discovery and development.

Isolation and characterization of human intestinal bacteria capable of transforming the dietary carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine

2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) is a carcinogenic heterocyclic aromatic amine formed in meat products during cooking. Although the formation of hazardous PhIP metabolites by mammalian enzymes has been extensively reported, research on the putative involvement of the human intestinal microbiota in PhIP metabolism remains scarce. In this study, the in vitro conversion of PhIP into its microbial derivate, 7-hydroxy-5-methyl-3-phenyl-6,7,8,9-tetrahydropyrido[3',2':4,5]imidazo[1,2-a]pyrimidin-5-ium chloride (PhIP-M1), by fecal samples from 18 human volunteers was investigated. High-performance liquid chromatography analysis showed that all human fecal samples transformed PhIP but with efficiencies ranging from 1.8 to 96% after 72 h of incubation. Two PhIP-transforming strains, PhIP-M1-a and PhIP-M1-b, were isolated from human feces and identified by fluorescent amplified fragment length polymorphism and pheS sequence analyses as Enterococcus faecium strains. Some strains from culture collections belonging to the species E. durans, E. avium, E. faecium, and Lactobacillus reuteri were also able to perform this transformation. Yeast extract, special peptone, and meat extract supported PhIP transformation by the enriched E. faecium strains, while tryptone, monomeric sugars, starch, and cellulose did not. Glycerol was identified as a fecal matrix constituent required for PhIP transformation. Abiotic synthesis of PhIP-M1 and quantification of the glycerol metabolite 3-hydroxypropionaldehyde (3-HPA) confirmed that the anaerobic fermentation of glycerol via 3-HPA is the critical bacterial transformation process responsible for the formation of PhIP-M1. Whether it is a detoxification is still a matter of debate, since PhIP-M1 has been shown to be cytotoxic toward Caco-2 cells but is not mutagenic in the Ames assay.

Enhanced Uptake of Glycerol by Butyrate Treatment in HCT-15 Human Colon Cancer Cell Line

The HCT-15 human colon cancer cell line has a Na(+)-dependent carrier-mediated transport system for the uptake of glycerol. A similar transport system has been suggested to be present also in the small intestine and is of interest with regard to its role in the absorption of glycerol and possibly some structurally related compounds. To help clarifying functional characteristics of such glycerol transport systems, we examined the effect of butyrate, an agent known to facilitate the differentiation of cells, on glycerol uptake in HCT-15 cells. The uptake of glycerol (0.4 microM) was found to be about 5-fold greater in HCT-15 cells pretreated with butyrate (2 mM) for 24 h than in those untreated. The increase in the uptake by the butyrate treatment was due to an increase in the maximum transport rate. The effect of butyrate was almost completely suppressed when actinomycin D, an inhibitor of gene transcription, and cycloheximide, an inhibitor of protein synthesis, were added to the medium during the butyrate treatment. These results support the suggestion that a specific carrier protein is involved in glycerol uptake by HCT-15 cells and the carrier protein is one of those inducible by butyrate-induced cell differentiation.