SCFAs: The Enigma of Weak Electrolyte Transport in the Colon

Butyrate to combat obesity and obesity-associated metabolic disorders: Current status and future implications for therapeutic use

Evidence is increasing that disturbances in the gut microbiome may play a significant role in the etiology of obesity and type 2 diabetes. The short chain fatty acid butyrate, a major end product of the bacterial fermentation of indigestible carbohydrates, is reputed to have anti-inflammatory properties and positive effects on body weight control and insulin sensitivity. However, whether butyrate has therapeutic potential for the treatment and prevention of obesity and obesity-related complications remains to be elucidated. Overall, animal studies strongly indicate that butyrate administered via various routes (e.g., orally) positively affects adipose tissue metabolism and functioning, energy and substrate metabolism, systemic and tissue-specific inflammation, and insulin sensitivity and body weight control. A limited number of human studies demonstrated interindividual differences in clinical effectiveness suggesting that outcomes may depend on the metabolic, microbial, and lifestyle-related characteristics of the target population. Hence, despite abundant evidence from animal data, support of human data is urgently required for the implementation of evidence-based oral and gut-derived butyrate interventions. To increase the efficacy of butyrate-focused interventions, future research should investigate which factors impact treatment outcomes including baseline gut microbial activity and functionality, thereby optimizing targeted-interventions and identifying individuals that merit most from such interventions.

Sodium butyrate reduces endoplasmic reticulum stress by modulating CHOP and empowers favorable anti-inflammatory adipose tissue immune-metabolism in HFD fed mice model of obesity

Over the past decade, the gut microbiome has been linked to several diseases including gastrointestinal diseases, cancer, immune disorder and metabolic syndrome. Shifts in the gut bacterial population affect the overall metabolic health status leading towards obesity and Type II diabetes mellitus. Secondary metabolites secreted by the gut microbiome interact with various host-sensing signalling pathways and are responsible for functional modulation of immune resident cells in metabolic tissues (Blüher, 2019). Of these, short- chain fatty acids (SCFAs) i.e., acetate, propionate and butyrate have been significantly correlated with the disposition of diabetes and metabolic disorder. The altered gut microbial population depletes the intestinal barrier causing entry of LPS into circulation and towards metabolic tissues triggering pro-inflammatory responses. As butyrate has been known to maintain intestinal integrity, we aimed to assess the apparent effect of externally given sodium butyrate [NaB] on immuno-metabolic profiling of adipose tissue, and its association with metabolic and inflammatory status of adipose tissue. To assess this, we put groups of C57BL/6 mice i.e., Control fed with a regular chow diet and another group that was fed on a high fat diet (HFD, 60%) for 8 weeks. Following this, the HFD group were further subdivided into two groups one fed with HFD and the other with HFD + NaB (5%w/w) for another 8 weeks. Body composition, weight gain, body adiposity and biochemical parameters were assessed. NaB fed group showed an improved metabolic profile compared to HFD fed group. Administration of NaB also improved glucose tolerance capacity and insulin sensitivity as determined by IPGTT and ITT profiles. Earlier reports have shown gut leakage and increased LPS in circulation is the primary cause of setting up inflammation at the tissue level. Our studies exhibited that, NaB increased the expression of tight junction proteins of intestinal linings and thereby enhanced intestinal barrier integrity. The FITC dextran permeability assay further confirmed this enhanced intestinal barrier integrity. We assessed the quantitative and relative population of different types of resident immune cells from a stromal vascular fraction of adipose tissue. Flow cytometry studies revealed significantly increased M2 (CD206+ ) macrophages and Tregs (CD25+ ) relative to the M1 macrophage population and CD4+ T cells respectively in NaB treated mice, suggesting its potential role in alleviating the inflammatory profile. In a nutshell, taken together better glucose tolerance, better gut health, reduced inflammatory adipose tissue immune cells, suggest potential beneficial role of sodium butyrate in alleviating overall inflammation and metabolic dysfunction associated with obesity.

Microbial short-chain fatty acids: a bridge between dietary fibers and poultry gut health

The maintenance of poultry gut health is complex depending on the intricate balance among diet, the commensal microbiota, and the mucosa, including the gut epithelium and the superimposing mucus layer. Changes in microflora composition and abundance can confer beneficial or detrimental effects on fowl. Antibiotics have devastating impacts on altering the landscape of gut microbiota, which further leads to antibiotic resistance or spread the pathogenic populations. By eliciting the landscape of gut microbiota, strategies should be made to break down the regulatory signals of pathogenic bacteria. The optional strategy of conferring dietary fibers (DFs) can be used to counterbalance the gut microbiota. DFs are the non-starch carbohydrates indigestible by host endogenous enzymes but can be fermented by symbiotic microbiota to produce short-chain fatty acids (SCFAs). This is one of the primary modes through which the gut microbiota interacts and communicate with the host. The majority of SCFAs are produced in the large intestine (particularly in the caecum), where they are taken up by the enterocytes or transported through portal vein circulation into the bloodstream. Recent shreds of evidence have elucidated that SCFAs affect the gut and modulate the tissues and organs either by activating G-protein-coupled receptors or affecting epigenetic modifications in the genome through inducing histone acetylase activities and inhibiting histone deacetylases. Thus, in this way, SCFAs vastly influence poultry health by promoting energy regulation, mucosal integrity, immune homeostasis, and immune maturation. In this review article, we will focus on DFs, which directly interact with gut microbes and lead to the production of SCFAs. Further, we will discuss the current molecular mechanisms of how SCFAs are generated, transported, and modulated the pro-and anti-inflammatory immune responses against pathogens and host physiology and gut health.

Gut-microbiota derived bioactive metabolites and their functions in host physiology

Every individual harbours a complex, diverse and mutualistic microbial flora in their intestine and over the time it became an integral part of the body, affecting a plethora of activities of the host. Interaction between host and gut-microbiota affects several aspects of host physiology. Gut-microbiota affects host metabolism by fermenting unabsorbed/undigested carbohydrates in the large intestine. Not only the metabolic functions, any disturbances in the composition of the gut-microbiota during first 2-3 years of life may impact on the brain development and later affects cognition and behaviour. Thus, gut-dysbiosis causes certain serious pathological conditions in the host including metabolic disorders, inflammatory bowel disease and mood alterations, etc. Microbial-metabolites in recent times have emerged as key mediators and are responsible for microbiota induced beneficial effects on host. This review provides an overview of the mechanism of microbial-metabolite production, their respective physiological functions and the impact of gut-microbiome in health and diseases. Metabolites from dietary fibres, aromatic amino acids such as tryptophan, primary bile acids and others are the potential substances and link microbiota to host physiology. Many of these metabolites act as signalling molecules to a number of cells types and also help in the secretion of hormones. Moreover, interaction of microbiota derived metabolites with their host, immunity boosting mechanisms, protection against pathogens and modulation of metabolism is also highlighted here. Understanding all these functional attributes of metabolites produced from gut-microbiota may lead to the opening of a new avenue for preventing and developing potent therapies against several diseases.

Participation of Short-Chain Fatty Acids and Their Receptors in Gut Inflammation and Colon Cancer

Short-chain fatty acids (SCFAs) are the main metabolites produced by the bacterial fermentation of dietary fiber, and they play a critical role in the maintenance of intestinal health. SCFAs are also essential for modulating different processes, and they have anti-inflammatory properties and immunomodulatory effects. As the inflammatory process predisposes the development of cancer and promotes all stages of tumorigenesis, an antitumor effect has also been associated with SCFAs. This is strongly supported by epidemiological studies showing that a diet rich in fiber is linked to a reduced risk of colon cancer and has significant clinical benefits in patients with inflammatory bowel disease (IBD). SCFAs may signal through the metabolite-sensing G protein-coupled receptors free fatty acid receptor 3 [FFAR3 or G protein-coupled receptor 41 (GPR41)], FFAR2 (GPR43), and GPR109A (also known as hydroxycarboxylic acid receptor 2 or HCAR2) expressed in the gut epithelium and immune cells. This review summarizes the existing knowledge regarding the SCFA-mediated suppression of inflammation and carcinogenesis in IBD and colon cancer.

Dietary metabolism, the gut microbiome, and heart failure

Advances in our understanding of how the gut microbiota contributes to human health and diseases have expanded our insight into how microbial composition and function affect the human host. Heart failure is associated with splanchnic circulation congestion, leading to bowel wall oedema and impaired intestinal barrier function. This situation is thought to heighten the overall inflammatory state via increased bacterial translocation and the presence of bacterial products in the systemic blood circulation. Several metabolites produced by gut microorganisms from dietary metabolism have been linked to pathologies such as atherosclerosis, hypertension, heart failure, chronic kidney disease, obesity, and type 2 diabetes mellitus. These findings suggest that the gut microbiome functions like an endocrine organ by generating bioactive metabolites that can directly or indirectly affect host physiology. In this Review, we discuss several newly discovered gut microbial metabolic pathways, including the production of trimethylamine and trimethylamine N-oxide, short-chain fatty acids, and secondary bile acids, that seem to participate in the development and progression of cardiovascular diseases, including heart failure. We also discuss the gut microbiome as a novel therapeutic target for the treatment of cardiovascular disease, and potential strategies for targeting intestinal microbial processes.

Metabiotics: One Step ahead of Probiotics; an Insight into Mechanisms Involved in Anticancerous Effect in Colorectal Cancer

Colorectal cancer is closely associated with environment, diet and lifestyle. Normally it is treated with surgery, radiotherapy or chemotherapy but increasing systemic toxicity, resistance and recurrence is prompting scientists to devise new potent and safer alternate prophylactic or therapeutic strategies. Among these, probiotics, prebiotics, synbiotics, and metabiotics are being considered as the promising candidates. Metabiotics or probiotic derived factors can optimize various physiological functions of the host and offer an additional advantage to be utilized even in immunosuppressed individuals. Interestingly, anti colon cancer potential of probiotic strains has been attributable to metabiotics that have epigenetic, antimutagenic, immunomodulatory, apoptotic, and antimetastatic effects. Thus, it's time to move one step further to utilize metabiotics more smartly by avoiding the risks associated with probiotics even in certain normal/or immuno compromised host. Here, an attempt is made to provide insight into the adverse effects associated with probiotics and beneficial aspects of metabiotics with main emphasis on the modulatory mechanisms involved in colon cancer.

The short-chain fatty acid uptake fluxes by mice on a guar gum supplemented diet associate with amelioration of major biomarkers of the metabolic syndrome

Studies with dietary supplementation of various types of fibers have shown beneficial effects on symptoms of the metabolic syndrome. Short-chain fatty acids (SCFAs), the main products of intestinal bacterial fermentation of dietary fiber, have been suggested to play a key role. Whether the concentration of SCFAs or their metabolism drives these beneficial effects is not yet clear. In this study we investigated the SCFA concentrations and in vivo host uptake fluxes in the absence or presence of the dietary fiber guar gum. C57Bl/6J mice were fed a high-fat diet supplemented with 0%, 5%, 7.5% or 10% of the fiber guar gum. To determine the effect on SCFA metabolism, ¹³C-labeled acetate, propionate or butyrate were infused into the cecum of mice for 6 h and the isotopic enrichment of cecal SCFAs was measured. The in vivo production, uptake and bacterial interconversion of acetate, propionate and butyrate were calculated by combining the data from the three infusion experiments in a single steady-state isotope model. Guar gum treatment decreased markers of the metabolic syndrome (body weight, adipose weight, triglycerides, glucose and insulin levels and HOMA-IR) in a dose-dependent manner. In addition, hepatic mRNA expression of genes involved in gluconeogenesis and fatty acid synthesis decreased dose-dependently by guar gum treatment. Cecal SCFA concentrations were increased compared to the control group, but no differences were observed between the different guar gum doses. Thus, no significant correlation was found between cecal SCFA concentrations and metabolic markers. In contrast, in vivo SCFA uptake fluxes by the host correlated linearly with metabolic markers. We argue that in vivo SCFA fluxes, and not concentrations, govern the protection from the metabolic syndrome by dietary fibers.

The effects of propionate and valerate on insulin responsiveness for glucose uptake in 3T3-L1 adipocytes and C2C12 myotubes via G protein-coupled receptor 41

Since insulin resistance can lead to hyperglycemia, improving glucose uptake into target tissues is critical for regulating blood glucose levels. Among the free fatty acid receptor (FFAR) family of G protein-coupled receptors, GPR41 is known to be the Gα_i/o-coupled receptor for short-chain fatty acids (SCFAs) such as propionic acid (C3) and valeric acid (C5). This study aimed to investigate the role of GPR41 in modulating basal and insulin-stimulated glucose uptake in insulin-sensitive cells including adipocytes and skeletal muscle cells. Expression of GPR41 mRNA and protein was increased with maximal expression at differentiation day 8 for 3T3-L1 adipocytes and day 6 for C2C12 myotubes. GPR41 protein was also expressed in adipose tissues and skeletal muscle. After analyzing dose-response relationship, 300 µM propionic acid or 500 µM valeric acid for 30 min incubation was used for the measurement of glucose uptake. Both propionic acid and valeric acid increased insulin-stimulated glucose uptake in 3T3-L1 adipocyte, which did not occur in cells transfected with siRNA for GPR41 (siGPR41). In C2C12 myotubes, these SCFAs increased basal glucose uptake, but did not potentiate insulin-stimulated glucose uptake, and siGPR41 treatment reduced valerate-stimulated basal glucose uptake. Therefore, these findings indicate that GPR41 plays a role in insulin responsiveness enhanced by both propionic and valeric acids on glucose uptake in 3T3-L1 adipocytes and C2C12 myotubes, and in valerate-induced increase in basal glucose uptake in C2C12 myotubes.

β-Hydroxybutyrate modulates N-type calcium channels in rat sympathetic neurons by acting as an agonist for the G-protein-coupled receptor FFA3

Free fatty acids receptor 3 (FFA3, GPR41) and 2 (FFA2, GPR43), for which the short-chain fatty acids (SCFAs) acetate and propionate are agonist, have emerged as important G-protein-coupled receptors influenced by diet and gut flora composition. A recent study (Kimura et al., 2011) demonstrated functional expression of FFA3 in the rodent sympathetic nervous system (SNS) providing a potential link between nutritional status and autonomic function. However, little is known of the source of endogenous ligands, signaling pathways, or effectors in sympathetic neurons. In this study, we found that FFA3 and FFA2 are unevenly expressed in the rat SNS with higher transcript levels in prevertebral (e.g., celiac-superior mesenteric and major pelvic) versus paravertebral (e.g., superior cervical and stellate) ganglia. FFA3, whether heterologously or natively expressed, coupled via PTX-sensitive G-proteins to produce voltage-dependent inhibition of N-type Ca(2+) channels (Cav2.2) in sympathetic neurons. In addition to acetate and propionate, we show that β-hydroxybutyrate (BHB), a metabolite produced during ketogenic conditions, is also an FFA3 agonist. This contrasts with previous interpretations of BHB as an antagonist at FFA3. Together, these results indicate that endogenous BHB levels, especially when elevated under certain conditions, such as starvation, diabetic ketoacidosis, and ketogenic diets, play a potentially important role in regulating the activity of the SNS through FFA3.

Epithelia of the ovine and bovine forestomach express basolateral maxi-anion channels permeable to the anions of short-chain fatty acids

It has long been established that the absorption of short-chain fatty acids (SCFA) across epithelia stimulates sodium proton exchange. The apically released protons are not available as countercations for the basolateral efflux of SCFA anions and a suitable transport model is lacking. Patch clamp and microelectrode techniques were used to characterize an anion conductance expressed by cultured cells of the sheep and bovine rumen and the sheep omasum and to localize the conductance in the intact tissue. Cells were filled with a Na-gluconate solution and superfused with sodium salts of acetate, propionate, butyrate, or lactate. Reversal potential rose and whole cell current at +100 mV decreased with the size of the anion. Anion-induced currents could be blocked by diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS), NPPB (200 μmol l(-1)), or pCMB (1 mmol l(-1)). In patches of bovine ruminal cells, single channels were observed with a conductance for chloride (327 ± 11 pS), acetate (115 ± 8 pS), propionate (102 ± 10 pS), butyrate (81 ± 2 pS), and gluconate (44 ± 3 pS). Channels expressed by sheep rumen and omasum were similar. Microelectrode experiments suggest basolateral localization. In conclusion, forestomach epithelia express basolateral maxi-anion channels with a permeability sequence of chloride > acetate > propionate > butyrate. SCFA absorption may resemble functionally coupled transport of NaCl, with the Na(+)/K(+)-ATPase driving the basolateral efflux of the anion through a channel. Since protons are apically extruded, the model accurately predicts that influx of buffers with saliva is essential for the pH homeostasis of the ruminant forestomach.

The role of short-chain fatty acids in the interplay between diet, gut microbiota, and host energy metabolism

Short-chain fatty acids (SCFAs), the end products of fermentation of dietary fibers by the anaerobic intestinal microbiota, have been shown to exert multiple beneficial effects on mammalian energy metabolism. The mechanisms underlying these effects are the subject of intensive research and encompass the complex interplay between diet, gut microbiota, and host energy metabolism. This review summarizes the role of SCFAs in host energy metabolism, starting from the production by the gut microbiota to the uptake by the host and ending with the effects on host metabolism. There are interesting leads on the underlying molecular mechanisms, but there are also many apparently contradictory results. A coherent understanding of the multilevel network in which SCFAs exert their effects is hampered by the lack of quantitative data on actual fluxes of SCFAs and metabolic processes regulated by SCFAs. In this review we address questions that, when answered, will bring us a great step forward in elucidating the role of SCFAs in mammalian energy metabolism.

Short-chain fatty acids induce pro-inflammatory cytokine production alone and in combination with toll-like receptor ligands

PROBLEM

Short-chain fatty acids (SCFAs), produced at relatively high levels by anaerobic bacteria in bacterial vaginosis (BV), are believed to be anti-inflammatory. BV, a common alteration in the genital microbiota associated with increased susceptibility to HIV infection, is characterized by increased levels of both pro-inflammatory cytokines and SCFAs. We investigated how SCFAs alone or together with Toll-like receptor (TLR) ligands affected pro-inflammatory cytokine secretion.

METHOD OF STUDY

Cytokines were measured by ELISA. Flow was used for phenotyping and reactive oxygen species (ROS) measurement.

RESULTS

Short-chain fatty acids, at 20 mM, induced interleukin (IL)-8, IL-6, and IL-1β release, while lower levels (0.02-2 mM) did not induce cytokine secretion. Levels >20 mM were toxic to cells. Interestingly, lower levels of SCFAs significantly enhanced TLR2 ligand- and TLR7 ligand-induced production of IL-8 and TNFα in a time- and dose-dependent manner, but had little effect on lipopolysaccharide-induced cytokine release. SCFAs mediated their effects on pro-inflammatory cytokine production at least in part by inducing the generation of ROS.

CONCLUSION

Our data suggest that SCFAs, especially when combined with specific TLR ligands, contribute to a pro-inflammatory milieu in the lower genital tract and help further our understanding of how BV affects susceptibility to microbial infections.

Sodium-coupled transport of the short chain fatty acid butyrate by SLC5A8 and its relevance to colon cancer

INTRODUCTION

SLC5A8, expressed predominantly in the colon, is a Na(+)-coupled transporter for short-chain fatty acids. In this paper, we report on the characterization of butyrate transport by SLC5A8 and the relevance of SLC5A8-mediated butyrate transport to colon cancer.

RESULTS

SLC5A8 transports butyrate via a Na(+)-dependent electrogenic process. Na(+) activation of the transport process exhibits sigmoidal kinetics, indicating involvement of more than one Na(+) in the activation process. SLC5A8 is silenced in colon cancer in humans, in a mouse model of intestinal/colon cancer, and in colon cancer cell lines. The tumor-associated silencing of SLC5A8 involves DNA methylation by DNA methyltransferase 1. Reexpression of SLC5A8 in colon cancer cells leads to apoptosis but only in the presence of butyrate. SLC5A8-mediated entry of butyrate into cancer cells is associated with inhibition of histone deacetylation. The changes in gene expression in SLC5A8/butyrate-induced apoptosis include upregulation of pro-apoptotic genes and downregulation of anti-apoptotic genes. In addition, the expression of phosphatidylinositol-3-kinase subunits is affected differentially, with downregulation of p85alpha and upregulation of p55alpha and p50alpha.

CONCLUSION

These studies show that SLC5A8 mediates the tumor-suppressive effects of the bacterial fermentation product butyrate in the colon.

Organic anions and potassium salts in nutrition and metabolism

The present review examines the importance of dietary organic anions in preventive nutrition. Organic anions are chiefly supplied by plant foods, as partially neutralised K salts such as potassium citrate, potassium malate and, to a lesser extent, oxalate or tartrate salts. Animal products may also supply K anions, essentially as phosphate, but also as lactate as a result of fermentative or maturation processes, but these K salts have little alkalinising significance. Citrate and malate anions are absorbed in the upper digestive tract, while a substantial proportion is probably metabolised in the splanchnic area. Whatever their site of metabolism, these anions finally yield KHCO3which is used by the kidneys to neutralise fixed acidity. This acidity essentially reflects the oxidation of excess S amino acids to sulfate ions, which is mainly related to the dietary protein level. Failure to neutralise acidity leads to low-grade metabolic acidosis, with possible long-term deleterious effects on bone Ca status and on protein status. Furthermore, low-grade acidosis is liable to affect other metabolic processes, such as peroxidation of biological structures. These metabolic disturbances could be connected with the relatively high incidence of osteoporosis and muscle-protein wasting problems observed in ageing individuals in Europe and Northern America. Providing a sufficient supply of K organic anions through fruit and vegetable intake should be recommended, fostering the actual motivational campaigns ('five (or ten) per d') already launched to promote the intake of plant foods rich in complex carbohydrates and various micronutrients.

Acetylation of the Entamoeba histone H4 N-terminal domain is influenced by short-chain fatty acids that enter trophozoites in a pH-dependent manner

Treatment of higher eukaryotic cells with short-chain fatty acids (SCFA) such as butyrate causes decreased levels of histone deacetylase (HDAC) activity and hyperacetylation of histones, and thereby affects gene expression, cell growth and differentiation. Entamoeba parasites encounter high levels of SCFA in the host colon, and in vitro these compounds allow trophozoite stage parasites to multiply but prevent their differentiation into infectious cysts. The Entamoeba invadens IP-1 histone H4 protein has an unusual number of lysines in its N-terminus, and these become hyperacetylated in trophozoites exposed to the HDAC inhibitors trichostatin A (TSA) or HC-toxin, but not in trophozoites exposed to butyrate. We have now found that several other commonly studied isolates of Entamoeba parasites also have an extended set of histone H4 acetylation sites that become hyperacetylated in response to TSA, but hypoacetylated in response to butyrate, suggesting an unusual sensitivity of this parasite's histone modifying enzymes to SCFA. Butyrate was found to enter trophozoites in a pH-dependent manner consistent with diffusive entry of the un-ionised form of the fatty acid into the amoebae. Transit of the Entamoeba organism through areas of the host intestine with distinct pH and SCFA concentrations would therefore result in very different levels of SCFA within the parasite. Entamoeba appears to have acquired unique alterations of its histone acetylation mechanism that may allow for its growth in the presence of varying amounts of the bacterial fermentation products.

Predicting pharmacokinetic food effects using biorelevant solubility media and physiologically based modelling

BACKGROUND

Food-induced changes in gastric emptying time, gastric pH and/or intestinal fluid composition may have an impact on the pharmacokinetics of drugs. The aim of this work was to use mathematical models describing physiology in fed and fasted states together with biorelevant solubility and degradation data to simulate food effects for six compounds from recent Roche projects.

METHODS

The solubility of each compound was measured in different biorelevant media: simulated human gastric fluid for the fasted and fed state, simulated human intestinal fluid for the fasted, fed and high-fat state, and simulated human colonic fluid for the upper and the lower colon. A physiologically based absorption model was developed in GastroPlustrade mark for each compound using permeability, solubility, metabolism and distribution data. By incorporating the appropriate physiological parameters and solubility data into the model, the oral pharmacokinetics of each drug was simulated under fasted, fed and/or high-fat conditions. Predicted and observed plasma concentration-time profiles and food effects were compared for a range of doses to assess the accuracy of the simulations.

RESULTS

The models were able to distinguish between minor and significant food effects. The simulation captured well the magnitude of the food effects and for the six compounds correctly predicted the observed plasma exposure in fasted, fed and high-fat conditions.

CONCLUSION

Biorelevant solubility tests can be used together with physiologically based absorption models to predict clinical food effects caused by solubility and/or dissolution rate limitations.

Intestinal sugar transport

Carbohydrates are an important component of the diet. The carbohydrates that we ingest range from simple monosaccharides (glucose, fructose and galactose) to disaccharides (lactose, sucrose) to complex polysaccharides. Most carbohydrates are digested by salivary and pancreatic amylases, and are further broken down into monosaccharides by enzymes in the brush border membrane (BBM) of enterocytes. For example, lactase-phloridzin hydrolase and sucrase-isomaltase are two disaccharidases involved in the hydrolysis of nutritionally important disaccharides. Once monosaccharides are presented to the BBM, mature enterocytes expressing nutrient transporters transport the sugars into the enterocytes. This paper reviews the early studies that contributed to the development of a working model of intestinal sugar transport, and details the recent advances made in understanding the process by which sugars are absorbed in the intestine.

The human tumour suppressor gene SLC5A8 expresses a Na+-monocarboxylate cotransporter

The orphan cotransport protein expressed by the SLC5A8 gene has been shown to play a role in controlling the growth of colon cancers, and the silencing of this gene is a common and early event in human colon neoplasia. We expressed this protein in Xenopus laevis oocytes and have found that it transports small monocarboxylic acids. The electrogenic activity of the cotransporter, which we have named SMCT (sodium monocarboxylate transporter), was dependent on external Na(+) and was compatible with a 3 : 1 stoichiometry between Na(+) and monocarboxylates. A portion of the SMCT-mediated current was also Cl(-) dependent, but Cl(-) was not cotransported. SMCT transports a variety of monocarboxylates (similar to unrelated monocarboxylate transport proteins) and most transported monocarboxylates demonstrated K(m) values near 100 microm, apart from acetate and d-lactate, for which the protein showed less affinity. SMCT was strongly inhibited by 1 mm probenecid or ibuprofen. In the absence of external substrate, a Na(+)-independent leak current was also observed to pass through SMCT. SMCT activity was strongly inhibited after prolonged exposure to high external concentrations of monocarboxylates. The transport of monocarboxylates in anionic form was confirmed by the observation of a concomitant alkalinization of the cytosol. SMCT, being expressed in colon and kidney, represents a novel means by which Na(+), short-chain fatty acids and other monocarboxylates are transported in these tissues. The significance of a Na(+)-monocarboxylate transporter to colon cancer presumably stems from the transport of butyrate, which is well known for having anti-proliferative and apoptosis-inducing activity in colon epithelial cells.