Expression and distribution of monocarboxylate transporter 1 (MCT1) in the gastrointestinal tract of calves

Lipoic acid-mediated oral drug delivery system utilizing changes on cell surface thiol expression for the treatment of diabetes and inflammatory diseases

Lipoic acid (LA), which has good safety and oral absorption, is obtained from various plant-based food sources and needs to be supplemented through human diet. Moreover, substances with a disulfide structure can enter cells through dynamic covalent disulfide exchange with thiol groups on the cell membrane surface. Based on these factors, we constructed LA-modified nanoparticles (LA NPs). Our results showed that LA NPs can be internalized into intestinal epithelial cells through surface thiols, followed by intracellular transcytosis via the endoplasmic reticulum-Golgi pathway. Further mechanistic studies indicated that disulfide bonds within the structure of LA play a critical role in this transport process. In a type I diabetes rat model, the oral administration of insulin-loaded LA NPs exhibited a more potent hypoglycemic effect, with a pharmacokinetic bioavailability of 5.42 ± 0.53%, representing a 1.6 fold enhancement compared to unmodified PEG NPs. Furthermore, a significant upregulation of surface thiols in inflammatory macrophages was reported. Thus, we turned our direction to investigate the uptake behavior of inflammatory macrophages with increased surface thiols towards LA NPs. Inflammatory macrophages showed a 2.6 fold increased uptake of LA NPs compared to non-inflammatory macrophages. Surprisingly, we also discovered that the antioxidant resveratrol facilitates the uptake of LA NPs in a concentration-dependent manner. This is mainly attributed to an increase in glutathione, which is involved in thiol uptake. Consequently, we employed LA NPs loaded with resveratrol for the treatment of colitis and observed a significant alleviation of colitis symptoms. These results suggest that leveraging the variations of thiol expression levels on cell surfaces under both healthy and diseased states through an oral drug delivery system mediated by the small-molecule nutrient LA can be employed for the treatment of diabetes and certain inflammatory diseases.

Sex-Related Differences in UT-B Urea Transporter Abundance in Fallow Deer Rumen

Rumen studies have focused almost exclusively on livestock species under strictly regimented diets. This means that the ruminal condition of free-living and free-feeding wildlife remains practically unstudied. Urea nitrogen salvaging, a process by which urea is passed into the rumen, to both provide a valuable source of nitrogen for bacterial growth and to buffer the potentially harmful acidic effects of bacterial short chain fatty acids, has remained unexplored in wild ruminants, such as deer. UT-B2 transporters are the key proteins reported to facilitate the transepithelial ruminal urea transport. In this study, we investigate the expression, abundance and localisation of urea transporters in the rumen of a semi-wild fallow deer (Dama dama) population. Physical measurements confirmed that males had larger rumen than females, while adults had longer papillae than juveniles. Initial RT-PCR experiments confirmed the expression of UT-B2, while immunolocalisation studies revealed that strong UT-B staining was present in the stratum basale of deer rumen. Western blotting analysis demonstrated that a 50 kDa UT-B2 protein was significantly more abundant in adult females compared to adult males. This study confirms the presence of UT-B2 urea transporters in deer rumen and suggests that sex-related differences occur, bringing new insight into our understanding of rumen physiology.

Canola meal or soybean meal as protein source and the effect of microencapsulated sodium butyrate supplementation in calf starter mixture. II. Development of the gastrointestinal tract

The aim of this study was to assess the effect of protein source, either soybean meal (SM) or canola meal (CM), and microencapsulated sodium butyrate (MSB) supplementation in a pelleted starter mixture on the development of the gastrointestinal tract (GIT) in dairy calves. Twenty-eight bull calves (8.7 ± 0.8 d of age and 43.0 ± 4.4 kg; mean ± SD) were assigned to 1 of 4 treatments in a 2 × 2 factorial arrangement: CM as a main source of protein without or with MSB or SM without or with MSB. Calves were fed starters ad libitum and exposed to a gradual weaning program, with weaning taking place on 51.7 ± 0.8 d of age. Calves were observed for an additional 3 wk after weaning and slaughtered on d 72.1 ± 0.9 of age, after which the GIT was dissected. Morphometric measurements were recorded, and samples for determination of ruminal fermentation, histology, gene expression, and brush border enzyme activities were collected. Canola meal use in the starter mixture increased abomasal tissue weight, jejunal tissue weight and length, and mRNA expression of SLC16A4 (formerly known as MCT4) and FFAR2 (GPR43) in the ruminal epithelium, and decreased ruminal ammonia and mRNA expression of SLC15A2 (PEPT2) and SLC6A14 (ATB0+) in the proximal small intestine and ileum, respectively. However, MSB inclusion in the starter mixture decreased ruminal papillae length, ruminal epithelial surface, and ruminal epithelium dry weight, while increasing mRNA expression of SLC16A1 (MCT1) in ruminal epithelia. Reduced ruminal surface area associated with MSB supplementation was the most apparent when MSB was combined with CM in the starter mixture. Additionally, MSB supplementation decreased the thickness of omasal epithelium, omasal epithelium living strata, and stratum corneum, and increased duodenal and ileal aminopeptidase A enzymatic activity and ileal aminopeptidase N enzymatic activity. Overall, CM might increase growth of the GIT of calves, particularly of the small intestine, but may negatively affect intestinal epithelium function and peptide and AA absorption. Supplementation of MSB has a negative effect on the ruminal and omasal epithelium development, particularly when combined in a starter mixture with CM.

MicroRNA Sequencing Reveals the Effect of Different Levels of Non-Fibrous Carbohydrate/Neutral Detergent Fiber on Rumen Development in Calves

Simple Summary

By histological sectioning and staining of rumen tissues from calves fed with a high or low ratio of non-fibrous carbohydrate/neutral detergent fiber diet, we found that the length and width of papillae were significantly affected by the ratio. From microRNA expression analysis we found cell proliferation, differentiation, physical and nutrient stimuli processes participate in the development of the rumen. In addition, bta-miR-128 was found to affect rumen development by negatively regulating PPARG and SLC16A1. Our findings provided an important resource for the continuing study of rumen development and absorption.

Abstract

Rumen development in calves is affected by many factors, including dietary composition. MicroRNAs (miRNAs) are known to function in the development of the rumen in cattle, what is not known is how these miRNAs function in rumen development of calves fed with high and low ratios of non-fibrous carbohydrate (NFC)/neutral detergent fiber (NDF). A total of six healthy Charolais hybrids bull calves of similar weight were divided into two groups; three calves were fed a mixed diet with NFC/NDF = 1.35 (H group), and three were fed a mixed diet with NFC/NDF = 0.80 (L group). After 105 days on the diet, calves were sacrificed and rumen tissues were collected. Tissues were subjected to histological observation and miRNA expression analysis. Functional enrichment analysis was conducted on the target genes of the miRNAs. Targeting and regulatory relationships were verified by luciferase reporter assay and quantitative PCR (qPCR). We found that the length of rumen papilla in the L group was significantly greater than that in the H group, while the width of rumen papilla in H group was significantly greater than that that in L group. We identified 896 miRNAs; 540 known miRNAs, and 356 novel predicted miRNAs. After statistical testing, we identified 24 differentially expressed miRNAs (DEmiRNAs). miRNA-mRNA-cluster network analysis and literature reviews revealed that cell proliferation, differentiation, physical and nutrient stimuli processes participate in rumen development under different NFC/NDF levels. The regulatory relationships between three DEmiRNAs and five target genes were verified by examining the levels of expression. The binding sites on bta-miR-128 for the peroxisome proliferator activated receptor gamma (PPARG) and solute carrier family 16 member 1 (SLC16A1) genes were investigated using a dual luciferase assay. The results of this study provide insight into the role of miRNAs in rumen development in calves under different NFC/NDF levels.

Form of calf diet and the rumen. II: Impact on volatile fatty acid absorption

Diet is known to affect rumen growth and development. Calves fed an all-liquid diet have smaller and less developed rumens and a decreased ability to absorb volatile fatty acids (VFA) compared to calves fed both liquid and dry feed. However, it is unknown how rumens respond when challenged with a defined concentration of VFA. The objective of this study was to assess the effects of 2 different feeding programs on VFA absorption in preweaned calves. Neonatal Holstein bull calves were individually housed and randomly assigned to 1 of 2 diets. The diets were milk replacer only (MRO; n = 5) or milk replacer with starter (MRS; n = 6). Diets were isoenergetic (3.87 ± 0.06 Mcal of metabolizable energy per day) and isonitrogenous (0.17 ± 0.003 kg/d of apparent digestible protein). Milk replacer was 22% crude protein, 21.5% fat (dry matter basis). The textured calf starter was 21.5% crude protein (dry matter basis). Feed and ad libitum water intakes were recorded daily. Calves were exposed to a defined concentration of VFA buffer (acetate 143 mM, propionate 100 mM, butyrate 40.5 mM) 6 h before euthanasia on d 43 ± 1. Rumen fluid samples were obtained every 15 to 30 min for 6 h to measure the rate of VFA absorption. Rumen tissues were obtained from the ventral sac region and processed for morphological and immunohistochemical analyses of the VFA transporters monocarboxylate transporter 1 (MCT1) and 4 (MCT4). Body growth did not differ between diets, but empty reticulorumens were heavier in MRS than MRO calves (0.67 vs. 0.39 ± 0.04 kg) and MRS calves had larger papillae areas (0.76 vs. 15 ± 0.08 mm²). We observed no differences between diets in terms of the abundance of MCT1 and MCT4 per unit area. These results indicate that the extrapolated increase in total abundance of MCT1 or MCT4 in MRS calves was not due to increased transporter density per unit area. Modeled VFA absorption metrics (flux, mmol/h, or 6 h absorbed VFA in mmol) were not different across diets. These results demonstrate that the form of calfhood diet, whether solely MR or MR and starter, does not alter VFA absorption capacity when the rumen is exposed to a defined concentration of VFA at 6 wk of age.

Propionate and butyrate induce gene expression of monocarboxylate transporter 4 and cluster of differentiation 147 in cultured rumen epithelial cells derived from preweaning dairy calves

Short-chain fatty acids (SCFAs) are the main source of energy for postweaning ruminants. The monocarboxylic acid transporters, MCT1 and MCT4, are thought to contribute to the absorption of SCFAs from the surface of the rumen following weaning. The present study measured changes in MCT1 and MCT4 expression in ruminal epithelial cells isolated from male preweaning (22 to 34 d old, n = 6) and postweaning (55 to 58 d old, n = 8) calves after euthanasia and sought to examine whether SCFAs stimulate the expression of these transporters. In the current study, cluster of differentiation 147 (CD147) gene expression in the rumen was also investigated since CD147 has been considered to act as ancillary protein for MCT1 and MCT4 to express their correct function. The gene expression levels of MCT1, MCT4, and CD147 in the rumen were found to be significantly higher in postweaning calves than in preweaning calves. Strong MCT1 immunoreactivity was detected in both the stratum basale (SB) and the stratum spinosum (SS) in postweaning ruminal epithelium. Expression of MCT1 in preweaning calves was localized to a specific region of the SB and of the SS. MCT4-immunopositive cells were detected in the stratum corneum (SC) of the ruminal epithelium in postweaning calves. However, only a low level of signal was detected in the SC of preweaning animals. Furthermore, in vitro experiments, ruminal epithelial cells were incubated for 24 h with acetate (0.04, 0.4, and 4 mM), propionate (0.2, 2, and 20 mM), butyrate (0.1, 1, and 10 mM), or β-hydroxybutyrate (BHBA; 0.1, 1, and 10 mM), respectively. Both propionate and butyrate induced an increase in the gene expression levels of MCT4 and CD147, but did not affect MCT1 gene expression. There are no significant effects of acetate and BHBA treatment on these gene expressions. Taken together, these results suggest that an increase in MCT4 and CD147 gene expression in the ruminal epithelium of postweaning calves is likely to be due to the effects of propionate and butyrate derived from a solid-based diet, which may contribute to ruminal development following weaning.

Expression of Monocarboxylate Transporter 1 in Oral and Ocular Canine Melanocytic Tumors

Solid tumors are composed of a heterogeneous population of cells surviving in various concentrations of oxygen. In a hypoxic environment, tumor cells generally up-regulate glycolysis and, therefore, generate more lactate that must be expelled from the cell through proton transporters to prevent intracellular acidosis. Monocarboxylate transporter 1 (MCT1) is a major proton transporter in mammalian cells that transports monocarboxylates, such as lactate and pyruvate, together with a proton across the plasma membrane. Melanocytic neoplasia occurs frequently in dogs, but the prognosis is highly site-dependent. In this study, 50 oral canine melanomas, which were subdivided into 3 histologic subtypes, and 17 ocular canine melanocytic neoplasms (14 melanocytomas and 3 melanomas) were used to examine and compare MCT1 expression. Immunohistochemistry using a polyclonal chicken anti-rat MCT1 antibody showed that most oral melanoma exhibited cell membrane staining, although there were no significant differences observed among the 3 histologic subtypes. In contrast, the majority of ocular melanocytic tumors were not immunoreactive. Additionally, we documented the presence of a 45-kDa band in cell membrane protein Western blots, and sequencing of a reverse transcriptase polymerase chain reaction band of expected size confirmed its identity as a partial canine MCT1 transcript in 3 oral tumors. Increased MCT1 expression in oral melanomas compared with ocular melanocytic tumors may reflect the very different biology between these tumors in dogs. These results are the first to document canine MCT1 expression in canine tumors and suggest that increased MCT1 expression may provide a potential therapeutic target for oral melanoma.

UT-B Urea Transporter Localization in the Bovine Gastrointestinal Tract

Facilitative UT-B urea transporters play an important role in the urea nitrogen salvaging process that occurs in the gastrointestinal tract of mammals, particularly ruminants. Gastrointestinal UT-B transporters have previously been reported in various ruminant species-including cow, sheep and goat. In this present study, UT-B transporter localization was investigated in tissues throughout the bovine gastrointestinal tract. RT-PCR analysis showed that UT-B2 was the predominant UT-B mRNA transcript expressed in dorsal, ventral and cranial ruminal sacs, while alternative UT-B transcripts were present in other gastrointestinal tissues. Immunoblotting analysis detected a strong, glycosylated ~50 kDa UT-B2 protein in all three ruminal sacs. Immunolocalization studies showed that UT-B2 protein was predominantly localized to the plasma membrane of cells in the stratum basale layer of all ruminal sac papillae. In contrast, other UT-B protein staining was detected in the basolateral membranes of the surface epithelial cells lining the abomasum, colon and rectum. Overall, these findings confirm that UT-B2 cellular localization is similar in all ruminal sacs and that other UT-B proteins are located in epithelial cells lining various tissues in the bovine gastrointestinal tract.

Differences in monocarboxylic acid transporter type 1 expression in rumen epithelium of newborn calves due to age and milk or milk replacer feeding

The aim of the present study was to investigate whether besides age and solid feed intake, monocarboxylic acid transporter type 1 (MCT1) expression in the rumen epithelium of calves is affected by liquid feed type [whole milk (WM) or milk replacer (MR)]. Thirty bull calves at the mean age of 5 days were randomly allocated to five experimental groups (six calves/group). Six calves were slaughtered immediately after allocation to the trial (5 days of life), eighteen calves were fed MR and slaughtered at week intervals (on 12, 19, 26 days of life respectively), and six calves were fed WM and slaughtered at the 26 days of life. MCT1 protein abundance and the MCT1 mRNA level were investigated in the dorsal and ventral sack of the rumen. Solid feed intake and short-chain fatty acids (SCFA) concentration in the rumen fluid increased linearly with calves' age. The amount of the MCT1 protein and mRNA in the dorsal sac of rumen as well as the amount of MCT1 protein in the cranial ventral sac of rumen also increased linearly with calves' age. Calves fed WM had greater solid feed intake in the last week of the study as compared to calves fed MR, but SCFA concentration in the rumen fluid was not different. MCT1 mRNA expression in the cranial dorsal sac of rumen and protein MCT1 expression in both dorsal and ventral cranial sack of the rumen were higher in calves fed WM as compared to calves fed MR. This study confirmed age-dependent changes of MCT1 expression in the rumen epithelium of newborn calves and showed that its expression might be affected by liquid feed type.

Epigenetic effects of dietary butyrate on hepatic histone acetylation and enzymes of biotransformation in chicken

The aim of the study was to investigate the in vivo epigenetic influences of dietary butyrate supplementation on the acetylation state of core histones and the activity of drug-metabolising microsomal cytochrome P450 (CYP) enzymes in the liver of broiler chickens in the starter period. One-day-old Ross 308 broilers were fed a starter diet without or with sodium butyrate (1.5 g/kg feed) for 21 days. After slaughtering, nucleus and microsome fractions were isolated from the exsanguinated liver by multi-step differential centrifugation. Histone acetylation level was detected from hepatocyte nuclei by Western blotting, while microsomal CYP activity was examined by specific enzyme assays. Hyperacetylation of hepatic histone H2A at lysine 5 was observed after butyrate supplementation, providing modifications in the epigenetic regulation of cell function. No significant changes could be found in the acetylation state of the other core histones at the acetylation sites examined. Furthermore, butyrate did not cause any changes in the drugmetabolising activity of hepatic microsomal CYP2H and CYP3A37 enzymes, which are mainly involved in the biotransformation of most xenobiotics in chicken. These data indicate that supplementation of the diet with butyrate probably does not have any pharmacokinetic interactions with simultaneously applied xenobiotics.

Post-natal changes in MCT1 expression in the forestomach of calves

The monocarboxylate transporter 1 (MCT1) has been demonstrated to be involved in the transfer of short-chain fatty acids (SCFA) and/or their intraepithelial metabolites from the rumen to the blood. As MCT1 plays a role in SCFA transfer, it is assumed that SCFA are the main substrates influencing its expression. However, there are hints that MCT1 may also be expressed during the early life of the animal when SCFA are not released in the forestomach. To figure out whether MCT1 expression in the forestomach is influenced independently of SCFA during that period, we studied post-natal MCT1 expression immunohistochemically in the epithelia of omasum, atrium ruminis, saccus dorsalis ruminis, saccus ventralis ruminis and reticulum of calves born preterm and at term. The calves were nourished by colostrum or by milk-based formula diet. MCT1 could be found in all the forestomach compartments tested, even in preterm calves. The protein was mainly oriented to the luminal side in the immature epithelium 24 h after birth. Orientation to the blood side of the cells developed during the first 4 days after birth. In the rumen epithelia (but not in the other forestomach compartments tested), orientation of MCT1 to the blood side of the cells was paralleled by an increase in the overall expression rate during the first 4 days after birth. As lactate levels were very high directly after birth, a lactate-dependent substrate induction may have been the underlying mechanism. However, non-specific changes due to general differential processes might also be the cause. Both early upregulation of MCT1 and high blood lactate levels may provide the epithelia with lactate as energy source.

Rumen epithelial adaptation to high-grain diets involves the coordinated regulation of genes involved in cholesterol homeostasis

The molecular mechanisms underlying rumen epithelial adaption to high-grain (HG) diets are unknown. To gain insight into the metabolic mechanisms governing epithelial adaptation, mature nonlactating dairy cattle (n = 4) were transitioned from a high-forage diet (HF, 0% grain) to an HG diet (65% grain). After the cattle were fed the HG diet for 3 wk, they returned to the original HF diet, which they were fed for an additional 3 wk. Continuous ruminal pH, ruminal short chain fatty acids, and plasma β-hydroxybutyrate were measured on a weekly basis, and rumen papillae were biopsied from the ventral sac to assess alterations in mRNA expression profiles. The subacute form of ruminal acidosis was diagnosed during the first week of the HG period (4.6 ± 1.6 h/day <pH 5.6), but not during weeks 2 and 3, thereby indicating ruminal adaption to the HG diet. Changes in the mRNA expression profile of rumen papillae were initially examined using Bovine Affymetrix microarrays; a total of 521 differentially expressed genes (false discovery rate P < 0.08) were uncovered from the first to third week of the HG period. Ingenuity Pathway Analysis of microarray results revealed that enzymes involved in cholesterol synthesis were coordinately downregulated from the first to third week of the HG period. In addition, the LXR/RXR activation pathway was significant and included several genes involved in intracellular cholesterol homeostasis. The differential expression signature of eight genes representing the key regulatory points of cholesterol homeostasis was confirmed by quantitative real-time PCR. Based upon our pathway and network results we propose a model to explain cellular events during rumen epithelial adaptation to HG diets and thus provide molecular targets that may be useful in the treatment and prevention of ruminal acidosis.

Short-chain fatty acid signaling pathways in bovine mammary epithelial cells

GPR41 and 43 have recently been identified as G-protein-coupled cell-surface receptors for short-chain fatty acids (SCFAs). Bovine orthologs of GPR41 and 43 (bGPR41 and 43) mRNA were detected by RT-PCR in cloned bovine mammary epithelial cells (bMEC) and various lactation stages of bovine mammary gland. Acetate and propionate caused an increase in intracellular Ca(2+) concentrations in these cells that was blocked by the treatment with pertussis toxin (PTX). SCFAs significantly reduced forskolin-induced cAMP concentrations in these cells. The phosphorylation of mitogen-activated protein kinase (MAPK) p38 was selectively increased by SCFAs. The downstream substrate heat shock protein 27 (HSP27) was also phosphorylated by SCFAs at Ser-78 and -82, but not -15. These results suggest that bGPR41 mainly, but not bGPR43, mediate SCFA signaling in mammary epithelial cells and thereby plays some important role in mammary gland.

The SLC16 monocaboxylate transporter family

1. The monocarboxylate transporter (MCT, SLC16) family comprises 14 members, of which to date only MCT1-4 have been shown to carry monocarboxylates, transporting important metabolic compounds such as lactate, pyruvate and ketone bodies in a proton-coupled manner. The transport of such compounds is fundamental for metabolism, and the tissue locations, properties and regulation of these isoforms is discussed. 2. Of the other members of the MCT family, MCT8 (a thyroid hormone transporter) and TAT1 (an aromatic amino acid transporter) have been characterized more recently, and their physiological roles are reviewed herein. The endogenous substrates and functions of the remaining members of the MCT family await elucidation. 3. The MCT proteins have the typical twelve transmembrane-spanning domain (TMD) topology of membrane transporter proteins, and their structure-function relationship is discussed, especially in relation to the future impact of the single nucleotide polymorphism (SNP) databases and, given their ability to transport pharmacologically relevant compounds, the potential impact for pharmacogenomics.

Expression of monocarboxylate transporter 1 (MCT1) in the dog intestine

In this study, the expression and distribution of monocarboxyolate transporter 1 (MCT1) along the intestines (duodenum, jejunum, ileum, cecum, colon and rectum) of dogs were investigated at both the mRNA and protein levels. The expression of MCT1 protein and its distribution were confirmed by Western blotting and immunohistochemical staining using the antibody for MCT1. We identified mRNA coding for MCT1 and a 43-kDa band of MCT1 protein in all regions from the duodenum to the rectum. Immunoreactive staining for MCT1 was also observed in epithelial cells throughout the intestines. MCT1 immunoreactivity was greater in the large intestine than in the small intestine. MCT1 protein was predominantly expressed on the basolateral membranes along intestinal epithelial cells, suggesting that MCT1 may play an important role in lactate efflux and transport of short-chain fatty acids (SCFAs) to the bloodstream across the basolateral membranes of the dog intestine.

Cellular expression of monocarboxylate transporters (MCT) in the digestive tract of the mouse, rat, and humans, with special reference to slc5a8

Short-chain fatty acids (SCFA) are monocarboxylates produced by bacterial fermentation that play a crucial role in maintaining homeostasis in the large intestine. Two major transporters for SCFA, monocarboxylate transporter (MCT) and slc5a8 (or SMCT), exist in the digestive tract. The present histochemical study using in situ hybridization and immunohistochemistry revealed the distribution and subcellular localization of the MCT family in the digestive tract of mice, rats, and humans, comparing these with that of slc5a8. The expression of mucosal MCT1 in the mouse and rat was most intense in the cecum, followed by the colon, but low in the stomach and small intestine. Among other MCT subtypes, only MCT2 was detected in the parietal cell region of the gastric mucosa. Slc5a8 had predominant expression sites in the distal half of the large bowel and in the most terminal ileum. The mucosal MCT1 was localized in the basolateral membrane of enterocytes, while slc5a8 was restricted to the apical cell membrane, suggesting the involvement of slc5a8 in the uptake of luminal SCFA, and of MCT1 in the efflux of SCFA and monocarboxylate metabolites towards blood circulation. The large intestine expressed both types of the transporter, but their distribution patterns differed along the longitudinal axis of the intestine and along the perpendicular axis of the mucosa.

Monocarboxylate transporter 1 (MCT1) in the liver of pre-ruminant and adult bovines

This study investigated the distribution and expression of monocarboxylate transporter 1 (MCT1) in the livers of pre-ruminant calves and adult bovines (bulls and cows), using different molecular biological techniques. Reverse transcription-polymerase chain reaction (RT-PCR) verified the presence of mRNA encoding for MCT1 in both pre-ruminant and adult bovine livers. Immunohistochemically, MCT1 was clearly demonstrated on the sinusoidal surfaces of bovine hepatocytes but its expression varied widely between pre-ruminants and adult bovines. In pre-ruminants, a faint hepatocellular expression of MCT1 was observed in a few hepatocytes, whereas an intense immunoreactive staining for MCT1 was shown in the majority of adult bovine hepatocytes. Western blot analysis also confirmed the results of the immunohistochemistry. Quantitative immunoblotting, as estimated by densitometric analysis, showed that the level of MCT1 in the liver of adult bovines was 8-9-fold greater (P<0.01) than that in pre-ruminant calf livers although no significant differences were detected between bulls and cows. The results demonstrated that MCT1 may play a crucial role in the transport of propionate in bovine liver, suggesting that MCT1 expression may be influenced by developmental and metabolic regulations.

Phenolic acids are absorbed from the rat stomach with different absorption rates

The intestinal absorption characteristics of phenolic acids (PAs) have been elucidated in terms of their affinity for the monocarboxylic acid transporter (MCT). Recently, the involvement of the stomach has been implicated in the absorption of polyphenols. The present work demonstrates that the gastric absorption efficiency of each PA is apparently different between various PAs. Various PAs with different affinities for MCT were administered (2.25 mumol) to rat stomach, and then the plasma concentration of the PA was measured. The plasma concentration of ferulic acid (FA) peaked 5 min after administration in the stomach. At 5 min after administration, the plasma concentration of each PA increased in the order: gallic acid = chlorogenic acid < caffeic acid < p-coumaric acid = FA. This order matches their respective affinity for MCT in Caco-2 cells, which we have demonstrated in previous studies. These results indicated that MCT might be involved in the gastric absorption of PAs, similar to the intestinal absorption.

Expression and localization of monocarboxylate transporters and sodium/proton exchangers in bovine rumen epithelium

Monocarboxylate-H+ cotransporters, such as monocarboxylate transporter (MCT) SLC16A, have been suggested to mediate transruminal fluxes of short-chain fatty acids, ketone bodies, and lactate. Using an RT-PCR approach, we demonstrate expression of MCT1 (SLC16A1) and MCT2 (SLC16A7) mRNA in isolated bovine rumen epithelium. cDNA sequence from these PCR products combined with overlapping expressed sequence tag data allowed compilation of the complete open reading frames for MCT1 and MCT2. Immunohistochemical localization of MCT1 shows plasma membrane staining in cells of the stratum basale, with intense staining of the basal aspects of the cells. Immunostaining decreased in the cell layers toward the rumen lumen, with weak staining in the stratum spinsoum. Immunostaining in the stratum granulosum and stratum corneum was essentially negative. Since monocarboxylate transport will load the cytosol with acid, expression and location of Na+/H+ exchanger (NHE) family members within the rumen epithelium were determined. RT-PCR demonstrates expression of multiple NHE family members, including NHE1, NHE2, NHE3, and NHE8. In contrast to MCT1, immunostaining showed that NHE1 was predominantly localized to the stratum granulosum, with a progressive decrease toward the stratum basale. NHE2 immunostaining was observed mainly at an intracellular location in the stratum basale, stratum spinosum, and stratum granulosum. Given the anatomic localization of MCT1, NHE1, and NHE2, the mechanism of transruminal short-chain fatty acid, ketone body, and lactate transfer is discussed in relation to a functional model of the rumen epithelium comprising an apical permeability barrier at the stratum granulosum, with a cell syncitium linking the stratum granulosum to the blood-facing stratum basale.

Monocarboxylate transporter 1 (MCT1) plays a direct role in short-chain fatty acids absorption in caprine rumen

Despite the importance of short-chain fatty acids (SCFA) in maintaining the ruminant physiology, the mechanism of SCFA absorption is still not fully studied. The goal of this study was to elucidate the possible involvement of monocarboxylate transporter 1 (MCT1) in the mechanism of SCFA transport in the caprine rumen, and to delineate the precise cellular localization and the level of MCT1 protein along the entire caprine gastrointestinal tract. RT-PCR revealed the presence of mRNA encoding for MCT1 in all regions of the caprine gastrointestinal tract. Quantitative Western blot analysis showed that the level of MCT1 protein was in the order of rumen >/= reticulum > omasum > caecum > proximal colon > distal colon > abomasum > small intestine. Immunohistochemistry and immunofluorescence confocal analyses revealed widespread immunoreactive positivities for MCT1 in the caprine stomach and large intestine. Amongst the stratified squamous epithelial cells of the forestomach, MCT1 was predominantly expressed on the cell boundaries of the stratum basale and stratum spinosum. Double-immunofluorescence confocal laser-scanning microscopy confirmed the co-localization of MCT1 with its ancillary protein, CD147 in the caprine gastrointestinal tract. In vivo and in vitro functional studies, under the influence of the MCT1 inhibitors, p-chloromercuribenzoate (pCMB) and p-chloromercuribenzoic acid (pCMBA), demonstrated significant inhibitory effect on acetate and propionate transport in the rumen. This study provides evidence, for the first time in ruminants, that MCT1 has a direct role in the transepithelial transport and efflux of the SCFA across the stratum spinosum and stratum basale of the forestomach toward the blood side.

Monocarboxylate transporter 1 (MCT1) mediates transport of short-chain fatty acids in bovine caecum

The present study was undertaken to investigate the functional role of monocarboxylate transporter 1 (MCT1) in the ruminant large intestine. Messenger RNA encoding for MCT1 was verified by reverse transcriptase-polymerase chain reaction in caecum, proximal colon and distal colon of adult cattle. Both immunohistochemistry and confocal laser microscopy verified that the MCT1 protein was abundant in the surface epithelium of the large intestine, and the amount decreased from the opening of the crypt to its base. In the immunopositive cells, MCT1 was primarily localized in the basolateral membranes of epithelium lining the large intestine. Western blotting indicated that the levels of MCT1 protein were highest in the caecum, followed by proximal colon and then distal colon. In vitro studies were conducted to elucidate the possible involvement of MCT1 in the transport of short-chain fatty acids (SCFA) across the isolated mucosal sheets of cattle caecum using the Ussing chamber technique. Acetate absorption was found to be pH dependent, and the rate of acetate absorption increased as pH decreased. The serosal application of the MCT1 inhibitor 'p-chloromercuribenzoic acid (pCMB)' significantly reduced the transport of acetate across the caecal epithelium of cows. In addition, the transport of acetate was significantly reduced in the presence of its analogue, propionate, indicating that acetate and propionate compete for binding to the same transporter. The results show that MCT1 is a major route for SCFA efflux across the basolateral membrane of bovine large intestine and that it could play a role in the regulation of intracellular pH.