Cytokine regulation of OCTN2 expression and activity in small and large intestine

Beyond nanoparticle-based oral drug delivery: transporter-mediated absorption and disease targeting

Various strategies at the microscale/nanoscale have been developed to improve oral absorption of therapeutics. Among them, gastrointestinal (GI)-transporter/receptor-mediated nanosized drug delivery systems (NDDSs) have drawn attention due to their many benefits, such as improved water solubility, improved chemical/physical stability, improved oral absorption, and improved targetability of their payloads. Their therapeutic potential in disease animal models (e.g., solid tumors, virus-infected lungs, metastasis, diabetes, and so on) has been investigated, and could be expanded to disease targeting after systemic/lymphatic circulation, although the detailed paths and mechanisms of endocytosis, endosomal escape, intracellular trafficking, and exocytosis through the epithelial cell lining in the GI tract are still unclear. Thus, this review summarizes and discusses potential GI transporters/receptors, their absorption and distribution, in vivo studies, and potential sequential targeting (e.g., oral absorption and disease targeting in organs/tissues).

Inflammation and Organic Cation Transporters Novel (OCTNs)

Inflammation is a physiological condition characterized by a complex interplay between different cells handled by metabolites and specific inflammatory-related molecules. In some pathological situations, inflammation persists underlying and worsening the pathological state. Over the years, two membrane transporters namely OCTN1 (SLC22A4) and OCTN2 (SLC22A5) have been shown to play specific roles in inflammation. These transporters form the OCTN subfamily within the larger SLC22 family. The link between these proteins and inflammation has been proposed based on their link to some chronic inflammatory diseases such as asthma, Crohn's disease (CD), and rheumatoid arthritis (RA). Moreover, the two transporters show the ability to mediate the transport of several compounds including carnitine, carnitine derivatives, acetylcholine, ergothioneine, and gut microbiota by-products, which have been specifically associated with inflammation for their anti- or proinflammatory action. Therefore, the absorption and distribution of these molecules rely on the presence of OCTN1 and OCTN2, whose expression is modulated by inflammatory cytokines and transcription factors typically activated by inflammation. In the present review, we wish to provide a state of the art on OCTN1 and OCTN2 transport function and regulation in relationships with inflammation and inflammatory diseases focusing on the metabolic signature collected in different body districts and gene polymorphisms related to inflammatory diseases.

PGC-1α and MEF2 Regulate the Transcription of the Carnitine Transporter OCTN2 Gene in C2C12 Cells and in Mouse Skeletal Muscle

OCTN2 (SLC22A5) is a carnitine transporter whose main function is the active transport of carnitine into cells. In skeletal muscle and other organs, the regulation of the SLC22A5 gene transcription has been shown to depend on the nuclear transcription factor PPAR-α. Due to the observation that the muscle OCTN2 mRNA level is maintained in PPAR-α knock-out mice and that PGC-1α overexpression in C2C12 myoblasts increases OCTN2 mRNA expression, we suspected additional regulatory pathways for SLC22A5 gene transcription. Indeed, we detected several binding sites of the myocyte-enhancing factor MEF2 in the upstream region of the SLC22A5 gene, and MEF2C/MEF2D stimulated the activity of the OCTN2 promoter in gene reporter assays. This stimulation was increased by PGC-1α and was blunted for a SLC22A5 promoter fragment with a mutated MEF2 binding site. Further, we demonstrated the specific binding of MEF2 to the SLC22A5 gene promoter, and a supershift of the MEF2/DNA complex in electrophoretic mobility shift assays. In immunoprecipitation experiments, we could demonstrate the interaction between PGC-1α and MEF2. In addition, SB203580, a specific inhibitor of p38 MAPK, blocked and interferon-γ stimulated the transcriptional activity of the SLC22A5 gene promoter. Finally, mice with muscle-specific overexpression of OCTN2 showed an increase in OCTN2 mRNA and protein expression in skeletal muscle. In conclusion, we detected and characterized a second stimulatory pathway of SLC22A5 gene transcription in skeletal muscle, which involves the nuclear transcription factor MEF2 and co-stimulation by PGC-1α and which is controlled by the p38 MAPK signaling cascade.

Pharmacogenetic Variation and Its Clinical Relevance in a Latin American Rural Population

Latin-American populations have been largely underrepresented in genomic studies of drug response and disease susceptibility. In this paper, we present a genome-wide Chilean dataset from Talca based on the Illumina Global Screening Array. This let us to compare the frequency of gene variants involved in response to drugs among our population and others, taking data from the 1000 Genomes Project. We found four single-nucleotide polymorphisms with low prevalence in Chileans when compared with African, Amerindian, East and South Asian, and European populations: rs2819742 (RYR2), rs2631367 (SLC22A5), rs1063320 (HLA-G), and rs1042522 (TP53). Moreover, two markers showed significant differences between lower and higher proportion of Mapuche ancestry groups: rs1719247 (located in an intergenic region in chromosome 15; p-value = 6.17 × 10⁻⁵, Bonferroni corrected p-value = 0.02) and rs738409 (A nonsynonymous gene variant in the PNPLA3 gene; p-value = 9.02 × 10⁻⁵, Bonferroni corrected p-value = 0.04). All of these polymorphisms have been shown to be associated with diverse pathologies, such as asthma, cancer, or chronic hepatitis B, or to be involved in a different response to drugs, such as metformin, HMG-CoA reductase inhibitors, or simvastatin. The present work provides a pharmacogenetic landscape of an understudied Latin American rural population and supports the notion that pharmacogenetic studies in admixed populations should consider ancestry for a higher accuracy of the results. Our study stresses the relevance of the pharmacogenomic research to provide guidance for a better choice of the best treatment for each individual in a population with admixed ancestry.

Downregulation of OCTN2 by cytokines plays an important role in the progression of inflammatory bowel disease

Inflammatory bowel diseases (IBD) are characterized by chronic relapsing disorders of the gastrointestinal tract. OCTN2 (SLC22A5) and its substrate l-carnitine (l-Car) play crucial roles in maintaining normal intestinal function. An aim of this study was to delineate the expression alteration of OCTN2 in IBD and its underlying mechanism. We also investigated the impact of OCTN2 on IBD progression and the possibility of improving IBD through OCTN2 regulation. Our results showed decreased OCTN2 expression levels and l-Car content in inflamed colon tissues of IBD patients and mice, which negatively correlated with the degree of colonic inflammation in IBD mice. Mixed proinflammatory cytokines TNF-α, IL-1β and IFNγ downregulated the expression of OCTN2 and subsequently reduced the l-Car content through PPARγ/RXRα pathways in FHC cells. OCTN2 silencing reduced the proliferation rate of the colon cells, whereas OCTN2 overexpression increased the proliferation rate. Furthermore, the ability of PPARγ agonist, luteolin, to increase OCTN2 expression resulted in the alleviation of colonic inflammatory responses. In conclusion, OCTN2 was downregulated in IBD by proinflammatory cytokines via the PPARγ/RXRα pathways, which reduced l-Car concentration and subsequently induced IBD deterioration. Upregulation of OCTN2 by the PPARγ agonist alleviated colonic inflammation. Our findings suggest that, OCTN2 may serve as a therapeutic target for IBD therapy.

Diosgenin and 4-Hydroxyisoleucine from Fenugreek Are Regulators of Genes Involved in Lipid Metabolism in The Human Colorectal Cancer Cell Line SW480

Objective

Diosignin and 4-hydroxy-L-isulosine (4-OH-Ile) are the two active ingredients of Fenugreek (Trigonella foenumgraecum). Thus, in this study, we examined the effects of hydroalcoholic extract of fenugreek seeds (HEFS), diosgenin and 4-OH-Ile on the expression of acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), peroxisome proliferator-activated receptor gamma (PPARγ) and low-density lipoprotein (LDL) receptor (LDLR) which are involved in lipid metabolism in SW480 cell line.

Materials and Methods

In this experimental study, SW480 cells were cultured in RPMI-1640 medium and treated with HEFS, diosignin, 4-OH-Ile or orlistat for 24 and 48 hours. Inhibitory concentration of 20% (IC20) was calculated using MTT method and cells were then pre-treated with the IC20 concentrations for 24 and 48 hours before RNA extraction and cDNA synthesis. Changes in the expression of ACC, FAS, PPARγ and LDLR genes were assayed by employing the real time-polymerase chain reaction (PCR) method.

Results

Our results showed a significant down-regulation in the expression of ACC (P<0.001 and P<0.001 after 24 and 48 hours, respectively) and FAS genes (P<0.001 and P<0.001 after 24 and 48 hours, respectively) in SW480 cells treated with HEFS, diosignin, 4-OH-Ile, or orlistat, but significant up-regulation in the expression of PPARγ (P<0.001 and P<0.001 after 24 and 48 hours, respectively) and LDLR (P=0.005 and P=0.001 after 24 and 48 hours, respectively).

Conclusion

According to the results of the present study, HEFS, diosgenin and 4-OH-Ile up or down-regulate the expression of some predominant genes involved in lipid metabolism pathway, similar to that observed for orlistat. These types of regulatory effects are presumably proper for the treatment of obesity and overweight.

A cross-disease meta-GWAS identifies four new susceptibility loci shared between systemic sclerosis and Crohn's disease

Genome-wide association studies (GWASs) have identified a number of genetic risk loci associated with systemic sclerosis (SSc) and Crohn’s disease (CD), some of which confer susceptibility to both diseases. In order to identify new risk loci shared between these two immune-mediated disorders, we performed a cross-disease meta-analysis including GWAS data from 5,734 SSc patients, 4,588 CD patients and 14,568 controls of European origin. We identified 4 new loci shared between SSc and CD, IL12RB2, IRF1/SLC22A5, STAT3 and an intergenic locus at 6p21.31. Pleiotropic variants within these loci showed opposite allelic effects in the two analysed diseases and all of them showed a significant effect on gene expression. In addition, an enrichment in the IL-12 family and type I interferon signaling pathways was observed among the set of SSc-CD common genetic risk loci. In conclusion, through the first cross-disease meta-analysis of SSc and CD, we identified genetic variants with pleiotropic effects on two clinically distinct immune-mediated disorders. The fact that all these pleiotropic SNPs have opposite allelic effects in SSc and CD reveals the complexity of the molecular mechanisms by which polymorphisms affect diseases.

Exosomes in inflammation and role as biomarkers

Exosomes are endosomal-derived nano-vesicles. They are considered vehicles through which donor cells transfer proteins, lipids and nucleic acids to target cells thus influencing their metabolism. Exosomes are involved in inflammatory processes that play a pivotal role in a large number of pathologic states including cancer, inflammatory bowel diseases, type 2 diabetes, obesity, rheumatoid arthritis and neurodegenerative diseases. The association between inflammation and change in nature or expression level of some exosomal cargos is the fundamental step for identifying possible novel biomarkers of inflammatory-based diseases. A novel interesting exosome cargo is the SLC22A5 transport protein whose level in exosomes is regulated by the pro-inflammatory cytokine INF-γ. The advantage of using exosomes as a biomarker vehicle consists of their ease of collection from body fluids such as urine and saliva as they may represent a non-invasive means for screening human pathology.

Regulation of carnitine status in ruminants and efficacy of carnitine supplementation on performance and health aspects of ruminant livestock: a review

Carnitine has long been known to play a critical role for energy metabolism. Due to this, a large number of studies have been carried out to investigate the potential of supplemental carnitine in improving performance of livestock animals including ruminants, with however largely inconsistent results. An important issue that has to be considered when using carnitine as a feed additive is that the efficacy of supplemental carnitine is probably dependent on the animal's carnitine status, which is affected by endogenous carnitine synthesis, carnitine uptake from the gastrointestinal tract and carnitine excretion. The present review aims to summarise the current knowledge of the regulation of carnitine status and carnitine homeostasis in ruminants, and comprehensively evaluate the efficacy of carnitine supplementation on performance and/or health in ruminant livestock by comparing the outcomes of studies with carnitine supplementation in dairy cattle, growing and finishing cattle and sheep. While most of the studies show that supplemental carnitine, even in ruminally unprotected form, is bioavailable in ruminants, its effect on either milk or growth performance is largely disappointing. However, supplemental carnitine appears to be a useful strategy to offer protection against ammonia toxicity caused by consumption of high levels of non-protein N or forages with high levels of soluble N both, in cattle and sheep.

Characterization of Exosomal SLC22A5 (OCTN2) carnitine transporter

Exosomes are extracellular vesicles involved in cell-to-cell communication. Previous large scale proteomics revealed that they contain SLC proteins. However, no data on the function of exosomal SLCs is available, so far. An SLC localized in exosomes was here characterized for the first time: the carnitine transporter OCTN2 (SLC22A5). The protein was detected by Western Blot analysis in HEK293 exosomes. To investigate the functional properties of the exosomal OCTN2, the proteins extracted from vesicles were reconstituted into proteolipsomes and the transport function was measured as uptake of ³H-carnitine. Transport was stimulated by sodium and was dependent on pH. ³H-carnitine uptake was inhibited by Acetyl-carnitine, but not by Asn, Gln and Arg thus excluding interference by ATB^0,+, an amino acid transporter which also recognizes carnitine. Cardiolipin failed to stimulate transport, excluding the activity of the mitochondrial Carnitine/acylcarnitine transporter. Increased level of exosomal OCTN2 was induced by treatment of HEK293 with the pro-inflammatory cytokine INFγ. All data concurred to demonstrate that OCTN2 present in exosomes is fully functional and is in its native conformation. Functional OCTN2 was detected also in human urinary exosomes, thus suggesting the OCTN2 exosomal protein as a candidate biomarker for inflammation related pathologies.

Reduced active transcellular intestinal oxalate secretion contributes to the pathogenesis of obesity-associated hyperoxaluria

Most kidney stones are composed of calcium oxalate, and minor changes in urine oxalate affect the stone risk. Obesity is a risk factor for kidney stones and a positive correlation of unknown etiology between increased body size, and elevated urinary oxalate excretion has been reported. Here, we used obese ob/ob (ob) mice to elucidate the pathogenesis of obesity-associated hyperoxaluria. These ob mice have significant hyperoxaluria (3.3-fold) compared with control mice, which is not due to overeating as shown by pair-feeding studies. Dietary oxalate removal greatly ameliorated this hyperoxaluria, confirming that it is largely enteric in origin. Transporter SLC26A6 (A6) plays an essential role in active transcellular intestinal oxalate secretion, and ob mice have significantly reduced jejunal A6 mRNA (- 80%) and total protein (- 62%) expression. While net oxalate secretion was observed in control jejunal tissues mounted in Ussing chambers, net absorption was seen in ob tissues, due to significantly reduced secretion. We hypothesized that the obesity-associated increase in intestinal and systemic inflammation, as reflected by elevated proinflammatory cytokines, suppresses A6-mediated intestinal oxalate secretion and contributes to obesity-associated hyperoxaluria. Indeed, proinflammatory cytokines (elevated in ob mice) significantly decreased intestinal oxalate transport in vitro by reducing A6 mRNA and total protein expression. Proinflammatory cytokines also significantly reduced active mouse jejunal oxalate secretion, converting oxalate transport from net secretion in vehicle-treated tissues to net absorption in proinflammatory cytokines-treated tissues. Thus, reduced active intestinal oxalate secretion, likely secondary to local and systemic inflammation, contributes to the pathogenesis of obesity-associated hyperoxaluria. Hence, proinflammatory cytokines represent potential therapeutic targets.

Dual targeting of l-carnitine-conjugated nanoparticles to OCTN2 and ATB0,+ to deliver chemotherapeutic agents for colon cancer therapy

l-Carnitine, obligatory for oxidation of fatty acids, is transported into cells by the Na+-coupled transporter OCTN2 and the Na+/Cl--coupled transporter ATB0,+. Here we investigated the potential of L-carnitine-conjugated poly(lactic-co-glycolic acid) (PLGA) nanoparticles (LC-PLGA NPs) to deliver chemotherapeutic drugs into cancer cells by targeting the nanoparticles to both OCTN2 and ATB0,+. The cellular uptake of LC-PLGA NPs in the breast cancer cell line MCF7 and the colon cancer cell line Caco-2 was increased compared to unmodified nanoparticles, but decreased in the absence of co-transporting ions (Na+ and/or Cl-) or in the presence of competitive substrates for the two transporters. Studies with fluorescently labeled nanoparticles showed their colocalization with both OCTN2 and ATB0,+, confirming the involvement of both transporters in the cellular uptake of LC-PLGA NPs. As the expression levels of OCTN2 and ATB0,+ are higher in colon cancer cells than in normal colon cells, LC-PLGA NPs can be used to deliver chemotherapeutic drugs selectively into cancer cells for colon cancer therapy. With 5-fluorouracil-loaded LC-PLGA NPs, we were able to demonstrate significant increases in the uptake efficiency and cytotoxicity in colon cancer cells that were positive for OCTN2 and ATB0,+. In a 3D spheroid model of tumor growth, LC-PLGA NPs showed increased uptake and enhanced antitumor efficacy. These findings indicate that dual-targeting LC-PLGA NPs to OCTN2 and ATB0,+ has great potential to deliver chemotherapeutic drugs for colon cancer therapy. Dual targeting LC-PLGA NPs to OCTN2 and ATB0,+ can selectively deliver chemotherapeutics to colon cancer cells where both transporters are overexpressed, preventing targeting to normal cells and thus avoiding off-target side effects.

Enhanced expression of Organic Cation Transporters in bronchial epithelial cell layers following insults associated with asthma - Impact on salbutamol transport

Increasing evidence suggests Organic Cation Transporters (OCT) might facilitate the absorption of inhaled bronchodilators, including salbutamol, across the lung epithelium. This is essentially scarred and inflamed in asthma. Accordingly, the impact of epithelial insults relevant to asthma on OCT expression and salbutamol transport was evaluated in air-liquid interfaced layers of the human broncho-epithelial cell line Calu-3. These were physically injured and allowed to recover for 48h or exposed to the pro-inflammatory stimulant lipopolysaccharide (LPS) for 48h and the aeroallergen house dust mite (HDM) for 8h twice over 48h. Increases in transporter expression were measured following each treatment, with the protein levels of the OCTN2 subtype consistently raised by at least 50%. Interestingly, OCT upregulation upon LPS and HDM challenges were dependent on an inflammatory event occurring in the cell layers. Salbutamol permeability was higher in LPS exposed layers than in their untreated counterparts and in both cases, was sensitive to the OCT inhibitor tetraethylammonium. This study is the first to show epithelial injury, inflammation and allergen abuse upregulate OCT in bronchial epithelial cells, which might have an impact on the absorption of their substrates in diseased lungs.

Identification of OCTN2 variants and their association with phenotypes of Crohn's disease in a Korean population

Crohn’s disease (CD) is a chronic inflammatory bowel disease and a genetic variant in the OCTN2, g.-207G > C is significantly associated with CD susceptibility. This study was aimed to identify novel OCTN2 functional promoter variants and their roles in transcriptional regulation using various in vitro assays. In addition, we investigated the association between OCTN2 genotypes and CD through genetic analysis using DNA samples from 193 patients with CD and 281 healthy controls. Among the three major promoter haplotypes of OCTN2 identified, one haplotype, H3, showed a significant decrease in promoter activity: two polymorphisms in H3 were associated with a significant reduction in promoter activity. In particular, we found that the reduced transcriptional activity of those two polymorphisms results from a reduction in the binding affinity of the activators, NF-E2 and YY1, to the OCTN2 promoter. The functional haplotype of the OCTN2 promoter was associated with clinical course of CD such as the disease behavior and need for surgery. However, genetic variants or haplotypes of OCTN2 did not affect the susceptibility to CD. Our results suggest that a common promoter haplotype of OCTN2 regulates the transcriptional rate of OCTN2 and influences the clinical course of CD.

Host-microbiome interaction in Crohn’s disease: A familiar or familial issue?

An impaired interaction between the gut and the intestinal microbiome is likely to be the key element in the pathogenesis of Crohn’s disease (CD). Family studies have provided invaluable information on CD pathogenesis and on its etiology. Relatives share the same genetic risk of developing the disease as affected subjects. Relatives also exhibit similar features relating to their host-microbiome interaction, namely genetic variants in loci involved in detecting bacteria, a greater sero-reactivity to microbial components, and an impaired intestinal permeability. The burden of environmental factors such as cigarette smoking and dysbiosis also seems to be particularly relevant in these genetically predisposed subjects. Diet is emerging as an important factor and could account for the changing epidemiology of CD in recent years. Despite the pivotal role of genetics in the disease’s pathogenesis (especially in familial CD), screening tests in healthy relatives cannot be recommended.

The pro-inflammatory cytokine tumor necrosis factor α stimulates expression of the carnitine transporter OCTN2 (novel organic cation transporter 2) and carnitine uptake via nuclear factor-κB in Madin-Darby bovine kidney cells

Carnitine uptake into tissues is mediated mainly by the novel organic cation transporter 2 (OCTN2), whose expression is upregulated in the liver of early-lactating dairy cows. It has been shown recently that pro-inflammatory cytokines, including tumor necrosis factor α (TNFα), stimulate OCTN2 expression and carnitine uptake in intestinal cells and inflamed intestinal mucosa. Given that many early-lactating dairy cows show typical signs of hepatic and systemic inflammation, such as elevated concentrations of circulating TNFα and activation of the key regulator of inflammation, nuclear factor κB (NF-κB), in tissues, it is possible that upregulation of OCTN2 and increase of carnitine uptake by TNFα is mediated by NF-κB, a mechanism that might contribute to the upregulation of OCNT2 in the liver of early-lactating dairy cows. Thus, in the present study, we tested the hypothesis that TNFα stimulates OCTN2 gene expression and carnitine uptake via NF-κB in the bovine Madin-Darby bovine kidney (MDBK) cell line. Treatment with TNFα caused activation of NF-κB, increased the mRNA and protein concentration of OCTN2, and stimulated the uptake of carnitine in MDBK cells. In contrast, combined treatment of MDBK cells with TNFα and the NF-κB inhibitor BAY 11-7085 completely blocked the effect of TNFα on OCTN2 mRNA and protein concentration and uptake of carnitine. These findings suggest that the bovine OCTN2 gene and carnitine uptake are regulated by NF-κB. Future studies are required to show the in vivo relevance of this regulatory mechanism in cattle.

Host-microbe interactions via membrane transport systems

Living organisms take in essential molecules and get rid of wastes effectively through the selective transport of materials. Especially in the digestive tract, advanced transport systems are indispensable for the absorption of nutrients and elimination of waste products. These transport pathways control physiological functions by modulating the ionic environment inside and outside the cells. Moreover, recent studies have shown the importance of the expression of trafficking-related molecules and the population of gut microbiota. We found that the molecules secreted from microorganisms are imported into the cells via transporters or endocytosis and that they activate cell survival pathways of intestinal epithelial cells. These findings indicate that the interactions between the gut microbiota and host cells are mediated, at least partly, by the membrane transport systems. In addition, it is well known that the breakdown of transport systems induces various diseases. This review highlights the significance of the transport systems as the pathogenic molecules and therapeutic targets in gastrointestinal disorders. For example, abnormal expression of the genes encoding membrane transport-related molecules is frequently involved in digestive diseases, such as colorectal cancer and inflammatory bowel disease. We herein review the significance of these molecules as pathogenic and therapeutic targets for digestive diseases.

Advances in inflammatory bowel disease pathogenesis: linking host genetics and the microbiome

Studies of the genetics underlying inflammatory bowel diseases have increased our understanding of the pathways involved in both ulcerative colitis and Crohn's disease and focused attention on the role of the microbiome in these diseases. Full understanding of pathogenesis will require a comprehensive grasp of the delicate homeostasis between gut bacteria and the human host. In this review, we present current evidence of microbiome-gene interactions in the context of other known risk factors and mechanisms, and describe the next steps necessary to pair genetic variant and microbiome sequencing data from patient cohorts. We discuss the concept of dysbiosis, proposing that the functional composition of the gut microbiome may provide a more consistent definition of dysbiosis and may more readily provide evidence of genome-microbiome interactions in future exploratory studies.

Enzymes involved in L-carnitine biosynthesis are expressed by small intestinal enterocytes in mice: implications for gut health

BACKGROUND

Carnitine is essential for mitochondrial β-oxidation of long-chain fatty acids. Deficiency of carnitine leads to severe gut atrophy, ulceration and inflammation in animal models of carnitine deficiency. Genetic studies in large populations have linked mutations in the carnitine transporters OCTN1 and OCTN2 with Crohn's disease (CD), while other studies at the same time have failed to show a similar association and report normal serum carnitine levels in CD patients.

METHODS

In this report, we have studied the expression of carnitine-synthesizing enzymes in intestinal epithelial cells to determine the capability of these cells to synthesize carnitine de novo. We studied expression of five enzymes involved in carnitine biosynthesis, namely 6-N-trimethyllysine dioxygenase (TMLD), 4-trimethylaminobutyraldehyde dehydrogenase (TMABADH), serine hydroxymethyltransferase 1 and 2 (SHMT1 and 2) and γ-butyrobetaine hydroxylase (BBH) by real-time PCR in mice (C3H strain). We also measured activity of γ-BBH in the intestine using an ex vivo assay and localized its expression by in situ hybridization.

RESULTS

Our investigations show that mouse intestinal epithelium expresses all five enzymes required for de novo carnitine biosynthesis; the expression is localized mainly in villous surface epithelial cells throughout the intestine. The final rate-limiting enzyme γ-BBH is highly active in the small intestine; its activity was 9.7 ± 3.5 pmol/mg/min, compared to 22.7 ± 7.3 pmol/mg/min in the liver.

CONCLUSIONS

We conclude that mouse gut epithelium is able to synthesize carnitine de novo. This capacity to synthesize carnitine in the intestine may play an important role in gut health and can help explain lack of clinical carnitine deficiency signs in subjects with mutations with OCTN transporters.

OCTN cation transporters in health and disease: role as drug targets and assay development

The three members of the organic cation transporter novel subfamily are known to be involved in interactions with xenobiotic compounds. These proteins are characterized by 12 transmembrane segments connected by nine short loops and two large hydrophilic loops. It has been recently pointed out that acetylcholine is a physiological substrate of OCTN1. Its transport could be involved in nonneuronal cholinergic functions. OCTN2 maintains the carnitine homeostasis, resulting from intestinal absorption, distribution to tissues, and renal excretion/reabsorption. OCTN3, identified only in mouse, mediates also carnitine transport. OCTN1 and OCTN2 are associated with several pathologies, such as inflammatory bowel disease, primary carnitine deficiency, diabetes, neurological disorders, and cancer, thus representing useful pharmacological targets. The function and interaction with drugs of OCTNs have been studied in intact cell systems and in proteoliposomes. The latter experimental model enables reduced interference from other transporters or enzyme pathways. Using proteoliposomes, the molecular bases of toxicity of some drugs have recently been revealed. Therefore, proteoliposomes represent a promising experimental tool suitable for large-scale molecular screening of interactions of OCTNs with chemicals regarding human health.

NHE3 regulatory factor 1 (NHERF1) modulates intestinal sodium-dependent phosphate transporter (NaPi-2b) expression in apical microvilli

P(i) uptake in the small intestine occurs predominantly through the NaPi-2b (SLC34a2) co-transporter. NaPi-2b is regulated by changes in dietary P(i) but the mechanisms underlying this regulation are largely undetermined. Sequence analyses show NaPi-2b has a PDZ binding motif at its C terminus. Immunofluorescence imaging shows NaPi-2b and two PDZ domain containing proteins, NHERF1 and PDZK1, are expressed in the apical microvillar domain of rat small intestine enterocytes. Co-immunoprecipitation studies in rat enterocytes show that NHERF1 associates with NaPi-2b but not PDZK1. In HEK co-expression studies, GFP-NaPi-2b co-precipitates with FLAG-NHERF1. This interaction is markedly diminished when the C-terminal four amino acids are truncated from NaPi-2b. FLIM-FRET analyses using tagged proteins in CACO-2(BBE) cells show a distinct phasor shift between NaPi-2b and NHERF1 but not between NaPi-2b and the PDZK1 pair. This shift demonstrates that NaPi-2b and NHERF1 reside within 10 nm of each other. NHERF1(-/-) mice, but not PDZK1(-/-) mice, had a diminished adaptation of NaPi-2b expression in response to a low P(i) diet. Together these studies demonstrate that NHERF1 associates with NaPi-2b in enterocytes and regulates NaPi-2b adaptation.

Selective regulation of cardiac organic cation transporter novel type 2 (OCTN2) in dilated cardiomyopathy

Organic cation transporters (OCT1-3 and OCTN1/2) facilitate cardiac uptake of endogenous compounds and numerous drugs. Genetic variants of OCTN2, for example, reduce uptake of carnitine, leading to heart failure. Whether expression and function of OCTs and OCTNs are altered by disease has not been explored in detail. We therefore studied cardiac expression, heart failure-dependent regulation, and affinity to cardiovascular drugs of these transporters. Cardiac transporter mRNA levels were OCTN2>OCT3>OCTN1>OCT1 (OCT2 was not detected). Proteins were localized in vascular structures (OCT3/OCTN2/OCTN1) and cardiomyocytes (OCT1/OCTN1). Functional studies revealed a specific drug-interaction profile with pronounced inhibition of OCT1 function, for example, carvedilol [half maximal inhibitory concentration (IC₅₀), 1.4 μmol/L], diltiazem (IC₅₀, 1.7 μmol/L), or propafenone (IC₅₀, 1.0 μmol/L). With use of the cardiomyopathy model of coxsackievirus-infected mice, Octn2mRNA expression was significantly reduced (56% of controls, 8 days after infection). Accordingly, in endomyocardial biopsy specimens OCTN2 expression was significantly reduced in patients with dilated cardiomyopathy, whereas the expression of OCT1-3 and OCTN1 was not affected. For OCTN2 we observed a significant correlation between expression and left ventricular ejection fraction (r = 0.53, P < 0.0001) and the presence of cardiac CD3⁺ T cells (r = -0.45, P < 0.05), respectively. OCT1, OCT3, OCTN1, and OCTN2 are expressed in the human heart and interact with cardiovascular drugs. OCTN2 expression is selectively reduced in dilated cardiomyopathy patients and predicts the impairment of cardiac function.