beta-alanine and alpha-fluoro-beta-alanine concentrative transport in rat hepatocytes is mediated by GABA transporter GAT-2

Fabrication and Use of Poly(d,l-lactide-co-glycolide)-Based Formulations Designed for Modified Release of 5-Fluorouracil

5-fluorouracil (5-FU) is a chemotherapeutic agent that has been used for the treatment of a variety of malignancies since its initial introduction to the clinic in 1957. Owing to its short biological half-life, multiple dosings are generally required to maintain effective 5-FU plasma concentrations throughout the therapeutic period. Clinical studies have shown that continuous 5-FU administration is generally superior to bolus injection as exhibited by lower toxicities and increased therapeutic efficacy. Optimal therapeutic efficacy, however, is often compromised by the limiting therapeutic index. Whilst oral formulations are also used, these suffer from the drawbacks of variable bioavailability and first-pass metabolism. As a result, sustained release formulations of 5-FU have been investigated in an effort to mimic the kinetics of continuous infusion particularly for situations where local delivery is considered appropriate. The biocompatible, biodegradable, and highly tunable synthetic polymer, poly(d,l-lactide-co-glycolide) (PLGA), is widely used as a vector for sustained drug delivery, however, issues such as insufficient loading and inappropriate burst release kinetics have dogged progress into the clinic for small hydrophilic drugs such as 5-FU. This review provides introductory information about the mechanism of action, pharmacokinetic and physicochemical properties, and clinical use of 5-FU that have contributed to the development of PLGA-based 5-FU release platforms. In addition, this review provides information on fabrication methods used for a range of 5-FU-loaded PLGA formulations and discusses factors affecting the release kinetics of 5-FU as well as the in vitro and in vivo antitumor or antiproliferative efficacy of these platforms.

Impact of SLC6A Transporters in Physiological Taurine Transport at the Blood-Retinal Barrier and in the Liver

Cumulative studies showed that taurine (2-aminoethanesulfonic acid) contributes to a variety of physiological events. Transport study suggested the cellular taurine transport in an Na⁺- and Cl^--dependent manner, and the several members of SLC6A family have been shown as taurine transporter. At the inner blood-retinal barrier (BRB), taurine transporter (TauT/SLC6A) is involved in the transport of taurine to the retina from the circulating blood. The involvement of TauT is also suggested in γ-aminobutyric acid (GABA) transport at the inner BRB, and its role is assumed in the elimination of GABA from the retinal interstitial fluid. In the retina, taurine is thought to be a major organic osmolyte, and its influx and efflux through TauT and volume-sensitive organic osmolyte and anion channel (VSOAC) in Müller cells regulate the osmolarity in the retinal microenvironment to maintain a healthy retina. In the liver, hepatocytes take up taurine via GABA transporter 2 (GAT2/SLC6A13, the orthologue of mouse GAT3) expressed at the sinusoidal membrane of periportal hepatocytes, contributing to the metabolism of bile acid. Site-directed mutagenesis study suggests amino acid residues that are crucial in the recognition of substrates by GATs and TauT. The evidence suggests the physiological impact of taurine transporters in tissues.

Transport of haloacids across biological membranes

Haloacids are considered to be environmental pollutants, but some of them have also been tested in clinical research. The way that haloacids are transported across biological membranes is important for both biodegradation and drug delivery purposes. In this review, we will first summarize putative haloacids transporters and the information about haloacids transport when studying carboxylates transporters. We will then introduce MCT1 and SLC5A8, which are respective transporter for antitumor agent 3-bromopyruvic acid and dichloroacetic acid, and monochloroacetic acid transporters Deh4p and Dehp2 from a haloacids-degrading bacterium. Phylogenetic analysis of these haloacids transporters and other monocarboxylate transporters reveals their evolutionary relationships. Haloacids transporters are not studied to the extent that they deserve compared with their great application potentials, thus future inter-discipline research are desired to better characterize their transport mechanisms for potential applications in both environmental and clinical fields.

β-Aminoisobutyric acid induces browning of white fat and hepatic β-oxidation and is inversely correlated with cardiometabolic risk factors

The transcriptional coactivator peroxisome proliferator-activated receptor-gamma coactivator-1α (PGC-1α) regulates metabolic genes in skeletal muscle and contributes to the response of muscle to exercise. Muscle PGC-1α transgenic expression and exercise both increase the expression of thermogenic genes within white adipose. How the PGC-1α-mediated response to exercise in muscle conveys signals to other tissues remains incompletely defined. We employed a metabolomic approach to examine metabolites secreted from myocytes with forced expression of PGC-1α, and identified β-aminoisobutyric acid (BAIBA) as a small molecule myokine. BAIBA increases the expression of brown adipocyte-specific genes in white adipocytes and β-oxidation in hepatocytes both in vitro and in vivo through a PPARα-mediated mechanism, induces a brown adipose-like phenotype in human pluripotent stem cells, and improves glucose homeostasis in mice. In humans, plasma BAIBA concentrations are increased with exercise and inversely associated with metabolic risk factors. BAIBA may thus contribute to exercise-induced protection from metabolic diseases.

Involvement of γ-aminobutyric acid transporter 2 in the hepatic uptake of taurine in rats

Taurine is essential for the hepatic synthesis of bile salts and, although taurine is synthesized mainly in pericentral hepatocytes, taurine and taurine-conjugated bile acids are abundant in periportal hepatocytes. One possible explanation for this discrepancy is that the active supply of taurine to hepatocytes from the blood stream is a key regulatory factor. The purpose of the present study is to investigate and identify the transporter responsible for taurine uptake by periportal hepatocytes. An in vivo bolus injection of [(3)H]taurine into the rat portal vein demonstrated that 25% of the injected [(3)H]taurine was taken up by the liver on a single pass. The in vivo uptake was significantly inhibited by GABA, taurine, β-alanine, and nipecotic acid, a GABA transporter (GAT) inhibitor, each at a concentration of 10 mM. The characteristics of Na(+)- and Cl(-)-dependent [(3)H]taurine uptake by freshly isolated rat hepatocytes were consistent with those of GAT2 (solute carrier SLC6A13). Indeed, the K(m) value of the saturable uptake (594 μM) was close to that of mouse SLC6A13-mediated taurine transport. Although GABA, taurine, and β-alanine inhibited the [(3)H]taurine uptake by > 50%, each at a concentration of 10 mM, GABA caused a marked inhibition with an IC(50) value of 95 μM. The [(3)H]taurine uptake exhibited a significant reduction when the GAT2 gene was silenced. Immunohistochemical analysis showed that GAT2 was localized on the sinusoidal membrane of the hepatocytes predominantly in the periportal region. These results suggest that GAT2 is responsible for taurine transport from the circulating blood to hepatocytes predominantly in the periportal region.

γ-Aminobutyric acid transporter 2 mediates the hepatic uptake of guanidinoacetate, the creatine biosynthetic precursor, in rats

Guanidinoacetic acid (GAA) is the biosynthetic precursor of creatine which is involved in storage and transmission of phosphate-bound energy. Hepatocytes readily convert GAA to creatine, raising the possibility that the active uptake of GAA by hepatocytes is a regulatory factor. The purpose of this study is to investigate and identify the transporter responsible for GAA uptake by hepatocytes. The characteristics of [(14)C]GAA uptake by hepatocytes were elucidated using the in vivo liver uptake method, freshly isolated rat hepatocytes, an expression system of Xenopus laevis oocytes, gene knockdown, and an immunohistochemical technique. In vivo injection of [(14)C]GAA into the rat femoral vein and portal vein results in the rapid uptake of [(14)C]GAA by the liver. The uptake was markedly inhibited by γ-aminobutyric acid (GABA) and nipecotinic acid, an inhibitor of GABA transporters (GATs). The characteristics of Na(+)- and Cl(-)-dependent [(14)C]GAA uptake by freshly isolated rat hepatocytes were consistent with those of GAT2. The Km value of the GAA uptake (134 µM) was close to that of GAT2-mediated GAA transport (78.9 µM). GABA caused a marked inhibition with an IC(50) value of 8.81 µM. The [(14)C]GAA uptake exhibited a significant reduction corresponding to the reduction in GAT2 protein expression. GAT2 was localized on the sinusoidal membrane of the hepatocytes predominantly in the periportal region. This distribution pattern was consistent with that of the creatine biosynthetic enzyme, S-adenosylmethionine:guanidinoacetate N-methyltransferase. GAT2 makes a major contribution to the sinusoidal GAA uptake by periportal hepatocytes, thus regulating creatine biosynthesis in the liver.

δ-Aminolevulinic acid and its methyl ester induce the formation of Protoporphyrin IX in cultured sensory neurones

Application of δ-aminolevulinic acid (ALA) or its methyl ester (MAL) onto cutaneous tumours increases intracellular Protoporphyrin IX (PpIX), serving as photosensitizer in photodynamic therapy (PDT). While PDT is highly effective as treatment of neoplastic skin lesions, it may induce severe pain in some patients. Here, we investigated ALA and MAL uptake and PpIX formation in sensory neurones as potential contributor to the pain. PpIX formation was induced in cultured sensory neurones from rat dorsal root ganglion by incubation with ALA or MAL. Using inhibitors of GABA transporters (GAT), a pharmacological profile of ALA and MAL uptake was assessed. GAT mRNA expression in the cultures was determined by RT-PCR. Cultured sensory neurones synthesised Protoporphyrin IX (PpIX) from extracellularly administered ALA and MAL. PpIX formation was dose- and time-dependent with considerably different kinetics for both compounds. While partial inhibition occurred using L-arginine, PpIX formation from both ALA and MAL could be fully blocked by the GABA-Transporter (GAT)-2/3 inhibitor (S)-SNAP 5114 with similar K (i) (ALA: 195 ± 6 μM; MAL: 129 ± 13 μM). GAT-1 and GAT-3 could be detected in sensory neurons using RT-PCR on mRNA level and using [³H]-GABA uptake on protein level. Cultured sensory neurones take up ALA and MAL and synthesize PpIX from both, enabling a direct impact of photodynamic therapy on cutaneous free nerve endings. The pharmacological profile of ALA and MAL uptake in our test system was very similar and suggests uptake via GABA and amino acid transporters.

Hepatic uptake of gamma-butyrobetaine, a precursor of carnitine biosynthesis, in rats

Gamma-butyrobetaine (GBB) is a precursor in the biosynthesis of carnitine, which plays an important role in the beta-oxidation of fatty acids, and is converted to carnitine by gamma-butyrobetaine dioxygenase (BBD) predominantly in liver. We investigated the molecular mechanism of hepatic uptake of GBB in rat hepatocytes. Cellular localization of rat Octn2 (rOctn2:Slc22A5) was studied by Western blot analysis. Uptake of deuterated GBB (d(3)-GBB) was examined in HEK293 cells expressing rOctn2 (HEK293/rOctn2) and freshly isolated rat hepatocytes. d(3)-GBB was quantified by use of liquid chromatography-tandem mass spectrometry. Western blot analysis demonstrated an expression of OCTN2 protein in hepatic basolateral membrane but not in bile canalicular membrane fraction. Furthermore, we found that d(3)-GBB was taken up by rOctn2 in an Na(+)-dependent manner with K(m) value of 13 microM. The apparent K(m) value for d(3)-GBB transport in freshly isolated rat hepatocytes was 9 microM. d(3)-GBB uptake by the rat hepatocytes was inhibited by gamma-aminobutyric acid (GABA) to 30% of the control, whereas it was inhibited by carnitine to 62% of the control, even at 500 microM. Furthermore, d(3)-GBB uptake by rat hepatocytes was decreased by 45% with rat Gat2 (Slc6A13, a major liver GABA transporter) silenced by the microRNA method. Accordingly, the present study clearly demonstrates that GBB is taken up by hepatocytes for carnitine biosynthesis not only via Octn2 but also via the GABA transporter, possibly Gat2.

SNAT2 amino acid transporter is regulated by amino acids of the SLC6 gamma-aminobutyric acid transporter subfamily in neocortical neurons and may play no role in delivering glutamine for glutamatergic transmission

System A transporters SNAT1 and SNAT2 mediate uptake of neutral alpha-amino acids (e.g. glutamine, alanine, and proline) and are expressed in central neurons. We tested the hypothesis that SNAT2 is required to support neurotransmitter glutamate synthesis by examining spontaneous excitatory activity after inducing or repressing SNAT2 expression for prolonged periods. We stimulated de novo synthesis of SNAT2 mRNA and increased SNAT2 mRNA stability and total SNAT2 protein and functional activity, whereas SNAT1 expression was unaffected. Increased endogenous SNAT2 expression did not affect spontaneous excitatory action-potential frequency over control. Long term glutamine exposure strongly repressed SNAT2 expression but increased excitatory action-potential frequency. Quantal size was not altered following SNAT2 induction or repression. These results suggest that spontaneous glutamatergic transmission in pyramidal neurons does not rely on SNAT2. To our surprise, repression of SNAT2 activity was not limited to System A substrates. Taurine, gamma-aminobutyric acid, and beta-alanine (substrates of the SLC6 gamma-aminobutyric acid transporter family) repressed SNAT2 expression more potently (10x) than did System A substrates; however, the responses to System A substrates were more rapid. Since ATF4 (activating transcription factor 4) and CCAAT/enhancer-binding protein are known to bind to an amino acid response element within the SNAT2 promoter and mediate induction of SNAT2 in peripheral cell lines, we tested whether either factor was similarly induced by amino acid deprivation in neurons. We found that glutamine and taurine repressed the induction of both transcription factors. Our data revealed that SNAT2 expression is constitutively low in neurons under physiological conditions but potently induced, together with the taurine transporter TauT, in response to depletion of neutral amino acids.

The effect of 4 weeks beta-alanine supplementation and isokinetic training on carnosine concentrations in type I and II human skeletal muscle fibres

Seven male students were supplemented with beta-alanine (beta-ALG) for 4 weeks (6.4 g day(-1)) and seven with a matching placebo (PLG). Subjects undertook 4 weeks of isokinetic training with the right leg (T) whilst the left leg was untrained (UT), serving as a control. Each training session consisted of 10 x 10 maximal 90 degrees extension and flexion contractions at 180 degrees /s using a Kin-Com isokinetic dynamometer, with 1 min rest between bouts. Muscle biopsies were taken from the vastus lateralis immediately before and at the end of the supplementation period. Following freeze drying muscle fibres were dissected and characterised by their MHC profile, as type I, IIa, IIx, or as hybrids of these. Carnosine was measured by HPLC. There was a significant increase in carnosine in both T and UT legs of the beta-ALG (9.63 +/- 3.92 mmol kg(-1) dry muscle and 6.55 +/- 2.36 mmol kg(-1) dry muscle respectively). There was a significant increase in the carnosine content of all fibre phentotypes, with no significant difference between types. There were no significant differences in the changes in muscle or in fibres between the T and UT legs. In contrast there was no significant change in the carnosine content in either the T or UT legs with placebo. The results indicate that 4 weeks training has no effect on the muscle carnosine content. Whilst an increase was seen with beta-alanine supplementation, this was not further influenced by training. These findings suggest that beta-alanine availability is the main factor regulating muscle carnosine synthesis.