The Orally Active Antihyperglycemic Drug β-Guanidinopropionic Acid Is Transported by the Human Proton-Coupled Amino Acid Transporter hPAT1

Guanidinoacetic acid in human nutrition: Beyond creatine synthesis

Guanidinoacetic acid (GAA) is a nutrient that has been used in human nutrition since the early 1950s. Recommended for its role in creatine biosynthesis, GAA demonstrated beneficial energy-boosting effects in various clinical conditions. Dietary GAA has also been suggested to trigger several creatine-independent mechanisms. Besides acting as a direct precursor of high-energy phosphagen creatine, dietary GAA is suggested to reduce blood glucose concentration by acting as an insulinotropic food compound, spare amino acid arginine for other metabolic purposes (including protein synthesis), modulate taste, and perhaps alter methylation and fat deposition in various organs including the liver. GAA as a food component can have several important metabolic roles beyond creatine biosynthesis; future studies are highly warranted to address GAA overall role in human nutrition.

Glycosylation affects the stability and subcellular distribution of human PAT1 protein

The amino acid transporter PAT1 is typically expressed on the lysosome and plasma membranes in various human tissues. Glycosylation has been shown to be critical for the cell surface expression of PAT1, but not for its stability, in Xenopus oocytes. Here, we report that the glycosylation-deficient mutant of PAT1 (PAT1^3NQ ) is unstable and is degraded mainly via the endoplasmic reticulum-associated degradation pathway in HEK293 cells. Interestingly, PAT1^3NQ binds preferentially to the plasma membrane rather than to the lysosome. Consistent with this altered distribution, overexpression of PAT1^3NQ fails to inhibit the mechanistic target of rapamycin complex 1 (mTORC1). Our data suggest that glycosylation affects the stability and localization of PAT1 in HEK293 cells and the subcellular distribution of PAT1 is a factor affecting mTORC1 activity.

β-Guanidinopropionic acid extends the lifespan of Drosophila melanogaster via an AMP-activated protein kinase-dependent increase in autophagy

Previous studies have demonstrated that AMP‐activated protein kinase (AMPK) controls autophagy through the mammalian target of rapamycin (mTOR) and Unc‐51 like kinase 1 (ULK1/Atg1) signaling, which augments the quality of cellular housekeeping, and that β‐guanidinopropionic acid (β‐GPA), a creatine analog, leads to a chronic activation of AMPK. However, the relationship between β‐GPA and aging remains elusive. In this study, we hypothesized that feeding β‐GPA to adult Drosophila produces the lifespan extension via activation of AMPK‐dependent autophagy. It was found that dietary administration of β‐GPA at a concentration higher than 900 mm induced a significant extension of the lifespan of Drosophila melanogaster in repeated experiments. Furthermore, we found that Atg8 protein, the homolog of microtubule‐associated protein 1A/1B‐light chain 3 (LC3) and a biomarker of autophagy in Drosophila, was significantly upregulated by β‐GPA treatment, indicating that autophagic activity plays a role in the effect of β‐GPA. On the other hand, when the expression of Atg5 protein, an essential protein for autophagy, was reduced by RNA interference (RNAi), the effect of β‐GPA on lifespan extension was abolished. Moreover, we found that AMPK was also involved in this process. β‐GPA treatment significantly elevated the expression of phospho‐T172‐AMPK levels, while inhibition of AMPK by either AMPK‐RNAi or compound C significantly attenuated the expression of autophagy‐related proteins and lifespan extension in Drosophila. Taken together, our results suggest that β‐GPA can induce an extension of the lifespan of Drosophila via AMPK‐Atg1‐autophagy signaling pathway.

Sertraline inhibits the transport of PAT1 substrates in vivo and in vitro

BACKGROUND AND PURPOSE

Intestinal nutrient transporters may mediate the uptake of drugs. The aim of this study was to investigate whether sertraline interacts with the intestinal proton-coupled amino acid transporter 1 PAT1 (SLC36A1).

EXPERIMENTAL APPROACH

In vitro investigations of interactions between sertraline and human (h)PAT1, hSGLT1 (sodium-glucose linked transporter 1) and hPepT1 (proton-coupled di-/tri-peptide transporter 1) were conducted in Caco-2 cells using radiolabelled substrates. In vivo pharmacokinetic investigations were conducted in male Sprague-Dawley rats using gaboxadol (10 mg·kg(-1), p.o.) as a PAT1 substrate and sertraline (0-30.6 mg·kg(-1)). Gaboxadol was quantified by hydrophilic interaction chromatography followed by MS/MS detection.

KEY RESULTS

Sertraline inhibited hPAT1-mediated L-[(3)H]-Pro uptake in Caco-2 cells. This interaction between sertraline and PAT1 appeared to be non-competitive. The uptake of the hSGLT1 substrate [(14)C]-α-methyl-D-glycopyranoside and the hPepT1 substrate [(14)C]-Gly-Sar in Caco-2 cells was also decreased in the presence of 0.3 mM sertraline. In rats, the administration of sertraline (0.1-10 mM, corresponding to 0.3-30.6 mg·kg(-1), p.o.) significantly reduced the maximal gaboxadol plasma concentration and AUC after its administration p.o.

CONCLUSIONS AND IMPLICATIONS

Sertraline is an apparent non-competitive inhibitor of hPAT1-mediated transport in vitro. This inhibitory effect of sertraline is not specific to hPAT1 as substrate transport via hPepT1 and hSGLT1 was also reduced in the presence of sertraline. In vivo, sertraline reduced the amount of gaboxadol absorbed, suggesting that the inhibitory effect of sertraline on PAT1 occurs both in vitro and in vivo. Hence, sertraline could alter the bioavailability of drugs absorbed via PAT1.

Pharmacokinetic aspects of the anti-epileptic drug substance vigabatrin: focus on transporter interactions

Drug transporters in various tissues, such as intestine, kidney, liver and brain, are recognized as important mediators of absorption, distribution, metabolism and excretion of drug substances. This review gives a current status on the transporter(s) mediating the absorption, distribution, metabolism and excretion properties of the anti-epileptic drug substance vigabatrin. For orally administered drugs, like vigabatrin, the absorption from the intestine is a prerequisite for the bioavailability. Therefore, transporter(s) involved in the intestinal absorption of vigabatrin in vitro and in vivo are discussed in detail. Special focus is on the contribution of the proton-coupled amino acid transporter 1 (PAT1) for intestinal vigabatrin absorption. Furthermore, the review gives an overview of the pharmacokinetic parameters of vigabatrin across different species and drug–food and drug–drug interactions involving vigabatrin.

The effect of the creatine analogue beta-guanidinopropionic acid on energy metabolism: a systematic review

Background

Creatine kinase plays a key role in cellular energy transport. The enzyme transfers high-energy phosphoryl groups from mitochondria to subcellular sites of ATP hydrolysis, where it buffers ADP concentration by catalyzing the reversible transfer of the high-energy phosphate moiety (P) between creatine and ADP. Cellular creatine uptake is competitively inhibited by beta-guanidinopropionic acid. This substance is marked as safe for human use, but the effects are unclear. Therefore, we systematically reviewed the effect of beta-guanidinopropionic acid on energy metabolism and function of tissues with high energy demands.

Methods

We performed a systematic review and searched the electronic databases Pubmed, EMBASE, the Cochrane Library, and LILACS from their inception through March 2011. Furthermore, we searched the internet and explored references from textbooks and reviews.

Results

After applying the inclusion criteria, we retrieved 131 publications, mainly considering the effect of chronic oral administration of beta-guanidinopropionic acid (0.5 to 3.5%) on skeletal muscle, the cardiovascular system, and brain tissue in animals. Beta-guanidinopropionic acid decreased intracellular creatine and phosphocreatine in all tissues studied. In skeletal muscle, this effect induced a shift from glycolytic to oxidative metabolism, increased cellular glucose uptake and increased fatigue tolerance. In heart tissue this shift to mitochondrial metabolism was less pronounced. Myocardial contractility was modestly reduced, including a decreased ventricular developed pressure, albeit with unchanged cardiac output. In brain tissue adaptations in energy metabolism resulted in enhanced ATP stability and survival during hypoxia.

Conclusion

Chronic beta-guanidinopropionic acid increases fatigue tolerance of skeletal muscle and survival during ischaemia in animal studies, with modestly reduced myocardial contractility. Because it is marked as safe for human use, there is a need for human data.

Transport of the areca nut alkaloid arecaidine by the human proton-coupled amino acid transporter 1 (hPAT1)

Objectives

The pyridine alkaloid arecaidine is an ingredient of areca nut preparations. It is responsible for many physiological effects observed during areca nut chewing. However, the mechanism underlying its oral bioavailability has not yet been studied. We investigated whether the H+-coupled amino acid transporter 1 (PAT1, SLC36A1), which is expressed in the intestinal epithelium, accepts arecaidine, arecoline, isoguvacine and other derivatives as substrates.

Methods

Inhibition of l-[3H]proline uptake by arecaidine and derivatives was determined in Caco-2 cells expressing hPAT1 constitutively and in HeLa cells transiently transfected with hPAT1-cDNA. Transmembrane transport of arecaidine and derivatives was measured electrophysiologically in Xenopus laevis oocytes.

Key findings

Arecaidine, guvacine and isoguvacine but not arecoline strongly inhibited the uptake of l-[3H]proline into Caco-2 cells. Kinetic analyses revealed the competitive manner of l-proline uptake inhibition by arecaidine. In HeLa cells transfected with hPAT1-cDNA an affinity constant of 3.8 mm was obtained for arecaidine. Electrophysiological measurements at hPAT1-expressing X. laevis oocytes demonstrated that arecaidine, guvacine and isoguvacine are transported by hPAT1 in an electrogenic manner.

Conclusion

We conclude that hPAT1 transports arecaidine, guvacine and isoguvacine across the apical membrane of enterocytes and that hPAT1 might be responsible for the intestinal absorption of these drug candidates.

The SLC36 family of proton-coupled amino acid transporters and their potential role in drug transport

Members of the solute carrier (SLC) 36 family are involved in transmembrane movement of amino acids and derivatives. SLC36 consists of four members. SLC36A1 and SLC36A2 both function as H(+) -coupled amino acid symporters. SLC36A1 is expressed at the luminal surface of the small intestine but is also commonly found in lysosomes in many cell types (including neurones), suggesting that it is a multipurpose carrier with distinct roles in different cells including absorption in the small intestine and as an efflux pathway following intralysosomal protein breakdown. SLC36A1 has a relatively low affinity (K(m) 1-10 mM) for its substrates, which include zwitterionic amino and imino acids, heterocyclic amino acids and amino acid-based drugs and derivatives used experimentally and/or clinically to treat epilepsy, schizophrenia, bacterial infections, hyperglycaemia and cancer. SLC36A2 is expressed at the apical surface of the human renal proximal tubule where it functions in the reabsorption of glycine, proline and hydroxyproline. SLC36A2 also transports amino acid derivatives but has a narrower substrate selectivity and higher affinity (K(m) 0.1-0.7 mM) than SLC36A1. Mutations in SLC36A2 lead to hyperglycinuria and iminoglycinuria. SLC36A3 is expressed only in testes and is an orphan transporter with no known function. SLC36A4 is widely distributed at the mRNA level and is a high-affinity (K(m) 2-3 µM) transporter for proline and tryptophan. We have much to learn about this family of transporters, but from current knowledge, it seems likely that their function will influence the pharmacokinetic profiles of amino acid-based drugs by mediating transport in both the small intestine and kidney.

Hijacking solute carriers for proton-coupled drug transport

The physiological role of mammalian solute carrier (SLC) proteins is to mediate transmembrane movement of electrolytes, nutrients, micronutrients, vitamins, and endogenous metabolites from one cellular compartment to another. Many transporters in the small intestine, kidney, and solid tumors are H(+)-coupled, driven by local H(+)-electrochemical gradients, and transport numerous drugs. These transporters include PepT1 and PepT2 (SLC15A1/2), PCFT (SLC46A1), PAT1 (SLC36A1), OAT10 (SLC22A13), OATP2B1 (SLCO2B1), MCT1 (SLC16A1), and MATE1 and MATE2-K (SLC47A1/2).

The proton-coupled amino acid transporter, SLC36A1 (hPAT1), transports Gly-Gly, Gly-Sar and other Gly-Gly mimetics

BACKGROUND AND PURPOSE The intestinal proton-coupled amino acid transporter, SLC36A1, transports zwitterionic α-amino acids and drugs such as vigabatrin, gaboxadol and δ-aminolevulinic acid. We hypothesize that SLC36A1 might also transport some dipeptides. The aim of the present study was to investigate SLC36A1-mediated transport of Gly-Gly and Gly-Gly mimetics, and to investigate Gly-Sar transport via SLC36A1 and the proton-coupled dipeptide/tripeptide transporter, SLC15A1 in Caco-2 cells. EXPERIMENTAL APPROACH Transport of a compound via SLC36A1 was determined by its ability to induce an increase in the inward current of two-electrode voltage clamped SLC36A1 cRNA-injected Xenopus laevis oocytes. SLC36A1-mediated L-[³H]Pro uptake in Caco-2 cells was measured in the absence and presence of Gly-Gly or Gly-Sar. In addition, apical [¹⁴C]Gly-Sar uptake was measured in the absence and presence of the SLC36A1 inhibitor 5-hydroxy-L-tryptophan (5-HTP) or the SLC15A1 inhibitor L-4,4'-biphenylalanyl-L-proline (Bip-Pro). KEY RESULTS In SLC36A1-expressing oocytes, an inward current was induced by Gly-Sar, Gly-Gly, δ-aminolevulinic acid, β-aminoethylglycine, δ-aminopentanoic acid, GABA, Gly and Pro, whereas Val, Leu, mannitol, 5-HTP and the dipeptides Gly-Ala, Gly-Pro and Gly-Phe did not evoke currents. In Caco-2 cell monolayers, the apical uptake of 30 mM Gly-Sar was inhibited by 20 and 22% in the presence of 5-HTP or Bip-Pro, respectively, and by 48% in the presence of both. CONCLUSION AND IMPLICATIONS Our results suggest that whereas Gly-Gly amid bond bioisosteres are widely accepted by the hPAT1 carrier, dipeptides in general are not; and therefore, Gly-Sar might structurally define the size limit of dipeptide transport via SLC36A1.

SLC36A4 (hPAT4) is a high affinity amino acid transporter when expressed in Xenopus laevis oocytes

The SLC36 family of transporters consists of four genes, two of which, SLC36A1 and SLC36A2, have been demonstrated to code for human proton-coupled amino acid transporters or hPATs. Here we report the characterization of the fourth member of the family, SLC36A4 or hPAT4, which when expressed in Xenopus laevis oocytes also encodes a plasma membrane amino acid transporter, but one that is not proton-coupled and has a very high substrate affinity for the amino acids proline and tryptophan. hPAT4 in Xenopus oocytes mediated sodium-independent, electroneutral uptake of [(3)H]proline, with the highest rate of uptake when the uptake medium pH was 7.4 and an affinity of 3.13 μM. Tryptophan was also an excellently transported substrate with a similarly high affinity (1.72 μM). Other amino acids that inhibited [(3)H]proline were isoleucine (K(i) 0.23 mM), glutamine (0.43 mM), methionine (0.44 mM), and alanine (1.48 mM), and with lower affinity, glycine, threonine, and cysteine (K(i) >5 mM for all). Of the amino acids directly tested for transport, only proline, tryptophan, and alanine showed significant uptake, whereas glycine and cysteine did not. Of the non-proteogenic amino acids and drugs tested, only sarcosine produced inhibition (K(i) 1.09 mM), whereas γ-aminobutyric acid (GABA), β-alanine, L-Dopa, D-serine, and δ-aminolevulinic acid were without effect on [(3)H]proline uptake. This characterization of hPAT4 as a very high affinity/low capacity non-proton-coupled amino acid transporter raises questions about its physiological role, especially as the transport characteristics of hPAT4 are very similar to the Drosophila orthologue PATH, an amino acid "transceptor" that plays a role in nutrient sensing.