Substrates of the human oligopeptide transporter hPEPT2

Critical Roles of the Cysteine-Glutathione Axis in the Production of γ-Glutamyl Peptides in the Nervous System

γ-Glutamyl moiety that is attached to the cysteine (Cys) residue in glutathione (GSH) protects it from peptidase-mediated degradation. The sulfhydryl group of the Cys residue represents most of the functions of GSH, which include electron donation to peroxidases, protection of reactive sulfhydryl in proteins via glutaredoxin, and glutathione conjugation of xenobiotics, whereas Cys-derived sulfur is also a pivotal component of some redox-responsive molecules. The amount of Cys that is available tends to restrict the capacity of GSH synthesis. In in vitro systems, cystine is the major form in the extracellular milieu, and a specific cystine transporter, xCT, is essential for survival in most lines of cells and in many primary cultivated cells as well. A reduction in the supply of Cys causes GPX4 to be inhibited due to insufficient GSH synthesis, which leads to iron-dependent necrotic cell death, ferroptosis. Cells generally cannot take up GSH without the removal of γ-glutamyl moiety by γ-glutamyl transferase (GGT) on the cell surface. Meanwhile, the Cys-GSH axis is essentially common to certain types of cells; primarily, neuronal cells that contain a unique metabolic system for intercellular communication concerning γ-glutamyl peptides. After a general description of metabolic processes concerning the Cys-GSH axis, we provide an overview and discuss the significance of GSH-related compounds in the nervous system.

Transporter-Mediated Drug Delivery

Transmembrane transport of small organic and inorganic molecules is one of the cornerstones of cellular metabolism. Among transmembrane transporters, solute carrier (SLC) proteins form the largest, albeit very diverse, superfamily with over 400 members. It was recognized early on that xenobiotics can directly interact with SLCs and that this interaction can fundamentally determine their efficacy, including bioavailability and intertissue distribution. Apart from the well-established prodrug strategy, the chemical ligation of transporter substrates to nanoparticles of various chemical compositions has recently been used as a means to enhance their targeting and absorption. In this review, we summarize efforts in drug design exploiting interactions with specific SLC transporters to optimize their therapeutic effects. Furthermore, we describe current and future challenges as well as new directions for the advanced development of therapeutics that target SLC transporters.

Biology of Peptide Transporter 2 in Mammals: New Insights into Its Function, Structure and Regulation

Peptide transporter 2 (PepT2) in mammals plays essential roles in the reabsorption and conservation of peptide-bound amino acids in the kidney and in maintaining neuropeptide homeostasis in the brain. It is also of significant medical and pharmacological significance in the absorption and disposing of peptide-like drugs, including angiotensin-converting enzyme inhibitors, β-lactam antibiotics and antiviral prodrugs. Understanding the structure, function and regulation of PepT2 is of emerging interest in nutrition, medical and pharmacological research. In this review, we provide a comprehensive overview of the structure, substrate preferences and localization of PepT2 in mammals. As PepT2 is expressed in various organs, its function in the liver, kidney, brain, heart, lung and mammary gland has also been addressed. Finally, the regulatory factors that affect the expression and function of PepT2, such as transcriptional activation and posttranslational modification, are also discussed.

The ubiquitin ligase Nedd4-2 mediates the regulation of PepT2 by mTORC1 in bovine mammary epithelial cells

Peptide transporter 2 (PepT2) transports short peptides from the blood into bovine mammary epithelial cells (BMEC) to stimulate milk protein synthesis. Despite the fact that the effect of PepT2 is acknowledged in BMEC, little is known about its regulation. This study was completed to investigate the role of mammalian target of the rapamycin (mTOR) signaling in regulating the expression and function of PepT2 in BMEC. The regulation of PepT2 by mTOR in BMEC was studied in vitro using peptide transport assay, gene silencing, Western blot. The membrane expression of PepT2 and the uptake of β-Ala-Lys-N-7-amino-4-methylcoumarin-3-acetic acid (β-Ala-Lys-AMCA), a model dipeptide, in BMEC were reduced by rapamycin (a mTOR inhibitor) and silencing of either mTOR complex 1 (mTORC1) or mTOR complex 2 (mTORC2), stimulated by DEP domain-containing mTOR-interacting protein (DEPTOR, endogenous inhibitor of mTORC1 and mTORC2) silencing. The trafficking of PepT2 to the membrane and the uptake of β-Ala-Lys-AMCA was promoted by neuronal precursor cell-expressed developmentally down-regulated 4 isoform 2 (Nedd4-2) silencing. The effects of knockdown of mTORC1, but not mTORC2, on cell membrane expression and transport activity of PepT2 was abolished by Nedd4-2 silencing. With immunofluorescence staining, PepT2 was identified to be interacting with Nedd4-2. The Nedd4-2 expression and the interaction between PepT2 and Nedd4-2 was increased through mTORC1 knockdown, indicating an increased ubiquitination of PepT2. The results revealed that mTORC1 can regulate the expression and function of PepT2 through Nedd4-2 in BMEC.

Functional characterization of Atlantic salmon (Salmo salar L.) PepT2 transporters

Abstract

The high‐affinity/low‐capacity system Slc15a2 (PepT2) is responsible for the reuptake of di/tripeptides from the renal proximal tubule, but it also operates in many other tissues and organs. Information regarding PepT2 in teleost fish is limited and, to date, functional data are available from the zebrafish (Danio rerio) only. Here, we report the identification of two slc15a2 genes in the Atlantic salmon (Salmo salar) genome, namely slc15a2a and slc15a2b. The two encoded PepT2 proteins share 87% identity and resemble both structurally and functionally the canonical vertebrate PepT2 system. The mRNA tissue distribution analyses reveal a widespread distribution of slc15a2a transcripts, being more abundant in the brain and gills, while slc15a2b transcripts are mainly expressed in the kidney and the distal part of the gastrointestinal tract. The function of the two transporters was investigated by heterologous expression in Xenopus laevis oocytes and two‐electrode voltage‐clamp recordings of transport and presteady‐state currents. Both PepT2a and PepT2b in the presence of Gly‐Gln elicit pH‐dependent and Na⁺ independent inward currents. The biophysical and kinetic analysis of the recorded currents defined the transport properties, confirming that the two Atlantic salmon PepT2 proteins behave as high‐affinity/low‐capacity transporters. The recent structures and the previous kinetic schemes of rat and human PepT2 qualitatively account for the characteristics of the two Atlantic salmon proteins. This study is the first to report on the functional expression of two PepT2‐type transporters that operate in the same vertebrate organism as a result of (a) gene duplication process(es).

Key points

Two slc15a2‐type genes, slc15a2a and slc15a2b coding for PepT2‐type peptide transporters were found in the Atlantic salmon.

slc15a2a transcripts, widely distributed in the fish tissues, are abundant in the brain and gills, while slc15a2b transcripts are mainly expressed in the kidney and distal gastrointestinal tract.

Amino acids involved in vertebrate Slc15 transport function are conserved in PepT2a and PepT2b proteins.

Detailed kinetic analysis indicates that both PepT2a and PepT2b operate as high‐affinity transporters.

The kinetic schemes and structures proposed for the mammalian models of PepT2 are suitable to explain the function of the two Atlantic salmon transporters.

Finding a cell-permeable compound to inhibit inflammatory cytokines: Uptake, biotransformation, and anti-cytokine activity of javamide-I/-II esters

AIM

Currently, there is limited information available about cell-permeability and anti-cytokine activity of javamide-I/-II esters in monocyte/macrophage-like cells. Therefore, the aim of this study was to investigate their cell-permeability and anti-cytokine activity in the cells.

MATERIALS AND METHODS

The uptake of javamide-I/-II and esters was studied in THP-1 cells and PBMCs. Also, kinetic and inhibition studies were conducted using THP-1 cells. Western Blot was performed to determine the level of ATF-2 phosphorylation in THP-1 cells, and ELISA assays were carried out to measure TNF-alpha, MCP-1, IL-1beta and IL-8 levels in PBMCs.

KEY FINDINGS

In THP-1 cells, the uptake of javamide-I/-II esters was significantly higher than javamide-I/-II (P < 0.001), and the K_m for javamide-I ester was 27 μM. Also, the uptake of the esters was inhibited by PepT2 substrate/blocker. In THP-1 cells, javamide-I/-II esters were also biotransformed into javamide-I/-II. Furthermore, javamide-I ester could inhibit ATF-2 phosphorylation better than javamide-I in the cells, suggesting that the ester could be transported inside the cells better than javamide-I. Similarly, javamide-I/-II esters were found to be transported and biotransformed in PBMCs involved in inflammation processes. As anticipated, the esters were found to inhibit TNF-alpha and MCP-1 significantly in PBMCs (P < 0.005). Especially, javamide-I ester inhibited TNF-alpha, MCP-1, IL-1beta and IL-8 with IC₅₀ values of 1.79, 0.88, 0.91 and 2.57 μM in PBMCs.

SIGNIFICANCE

Javamide-I/-II esters can be transported, biotransformed and inhibit inflammatory cytokines significantly in monocyte/macrophage-like cells, suggesting that they may be utilized as a potent cell-permeable carrier to inhibit inflammatory cytokines in the cells.

CHEMICAL COMPOUNDS

Javamide-I, javamide-I-O-methyl ester, javamide-II, javamide-II-O-methyl ester, tryptophan, coumaric acid, caffeic acid, GlySar, enalapril.

Small Airway Susceptibility to Chemical and Particle Injury

Small airways (SA) in humans are commonly defined as those conducting airways <2 mm in diameter. They are susceptible to particle- and chemical-induced injury and play a major role in the development of airway disease such as COPD and asthma. Susceptibility to injury can be attributed in part to structural features including airflow dynamics and tissue architecture, but recent evidence may indicate a more prominent role for cellular composition in directing toxicological responses. Animal studies support the hypothesis that inherent cellular differences across the tracheobronchial tree, including metabolic CYP450 expression in the distal conducting airways, can influence SA susceptibility to injury. Currently, there is insufficient information in humans to make similar conclusions, prompting further necessary work in this area. An understanding of why the SA are more susceptible to certain chemical and particle exposures than other airway regions is fundamental to our ability to identify hazardous materials, their properties, and accompanying exposure scenarios that compromise lung function. It is also important for the ability to develop appropriate models for toxicity testing. Moreover, it is central to our understanding of SA disease aetiology and how interventional strategies for treatment may be developed. In this review, we will document the structural and cellular airway regional differences that are likely to influence airway susceptibility to injury, including the role of secretory club cells. We will also describe recent advances in single-cell sequencing of human airways, which have provided unprecedented details of cell phenotype, likely to impact airway chemical and particle injury.

Carnosine dipeptidase II (CNDP2) protects cells under cysteine insufficiency by hydrolyzing glutathione-related peptides

The knockout (KO) of the cystine transporter xCT causes ferroptosis, a type of iron-dependent necrotic cell death, in mouse embryonic fibroblasts, but this does not occur in macrophages. In this study, we explored the gene that supports cell survival under a xCT deficiency using a proteomics approach. Analysis of macrophage-derived peptides that were tagged with iTRAQ by liquid chromatography-mass spectrometry revealed a robust elevation in the levels of carnosine dipeptidase II (CNDP2) in xCT KO macrophages. The elevation in the CNDP2 protein levels was confirmed by immunoblot analyses and this elevation was accompanied by an increase in hydrolytic activity towards cysteinylglycine, the intermediate degradation product of glutathione after the removal of the γ-glutamyl group, in xCT KO macrophages. Supplementation of the cystine-free media of Hepa1-6 cells with glutathione or cysteinylglycine extended their survival, whereas the inclusion of bestatin, an inhibitor of CNDP2, counteracted the effects of these compounds. We established CNDP2 KO mice by means of the CRISPR/Cas9 system and found a decrease in dipeptidase activity in the liver, kidney, and brain. An acetaminophen overdose (350 mg/kg) showed not only aggravated hepatic damage but also renal injury in the CNDP2 KO mice, which was not evident in the wild-type mice that were receiving the same dose. The aggravated renal damage in the CNDP2 KO mice was consistent with the presence of abundant levels of CNDP2 in the kidney, the organ prone to developing ferroptosis. These collective data imply that cytosolic CNDP2, in conjugation with the removal of the γ-glutamyl group, recruits Cys from extracellular GSH and supports redox homeostasis of cells, particularly in epithelial cells of proximal tubules that are continuously exposed to oxidative insult from metabolic wastes that are produced in the body.

A mechanism for matrikine regulation in acute inflammatory lung injury

Proline-glycine-proline (PGP) and its acetylated form (Ac-PGP) are neutrophil chemoattractants generated by collagen degradation, and they have been shown to play a role in chronic inflammatory disease. However, the mechanism for matrikine regulation in acute inflammation has not been well established. Here, we show that these peptides are actively transported from the lung by the oligopeptide transporter, PEPT2. Following intratracheal instillation of Ac-PGP in a mouse model, there was a rapid decline in concentration of the labeled peptide in the bronchoalveolar lavage (BAL) over time and redistribution to extrapulmonary sites. In vitro knockdown of the PEPT2 transporter in airway epithelia or use of a competitive inhibitor of PEPT2, cefadroxil, significantly reduced uptake of Ac-PGP. Animals that received intratracheal Ac-PGP plus cefadroxil had higher levels of Ac-PGP in BAL and lung tissue. Utilizing an acute LPS-induced lung injury model, we demonstrate that PEPT2 blockade enhanced pulmonary Ac-PGP levels and lung inflammation. We further validated this effect using clinical samples from patients with acute lung injury in coculture with airway epithelia. This is the first study to our knowledge to determine the in vitro and in vivo significance of active matrikine transport as a mechanism of modulating acute inflammation and to demonstrate that it may serve as a potential therapeutic target.

Dysregulation of solute carrier transporters in malaria-infected pregnant mice

AIMS

Malaria in pregnancy (MiP) alters the expression of ATP-binding cassette efflux transporters in maternal and foetal tissues, as well as the placenta. Malaria induces oxidative stress, and pregnancy is associated with arginine deficiency. We hypothesized that reducing oxidative stress during MiP by supplementation with L-arginine, a NO precursor, would attenuate transcriptional changes in a second superfamily of transporters, solute carrier (SLC) transporters, and improve pregnancy outcomes.

METHODS AND RESULTS

Pregnant BALB/c mice receiving L-arginine (1.2%) in water, or water alone, were infected with Plasmodium berghei ANKA on gestational day 13 and sacrificed on gestational day 19. Compared to controls, the mRNA of numerous SLC transporters was downregulated in maternal and foetal tissues, as well as in the placentas of infected mice. While supplementation with L-arginine did improve foetal viability, it did not improve the mRNA expression of oxidative stress markers, transporters nor other indices of foetal and maternal health. Moreover, amino acid uptake transporters were downregulated upon infection, which could potentially contribute to decreased foetal birthweight.

CONCLUSIONS

Malaria in pregnancy significantly alters the expression of SLC transporters in maternal and foetal tissues as well as the placenta, regardless of L-arginine supplementation. Further studies to investigate methods of reducing oxidative stress in MiP are warranted.

Organic and Peptidyl Constituents of Snake Venoms: The Picture Is Vastly More Complex Than We Imagined

Small metabolites and peptides in 17 snake venoms (Elapidae, Viperinae, and Crotalinae), were quantified using liquid chromatography-mass spectrometry. Each venom contains >900 metabolites and peptides. Many small organic compounds are present at levels that are probably significant in prey envenomation, given that their known pharmacologies are consistent with snake envenomation strategies. Metabolites included purine nucleosides and their bases, neurotransmitters, neuromodulators, guanidino compounds, carboxylic acids, amines, mono- and disaccharides, and amino acids. Peptides of 2⁻15 amino acids are also present in significant quantities, particularly in crotaline and viperine venoms. Some constituents are specific to individual taxa, while others are broadly distributed. Some of the latter appear to support high anabolic activity in the gland, rather than having toxic functions. Overall, the most abundant organic metabolite was citric acid, owing to its predominance in viperine and crotaline venoms, where it chelates divalent cations to prevent venom degradation by venom metalloproteases and damage to glandular tissue by phospholipases. However, in terms of their concentrations in individual venoms, adenosine, adenine, were most abundant, owing to their high titers in Dendroaspis polylepis venom, although hypoxanthine, guanosine, inosine, and guanine all numbered among the 50 most abundant organic constituents. A purine not previously reported in venoms, ethyl adenosine carboxylate, was discovered in D. polylepis venom, where it probably contributes to the profound hypotension caused by this venom. Acetylcholine was present in significant quantities only in this highly excitotoxic venom, while 4-guanidinobutyric acid and 5-guanidino-2-oxopentanoic acid were present in all venoms.

The zinc form of carnosine dipeptidase 2 (CN2) has dipeptidase activity but its substrate specificity is different from that of the manganese form

Carnosine dipeptidase II (CN2), a metallopeptidase present in the cytosol of various vertebrate tissues, catalyzes the hydrolysis of carnosine and several other dipeptides in the presence of Mn²⁺. Although the metal-binding center of mouse CN2 is also able to associate with Zn²⁺in vitro, it was not known whether the zinc form of CN2 has any enzymatic activity. In the present study, we show that Zn²⁺ has a higher affinity for binding to CN2 than Mn²⁺, as evidenced by native mass spectrometry. The issue of whether the zinc form of CN2 has enzymatic activity was also examined using various dipeptides as substrates. The findings indicate that the zinc form of CN2 catalyzes the hydrolysis of several different dipeptides including Leu-His, Met-His and Ala-His at a reaction rate comparable to that for its manganese form. On the other hand, the zinc form of CN2 did not catalyze the hydrolysis of carnosine and several other dipeptides that are hydrolyzed by the manganese form of CN2. Substrate specificity was also examined in HEK293T cells expressing CN2, and the findings indicate that Leu-His, Met-His, but not carnosine, were hydrolyzed in the cell culture. These results suggest that the zinc form of CN2 is an active enzyme, but with a different substrate specificity from that of the manganese form.

Quantification of Transporter and Receptor Proteins in Dog Brain Capillaries and Choroid Plexus: Relevance for the Distribution in Brain and CSF of Selected BCRP and P-gp Substrates

Transporters at the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB) play a pivotal role as gatekeepers for efflux or uptake of endogenous and exogenous molecules. The protein expression of a number of them has already been determined in the brains of rodents, nonhuman primates, and humans using quantitative targeted absolute proteomics (QTAP). The dog is an important animal model for drug discovery and development, especially for safety evaluations. The purpose of the present study was to clarify the relevance of the transporter protein expression for drug distribution in the dog brain and CSF. We used QTAP to examine the protein expression of 17 selected transporters and receptors at the dog BBB and BCSFB. For the first time, we directly linked the expression of two efflux transporters, P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), to regional brain and CSF distribution using specific substrates. Two cocktails, each containing one P-gp substrate (quinidine or apafant) and one BCRP substrate (dantrolene or daidzein) were infused intravenously prior to collection of the brain. Transporter expression varied only slightly between the capillaries of different brain regions and did not result in region-specific distribution of the investigated substrates. There were, however, distinct differences between brain capillaries and choroid plexus. Largest differences were observed for BCRP and P-gp: both were highly expressed in brain capillaries, but no BCRP and only low amounts of P-gp were detected in the choroid plexus. K_p,uu,brain and K_p,uu,CSF of both P-gp substrates were indicative of drug efflux. Also, K_p,uu,brain for the BCRP substrates was low. In contrast, K_p,uu,CSF for both BCRP substrates was close to unity, resulting in K_p,uu,CSF/K_p,uu,brain ratios of 7 and 8, respectively. We conclude that the drug transporter expression profiles differ between the BBB and BCSFB in dogs, that there are species differences in the expression profiles, and that CSF is not a suitable surrogate for unbound brain concentrations of BCRP substrates in dogs.

Insights into solute carriers: physiological functions and implications in disease and pharmacokinetics

SLCs transport many endogenous and exogenous compounds including drugs; SLCs dysfunction has implications in pharmacokinetics, drug toxicity or lack of efficacy.