Inhibition of the OCTN2 carnitine transporter by HgCl2and methylmercury in the proteoliposome experimental model: insights in the mechanism of toxicity

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.

Disclosing the Preferential Mercury Chelation by SeCys Containing Peptides over Their Cys Analogues

Methylmercury, mercury (II), and mercury (I) chlorides were found to react with vasopressin, a nonapeptide hormone cyclized by two cysteine residues, and its mono- and diselenium analogues to form several mercury-peptide adducts. The replacement of Cys by SeCys in vasopressin increased the reactivity toward methylmercury, with the predominant formation of -Se/S-Hg-Se-bridged structures and the consequent demethylation of methylmercury. In competitive experiments, CH3HgCl reacted preferentially with the diselenium analogue rather than with vasopressin. The diselenium peptide also showed the capability to displace the CH3Hg moiety bound to S in vasopressin. These results open a promising perspective for the use of selenopeptides for methylmercury chelation and detoxification strategies.

Acetylcholinesterase and human cancers

The enzyme acetylcholinesterase (AChE) is a serine hydrolase whose primary function is to degrade acetylcholine (ACh) and terminate neurotransmission. Apart from its role in synaptic transmission, AChE has several "non-classical" functions in non-neuronal cells. AChE is involved in cellular growth, apoptosis, drug resistance pathways, response to stress signals and inflammation. The observation that the functional activity of AChE is altered in human tumors (relative to adjacent matched normal tissue) has raised several intriguing questions about its role in the pathophysiology of human cancers. Published reports show that AChE is a vital regulator of oncogenic signaling pathways involving proliferation, differentiation, cell-cell adhesion, migration, invasion and metastasis of primary tumors. The objective of this book chapter is to provide a comprehensive overview of the contributions of the AChE-signaling pathway in the growth of progression of human cancers. The AChE isoforms, AChE-T, AChE-R and AChE-S are robustly expressed in human cancer cell lines as well in human tumors (isolated from patients). Traditionally, AChE-modulators have been used in the clinic for treatment of neurodegenerative disorders. Emerging studies reveal that these drugs could be repurposed for the treatment of human cancers. The discovery of potent, selective AChE ligands will provide new knowledge about AChE-regulatory pathways in human cancers and foster the hope of novel therapies for this disease.

Sulfhydryl groups as targets of mercury toxicity

The present study addresses existing data on the affinity and conjugation of sulfhydryl (thiol; -SH) groups of low- and high-molecular-weight biological ligands with mercury (Hg). The consequences of these interactions with special emphasis on pathways of Hg toxicity are highlighted. Cysteine (Cys) is considered the primary target of Hg, and link its sensitivity with thiol groups and cellular damage. In vivo, Hg complexes play a key role in Hg metabolism. Due to the increased affinity of Hg to SH groups in Cys residues, glutathione (GSH) is reactive. The geometry of Hg(II) glutathionates is less understood than that with Cys. Both Cys and GSH Hg-conjugates are important in Hg transport. The binding of Hg to Cys mediates multiple toxic effects of Hg, especially inhibitory effects on enzymes and other proteins that contain free Cys residues. In blood plasma, albumin is the main Hg-binding (Hg2+, CH3Hg+, C2H5Hg+, C6H5Hg+) protein. At the Cys34 residue, Hg2+ binds to albumin, whereas other metals likely are bound at the N-terminal site and multi-metal binding sites. In addition to albumin, Hg binds to multiple Cys-containing enzymes (including manganese-superoxide dismutase (Mn-SOD), arginase I, sorbitol dehydrogenase, and δ-aminolevulinate dehydratase, etc.) involved in multiple processes. The affinity of Hg for thiol groups may also underlie the pathways of Hg toxicity. In particular, Hg-SH may contribute to apoptosis modulation by interfering with Akt/CREB, Keap1/Nrf2, NF-κB, and mitochondrial pathways. Mercury-induced oxidative stress may ensue from Cys-Hg binding and inhibition of Mn-SOD (Cys196), thioredoxin reductase (TrxR) (Cys497) activity, as well as limiting GSH (GS-HgCH3) and Trx (Cys32, 35, 62, 65, 73) availability. Moreover, Hg-thiol interaction also is crucial in the neurotoxicity of Hg by modulating the cytoskeleton and neuronal receptors, to name a few. However, existing data on the role of Hg-SH binding in the Hg toxicity remains poorly defined. Therefore, more research is needed to understand better the role of Hg-thiol binding in the molecular pathways of Hg toxicology and the critical role of thiols to counteract negative effects of Hg overload.

Exploiting Cysteine Residues of SLC Membrane Transporters as Targets for Drugs

The observation that cysteine is the top gainer amino acid during evolution attracted the attention of scientists dealing with protein chemistry. The thiol group of cysteine, indeed, is a potential site for several types of reactions with variable specificity and strength. This feature proved to be promising also in the field of membrane transporters that represent boundary proteins fundamental for cell homeostasis. These proteins are classified, according to the driving force for transport, in primary or secondary active transporters. Another frequently used classification is nowadays based on phylogenesis. Two major groups are identified that take into account both criteria: the ABC and the SLC transporters, the second being much more numerous. The cellular localization of the transporters makes them very attractive for drug design. Moreover, the presence of at least one cysteine residue in all the annotated SLC transporters, so far, highlights the possibility of using the thiol (SH) residue for covalent drug targeting. Even if a delay exists in this research field due to the scarce knowledge of structure/function relationships, the setup of novel experimental tools for studying SLC proteins of plasma and organelle membranes opens an important perspective in pharmacology.

OCTN: A Small Transporter Subfamily with Great Relevance to Human Pathophysiology, Drug Discovery, and Diagnostics

OCTN is a small subfamily of membrane transport proteins that belongs to the larger SLC22 family. Two of the three members of the subfamily, namely, OCTN2 and OCTN1, are present in humans. OCTN2 plays a crucial role in the absorption of carnitine from diet and in its distribution to tissues, as demonstrated by the occurrence of severe pathologies caused by malfunctioning or altered expression of this transporter. These findings suggest avoiding a strict vegetarian diet during pregnancy and in childhood. Other roles of OCTN2 are related to the traffic of carnitine derivatives in many tissues. The role of OCTN1 is still unclear, despite the identification of some substrates such as ergothioneine, acetylcholine, and choline. Plausibly, the transporter acts on the control of inflammation and oxidative stress, even though knockout mice do not display phenotypes. A clear role of both transporters has been revealed in drug interaction and delivery. The polyspecificity of the OCTNs is at the base of the interactions with drugs. Interestingly, OCTN2 has been recently exploited in the prodrug approach and in diagnostics. A promising application derives from the localization of OCTN2 in exosomes that represent a noninvasive diagnostic tool.

Multiple organic cation transporters contribute to the renal transport of sulpiride

Sulpiride, a selective dopamine D2 receptor blocker, is used widely for the treatment of schizophrenia, depression and gastric/duodenal ulcers. Because the great majority of sulpiride is positively charged at physiological pH 7.4, and ~70% of the dose recovered in urine is in the unchanged form after human intravenous administration of sulpiride, it is believed that transporters play an important role in the renal excretion of sulpiride. The aim of the present study was to explore which transporters contribute to the renal disposition of sulpiride. The results demonstrated that sulpiride was a substrate of human carnitine/organic cation transporter 1 (hOCTN1) and 2 (hOCTN2), human organic cation transporter 2 (hOCT2), human multidrug and toxin efflux extrusion protein 1 (hMATE1) and 2-K (hMATE2-K). Sulpiride accumulation from the basolateral (BL) to the apical (AP) side in MDCK-hOCT2/pcDNA3.1 cell monolayers was much greater than that in MDCK-hOCT2/hMATE1 cells, and cimetidine dramatically reduced the intracellular accumulation of sulpiride from BL to AP. In addition, the accumulation of sulpiride in mouse primary renal tubular cells (mPRTCs) was markedly reduced by inhibitors of Oct2 and Octns. The results implied that OCTN1, OCTN2, OCT2, MATE1 and MATE2-K probably contributed to the renal transfer of sulpiride, in which OCT2 mediated the uptake of sulpiride from the bloodstream to the proximal tubular cells, while MATEs contributed to the sulpiride efflux from the proximal tubular cells to the renal lumen, and OCTNs participated in both renal secretion and reabsorption.

Alkyl Mercury-Induced Toxicity: Multiple Mechanisms of Action

There are a number of mechanisms by which alkylmercury compounds cause toxic action in the body. Collectively, published studies reveal that there are some similarities between the mechanisms of the toxic action of the mono-alkyl mercury compounds methylmercury (MeHg) and ethylmercury (EtHg). This paper represents a summary of some of the studies regarding these mechanisms of action in order to facilitate the understanding of the many varied effects of alkylmercurials in the human body. The similarities in mechanisms of toxicity for MeHg and EtHg are presented and compared. The difference in manifested toxicity of MeHg and EtHg are likely the result of the differences in exposure, metabolism, and elimination from the body, rather than differences in mechanisms of action between the two.

Species specificity profiling of rat and human organic cation/carnitine transporter Slc22a5/SLC22A5 (Octn2/OCTN2)

The purpose of this study was to characterize the uptake of carnitine, the physiological substrate, and the uptake of 3-(2,2,2-trimethylhydrazinium)propionate, a consensus substrate by rat Octn2 and human OCTN2 transporters as well as to characterize drug-mediated inhibition of l-carnitine uptake by the rat and human orthologs overexpressed in CHO-K1 cells. l-carnitine and 3-(2,2,2-trimethylhydrazinium)propionate were found to be a lower affinity substrate for rat Octn2 (K_M = 32.66 ± 5.11 μM and 23.62 ± 4.99 μM respectively) than for human OCTN2 (K_M = 3.08 ± 0.74 μM and 7.98 ± 0.63 μM). The intrinsic clearance (CL_int) value for carnitine was higher for the human than for the rat transporter (22.82 ± 5.57 ml/min*mg vs 4.008 ± 0.675 ml/min*mg). For 3-(2,2,2-trimethylhydrazinium)propionate, in contrast, the CL_int value for rat Octn2 was higher than for human OCTN2 (323.9 ± 72.8 ml/min*mg vs 65.11 ± 5.33 ml/min*mg). Furthermore, many pharmacologically important drugs were shown to affect l-carnitine transport by Octn2/OCTN2. The correlation between the IC₅₀ datasets for the rat and human transporter resulted in an r value of 0.47 (p > 0.05). However, the greatest difference was less than seven-fold and 13 of 15 compounds yielded a difference less than 3-fold. Thus, the transporters from these two species showed an overlapping but somewhat different substrate and inhibitor specificity.

Acetylcholine and acetylcarnitine transport in peritoneum: Role of the SLC22A4 (OCTN1) transporter

A suitable experimental tool based on proteoliposomes for assaying Organic Cation Transporter Novel member 1 (OCTN1) of peritoneum was pointed out. OCTN1, recently acknowledged as acetylcholine transporter, was immunodetected in rat peritoneum. Transport was assayed following flux of radiolabelled TEA, acetylcholine or acetylcarnitine in proteoliposomes reconstituted with peritoneum extract. OCTN1 mediated, besides TEA, also acetylcholine and a slower acetylcarnitine transport. External sodium inhibited acetylcholine uptake but not its release from proteoliposomes. Differently, sodium did not affect acetylcarnitine uptake. These results suggested that physiologically, acetylcholine should be released while acetylcarnitine was taken up by peritoneum cells. Transport was impaired by OCTN1 inhibitors, butyrobetaine, spermine, and choline. Biotin was also found as acetylcholine transport inhibitor. Anti-OCTN1 antibody specifically inhibited acetylcholine transport confirming the involvement of OCTN1. The transporter was also immunodetected in human mesothelial primary cells. Extract from these cells was reconstituted in proteoliposomes. Transport features very similar to those found with rat peritoneum were observed. Validation of the proteoliposome model for peritoneal transport study was then achieved assaying transport in intact mesothelial cells. TEA, butyrobetaine and Na(+) inhibited acetylcholine transport in intact cells while efflux was Na(+) insensitive. Therefore transport features in intact cells overlapped those found in proteoliposomes.

Mitochondrial carnitine/acylcarnitine transporter, a novel target of mercury toxicity

The effect of Hg(2+) and CH3Hg(+) on the mitochondrial carnitine/acylcarnitine transporter (CACT) has been studied on the recombinant protein and on the CACT extracted from HeLa cells or Zebrafish and reconstituted in proteoliposomes. Transport was abolished upon treatment of the recombinant CACT in proteoliposomes by Hg(2+) or CH3Hg(+). Inhibition was reversed by the SH reducing agent 1,4-dithioerythritol, GSH, and N-acetylcysteine. IC50 for Hg(2+) and CH3Hg(+) of 90 nM and 137 nM, respectively, were measured by dose-response analyses. Inhibition was abolished in the C-less CACT mutant. Strong reduction of inhibition by both reagents was observed in the C136A and some reduction in the C155A mutants. Inhibition similar to that of the WT was observed in the C23V/C58V/C89S/C155V/C283S mutant, containing only C136. Optimal inhibition by Hg(2+)was found in the four replacement mutants C23V/C58V/C89S/C283S containing both C136 and C155 indicating cross-reaction of Hg(2+) with the two Cys residues. Inhibition kinetic analysis showed mixed inhibition by Hg(2+) or competitive inhibition by CH3Hg(+). HeLa cells or Zebrafish were treated with the more potent inhibitor. Ten micromolar HgCl2 caused clear impairment of viability of HeLa cells. The transport assay in proteoliposomes with CACT extracted from treated cells showed that the transporter was inactivated and that DTE rescued the activity. Nearly identical results were observed with Zebrafish upon extraction of the CACT from the liver of the treated animals that, indeed, showed accumulation of the mercurial compound.

Functional and molecular effects of mercury compounds on the human OCTN1 cation transporter: C50 and C136 are the targets for potent inhibition

The effect of mercury compounds has been tested on the organic cation transporter, hOCTN1. MeHg(+), Hg(2+), or Cd(2+) caused strong inhibition of transport. 1,4-Dithioerythritol (DTE), cysteine (Cys), and N-acetyl-l-cysteine reversed (NAC) the inhibition at different extents. 2-Aminoethyl methanethiosulfonate hydrobromide (MTSEA), a prototype SH reagent, exerted inhibition of transport similar to that observed for the mercurial agents. To investigate the mechanism of action of mercurials, mutants of hOCTN1 in which each of the Cys residues was substituted by Ala have been constructed, over-expressed in Escherichia coli, and purified. Tetraethylammonium chloride (TEA) uptake mediated by each mutant in proteoliposomes was comparable to that of wild type (WT). IC50 values of the WT and mutants for the mercury compounds were derived from dose-response analyses. The mutants C50A and C136A showed significant increase of IC50 indicating that the 2 Cys residues were involved in the interaction with the mercury compounds and inhibition of the transporter. The double mutant C50A/C136A was constructed; the lack of inhibition confirmed that the 2 Cys residues are the targets of mercury compounds. MTSEA showed similar behavior with respect to the mercurial reagents with the difference that increased IC50 was observed also in the C81A mutant. Similar results were obtained when transport was measured as acetylcholine uptake. Ethyl mercury (Thimerosal) inhibited hOCTN1 as well. C50A, C50A/C136A and, at very lower extent, C136A showed increased IC50 indicating that C50 was the major target of this mercury compound. The homology model of hOCTN1 was built using as template PiPT and validated by the experimental data on mutant proteins.

Nimesulide binding site in the B0AT1 (SLC6A19) amino acid transporter. Mechanism of inhibition revealed by proteoliposome transport assay and molecular modelling

The effect of pharmaceutical compounds on the rat kidney B0AT1 transporter in proteoliposomes has been screened. To this aim, inhibition of the transport activity by the different compounds was measured on Na(+)-[(3)H]glutamine co-transport in the presence of membrane potential positive outside. Most of the tested drugs had no effect on the transport activity. Some compounds exhibited inhibitory effects from 5 to 88% at concentration of 300μM. Among the tested compounds, only the anti-inflammatory drug nimesulide exerted potent inhibition on B0AT1. From dose response analysis, an IC50 value of 23μM was found. Inhibition kinetic analysis was performed: noncompetitive inhibition of the glutamine transport was observed while competitive behaviour was found when the inhibition was analyzed with respect to the Na(+) concentration. Several molecules harbouring functional groups of nimesulide (analogues) were tested as inhibitors. None among the tested molecules has the capacity to inhibit the transport with the exception of the compound NS-398, whose chemical structure is very close to that of whole nimesulide. The IC50 for this compound was 131μM. Inhibition kinetics showed behaviour of NS-398 identical to that of nimesulide, i.e., noncompetitive inhibition respect to glutamine and competitive inhibition respect to Na(+). Molecular docking of nimesulide suggested that this drug is able to bind B0AT1 in an external dedicated binding site and that its binding produces a steric hindrance effect of the protein translocation path abolishing the transporter activity.

Proteoliposomes as tool for assaying membrane transporter functions and interactions with xenobiotics

Proteoliposomes represent a suitable and up to date tool for studying membrane transporters which physiologically mediate absorption, excretion, trafficking and reabsorption of nutrients and metabolites. Using recently developed reconstitution strategies, transporters can be inserted in artificial bilayers with the same orientation as in the cell membranes and in the absence of other interfering molecular systems. These methodologies are very suitable for studying kinetic parameters and molecular mechanisms. After the first applications on mitochondrial transporters, in the last decade, proteoliposomes obtained with optimized methodologies have been used for studying plasma membrane transporters and defining their functional and kinetic properties and structure/function relationships. A lot of information has been obtained which has clarified and completed the knowledge on several transporters among which the OCTN sub-family members, transporters for neutral amino acid, B0AT1 and ASCT2, and others. Transporters can mediate absorption of substrate-like derivatives or drugs, improving their bioavailability or can interact with these compounds or other xenobiotics, leading to side/toxic effects. Therefore, proteoliposomes have recently been used for studying the interaction of some plasma membrane and mitochondrial transporters with toxic compounds, such as mercurials, H₂O₂ and some drugs. Several mechanisms have been defined and in some cases the amino acid residues responsible for the interaction have been identified. The data obtained indicate proteoliposomes as a novel and potentially important tool in drug discovery.

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.