Drug Transporters At Brain Barriers

Disease-Induced Modulation of Drug Transporters at the Blood-Brain Barrier Level

The blood–brain barrier (BBB) is a highly selective and restrictive semipermeable network of cells and blood vessel constituents. All components of the neurovascular unit give to the BBB its crucial and protective function, i.e., to regulate homeostasis in the central nervous system (CNS) by removing substances from the endothelial compartment and supplying the brain with nutrients and other endogenous compounds. Many transporters have been identified that play a role in maintaining BBB integrity and homeostasis. As such, the restrictive nature of the BBB provides an obstacle for drug delivery to the CNS. Nevertheless, according to their physicochemical or pharmacological properties, drugs may reach the CNS by passive diffusion or be subjected to putative influx and/or efflux through BBB membrane transporters, allowing or limiting their distribution to the CNS. Drug transporters functionally expressed on various compartments of the BBB involve numerous proteins from either the ATP-binding cassette (ABC) or the solute carrier (SLC) superfamilies. Pathophysiological stressors, age, and age-associated disorders may alter the expression level and functionality of transporter protein elements that modulate drug distribution and accumulation into the brain, namely, drug efficacy and toxicity. This review focuses and sheds light on the influence of inflammatory conditions and diseases such as Alzheimer’s disease, epilepsy, and stroke on the expression and functionality of the BBB drug transporters, the consequential modulation of drug distribution to the brain, and their impact on drug efficacy and toxicity.

Regulation of brain drug metabolizing enzymes and transporters by nuclear receptors

Nuclear receptors (NRs) belong to a family of ligand-dependent transcription factors. The target genes of NRs include many drug metabolizing enzymes and transporters. The central nervous system (CNS) bears the expression of NRs, drug metabolizing enzymes and transporters. NRs that express in the brain can be divided into three groups according to their characteristics of ligand binding: steroid hormone receptors, non-steroid hormone receptors, and orphan receptors. The NR-mediated regulation of drug metabolizing enzymes and transporters plays important roles in the metabolism and disposition of drugs in the CNS and the penetration of endogenous and exogenous substances through the blood-brain barrier (BBB). NR-mediated regulation of drug metabolizing enzymes and transporters can cause the toxicological effects of xenobiotics in the CNS and also lead to drug resistance in the centrum. The regulatory pathways of drug metabolizing enzymes and transporters can provide new strategies for selective regulation of the BBB permeability and drug metabolism in the brain. This review focuses on the importance of NR-mediated regulation of drug metabolizing enzymes and transporters in the CNS and the implications of this regulation in the therapeutic effect of CNS drugs and CNS side effects of drugs and other xenotoxicants.

Innate Immunity Cells and the Neurovascular Unit

Recent studies have clarified many still unknown aspects related to innate immunity and the blood-brain barrier relationship. They have also confirmed the close links between effector immune system cells, such as granulocytes, macrophages, microglia, natural killer cells and mast cells, and barrier functionality. The latter, in turn, is able to influence not only the entry of the cells of the immune system into the nervous tissue, but also their own activation. Interestingly, these two components and their interactions play a role of great importance not only in infectious diseases, but in almost all the pathologies of the central nervous system. In this paper, we review the main aspects in the field of vascular diseases (cerebral ischemia), of primitive and secondary neoplasms of Central Nervous System CNS, of CNS infectious diseases, of most common neurodegenerative diseases, in epilepsy and in demyelinating diseases (multiple sclerosis). Neuroinflammation phenomena are constantly present in all diseases; in every different pathological state, a variety of innate immunity cells responds to specific stimuli, differentiating their action, which can influence the blood-brain barrier permeability. This, in turn, undergoes anatomical and functional modifications, allowing the stabilization or the progression of the pathological processes.

Impact of aging, Alzheimer's disease and Parkinson's disease on the blood-brain barrier transport of therapeutics

Older people are at a greater risk of medicine-induced toxicity resulting from either increased drug sensitivity or age-related pharmacokinetic changes. The scenario is further complicated with the two most prevalent age-related neurodegenerative diseases, Alzheimer's disease (AD) and Parkinson's disease (PD). With aging, AD and PD, there is growing evidence of altered structure and function of the blood-brain barrier (BBB), including modifications to tight junctions and efflux transporters, such as P-glycoprotein. The subsequent impact on CNS drug exposure and risk of neurotoxicity from systemically-acting medicines is less well characterized. The purpose of this review, therefore, is to provide an overview of the multiple changes that occur to the BBB as a result of aging, AD and PD, and the impact that such changes have on CNS exposure of drugs, based on studies conducted in aged rodents or rodent models of disease, and in elderly people with and without AD or PD.

Signaling pathways regulating blood-tissue barriers - Lesson from the testis

Signaling pathways that regulate blood-tissue barriers are important for studying the biology of various blood-tissue barriers. This information, if deciphered and better understood, will provide better therapeutic management of diseases particularly in organs that are sealed by the corresponding blood-tissue barriers from systemic circulation, such as the brain and the testis. These barriers block the access of antibiotics and/or chemotherapeutical agents across the corresponding barriers. Studies in the last decade using the blood-testis barrier (BTB) in rats have demonstrated the presence of several signaling pathways that are crucial to modulate BTB function. Herein, we critically evaluate these findings and provide hypothetical models regarding the underlying mechanisms by which these signaling molecules/pathways modulate BTB dynamics. This information should be carefully evaluated to examine their applicability in other tissue barriers which shall benefit future functional studies in the field. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Platinum-induced neurotoxicity and preventive strategies: past, present, and future

Neurotoxicity is a burdensome side effect of platinum-based chemotherapy that prevents administration of the full efficacious dosage and often leads to treatment withdrawal. Peripheral sensory neurotoxicity varies from paresthesia in fingers to ataxic gait, which might be transient or irreversible. Because the number of patients being treated with these neurotoxic agents is still increasing, the need for understanding the pathogenesis of this dramatic side effect is critical. Platinum derivatives, such as cisplatin and carboplatin, harm mainly peripheral nerves and dorsal root ganglia neurons, possibly because of progressive DNA-adduct accumulation and inhibition of DNA repair pathways (e.g., extracellular signal-regulated kinase 1/2, c-Jun N-terminal kinase/stress-activated protein kinase, and p38 mitogen-activated protein kinass), which finally mediate apoptosis. Oxaliplatin, with a completely different pharmacokinetic profile, may also alter calcium-sensitive voltage-gated sodium channel kinetics through a calcium ion immobilization by oxalate residue as a calcium chelator and cause acute neurotoxicity. Polymorphisms in several genes, such as voltage-gated sodium channel genes or genes affecting the activity of pivotal metal transporters (e.g., organic cation transporters, organic cation/carnitine transporters, and some metal transporters, such as the copper transporters, and multidrug resistance-associated proteins), can also influence drug neurotoxicity and treatment response. However, most pharmacogenetics studies need to be elucidated by robust evidence. There are supportive reports about the effectiveness of several neuroprotective agents (e.g., vitamin E, glutathione, amifostine, xaliproden, and venlafaxine), but dose adjustment and/or drug withdrawal seem to be the most frequently used methods in the management of platinum-induced peripheral neurotoxicity. To develop alternative options in the treatment of platinum-induced neuropathy, studies on in vitro models and appropriate trials planning should be integrated into the future design of neuroprotective strategies to find the best patient-oriented solution.

Role of nuclear factor-erythroid 2-related factor 2 (Nrf2) in the transcriptional regulation of brain ABC transporters during acute acetaminophen (APAP) intoxication in mice

Changes in expression of liver ABC transporters have been described during acute APAP intoxication. However, the effect of APAP on brain ABC transporters is poorly understood. The aim of this study was to evaluate the effect of APAP on brain ABC transporters expression and the role of the oxidative stress sensor Nrf2. Male C57BL/6J mice were administered APAP (400mg/kg) for analysis of brain mRNA and protein expression of Mrp1-6, Bcrp and P-gp. The results show induction of P-gp, Mrp2 and Mrp4 proteins, with no changes in Bcrp, Mrp1 or Mrp5-6. The protein values were accompanied by corresponding changes in mRNA levels. Additionally, brain Nrf2 nuclear translocation and expression of two Nrf2 target genes,

NAD(P)H

quinone oxidoreductase 1 (Nqo1) and Hemoxygenase 1 (Ho-1), was evaluated at 6, 12 and 24h after APAP treatment. Nrf2 nuclear content increased by 58% at 12h after APAP along with significant increments in mRNA and protein expression of Nqo1 and Ho-1. Furthermore, APAP treated Nrf2 knockout mice did not increase mRNA or protein expression of Mrp2 and Mrp4 as observed in wildtypes. In contrast, P-gp induction by APAP was observed in both genotypes. In conclusion, acute APAP intoxication induces protein expression of brain P-gp, Mrp2 and Mrp4. This study also suggests that brain changes in Mrp2 and Mrp4 expression may be due to in situ Nrf2 activation by APAP, while P-gp induction is independent of Nrf2 function. The functional consequences of these changes in brain ABC transporters by APAP deserve further attention.

(125)I-Labelled 2-Iodoestrone-3-sulfate: synthesis, characterization and OATP mediated transport studies in hormone dependent and independent breast cancer cells

INTRODUCTION

Organic Anion Transporting Polypeptides (OATP) are a family of membrane associated transporters that facilitate estrone-3-sulphate (E3S) uptake by hormone dependent, post-menopausal breast cancers. We have established E3S as a potential ligand for targeting hormone dependent breast cancer cells, and in this study sought to prepare and investigate radioiodinated E3S as a tool to study the OATP system.

METHODS

2- and 4-Iodoestrone-3-sulfates were prepared from estrone via aromatic iodination followed by a rapid and high yielding sulfation procedure. The resulting isomers were separated by preparative HPLC and verified by (1)H NMR and analytical HPLC. Transport studies of 2- and 4-[(125)I]-E3S were conducted in hormone dependent (i.e. MCF-7) and hormone independent (i.e. MDA-MB-231) breast cancer cells in the presence or absence of the specific transport inhibitor, bromosulfophthalein (BSP). Cellular localization of OATP1A2, OATP2B1, OATP3A1 and OATP4A1 were determined by immunofluorescence analysis using anti-Na(+)/K(+) ATPase-α (1:100 dilution) and DAPI as plasma membrane and nuclear markers, respectively.

RESULTS

Significantly (p<0.01) higher total accumulation of 2-[(125)I]-E3S was observed in hormone dependent MCF-7 as compared to hormone independent MDA-MB-231 breast cancer cells. In contrast 4-[(125)I]-E3S did not show cellular accumulation in either case. The efficiency of 2-[(125)I]-E3S transport (expressed as a ratio of Vmax/Km) was 2.4 times greater in the MCF-7 as compared to the MDA-MB-231 breast cancer cells. OATP1A2, OATP3A1 and OATP4A1 expression was localized in plasma membranes of MCF-7 and MDA-MB-231 cells confirming the functional role of these transporters in radioiodinated E3S cellular uptake.

CONCLUSION

An efficient method for the preparation of 2- and 4-[(125)I]-E3S was developed and where the former demonstrated potential as an in vitro probe for the OATP system. The new E3S probe can be used to study the OATP system and as a platform to create radiopharmaceuticals for imaging breast cancer.

Focal adhesion kinase is a regulator of F-actin dynamics: New insights from studies in the testis

During spermatogenesis, spermatogonia (2n, diploid) undergo a series of mitotic divisions as well as differentiation to become spermatocytes, which enter meiosis I to be followed by meiosis II to form round spermatids (1n, haploid), and then differentiate into spermatozoa (1n, haploid) via spermiogenesis. These events take place in the epithelium of the seminiferous tubule, involving extensive junction restructuring at the Sertoli-Sertoli and Sertoli-germ cell interface to allow the transport of developing germ cells across the epithelium. Although structural aspects of these cell-cell junctions have been studied, the underlying mechanism(s) that governs these events has yet to be explored. Earlier studies have shown that a non-receptor protein tyrosine kinase known as focal adhesion kinase (FAK) is a likely regulator of these events due to the stage-specific and spatiotemporal expression of its various phosphorylated/activated forms at the testis-specific anchoring junctions in the testis, as well as its association with actin regulatory proteins. Recent studies have shown that FAK, in particular its two activated phosphorylated forms p-FAK-Tyr⁴⁰⁷ and p-FAK-Tyr³⁹⁷, are crucial regulators in modulating junction restructuring at the Sertoli cell-cell interface at the blood-testis barrier (BTB) known as the basal ectoplasmic specialization (basal ES), as well as at the Sertoli-spermatid interface called apical ES during spermiogenesis via its effects on the filamentous (F)-actin organization at the ES. We herein summarize and critically evaluate the current knowledge regarding the physiological significance of FAK in regulating BTB and apical ES dynamics by governing the conversion of actin filaments at the ES from a “bundled” to a “de-bundled/branched” configuration and vice versa. We also provide a molecular model on the role of FAK in regulating these events based on the latest findings in the field.

Role of nuclear receptors in the regulation of drug transporters in the brain

ATP-binding cassette membrane-associated drug efflux transporters and solute carrier influx transporters, expressed at the blood-brain barrier, blood-cerebrospinal fluid barrier, and in brain parenchyma, are important determinants of drug disposition in the central nervous system. Targeting the regulatory pathways that govern the expression of these transporters could provide novel approaches to selectively alter drug permeability into the brain. Nuclear receptors are ligand-activated transcription factors which regulate the gene expression of several metabolic enzymes and drug efflux/influx transporters. Although efforts have primarily been focused on investigating these regulatory pathways in peripheral organs (i.e., liver and intestine), recent findings demonstrate their significance in the brain. This review addresses the role of nuclear receptors in the regulation of drug transporter functional expression in the brain. An in-depth understanding of these pathways could guide the development of novel pharmacotherapy with either enhanced efficacy in the central nervous system or minimal associated neurotoxicity.

Breast cancer resistance protein (Bcrp) and the testis--an unexpected turn of events

Breast cancer resistance protein (Bcrp) is an ATP-dependent efflux drug transporter. It has a diverse spectrum of hydrophilic and hydrophobic substrates ranging from anticancer, antiviral and antihypertensive drugs, to organic anions, antibiotics, phytoestrogens (e.g., genistein, daidzein, coumestrol), xenoestrogens and steroids (e.g., dehydroepiandrosterone sulfate). Bcrp is an integral membrane protein in cancer and normal cells within multiple organs (e.g., brain, placenta, intestine and testis) that maintains cellular homeostasis by extruding drugs and harmful substances from the inside of cells. In the brain, Bcrp is a major component of the blood-brain barrier located on endothelial cells near tight junctions (TJs). However, Bcrp is absent at the Sertoli cell blood-testis barrier (BTB); instead, it is localized almost exclusively to the endothelial TJ in microvessels in the interstitium and the peritubular myoid cells in the tunica propria. Recent studies have shown that Bcrp is also expressed stage specifically and spatiotemporally by Sertoli and germ cells in the seminiferous epithelium of rat testes, limited only to a testis-specific cell adhesion ultrastructure known as the apical ectoplasmic specialisation (ES) in stage VI-early VIII tubules. These findings suggest that Bcrp is equipped by late spermatids and Sertoli cells to protect late-stage spermatids completing spermiogenesis. Furthermore, Bcrp was found to be associated with F (filamentous)-actin and several actin regulatory proteins at the apical ES and might be involved in the organisation of actin filaments at the apical ES in stage VII-VIII tubules. These findings will be carefully evaluated in this brief review.