Membrane transport of dietary phenethyl isothiocyanate by ABCG2 (breast cancer resistance protein)

The role of oxidative stress in blood-brain barrier disruption during ischemic stroke: Antioxidants in clinical trials

Ischemic stroke is one of the leading causes of death and disability. Occlusion and reperfusion of cerebral blood vessels (i.e., ischemia/reperfusion (I/R) injury) generates reactive oxygen species (ROS) that contribute to brain cell death and dysfunction of the blood-brain barrier (BBB) via oxidative stress. BBB disruption influences the pathogenesis of ischemic stroke by contributing to cerebral edema, hemorrhagic transformation, and extravasation of circulating neurotoxic proteins. An improved understanding of mechanisms for ROS-associated alterations in BBB function during ischemia/reperfusion (I/R) injury can lead to improved treatment paradigms for ischemic stroke. Unfortunately, progress in developing ROS targeted therapeutics that are effective for stroke treatment has been slow. Here, we review how ROS are produced in response to I/R injury, their effects on BBB integrity (i.e., tight junction protein complexes, transporters), and the utilization of antioxidant treatments in ischemic stroke clinical trials. Overall, knowledge in this area provides a strong translational framework for discovery of novel drugs for stroke and/or improved strategies to mitigate I/R injury in stroke patients.

Transport Mechanisms at the Blood–Brain Barrier and in Cellular Compartments of the Neurovascular Unit: Focus on CNS Delivery of Small Molecule Drugs

Ischemic stroke is a primary origin of morbidity and mortality in the United States and around the world. Indeed, several research projects have attempted to discover new drugs or repurpose existing therapeutics to advance stroke pharmacotherapy. Many of these preclinical stroke studies have reported positive results for neuroprotective agents; however, only one compound (3K3A-activated protein C (3K3A-APC)) has advanced to Phase III clinical trial evaluation. One reason for these many failures is the lack of consideration of transport mechanisms at the blood–brain barrier (BBB) and neurovascular unit (NVU). These endogenous transport processes function as a “gateway” that is a primary determinant of efficacious brain concentrations for centrally acting drugs. Despite the knowledge that some neuroprotective agents (i.e., statins and memantine) are substrates for these endogenous BBB transporters, preclinical stroke studies have largely ignored the role of transporters in CNS drug disposition. Here, we review the current knowledge on specific BBB transporters that either limit drug uptake into the brain (i.e., ATP-binding cassette (ABC) transporters) or can be targeted for optimized drug delivery (i.e., solute carrier (SLC) transporters). Additionally, we highlight the current knowledge on transporter expression in astrocytes, microglia, pericytes, and neurons with an emphasis on transport mechanisms in these cell types that can influence drug distribution within the brain.

A principal mechanism for the cancer chemopreventive activity of phenethyl isothiocyanate is modulation of carcinogen metabolism

Isothiocyanates are small molecules characterized by high chemical reactivity that allows them to interact readily with cellular constituents eliciting a plethora of biological activities. They are present exclusively in cruciferous vegetables, as glucosinolates, the intake of which has been associated with cancer chemoprevention. When the physical structure of these vegetables is disturbed, e.g. during mastication, the enzyme myrosinase is released and converts the glucosinolates to isothiocyanates (R-N=C=S), where R can be aliphatic or aromatic. Although sulforaphane, an aliphatic isothiocyanate, has received most attention worldwide, the most extensively studied aromatic isothiocyanate is phenethyl isothiocyanate (PEITC), and there are substantial differences in biological activity between the two sub-classes. In animal cancer models, PEITC effectively antagonized the carcinogenicity of chemicals, especially nitrosocompounds. A principal mechanism of their action is to protect the integrity of DNA by decreasing the levels of the genotoxic metabolites of chemical carcinogens. Extensive studies established that PEITC modulates the metabolism of the tobacco-specific carcinogenic nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) by inhibiting its cytochrome P450-mediated bioactivation. Moreover, PEITC is a potent inducer of detoxification enzymes such as quinone reductase, glutathione S-transferase and glucuronosyl transferase. PEITC is rapidly absorbed and is characterized by a large bioavailability; Cmax concentrations achieved in plasma after dietary intake are sufficient to modulate carcinogen metabolism. PEITC is primarily metabolized by glutathione conjugation and is excreted in the urine and bile as the mercapturate. The ability of PEITC to perturb carcinogen metabolism through modulation of cytochrome P450 and phase II detoxification enzymes is comprehensively and critically reviewed.

Advances in molecular signaling mechanisms of β-phenethyl isothiocyanate antitumor effects

β-Phenethyl isothiocyanate (PEITC) is an important phytochemical from cruciferous vegetables and is being evaluated for chemotherapeutic activity in early phase clinical trials. Moreover, studies in cell culture and in animals found that the anticarcinogenic activities of PEITC involved all the major stages of tumor growth: initiation, promotion, and progression. A number of mechanisms have been proposed for the chemopreventive activities of this compound. Here, we focus on the major molecular signaling pathways for the anticancer activities of PEITC. These include (1) activation of apoptosis pathways; (2) induction of cell cycle arrest; and (3) inhibition of the survival pathways. Furthermore, we also discussed the regulation of drug-metabolizing enzymes, including cytochrome P450s, metabolizing enzymes, and multidrug resistance.

Cruciferous vegetables, isothiocyanates, and prevention of bladder cancer

Approximately 80% of human bladder cancers (BC) are non-muscle invasive when first diagnosed and are usually treated by transurethral tumor resection. But 50-80% of patients experience cancer recurrence. Agents for prevention of primary BC have yet to be identified. Existing prophylactics against BC recurrence, e.g., Bacillus Calmette-Guerin (BCG), have limited efficacy and utility; they engender significant side effects and require urethral catheterization. Many cruciferous vegetables, rich sources of isothiocyanates (ITCs), are commonly consumed by humans. Many ITCs possess promising chemopreventive activities against BC and its recurrence. Moreover, orally ingested ITCs are selectively delivered to bladder via urinary excretion. This review is focused on urinary delivery of ITCs to the bladder, their cellular uptake, their chemopreventive activities in preclinical and epidemiological studies that are particularly relevant to prevention of BC recurrence and progression, and their chemopreventive mechanisms in BC cells and tissues.

Phenethyl isothiocyanate: a comprehensive review of anti-cancer mechanisms

The epidemiological evidence suggests a strong inverse relationship between dietary intake of cruciferous vegetables and the incidence of cancer. Among other constituents of cruciferous vegetables, isothiocyanates (ITC) are the main bioactive chemicals present. Phenethyl isothiocyanate (PEITC) is present as gluconasturtiin in many cruciferous vegetables with remarkable anti-cancer effects. PEITC is known to not only prevent the initiation phase of carcinogenesis process but also to inhibit the progression of tumorigenesis. PEITC targets multiple proteins to suppress various cancer-promoting mechanisms such as cell proliferation, progression and metastasis. Pre-clinical evidence suggests that combination of PEITC with conventional anti-cancer agents is also highly effective in improving overall efficacy. Based on accumulating evidence, PEITC appears to be a promising agent for cancer therapy and is already under clinical trials for leukemia and lung cancer. This is the first review which provides a comprehensive analysis of known targets and mechanisms along with a critical evaluation of PEITC as a future anti-cancer agent.

Pharmacokinetics and pharmacodynamics of phenethyl isothiocyanate: implications in breast cancer prevention

Phenethyl isothiocyanate (PEITC)-a naturally occurring isothiocyanate in cruciferous vegetables-has been extensively studied as a chemopreventive agent in several preclinical species and in humans. Pharmacokinetic features of unchanged PEITC are (I) linear and first-order absorption, (II) high protein binding and capacity-limited tissue distribution, and (III) reversible metabolism and capacity-limited hepatic elimination. Membrane transport of PEITC is mediated by BCRP, multidrug resistance-associated protein (MRP) 1, and MRP2 transporters belonging to the ATP-binding-cassette (ABC) family. PEITC is metabolized by glutathione S-transferase (GST) in the liver, with the glutathione conjugate of PEITC undergoing further conversion to mercapturic acid by N-acetyl transferase in rats and humans. PEITC modulates the activity and expression of numerous phase I and phase II drug-metabolizing enzymes and can inhibit the metabolism of procarcinogens to form carcinogens and increase carcinogen elimination. In recent years, several in vitro and in vivo studies have elucidated molecular mechanisms underlying the pharmacodynamics of PEITC in breast cancer that include cancer cell apoptosis by upregulation of apoptotic genes, cell cycle arrest at G2/M phase by generation of reactive oxygen species and depletion of intracellular glutathione, downregulation of the estrogen receptor, decrease in sensitivity to estrogen, and inhibition of tumor metastasis. Inhibition of angiogenesis is one of the recently reported mechanisms of breast cancer prevention by PEITC. Complex pharmacokinetics and pharmacodynamics of PEITC necessitate a systems-biology approach in parallel with PK/PD modeling to develop PEITC as a therapeutic agent for treating cancers.

Engineering glucosinolates in plants: current knowledge and potential uses

Glucosinolates (GSL) and their derivatives are well known for the characteristic roles they play in plant defense as signaling molecules and as bioactive compounds for human health. More than 130 GSLs have been reported so far, and most of them belong to the Brassicaceae family. Several enzymes and transcription factors involved in the GSL biosynthesis have been studied in the model plant, Arabidopsis, and in a few other Brassica crop species. Recent studies in GSL research have defined the regulation, distribution, and degradation of GSL biosynthetic pathways; however, the underlying mechanism behind transportation of GSLs in plants is still largely unknown. This review highlights the recent advances in the metabolic engineering of GSLs in plants and discusses their potential applications.

The molecular basis that unifies the metabolism, cellular uptake and chemopreventive activities of dietary isothiocyanates

Organic isothiocyanates (ITCs), which are characterized by the presence of an -N=C=S group, are among the most extensively studied cancer chemopreventive agents and show highly promising chemopreventive activities. Numerous studies have shown that ITCs can inhibit both carcinogenesis and cancer growth in a variety of animal models. Many cruciferous vegetables, which are commonly consumed by humans, are rich sources of these compounds. Of particular interest are their high bioavailability, their shared metabolic profile and their ability to target a wide array of cancer-related cellular proteins. This review is focused on discussing the molecular basis of these intriguing properties of ITCs, with a particular emphasis on the concept that cellular uptake and metabolism of ITCs and at least some of their major chemopreventive activities are all initiated through direct reaction of the carbon atom of the -N=C=S group of the ITCs with cysteine sulfhydryl groups of glutathione (GSH) and of proteins. This knowledge deepens our understanding about the biological activities of ITCs and may facilitate further research and development of these compounds for cancer prevention and treatment.

Proteins as binding targets of isothiocyanates in cancer prevention

Isothiocyanates are versatile cancer-preventive compounds. Evidence from animal studies indicates that the anticarcinogenic activities of ITCs involve all the major stages of tumor growth: initiation, promotion and progression. Epidemiological studies have also shown that dietary intake of ITCs is associated with reduced risk of certain human cancers. A number of mechanisms have been proposed for the chemopreventive activities of ITCs. To identify the molecular targets of ITCs is a first step to understand the molecular mechanisms of ITCs. Studies in recent years have shown that the covalent binding to certain protein targets by ITCs seems to play an important role in ITC-induced apoptosis and cell growth inhibition and other cellular effects. The knowledge gained from these studies may be used to guide future design and screen of better and more efficacious compounds. In this review, we intend to cover all potential protein targets of ITCs so far studied and summarize what are known about their binding sites and the potential biological consequences. In the end, we also offer discussions to shed light onto the relationship between protein binding and reactive oxygen species generation by ITCs.

ABC transporters and isothiocyanates: potential for pharmacokinetic diet-drug interactions

Isothiocyanates, a class of anti-cancer agents, are derived from cruciferous vegetables such as broccoli, cabbage and watercress, and have demonstrated chemopreventive activity in a number of cancer models and epidemiologic studies. Due to public interest in cancer prevention and alternative therapies in cancer, the consumption of herbal supplements and vegetables containing these compounds is widespread and increasing. Isothiocyanates interact with ATP-binding cassette (ABC) efflux transporters such as P-glycoprotein, MRP1, MRP2 and BCRP, and may influence the pharmacokinetics of substrates of these transporters. This review discusses the pharmacokinetic properties of isothiocyanates, their interactions with ABC transporters, and presents some data describing the potential for isothiocyanate-mediated diet-drug interactions.

Role of membrane transporters in the safety profile of drugs

It has increasingly been recognized that few molecules move across the cell membrane without the assistance of transporter proteins. Large superfamilies of transporter proteins have been identified in every living cell, including microorganisms and mitochondria. This report reviews the role of transporters in physiology and pharmacology, and identifies where this may have an impact on drug efficacy and toxicity. This new understanding will require a fresh appreciation of pharmacokinetics and drug effects, as the current paradigms are based largely on the assumption that drug molecules have a reasonable unrestricted permeability across membranes. Rather than just focusing on clearance changes and central compartment pharmacokinetics, it will become increasingly necessary to examine the peripheral tissue distribution of drugs to more accurately predict drug efficacy and toxicity.

The H2 receptor antagonist nizatidine is a P-glycoprotein substrate: characterization of its intestinal epithelial cell efflux transport

The aim of this study was to elucidate the intestinal epithelial cell efflux transport processes that are involved in the intestinal transport of the H(2) receptor antagonist nizatidine. The intestinal epithelial efflux transport mechanisms of nizatidine were investigated and characterized across Caco-2 cell monolayers, in the concentration range 0.05-10 mM in both apical-basolateral (AP-BL) and BL-AP directions, and the transport constants of P-glycoprotein (P-gp) efflux activity were calculated. The concentration-dependent effects of various P-gp (verapamil, quinidine, erythromycin, ketoconazole, and cyclosporine A), multidrug resistant-associated protein 2 (MRP2; MK-571, probenecid, indomethacin, and p-aminohipuric acid), and breast cancer resistance protein (BCRP; Fumitremorgin C) inhibitors on nizatidine bidirectional transport were examined. Nizatidine exhibited 7.7-fold higher BL-AP than AP-BL Caco-2 permeability, indicative of net mucosal secretion. All P-gp inhibitors investigated displayed concentration-dependent inhibition on nizatidine secretion in both directions. The IC(50) of verapamil on nizatidine P-gp secretion was 1.2 x 10(-2) mM. In the absence of inhibitors, nizatidine displayed concentration-dependent secretion, with one saturable (J(max) = 5.7 x 10(-3) nmol cm(-2) s(-1) and K(m) = 2.2 mM) and one nonsaturable component (K(d) = 7 x 10(-4) microL cm(-2) s(-1)). Under complete P-gp inhibition, nizatidine exhibited linear secretory flux, with a slope similar to the nonsaturable component. V(max) and K(m) estimated for nizatidine P-gp-mediated secretion were 4 x 10(-3) nmol cm(-2) s(-1) and 1.2 mM, respectively. No effect was obtained with the MRP2 or the BCRP inhibitors. Being a drug commonly used in pediatrics, adults, and elderly, nizatidine susceptibility to efflux transport by P-gp revealed in this paper may be of significance in its absorption, distribution, and clearance, as well as possible drug-drug interactions.

Phenethyl isothiocyanate induced apoptosis via down regulation of Bcl-2/XIAP and triggering of the mitochondrial pathway in MCF-7 cells

Although isothiocyanates have been shown to inhibit carcinogen-induced tumorigenesis, no studies have been made to determine their therapeutic potential for the treatment of breast cancer. In the present study, we evaluated the apoptotic activities of phenethyl isothiocyanate (PEITC) in human breast cancer MCF-7 cells. Exposure to PEITC potently reduced cell viability. In addition, DNA fragments and TUNEL positive nuclei were detected in PEITC-treated cells. Furthermore, PEITC induced apoptosis via activation of caspases 7 and 9 and the cleavage of PARP, and these effects were reversed by treatment with the caspase inhibitor, Z-VAD-fmk. PEITC also caused a decrease in the levels of Bcl-2 with a concomitant increase in Bax levels, which resulted in the release of cytochrome c. XIAP suppression and Smac translocation also contributed to the PEITC-induced apoptosis. However, PEITC did not increase the expressions of p53 and p21. Taken together, the results of this study demonstrate that PEITC significantly induces apoptosis via a mitochondrial pathway. Specifically, PEITC induced a change in the Bax/Bcl-2 ratios, XIAP levels and Smac translocation that was conjunction with the release of cytochrome c and following caspase activation. Therefore, PEITC has the potential for use as a therapeutic agent for the treatment of breast cancer.

Modulation of breast cancer resistance protein (BCRP/ABCG2) by non-basic chalcone analogues

Chalcones are biosynthetic precursors of flavonoids found to possess cytotoxic and chemopreventive activities. In this study, 17 non-basic chalcone analogues were synthesized and evaluated for their ability to modulate the function of either the human wild-type (482R) or mutant (482T) breast cancer resistance protein (BCRP/ABCG2) stably expressed in breast cancer MDA-MB-231 cells. At 5microM, chalcones with 2,4-dimethoxy groups or 2,4-dihydroxyl groups on ring A were found to increase mitoxantrone accumulation to a greater extent than an established BCRP inhibitor, fumitremorgin C. At the same time, these chalcones had negligible effect on calcein accumulation in P-glycoprotein overexpressing MDCKII cells, indicating their potential as selective BCRP inhibitors. Functionally, these compounds were able to increase the sensitivity of BCRP-overexpressing cancer cells to mitoxantrone by 2-5-fold. The effect of chalcone compounds on both wild-type and mutant BCRP ATPase activity was also examined and variable effects were observed. A stimulatory effect was mostly observed with chalcones with 2,4-dimethoxy substitution on ring A which were earmarked as good BCRP inhibitors in the MX accumulation and cytotoxicity assays. These findings underscore the potential of methoxylated and hydroxylated chalcones as selective and potent inhibitors of BCRP whose mode of action may not involve the inhibition of ATPase activity.

Transport of dietary phenethyl isothiocyanate is mediated by multidrug resistance protein 2 but not P-glycoprotein

We demonstrated recently that phenethyl isothiocyanate (PEITC), a potent anticarcinogen present in cruciferous vegetables, inhibited P-glycoprotein (P-gp) and multidrug resistance protein 1 (MRP1) and that MRP1 can transport PEITC and/or its metabolites. In this study, we have examined whether PEITC is transported by P-gp and MRP2, two transporters with high expression in human intestine, liver and kidney. Using (14)C-PEITC, no significant difference was observed for the intracellular accumulation of PEITC in human breast cancer MCF-7/sensitive (control) and MCF-7/ADR (P-gp overexpressing) cells at PEITC concentrations of 1, 10 and 50 microM. Moreover, the presence of verapamil or PSC833, two P-gp inhibitors, had no significant effect on the intracellular accumulation of PEITC in P-gp overexpressing MCF-7/ADR and MDA435/LCC6MDR1 cells, indicating that PEITC may not be a substrate for P-gp. In contrast, (14)C-PEITC intracellular accumulation in the kidney epithelial MDCK II/MRP2 cells (transfected with human MRP2) was significantly lower than in the wild-type MDCK II/wt cells at PEITC concentrations of 1, 5, 10 and 50 microM. The presence of MK571, an MRP inhibitor, significantly enhanced (14)C-PEITC accumulation in MDCK II/MRP2 but not MDCK II/wt cells. Furthermore, depletion of intracellular glutathione (GSH) following treatment with buthionine sulphoximine, an inhibitor of GSH biosynthesis, significantly increased (14)C-PEITC intracellular accumulation in a concentration-dependent manner. Transcellular transport studies also demonstrated that depletion of intracellular GSH reduced the mean ratio of basal-to-apical transport to apical-to-basal transport of PEITC in MDCK II/MRP2, but not MDCK II/wt cell monolayers. These results indicate that GSH plays an important role in the MRP2-mediated transport of PEITC. The findings provide new information concerning the interactions between PEITC and membrane transporters and suggest the possibility of PEITC interactions with xenobiotics that are MRP2 substrates.