ATP-Binding-Cassette (ABC) transporters are primary active pumps that typically couple the binding and hydrolysis of ATP to the translocation of compounds across cellular membranes. Some, like ABCB1, ABCC1 and ABCC3, are polyspecific and can efflux clinically important drugs which may contribute to their therapeutic failure. In this study I have investigated (1) the mechanism of ABC transporter function, (2) studied the potential for regulation by ubiquitin ligases (both using ABCB1 as a model), and (3) tested the involvement of ABCC1 and ABCC3 in autocrine signalling in cancer. (1) In 1966, Jardetzky et. al [1] proposed that membrane pumps function by exposing their ligand-binding pocket alternately on different sides of the membrane. For ABC transporters, this coupling of the aspect and affinity of the ligand-binding cavities of the two transmembrane domains (TMDs) to the ATP catalytic cycle of the two nucleotide-binding domains (NBDs) is fundamental to the transport mechanism but is poorly understood at the molecular level. Structure data suggest signals are transduced through intracellular loops of the TMDs which slot into grooves on the top surface of the NBDs. At the base of these grooves is the Q-loop. By analysing the function of Q-loop mutants in combination with ligand binding cavity mutants I have discovered that the Q-loops are crucial to the transport cycle and that they are required to couple ligand binding to conformational changes at the NBDs necessary to drive the transporter into an inward closed state. 4 (2) ABCB1 is known to be a key component of chemical barrier separating the circulation from the cerebrospinal fluid. It has also been reported to transport β-amyloid across the lumenal membrane and into the circulation. Loss of ABCB1 from the barrier with age has therefore been suggested to play a role in Alzheimer’s Disease. The ubiquitin ligase Nedd4-1 has been implicated in the post-translational regulation of ABCB1 abundance in cells. Here, I report that ABCB1 can be ubiquitinated by Nedd4-1 in vitro and identify the residues modified (by mass spectrometry). (3) Lysophosphatidylinositol (LPI) is an autocrine metabolite produced by cancer cells that binds to the G-protein coupled transmembrane receptor GPR55 on the surface of cells. Stimulation of GPR55 activates a signalling cascade that induces proliferation and metastases of the cancer cells. How LPI is released from the cells was not known. In this study I show that ABCC1 and ABCC3, which are known to be expressed in ovarian and pancreatic cancers, can transport LPI into inside-out vesicles suggesting a new role for these “drug resistance” transporters in cancer biology.

Human multiple myeloma is an incurable hematological malignancy characterized by the proliferation of plasma cells in the bone marrow. Myeloma represents approximately 20% of all blood cancers. In this research we have explored examples of both intrinsic and acquired drug resistance in myeloma. Topoisomerases are enzymes that are critical for cell division, especially in rapidly dividing cells such as are found in cancer. Topoisomerase poisons are a common group of drugs that are used to treat cancer. Topoisomerase I and II poisons used in the treatment of multiple myeloma include topotecan, mitoxantrone, doxorubicin, and etoposide In order for topoisomerase drugs to be effective, the enzyme must be in direct contact with the DNA. In chapters one and two we examined the export of topoisomerase II alpha from the nucleus as a mechanism of drug resistance. High density cells, similar to those found in the bone marrow, export topoisomerase II alpha from the nucleus to the cytoplasm, rendering the cell drug resistant. We found that blocking nuclear export using the CRM1 inhibitor ratjadone C, or CRM1 specific siRNA, could sensitize high density cells to topoisomerase drugs. Sensitization to topoisomerase inhibitors was correlated with increased topoisomerase/DNA complexes and increased DNA strand breaks. This method of sensitizing human myeloma cells suggests a new therapeutic approach to this disease. In chapter three we examined the role of the molecular transporter ABCG2 in drug resistance in multiple myeloma. We found that ABCG2 expression in myeloma cell lines increased after exposure to topotecan or doxorubicin. Myeloma patients treated with topotecan had an increase in ABCG2 mRNA and protein expression after drug treatment and at relapse. We found that expression of ABCG2 is regulated, at least in part, by promoter methylation both in cell lines and in patient plasma cells. Demethylation of the promoter increased ABCG2 mRNA and protein expression. These findings suggest that ABCG2 is expressed and functional in human myeloma cells, regulated by promoter methylation, affected by cell density, upregulated in response to chemotherapy, and may contribute to drug resistance.

<p>The appearance of adverse drug reactions is a common reason for hospitalization in Western countries. Research on underlying biological mechanisms for interindividual variability in drug response aims to better identify patients with exceptional genetic traits, disease conditions or risk of drug-drug interactions and thereby help to prevent adverse drug reactions. </p><p>Active transport mechanisms are involved in the absorption and disposition of several therapeutic agents. The main objective of this thesis was to investigate factors potentially affecting transport proteins and thus contributing to variability in drug absorption and disposition. Studies of physiological, genetic, environmental, and pathological factors were included. The main focus was the two ATP-binding cassette (ABC) transporters: P-glycoprotein 170 (Pgp) and Breast Cancer Resistance Protein (BCRP). </p><p>Quantification of transport protein mRNAs along the human intestine indicated that eight of the nine investigated drug transporters were expressed in a region-dependent manner. Effects of drug-drug interactions may therefore vary depending on the site of absorption. The genetic aspect was illustrated by identification of sequence variation in the gene encoding BCRP, the most highly expressed ABC transporter along the human intestine. Drug-drug interactions are important environmental causes of interindividual variability. An evaluation of the effects of Pgp-mediated drug-drug interactions showed that patients receiving Pgp inhibitors had elevated serum concentrations of the Pgp substrate digoxin and that digoxin concentrations were positively correlated with the number of co-administered Pgp inhibitors. The final topic in this thesis was that of drug-disease interactions. BCRP and Pgp were down-regulated during active inflammation in patients with ulcerative colitis. This may contribute to altered concentrations of drug in the intestinal mucosa during periods of inflammation and possibly to changes in drug absorption.</p><p>To summarize, results of this thesis emphasize the complex background to the interindividual variability of drug transport proteins, where physiological, genetic, environmental and pathological factors all can contribute.</p>

The appearance of adverse drug reactions is a common reason for hospitalization in Western countries. Research on underlying biological mechanisms for interindividual variability in drug response aims to better identify patients with exceptional genetic traits, disease conditions or risk of drug-drug interactions and thereby help to prevent adverse drug reactions. Active transport mechanisms are involved in the absorption and disposition of several therapeutic agents. The main objective of this thesis was to investigate factors potentially affecting transport proteins and thus contributing to variability in drug absorption and disposition. Studies of physiological, genetic, environmental, and pathological factors were included. The main focus was the two ATP-binding cassette (ABC) transporters: P-glycoprotein 170 (Pgp) and Breast Cancer Resistance Protein (BCRP). Quantification of transport protein mRNAs along the human intestine indicated that eight of the nine investigated drug transporters were expressed in a region-dependent manner. Effects of drug-drug interactions may therefore vary depending on the site of absorption. The genetic aspect was illustrated by identification of sequence variation in the gene encoding BCRP, the most highly expressed ABC transporter along the human intestine. Drug-drug interactions are important environmental causes of interindividual variability. An evaluation of the effects of Pgp-mediated drug-drug interactions showed that patients receiving Pgp inhibitors had elevated serum concentrations of the Pgp substrate digoxin and that digoxin concentrations were positively correlated with the number of co-administered Pgp inhibitors. The final topic in this thesis was that of drug-disease interactions. BCRP and Pgp were down-regulated during active inflammation in patients with ulcerative colitis. This may contribute to altered concentrations of drug in the intestinal mucosa during periods of inflammation and possibly to changes in drug absorption. To summarize, results of this thesis emphasize the complex background to the interindividual variability of drug transport proteins, where physiological, genetic, environmental and pathological factors all can contribute.

Biliary excretion is an important elimination route for many drugs and metabolites. For such compounds, it is important to know the extent of excretion and drug exposure in the bile, e.g., for the risk assessment of drug interactions, liver toxicity and the effects of genetic variants. In this thesis, duodenal aspiration of bile was performed in healthy volunteers and complemented with experiments in an in vivo model in pigs to increase the understanding of the intestinal and hepatobiliary disposition of two direct thrombin inhibitors. The compounds investigated, ximelagatran and AZD0837, are both prodrugs that require bioactivation to exert their pharmacological effect. Upon co-administration with erythromycin and ketoconazole, respectively, altered plasma exposure to ximelagatran and AZD0837 and their respective metabolites has been observed. The main objective of this thesis was to characterize the biliary excretion of the compounds, and investigate whether this elimination route explains the observed drug-drug interactions. High plasma-to-bile AUC ratios were observed, in particular for ximelagatran, its active metabolite melagatran, and AR-H067637, the active metabolite of AZD0837. These high ratios indicate the involvement of active transporters in the biliary excretion of the compounds, which is important since transporters constitute possible sites for drug interactions. The effects of erythromycin and ketoconazole on the plasma exposure of the prodrugs and metabolites were confirmed in both the pig and the clinical studies. The changes seen in plasma for ximelagatran and its metabolites were partly explained by reduced biliary clearance. Inhibited CYP3A4 metabolism likely caused the elevated plasma levels of AZD0837, whereas reduced biliary clearance was seen for AR-H067637 suggesting an effect on its excretion into bile. In summary, the studies led to mechanistic insights in the hepatobiliary disposition of ximelagatran and AZD0837, and demonstrate the value of combined clinical and animal studies for the investigation of the biliary drug excretion.

ABCG2 est un transporteur de la famille ABC impliqué dans le phénotype de résistance aux drogues développé par certaines cellules, par exemple les cellules cancéreuses. Ce transporteur a aussi un rôle physiologique de détoxication de composés endogènes, notamment les porphyrines, molécules indispensables mais qui présentent une toxicité potentielle. Cette toxicité nécessite une prise en charge particulière, évitant à ces composés d'être libres en solution. Dans ce contexte, nous avons fait l'hypothèse qu'ABCG2 pourrait participer à cette détoxication en limitant l'accumulation des porphyrines dans les cellules en les présentant à un partenaire extracellulaire. Nous montrons qu'ABCG2 transporte de l'hème ainsi que certains de ses dérivés et précurseurs et que ces porphyrines, contrairement aux autres substrats d'ABCG2, se fixent sur un domaine extracellulaire spécifique d'ABCG2, ECL3, composé d'environ 70 acides aminés. L'affinité d'ECL3 pour les porphyrines est de 0,5 à 3,5 μM, suffisamment affine pour permettre leur fixation après transport.Nous montrons aussi que l'albumine sérique humaine, impliquée dans la détoxication de l'hème, récupère les porphyrines fixées sur ECL3 par une interaction directe avec ABCG2. L'ensemble de ce travail a donc permis d'une part de mieux comprendre le rôle d'ABCG2 dans la régulation de l'homéostasie des porphyrines, notamment l'hème, et d'autre part, de façon originale, d'identifier le mécanisme moléculaire par lequel cette détoxication s'effectue.

Dissertation (Ph.D.)--University of South Florida, 2009. / Title from PDF of title page. Document formatted into pages; contains 86 pages. Includes vita. Includes bibliographical references.

Dissertation (Ph.D.)--University of South Florida, 2009. / Includes vita. Includes bibliographical references. Also available online.

The androgen receptor’s (AR) resurgence following treatment leads to castration resistant prostate cancer (CRPC). Studies show that the aryl hydrocarbon receptor (AhR) regulates AR signaling, is constitutively active, and enhances AR signaling in CRPC. AhR has ligands with carcinogenic properties and interacts with phytochemicals with anti-tumorigenic properties. Curcumin inhibits AhR activity and multidrug transporter ABCG2 activity, which mediates substrates out of the cell. Elevated ABCG2 expression causes resistance to anticancer drugs. AhR transcriptionally activates ABCG2 and our hypothesis is that inhibition of AhR activity by curcumin will decrease ABCG2 expression and activity in CRPC cells. C4-2 cells were treated with increasing concentrations of curcumin (0, 10, 25, 50µM) and CH223191 (50µM). Results show that curcumin decreases AhR, CYP1B1 and ABCG2 gene expression. Higher concentrations of curcumin diminish AhR and ABCG2 protein expression, ABCG2 activity, and cell proliferation. These results will help reveal a role for AhR in drug resistance.

Récemment, des études menées chez des patients atteints de la maladie d’Alzheimer (MA) suggèrent un rôle important de la clairance cérébrale des peptides bêta-amyloïde (Abeta) dans la physiopathologie de la MA. Les échanges de peptide Abeta entre le cerveau et le sang peuvent se faire à travers la barrière hémato-encéphalique (BHE). De nombreux transporteurs sont exprimés au niveau de la BHE, telles les protéines ABC (ATP-Binding Casette) et SLC (Solute Carriers). Il a été montré que l’influx du peptide Abeta à travers la BHE était partiellement médié par le récepteur RAGE (Receptor for Advanced Glycation End products) et son efflux par le récepteur LRP-1 (Low density lipoprotein receptor-related protein 1). De plus, l’implication de transporteurs ABC/SLC dans le passage cérébral du peptide Abeta a été suggérée. Il paraît donc important de caractériser les transporteurs ABC et/ou SLC impliqués dans le transport du peptide Abeta à travers la BHE. D’autre part, l’on peut se demander si, dans le cadre de la MA, la BHE subit des modifications, en termes d’étanchéité, d’expression de transporteurs, de mécanismes de transport, et si, dans ce cas, il y a une modification du transport du peptide Abeta à travers la BHE altérée. Nous avons d’abord montré que des transporteurs ABC et SLC étaient respectivement impliqués dans l’efflux et l’influx des peptides Abeta à travers la BHE. Concernant l’efflux, outre l’Abcb1, nous avons montré qu’Abcg2 et Abcg4 étaient impliquées dans la clairance cérébrale des peptides Abeta. Concernant l’influx, nous avons montré qu’Oatp1a4 pourrait jouer un rôle important dans la pénétration cérébrale des peptides Abeta. De plus, Abca1, principal transporteur ABC impliqué dans le transport du cholestérol, régule indirectement les taux cérébraux d’Abeta. En particulier, nous avons identifié la L-thyroxine et la rosuvastatine comme de puissants inhibiteurs respectifs de l’efflux et de l’influx cérébral d’Abeta. L’ensemble de ces transporteurs d’influx et d’efflux fixe ainsi la clairance cérébrale des peptides Abeta à travers la BHE. Or ces transporteurs sont régulés chez les souris 3xTg-AD (modèle de souris triple transgénique pour la MA exprimant à la fois les pathologies amyloïde et tau), dans des phases précoces et/ou tardives de la MA. Précocement, l’expression de Rage et d’Abca1 sont fortement augmentées au niveau de la BHE chez les souris 3xTg-AD. L’augmentation de Rage dès l’âge de 3 mois laisse supposer une augmentation très précoce de l’influx du peptide Abeta à travers la BHE. Mais cet influx semble être contre-balancé par l’augmentation concomitante d’Abcg4. Quant à Abca1, ne transportant pas directement le peptide Abeta, le rôle de son augmentation graduelle au cours du développement de la MA reste à déterminer. L’ensemble de ces régulations n’étant pas suffisantes pour empêcher l’accumulation cérébrale d’Abeta, des régulations plus tardives semblent se mettre en place, avec notamment l’augmentation de l’expression d’Abcb1 et d’Abcg2, et la diminution d’Oatp1a4. Ces mécanismes semblent donc correspondre à des phénomènes compensatoires ayant pour objectif d’augmenter la clairance cérébrale d’Abeta. Enfin, nous avons montré que l’intégrité physique de la BHE n’était pas altérée chez ces souris 3xTg-AD âgées de 3 à 18 mois. De plus, nos résultats ont montré que le volume vasculaire était diminué de manière précoce, notamment au niveau de l’hippocampe, chez les souris 3xTg-AD par rapport à leurs contrôles. Ce phénomène n’a pas été retrouvé chez les souris APP/PS1 n’exprimant que la pathologie amyloïde. Ces résultats suggèrent un rôle causal et précoce de la protéine tau hyperphosphorylée dans la pathologie de la MA. En conclusion, nos résultats soulignent l’importance de la BHE dans la physiopathologie de la MA. Ce travail de thèse ouvre des perspectives thérapeutiques, mais aussi des pistes pour la compréhension des mécanismes conduisant à une régulation de ces systèmes de transport dans la MA. / Recent studies in Alzheimer's disease (AD) patients have suggested an important role of cerebral clearance of Abeta peptide in the pathogenesis of AD. The blood-brain barrier (BBB) represents a major pathway for exchanges of Abeta between the brain and the peripheral circulation. Many transporters are expressed at the BBB, such as the ABC (ATP-Binding Casette) and SLC (Solute Carriers) proteins. It has been shown that the influx of Abeta peptide across the BBB was partially mediated by the receptor RAGE (Receptor for Advanced Glycation End products) and its efflux by the LRP-1 receptor (low density lipoprotein receptor-related protein 1). On the other hand, the involvement of ABC/SLC transporters in the brain efflux/influx of Abeta peptide has been suggested. It was therefore important to characterize the ABC/SLC transporters involved in the transport of Abeta peptide across the BBB. In addition, the disorders of the BBB have always been suggested in neurodegenerative diseases. The question is whether, in the context of AD, the BBB undergoes changes in terms of integrity, expression of transporters, transport mechanisms, and if, in this case, there is a change in the transport of Abeta peptide across the impaired BBB. We first showed that the BBB regulated the exchange of blood-brain Abeta peptides. Thus, the involvement of efflux (ABCG2 and ABCG4) and influx (Oatp1a4) transporters allows this equilibrium of Abeta peptides between the blood and the brain parenchyma. In addition, ABCA1, the main ABC transporter involved in cholesterol transport, regulates indirectly the brain levels of Abeta. We also identified the L-thyroxine and rosuvastatin as potent inhibitors of the efflux and influx transport of brain Abeta, respectively. All these influx and efflux transporters could control the transport of Abeta peptide across the BBB. However, these transporters are regulated in 3xTg-AD mice (triple transgenic mouse model for AD expressing both amyloid and tau pathologies) in the early and/or late stages of AD. Early, the expression of Abca1 and Rage are strongly increased at the BBB in 3xTg-AD mice. The high expression levels of Rage at the age of 3 months suggest an early increase in the influx transport of Abeta peptide across the BBB. But this increase seems to be compensated by the concomitant increase of Abcg4. As Abca1 does not directly mediate the transport of Abeta peptide, the role of its gradual increase in the development of AD remains to be determined. As all these regulations are not sufficient to prevent the accumulation of cerebral Abeta, the late regulations seem to develop, including increased expression of Abcb1 and Abcg2, and decreased expression of Oatp1a4. These mechanisms seem to correspond to compensatory phenomena with the objective to increase the cerebral clearance of Abeta. Finally, we have shown that the physical integrity of the BBB was not altered in 3xTg-AD mice aging from 3 to18 months. In addition, our results showed that the cerebral vascular volume was reduced early, especially in the hippocampus of 3xTg-AD mice compared to their age-matched controls. This phenomenon was not found in APP/PS1 mice expressing only the amyloid pathology. These results suggest a causal and early role of hyperphosphorylated tau in AD pathology.In conclusion, our results show the importance of the BBB and particularly of Abcg2, Abcg4 and Oatp1a4 transporters in the pathophysiology of AD. Knowledge of these transporters not only opens up therapeutic or prophylactic purposes, but also leads to the further understanding of the regulation mechanisms of these transport systems in AD.