Pharmacology and Toxicology

Mechanistic studies of the Nrf2/Keap1 signaling pathway Oxidative stress, an imbalance between production and removal of reactive oxygen species, can damage biological macromolecules including DNA, proteins and lipids ( Oxidative damage to biological macromolecules can have profound effects on cellular functions and has been implicated in cancer, inflammation, neurodegenerative diseases, cardiovascular diseases and aging. Eukaryotic cells have evolved anti-oxidant defense mechanisms to neutralize reactive oxygen species (ROS) and maintain cellular redox homeostasis. One of the most important cellular defense mechanisms against ROS and electrophilic intermediates is mediated through the ARE (antioxidant responsive element, or electrophile responsive element) sequence in the promoter regions of phase II and antioxidant genes. The ARE-dependent cellular defense system is controlled by the transcription factor Nrf2. Recent advances in the mechanistic studies of this pathway have provided the following models for Nrf2 regulation: Keap1, a key player in the activation of this pathway, has been identified to function as a molecular switch to turn on and off the Nrf2-mediated antioxidant response. Under basal condition, Keap1 is in the off position and functions as an E3 ubiquitin ligase, constantly targeting Nrf2 for ubiquitination and degradation. As a consequence, the constitutive levels of Nrf2 are very low. The switch is turned on when oxidative stress or chemopreventive compounds inhibit the activity of the Keap1-Cul3-Rbx1 E3 ubiquitin ligase, resulting in increased levels of Nrf2 and activation of its downstream target genes. The switch is turned off again upon recovery of cellular redox homeostasis; Keap1 travels into the nucleus to remove Nrf2 from the ARE. The Nrf2-Keap1 complex is then transported out of the nucleus by the nuclear export signal (NES) in Keap1. In the cytosol, the Nrf2-Keap1 complex associates with the Cul3-Rbx1 core ubiquitin machinery, resulting in degradation of Nrf2. We are currently working on the detailed steps of the Nrf2-Keap1-ARE pathway in response to oxidative stress and to chemopreventive compounds The protective role of Nrf2 in arsenic-induced toxicity and carcinogenicity Another direction of our research is to understand the molecular mechanisms of toxicity/carcinogenicity of environmental pollutants and the endogenous cellular defense systems to cope with pollutants. Drinking water contaminated with arsenic is a worldwide public health issue. Arsenic has been classified as a human carcinogen that induces tumors in the skin, lung, and bladder. Arsenic damages biological systems through multiple mechanisms, one of them being reactive oxygen species. The ARE-Nrf2-Keap1 signaling pathway, activated by compounds possessing anti-cancer properties, has been clearly demonstrated to have profound effects on tumorigenesis. More significantly, Nrf2 knockout mice display increased sensitivity to chemical toxicants and carcinogens and are refractory to the protective actions of chemopreventive compounds. Therefore, we hypothesize that activation of the ARE-Nrf2-Keap1 pathway acts as an endogenous protective system against arsenic-induced toxicity and carcinogenicity. The following Specific Aims are intended to further elucidate the mechanism of Nrf2-activation in protection from arsenic-induced toxicity/tumorigenicity. We will (1) determine the protective role of the ARE-Nrf2-Keap1 pathway in arsenic-induced toxicity and cell transformation using a model cell line UROtsa, (2) define the molecular mechanisms of activation of the ARE-Nrf2-Keap1 pathway by arsenic, sulforaphane, and tBHQ, and (3) define the protective role of the ARE-Nrf2-Keap1 pathway in arsenic-induced toxicity and tumorigenicity using Nrf2 knockout mouse as a model. So far, we have demonstrated a protective role of Nrf2 against arsenic-induced toxicity using cell culture and Nrf2-/- mouse model. We have provided evidence demonstrating that Nrf2 protects against liver and bladder injury in response to six weeks of arsenic exposure in a mouse model. Nrf2−/− mice displayed more severe pathological changes in the liver and bladder, compared to Nrf2+/+ mice. Furthermore, Nrf2−/− mice were more sensitive to arsenic-induced DNA hypomethylation, oxidative DNA damage, and apoptotic cell death. Recently, we submitted another manuscript to Toxicological and Applied Pharmacology, reporting our long-term study of the effect of Nrf2 on arsenic-mediated cell transformation and tumor formation. In this study, we provide evidence demonstrating the importance of Nrf2 activation in preventing the carcinogenetic effects induced by long-term exposure to low-dose arsenic both in vitro and in vivo. The UROtsa cell line was used to show that daily exposure to the Nrf2 inducer, tBHQ, alleviated arsenic-induced hypomethylation and cell transformation. Moreover, tBHQ treatment reduced tumorigenicity of arsenic-transformed cells in SCID mice. Chronic treatment with arsenic also compromised the Nrf2-dependent defense response in the bladder epithelium in Nrf2+/+ implicating the important role of Nrf2 in protecting against arsenic-induced carcinogenicity. This study supports the advantages of using dietary supplements specifically targeting Nrf2 as a chemopreventive strategy to protect humans from various environmental insults that may occur on a daily basis. Identification and development of Nrf2 activators into dietary supplements/ for disease preventio Identification and development of Nrf2 inhibitors into therapeutic drugs to enhance the efficacy of cancer treatment High-throughput screening of Nrf2 activators or inhibitors: we are screening a chemical library and a natural product library to identify compounds that are able to activate or inhibit ARE-luciferase activity using a stable cell line established in our laboratory, MDA-MB-231-ARE-Luc. Based on the critical role of Nrf2 in disease prevention, using Nrf2 activators to boost our antioxidant response represents an innovative strategy to enhance resistance to environmental insults. Once Nrf2 activators are identified and the specificity of these compounds in activating Nrf2 is validated, the compounds will then be tested for the mechanism by which they confer cellular protection and the feasibility of using these compounds for disease prevention using various disease models. On the other hand, recent findings point to the “dark side” of Nrf2, as studies have shown that Nrf2 promotes cancer formation and contributes to chemoresistance. Using a genetic approach, we have provided evidence that the level of Nrf2 correlates well with cancer cell resistance to several therapeutic drugs, demonstrating that Nrf2 is likely responsible for chemoresistance. More recently, we have reported a study on Nrf2 expression in endometrial cancer patients (117 cases). We found no detectable Nrf2 expression in complex hyperplasia, 28% Nrf2 positive cases in endometrial endometrioid carcinoma (type I), and 89% Nrf2 positive cases in endometrial serous carcinoma (type II). Please note that type II endometrial cancer is the most malignant and recurrent carcinoma among various female genital malignancies. Furthermore, inhibition of Nrf2 by overexpressing Keap1 sensitized SPEC-2 cells, which are derived from type II endometrial cancer, or SPEC-2 xenografts to cisplatin using both cultured cells and SCID mouse models. These studies demonstrate that Nrf2 contributes to chemoresistance in many cancers originating from different organs and illustrate the urgent need for identification of Nrf2 inhibitors and for the development of Nrf2 inhibitors into druggable compounds to enhance the efficacy of cancer treatment. We have identified the very first Nrf2 inhibitor and characterized its use to sensitize cancer cells to chemotherapy. We just submitted a manuscript to Science reporting our discovery. The following is the abstract of the manuscript: “The major obstacle in cancer treatment is the resistance of cancer cells to chemotherapy. Nrf2 is a transcription factor that regulates a cellular defense response and is ubiquitously expressed at low basal levels in normal tissues due to Keap1-dependent ubiquitination and proteasomal degradation. Recently, Nrf2 has emerged as an important contributor to chemoresistance. High constitutive expression of Nrf2 was found in many types of cancers, creating an environment conducive to cancer cell survival. We have identified brusatol as a selective Nrf2 inhibitor that is able to sensitize cancer cells and xenografts to chemotherapeutic drugs by enhancing the degradation of Nrf2 and inhibiting the Nrf2-dependent antioxidant response. These results suggest that brusatol can be developed into an adjuvant drug to enhance the efficacy of cancer treatments”. The importance of this project: (i) the use of an Nrf2 inhibitor to enhance the efficacy of cancer therapeutics represents a novel approach to caner treatment. Nrf2 inhibitors may be used in a broad spectrum across many types of cancers and chemotherapeutic drugs to increase the effectiveness of cancer treatment. Development of brusatol into an adjuvant for clinical use to sensitizer many cancer types to treatment will have an enormous impact on human health worldwide. (ii) Brusatol will be extremely useful for the mechanistic investigation of Nrf2 regulation by complex cellular networks. Cross talk between the Nrf2 signaling pathway and others During the last couple of years, crosstalk between the Nrf2 pathway and other important pathways has emerged. Our group has identified two separate branches that converge with the Nrf2 pathway, the p53-p21(Cip1/WAF1) pathway and the autophagy pathway. Crosstalk is mediated by the direct interaction between p21 and Nrf2, and Keap1 with p62, respectively. In the p53-p21 study, we provide molecular and genetic evidence suggesting that the previously suggested antioxidant function of p53 or p21 is mediated through activation of the Nrf2 pathway. Mechanistically, p21 is able to stabilize Nrf2 by competing away Keap1, thus, activating the Nrf2-mediated antioxidant response. Therefore, the interaction between Nrf2 and p21 represents a fine-tuning mechanism between life and death according to the level of stress. In the study with p62 and Nrf2, we reported a novel mechanism of Nrf2 activation by autophagy deficiency through a direct interaction between Keap1 and p62. In response to stress, cells can utilize several cellular processes, such as autophagy, a bulk-lysosomal degradation pathway, to mitigate damages and increase the chances of cell survival. Deregulation of autophagy causes upregulation of p62 and the formation of p62-containing aggregates, which are associated with neurodegenerative diseases and cancer. Accumulation of endogenous p62 or ectopic expression of p62 sequesters Keap1 into aggregates, resulting in the inhibition of Keap1-mediated Nrf2 ubiquitination and its subsequent degradation by the proteasome. In contrast, overexpression of mutated p62, which loses its ability to interact with Keap1, had no effect on Nrf2 stability, demonstrating that p62-mediated Nrf2 upregulation is Keap1-dependent. These findings demonstrate that autophagy deficiency activates the Nrf2 pathway in a non-canonical cysteine-independent mechanism. These work was published in Molecular Cell and Molecular and Cellular Biology, respectively, both are high profile journals. Furthermore, both articles were highlighted in Molecular Cell and Science Signaling, respectively, indicating the importance and high impact of the projects.

Richard Vaillancourt, PhD, studies the regulation of serine/threonine protein kinase pathways that function in stress-related signal transduction pathways. These intracellular serine/threonine protein kinase pathways, which are referred to as mitogen-activated protein (MAP) kinase pathways, are activated by a number of hormones, growth factors, cytokines, and environmental agents. Currently, at least five MAP kinase pathways have been identified, and there are many protein kinases that function within a defined MAP pathway. One role for these sequential kinase pathways is to transmit an extracellular signal from the plasma membrane to the nucleus. Simply stated, these sequential protein kinase pathways provide the cell with an intracellular signal, which elicits a biological response that is appropriate for the type of stimulus. The cytoplasmic kinases that transmit the signal from the plasma membrane to various MAP kinase proteins include the MAP/Extracellular signal-regulated kinase (ERK) Kinase Kinase (MEKK) proteins. To date, at least four MEKK proteins have been identified based on a homology to similar protein kinases found in the budding yeast, Saccharomyces cerevisiae. However, the extracellular molecules that regulate the MEKK proteins remain largely undefined in mammalian cells. A major focus in Dr. Vaillancourt's lab is to characterize the role of MEKK3 and MEKK4 in cellular signal transduction pathways. Current research focuses on the regulation of MEKK3 by the serine/threonine kinase, Akt, which functions in cell survival pathways and the inhibition of apoptosis. In another project, Dr. Vaillancourt and his team are characterizing the regulation of MEKK4 in response to arsenic in human keratinocytes. Finally, they are also studying the role of the PITSLRE protein kinase in the regulation of tyrosine hydroxylase, as it relates to nicotine signal transduction.

Catharine Smith, PhD, focuses on epigenetic mechanisms of gene expression, particularly their regulation through signaling pathways and their modulation by anti-cancer drugs. Epigenetic mechanisms play a very important role in transcriptional regulation of genes but the specifics of these mechanisms require ongoing study. A very exciting new area of research focuses on how these mechanisms are disrupted during tumorigenesis but may also be harnessed to treat cancer. Signaling pathways control the expression of key genes in non-cancerous cells but are often misregulated during the process of oncogenesis. Chromatin proteins and transcription factors that interact with chromatin are often targets of these pathways. Two projects in the lab are directed at the interface of signaling pathways and chromatin. First, Dr. Smith is interested in the mechanism by which the female reproductive steroid, progesterone, regulates target genes in the physiological context of chromatin. Chronic progestin exposure has been linked to increased incidence of breast cancer in post-menopausal women on hormone-replacement therapy. However, the function of the progesterone receptor in mammary tissue and its role in oncogenesis are not well understood. Current studies in this area are directed at the role of chaperone proteins in determining how the progesterone receptor functions at target genes in chromatin and how it is impacted by other signaling pathways.Second, her lab has discovered a novel cAMP signaling pathway that regulates cell cycle progression and are focused on identifying specific components and targets of this pathway.Third, histone deacetylases (HDACs) are key transcriptional regulatory proteins. Inhibitors that target these enzymes have shown great promise as anti-cancer drugs and are currently in clinical trials. However, a lack of knowledge of HDAC biology has made it difficult to predict which tumors will respond to these drugs. HDACs are known to participate in gene repression, but recent work indicates that they are also transcriptional coactivators. Further studies on the mechanism of gene repression through HDAC inhibitors will provide insight into the role of these enzymes as coactivators.

Walter T. Klimecki, DVM, PhD, is an Associate Professor in the Department of Pharmacology and Toxicology in the College of Pharmacy at the University of Arizona. Dr. Klimecki holds joint appointments in the College of Medicine, the College of Public Health, and the Arizona Respiratory Center. He is a Full Member of the Southwest Environmental Health Sciences Center (SWEHSC) where, together with BIO5 director Martinez and BIO5 Statistics Consulting Service director Billheimer, he leads the Integrative Health Sciences (IHS) Center at SWEHSC. The IHS is a translational research support core at SWEHSC, focused on lowering the “activation energy” for translational research.Dr. Klimecki’s research focuses on the toxicology of metals in the environment, an issue particularly relevant in our mining-intensive state. His research work has encompassed a wide range of experimental approaches, from epidemiological studies of arsenic-exposed human populations, to laboratory models including cell culture and rodents. Using cutting edge genetics tools, Dr. Klimecki’s group recently published the first report of an association between human ancestry and response to environmental toxicants. In this provocative work, his group found that individuals whose genomes were comprised of DNA with its origins in the indigenous American populations processed ingested arsenic in a less harmful manner than did individuals whose genomes had their origins in Europe. Using laboratory models his group made ground-breaking discoveries of the impact of arsenic exposure on a process known as autophagy, in which cells digest parts of their own machinery in a sort of “cash for clunkers” arrangement. The ability of arsenic to perturb this process is only now being appreciated by the toxicology community, thanks to the work of the Klimecki Lab. Dr. Klimecki was recently elected as a Vice President-elect to the Metals Specialty Section of the Society of Toxicology, the preeminent scientific toxicology organization in the world. Dr. Klimecki’s research is highly collaborative: his grants and publications have included many BIO5 members, including BIO5 director Fernando Martinez, and BIO5 members Donata Vercelli, Dean Billheimer, and Marilyn Halonen.

Laurence Hurley, PhD, embraces an overall objective to design and develop novel antitumor agents that will extend the productive lives of patients who have cancer. His research program in medicinal chemistry depends upon a structure-based approach to drug design that is intertwined with a clinical oncology program in cancer therapeutics directed by Professor Daniel Von Hoff at TGen at the Mayo Clinic in Scottsdale. Dr. Hurley directs a research group that consists of a team of graduate and postdoctoral students with expertise in structural and synthetic chemistry working alongside students in biochemistry and molecular biology. NMR and in vivo evaluations of novel agents are carried out in collaboration with other research groups in the Arizona Cancer Center. At present, they have a number of different groups of compounds that target a variety of intracellular receptors. These receptors include: (1) transcriptional regulatory elements, (2) those involved in cell signaling pathways, and (3) protein-DNA complexes, including transcriptional factor-DNA complexes.In close collaboration with Dr. Gary Flynn in Medicinal Chemistry, he has an ongoing program to target a number of important kinases, including aurora kinases A and B, p38, and B-raf. These studies involve structure-based approaches as well as virtual screening. Molecular modeling and synthetic medicinal chemistry are important tools.The protein–DNA complexes involved in transcriptional activation of promoter complexes using secondary DNA structures are also targets for drug design.

Christopher Hulme, PhD, focuses on small molecule drug design and developing enabling chemical methodologies to expedite the drug discovery process. Target families of particular current interest for the group are kinases, protein-protein interactions and emerging DNA receptors for indications in oncology. Such efforts are highly collaborative in nature and students will be exposed to the full array of design hurdles involved in progressing molecules along the value chain to clinical evaluation. These efforts will be aided by the group’s interest in both microwave assisted organic synthesis (MAOS) and flow chemistry. Both technologies enable ‘High-throughput Medicinal Chemistry’ (HTMC) and will be supported by similar High-throughput Purification capabilities.The group also has a long standing interest in the development of new reactions that produce biologically relevant molecules in an efficient manner. Front loading screening collections with molecules possessing high ‘iterative efficiency potential’ is critical for expediting the drug discovery process. The discovery of such tools that perturb cellular systems is of high value to the scientific community and may be facilitated by rapid forays into MCR space that can produce a multitude of novel scaffolds with appropriate decoration for evaluation with a variety of different screening paradigms.Novel hypervalent iodine mediated C-H activation methodologies is also an active area of interest. Probing the scope of the transformation below and investigating applications toward the synthesis of new peptidomimetics will be an additional pursuit in the Hulme group.

Bernard Futscher, PhD, and his lab focus on the molecular origins of human cancer. More specifically, the lab group has 3 inter-related research objectives based on the underlying concept that developing an in-depth understanding of epigenetic mechanismsresponsible for governing cell fate will allow for the development of more effective strategies for the prevention, treatment, and cure of cancer. First, they wish to identify which epigenetic mechanisms participate in the transcriptional control of genes important to growth and differentiation. Second, they seek to determine how these epigenetic mechanisms, and therefore epigenetic homeostasis, become compromised during oncogenesis. Third, using a new and more complete understanding of epigenetic control of the genome, Dr. Futscher and his team are developing rational new therapeutic strategies that seek to repair these defects in the cancer cell and transcriptionally reprogram the malignant cancer cell to a benign state. To reach their objectives, a variety of in vitro models of cancer have been developed to address emerging hypotheses that are inferred from the literature in basic and clinical science as well as our own data. Results from these in vitro studies are then translated to the clinical situation to determine their meaning in the actual clinical face of the disease. Similarly, they attempt to take information obtained from the genome-wide assessment of clinical specimens in order to help guide our thinking and develop new hypotheses that can be tested experimentally in our in vitro models.

Numerous drug-induced and environmental exposure-related toxicities are the result of inter-individual variation in the ADME processes of absorption, distribution, metabolism and elimination that control the fate of these compounds from the body. Alterations in these processes provide the mechanistic basis for individual variability in response to drugs and environmental exposures. A common perception is that variability in response is due to genetic polymorphisms within the drug metabolizing enzyme and transporter genes. While there are numerous examples of these differences that play a major role in the susceptibility of genetic subpopulations for specific toxicities, the potential for transient phenotypic conversion due to temporary environmental changes, such as inflammation and disease, are often overlooked.Due to the ensuing liver damage caused by the progressive stages of NAFLD, gene expression patterns can change dramatically resulting in a phenoconversion resembling genetic polymorphisms. Because the liver plays such a key role in the metabolism and disposition of xenobiotics, this temporary phenoconversion could lead to the inability of patients to properly metabolize and excrete drugs and environmental toxicants, increasing the risk of some adverse drug reactions and environmental toxicities.

Yin Chen, PhD. is an Assistant Professor in Pharmacology and Toxicology in the College of Pharmacy at UA. Dr. Chen’s research focus is on epithelial biology. He was a research faculty in University of California, Davis and an Assistant Investigator in Chemical Industry Institute of Toxicology (former CIIT and later Hamner Institute). His long-term research objective is to understand the dysfunction of airway epithelial mucosa in the pathogenesis of a variety of acute and chronic airway diseases. His current research programs are: a) understanding the molecular mechanisms underlying airway mucous cell development and mucous cell metaplasia in chronic diseases including cancer, COPD and asthma; (b) understanding the function and regulation of novel COPD associated genes and developing novel compounds to treat COPD; (c) understanding the impact of fungal exposure on airway innate immunity and its contribution to the development and exacerbation of asthma. Dr. Chen has more than 30 publications including peer-reviewed research articles, reviews and book chapters. He has served as the PI on one R01, two R21, one Flight Attendant Medical Institute (FAMRI) Innovative Clinical Award and one Arizona Biomedical Research Commission Award. He has also served as co-PI on two R01 and one P01 grants. He has built a long productive track record in studying airway mucus production and respiratory viral infection using primary airway epithelial cell model. He routinely cultivate and use primary epithelial cells from eye, salivary gland, airway surface and submucosal gland in different species (e.g. human, monkey, pig, rat and mouse) as our in vitro model to study mucin genes. The differentiated primary culture model demonstrates pseudostratified morphology, is composed of ciliated, non-ciliated, and goblet cells, and has a transepithelial barrier with high electro-resistance. He has also established in vivo exposure system to study the pulmonary effect of the exposure to particulates, pathogens and gases. Using this system, he has developed various airway disease models including CS-induced COPD model, ovalbumin-induced asthma model, fungal-induced asthma model and several infection models such as H1N1, rhinovirus, Aspergillus, and Alternaria.