The development of resistance to chemotherapeutic drugs is the main obstacle to the successful treatment of many cancers, including childhood neuroblastoma, the most common solid tumour of infants. One factor that may play a role in determining response of neuroblastoma tumours to therapeutic agents is the p53 tumour suppressor gene. A number of previous studies have suggested that this tumour suppressor protein is inactive in neuroblastoma due to its cytoplasmic sequestration. This thesis therefore has examined the functionality of p53 and its role in determining drug response of neuroblastoma cells. An initial study was undertaken that characterised an unusually broad multidrug resistance (MDR) phenotype of a neuroblastoma cell line (IMR/KAT100). The results demonstrated that the MDR phenotype of the IMR/KAT100 cells was associated with the acquisition of mutant p53. To explore the role of p53 in drug resistance further, p53-deficient variants in cell lines with wild-type p53 were generated by transduction of p53-suppressive constructs encoding either shRNA or a dominant-negative p53 mutant. Analysis of these cells indicated that: (i) in contrast to previous reports, wild-type p53 was fully functional in all neuroblastoma lines tested, as evidenced by its activation and nuclear translocation in response to DNA damage, transactivation of target genes and control of cell cycle checkpoints; (ii) inactivation of p53 in neuroblastoma cells resulted in establishment of an MDR phenotype; (iii) knockdown of mutant p53 did not revert the drug resistance phenotype, suggesting it is determined by loss of wild-type function rather than gain of mutant function; (iv) p53-dependent cell senescence, the primary response of S-type neuroblastoma cells to DNA damage, is replaced, after p53 inactivation, by mitotic catastrophe and subsequent apoptosis. In contrast to neuroblastoma, p53 suppression had no effect or increased drug susceptibility in several other tumour cell types, indicating the importance of tissue context for p53- mediated modulation of tumour cell sensitivity to treatment. Taken together, these data provide strong evidence for p53 having a role in mediating drug resistance in neuroblastoma and suggest that p53 status may be an important prognostic marker of treatment response in this disease.

Neuroblastoma (NB) is the most common solid tumor of early infancy. Despite a significant improvement in the general survival rate for children with cancer, the prognosis of high-risk NB remains low, at about 30%, despite the use of intensive chemo-radiotherapy followed by differentiation therapy with retinoic acid (RA). Relapses in this category of NB are often due to the emergence of multi-drug and RA-resistant minimal residual cancer cells. The use of natural 13-cis RA, as a single chemo-preventive agent, has improved the survival rate to 50% for high-risk NB patients. However, the prevalence of RA-resistance is high in high-risk NB, and in solid cancers, in general. RA-resistance in cancer cells is mediated by a number of factors. Loss of RA-induced expression of the putative tumor suppressor gene, retinoic acid receptor-beta (RARβ), is one of the most common factors that have been reported in RAresistant phenotypes of a wide range of cancer cells. The transcriptional regulation of RAR(β) gene and other retinoid responsive-genes is believed to be regulated by the ligand-dependent transactivation of the homo- or heterodimer complexes of the retinoic acid receptor (RAR) and retinoid X receptor (RXR) subtypes, namely alpha (α), beta (β) and gamma (γ). It is believed that the anti-cancer activities of natural all-trans RA and 13-cis RA are mediated through activation of RAR-complexes. The loss of RA-induced RAR β expression can be caused by aberrant recruitment of chromatin structure modifying enzymes, histone deacetylases (HDACs), which have major roles in the global regulation of gene transcription. However, the mechanism of RA-resistance in NB cells is unclear. This thesis set out to identify the molecular mechanism of RA-resistance and to develop a new therapeutic approach to overcome RA-resistance in NB cells. The data in this thesis demonstrated that deregulation or over-expression of proto-oncogene MYCN caused a total RA-resistance in NB cells in vitro and in vivo, despite the strong induction of RARI3 expression. The data also indicated that the activation of RAR-dependent pathways by aRA or 13RA alone is not sufficient to overcome MYCN-mediated RA-resistance in NB cells. In the light of this observation, this thesis went on to examine whether combined targeting activation of RAR and RXR subtypes with receptor specific ligands could enhance the therapeutic efficacy of the retinoid signaling pathway. NB cells were treated with a panel of receptor-specific retinoids, namely aRA, l3RA, 9RA (RAR-specific), CD 417, CD 2314 (RARβ-specific), CD 666 (RARγ-specific), CD 336 (RARα-specific), CD 3640, CD 2872 (RXR-specific), as a single agent or in combination at a low concentration of 0.1 ??M. The results showed that combined targeting activation of RARα and RXR was not only the most effective combination, but also overcame MYCN-mediated RA-resistance in NB cells in vitro.Collectively, these data demonstrated the combined targeting activation of RAR and RXRs as a new approach to enhance the efficacy of retinoid therapy and overcome RA-resistance in the treatment of high-risk NB, and other cancers. The emerging therapeutic potential of HDAC inhibitors (HDACi) as front line anti-cancer agents, or adjuvants to other agents such as RA, has suggested a new approach in the treatment of cancer. However, the molecular mechanism of the remarkably specific anticancer actions of HDACi is still largely speculation. The data presented in this study was the first to demonstrate a novel sequential order and the dosage-dependent roles of basal p21Wafl expression and G2/M arrest as protective mechanisms against HDACi-induced apoptosis. In addition, this thesis also examined and compared the therapeutic efficacy of HDACi as a single agent and in combination with other anti-cancer agents such as RA, IFNα and chemotherapeutic agents. Evaluation of the therapeutic effects of combinations of aRA, IFN and HDACi showed that combination of HDACi and IFNα exhibited the strongest synergy against NB cells in vitro. Treatment of MYCN transgenic mice, which consistently develop abdominal NB tumors that closely mirror the human disease in both physiological and biological aspects, with hydroxamic acid-based HDACi, trichostatin A (TSA), alone reduced tumor growth by nearly 50%, compared to the solvent control and IFNα alone, which had no effect on NB tumor growth. The most exciting finding was that the combination of HDACi and IFNα synergistically reduced tumor mass and angiogenesis by over 80% without any apparent systemic side-effects. The therapeutic effect of treatment with HDACi correlated with the induction of acetylation of histone 4 protein (H4) in both tumor and organ tissues, indicating a wide therapeutic index for HDACi in vivo. Collectively, the data in this study have demonstrated basal p21 Wafl expression as a potential marker of sensitivity to HDACi-based therapy, and the therapeutic efficacy of a novel combination of HDACi with IFNα in vivo. These preclinical data have provided an evidence-based rationale for a clinical trial of the combination of HDACi and IFNα in the treatment of high risk NB.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Also available online.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

Thesis (Ph.D.)--University of Alberta, 2009. / A thesis submitted to the Faculty of Graduate Studies and Research in partial fulfillment of the requirements for the degree of Doctor of Philosophy, Medical Sciences, Laboratory Medicine and Pathology. Title from pdf file main screen (viewed on October 31, 2009). Includes bibliographical references.

Despite rapid advances in clinical oncology, acquired drug resistance still poses a significant threat to the long-term efficacy of current treatment regimens. Because most chemotherapy drugs aim to activate apoptosis in cancer cells, expansion of the pharmacopeia to include treatments targeting novel tumor cell death mechanisms is a promising anti-cancer strategy. Induction of ferroptosis, an iron-dependent form of regulated cell death, shows particular therapeutic potential as aggressive metastatic and drug-resistant cancer cell states have been demonstrated to possess an exquisite dependency on glutathione peroxidase 4 (GPX4), a key suppressor of the ferroptotic cell death pathway. However, current GPX4 inhibitors are limited by poor pharmacokinetic properties that preclude their clinical use. The development of novel drug-like GPX4 inhibitors would benefit from the discovery of new chemical scaffolds to both enhance our understanding of the structural basis of small molecule binding and inhibition as well as facilitate the rational design of future GPX4-targeted therapeutics. In this dissertation, we employed three high-throughput screening strategies to identify novel scaffolds of interest for GPX4 inhibitor development. First, a Lead-Optimized Compound (LOC) library was screened and we conducted further characterization and structure-activity relationship (SAR) studies on hit compound LOC880. Compared to the original hit, analogs QW-095 and QW-105 showed improved binding affinity and GPX4 inhibitory activity in vitro and also induced lipid peroxidation in cells suggestive of ferroptotic death. Further enhancement of the potency and ferroptosis specificity of this scaffold is still needed, but the potentially noncovalent and allosteric mechanism of action presents a novel approach for targeting GPX4. Second, we conducted extensive SAR analyses on another promising hit from the LOC library screen, LOC1886, which led to the identification of the lead compound QW-314. This analog showed significantly improved potency and ferroptosis specificity in multiple cancer cell contexts, including a drug-tolerant persister cell model of minimal residual disease. Characteristic markers of GPX4 inhibition and ferroptosis are also observed in cells treated with QW-314, including GPX4 protein degradation and induction of lipid peroxidation, and QW-314 exhibited excellent selectivity for GPX4 over another glutathione peroxidase family selenoprotein GPX1 in an in vitro assay using cell lysates. Moreover, we determined a baseline of pharmacokinetic measures including aqueous solubility and metabolic stability in human and mouse liver microsomes for further medicinal chemistry optimization. Lastly, we screened a DNA-encoded library (DEL) and an Enamine Diversity library, identifying 10 additional chemical starting points for future investigation.

Tumors employ a variety of genetic resistance mechanisms to evade immune responses and immunotherapies such as PD-1 blockade. The pleiotropic cytokine interferon-gamma (IFNγ) is a potent immune effector and critical for patient response to PD-1 blockade, yet conventional systemic delivery is hindered by severe dose limiting toxicities. As such, the effects of exogenously introduced IFNγ either as monotherapy or in combination with PD-1 blockade in the context of different tumor genetic background remain poorly understood. Synthetic biology allows programming of microbes for tumor-specific delivery of therapeutic candidates that are otherwise not possible using conventional administration strategies. Herein, we engineered a strain of probiotic bacteria that home to tumors and locally release IFNγ. We validated the efficacy of our therapeutic strain, either as monotherapy or in combination with PD-1 blockade, in multiple murine tumor models. Within this dissertation, we demonstrate that a single intratumoral injection of these IFNγ-producing bacteria is sufficient to drive systemic tumor antigen–specific antitumor immunity, without observable toxicity. Although cancer cells employ various resistance mechanisms to evade immune responses, bacteria-derived IFNγ additionally overcomes primary resistance to PD-1 blockade via activation of cytotoxic CD4⁺Foxp3⁻ and CD8⁺ T cells. Moreover, by activating NK cells, bacteria-derived IFNγ also overcome acquired resistance mechanisms to PD-1 blockade, specifically loss of function mutations in IFNγ signaling and antigen presentation pathways. Collectively, this dissertation highlights the promise of combining IFNγ-producing bacteria with PD-1 blockade as a therapeutic strategy for overcoming immunotherapy-resistant, locally advanced, and metastatic disease.

published_or_final_version / abstract / Anatomy / Doctoral / Doctor of Philosophy

by Alexander Kreymerman. / Thesis (M.S.)--Florida Atlantic University, 2011. / Includes bibliography. / Electronic reproduction. Boca Raton, Fla., 2011. Mode of access: World Wide Web. / Sulindac is a known NSAID that has also been shown to have anti-cancer activity that is not related to its ability to inhibit COX 1 and 2. During the past 15 years there have been a large number of studies attempting to elucidate its mechanism of action. Our laboratory has shown that sulindac can both protect normal cells and enhance the killing of cancer cells under oxidative stress from H2O2 and TBHP. However, except for mitochondrial dysfunction and ROS production, the mechanism by which sulindac sensitized the cancer cells to oxidative stress remains unknown. Results of this research project suggest that the effect of sulindac and oxidative stress not only involves mitochondrial ROS production, but also aspects of the preconditioning response. In normal cells this leads to survival by a preconditioning pathway, likely involving PKCε. . However, cancer cells react by initiating a pathway leading to apoptosis involving PKCδ.