Studies On The Mechanisms Of Breast Cancer Cell Resistance And The Reversal Strategies To Chemotherapy And Radiotherapy

The previous50years have seen numerous advances in the development of chemotherapeutic agents as well as big improvement in treatment of tumors. However, a significant proportion of cancers are inherently unaffected by the administration of anticancer drugs. Furthermore, another considerable proportion of patients undergoing chemotherapy display an initial reduction in tumor size only to relapse with a marked resistance to a variety of drugs. Both phenomena are brought about by a resistant phenotype, which may present perhaps the single greatest barrier to successful chemotherapy. Biological mechanisms contributing to drug resistance may be present de novo or arise after exposure to anticancer drugs. At present, drug resistance is considered as a multifactorial phenomenon involving several major mechanisms, such as inadequate access of the drug to the tumor, increased energy-dependent efflux (e.g. ABC transporters) of chemotherapeutic drugs, treatment-induced cell cycle arrest which protects the cancer cells from other phase specific agents, etc. However, the mechanisms mediating tumor cell resistance are not completely clear.In recent years, both clinical and experimental studies suggested that steroid hormones and their receptors might also affect the therapeutic efficacy of antineoplastic drugs. Previous studies from our laboratory showed that glucocorticoids, such as dexamethasone, could significantly interfere with the antitumor activities of paclitaxel in vitro and in vivo. Recently, we established a pair of isogenic ER-/ER+cell lines (BC-V/BC-ER) by stable transfection of ERα expression vectors into ER-human breast cancer BCap37cells. Our studies with this pair of isogenic tumor cell lines, together with data obtained in other breast tumor cells expressing endogenous ER, demonstrated that estrogen (EST) significantly reduced the overall cytotoxicity of paclitaxel in BC-ER but had no influence on the BC-V. Further analyses indicate that expression of ER in BCap37cells mainly interferes with the apoptotic cell death but not mitotic arrest induced by paclitaxel. Moreover, we found that the addition of fulvestrant, a selective ER down-regulator, could completely reverse the above resistance observed in BC-ER cells, and sensitize ER+BC-ER, MCF-7and T47D cell lines to paclitaxel. Considering that ER is present in65%of human breast tumors, our findings may provide valuable information for understanding the resistance of ER+tumors to treatment as well as finding strategies to reverse the ER-mediated resistance.On the other hand, tumor is considered as a multi-factoral and multi-stage disease. Combination therapy with multiple drugs or different modalities is therefore common practice in the treatment of cancer. However, when anticancer agents with similar or different modes of action are combined, the outcome can be synergistic, additive or antagonistic. It was theorized that the combination of antimicrotubule drugs, such as paclitaxel, with radiation would behave synergistically due to the ability of paclitaxel to arrest cells at the radiosensitive G2/M phase. However, previous clinical and preclinical studies have produced contradictory results. Some investigations supported the above hypothesis, while others suggested that the combination of antimicrotubule drugs and radiation might not result in any synergistic or additive effects. Our laboratory has recently evaluated the therapeutic effect of two classes of antimicrotubule drugs, taxanes and vinca alkaloids, with or without the combination of y-radiation in human breast cancer BCap37and epidermoid carcinoma KB cell lines (Clin Cancer Res; Int J Radiat Oncol Biol Phys). Our results showed that the overall cytotoxicity generated by the combination of drug and radiation was significantly less than that produced by the drug alone. Further analyses revealed that radiation interfered with the effects of antimicrotubule drugs on both mitotic arrest and apoptosis through induction of prolonged G2cell cycle arrest. These findings indicated that the G2/M arrest is not sufficient to produce cooperative interaction between radiation and antimicrotubule drugs. In contrast, the combination of radiation and antimicrotubule drugs may result in an antagonistic effect rather than an additive or synergistic effect.Based on our previous studies, herein we conducted two parts of research work to elucidate the mechanisms mediating tumor cell resistance, as well as explorer the potential strategies to reverse the resistance. First, we investigated the possible influence of ER on the therapeutic effects of vinblastine and vinorelbine on breast cancer cells and explored whether combination of fulvestrant may enhance the sensitivity of ER+tumor cells to these chemotherapeutic agents. Second, we previously demonstrated that radiation arrests tumor cells at G2phase, which in turn prevents the cytotoxic effects of antimicrotubule drugs. These findings have raised a clinically relevant question as to how this inhibitory effect of radiation on chemotherapy could be repressed. Herein we tested our hypothesis that G2abrogators (e.g. UCN-01) would attenuate the above antagonistic interaction and improve the therapeutic efficacy of combination therapy between radiation and antimicrotubule drugs.Through comparing ER+BC-ER and T47D, with ER-BC-V and MDA-MB-468human breast tumor cell lines, a series of assays were applied to determine the sensitivity of above tumor cell lines to vinblastine and vinorelbine in the presence or absence of EST and/or fulvestrant. EST showed no effect on the sensitivity of ER-cell lines to the treatment of vincristine or vinblastine, but it significantly reduced the sensitivity of ER+cell lines to these drugs. Further analyses show that E2/ER has little effect on vinca alkaloids-induced mitotic arrest, but dramatically affects their ability to induce tumor cell apoptosis. Moreover, through a series of assays, we also demonstrated that the combination of fulvestrant could reverse the resistance of ER+breast tumor cells to vinca alkaloids and even produce synergistic effect.Meanwhile, our data demonstrated that in BCap37and KB cell lines, through attenuation of radiation-induced inhibitory G2arrest, UCN-01indeed abates the inhibitory effect of radiation on paclitaxle and vincristine, and significantly enhances the cytotoxicity of radiation alone, drug alone and most importantly, their various combinations. As we know, this is the first report investigating the potential impact of G2abrogators on chemoradiotherapy. Our findings have suggested the potential use of G2abrogators in combination therapy between radiation and chemotherapy for cancer treatment. Considering that G2abrogators are currently under extensive evaluation for cancer treatment, both as a single agent and in combinations, our findings also provide valuable information for this class of promising compounds.In brief, the findings obtained from this research project have provided valuable information for clarifying the mechanisms of tumor cell resistance to cancer therapy, finding strategies to reverse the resistance, as well as optimizing the combination treatment for cancer patients.