Open Access Dissertation
Date of Award
Q. Ping Dou
Although considerable progress has been achieved in the field of cancer therapeutics, primary or acquired drug resistance remains a fundamental cause of therapeutic failure in cancer therapy. Among different mechanisms characterized that are responsible for tumor drug resistance, there is increasing evidence suggesting that dysregulation of gene expression, especially oncogene or tumor suppressor gene expression, at either gene transcription or protein synthesis level, can contribute to the drug-resistant phenotype. AMP-activated protein kinase (AMPK) is a well-known major cellular energy sensor, which negatively regulates metabolic pathways such as protein synthesis, fatty acid oxidation and glucose consumption. Activation of AMPK may suppress metabolic activities that are in favor of assisting tumor cell growth and resistance to various anti-tumor drugs. Along this line, I hypothesized that activation of AMPK signaling could help overcoming tumor drug resistance. The data presented in this dissertation strongly support this hypothesis.
The hypothesis was investigated in two different types of cancers with resistance to two different types of drugs. The first model system I used to test my hypothesis is prostate cancer cell models. By using androgen-dependent, androgen receptor (AR)-positive LNCaP cell line and its androgen-independent, AR-positive derivative C4-2B cell line, I found that both cell lines responded to pharmacological AMPK activator metformin, regardless of their androgen dependency. Activation of AMPK by metformin caused AR protein level decrease through suppression of AR mRNA expression and promotion of AR protein degradation. On the other hand, I found that AR is an inhibitor of AMPK signaling-mediated growth suppression and cell death in prostate cancer cells. These findings suggest that combination of AR inhibition therapy with metformin or other AMPK activators may benefit the therapeutic outcome of AR-positive prostate cancer.
The hypothesis has also been studied in multiple myeloma cell models in which paired parental bortezomib-sensitive multiple myeloma cells and their bortezomib-resistant counterparts generated by chronic drug exposure were used. In this study, I found that paired bortezomib-sensitive and -resistant multiple myeloma cells were about equally sensitive to AMPK activators metformin and AICAR. Although carfilzomib is developed as next-generation proteasome inhibitor to overcome bortezomib resistance; the two bortezomib-resistant multiple myeloma cell lines tested in this study exhibited cross-resistance to carfilzomib. I also found that AMPK signaling is suppressed in bortezomib-resistant multiple myeloma cells and that the suppressed AMPK signaling can be elevated by challenging with an AMPK activator. Finally, I found that AMPK activators were able to overcome not only resistance to bortezomib but also cross-resistance to carfilzomib in bortezomib-resistant multiple myeloma cells. These findings support the further investigation of AMPK signaling in multiple myeloma patient samples and in vivo evaluation of metformin use in multiple myeloma mouse models.
Originating from the observation that decrease of AR protein level is a critical step for apoptosis induction in prostate cancer cells, I studied the strategy of destabilizing Bcr-Abl oncoprotein in the scenario of chronic myeloid leukemia (CML). Bcr-Abl is crucial for the pathogenesis of CML by acting as a proliferation activator and apoptosis suppressor. Our laboratory has previously shown that some proteasome inhibitors can efficiently reduce Bcr-Abl protein level. In my study, I examined the effect of celastrol, a natural product with potent proteasome inhibitory activity, on destabilizing Bcr-Abl protein, and explored the potential combination therapies for Bcr-Abl-driven leukemia. I found that (i) celastrol induced apoptosis and Bcr-Abl degradation in a time-dependent manner; (ii) celastrol-induced apoptosis was not blocked by newly synthesized Bcr-Abl protein once the cells were committed; and (iii) celastrol-induced Bcr-Abl degradation and apoptosis were not prevented by selected protease inhibitors or their mixture under the selected experimental conditions. These findings shed light on the mechanism how celastrol inhibits Bcr-Abl protein expression/function and provide support for the potential application of celastrol in the CML treatment.
Taken together, the studies presented in this dissertation will definitely help elucidating the role of AMPK signaling in cancer cells and promote the development of alternative strategies against drug resistance.
Shen, Min, "Overcoming Tumor Drug Resistance By Activating Amp-Activated Protein Kinase And Destabilizing Oncoproteins" (2013). Wayne State University Dissertations. 796.