Cell Cycle Regulation Changes in Drug Resistant Cancers.

Cell cycle perturbation is a common event in tumorigenesis and the development of anticancer drug resistance. Acquired drug resistance is a significant hindrance in achieving effective cancer treatment. Thus, understanding of mechanisms underlying acquired drug resistance is vital to develop strategies to overcome those processes and to improve the survival rates of cancer patients. Moreover, understanding drug resistance can lead to the discovery of potential druggable targets that can contribute towards increasing success of cancer intervention. In this study, molecular mechanisms leading to acquired drug resistance in various cancer cell lines were investigated. Informed by cDNA microarray screens, the cell cycle regulators p57Kip2, Weel and Stratifin (SFN) were shown to be frequently deregulated in drug resistance. Using Western blotting and qPCR, p57Kip2 was shown to be downregulated in platinum-resistant ovarian cancer cell lines PEO1CarbR and PEOlCisR in comparison with the platinum-sensitive counterpart. p57Kip2 downregulation was due to epigenetic silencing (hypermethylation of gene promoter region). Thus, p57Kip2 silencing with siRNA in otherwise drug sensitive cells recapitulated platinum resistance. Interestingly, p57Kip2 downregulation conferred collateral sensitivity towards the synthetic CDK inhibitor drug Seliciclib (CYC202). A similar approach was adopted in the study of Weel and SFN in taxane resistance. Weel and SFN were downregulated in Taxol-resistant SK-OV-3 and A2780 cell lines compared to drug-sensitive counterparts. Silencing of Weel in Taxol-sensitive SK-OV-3 decreased the apoptotic response to Taxol treatment, but this effect was not seen using SFN silencing. I conclude that Weel plays an important role in response to Taxol. However, p53 status is also associated with this response. Additionally, examination of several drug resistant and sensitive pairs of cancer cell lines indicated that DNA damage checkpoint and cell cycle regulation are significant factors in mediating anticancer drug resistance. Based on the results, the ATR-Chkl-Weel pathway was shown to be the most deregulated pathway in drug-resistant cancer cells. I conclude that deregulation of cell cycle is essential to the adaptive responses and enabling characteristics that result in the drug resistant phenotype