In this manuscript, we describe a method to generate Docetaxel-resistant prostate cancer cell model systems that allows for the study of mechanisms contributing to acquisition of the chemoresistance phenotype. While this approach is highly reliable and reproducible, some potential limitations should be considered. Since only a small fraction of the bulk population of cells will exhibit chemoresistance28, it is recommended to start with a large population of cells (we usually plate 20 flasks of 150 cm2, which account for approximately 1.2 x 108 cells). Key steps of the protocol should be considered to ensure success of the procedure. First, it is very important to use the same Docetaxel freshly prepared stock for long term usage (10 mM stock aliquots can be used for years when stored properly in at -80 °C). Slight changes in the Docetaxel aliquot can impact the results of IC50, generation of the cell line timing and the colony formation results. Second, it is crucial to start the protocol determining the IC50 in each individual cell line, since variations can occur when using a new batch of cells. Third, it is essential to monitor that the drug treatment is effective by checking cells under the microscope frequently so any errors can be detected quickly and corrected. Finally, we recommend to always keep a stock of intermediate steps in case of contamination or unexpected problems that could affect the viability of the cells in the middle of the protocol. Importantly, our protocol aims to generate models of drug resistance by exposing prostate cancer cells to high doses of Docetaxel for 72 h followed by long recovery periods, rather than lower constant concentrations of the drug in an attempt to mimic the best clinical scenario reported for prostate cancer treatment with this taxane agent15,16.
Additionally, it should be noted that drug-resistant cells exhibit a high plasticity, which may lead to a progressive loss of acquired resistance properties over time. Therefore, extending the use of these cells for more than 2-3 months in culture is not recommended. If a permanent supply is needed it is advisable to continuously generate new batches of resistant cells. However, if batches of cells have been cultured for an extended time, colony formation assays and qPCR can be used to reevaluate their resistance. Another alternative is to boost waning resistance by treating the cells with a high dose of Docetaxel for 72 h (500-1,000 nM). After a recovery period of one to two weeks, the phenotype can be re-tested by qPCR and colony formation assays. It is also possible, although not recommended, to store aliquots of treated cells in liquid nitrogen (in FBS, 10% DMSO). Please note that after a freeze-thaw cycle, the chemoresistance phenotype should always be re-evaluated with the aforementioned methods.
Despite these caveats, we28,29 and others30,31,32,33 were able to successfully employ these model systems to identify key novel cellular and molecular mechanisms leading to elevated tumor-initiating capacity, cell survival, and aggressiveness in Docetaxel-resistant cells. Using these cancer cell model systems, we discovered that cells exhibiting an undifferentiated phenotype, characterized by the absence of epithelial and prostate-related differentiation markers and overexpression of targetable developmental (Notch and Hedgehog) signaling pathways, survive chemotherapeutic exposure28. In a second study, using these models together with publicly available patient gene data sets24,25, we uncovered that the pioneer transcription factor GATA2 is highly overexpressed in metastatic castration-resistant chemotherapy-resistant prostate cancer cells. GATA2 increases the survival of cells by directly activating an IGF2-dependent downstream kinase signaling network29. Notably, these studies helped to identify novel key roles of GATA2 in advanced metastatic prostate cancer aggressiveness34.
The development of drug resistance is a problem that is not limited to Docetaxel and prostate cancer, as it is prevalent among many other types of cancers treated with a wide array of chemotherapeutic drugs. Indeed, similar restrictions on the availability of experimental models apply, and it is conceivable that our protocol can be adapted to study other cancer types and their respective chemoresistance mechanisms.
The generation of Docetaxel-resistant cells as described in this protocol can be used as a functional tool to identify novel relevant molecular and cellular mechanisms of chemoresistance. This opens the door to finding new targets, upon which the development of new treatments for highly malignant prostate cancer might articulate, pushing once more the boundaries of what we know about this disease and how we treat its patients.