The regrowth kinetic of the surviving population is independent of acute and chronic responses to temozolomide in glioblastoma cell lines.

Chemotherapy acts on cancer cells by producing multiple effects on a cell population including cell cycle arrest, necrosis, apoptosis and senescence. However, often a subpopulation of cells survives and the behavior of this subpopulation, which is responsible for cancer recurrence, remains obscure. Here we investigated the in vitro short- and long-term responses of six glioblastoma cell lines to clinically relevant doses of temozolomide for 5 days followed by 23 days of recovery, mimicking the standard schedule used in glioblastoma patient for this drug. These cells presented different profiles of sensitivity to temozolomide with varying levels of cell cycle arrest, autophagy and senescence, followed by a regrowth of the surviving cells. The initial reduction in cell number and the subsequent regrowth was analyzed with four new parameters applied to Cumulative Population Doubling (CPD) curves that describe the overall sensitivity of the population and the characteristic of the regrowth: the relative end point CPD (RendCPD); the relative Area Under Curve (rAUC); the Relative Time to Cross a Threshold (RTCT); and the Relative Proliferation Rate (RPR). Surprisingly, the kinetics of regrowth were not predicted by the mechanisms activated after treatment nor by the acute or overall sensitivity. With this study we added new parameters that describe key responses of glioblastoma cell populations to temozolomide treatment. These parameters can also be applied to other cell types and treatments and will help to understand the behavior of the surviving cancer cells after treatment and shed light on studies of cancer resistance and recurrence.

Cancer Cell Fitness Is Dynamic.

Several phenotypes that impact the capacity of cancer cells to survive and proliferate are dynamic. Here we used the number of cells in colonies as an assessment of fitness and devised a novel method called Dynamic Fitness Analysis (DynaFit) to measure the dynamics in fitness over the course of colony formation. DynaFit is based on the variance in growth rate of a population of founder cells compared with the variance in growth rate of colonies with different sizes. DynaFit revealed that cell fitness in cancer cell lines, primary cancer cells, and fibroblasts under unhindered growth conditions is dynamic. Key cellular mechanisms such as ERK signaling and cell-cycle synchronization differed significantly among cells in colonies after 2 to 4 generations and became indistinguishable from randomly sampled cells regarding these features. In the presence of cytotoxic agents, colonies reduced their variance in growth rate when compared with their founder cell, indicating a dynamic nature in the capacity to survive and proliferate in the presence of a drug. This finding was supported by measurable differences in DNA damage and induction of senescence among cells of colonies. The presence of epigenetic modulators during the formation of colonies stabilized their fitness for at least four generations. Collectively, these results support the understanding that cancer cell fitness is dynamic and its modulation is a fundamental aspect to be considered in comprehending cancer cell biology and its response to therapeutic interventions. SIGNIFICANCE: Cancer cell fitness is dynamic over the course of the formation of colonies. This dynamic behavior is mediated by asymmetric mitosis, ERK activity, cell-cycle duration, and DNA repair capacity in the absence or presence of a drug.