Investigation of the dose- and time-dependence of the induction of different types of cell death in a small‑cell lung cancer cell line: Implementation of the repairable-conditionally repairable model

The purpose of this study was to quantify and model various types of cell death for a small‑cell lung cancer (SCLC) cell line (U1690) after exposure to a 137Cs source and as well as to compare the linear-quadratic (LQ) and repairable-conditionally repairable model (RCR). This study is based on four different experiments that were taken place at Cancer Centrum Karolinska (CCK). A human small‑cell lung cancer (SCLC) cell line after the exposure to a 137Cs source was used for the extraction of the clonogenic cell survival curve. Additionally, for the determination and quantification of various modes of cell death the method of fluorescence staining was implemented, where the cell deaths were categorized based on morphological characteristics. The percentage of cells in each phase of the cell cycle was investigated with flow cytometry analysis. The quantification of senescent cells was performed by staining the samples with senescence-associated β-galactosidase (SA-β-Gal) solution and then scoring as senescent cells those that had incorporated the substance. These data were introduced into a maximum likelihood fitting to calculate the best estimates of the parameters used by the examined model. In this model, the modes of cell death are divided into three categories: apoptotic, senescent and other types of cell death (necrotic/apoptotic, necrotic, micronuclei and giant). In the clonogenic cell survival assay, the fitting of the RCR model gives a χ2‑value of 6.10 whereas for the LQ model became 9.61. In the fluorescence microscopy and senescence assay, the probability of the three different modes of cell death on day 2 seems to increases with a dose up to about 10 Gy where there is saturation. On day 7 a significant induction of apoptosis in a dose- and time-dependent manner was evident, whereas senescence was slightly increased in response to dose but not to time. As for the ‘other types of cell death’ mode on day 7 showed a higher probability than the one on day 2 and as well as a prominent dose-dependence. The RCR model fits better to the experimental data than the LQ model. On day 2 there is a slight increase of the apoptotic and senescent probability with dose. On the other hand, on day 7 the shape of the curve of apoptosis differs and a sigmoidal increase with dose is observed. At both time-points, the present model fits the data reasonably well. Due to the fact that the clonogenic survival does not coincide with the one extracted from the fluorescence microscopy, a more accurate way to quantify cell death needs to be used, e.g. computerized video time-lapse (CVTL).