Analysis of radiation-induced changes to human melanoma cultures using a mathematical model

Tumour cells respond to ionizing radiation by cycle arrest, cell death or repair and possible regrowth. We have developed a dynamic mathematical model of the cell cycle to incorporate transition probabilities for entry into DNA replication and mitosis. In this study, we used the model to analyse effects of radiation on cultures of five human melanoma cell lines.

Cell lines were irradiated (9 Gy) prior to further culture and harvesting at multiple points up to 96 h later. Cells were fixed, stained with propidium iodide and analysed for G(1)-, S- and G(2)/M-phase cells by flow cytometry. Data for all time points were fitted to a mathematical model. To provide unique solutions, cultures were grown in the presence and absence of the mitotic poison paclitaxel, added to prevent cell division.

The model demonstrated that irradiation at 9 Gy induced G(2)-phase arrest in all lines for at least 96 h. Two cell lines with wild-type p53 status additionally exhibited G(1)-phase arrest with recovery over 15 h, as well as evidence of cell loss. Resumption of cycling of surviving cells, as indicated by increases in G(1)/S and G(2)/M-phase transitions, was broadly comparable with results of clonogenic assays.

The results, combined with existing data from clonogenic survival assays, support the hypothesis that a dominant effect of radiation in these melanoma lines is the induction of long-term cell cycle arrest.