EFFECTS OF IONIZING-RADIATION AND UV-B-RADIATION ON PROTEINS CONTROLLING CELL-CYCLE PROGRESSION IN HUMAN-CELLS - COMPARISON OF THE MCF-7 ADENOCARCINOMA AND THE SCL-2 SQUAMOUS-CELL CARCINOMA CELL-LINE

MCF-7 and SCL-2 cells were irradiated with UV B-radiation or with Cs-1 37 gamma-radiation, in order to investigate cell cycle checkpoint cont rol mechanisms. Effects of both qualities of radiation were investigat ed for the two cell lines in regard to p53 protein levels, and alterat ions in Cdk1 (cyclin dependent kinase 1) and Cdk2 phosphorylation were monitored. SCL-2 cells constitutively overexpressed a form of p53 pro tein whose abundance remained unchanged after irradiation, whereas MCF -7 cells expressed wild type p53 whose abundance increased after irrad iation. Accordingly, MCF-7 cells showed a strong G1 phase arrest, wher eas SCL-2 cells were only delayed in S phase (after UV B-irradiation) and arrested in G2 phase (after gamma-irradiation and UV B-irradiation ), as monitored by flow cytometry. In MCF-7 cells increased p53 levels were observed for up to 30 h after gamma-irradiation and up to 20 h a fter UV B-irradiation. Only in SCL-2 cells was there a significant rad iation induced inactivation of Cdk1 by hyperphosphorylation. This effe ct was prevented by culturing cells in the presence of caffeine after irradiation. After UV B-irradiation the inactivation of Cdk1 was less pronounced and only partially diminished in the presence of caffeine. No alteration in Cdk2 phosphorylation was observed after irradiation i n either cell line.