Caffeine-suppressed ATM pathway leads to decreased p53 phosphorylation and increased programmed cell death in gamma-irradiated leukaemic molt-4 cells

Ionising radiation (IR) is one of the main treatment modalities in oncology. However, we still search for substances which can radio-sensitize tumour cells. In this study we used caffeine, a non-specific ataxia-telangiectasia mutated kinase (ATM) inhibitor, and studied its effect on the activation of the proteins involved in cell cycle control and the induction of apoptosis in human T-lymphocyte leukaemic MOLT-4 cells (p53 wt). We evaluated the expression of the tumour-suppressor p53 (itself and phosphorylated on Ser(15) and Ser(392)), the cell cycle regulator p21, and the anti-apoptotic protein myeloid cell leukemia 1 (Mcl-1). After treatment with 2 mM caffeine, the cells were irradiated by 1 or 3 Gy, lysed and the proteins detected by Western-blotting. Apoptosis was determined by flow-cytometric annexin V/propidium iodine detection. Irradiation by 1 or 3 Gy induced p53 phosphorylation at Ser(15) and Ser(392) after 2 h with maximum after 4 h. Adding caffeine significantly inhibited Ser(15) phosphorylation, which is ATM-dependent but surprisingly also Ser(392) phosphorylation, which is ATM-independent, suggesting that caffeine might have another cellular target (protein kinase). Similarly, caffeine caused a substantial decrease in p21 in combination with both doses of IR and also Mcl-1 was down-regulated. Three days after irradiation, caffeine significantly increased induction of apoptosis. The ATM/p53 pathway was suppressed by caffeine, which led to increased apoptosis accompanied by a p53-independent decrease in Mcl-1. It also caused down-regulation of p21, which possibly contributed to the shortened cell cycle arrest necessary for effective DNA repair and thus impeded radio-resistance. Caffeine promotes the cytotoxic effect of ionising radiation and provides a possible platform for the development of new anti-cancer therapeutics known as radio-sensitizers.