ADP sensitizes ZL55 cells to the activity of anticancer cisplatin

We used the human mesothelioma ZL55 cells, which are a reliable model for investigating the therapeutic potential of nucleotides in MPM clinical trials. In addition we attempted to potentiate the cytotoxic effects of cisplatin by ADP. In ZL55 cells ADP is able to block cellular proliferation in a time and dose dependent manner, but evaluating the major apoptotic markers, it is seen that the mechanism underlying this effect is not apoptosis. Cell cycle analysis revealed that inhibition of proliferation of ZL55 cells by ADP was mediated by block of cell cycle progression. Flow cytometry analysis of subconfluent ZL55 cell cultures showed that treatment with ADP (10–100μM) for 24 h caused a concentration dependent increase of cells in the G0/G1 phases. ADP also induced p53 protein levels and increased the levels of p53, p21 and p27 mRNA, whilst cisplatin did not. Cisplatin plus ADP were more efficacious than cisplatin alone in inducing apoptosis characterized by: (a) mitochondria depolarization, (b) increase of bax expression and its cytosol-to-mitochondria translocation and decrease of Bcl-2 expression, (c) activation of caspase-7 and -9 and cleavage of PARP-1. Therefore, the antiproliferative effect of ADP is due to the cell cycle block and increases cisplatin cytotoxicity.