Arginine deiminase enhances MCF-7 cell radiosensitivity by inducing changes in the expression of cell cycle-related proteins

After successful clinical application, arginine deiminase (ADI) has been proposed to be a new cancer therapeutic. In the present study, we examined the effect of ADI in combination with ionizing radiation (IR) on MCF-7 cell growth and clonogenic cell death. Cell growth was inhibited by IR in a dose-dependent manner and ADI enhanced the radiosensitivity. ADI itself did not suppress the growth of MCF-7 cells due to the high level of expression of argininosuccinate synthetase (ASS), which convert citrulline, a product of arginine degradation by ADI, to arginine. Previously, it was suggested that ammonia, another product of arginine degradation by ADI, is the main cause of the growth inhibition of irradiated hepatoma cells contaminated with ADI-expressing mycoplasma [van Rijn et al. (2003)]. However, we found that ammonia is not the only factor that enhances radiosensitivity, as enhancement was also observed in the absence of ammonia. In order to identify the enhancing effect, levels of ASS and proteins related to the cell cycle were examined. ASS was unchanged by ADI plus IR, but p21 (a CDK inhibitor) was upregulated and c-Myc downregulated. These findings indicate that changes in the expressions of cell cycle proteins are involved in the enhancement of radiosensitivity by ADI. We suggest that ADI is a potential adjunct to cancer therapy.