4. Conclusions 
To assess the effects of PpIX administration prior to X-ray irradiation on cancer cells in vitro, we analyzed gene expression by microarray. We evaluated gene expression differences between HeLa cell groups treated with PpIX (PpIXT), X-ray irradiation (XT) and both treatments combined (PpIX-XT) at sub-lethal doses in cells with limited proliferation capacity. Under these conditions, we confirmed the intracellular accumulation of porphyrin, as well as ROS generation. XT and PpIX-XT induced systematic changes in the expression of genes related to cell-cycle arrest and inhibition of DNA replication. However, the changes in gene expression associated with PpIX administration alone, without X-ray irradiation, were less extensive and not systematic. Interestingly, PpIX-XT and XT differed in the number of genes that showed altered expression profiles relative to controls, but qualitative gene expression remained the same between the two groups. The increase in intracellular ROS levels after PpIX and/or X-ray treatment may have led to differences in clonogenic survival between different treatment groups, and this may have resulted from differences in gene expression between the PpIX-XT and XT groups. Our data highlight the complex mechanism by which PpIX enhances ROS generation, resulting in a decrease in clonogenic survival via DNA damage-induced cell-cycle arrest. The present study elucidates the cellular responses to PpIX, which acts as a radiosensitizer under non-lethal and limited proliferation conditions. Our findings could be useful in improving the efficacy of radiotherapy to treat human cancers.