J Oral Pathol Med (2008) Background: Although hydrogen peroxide may play an important role in the development of cancer, it can be an efficient inducer of apoptosis in cancer cells; the exact mechanism by which this action occurs is not completely understood in oral cancer cells.Method: In this study, the mechanisms by which H2O2 inhibited growth and induced apoptosis were differentially investigated using HPV-immortalized human oral keratinocytes (IHOK) and oral cancer cells (HN4).Results: H2O2 treatment sensitively and dose-dependently induced growth inhibition and typical apoptosis in IHOK and HN4 cells, as demonstrated by a decreased level of cell viability, an increased population of cells in the sub-G0/G1 phase, ladder formation of the genomic DNA, chromatin condensation and accumulation of Annexin V+/PI+ cells. Furthermore, the expression of Bax, p53 and p21WAF1/CIP1 increased, whereas the expression of Bcl-2 decreased in immortalized and malignant keratinocytes that were treated with H2O2. In addition, cytochrome-c from the mitochondria was observed in H2O2-treated IHOK and oral cancer cells, and this was accompanied by the activation of caspase-3 and -9. Additionally, H2O2 treatment induced upregulation of CHOP, GRP78 and several representative endoplasmic reticulum (ER) stress-responsive proteins, including heme oxygenase-1.Conclusion: Overall, these results suggest that H2O2 triggers apoptosis via the mitochondrial and ER stress pathway in IHOK and HN4 cells, and that increasing the cellular levels of H2O2 sufficiently may lead to selective killing of oral cancer cells and therefore be therapeutically useful. (Source: Journal of Oral Pathology and Medicine)