Background: Heme oxygenase-1 (HO-1) exhibits cytoprotective effects in many different cell types and is induced by nicotine exposure in human gingival fibroblasts. However, the role of HO-1 in cancer cells exposed to nicotine has not previously been described.
Methods: We investigated the effects of nicotine on HO-1 protein expression and cell viability in immortalized (IHOK) and malignant (HN12) human oral keratinocyte cells using the 3,4,5-dimethylthiazol-2-yl-2,5-diphenyl tetrazolium bromide assay and Western blotting. We also examined the involvement of the phosphoinositide-3-kinase (PI3K), mitogen-activated protein kinase (MAPK), and nuclear factor-kappaB (NF-kappaB) signaling pathways in nicotine-induced cytotoxicity and HO-1 levels in IHOK and HN12 cells.
Results: Nicotine-induced HO-1 production and had cytotoxic effects on cells in both a concentration- and time-dependent manner. Nicotine-induced cytotoxicity and accumulation of HO-1 were greater in IHOK cells than in HN12 cells. Molecular inhibitors of the ERK, p38 MAP kinase, PI3 K, and NF-kappaB signaling pathways blocked the cytotoxic effects and induction of HO-1 expression by nicotine. Treatment with antioxidants (bilirubin, N-acetylcysteine) protected cells against nicotine-induced cytotoxicity and blocked the upregulation of HO-1, the effects of which were more pronounced in IHOK cells than in HN12 cells.
Conclusions: Collectively, these results suggest that HO-1 plays a principal role in the protective response to nicotine in oral cancer and immortalized keratinocytes. Moreover, the addition of exogenous antioxidants may help to protect oral epithelial cells as chemopreventive agents against nicotine-induced oxidative stress.