Dual effect of nitric oxide in immortalized and malignant human oral keratinocytes: induction of apoptosis and differentiation

Background: Nitric oxide (NO) is known to act cytostatically on several tumor cell when functioning as an effector molecule of activated macrophages, but the differential effects of NO on immortalized and malignant oral keratinocytes have not been examined. Methods: We investigated the influence of NO on the proliferation, cell cycle, apoptosis, and differentiation of immortalized human oral keratinocytes (IHOK) and primary oral cancer cells (HN4) using the 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide (MTT) assay, sulforhodamine B (SRB) assay, flow cytometry, nuclear DNA staining, and Western blotting. Results: The MTT and SRB assays indicated inhibited growth of IHOK and HN4 cells that were treated with sodium nitroprusside (SNP) at concentrations higher than 1 mM but not at lower SNP concentrations. The higher concentrations of SNP up‐regulated the apoptosis‐related protein expression, which is consistent with the analyses of sub‐G 1 phase arrest, annexin V‐FITC (fluorescein isothiocynate) staining, nuclear staining, and DNA fragmentation. On the other hand, the lower concentrations of SNP enhanced the expression of keratinocyte differentiation markers in IHOK and HN4 cells. Conclusions: These data suggest that high concentrations of NO can inhibit the growth of IHOK and HN4 cells through the induction of apoptosis, while low concentrations of NO can induce cytodifferentiation. The dual effects of NO, namely, the induction of apoptosis or cytodifferentiation, have important implications for the possible anti‐oral cancer treatment.