Effects of exogenous nitric oxide on oral squamous cell carcinoma: an in vitro study

Nitric oxide (NO) is a newly found unstable free radical gas, serving as an important mediator, messenger, and signal transduction molecule and involved in a variety of pathophysiologic processes. Recently, NO has been reported to have cytotoxic effects on several tumor cells as an effector molecule of activated macrophage. The objective of this study was to investigate the effects of exogenous NO on oral squamous cell carcinoma cell line and to try to clarify the possible mechanisms by which it kills tumor cells.

TSCCa cell line, established from a patient with oral squamous cell carcinoma of the tongue, was exposed to various concentrations of exogenous NO that were released from an NO donor, sodium nitroprusside (SNP), for 48 hours. Nitrite/nitrate levels in the culture supernatant were determined with a commercial available NO kit. Both morphologic and ultrastructural changes were evaluated by reverse phase contrast microscopy or transmission electron microscopy. The DNA was harvested from SNP-treated or untreated TSCCa cells and assessed by agarose gel electrophoresis.

SNP released NO into medium in a dose-dependent manner. NO had a concentration-dependent cytotoxicity against TSCCa cells. NO induced tumor cell death through apoptosis, which was characterized by incompleteness of nuclear membrane, disappearance of nucleole and nuclear condensation, chromatin margination, or chromatin homogenization. Agarose gel electrophoresis showed a typical internucleosomal DNA cleavage pattern (DNA ladder), a reliable indicator of apoptosis.

Our results suggested that NO had a tumoricidal potential against oral cancer cells. NO might exert its cytotoxicity as an effector molecule of activated microphage through at least apoptosis.