Reduced tumorigenicity of murine hepatoma cells after treatment with antioxidants and melatonin

The tumor growth of murine hepatoma cells MH22a treated with N-acetylcysteine (NAC, 10 mM) and alpha-lipoic acid (ALA, 1.25 mM) antioxidants or hormone melatonin (1 μM) and transplanted into syngeneic (C3HA) mice has been studied. NAC, ALA, or melatonin treatment for 20 h reduced the tumor development and the number of dead mice. Melatonin produced the most pronounced effect. Tumors appeared in 10 days in 100% of control mice injected with untreated cells; the injection of cells pretreated by NAC or ALA generated tumors in 40 and 53% of mice, respectively. Cells pretreated with melatonin produced tumors 18–20 days after injection; 67% of control mice died in 36 days (the observation period). The mortality rate was 20 and 53% if the injected cells were treated with NAC or ALA, respectively. No mice died during this period with melatonin-pretreated cells. We found that treatment with antioxidants delayed (NAC) or completely inhibited (ALA) the progression of the cell cycle of murine hepatoma cells. After the antioxidant removal, the cell cycle was restored. Melatonin did not affect the cell cycle phase distribution. We conclude that there is no direct correlation between the loss of tumorigenic properties and the altered proliferative activity of hepatoma cells. Different mechanisms of antioxidants and melatonin action that underlie the transient normalization of the tumor phenotype are discussed.