Folate deficiency-induced oxidative stress and apoptosis are mediated via homocysteine-dependent overproduction of hydrogen peroxide and enhanced activation of NF-kappa B in human Hep G2 cells

Folate coenzymes are critical for de novo synthesis of purine and thymidine , and for interconversion of amino acids. Folate deficiency inhibits cellul ar proliferation, disturbs cell cycling, causes genetic damage and eventual ly results in cell death. Previously, we demonstrated that the demise of hu man hepatoma Hep G2 cells mediated by folate deficiency proceeded via a p53 -independent apoptosis, and the perturbation of intracellular calcium homeo stasis was also shown to be involved. To further delineate the mechanism as sociated with this observed phenomenon, Hep G2 cells were cultivated in the control or folate-deficient media (control media lacking folate, glycine, thymidine and hypoxanthine) for 4 weeks. At the end of this cultivation per iod, we found that TBARS (an index of lipid peroxidation) concentrations in the folate-deficient cells were drastically increased as compared to the c ontrol cells (0.04 vs 0.01 nmole/10(6) cells), indicating that a severe oxi dative stress of the former cells had occurred. This phenomenon was also sh own to coincide with the ability of these folate-deficient cells to elabora te increased amounts of H2O2 as compared to its folate-supplemented cells ( 2.87 vs 0.98 nmole/10(5) cells/h). Furthermore, the accelerated production of H2O2 by the folate-deficient cells was also closely correlated with the elevated homocysteine concentrations released in the culture medium (15.37 +/- 2.4 vs 3.58 +/- 2.4 mu mole/L; P < 0.001). Finally, we demonstrated tha t folate deficiency was indeed capable of activating a redox-sensitive tran scription factor, NF-<kappa>B, which is crucial in the control of a reactiv e oxygen species-mediated apoptosis. In summary, we show that folate defici ency-induced apoptosis is proceeded via the enhanced activation of NF-kappa B, which is the resulting form of the homocysteine-mediated overproduction of hydrogen peroxide. (C) 2001 Editions scientifiques et medicales Elsevier SAS.