Folate deficiency induces a cell cycle-specific apoptosis in HepG2 cells

The human hepatoma HepG2 cell line was chosen as a representative of solid tissue-derived cell systems in which folate metabolism and apoptosis induct ion have not been thoroughly investigated. HepG2 cells were cultivated in t he control or folate-deficient media (control media lacking of folate, glyc ine, thymidine and hypoxanthine) for 4 wk. This resulted in a decrease in i ntracellular folate levels to 32% of the control within 1 wk, which was fol lowed by growth arrest and greater cell death rates. These disturbances of folate deficiency coincided with apoptotic induction, as characteristically shown by nucleosomal DNA fragmentation of 180-200 base pair multimers, nuc lear chromatin condensation and positive terminal transferase-mediated dUTP nick end labeling assay. Apoptosis coincided with an accumulation of cells in S-phase, a subsequent G2/M phase block and a significant increase in me an protein content as evaluated by flow cytometric analyses employing a dou ble-staining method. The growth and cell cycle arrest under folate-deficien t conditions was independent of a change of p53 expression as measured by a n enzyme-linked immunosorbent assay. Supplementation of 2 mu mol/L folate n ormalized cell cycles and diminished DNA fragmentation. Taken together, the se data indicate that HepG2 cells cultivated in folate-deficient medium hav e a low folate concentration, decreased growth and viability, and increased apoptotic propensity. This occurrence of apoptosis was associated with a c ell cycle-specific mechanism and independent of p53-mediated pathway.