Flavin-containing oxidases regulate progression from G1 to S phase of the cell cycle in normal human diploid fibroblasts

The effects of flavin-containing oxidase enzymes on cellular progression from G1 to S phase of the cell cycle were examined in normal human diploid fibroblasts by use of the cumulative labeling index method. Significant transient arrests in G1 occurred when quiescent cell populations incubated with diphenyliodonium (DPI) for a 20 h period at concentrations as low as 0.05 μM were stimulated to proliferate by subculturing to low density in control growth medium. Quiescent cells pre-treated with DPI and subcultured in DPI-containing growth medium were permanently arrested in G1. Induction of this G1 checkpoint was associated with activation of the p53/p21Waf1 signaling pathway. Incubation of G0/G1 phase cells with DPI (0.05-0.2 μM) resulted in significant decreases in the expression levels of cyclin D1 and p21Waf1; no effect was detected on cyclin A expression. Collectively, these results suggest that DPI-sensitive signaling primarily regulates early G1 events that lead to S-phase transition. Furthermore, they support a critical role for membrane originating oxidants in the regulation of cellular proliferation pathways.