Regulation of normal cell cycle progression by flavin-containing oxidases

Mechanisms underlying the role of reactive oxygen species (ROS) generated by flavin-containing oxidases in regulating cell cycle progression were examined in human and rodent fibroblasts. Incubation of confluent cell cultures with nontoxic/nonclastogenic concentrations of the flavoprotein inhibitor, diphenyleneiodonium (DPI), reduced nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase activity and basal ROS levels, but increased proteolysis of cyclin D1, p21 Waf1 and phospho-p38 MAPK . When these cells were allowed to proliferate by subculture in DPI-free medium, an extensive G 1 delay was observed with concomitant activation of p53/p21 Waf1 signaling and reduced phosphorylation of mitogen-activated kinases. Compensation for decreased oxidant generation by simultaneous exposure to DPI and nontoxic doses of the ROS generators, γ -radiation or t -butyl-hydroperoxide, attenuated the G 1 delay. Whereas the DPI-induced G 1 checkpoint was completely dependent on PHOX91 , ATM and WAF1 , it was only partially dependent on P53 . Interestingly, G 1 to S progression was not affected when another flavin-containing enzyme, nitric oxide synthase, was inhibited nor was it associated with changes in mitochondrial membrane potential. Proliferating cells treated with DPI also experienced a significant but attenuated delay in G 2 . We propose that ATM performs a critical function in mediating normal cellular proliferation that is regulated by nonphagocytic NAD(P)H oxidase enzymes activity, which may serve as a novel target for arresting cancer cells in G 1 .