G1-arrested FaO cells re-enter the cell cycle upon stimulation with the rodent non-genotoxic hepatocarcinogen nafenopin

The peroxisome proliferators are rodent non-genotoxic hepatocarcinogens that suppress apoptosis and induce DNA replication, cell proliferation and liver tumours, In order to investigate the effect of peroxisome proliferators on cell cycle progression, we arrested the well-differentiated rat hepatoma cell line FaO in the G(1) phase of the cell cycle. Under these conditions, CDK2 and CDK4 protein expression remained unchanged compared with proliferating cells, but expression of cyclin D1 and p27(KIP1) was downregulated and cyclin E accumulated in the inactive form, G(1)-arrested cells were able to enter the cell cycle on addition of exogenous growth factors such as epidermal growth factor (EGF) or hepatocyte growth factor (HGF) and replicate their DNA within 12 to 24 h of re-stimulation. Upon release from G(1) arrest, CDK2 protein expression was down-regulated and, surprisingly, p27(KIP1) expression was restored, Cyclin D1 and phosphorylated cyclin E accumulated at 12 h but were degraded by 24 h after addition of EGF, Importantly, the peroxisome proliferator nafenopin and tumour necrosis factor alpha were able to induce DNA replication. Thus, the profile of expression of cell cycle regulatory proteins upon stimulation with nafenopin is comparable with that induced by growth factors such as EGF.