Critical role of p21 on olaquindox-induced mitochondrial apoptosis and S-phase arrest involves activation of PI3K/AKT and inhibition of Nrf2/HO-1pathway.

Olaquindox, a quinoxaline 1,4-di-N-oxide, is known as an antibacterial agent and feed additive to treat bacterial infections and promote animal growth. However, the potential mechanism of toxicity is still unknown. The present study aims to explore the molecular mechanism of p21 on olaquindox-induced mitochondrial apoptosis and S-phase arrest in human hepatoma G2 cells (HepG2). As a result, olaquindox promoted production of ROS, suppressed the protein expression p21 in p53-independent way and phosphorylated p21. Meanwhile, olaquindox activated AKT and Nrf2/HO-1 pathway, up-regulated Bax/Bcl-2 ratio, disrupted mitochondrial membrane potential (MMP) and subsequently caused cytochrome c release and a cascade activation of caspase, eventually induced apoptosis. Olaquindox could induce S-phase arrest in HepG2 cells involved with the increase of Cyclin A, Cyclin E and CDK 2. Furthermore, knockdown of p21 decreased cell viability, enhanced oxidative stress, aggravated olaquindox-induced mitochondrial apoptosis and S-phase arrest involvement of activating PI3K/AKT and inhibiting Nrf2/HO-1 pathway. PI3K/AKT inhibitor (LY294002) and HO-1inhibitor (ZnPP-IX) both increased olaquindox-induced apoptosis and S-phase arrest. In conclusion, knockdown of p21 increased olaquindox-induced mitochondrial apoptosis and S-phase arrest through further activating PI3K/AKT and inhibiting Nrf2/HO-1pathway. Our study provided new insights into the molecular mechanism of olaquindox and shed light on the role of p21.