Human papilloma virus type 16 E7 genes protect astrocytes against apoptotic and necrotic death induced by hydrogen peroxide

Hydrogen peroxide is considered to be a dose- and time-dependent mediator in apoptotic and necrotic death. In this study, we examined the signaling of the E6 and E7 proteins with respect to apoptosis or necrosis after H₂O₂ injury using an in vitro model with overexpressed E6 or E7 genes. For this purpose, the E6 and E7 gene expressing astrocytes were exposed to 10 μM and 200 μM H₂O₂ Solutions. Twentyfour hours after treatment with the lower dosage (10 μM H₂O₂), control, E6-expressing cells suffered about 45% injury and LXSN-expressing cells decreased by 67% as assessed by LDH release. However, E7-expressing cells showed less injury, resulting in 20-30% of LDH release. Astrocytes expressing E6, E7, LXSN and mock-infected cells showed a typical apoptotic death pattern on the DNA gel after treatment with a low-dose of H₂O₂ (10 μM), however they died from necrotic death after a high-dose (200 μM) H₂O₂. Overexpression of HPV-E7 genes protected the cells from apoptotic death after a low-dose of H₂O₂ and from necrotic death after a high-dose of H₂O₂, while the overexpression of E6 genes from the necrotic death. E7 expressing astrocytes showed higher catalase activity and the levels of E2F protein surged more than 100-folds compared with the control astrocytes. We believe that the activity of E7 protein to protect astrocytes from H₂O₂ injury was at least partly due to increased catalase, a scavenger protein.