E6 binds to p53, a major tumor suppressor in the cell that normally activates transcription of p21, an inhibitor of kinases that promote mitosis.
The E6 oncoproteins in HPV have been shown to be one of the main players in the progression to cervical cancer. E6 in HPVs that are at high risk of causing cancer can complex with p53, which is an important tumor suppressor that plays a role in directing the cellular response to genotoxic and cytotoxic stresses that threaten genomic stability. It functions as a sequence specific transcription activator that is necessary to regulate cell growth as well as tumor growth suppression. The p53 protein suppresses cell growth by transcriptionally activating p21, which inhibits the cell-cycle kinases critical for G1 progression and cell growth.
HPV E6 has also been shown to induce degradation of p53 through ubiquitin-dependent proteolysis. E6 forms a complex with the cellular ubiquitin protein ligase E6-AP, which is then able to bind and ubiquinate p53.
p53 usually acts to arrest G1 growth or to induce apoptosis in the cell to allow the host DNA to be repaired or for the cell to be eliminated if the DNA is irreparable. E6-expressing cells do not manifest a p53-mediated cellular response to DNA damage, which leads to genomic instability. E6 can also activate telomerase independent of p53, leading to immortalization of the infected cell. There is also evidence that E6 can induce abnormal centrosome duplication, leading to genomic instability and aneuploidy.
E2F is a cell-cycle activator that is regulated by the binding of pRB, a cellular tumor suppressor. HPV protein E7 can bind to pRB, rendering it unable to bind and regulate E2F, which then can freely activate the cell cycle to progress uncontrollably.
E7 can transactivate the E2 promoter, inducing DNA synthesis in quiescent cells. E2 normally binds to retinoblastoma tumor suppressor pRB. Its phosphorylation state is regulated through the cell cycle such that in normal conditions it is hypophosphorylated in G0 and G1 phase and is phosphorylated during S, G2, and M phase. pRB becomes phosphorylated by cyclin-dependent kinases at the G1 to S phase and remains phosphorylated until the M phase. The hypophosphorylated form is its active form in which it can inhibit cell-cycle progression. E7 binds to the hypophosphorylated form of pRB, functionally inactivating it and thus permiting cell-cycle progression of the cell into S phase. Thus, E7 is able to induce DNA synthesis and cellular proliferation.
pRB regulates members of the E2F family transcription factors. When E2F are bound to the hypophosphorylated form of pRB, E2F functions as a transcriptional repressor. When pRB is phosphorylated by cyclin-dependent kinase complexes, the pRB-E2F complex dissociates and E2F is free to act as a transcriptional activator. E2F acts as both an activator (G1 through S) and a repressor (M through G1) at different points in the cell cycle. When E7 binds pRB, pRB cannot bind to E2F and thus E2F freely activates the cell cycle.
It is known that viral E2 regulates the activity of E6 and E7, which are implicated in tumorigenesis. It is thought that in cervical keratinocytes that have been infected with HPV for a length of time, certain events occur in which the viral genome is integrated into the host genome, causing a deletion in the 3' region of the viral genome. This deletion results in the loss of the E2 region, leading to a loss of regulation of E7 transcription. Thus, after this integration event, E7 can be transcribed at higher rates and the risk for cancer increases tremendously.