The Human Papillomavirus Oncoprotein E7 Attenuates NF-κB Activation by Targeting the IκB Kinase Complex

Abstract

Infection with high-risk human papillomaviruses (HPV) can lead to the development of cervical carcinomas. This process critically depends on the virus-encoded E6 and E7 oncoproteins, which stimulate proliferation by manipulating the function of a variety of host key regulatory proteins. Here we show that both viral proteins dose-dependently interfere with the transcriptional activity of NF-B. A variety of experimental approaches revealed that a fraction of the E7 proteins is found in association with the IB kinase complex and attenuates induced kinase activity of IB kinase ␣ (IKK␣) and IKK␤, thus resulting in impaired IB␣ phosphorylation and degradation. Indirect immunofluorescence shows that E7 impairs TNF␣-induced nuclear translocation of NF-B, thus preventing NF-B from binding to its cognate DNA. While E7 obviates IKK activation in the cytoplasm, the E6 protein reduces NF-B p65-dependent transcriptional activity within the nucleus. We suggest that HPV oncogene-mediated suppression of NF-B activity contributes to HPV escape from the immune system. HPVs1 are small DNA viruses, and specific high-risk types such as the HPV type 16 (HPV16) or HPV18 are causative agents of some forms of anogenital and oral cancers (1). HPV16 encodes six early proteins including the major oncoproteins E6 and E7. Both proteins play a central role in the induction of benign proliferation and malignant transformation (2), and at least the persistence of E7 is necessary to maintain the transformed phenotype (3). These two oncoproteins are selectively and continuously expressed in HPV-induced tumors and manipulate cell proliferation upon physical and functional interaction with several master cell cycle regulators (4). E6 binds to p53 (5) and causes its ubiquitin-dependent degradation (6), thereby interfering with p53 functions in cell cycle control and apoptosis. In addition, the E6 protein binds to the protein kinase PKN (7) and other regulators including interferon regulatory factor 3 (8) and the proapoptotic Bak protein (9). The E7 protein interacts with so-called "pocket proteins" such as the retinoblastoma protein pRb, p107, and p130 (10), resulting in their enhanced phosphorylation and degradation (11). pRb destruction results in the release of E2F family transcription factors and subsequent activation of genes promoting cell proliferation (12). But the stimulatory effects of E7 on cell proliferation depends not only on its association with pRb (13, 14), because E7 targets the function of a plethora of regulators including cyclin E (15), acid alpha-glucosidase (16), and M2 pyruvate kinase (17). E7 also interferes with the activity of a variety of transcription factors such as AP-1 (18), interferon regulatory factor-1 (19), fork head domain transcription factor MPP2 (20), and TATA-box-binding protein (21). This multiplicity of interaction partners and additional levels of functional E7 regulation by phosphorylations (22), protein stability (23), and the oligomerization state (24) allow a highly complex and sophisticated manipulatio...

Human papillomavirus infection induces NF-κB activation in cervical cancer: A comparison with penile cancer

“…The expression of numerous cytokines, chemokines, growth factors and survival factors is NF-κB-dependent. NF-κB activation is modulated for HPV (18)(19)(20)(21)(22)(23). In this study, NF-κB was detected in the nucleus and/or cytoplasm in 96.4% of the HPV-positive cases.…”

“…Overexpression of viral E6 and E7 oncogenes reacts with the tumor suppressor gene products p53 and pRb proteins in the host cells, resulting in an induced cell immortalization, transformation and oncogenesis due to their interference with the cell cycle and apoptosis control (25). HPV E6 and E7 oncogenes are key regulatory proteins inside host cells and are associated with the transcriptional activity of NF-κB (19). Therefore, activation of NF-κB by viral oncogenes may be the mechanism of tumor formation in cervical cancer.…”

Relationship among human papillomavirus infection, p16INK4a, p53 and NF-κB activation in penile cancer from northern Thailand

“…NF-κB activity is modulated for many different viruses, such as HIV-1, HTLV-1, EBV, HBV, adenovirus and HPV (33)(34)(35)(36)(37)(38). NF-κB-dependent proliferation and protection from apoptosis are likely to have significant effects on the oncogenesis of HPV associated with cancers.…”

“…While E7 obviates IKK activation in the cytoplasm, the E6 protein reduces NF-κB p65-dependent transcriptional activity within the nucleus. It is suggested that the HPV oncogene-mediated suppression of NF-κB activity contributes to HPV escaping from the immune system (34).…”

Human papillomavirus type 16 E6 and E7 oncoproteins upregulate c-IAP2 gene expression and confer resistance to apoptosis

“…However, HPV16 E6/ E7-induced c-IAP2 transcription may not be mediated by TNF-a because HPV infection or E6 and E7 expression have been shown to reduce TNF-a expression in keratinocytes (Mota et al, 1999;Havard et al, 2002). Also, HPV16 E6 and E7 were reported to attenuate TNF-a-mediated NF-kB activation (Spitkovsky et al, 2002). TNF-a activates NF-kB through degradation of its inhibitor IkB, which allows NF-kB to translocate to the nucleus.…”