Repression of the Integrated Papillomavirus E6/E7 Promoter Is Required for Growth Suppression of Cervical Cancer Cells

Abstract

The human papillomavirus (HPV) E2 protein is an important regulator of viral E6 and E7 gene expression. E2 can repress the viral promoter for E6 and E7 expression as well as block progression of the cell cycle in cancer cells harboring the DNA of "high-risk" HPV types. Although the phenomenon of E2-mediated growth arrest of HeLa cells and other HPV-positive cancer cells has been well documented, the specific mechanism by which E2 affects cellular proliferation has not yet been elucidated. Here, we show that bovine papillomavirus (BPV) E2-induced growth arrest of HeLa cells requires the repression of the E6 and E7 promoter. This repression is specific for E2TA and not E2TR, a BPV E2 variant that lacks the N-terminal transactivation domain. We demonstrate that expression of HPV16 E6 and E7 from a heterologous promoter that is not regulated by E2 rescues HeLa cells from E2-mediated growth arrest. Our data indicate that the pathway of E2-mediated growth arrest of HeLa cells requires repression of E6 and E7 expression through an activity specified by the transactivation domain of E2TA.The papillomaviruses are small DNA viruses that infect a variety of mammalian species, including humans. Human papillomavirus (HPV) infections are quite common and cause a variety of benign proliferations including cutaneous warts, venereal warts, genital squamous intraepithelial lesions, and orolaryngeal and -pharyngeal papillomas, as well as other types of hyperkeratoses (41). Certain HPV types have also been linked to specific malignant lesions, most notably cervical cancer and other anogenital cancers.Of the over 80 HPV types identified, approximately 30 HPV types are associated with genital tract lesions. Subsets of the anogenital HPV types are classified as either "low risk" or "high risk" depending upon the potential of the associated lesions to progress to cancer. HPV6 and HPV11 are low-risk HPV types that are associated with low-grade squamous epithelial lesions (venereal warts or condyloma accuminata), which rarely progress to cancer. HPV16, HPV18, and others are high-risk HPV types that are associated with intraepithelial neoplasias or squamous intraepithelial lesions that can progress to cancer (54).It is now evident that most cervical neoplasias, whether intraepithelial or invasive, can be attributed largely to HPV infection. The DNA of a high risk HPV type can be found in over 90% of human cervical cancers (54). In addition, transfection of high-risk HPV genomic DNA can extend the life span of primary human genital keratinocytes (the normal cellular host of HPVs) and can contribute to cellular immortalization (13,39,51). This ability of the high-risk HPV types to immortalize keratinocytes is not a property of low-risk HPV types (13, 39).Oncogenic viruses often exploit the machinery of the cells they infect in order to promote their own survival and replication. As a part of this process, they interfere with normal cellular control mechanisms, leading to abnormal cell growth, genetic alterations and even malignant transform...

The human papillomavirus E6 and E7 inducible oncogene, hWAPL, exhibits potential as a therapeutic target

“…Because the papillomavirus E2 protein represses E6/E7 transcription by binding the HPV early promoter (Hwang et al, 1993(Hwang et al, , 1996Dowhanick et al, 1995;Francis et al, 2000), we analyse the effect of E2 on hWAPL expression. We observed a significant reduction of hWAPL protein levels after the infection of HPV-16 E6-and E7-expressing cervical cancer cell lines, such as CaSki and SiHa, with the retrovirus encoding E2 ( Figure 1B).…”

Repression of human papillomavirus oncogenes in HeLa cervical carcinoma cells causes the orderly reactivation of dormant tumor suppressor pathways

“…Introduction of an ectopic E2 gene into a number of cervical carcinoma cells lines results in a great reduction in E6͞E7 mRNA and in substantial growth inhibition (32-36, 38, 42). E2-mediated growth inhibition is observed only in cells containing HPV DNA, and E2-induced reduction in HeLa cell colony formation is prevented by constitutive expression of the HPV16 E6 and E7 genes (32,38,41,42), indicting that repression of E6͞E7 is required for the growth inhibitory effect. …”

Antiviral agent Cidofovir restores p53 function and enhances the radiosensitivity in HPV-associated cancers

“…Repression of viral oncoproteins expression would hence restore growth control in these cells by targeting E6 and E7. For this purpose, several approaches have been used such as antisense strategies (Steele et al, 1993;He et al, 1997), delivering HPV E2 gene to directly repress E6 and E7 transcription (Hwang et al, 1996;Francis et al, 2000), targeting E6 oncoprotein by peptide aptamers (Butz et al, 2000) and inhibiting E6/E7 transcription by nuclear export inhibitors (Hietanen et al, 2000). Although these strategies demonstrated that down-regulating E6/E7 could restore growth control, their ecacy was limited due to insucient gene delivery into tumors or to cytotoxic eects induced in normal cells.…”