Human papillomaviruses (HPVs) are responsible for benign and malignant epithelial lesions such as common warts, genital warts, and cervical cancer. HPVs are icosahedral viruses that contain circular 8-kb double-stranded DNA genomes and are strictly epitheliotropic (reviewed in reference 44
). Over 200 different HPVs have been characterized and are classified as mucosal or cutaneous. The majority of HPVs are low risk and usually result only in benign warts, while infections with high-risk types are associated with epithelial lesions that have a propensity for malignant progression in the oral cavity, the anogenital tract, and, most notably, the cervix (9
). High-risk HPVs are present in nearly all cervical carcinomas, the second most common malignancy in women worldwide (reviewed in reference 45
). HPV type 16 (HPV16) is by far the most prevalent of the high-risk types, followed by HPV types 18, 31, and others (reviewed in reference 46
). Although virtually all cervical carcinomas contain high-risk HPVs, only a small fraction of women infected with high-risk HPVs will develop cervical cancer. Therefore, in addition to HPV infection, cofactors must contribute to the development and progression of cervical cancer.
Numerous epidemiological studies implicate steroid hormones as cofactors in the initiation and progression of cervical neoplasia (reviewed in reference 31
). A large multinational WHO collaborative study has demonstrated an increased risk of cervical cancer with steroid hormone use (2
). Increased duration of oral contraceptive use was also reported to increase the relative risk of adenocarcinoma in situ of the cervix and contribute to the pathogenesis of these tumors in women (27
). In another study controlled for social, sexual, and cervical screening variables, the incidence of precursor and invasive lesions was highest among long-term oral contraceptive users (39
). A recent report evaluated the role of steroid hormones in association with HPV positivity in cervical carcinogenesis; the risk of cervical carcinoma was increased up to fourfold in HPV-positive women with histories of long-term use of oral contraceptives (32
Experimental evidence has also suggested a link between steroid hormones, HPV, and cervical cancer. Patterns of steroid hormone receptor expression are reported to be altered in HPV-associated lesions (12
). Additionally, HPV gene expression can be transcriptionally modulated by steroid hormones through response elements located in the upstream regulatory region of the viral genome (36
); estrogen has been shown to stimulate transcription of HPV16 in SiHa cervical carcinoma cells (29
), and treatment with progesterone was shown to alter growth and viral gene expression in HPV16-positive CaSki cervical cancer cells and HPV16-containing Hep2 cells (43
). However, the strongest experimental evidence that steroid hormones act as cofactors in cervical cancer has resulted from studies using a transgenic mouse model where HPV16 is expressed under the K14 promoter. Arbeit et al. showed that none of the K14-HPV16 transgenic mice spontaneously developed cervical cancer (irrespective of pregnancy number), whereas 100% of HPV16 transgenic mice developed cervical cancer upon long-term exposure to low doses of estrogen (3
). Interestingly, in this model, the HPV16 oncogenes were expressed under the control of a keratin promoter, implying that synergy between steroid hormones and HPV in cervical cancer cannot be explained solely by an increase in viral gene expression. Despite the evidence implicating steroid hormones as cofactors in cervical carcinogenesis, the molecular pathways that link steroid hormones and HPV in the initiation and/or progression of cervical cancer remain undefined.
A possible candidate linking steroid hormones and HPV in cervical cancer is steroid receptor coactivator 1 (SRC-1), a component of steroid hormone signaling. We report that the HPV16 E7 oncoprotein associates with SRC-1, a member of the p160 steroid receptor coactivator family required for full transcriptional activity of the steroid receptor superfamily (35
). Regulation of hormone-dependent gene expression by SRCs has been an area of intense study and has proved to be quite complex. SRCs, including SRC-1, modulate target gene expression by interaction with ligand-bound nuclear receptors, recruitment of histone acetyltransferases (HATs) and methylases, subsequent chromatin remodeling, and assembly of transcription factors (reviewed in reference 41
). Selective recruitment of SRCs by different nuclear receptors can form specific, unique coactivator complexes to confer specificity to transcriptional regulation (8
). Additionally, functionally distinct SRC-1 isoforms likely play different roles in estrogen receptor-mediated transcription (21
There is evidence to suggest that viral proteins can dysregulate SRC-1 transcriptional complexes, which have been shown to include the HATs p300, CREB-binding protein (CBP), and p300/CBP-associated factor (PCAF). SRC-1 is a very weak HAT, and SRC-1-associated HAT activity is derived largely by recruitment of other HATs, such as p300 and PCAF (reviewed in reference 41
). Adenovirus E1A, structurally and functionally similar to HPV E7, was shown to bind CBP and prevent assembly of an SRC-1 coactivation complex, a critical step in hormone-dependent gene expression (42
). More recently, p300, CBP, and PCAF were shown to be targets for the E1A proteins of all six adenovirus subgroups, demonstrating the importance of these interactions (38
). Interestingly, HPV E7s from both high- and low-risk HPV types have been previously reported to interact with p300 and PCAF (4
). However, our results suggest that HPV E7 associates directly with SRC-1, independently of p300 and PCAF, and dysregulates the function of SRC-1 transcriptional complexes. We also provide evidence that HPV E7 retains or relocalizes SRC-1 to the cytoplasm, removing SRC-1 from its transcriptional targets.
With the present study, we have begun to explore the molecular mechanisms by which steroid hormones act as cofactors with HPV in cervical carcinogenesis. In this report, we provide evidence for the dysregulation of SRC-1 localization and function by the HPV E7 oncoprotein.