The HPV16 E6 and E7 early genes are responsible for initiation of viral DNA replication and are believed to play a major role in virus-induced cervical neoplasia (38
). Proliferation and malignant phenotype of human cervical carcinoma cell cultures have been shown to depend on continuous expression of high-risk HPV oncogenes E6 and E7 (37
). Consequently, regulation of early gene expression has been studied intensively at the transcriptional level, and a number of both cis
regulatory factors have been found and characterized (28
). On the other hand, little is known about the translational regulation of E6 and E7 transcripts, possibly also because of the inefficient in vitro translation of HPV E7 mRNA in a rabbit reticulocyte lysate translation system (9
The present study reports three main findings. It explains E7 mRNA untranslatability in vitro as being mediated by a specific interaction between the rabbit α1-globin peptide fragment SEQIKA and the viral transcript; it shows that the E7 mRNA-binding SEQIKA sequence is also present in the human CK7 protein; and it demonstrates that the HPV16 E7 mRNA-CK7 interaction does occur in HPV16-positive squamous cancer SiHa cells.
These results have several important implications. mRNA-binding proteins generally inhibit mRNA translation and, in parallel, stabilize the transcripts. This twofold regulatory mechanism has the scope of protecting and storing particular mRNAs to be expressed at defined developmental stages (8
). In this context, the E7 mRNA-CK791-96
SEQIKA fragment interaction might contribute to explaining the protection of E7 mRNA during the differentiation program of the host keratinocyte. As a matter of fact, CK7 expression occurs in epithelial cells in parallel to differentiation (12
Moreover, CK7 is specifically localized in glandular (also called columnar) epithelium (27
). In the well-documented natural history of invasive squamous cell carcinoma of the cervix, it is a literature datum that the majority of invasive cervical neoplasms arise at the transition between the stratified squamous epithelium and the simple epithelium at the squamo-columnar junction. The cervical squamo-columnar junction is a region of cellular instability, and at certain critical physiological periods (i.e., pregnancy and menopause), the cells of the junctional epithelium undergo a process described as squamous metaplasia, in which the original simple cervical epithelium is replaced by squamous epithelium, with a concomitant change in the cytokeratin profile. The area in which metaplastic epithelium is found is known as the transformation zone (32
). Most cervical carcinomas arise in this transformation zone (20
); it is this transformation zone that must be biopsied to exclude carcinoma or precancerous lesions. This has led to the belief that neoplastic changes occur as a result of the action of oncogenic agents on metaplastic epithelium, resulting in a transformation event (14
). In this scenario, the spatial-temporal E7 protection ensured by CK7 might be a key event to understanding the transformation zone as the election area for the cervical carcinogenesis process.
In conclusion, the experiments reported here suggest new mechanisms for the regulation of HPV16 E7 expression which may be important during viral latency and activation and show that a previously hypothesized factor(s) might be the concomitant necessary cause for cervical cancer development following HPV infection (39
). The study of the precise determinants and mechanism of interaction between the CK791-96
SEQIKA fragment and HPV16 E7 mRNA might help in clarifying some aspects of cervical carcinogenesis and delineating possible peptide-based therapeutic approaches.