CELLULAR AND MOLECULAR ALTERATIONS IN HUMAN EPITHELIAL-CELLS TRANSFORMED BY RECOMBINANT HUMAN PAPILLOMAVIRUS DNA

Human papillomaviruses (HPVs) contribute to the development of benign and malignant cervical cancer: however, the exact role of papillomavir uses in the multistage carcinogenesis process is unclear. The developm ent of HPV-immortalized cervical and foreskin cell lines represents a useful model for studying the role of HPVs in cervical cancer. Studies with these cells show that HPV genes regulate epithelial cell growth and differentiation. Transfection of HPV types associated with invasiv e cervical cancer results in immortalization of human epithelial cells , whereas HPVs not associated with cancer are ineffective. The combina tion of E6 and E7 genes. which are normally retained and expressed in cervical carcinomas. is sufficient for immortalization; however, the E 7 gene alone induCeS immortality less efficiently. Although the immort alized cells actively express HPV oncoproteins observed in cervical ca ncer, after injection of immortal cells into nude mice, tumors are rar e, having been reported only for HPV-18. Immortalized cells are resist ant to terminal differentiation; in fact, HPVs may contribute to the c arcinogenic process by uncoupling the processes of cell growth and dif ferentiation. Host regulation of viral genes also is important in the malignant process. Endogenous cytokines modify HPV gene expression and influence the pathogenesis of HPV infection in the cervix. HPV gene e xpression is regulated by cellular transcriptional activators and repr essors. This normal regulation is altered by viral integration. HPVs b ecome integrated preferentially at chromosomal regions near fragile si tes and protooncogenes. In fact, immortality is associated with induct ion of structural rearrangements frequently affecting HPV integration sites. Structural and numerical alterations nonrandomly involve chromo somes 1, 11, 19, and 20. with chromosome 1 alteration being the most p redominant. Wild-type functions of Rb and p53 are necessary to control normal cell growth, and mutation or loss of these suppressor genes of ten contributes to cancer development. In HPV-containing carcinomas, p Rb and p53 were wild type. However, in carcinomas lacking HPV, both su ppressor genes were mutated. Functional inactivation of these tumor su ppressor genes by HPV oncoproteins E6 and E7 may explain this differen ce. Treatment of HPV-immortalized cells with ras or a subfragment of h erpes simplex virus (HSV) of HPV-immortalized cells resulted in locall y invasive carcinomas when the cells were implanted subcutaneously in nude mice. These experiments indicate that HPV integration and express ion are insufficient for malignancy but that HPVs do participate in th e multistep development of cancer.