Malignant transformation of human prostate epithelial cells by N-nitroso-N-methylurea

Prostate cancer is the most common malignancy and the second leading cause of male cancer death in the United States. In 1996, 317,000 new cases will be diagnosed, with over 40,000 deaths. It is estimated that 1 out of every 11 American men will eventually develop prostate cancer. Compared with all other cancers, the incidence of prostate cancer increases most rapidly with age. The genetic and environmental factors responsible for the high incidence of prostate cancers are largely unknown. In addition, unlike other major cancers such as breast, lung and colon, little is known at the molecular and genetic levels about prostate cancer. In vitro models of human prostate epithelial (HPE) cells provide a practical approach to analyze the molecular and genetic mechanisms underlying prostate tumorigenesis. To our knowledge, a chemical carcinogen-induced neoplastic transformation of HPE cell culture has not been described previously. We have recently reported the immortalization of normal adult HPE cells by DNA transfection of the HPV-18 genome and subsequent conversion of such nontumorigenic but immortalized cells (HPV-18 C-1) into tumorigenic cells by the introduction of an activated Ki-ras oncogene. The availability of a HPE cell line that could undergo neoplastic conversion in response to a ras oncogene led us to inquire whether this system might be useful in detecting chemical carcinogens for HPE cells. In the present study, we report the malignant transformation of HPV-18 C-1 cells following multiple treatment with the chemical carcinogen N-nitroso-N-methylurea (NMU), a prototype compound for nitrosamine metabolites in cigarette smoke. Such transformants showed morphological alteration, anchorage-independent growth in soft agar and induced carcinomas when transplanted into nude mice. No p53 or ras mutations were observed. Stepwise chromosomal changes in progression to tumorigenicity were observed. Loss of 'p' arms of chromosome 8 and chromosome 10 and gain of 'q' arm of chromosome 8 were only observed as the tumor grows. These findings provide the first evidence of malignant transformation of human prostate epithelial cells exposed to a chemical carcinogen. Our study demonstrates the malignant transformation of adult HPE cells in culture by a combination of HPV-18 and a chemical carcinogen and offers a unique in vitro model system for further analysis of molecular events underlying prostate carcinogenesis.

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