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Enhanced Cytochrome P450 (Cyp1b1) Expression, Aryl Hydrocarbon Hydroxylase Activity, Cytotoxicity, and Transformation of C3H 10T1/2 Cells by Dimethylbenz(a)anthracene in Conditioned Medium¹

Department of Radiological Health Sciences, Colorado State University, Fort Collins, Colorado 80523

The basal and benz(a)anthracene-induced aryl hydrocarbon hydroxylase activities of C3H 10T1/2 mouse embryo fibroblasts have been shown to vary with population growth. We report here that, in the case of dimethylbenz(a)anthracene, cytotoxicity and transformation (neoplastic/morphological transformation and focus formation) increased as a consequence of population growth and, at high cell densities, DNA adduct formation was elevated. Among the factors that may contribute to these changes, we have found that conditioning of the medium with population growth plays a significant role. Cells treated with medium conditioned by several days of cell growth supported increases in the dimethylbenz(a)anthracene induction of mRNA expression of a new mouse cytochrome P450 gene designated Cyp1b1, aryl hydrocarbon hydroxylase activity, cytotoxicity, and the frequency of neoplastic transformation. These results suggest a cause and effect relationship between the enhanced expression of Cyp1b1 due to medium conditioning and the enhanced expression of the cellular endpoints cytotoxicity and transformation.

¹ This research was supported by the Department of Health and Human Services, United States Public Health Service, via Grant CA47497 from the National Cancer Institute. The results contained herein are the responsibility of the authors and not the National Cancer Institute.

² Current address: LS-1, MS M888, Life Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545.

³ To whom requests for reprints should be addressed.

Received 1/31/94. Accepted 5/23/94.

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