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DNA Synthesis and Transformation Induced in Density-inhibited Cultures of Hamster Embryo Cells by the Carcinogen Benzo(a)pyrene¹

American National Red Cross, Blood Research Laboratory, Bethesda, Maryland 20014 [S. M.], and Department of Pathology, Jefferson Medical College of Thomas Jefferson University, Philadelphia, Pennsylvania 19107 [R. L.]

At concentrations that induce neoplastic transformation in vitro, benzo(a)pyrene (BP) elicited a round of DNA replication in confluent cultures of whole hamster embryos. The proportions of cells responding increased linearly over the dose range of 0.01 to 10 µg BP per ml, suggesting a one-hit mechanism of induction. Kinetic analysis after a dose of 10 µg BP per ml has indicated a G₁ and S period of 6 and 11 hr, respectively. However, the cells stimulated to initiate DNA synthesis (8 to 10% of the exposed population) were unable to enter mitosis due, presumably, to a G₂ block. Experimentation with WI 38 cells, which cannot metabolize BP, or with the noncarcinogenic derivative pyrene, and hamster embryo cultures, indicated that both metabolic activation and carcinogenicity of BP are necessary for its ability to elicit DNA syntheses. Two different types of BP-induced DNA synthesis have been identified autoradiographically with respect to their sensitivity to hydroxyurea (HU, 10 mM). The fraction of cells undergoing HU-resistant (repair) DNA synthesis represented 30% of the entire BP-stimulated cohort. Colonies with criss-cross or random orientation of cells have been obtained after plating the cultures treated at confluency with BP. The transformation frequency depended on the duration of carcinogen exposure. The transformation was maximal with the cells plated at the peak of HU-sensitive type of carcinogen-induced DNA synthesis, before HU-resistant (repair) DNA synthesis was terminated.

¹ This study was conducted under Grant 1R01 CA 12393-01 from the USPHS, NIH, and General Research Support Grant PR 5414, at the Department of Pathology, Thomas Jefferson University, Philadelphia Pa., while the senior author was the holder of a National Science Foundation Fellowship.

Received 1/29/74. Accepted 6/26/74.

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