Molecular Pharmacology, Vol 12, 966-976, Copyright © 1976 by the American Society for Pharmacology and Experimental Therapeutics

Induction and Decay of Aryl Hydrocarbon Hydroxylase Activity in Mouse 3T3 Cells

¹ Department of Pharmacology, The University of Michigan Medical School, Ann Arbor, Michigan 48109

Aryl hydrocarbon hydroxylase activity in mouse 3T3 cells is induced by 24 hr of exposure to 9 µM benz[a]anthracene or to 1 µM 3-methylcholanthrene; however, maximal activity induced by 3-methylcholanthrene is only about 33% of that induced by benz[a]anthracene. Induction by benz[a]anthracene is inhibited by simultaneous administration of 3-methylcholanthrene. Inducible aryl hydrocarbon hydroxylase activity attains a maximum during the late log-early confluence stage of culture growth and is lower in confluent, nonfed cells. Feeding confluent cells results in another peak of inducibility, concomitant with an increase in DNA synthesis. However, blockade of DNA synthesis with cytosine arabinoside does not block the peak of aryl hydrocarbon hydroxylase activity which occurs after feeding. After removal of inducer from cultures with maximally induced activity, two apparent decay rates are observed, one with a t_1/2 of 15-30 min and another with a t_1/2 of 4-6 hr. Cultures with submaximally induced activity show only the slow rate of decay. Addition of inducer in fresh medium to washed, maximally induced cultures stabilizes induced activity. Readdition of benz[a]-anthracene to cultures deprived of inducer for 1 or 4 hr results in reinduction of aryl hydrocarbon hydroxylase. This second increase in activity is faster than the initial rate of induction and is prevented by cycloheximide or actinomycin D. These data indicate the importance of culture growth phase, feeding schedule, and duration of exposure to inducer in the determination of aryl hydrocarbon hydroxylase activity in cultured 3T3 cells. In addition, it was observed that both control and induced levels of this enzyme are decreased in cultures contaminated with mycoplasma before cytopathic effects of the contamination are evident.

Note:
ACKNOWLEDGMENTS We wish to thank Dr. George Kenny (Department of Pathobiology, University of Washington) for the determinations indicating mycoplasma contamination, and Dr. William B. Pratt (Department of Pharmacology, University of Michigan) for his helpful advice.