Cytoplasmic suppression of tumorigenicity in reconstructed mouse cells.

Previous cybrid studies aimed at demonstrating cytoplasmic suppression of tumorigenicity have been generally inconclusive because of (a) the use of mutagens or carcinogens to introduce nuclear-coded and cytoplasmic-coded genetic markers and (b) dilution of putative cytoplasmic suppressors with tumorigenic cytoplasm of whole cells used in the cybrid construction. We have circumvented these potential problems by examining tumorigenicity in reconstructed cells made from tumorigenic karyoplasts and nontumorigenic cytoplasts and by using a ricin-antiricin selection to obtain the reconstructed cells. Karyoplasts from tumorigenic NIH/3T3 cells that were derived from a clone that had survived incubation with benzo(a)pyrene-trans-7,8-dihydrodiol-9,10-epoxy (anti) and been passaged 17 times were fused to NIH/3T3 cytoplasts derived from nontumorigenic cells. The cytoplasts were loaded with antiricin antibody prior to fusion. Ten clones which survived ricin selection were not tumorigenic in nude mice. These findings offer support for the presence of cytoplasmic factors in nontumorigenic mouse cells that suppress benzo(a)pyrene epoxide-induced tumorigenicity.