The isolation of potentially contact-inhibited cells from populations treated 

 with known carcinogenic agents is interesting for several reasons. Among the 

 more intriging ideas is the possibility that such cells represent back 

 transformation to the noncancerous state. The CHO cell possesses many of the 

 properties of transformed cells, including the loss of contact inhibition (42, 

 44). In accordance with the somatic cell mutation hypothesis for cancer (10, 

 22), agents inducing cell transformation through mutation should, in at least 

 some cases, be capable of inducing back transformation in individual cells via 

 true reverse mutation or via forward mutation at suppressor loci. Indeed, 

 spontaneous revertants of cells transformed to the malignant state in vitro by 

 viruses and chemicals have already been described (23, 24). Reversible 

 conversion of transformed cells to the contact-inhibited state has been 

 observed during exposure of such cells to dybutryl cyclic AMP (25) or 

 concanavalin A (11). It remains to be shown if the same agents capable of 

 inducing transformation form normal to malignant state can also reverse it, 

 such that the revertants are stable in the absence of inducing agent. Although 

 virtually all cell-transforming agents tested to date are also mutagens, it is not 

 yet possible to say if the phenomenon of cell transformation involves mutation 

 (29). Investigation of the properties of these cells is continuing. 



REFERENCES 



1. Abercrombie, M. 1970. Contact Inhibition in Tissue Culture. In Vitro. 6: 

 128-142. 



2. Abrahamson, S., and E.B. Lev^s. 1971. The Dectection of Mutagens in 

 Drosophila melanogaster . In Chemical Mutagens: Principles and Methods 

 for tlieir Dectection. Ed., A. Hollaender. Vol. 2. Plenum Press. New York. 

 461-484. 



3. Ames, B.N., et al. 1973. Carcinogens are Mutagens: A Simple Test 

 Combining Liver Homogenates for Activation and Bacteria for Detection. 

 Proc. Nat. Acad. Sci. USA. 70: 2281-2285. 



4. Arlett, C. 1972. Mutation Testing with Cultured Mammalian Cells. Lab. 

 Pract. 21:420-423. 



5. Arlett, C.F., and S.A. Harcourt. 1972. Expression Time and Spontaneous 

 Mutabihty in the Estimation of Induced Mutation Frequency Following 

 Treatment of Chinese Hamster Cells v^th Ultravioldet Light. Mutation 

 Research. 16: 301-306. 



6. Auerbach, C. 1969. Mutagen Specificity. Trans. Kansas Acad. Sci. 72: 

 273-295. 



96 



