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. Lewis. 1971. The Dectection of Mutagens in 
Drosophila melanogaster. In Chemical Mutagens: Principles and Methods 
for their 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 
Mutability in the Estimation of Induced Mutation Frequency Following 
Treatment of Chinese Hamster Cells with Ultravioldet Light. Mutation 
Research. 16: 301-306. 
6. Auerbach, C. 1969. Mutagen Specificity. Trans. Kansas Acad Sci 72' 
273-295. 
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