THE DIFFERENCE IN CONTACT INHIBITION OF CELL REPLICATION BETWEEN NORMAL CELLS AND CELLS TRANSFORMED BY DIFFERENT CARCINOGENS

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Abstract
The difference in contact inhibition of cell replication between normal cells and cells transformed by different carcinogens was measured by the ability of a sparse layer of cell marked with india ink to replicate on a confluent layer of unmarked cells. Hamster cells transformed by polyoma virus (PV) were not inhibited by other hamster or rat cells transformed by PV. Mouse cells transformed by Simian Virus 40 (SV 40) were not inhibited by other mouse or hamster cells transformed by SV40. Cells transformed by PV were able to inhibit and be inhibited by cells transformed by SV40. The specificity of the cell type induced by the transforming virus can thus be recognized by contact inhibition between transformed cells. Cells transformed by X-irradiation or carcinogenic hydrocarbons did not inhibit transformed cells from the same cell line. Two cell lines independently transformed by X-irradiation inhibited one another; a line transformed by benzo(a)pyrene inhibited a line transformed by 3-methylcholanthrene; and cells doubly transformed by X-irradiation and PV were inhibited by cells of both parental types. X-irradiation with 4000 R of cells transformed by viral or nonviral carcinogens was able to induce contact inhibition between the irradiated and un-irradiated transformed cells from the same cell line. Normal cells were able to inhibit and be inhibited by transformed cells and by normal cells. The comparison of normal and transformed cells has indicated that transformation results in a change in regulatory mechanism, in that normal cells can be contact-inhibited by the same and by a different cell type, whereas transformed cells can be inhibited only by a different cell type.