Modulation of the shedding of a rat tumor-associated antigen by growth regulation and anti-cancer drugs.

CE7 antigen is shed from A3 cell surfaces by cells grown in medium containing a sufficient (10%) amount of fetal calf serum (FCS), but shedding of the antigen decreases with a decrease in FCS content in the culture medium. However, the cells contain similar amounts of antigen as evidenced by Western blotting, indicating that low FCS levels interfere with antigen shedding but not antigen synthesis. Antigen expression by A3 cells treated with mitomycin C gradually shifted from negative to a strong positive with time, and on day 2, two peaks corresponding to negative and positive cells within the population can be observed. In contrast, A3 cells treated with bleomycin and cyclophosphamide shifted as a whole from negative to weakly positive. When A3 cells in media containing 10% FCS were incubated at 4 degrees C, although the cells did not proliferate, antigen expression could not be detected by flow cytometry.