The potent tumor promoter, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), is an effective modulator of DNA synthesis in bovine lymph node lymphocytes in culture. The effect of TPA on cellular proliferation depends to a great extent on the length of exposure and the mitogenic stimulus. Addition of TPA along with mitogenic lectins enhances DNA synthesis. Exposure to TPA for 2 days before addition of lectins depresses DNA synthesis. There is evidence that some inhbitory material other than TPA is formed during the longer incubation. Therefore, in this study, we used [3H]TPA to determine (i) the amount of material retained by the cells after preincubation and (ii) if TPA was metabolized during this culture period. We found that after incubation with 10-7 M, [3H]TPA, less than 3% of the radioactivity was retained by the cells. This was released by incubation in fresh medium. All of the cell associated material appeared to be TPA. However, the TPA in the medium was degraded by about 30% during a 2-day incubation to 12-O-tetradecanoyl-phorbol (TP), phorbol-13-acetate (PA) and phorbol. The source of the hydrolytic activity appeared to be serum because the same effect was seen in the absence of cells but was not seen in the absence of serum. These metabolites when added directly to the lymphocytes had no effect on DNA synthesis. Moreover, the amount of TPA retained by the cells and released into the medium was too small to account for the inhibitory activity of medium from TPA-treated cells. Studies are in progress to determine the nature of the inhibitory material after exposure to TPA.
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