Evidence is presented that inhibition of 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced ornithine decarboxylase (ODC; EC 126.96.36.199) by retinoic acid may involve inhibition of protein kinase C-mediated synthesis of ODC mRNA. A single application of 10 nmol of TPA to intact mouse skin led to an increase in the steady state levels of epidermal ODC mRNA; a maximal level of ODC mRNA occurred at about 3.5 h after TPA treatment. TPA-induced increase in ODC mRNA preceded the increase in epidermal ODC activity. Application of 17 nmol of retinoic acid 1 h before application of TPA to mouse skin inhibited the induction of both ODC mRNA and ODC activity. Using the DNA-excess filter hybridization technique, we found that TPA-increased steady state levels of ODC mRNA in primary culture of newborn mouse epidermal cells were the result of enhanced accumulation of newly synthesized ODC mRNA. Furthermore, in a pulse-chase experiment, we could not detect any difference in the half-life of ODC mRNA in epidermal cells after TPA or the vehicle dimethyl sulfoxide treatments; the half-life of ODC mRNA was about 7 h in both cases. Exposure of primary cultures of newborn epidermal cells to retinoic acid, in conjunction with TPA, inhibited the synthesis of ODC mRNA and failed to alter the half-life of ODC mRNA. These results implicate the role of transcription activation in TPA-induced ODC gene expression and indicate that retinoic acid may inhibit TPA-induced ODC gene transcription. We also found that protein kinase C may play a role in the mechanism of inhibition by retinoic acid of ODC gene expression. Supporting evidence is the finding that l-α-dioctanoylglycerol, an activator of protein kinase C, is a Stage II mouse skin tumor promoter and the application of retinoic acid 1 h before application of l-α-dioctanoylglycerol to mouse skin inhibited the induction of ODC activity and ODC mRNA as well as tumor promotion by l-α-dioctanoylglycerol. Taken together, one may conclude that the mechanism of inhibition of TPA-induced ODC by retinoic acid may involve the inhibition of protein kinase C-mediated accumulation of newly synthesized ODC mRNA.
Supported by United States Public Health Service Grant CA-42585 by the National Cancer Institute.