Retinoids antagonize the induction of ornithine decarboxylase activity by phorbol esters and phospholipase C in rat tracheal epithelial cells.

Abstract

In this study we examined the action of phorbol esters, several phospholipases and retinoids on the induction of ornithine decarboxylase (ODC) activity in rat tracheal epithelial cells. 12-O-Tetradecanoylphorbol-13-acetate (TPA) induces ODC activity in these cells in a dose-and time-dependent manner. This induction is inhibited by cycloheximide indicating a requirement for protein synthesis. Tracheal epithelial 2C5 cells contain two binding sites for phorbol esters, one with a high affinity KD,1 = 4.58 nM and one with a low affinity KD,2 = 344.8 nM. The ability of several phorbol esters to induce ODC correlates well with the described efficacy with which they bind to the receptor and is in agreement with the concept that phorbol ester receptors are involved in the induction of ODC. There is strong evidence that the phorbol ester receptor is the protein kinase C for which diacylglycerol is the physiological ligand. Treatment of cells with phospholipase C generates diacylglycerol and induces ODC activity in a dose- and time-dependent manner. Treatment with phospholipase A2 or D has no effect on ODC activity. These results support the concept that activation of protein kinase C is related to the induction of ODC activity. The induction of ODC by TPA as well as by phospholipase C is inhibited by retinoids. Specific cytosolic binding proteins for retinoids might be involved in at least some of the responses to these compounds. To examine whether the binding proteins are involved in the inhibition of ODC we determined the presence of these binding proteins and the structure-activity relationship of retinoids. Both retinol and retinoic acid-binding proteins can be detected in 2C5 cells, their levels are 1.06 and 3.36 pmoles/mg protein, respectively. The ability of several retinoids to inhibit ODC induction correlates well with their binding activity and support a role for these binding proteins in the action of retinoids on ODC induction.