Retinoids Inhibit the Oxidative Modification of Protein Kinase C Induced by Oxidant Tumor Promoters

Recently we reported that oxidant tumor promoters can induce the oxidative modification of protein kinase C (PKC) resulting in either activation or inactivation of the kinase (R. Gopalakrishna and W. B. Anderson, Arch. Biochem. Biophys. 285, 382-387, 1991). Since retinoids previously have been shown to antagonize the actions of tumor promoters, studies were carried out to determine if retinoids can inhibit the oxidative modification of PKC induced by tumor promoters. Prior treatment of BiG melanoma cells or C6 glioma cells with all-trans-retinoic acid (0.1 μM) for a short time period (15 to 60 min) followed by subsequent treatment with oxidants such as hydrogen peroxide resulted in a 30 to 70% decrease in the oxidative modification of PKC. This resulted in a decrease in oxidant-induced conversion of PKC from a Ca2+/lipid-dependent form (peak A) to a Ca2+/lipid-independent form (peak B). This retinoid-mediated protection also was observed with the reversible oxidative modification of PKC induced by m-periodate treatment of intact cells. To understand whether this protection offered by retinoids was caused by a direct influence of retinoids on PKC, experiments were carried out using the purified enzyme. The results of experiments using isolated PKC suggested that retinoids can act directly to protect the regulatory domain of PKC from oxidative modification induced by oxidants. However, high (1-10 μM) concentrations of retinoids are necessary to elicit this protection of isolated PKC. In contrast, in experiments with intact cells, only low (submicromolar) concentrations of retinoids are required to protect PKC from oxidation. The differences noted in the retinoid concentrations required to protect PKC from oxidant modification in the test tube versus in the intact cell may be due to increased retention of retinoids in the cell membrane by partitioning, or to other indirect actions of retinoids in the intact cells to decrease cellular oxidations. These results suggest that some of the anti-tumor promoter actions of retinoids may be mediated, in part, by inhibiting the oxidative modification of protein kinase C induced by oxidant tumor promoters.