We have previously found that retinoic acid stimulates the expression of protein kinase Cα (PKC) in B16 mouse melanoma cells. Because it has been reported that PKC can phosphorylate retinoic acid receptor (RAR) and alter its function, we determined whether changes in the level and/or activity of PKC could affect the expression or function of the RAR in B16 melanoma. Using in vivophosphorylation and band shift techniques, we could not demonstrate that altering PKC activity and/or protein level changed thein vivo phosphorylation of RARα. However activation of PKC resulted in increased RARα protein. Increased receptor protein correlated with a phorbol dibutyrate-stimulated increase in receptor activation function-2 (AF-2)-dependent transcriptional activity. Use of enzyme inhibitors and dominant-negative PKCs indicated that enzyme activity was required for elevation in the RARα. The PKC-mediated increase in RARα was due to a 2.5-fold increase in the half-life of this protein. In contrast, the down-regulation of PKC diminished RARα protein half-life and markedly inhibited AF-2-dependent transcriptional activity. The down-regulation of PKC also inhibited the binding of RAR to a retinoic acid response element and the retinoic acid induction of RARβ expression. These findings suggest that PKC can influence retinoic acid signaling by altering the stability of RAR protein without directly phosphorylating this receptor.
Boskovic G., Desai D., Niles R. M. (2002). Regulation of retinoic acid receptor α by protein kinase C in B16 mouse melanoma cells. J. Biol. Chem. 277: 26113-26119.