RESUMO
BACKGROUND: Activation of protein kinase A (PKA) occurs during retinoic acid (RA)-induced granulocytic differentiation of human promyelocytic leukemia HL60 cells. It is known that the RIIα regulatory subunit of PKA, is modified by RA (retinoylated) in the early stages of differentiation. We have investigated the effects of RA on PKA during cell differentiation in order to understand the potential significance of this process in the retinoylation of RIIα subunits. METHODS: Immunoblotting, immunoprecipitation, confocal microscopy, PCR, and PKA activity assays were employed for characterizing the effects of RA on PKA. RESULTS: We found that RA induces intracellular mobility of RIIα and the activation of PKA in HL60 cells. Increases in RIIα levels were observed in RA-treated HL60 cells. RA treatment altered intracellular localization of the PKA subunits, RIIα and Cα, and increased their protein levels in the nuclei as detected by both immunoblotting and immunostaining analyses. Coincident with the increase in nuclear Cα, RA-treated HL60 cells showed increases in both the protein phosphorylation activity of PKA and the levels of phosphorylated proteins in nuclear fractions as compared to control cells. In addition, RIIα protein was stabilized in RA-treated HL60 cells as compared to control cells. CONCLUSIONS: These results suggest that RA stabilizes RIIα protein and activates PKA in the nucleus, with a resultant increase in the phosphorylation of nuclear proteins. GENERAL SIGNIFICANCE: Our evidence suggests that retinoylation of PKA might contribute to its stabilization and activation and that this could potentially participate in RA's ability to induce granulocytic differentiation of HL60 cells.
