Antioxidant-induced nuclear translocation of CCAAT/enhancer-binding protein beta - A critical role for protein kinase A-mediated phosphorylation of Ser(299)

Alterations in intracellular oxidative status activate several signal transduction pathways resulting in distinct patterns of gene expression. Treatment of colorectal cancer cells with antioxidants can lead to apoptosis by induction of p21 through a mechanism involving CCAAT/enhancer-binding protein beta (C/EBP beta). Herein, we demonstrate that the antioxidant pyrrolidinedithio-carbamate activates cAMP-dependent protein kinase (PKA) in a colorectal cancer cell line DKO-1. Activation of PKA phosphorylates Ser(299) within C/EBP beta, which is essential for protein translocation to the nucleus. Pharmacological inhibition of PKA and mutation of Ser(299) to alanine blocks C/EBP beta nuclear translocation and induction of p21. Our results indicate that a cAMP-dependent phosphorylation of C/EBP beta at Ser(299) is critical for nuclear translocation of this protein and its subsequent transactivation of genes in response to antioxidant treatment.