Stimulation of c-Rel transcriptional activity by PKA catalytic subunit β

Nuclear factor κB (NF-κB) is a eukaryotic transcription factor which responds to different extracellular signals. It is involved in immune response, inflammation, and cell proliferation. Increased expression of c-Rel (or its viral homolog v-Rel), one component of the NF-κB factors, induces tumorigenesis in different systems. The activity of NF-κB can be regulated by protein kinase A (PKA) in a cAMP-independent manner. Our previous results showed that c-MYC induces the activity of PKA by inducing the transcription of the gene encoding the PKA catalytic subunit β (PKA-Cβ). Constitutive expression of PKA-Cβ in Rat1a cells induces their transformation. Here we show that CREB is unlikely to be a phosphorylation target of PKA-Cβ as characterized by different cell lines. Electrophoretic mobility shift assays showed that c-Rel is present as a significant component of the NF-κB factors in c-MYC overexpressing status. The transcriptional activity of c-Rel was significantly stimulated by PKA-Cβ. Coactivators p300/CBP are at least partially responsible for the enhanced activation mediated by c-Rel and PKA-Cβ. Interaction between c-Rel and PKA-Cβ was demonstrated using coimmunoprecipitation assays. Immunoprecipitation-in vitro phosphorylation assays showed the direct phosphorylation of c-Rel by PKA-Cβ. These results indicate that c-Rel is a reasonable phosphorylation target of PKA-Cβ, and that the transcriptional activity of c-Rel is stimulated by PKA-Cβ possibly through the interaction with p300/CBP.