Identification of CCAAT/enhancer-binding proteins as exchange protein activated by cAMP-activated transcription factors that mediate the induction of the SOCS-3 gene

The prototypical second messenger cAMP is a key regulator of immune and inflammatory responses. Its ability to inhibit interleukin (IL)-6 responses is due to induction of suppressor of cytokine signaling-3 (SOCS-3), a negative regulator of IL-6 receptor signaling. We have determined previously that SOCS-3 induction by cAMP occurs independently of cAMP-dependent protein kinase, instead requiring the recently identified cAMP sensor exchange protein activated by cAMP1 (EPAC1). Here we present evidence to suggest that the C/EBP family of transcription factors link EPAC1 activation to SOCS-3 induction. Firstly, selective activation of EPAC in human umbilical vein endothelial cells increased C/EBP DNA binding activity and recruitment of C/EBP beta to the SOCS-3 promoter. Secondly, knockdown of C/EBP beta and -delta isoforms abolished both SOCS-3 induction and inhibition of IL-6 signaling in response to cAMP. Thirdly, overexpression of C/EBP alpha, -beta, or -delta potentiated EPAC-mediated accumulation of SOCS-3. Finally, these effects were not restricted to human umbilical vein endothelial cells, because similar phenomena were observed in murine embryonic fibroblasts in which C/EBP beta or delta had been deleted. In summary, our findings constitute the first description of an EPAC-C/EBP pathway that can control cAMP-mediated changes in gene expression independently of protein kinase A.