ERK2-and p90Rsk2-dependent pathways regulate the CCAAT/enhancer-binding protein-β interaction with serum response factor

The serum response element (SRE) of the c-fos promoter is a convergence point for mitogenic signaling pathways. Several transcription factors regulate SRE, including serum response factor (SRF), ternary complex factors, and CCAAT/enhancer-binding protein-beta (C/ EBP beta). C/EBP beta can interact with both SRF and the ternary complex factor family member Elk-1, but only in response to activated Ras. Transactivation of the SRE by C/EBP beta is also greatly stimulated by Ras. The Ras effectors that signal to C/EBP beta are unknown. In this report, we demonstrate that a consensus MAPK site in C/EBP beta is necessary for Ras stimulation of both C/EBP beta -SRF interaction and transactivation of the SRE by C/EBP beta. To dissect signaling pathways activated downstream of Ras, different Ras effector constructs were analyzed. We show that activated forms of Raf and phosphatidylinositol 3-kinase stimulate C/EBP beta -SRF interaction. We also show a novel selectivity for the MAPK family member ERK2, where dominant-negative ERK2, but not dominant-negative ERK1, blocks Ras stimulation of C/EBP beta -SRF interaction. In addition, recombinant C/EBP beta protein is phosphorylated by ERK2, but not by ERK1, in vitro. Finally, we demonstrate a requirement for p90(Rsk2) in regulation of C/EBP beta -SRF interaction. These data show that multiple Ras effectors are required to regulate C/EBP beta and SRF association.