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-β (C/EBPβ). C/EBPβ 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/EBPβ is also greatly stimulated by Ras. The Ras effectors that signal to C/EBPβ are unknown. In this report, we demonstrate that a consensus MAPK site in C/EBPβ is necessary for Ras stimulation of both C/EBPβ-SRF interaction and transactivation of the SRE by C/EBPβ. 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/EBPβ-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/EBPβ-SRF interaction. In addition, recombinant C/EBPβ protein is phosphorylated by ERK2, but not by ERK1, in vitro. Finally, we demonstrate a requirement for p90^Rsk2 in regulation of C/EBPβ-SRF interaction. These data show that multiple Ras effectors are required to regulate C/EBPβ and SRF association.