| c-fos is a member of the family of immediate early genes whose transcription is rapidly induced by mitogenic signaling. The serum response element (SRE) in the c-fos promoter is a convergence point for several signaling pathways that regulate transcription of the c-fos gene. The SRE binds many transcription factors, such as serum response factor (SRF), ternary complex factors (TCFs), and CCAAT/Enhancer binding protein-beta (C/EBPβ). Both Ras- and Rho-dependent signaling pathways have been described to target the SRE. The Ras-dependent pathway activates the TCF family, while the target of the Rho pathway is unknown.; The goal of this study was to further characterize the role of C/EBPβ at the SRE. This was done by studying protein-protein interactions between C/EBPβ and other transcription factors at the SRE. It was shown that C/EBPβ interacts with both SRF and the TCF family member Elk-1 in vivo, but the interaction with each factor was dependent on activation of Ras-dependent signaling pathways. It was further shown that C/EBPβ, SRF, and Elk-1 were all necessary for maximal Ras-stimulated transactivation of the SRE. Due to the effects of Ras on regulation of C/EBPβ, the signaling pathways activated downstream of Ras to target C/EBPβ were also studied. The results showed that Ras was working through the Raf and PI3K pathways, and that ERK2 and p90-Rsk2 were necessary for the Ras-stimulation of C/EBPβ-SRF interaction. There was no role for either ERK1 or the Rho family of GTPases. Furthermore, ERK2, but not ERK1, can phosphorylate C/EBPβ in vitro, further supporting a specific role for ERK2. Therefore, in response to Ras-dependent signals, it is hypothesized that C/EBPβ is phosphorylated by ERK2 and p90-Rsk2, and this phosphorylation promotes the association of C/EBPβ with SRF and Elk-1. The complex of C/EBPβ, Elk-1, and SRF at the c-fos SRE can recruit co-activators and the basal transcriptional factors to create a competent transcriptional complex which results in transcription of the c-fos gene. |
