RSK-Mediated Phosphorylation in the C/EBPβ Leucine Zipper Regulates DNA Binding, Dimerization, and Growth Arrest Activity

Abstract

The bZIP transcription factor C/EBPβ is a target of Ras signaling that has been implicated in Ras-induced transformation and oncogene-induced senescence (OIS). To gain insights into Ras-C/EBPβ signaling, we investigated C/EBPβ activation by oncogenic Ras. We show that C/EBPβ DNA binding is autorepressed and becomes activated by the Ras-Raf-MEK-ERK-p90^RSK cascade. Inducible phosphorylation by RSK on Ser273 in the leucine zipper was required for DNA binding. In addition, three other modifications (phosphorylation on Tyr109 [p-Tyr109], p-Ser111, and monomethylation of Arg114 [me-Arg114]) within an N-terminal autoinhibitory domain were important for Ras-induced C/EBPβ activation and cytostatic activity. Apart from its role in DNA binding, Ser273 phosphorylation also creates an interhelical g↔e′ salt bridge with Lys268 that increases attractive electrostatic interactions between paired leucine zippers and promotes homodimerization. Mutating Ser273 to Ala or Lys268 to Glu decreased C/EBPβ homodimer formation, whereas heterodimerization with C/EBPγ was relatively unaffected. The S273A substitution also reduced the antiproliferative activity of C/EBPβ in Ras^V12-expressing fibroblasts and decreased binding to target cell cycle genes, while a phosphomimetic substitution (S273D) maintained growth arrest function. Our findings identify four novel C/EBPβ-activating modifications, including RSK-mediated phosphorylation of a bifunctional residue in the leucine zipper that regulates DNA binding and homodimerization and thereby promotes cell cycle arrest.

We thank the individuals mentioned in the text for generously providing plasmids and antibodies, Suzanne Specht for assistance with phosphopeptide mapping, Barb Shankle and Nancy Martin for genotyping and preparation of MEFs, Angie Hackley for animal handling, and Jiro Wada and Allan Kane for preparing figures.

We dedicate this paper to the memory of Barb Shankle.

This research was supported by the Intramural Research Program of the NIH, National Cancer Institute, Center for Cancer Research, and contract N01-CO-12400.

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