RSK2-Mediated ELK3 Activation Enhances Cell Transformation and Breast Cancer Cell Growth by Regulation of c-fos Promoter Activity.

Sun-Mi Yoo,Han Chang Kang,Yong-Yeon Cho,Hyun-Jung An,Joo Young Lee,Juhee Park,Cheol-Jung Lee,Hye Suk Lee,Sung-Jun Cho,Seung-Min Kim,Dae Joon Kim

doi:10.3390/ijms20081994

Abstract

Ribosomal S6 kinase 2 (RSK2), regulated by Ras/Raf/MEKs/ERKs, transmits upstream activation signals to downstream substrates including kinases and transcription and epigenetic factors. We observed that ELK members, including ELK1, 3, and 4, highly interacted with RSK2. We further observed that the RSK2-ELK3 interaction was mediated by N-terminal kinase and linker domains of RSK2, and the D and C domains of ELK3, resulting in the phosphorylation of ELK3. Importantly, RSK2-mediated ELK3 enhanced c-fos promoter activity. Notably, chemical inhibition of RSK2 signaling using kaempferol (a RSK2 inhibitor) or U0126 (a selective MEK inhibitor) suppressed EGF-induced c-fos promoter activity. Moreover, functional deletion of RSK2 by knockdown or knockout showed that RSK2 deficiency suppressed EGF-induced c-fos promoter activity, resulting in inhibition of AP-1 transactivation activity and Ras-mediated foci formation in NIH3T3 cells. Immunocytofluorescence assay demonstrated that RSK2 deficiency reduced ELK3 localization in the nucleus. In MDA-MB-231 breast cancer cells, knockdown of RSK2 or ELK3 suppressed cell proliferation with accumulation at the G1 cell cycle phase, resulting in inhibition of foci formation and anchorage-independent cancer colony growth in soft agar. Taken together, these results indicate that a novel RSK2/ELK3 signaling axis, by enhancing c-Fos-mediated AP-1 transactivation activity, has an essential role in cancer cell proliferation and colony growth.

Highlights

The E26 transformation-specific (ETS) proteins form a large family of transcription factors that regulate various functions in cells [1] through the formation of ternary complex factors (TCFs) with serum response factor and serum response elements [2,3]
The results in our previous study demonstrated that ribosomal S6 kinases (RSKs), including ribosomal S6 kinase 2 (RSK2), are located downstream of extracellular signal-regulated kinases (ERK) in the mitogen-activated protein kinase (MAPK) signaling pathway, and that ERK and RSK are spontaneously bound in the cytoplasm [25]
The results indicated that the strong binding of Ribosomal S6 kinase 2 (RSK2)-FL and RSK2-D1 with ELK3 was reduced in RSK2-D2 (Figure 2a), indicating that the N-terminal kinase domain (NTKD) of RSK2 might be involved in the interaction of RSK2 with ELK3

Summary

Introduction

The E26 transformation-specific (ETS) proteins form a large family of transcription factors that regulate various functions in cells [1] through the formation of ternary complex factors (TCFs) with serum response factor and serum response elements [2,3]. ELK3 was initially identified as a transcriptional repressor of c-fos gene expression [5], it has a role as a transactivator when it has been phosphorylated by the Ras-mediated mitogen-activated protein kinase (MAPK) signaling pathway [6]. Transcription factors are eventually activated, resulting in the regulation of various cellular behaviors including cell proliferation, transformation, migration, and death [10]

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Results

Discussion

Conclusion