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RSK2 phosphorylates T-bet to attenuate colon cancer metastasis and growth

  1. Zigang Donga,2
  1. aThe Hormel Institute, University of Minnesota, Austin, MN 55912;
  2. bChina-US (Henan) Hormel Cancer Institute, Zhengzhou 450008, China;
  3. cDepartment of Dermatology, Xiangya Hospital, Central South University, Changsha 410011, China;
  4. dSchool of Basic Medical Sciences, Zhengzhou University, Zhengzhou 450001, China
  1. Edited by Peter K. Vogt, The Scripps Research Institute, La Jolla, CA, and approved October 20, 2017 (received for review June 14, 2017)

Significance

Many patients with colorectal cancer die because of metastases in distant organs such as the liver and lungs, rather than from the primary tumor. A better molecular understanding of colorectal cancer has allowed for improved patient prognosis and the launching of precision medicine for treating metastatic colorectal cancer. Here we demonstrate that a deficiency of ribosomal S6 kinase 2 (RSK2) can result in dramatically decreased IFNγ secretion through an inappropriate phosphorylation status of T-bet, a modulator of IFNγ expression. Decreased IFNγ levels can lead to immune suppression, accelerating colon cancer-mediated liver and lung metastasis. We found that RSK2-mediated phosphorylation of T-bet at serines 498 and 502 is required for the inhibition of colon cancer metastasis and growth, through a positive regulation of RSK2/T-bet/IFNγ signaling.

Abstract

Metastasis is a major cause of cancer-related deaths. Approximately 80% of patients with colorectal cancer develop liver metastasis and 20% develop lung metastasis. We found that at different stages of colon cancer, IFNγ secretion from peripheral blood mononuclear cells was decreased compared with healthy controls. The ribosomal S6 kinase (RSK) family of kinases has multiple cellular functions, and we examined their roles in this observed IFNγ decrease. Flow cytometry analysis of wild-type (WT) and RSK2 knockout (KO) mice revealed significantly lower levels of IFNγ in the RSK2 KO mice compared with the WT mice. Since IFNγ is a component of immunity, which contributes to protection against metastatic carcinomas, we conducted a colon cancer liver metastasis experiment. We found significantly greater metastasis in RSK2 KO mice compared with WT mice. Transcription factor T-bet can directly activate Ifnγ gene transcription. In vitro kinase assay results showed that RSK2 phosphorylated T-bet at serines 498 and 502. We show that phosphorylation of T-bet by RSK2 is required for IFNγ expression, because knockdown of RSK2 expression or overexpression of mutant T-bet reduces IFNγ mRNA expression. To verify the function of the phosphorylation sites, we overexpressed a constitutively active mutant T-bet (S498E/S502E) in bone marrow. Mutant T-bet restored the IFNγ mRNA levels and dramatically reduced the metastasis rate in these mice. Overall, these results indicate that phosphorylation of T-bet is required for the inhibition of colon cancer metastasis and growth through a positive regulation of RSK2/T-bet/IFNγ signaling.

Footnotes

  • ?1K.Y., C.P., Yuwen Zhang, and T.A.Z. contributed equally to this work.

  • ?2To whom correspondence may be addressed. Email: zgdong{at}hi.umn.edu or b.li{at}louisville.edu.
  • Author contributions: K.Y., C.P., Ziming Dong, B.L., and Zigang Dong designed research; K.Y., C.P., Yuwen Zhang, T.A.Z., M.-H.L., S.-Y.L., E.R., H.C., J.R., L.W., Yi Zhang, G.G., W.H., W.-Y.M., and K.L. performed research; K.Y., C.P., T.A.Z., and H.C. analyzed data; and K.Y., A.M.B., and Zigang Dong wrote the paper.

  • The authors declare no conflict of interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at www.danielhellerman.com/lookup/suppl/doi:10.1073/pnas.1710756114/-/DCSupplemental.

Published under the PNAS license.

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