Abstract 3154: RSK2-mediated phosphorylation of stathmin promotes microtubule polymerization, providing a pro-invasive advantage to metastatic cancer cells
Authors: Alesi Gina , Li Dan , Jin Lingtao , Chen Georgia Z. , Shin Dong M. , Khuri Fadlo , Kang Sumin 
Published: May 2015

Abstract
Metastasis is responsible for over 90% of cancer related deaths. Breast cancer, lung cancer, and head and neck cancer commonly metastasize to vital organs resulting in poor clinical outcomes. However, the molecular processes underlying such cellular changes remain elusive. Therefore, identifying genes that induce cancer cell signaling changes and understanding how these changes control pro-metastatic mechanisms is necessary to develop molecular therapies for metastasis and to improve clinical prognosis for late-stage cancer patients. Protein kinases have been implicated in mediating pro-metastatic signaling in human cancers. We previously demonstrated that a serine/threonine kinase, p90 ribosomal S6 kinase 2 (RSK2) promotes cancer cell invasion and migration, anoikis resistance, and tumor metastasis in human cancers, including head and neck cancer, breast cancer, and lung cancer.
Here, we investigated the role of RSK2 in the regulation of microtubule dynamics and its potential implication in cancer cell invasion and tumor metastasis. SiRNA-mediated knockdown of RSK2 disrupted microtubules in diverse metastatic human cancer cell lines including lung cancer A549, breast cancer SKBR3, and head and neck cancer 212LN cells. Stathmin (STMN) is a microtubule destabilizing protein, which is an essential regulator of microtubule polymerization. We found that stable knockdown of RSK2 resulted in decreased phosphorylation of STMN at Serine 16 in the aforementioned metastatic cancer cell lines. Moreover, we found that RSK2 directly phosphorylated STMN at Ser16 in an in vitro RSK2 kinase assay using recombinant RSK2 and STMN proteins. We observed that phosphorylation of STMN by RSK2 resulted in reduced STMN-mediated microtubule depolymerization in vitro, while stable expression of phospho-deficient STMN S16A mutant but not STMN wild type (WT) led to decreased invasive potential of metastatic cancer cells. These data together suggest STMN as a potential novel RSK2 substrate/effector, which may contribute to RSK2-mediated pro-metastatic signaling.
Our finding demonstrates that RSK2 signals through STMN to regulate microtubule polymerization and cancer cell invasion via direct phosphorylation of STMN at Serine 16, providing a pro-invasive and metastatic advantage to cancer cells. Therefore, the RSK2-STMN pathway represents a promising therapeutic target in the clinical prognosis and treatment of metastatic human cancers.
Citation Format: Gina Alesi, Dan Li, Lingtao Jin, Georgia Z. Chen, Dong M. Shin, Fadlo Khuri, Sumin Kang. RSK2-mediated phosphorylation of stathmin promotes microtubule polymerization, providing a pro-invasive advantage to metastatic cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3154. doi:10.1158/1538-7445.AM2014-3154