Lung cancer is the commonest cancer killer worldwide. The appearance of distant metastasis is one of the main reasons for failing to cure patients with this disease. Thus, understanding the mechanisms regulating lung cancer metastasis should highlight novel therapeutic strategies to improve clinical outcome.
We performed an RNA interference screen for cell migration using A549 non-small cell lung cancer (NSCLC) cells. Seventy kinases modulating A549 cell motility were identified, including several members of the ribosomal-S6 kinase (Rsk) family. Indeed, Rsk1 silencing increased, while Rsk2 and Rsk4 downregulation decreased, cell migration. We then assessed the ability of our candidates to regulate A549 cell invasiveness in a 3-D invasion assay. We found that 38 of these similarly regulated cell migration and invasion, including Rsk1 and Rsk4 but not Rsk2. Further work demonstrated that the motility effects of Rsk1, but not Rsk2 or 4, silencing were reproduced in additional NSCLC cell lines. Hence, we focussed on Rsk1 as the principal Rsk isoform regulating NSCLC cell motility. In silico analysis and biochemical experimentation revealed that Rsk1 interacted with the actin regulators Vasp and Mena. This correlated with the ability of Rsk1 to phosphorylate Vasp on Thr-278, a site regulating Vasp-mediated actin dynamics. Furthermore, Vasp and Mena downregulation prevented the migratory effects of Rsk1 silencing. To assess the in vivo relevance of our findings, we developed a zebrafish metastasis model and showed that Rsk1 silencing enhanced the metastatic potential of A549 cells. Moreover, immunohistochemical staining of human isogenically-matched samples demonstrated that Rsk1 expression decreased, while Rsk2 or 4 expression increased, in metastatic versus primary lung cancer lesions. Also, patients with Rsk1-negative primary tumours
showed an increased number of metastases. Taken together, our findings establish that Rsk1 is a metastasis suppressor in NSCLC and may be a biomarker for the progression of this disease
