Steroid Receptor RNA Activator, a Long Noncoding RNA, Activates p38, Facilitates Epithelial-Mesenchymal Transformation, and Mediates Experimental Melanoma Metastasis

Melanoma metastasis signals dismal prognosis even with current checkpoint inhibitors. Long noncoding RNAs (lncRNAs) regulate dynamic metastasis in several cancers, including melanoma. We became interested in a lncRNA, steroid receptor RNA activator (SRA), because it is the first lncRNA also encoding a conserved protein, SRAP, and it regulates progression of prostate and breast cancers. We investigated how SRA mediates melanoma proliferation, migration, invasion, epithelial-mesenchymal transformation (EMT), and metastasis by RNA interference. The expression of SRAP was measured in melanoma tissue and in human and mouse B16 melanoma cells by immunofluorescence and PCR. The results showed that SRA knockdown decreased B16 cell and A375 cell proliferation and inhibited B16 cell migration significantly. Transwell analysis revealed that CCL21-mediated invasion was abolished in SRA-deficient B16 cells. In parallel, p38 dephosphorylation and reciprocal phosphorylation of B-Raf and mitogen-activated protein kinase kinase 1/2 were present in B16-SRA inhibited cells. The induction of EMT markers, β-catenin and N-cadherin, by CCL21 was reduced in B16-SRA inhibited cells, suggesting that SRA promotes the EMT process. In vivo experimental metastasis showed that B16-SRA inhibited cells formed significantly fewer tumor nodules in the lungs grossly and microscopically. In summary, our results showed that SRA expression is increased in melanoma tissue and that SRA mediates p38 activation, cell invasion, and proliferation and regulates EMT and distant metastasis.