The lncRNA LETS1 promotes TGF--induced EMT and cancer cell migration by transcriptionally activating a TR1-stabilizing mechanism

Transforming growth factor-beta (TGF-beta) signaling is a critical driver of epithelial-to-mesenchymal transition (EMT) and cancer progression. In SMAD-dependent TGF-beta signaling, activation of the TGF-beta receptor complex stimulates the phosphorylation of the intracellular receptor-associated SMADs (SMAD2 and SMAD3), which translocate to the nucleus to promote target gene expression. SMAD7 inhibits signaling through the pathway by promoting the polyubiquitination of the TGF-beta type I receptor (T beta RI). We identified an unannotated nuclear long noncoding RNA (lncRNA) that we designated LETS1 (lncRNA enforcing TGF-beta signaling 1) that was not only increased but also perpetuated by TGF-beta signaling. Loss of LETS1 attenuated TGF-beta-induced EMT and migration in breast and lung cancer cells in vitro and extravasation of the cells in a zebrafish xenograft model. LETS1 potentiated TGF-beta-SMAD signaling by stabilizing cell surface T beta RI, thereby forming a positive feedback loop. Specifically, LETS1 inhibited T beta RI polyubiquitination by binding to nuclear factor of activated T cells (NFAT5) and inducing the expression of the gene encoding the orphan nuclear receptor 4A1 (NR4A1), a component of a destruction complex for SMAD7. Overall, our findings characterize LETS1 as an EMT-promoting lncRNA that potentiates signaling through TGF-beta receptor complexes.