RNA demethylase ALKBH5 inhibits TGF-β-induced EMT by regulating TGF-β/SMAD signaling in non-small cell lung cancer

AlkB homolog 5 (ALKBH5) has been revealed as a key RNA N⁶ -methyladenosine (m⁶ A) demethylase that is implicated in development and diseases. However, the function of ALKBH5 in TGF-β-induced epithelial-mesenchymal transition (EMT) and tumor metastasis of non-small-cell lung cancer (NSCLC) remains unknown. Here, we firstly show that ALKBH5 expression is significantly reduced in metastatic NSCLC. ALKBH5 overexpression inhibits TGF-β-induced EMT and invasion of NSCLC cells, whereas ALKBH5 knockdown promotes the corresponding phenotypes. ALKBH5 overexpression suppresses TGF-β-stimulated NSCLC cell metastasis in vivo. ALKBH5 overexpression decreases the expression and mRNA stability of TGFβR2 and SMAD3 but increases those of SMAD6, while ALKBH5 knockdown causes the opposite results. Importantly, ALKBH5 overexpression or knockdown leads respectively to an attenuated or augmented phosphorylation of SMAD3, an indispensable downstream effector that activates TGF-β/SMAD signaling. Moreover, m⁶ A-binding proteins YTHDF1/3 promotes TGFβR2 and SMAD3 expression, and YTHDF2 inhibits SMAD6 expression. YTHDF1/2/3 facilitates TGF-β-stimulated EMT and invasion of NSCLC cells. Mechanistically, ALKBH5 affects TGFβR2, SMAD3 and SMAD6 expression and mRNA stability by erasing m⁶ A modification in NSCLC cells. ALKBH5 weakens YTHDF1/3-mediated TGFβR2 and SMAD3 mRNA stabilization, and abolishes YTHDF2-mediated SMAD6 mRNA degradation, supporting the notion that ALKBH5 inhibits TGF-β-induced EMT and invasion of NSCLC cells via YTHD1/2/3-mediated mechanism. Taken together, our findings highlight an important role of ALKBH5 in regulating TGF-β/SMAD signaling, and establish a mechanistic interaction of ALKBH5 with TGFβR2/SMAD3/SMAD6 for controlling TGF-β-induced EMT in NSCLCs.

Quaking 5 suppresses TGF-β-induced EMT and cell invasion in lung adenocarcinoma

Quaking (QKI) proteins belong to the signal transduction and activation of RNA (STAR) family of RNA-binding proteins that have multiple functions in RNA biology. Here, we show that QKI-5 is dramatically decreased in metastatic lung adenocarcinoma (LUAD). QKI-5 overexpression inhibits TGF-β-induced epithelial-mesenchymal transition (EMT) and invasion, whereas QKI-5 knockdown has the opposite effect. QKI-5 overexpression and silencing suppresses and promotes TGF-β-stimulated metastasis in vivo, respectively. QKI-5 inhibits TGF-β-induced EMT and invasion in a TGFβR1-dependent manner. KLF6 knockdown increases TGFβR1 expression and promotes TGF-β-induced EMT, which is partly abrogated by QKI-5 overexpression. Mechanistically, QKI-5 directly interacts with the TGFβR1 3' UTR and causes post-transcriptional degradation of TGFβR1 mRNA, thereby inhibiting TGF-β-induced SMAD3 phosphorylation and TGF-β/SMAD signaling. QKI-5 is positively regulated by KLF6 at the transcriptional level. In LUAD tissues, KLF6 is lowly expressed and positively correlated with QKI-5 expression, while TGFβR1 expression is up-regulated and inversely correlated with QKI-5 expression. We reveal a novel mechanism by which KLF6 transcriptionally regulates QKI-5 and suggest that targeting the KLF6/QKI-5/TGFβR1 axis is a promising targeting strategy for metastatic LUAD.