RASSF1A Represses TGFβ Oncogenic Signaling via YAP1/SMAD2 Translocation

Loss of the tumor suppressor RAS association domain family member 1 (RASSF1A), a component of several key cancer signaling pathways, has been shown to be caused by epigenetic inactivation or germline mutations and is correlated with the pathogenesis of solid cancers and poor patient prognosis. To identify signaling pathways that regulate RASSF1A at the protein level, Pefani and colleagues performed an integrated analysis of RNA sequencing and genomic methylation data from TCGA cohorts of patients with lung and breast cancers. Gene set enrichment analysis showed that tumors with decreased levels of RASSF1A expression and increased RASSF1 methylation exhibit lower TGFβ signaling in both tumor types, suggesting reduced TGFβ-mediated tumor suppression. TGFβ treatment reduced endogenous RASSF1A in lung cancer cell lines, and cotreatment with TGFβ and a proteasome inhibitor or a TGFβRI kinase inhibitor resulted in RASSF1A stabilization. Further, TGFβ treatment induced the interaction of RASSF1A with TGFβRI, and coimmunoprecipitation analyses showed that the E3 ubiquitin ligase ITCH, which has previously been shown to interact with TGFβRI and promote SMAD2 association with TGFβRI, binds to and ubiquitinates RASSF1A to drive RASSF1A degradation. Overexpression of RASSF1A reduced TGFβ-induced yes-associated protein 1 (YAP1)/SMAD2 binding and nuclear translocation of SMAD2, which resulted in the decreased TGFβ-dependent transcription of SMAD2 target genes, and increased the binding of YAP1 to the tumor suppressor p73. Consistent with these findings, ablation of RASSF1A increased TGFβ-induced YAP1/SMAD2 association, nuclear SMAD2, and expression of SMAD2 transcriptional targets. Similarly, induction of RASSF1A expression or depletion of ITCH significantly inhibited TGFβ-mediated invasion in vitro. Taken together, these results provide new insights into the tumor suppressor function of RASSF1A by elucidating the Hippo-dependent mechanism by which RASSF1A inhibits TGFβ-mediated oncogenic transcription, and demonstrate how epigenetic modifications can mediate a switch between tumor suppressive and protumorigenic signaling.

Pefani DE, Pankova D, Abraham AG, Grawenda AM, Vlahov N, Scrace S, et al. TGF-β targets the Hippo pathway scaffold RASSF1A to facilitate YAP/SMAD2 nuclear translocation. Mol Cell 2016 Jun 9 [Epub ahead of print].