Smad4 sequestered in SFPQ condensates prevents TGF-ß tumor-suppressive signaling

Loss of TGF-beta growth-inhibitory responses is a hallmark of human cancer. However, the molecular mechanisms underlying the TGF-beta resistance of cancer cells remain to be fully elucidated. Splicing factor proline- and glutamine-rich protein (SFPQ) is a prion-like RNA-binding protein that is frequently upregulated in human cancers, such as Hepatocellular carcinoma (HCC). In this study, we identified SFPQ as a potent suppressor of TGF-beta signaling. The ability of SFPQ to suppress TGF-beta responses depended on its prion-like domain (PrLD) that drives phase separation (LLPS). Mechanistically, SFPQ physically restrained Smad4 in its condensates, which excluded Smad4 from the Smad complex and chromatin occupancy, and thus functionally dampened Smad-dependent transcriptional responses. Accordingly, SFPQ deficiency or loss of LLPS rendered cells hypersensitive to TGF-beta responses. Together, our data reveal a unique function of SFPQ through LLPS that suppresses Smad transcriptional activation and TGF-beta tumor-suppressive activity. Overall design: mRNA profiles of six HCC cells siControl and siSFPQ cells with or without TGF-beta were generated by deep sequencing, using Illumina HiSeq X Ten. There are 24 samples for transcriptome sequencing (RNA-Seq).