The oncogenes yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are potent liver oncogenes. Because gene mutations cannot fully explain their nuclear enrichment, we aim to understand which mechanisms cause YAP/TAZ activation in liver cancer cells. The combination of proteomics and functional screening identified numerous apical cell polarity complex proteins interacting with YAP and TAZ. Co-immunoprecipitation (Co-IP) experiments confirmed that membrane protein palmitoylated 5 (MPP5; synonym: PALS1) physically interacts with YAP and TAZ. After removing different MPP5 protein domains, Co-IP analyses revealed that the PDZ domain plays a crucial role in YAP binding. The interaction between YAP and MPP5 in the cytoplasm of cancer cells was demonstrated by proximity ligation assays (PLAs). In human hepatocellular carcinoma (HCC) tissues, a reduction in apical MPP5 expression was observed, correlating with the nuclear accumulation of YAP and TAZ. Expression data analysis illustrated that MPP5 is inversely associated with YAP/TAZ target gene signatures in human HCCs. Low MPP5 levels define an HCC patient group with a poor clinical outcome. In summary, MPP5 facilitates the nuclear exclusion of YAP and TAZ in liver cancer. This qualifies MPP5 as a potential tumor-suppressor gene and explains how changes in cell polarity can foster tumorigenesis.

Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are key regulators of hepatic stellate cell (HSC) activation and liver fibrosis. While the canonical YAP/TAZ-TEAD axis is well-characterized, non-canonical interaction partners and downstream long non-coding RNAs (lncRNAs) that orchestrate fibrogenic programs in HSCs remain undefined. Using BioID and next-generation sequencing (NGS) in LX-2 cells, we identified YAP/TAZ-interactomes and YAP/TAZ-regulated lncRNAs. BioID results and proximity ligation assay (PLA) identified the transcription factor Kaiso as a TAZ interaction partner. Functionally, Kaiso knockdown suppressed key aspects of HSC activation in LX-2 cells. We next integrated our NGS results from YAP/TAZ-knockdown in LX-2 cells with publicly available Kaiso ChIP-seq data. This identified HIF1A-AS3 as a common downstream lncRNA. ChIP-qPCR confirmed direct binding of Kaiso to the HIF1A-AS3 promoter. Furthermore, HIF1A-AS3 silencing mirrored the functional effects of Kaiso knockdown. Global proteomic profiling following knockdown of TAZ, Kaiso, or HIF1A-AS3 revealed a network of commonly regulated proteins enriched for pathways central to HSC activation. In conclusion, we establish Kaiso as a novel TAZ interaction partner and identify HIF1A-AS3 as a shared downstream target of TAZ and Kaiso, suggesting a potential profibrotic regulatory network that promotes HSC activation and may represent a therapeutic target for liver fibrosis.