The cell polarity protein MPP5/PALS1 controls the subcellular localiza-tion of the oncogenes YAP and TAZ in hepatocellular carcinoma

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 both proteins. Co-immunoprecipitation (Co-IP) ex-periments confirmed that 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 of apical MPP5 expression was observed, positively correlating with the nuclear accumulation of YAP and TAZ. Expression data analysis illustrated that MPP5 is in-versely associated with YAP/TAZ target gene signatures in human HCCs. Low MPP5 levels de-fine an HCC patient group with poor clinical outcome. In summary, MPP5 facilitates the nuclear exclusion of YAP and TAZ in HCC cells. This qualifies MPP5 as a tumor-suppressor gene and ex-plains how changes in cell polarity can foster tumorigenesis. . MPP5 was inhibited by gene-specific siRNAs for 24 hours. Two out of six siRNAs were used, which caused the strongest inhibition effects (MMP5_3 and MPP5_5). Untreated cells (UTC) and nonsense-siRNA treated cells (NTC) were used as controls. Each biological sample consits or 3 replicates.