Loss of cell polarity defines a cancer-prone phenotype in liver epithelial cells

Background/Aims:

Loss of cell polarity has been shown to play an essential role in the cause and progression of carcinogenesis in cells of epithelial origin. Three polarity protein systems (PPS) are involved in the acquisition and maintenance of cell polarity: the apical Crumbs complex(Crumbs, Pals1,PatJ), the subapical Par complex(Par6, Par3,aPKC), and the basolateral Scribble complex(Lgl1/2, Dlg1, Scribble). It is now well accepted that the Hippo pathway and its transcriptional co-activators yes-associated protein (YAP)/TAZ (WWTR1) are well connected to PPS via partly unknown mechanisms. In this project we aim to define the crosstalk between PPS complexes formation and activity of the Hippo/YAP pathway.

Methods:

We compared immunofluorescence stains of cell junction markers such as tight junction (ZO1) and adherens junction (E-cadherin) in primary hepatocytes and HCC cells with a high (HepG2) and low (HLE, HLF) degree of cell polarity in 2D monolayer cultures and 3D collagen sandwich cultures. The expression and localization of characteristic PPS markers (Scribble, CRB3) were compared. We also analyzed the activity (phosphorylation and localization) of Hippo pathway constituents (YAP/TAZ) and compared their expression with PPS deregulation and mislocalization. Moreover, we analyzed the expression of selected factors (e.g., scribble) in primary human HCC samples.

Results:

Polarized HCC cells show a clear membrane localization of the well-known markers for polarity (ZO1, E-Cadherin) and also Scribble and CRB3, whereas in non-polarized HCC cells, these markers are mislocalized in the cytoplasm. In addition, a significant nuclear localization of YAP and TAZ was detectable in non-polarized cells, whereas the nuclear enrichment of YAP/TAZ is lower in polarized HCC cells. YAP nuclear enrichment can be inhibited by cell-cell contact in non-malignantly transformed epithelial cells but not in HCC cells. Surprisingly, high expression of Scribble significantly correlates with poor survival of HCC patients. In contrast, disruption of polarity in polarized HCC cells after silencing of Scribble using gene-specific siRNAs stimulated a significant nuclear translocation of YAP, dephosphorylation of AKT, and induction of a mesenchymal phenotype.

Conclusion/Outlook:

We demonstrate that the loss of cell polarity correlates with activation of the transcriptional co-activators of the Hippo pathway in HCC cells. First data indicate that the basolateral PPS protein Scribble is an important regulator for YAP/TAZ localization and that the loss of Scribble drives oncogenic YAP/TAZ activity. Because Scribble is highly expressed in human HCCs, we hypothesize that differential subcellular localization of Scribble may affect different signaling pathways: while membranous Scribble may bind and inhibit YAP, cytoplasmic Scribble might promote the PI3K/AKT pathway.

Corresponding author: Wan, Shan

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