Overexpression of CTNNA1 supports nuclear enrichment of the oncogene YAP in hepatocarcinogenesis

Background Hepatocytes are highly polarized cells and their physical contact with adjacent parenchymal cells is crucial to maintain liver homeostasis. Interestingly, changes of cell-cell contact and junctional structures are frequently observed in human hepatocellular carcinoma (HCC), however, how dysfunctional cellular interaction contributes to tumor formation and progression is poorly understood. In this study, we aim to identify junctional proteins that are dysregulated in liver cancer and that control activity of the oncogene yes-associated protein (YAP).

Methods Transcriptome expression data from human HCC patients (e.g. The Cancer Genome Atlas) was used to screen for dysregulated junctional factors in HCC tissues.1 For the same cohort, genomic gains and losses were analyzed. Candidates were confirmed by immunohistochemistry (tissue microarrays with 744 samples). RNAinterference was used for the genetic knock-down of genes in different liver cancer cells lines (Hep3B, HLF and HepG2) followed by functional analyses (MTT-, BrdU- and apoptosis assay). Protein binding studies were performed using co-immunoprecipitation (co-IP).

Results The adherens junction factor α-catenin (CTNNA1) was overexpressed in HCCs compared to normal/adjacent liver tissues with highest cytoplasmic expression in poorly differentiated tumors (r=0.29, p≤0.01). Elevated CTNNA1 levels statistically correlated with poor clinical outcome (p≤0.05). Interestingly, only cytoplasmic but not membranous CTNNA1 localization significantly associated with nuclear YAP levels. CTNNA1 gene amplification was rarely detected in human HCCs and therefore couldn’t explain its overexpression. CTNNA1 was mainly expressed at the membrane of Hep3B cells, while it predominantly showed cytoplasmic enrichment in HLF and HepG2 cells. CTNNA1 depletion decreased HCC cell viability and proliferation but had no obvious effect on cell apoptosis. CTNNA1 silencing led to reduced YAP protein but not transcript levels in HCC cells. Lastly, co-IP experiments revealed no direct interaction between CTNNA1 and YAP.

Conclusion CTNNA1 overexpression in HCC cells supports cell proliferation probably via indirect induction/stabilization of YAP. Currently, proteomics analysis after CTNNA1 co-IP aims to identify the mechanistic link between CTNNA1 and high nuclear YAP enrichment (BioID technology).

Publication History

Article published online:
04 January 2021

• Reference

• 1 Cancer Genome Atlas Research Network. Comprehensive and Integrative Genomic Characterization of Hepatocellular Carcinoma. Cell. 2017; 169 (07) : 1327-1341 e1323.