Background:
Cholangiocarcinoma (CC) is a highly aggressive tumor of the biliary tract and the
second most frequent primary liver cancer. Intrahepatic cholangiocarcinoma (iCC) is
the least common subtype, representing 10 – 25% of all cases. The significance of
the Hippo pathway effectors Yap (yes-associated protein) and TAZ (WWTR1; WW domain
containing transcription regulator 1) was already demonstrated in hepatocellular carcinoma; however, their relevance in
iCC is not well understood.
Methods:
Transcriptome data derived from 16 tumor samples (iCC) and seven corresponding nontumorous
tissues were analysed for the expression levels of YAP and TAZ as well as markers
of chromosomal instability (CIN). iCC tissue-microarrays (TMA) containing non-tumorous
liver tissues (n = 11), non-tumorous gall bladder tissues (n = 5) and iCC tissues
(n = 310) were stained immunohistochemically against YAP, MCM2 (minichromosome maintenance
complex component 2), p53, pH2AX1 (phospho-histone H2AX) and the proliferation marker
Ki-67. CIN signature genes (n = 16) and cell viability were analysed in iCC cell lines
(HUCCT-1 and Huh-28) after siRNA-mediated silencing of YAP/TAZ.
Results:
Gene expression profiling showed that both YAP and TAZ gene expression was significantly
increased in human iCC tissues in comparison to normal liver tissue. Moreover 22 out
of 25 of CIN25 signature genes were increased significantly in tumor tissues. At the
protein level, 43% of all iCCs showed a moderate to strong nuclear expression of YAP,
while no positivity was detected in nontumorous liver tissue. Equally, MCM2 (69%)
p53 (81%), and the CIN marker pH2AX1 (26%) were clearly expressed in iCC tissue. Importantly,
a significant correlation between the nuclear enrichment of YAP and MCM2 (rS = 0,436;
p < 0,001), p53 (rS = 0,326; p < 0,001) and pH2AX1 (rS = 0.341; p < 0.001) was detectable.
In vitro, YAP/TAZ inhibition diminished cell viability over time. Of note, a significant
reduction of known YAP target genes and CIN signature genes was detectable.
Conclusion:
Dysregulation of the Hippo/YAP/TAZ signaling axis is frequently observed in iCC cells.
In this tumor entity, YAP and TAZ may contribute to cancer initiation through the
induction of chromosomal instability.
