YAP1 is a key mediator of the Hippo pathway capable of exerting a profound effect on organ size as well as
tumorigenesis. Alternative
mRNA splicing of human YAP1 results in at least 8
protein isoforms that differ within the 2nd WW motif and the transcriptional activation domain. Methods: To investigate the
isoform-specific differences in their
mRNA expression, transcriptional activity and
tumor-promoting function, we cloned
cDNA encoding all of the eight YAP1
protein isoforms. Then, we examined their
mRNA expression, subcellular localization, transcriptional regulation properties, interactions with key regulatory partners, and protein stability in response to changes in cell density, as well as their effects on
pancreatic cancer cell
malignancy both in vitro and in vivo. Results: Multiple YAP1
mRNA isoforms are expressed in commonly used
pancreatic cancer lines as well as human
pancreatic cancer PDX lines. Based on the analysis of heterologous reporter and endogenous target genes, all YAP1
isoforms are capable of activating transcription, albeit to a different extent. Importantly, we unveiled a marked discrepancy between the
mRNA and
protein expression levels of the YAP1-1 and YAP1-2
isoforms. We further discovered that the YAP1-2
isoform, which contains two tandem WW motifs, is less stable at the
protein level, particularly at high cell densities. Mechanistically, we found that the presence of the 2nd WW motif in YAP1-2 facilitates the de novo formation of the YAP1-2/AMOT/LATS1 complex and contributes to a stronger binding of YAP1-2 to LATS1 and subsequently increased YAP1-2 ubiquitination and degradation by β-TRCP. Conclusion: Our data reveals a potent effect of YAP1-1 on
pancreatic cancer malignancy in vitro and in vivo and provides novel mechanistic insight into
isoform-specific and cell density-dependent regulation of YAP1 stability, as well as its impact on
cancer malignancy.