Oncogenic transcriptional co-activators YAP and TAZ regulate the expression of MCM helicase constituents in hepatocellular carcinoma

Overexpression of the Hippo pathway-regulated transcriptional co-activator Yes-Associated Protein (YAP) is frequently detected in human hepatocellular carcinoma (HCC) and correlates with poor clinical outcome of cancer patients [1]. In mice, expression of the constitutively active isoform YAPS127A induces hepatomegaly due to the induction of hepatocellular proliferation followed by tumor formation within 12 weeks. However, how oncogene YAP induces uncontrolled proliferation in liver cells is not known, yet. In addition, the potential synergistic function of TAZ (syn. WWTR1, WW Domain Containing Transcription Regulator 1), another Hippo pathway downstream effector, has not been defined so far.

Analyses of expression profiling data, derived from human liver cancer cells after siRNA-mediated inhibition of endogenous YAP, revealed that the Minichromosome Maintenance Complex Components (MCMs) family members MCM2 – 7 were positively regulated by YAP. MCMs are part of the pre-replication complex and are involved in the formation of the replication fork, which is essential for efficient DNA duplication followed by mitosis [2]. We confirmed that silencing of YAP by independent siRNAs reduced the expression of MCM2 – 7 at the transcript and MCM2 and MCM7 at the protein levels in different liver cancer cell lines. Interestingly, first in vitro data demonstrated that both, YAP and TAZ cooperatively regulate MCM2 – 7 expression in HCC cells after siRNA-mediated silencing. Administration of the YAP/TAZ inhibitor Verteporfin in human cancer cell lines reduced the expression of all MCMs in HCC cells. Vice versa, the inducible overexpression of YAPS127A in transgenic mice showed significantly increased levels of MCM2 – 7 associated with signs of pathological mitosis. In vitro analyses revealed that transcription factors of the TEAD family (TEAD1 and TEAD4) were the most relevant YAP binding partners involved in the regulation of MCMs. Using ChIP analysis, we exemplarily showed that YAP and TEAD4 directly bound the MCM2 promoter. Expression data derived from 242 HCC patients illustrated an association between YAP and MCM mRNA levels, with MCMs significantly correlating with worse overall survival and early cancer recurrence [3]. Furthermore, immunohistochemical stains of tissue micro-arrays revealed a significant correlation between YAP overexpression and nuclear MCM2 enrichment in human HCC tissues.

In summary, our results suggest that the Hippo pathway effectors YAP and TAZ support HCC proliferation through the direct and coordinated induction of MCM family members. Thus, combined inhibition of YAP and TAZ (e.g. by Verteporfin) or perturbation of MCM activity (e.g. Ciprofloxacin) might represent efficient therapeutic approaches for the treatment of HCC patients with YAP/TAZ overexpression.