Dissecting the Hippo signalling pathway in HPV-driven cervical cancer

Human papillomaviruses (HPVs) are a major cause of malignancy worldwide. HPV hijacks host signalling pathways to promote proliferation and cell survival, ultimately contributing to carcinogenesis. The Hippo pathway has been identified as a key signalling pathway in HPV+ cancers, with the downstream effector of this pathway YAP known to play a critical pro-oncogenic role. Recent work in the Macdonald group has confirmed literature and highlighted how the dysregulation of signalling pathways is important in cervical cancer tumourigenesis and found in particular that the Hippo signalling pathway is pivotal in transformation. Preliminary data found STK4, the key kinase of the Hippo signalling pathway to be lost in cervical cancer but the mechanism of how this occurred remained elusive.
Analysis of the expression level of critical components of the Hippo pathway in a panel of cervical cancer cell lines revealed that, in contrast to YAP, which was upregulated in all HPV+ cell lines on a protein level, TAZ expression was only significantly increased in HPV18+ cell lines. Significantly, we found that TAZ upregulation occurred at the transcript level. Further, TAZ mRNA was significantly upregulated in HPV18+, but not HPV16+, cervical disease clinical samples, increasing with disease severity. Subsequent investigation found TAZ mRNA was increased due to increased TAZ promoter activity via a HPV18 E7/EGFR/ERK(1/2)/SP1 signalling axis. Crucially, the overexpression of YAP was not able to rescue the proliferation defect in the TAZ knockdown cells, suggesting that YAP and TAZ have distinct targets, essential for growth in HPV cancer cell lines. RNA-seq confirmed this, revealing that TAZ and YAP have distinct target profiles, including a number of genes that have not previously been linked with cervical cancer. Thus, in HPV18+ cancers, both YAP and TAZ play non-redundant roles in regulating transformation. We chose three potential genes for further study, TOGARAM2, SSTR5 and IFIT2, which we confirmed as novel TAZ-dependent genes. We further showed TOGARAM2 is a potential novel oncogene while SSTR5 and IFIT2 are tumour suppressive in HPV18+ cervical cancer. In summary, this study identifies TAZ as a crucial oncogene in HPV18+ cervical cancer and unveils a novel transcriptional control network distinct to that of YAP.