MicroRNA-18a targeting of the STK4/MST1 tumour suppressor is necessary for transformation in HPV positive cervical cancer.

Ethan L Morgan,Adrian Whitehouse,Emma L Ryder,Katherine L Harper,Andrew Macdonald,Molly R Patterson,Christopher W Wasson,G Eric Blair,Siu Yi Lee,Yigen Li,Stephen Griffin

doi:10.1371/journal.ppat.1008624

Abstract

Human papillomaviruses (HPV) are a major cause of malignancy worldwide. They are the aetiological agents of almost all cervical cancers as well as a sub-set of other anogenital and head and neck cancers. Hijacking of host cellular pathways is essential for virus pathogenesis; however, a major challenge remains to identify key host targets and to define their contribution to HPV-driven malignancy. The Hippo pathway regulates epithelial homeostasis by down-regulating the function of the transcription factor YAP. Increased YAP expression has been observed in cervical cancer but the mechanisms driving this increase remain unclear. We found significant down-regulation of the master Hippo regulatory kinase STK4 (also termed MST1) in cervical disease samples and cervical cancer cell lines compared with healthy controls. Re-introduction of STK4 inhibited the proliferation of HPV positive cervical cells and this corresponded with decreased YAP nuclear localization and decreased YAP-dependent gene expression. The HPV E6 and E7 oncoproteins maintained low STK4 expression in cervical cancer cells by upregulating the oncomiR miR-18a, which directly targeted the STK4 mRNA 3’UTR. Interestingly, miR-18a knockdown increased STK4 expression and activated the Hippo pathway, significantly reducing cervical cancer cell proliferation. Our results identify STK4 as a key cervical cancer tumour suppressor, which is targeted via miR-18a in HPV positive tumours. Our study indicates that activation of the Hippo pathway may offer a therapeutically beneficial option for cervical cancer treatment.

Highlights

Human papillomaviruses (HPV) are a major cause of ano-genital cancers, accounting for 99.9% of all cervical cancer cases, and are linked to an increasing number of head and neck carcinomas [1,2,3]
serine/threonine protein kinase 4 (STK4) expression is decreased in HPV+ cervical cancer cell lines
Levels of the STK4 tumour suppressor protein were significantly reduced in HPV16+ and HPV18+ cell lines compared to normal human keratinocytes (NHKs) cells, whereas in HPV- C33A cells STK4 protein expression was similar to that observed in NHK cells (Fig 1A)

Summary

Introduction

Human papillomaviruses (HPV) are a major cause of ano-genital cancers, accounting for 99.9% of all cervical cancer cases, and are linked to an increasing number of head and neck carcinomas [1,2,3]. The core Hippo pathway is a kinase cascade comprised of the sterile 20-like family kinases serine/threonine protein kinase 4 (STK4) ( known as MST1) and STK3 ( known as MST2), the large tumour suppressors (LATS1 and LATS2) and the adaptor proteins Mps One Binder kinase activator (MOBs) and Salvador homologue 1 (SAV1) [11]. Activation of these kinases results in serine phosphorylation of the major Hippo targets yes-associated protein (YAP) and its paralogue the transcriptional co-activator with PDZ-binding (TAZ). In their non-phosphorylated state, YAP and TAZ traffic to the nucleus where they partner with TEAD transcription factors to regulate the expression of genes necessary for cell proliferation, differentiation and survival [12]

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