ΔNp63α Suppresses TGFB2 Expression and RHOA Activity to Drive Cell Proliferation in Squamous Cell Carcinomas

Abstract

SUMMARY The transcriptional repressor ΔNp63α is a potent oncogene widely overexpressed in squamous cell carcinomas (SCCs) of diverse tissue origins, where it promotes malignant cell proliferation and survival. We report here the results of a genome-wide CRISPR screen to identify pathways controlling ΔNp63α-de- pendent cell proliferation, which revealed that the small GTPase RHOA blocks cell division upon ΔNp63α knockdown. After ΔNp63α depletion, RHOA activity is increased, and cells undergo RHOA-depen- dent proliferation arrest along with transcriptome changes indicative of increased TGF-β signaling. Mechanistically, ΔNp63α represses transcription of TGFB2, which induces a cell cycle arrest that is partially dependent on RHOA. Ectopic TGFB2 activates RHOA and impairs SCC proliferation, and TGFB2 neutralization restores cell proliferation during ΔNp63α depletion. Genomic data from tumors demonstrate inactivation of RHOA and the TGFBR2 receptor and DNp63a overexpression in more than 80% of lung SCCs. These results reveal a signaling pathway controlling SCC proliferation that is potentially amenable to pharmacological intervention.

Maintaining protein stability of ∆Np63 via USP 28 is required by squamous cancer cells

“…As SCC cells require ∆Np63 to maintain proliferation (Abraham et al , ) and genetic depletion of USP28 by shRNA affected proliferation of human SCC cells (Fig ), we investigated the ability of AZ1 to hinder growth in A‐431. Twenty‐four hours post‐seeding, cells were grown in the presence of increasing concentrations of AZ1 for an additional 48 h, and cell numbers were measured by Hoechst immunofluorescence (Fig E).…”

“…SCC tumours have in common an inherent dependence on ∆Np63 expression (Rocco et al , ; Bergholz & Xiao, ). Previous work has established the role of ∆Np63 as a master transcription factor and regulator of SCC identity and proliferation (Abraham et al , ; Hamdan & Johnsen, ; Somerville et al , ). SCC tumour cells are addicted to ∆Np63 expression (Vivanco, ; Somerville et al , ), as depletion of ∆Np63 is not tolerated by these tumours and leads to rapid tumour regression (Ramsey et al , ).…”

ΔNp63 in squamous cell carcinoma: defining the oncogenic routes affecting epigenetic landscape and tumour microenvironment

“…More recently, Espinosa's group has exploited a genome-wide CRISPR-Cas9 screen to identify the pathway required for the DNp63-driven proliferation in lung SCC (Abraham et al, 2018). Upon DNp63 silencing, lung SCC cells undergo a block of proliferation which is dependent on the activation of the transforming growth factor beta (TGFb) signalling.…”

The USP28-ΔNp63 axis is a vulnerability of squamous tumours

“…SCC tumours have in common is an inherent dependence on ΔNp63 expression (Bergholz and Xiao, 2012; Rocco et al, 2006). Previous work has established the role of ΔNp63 as a master transcription factor and regulator of SCC identity (Abraham et al, 2018; Hamdan and Johnsen, 2018; Somerville et al, 2018). SCC tumour cells are addicted to ΔNp63 expression (Somerville et al, 2018; Vivanco, 2014), as depletion of ΔNp63 is not tolerated by these tumours and leads to rapid tumour regression (Ramsey et al, 2013).…”

“…Most transcription factors, including ΔNp63 (Abraham et al, 2018; Dang et al, 2017; Lambert et al, 2018), are considered as ‘non-druggable’ targets as their structure does not provide suitable domains for small molecule interactions. Modulation of their abundance by targeting mechanisms that control protein stability presents a viable option (Liu et al, 2015; Wang et al, 2018).…”