Search

Dysregulated gene expression is intrinsic to cell transformation, tumorigenesis and metastasis. Cancer-specific gene-expression profiles stem from gene regulatory networks fueled by genetic and epigenetic defects, and by abnormal signals of the tumor microenvironment. These oncogenic signals ultimately engage the transcriptional machinery on the cis -regulatory elements of a host of effector genes, through recruitment of transcription factors (TFs), co-activators and chromatin regulators. That said, whether gene -expression in cancer cells is the chaotic product of myriad regulations or rather a relatively ordered process orchestrated by few TFs (master regulators) has long remained enigmatic. Recent work on the YAP/TAZ co-activators has been instrumental to break new ground into this outstanding issue, revealing that tumor cells hijack growth programs that are active during development and regeneration through engagement of a small set of interconnected TFs and their nuclear partners.

Autophagy, besides ensuring energy metabolism and organelle renewal, is crucial for the biology of adult normal and cancer stem cells. However, it remains incompletely understood how autophagy connects to stemness factors and the nature of the microenvironmental signals that pattern autophagy in different cell types. Here we advance in these directions by reporting that YAP/TAZ transcriptionally control autophagy, being critical for autophagosomal degradation into autolysosomes. YAP/TAZ are downstream effectors of cellular mechanotransduction and indeed we found that cell mechanics, dictated by the physical property of the ECM and cytoskeletal tension, profoundly impact on autophagic flux in a YAP/TAZ-mediated manner. Functionally, by using pancreatic and mammary organoid cultures, we found that YAP/TAZ-regulated autophagy is essential in normal cells for YAP/TAZ-mediated dedifferentiation and acquisition of self-renewing properties. In tumor cells, the YAP/TAZ-autophagy connection is key to sustain transformed traits and for acquisition of a cancer stem cell state by otherwise more benign cells. Mechanistically, YAP/TAZ promote autophagic flux by directly promoting the expression of Armus, a RAB7-GAP required for autophagosome turnover and whose add-back rescues autophagy in YAP/TAZ-depleted cells. These findings expand the influence of YAP/TAZ mechanotransduction to the control of autophagy and, vice versa, the role of autophagy in YAP/TAZ biology, and suggest a mechanism to coordinate transcriptional rewiring with cytoplasmic restructuring during cell reprogramming.

Cancer cells rely on dysregulated gene expression. This establishes specific transcriptional addictions that may be therapeutically exploited. Yet, the mechanisms that are ultimately responsible for these addictions are poorly understood. Here, we investigated the transcriptional dependencies of transformed cells to the transcription factors YAP and TAZ. YAP/TAZ physically engage the general coactivator bromodomain-containing protein 4 (BRD4), dictating the genome-wide association of BRD4 to chromatin. YAP/TAZ flag a large set of enhancers with super-enhancer-like functional properties. YAP/TAZ-bound enhancers mediate the recruitment of BRD4 and RNA polymerase II at YAP/TAZ-regulated promoters, boosting the expression of a host of growth-regulating genes. Treatment with small-molecule inhibitors of BRD4 blunts YAP/TAZ pro-tumorigenic activity in several cell or tissue contexts, causes the regression of pre-established, YAP/TAZ-addicted neoplastic lesions and reverts drug resistance. This work sheds light on essential mediators, mechanisms and genome-wide regulatory elements that are responsible for transcriptional addiction in cancer and lays the groundwork for a rational use of BET inhibitors according to YAP/TAZ biology.

YAP/TAZ are nuclear effectors of the Hippo pathway regulating organ growth and tumorigenesis. Yet, their function as transcriptional regulators remains underinvestigated. By ChIP-seq analyses in breast cancer cells, we discovered that the YAP/TAZ transcriptional response is pervasively mediated by a dual element: TEAD factors, through which YAP/TAZ bind to DNA, co-occupying chromatin with activator protein-1 (AP-1, dimer of JUN and FOS proteins) at composite cis-regulatory elements harbouring both TEAD and AP-1 motifs. YAP/TAZ/TEAD and AP-1 form a complex that synergistically activates target genes directly involved in the control of S-phase entry and mitosis. This control occurs almost exclusively from distal enhancers that contact target promoters through chromatin looping. YAP/TAZ-induced oncogenic growth is strongly enhanced by gain of AP-1 and severely blunted by its loss. Conversely, AP-1-promoted skin tumorigenesis is prevented in YAP/TAZ conditional knockout mice. This work highlights a new layer of signalling integration, feeding on YAP/TAZ function at the chromatin level.

One of the biggest challenges in tumour research is the possibility to reprogram cancer cells towards less aggressive phenotypes. In this study, we reprogrammed primary Glioblastoma multiforme (GBM)-derived cells towards a more differentiated and less oncogenic phenotype by activating the Wnt pathway in a hypoxic microenvironment. Hypoxia usually correlates with malignant behaviours in cancer cells, but it has been recently involved, together with Wnt signalling, in the differentiation of embryonic and neural stem cells. Here, we demonstrate that treatment with Wnt ligands, or overexpression of β-catenin, mediate neuronal differentiation and halt proliferation in primary GBM cells. An hypoxic environment cooperates with Wnt-induced differentiation, in line with our finding that hypoxia inducible factor-1α (HIF-1α) is instrumental and required to sustain the expression of β-catenin transcriptional partners TCF-1 and LEF-1. In addition, we also found that Wnt-induced GBM cell differentiation inhibits Notch signalling, and thus gain of Wnt and loss of Notch cooperate in the activation of a pro-neuronal differentiation program. Intriguingly, the GBM sub-population enriched of cancer stem cells (CD133(+) fraction) is the primary target of the pro-differentiating effects mediated by the crosstalk between HIF-1α, Wnt, and Notch signalling. By using zebrafish transgenics and mutants as model systems to visualize and manipulate in vivo the Wnt pathway, we confirm that Wnt pathway activation is able to promote neuronal differentiation and inhibit Notch signalling of primary human GBM cells also in this in vivo set-up. In conclusion, these findings shed light on an unsuspected crosstalk between hypoxia, Wnt and Notch signalling in GBM, and suggest the potential to manipulate these microenvironmental signals to blunt GBM malignancy.

Correction to: Cell Death and Disease (2013) 4, e500; doi:10.1038/cddis.2013.32; published online 21 February 2013 Since the publication of this article the authors have noticed that the Authors Contribution section was omitted. The error has now been rectified. The article with the Authors Contribution appears online together with this corrigendum.

Defining the interplay between the genetic events and microenvironmental contexts necessary to initiate tumorigenesis in normal cells is a central endeavour in cancer biology. We found that receptor tyrosine kinase (RTK)–Ras oncogenes reprogram normal, freshly explanted primary mouse and human cells into tumour precursors, in a process requiring increased force transmission between oncogene-expressing cells and their surrounding extracellular matrix. Microenvironments approximating the normal softness of healthy tissues, or blunting cellular mechanotransduction, prevent oncogene-mediated cell reprogramming and tumour emergence. However, RTK–Ras oncogenes empower a disproportional cellular response to the mechanical properties of the cell’s environment, such that when cells experience even subtle supra-physiological extracellular-matrix rigidity they are converted into tumour-initiating cells. These regulations rely on YAP/TAZ mechanotransduction, and YAP/TAZ target genes account for a large fraction of the transcriptional responses downstream of oncogenic signalling. This work lays the groundwork for exploiting oncogenic mechanosignalling as a vulnerability at the onset of tumorigenesis, including tumour prevention strategies. Receptor tyrosine kinase (RTK)–Ras oncogenes have now been shown to reprogram normal primary human and mouse cells into tumour precursors by empowering cellular mechanotransduction, in a process requiring permissive extracellular-matrix rigidity and intracellular YAP/TAZ/Rac mechanical signalling sustained by activated oncogenes.

Inactivation of ARID1A and other components of the nuclear SWI/SNF protein complex occurs at very high frequencies in a variety of human malignancies, suggesting a widespread role for the SWI/SNF complex in tumour suppression1. However, the underlying mechanisms remain poorly understood. Here we show that ARID1A-containing SWI/SNF complex (ARID1A–SWI/SNF) operates as an inhibitor of the pro-oncogenic transcriptional coactivators YAP and TAZ2. Using a combination of gain- and loss-of-function approaches in several cellular contexts, we show that YAP/TAZ are necessary to induce the effects of the inactivation of the SWI/SNF complex, such as cell proliferation, acquisition of stem cell-like traits and liver tumorigenesis. We found that YAP/TAZ form a complex with SWI/SNF; this interaction is mediated by ARID1A and is alternative to the association of YAP/TAZ with the DNA-binding platform TEAD. Cellular mechanotransduction regulates the association between ARID1A–SWI/SNF and YAP/TAZ. The inhibitory interaction of ARID1A–SWI/SNF and YAP/TAZ is predominant in cells that experience low mechanical signalling, in which loss of ARID1A rescues the association between YAP/TAZ and TEAD. At high mechanical stress, nuclear F-actin binds to ARID1A–SWI/SNF, thereby preventing the formation of the ARID1A–SWI/SNF–YAP/TAZ complex, in favour of an association between TEAD and YAP/TAZ. We propose that a dual requirement must be met to fully enable the YAP/TAZ responses: promotion of nuclear accumulation of YAP/TAZ, for example, by loss of Hippo signalling, and inhibition of ARID1A–SWI/SNF, which can occur either through genetic inactivation or because of increased cell mechanics. This study offers a molecular framework in which mechanical signals that emerge at the tissue level together with genetic lesions activate YAP/TAZ to induce cell plasticity and tumorigenesis. The ARID1A-containing SWI/SNF complex operates as an inhibitor of the pro-oncogenic transcriptional coactivators YAP and TAZ; this interaction is regulated by cellular mechanotransduction.

How the behaviour of somatic stem cells (SCs) is influenced by mechanical signals remains a black-box in cell biology. Here we show that YAP/TAZ regulation by cell shape and rigidity of the extracellular matrix (ECM) dictates a pivotal SC decision: to remain undifferentiated and grow, or to activate a terminal differentiation programme. Notably, mechano-activation of YAP/TAZ promotes epidermal stemness by inhibition of Notch signalling, a key factor for epidermal differentiation. Conversely, YAP/TAZ inhibition by low mechanical forces induces Notch signalling and loss of SC traits. As such, mechano-dependent regulation of YAP/TAZ reflects into mechano-dependent regulation of Notch signalling. Mechanistically, at least in part, this is mediated by YAP/TAZ binding to distant enhancers activating the expression of Delta-like ligands, serving as ‘in cis' inhibitors of Notch. Thus YAP/TAZ mechanotransduction integrates with cell–cell communication pathways for fine-grained orchestration of SC decisions., Notch signalling is a fundamental negative regulator of epidermal stemness. Here, the authors show that cell mechanics through YAP/TAZ activity prevent primary human keratinocytes from differentiating by inhibiting cell-autonomous Notch signals.