Laurent Fattet

Matrix stiffness drives epithelial–mesenchymal transition and tumour metastasis through a TWIST1–G3BP2 mechanotransduction pathway

Matrix stiffness potently regulates cellular behaviour in various biological contexts. In breast tumours, the presence of dense clusters of collagen fibrils indicates increased matrix stiffness and correlates with poor survival. It is unclear how mechanical inputs are transduced into transcriptional outputs to drive tumour progression. Here we report that TWIST1 is an essential mechanomediator that promotes epithelial–mesenchymal transition (EMT) in response to increasing matrix stiffness. High matrix stiffness promotes nuclear translocation of TWIST1 by releasing TWIST1 from its cytoplasmic binding partner G3BP2. Loss of G3BP2 leads to constitutive TWIST1 nuclear localization and synergizes with increasing matrix stiffness to induce EMT and promote tumour invasion and metastasis. In human breast tumours, collagen fibre alignment, a marker of increasing matrix stiffness, and reduced expression of G3BP2 together predict poor survival. Our findings reveal a TWIST1–G3BP2 mechanotransduction pathway that responds to biomechanical signals from the tumour microenvironment to drive EMT, invasion and metastasis.

Molecular Pathways: Linking Tumor Microenvironment to Epithelial–Mesenchymal Transition in Metastasis

During tumor development, tumor cells constantly communicate with the surrounding microenvironment through both biochemical and biophysical cues. In particular, the tumor microenvironment can instruct carcinoma cells to undergo a morphogenesis program termed epithelial-to-mesenchymal transition (EMT) to facilitate local invasion and metastatic dissemination. Growing evidence uncovered a plethora of microenvironmental factors in promoting EMT, including proinflammatory cytokines secreted by locally activated stromal cells, hypoxia conditions, extracellular matrix components, and mechanical properties. Here, we review various biochemical and biophysical factors in the tumor microenvironment that directly impinge upon the EMT program. Specifically, cytokines such as TGFβ, TNFα, and IL6 and hypoxia are capable of inducing EMT in various tumors. Several extracellular matrix (ECM) proteins, including collagen-I, fibronectin, and hyaluronan, and ECM remodeling via extracellular lysyl oxidase are also implicated in regulating EMT. In preclinical studies and ongoing clinical trials, targeting these tumor microenvironmental signals has shown promises in halting tumor progression in various human cancers. Clin Cancer Res; 21(5); 962–8. ©2014 AACR.

Release of SR Proteins from CLK1 by SRPK1: A Symbiotic Kinase System for Phosphorylation Control of Pre-mRNA Splicing

Phosphorylation has been generally thought to activate the SR family of splicing factors for efficient splice-site recognition, but this idea is incompatible with an early observation that overexpression of an SR protein kinase, such as the CDC2-like kinase 1 (CLK1), weakens splice-site selection. Here, we report that CLK1 binds SR proteins but lacks the mechanism to release phosphorylated SR proteins, thus functionally inactivating the splicing factors. Interestingly, CLK1 overcomes this dilemma through a symbiotic relationship with the serine-arginine protein kinase 1 (SRPK1). We show that SRPK1 interacts with an RS-like domain in the N terminus of CLK1 to facilitate the release of phosphorylated SR proteins, which then promotes efficient splice-site recognition and subsequent spliceosome assembly. These findings reveal an unprecedented signaling mechanism by which two protein kinases fulfill separate catalytic features that are normally encoded in single kinases to institute phosphorylation control of pre-mRNA splicing in the nucleus.

The forces behind EMT and tumor metastasis

Metastasis is the primary cause of mortality for cancer patients. Understanding the molecular and cellular mechanisms that underlie metastatic spread is critical for the development of effective therapeutic approaches. Acquisition of migratory and invasive properties by tumors cells is mediated by induction of Epithelial-Mesenchymal Transition (EMT). This program is orchestrated by a network of transcription factors including Twist1, Snai1/2, and Zeb1/2. Recent studies have demonstrated the requirement of a dynamic regulation of EMT during tumor progression to first promote tumor cell dissemination and then allow metastatic outgrowth. However, we currently lack a complete understanding of how this process is induced and then repressed during the metastasis cascade.1 Past studies largely have focused on biochemical signals that induce EMT including TGF-β, hypoxia, and inflammatory cytokines. Landmark studies identified a functional role for the mechanical forces generated by stiffening matrix during tumor progression in driving tumor malignancy.2,3 How such biomechanical cues are recognized and subsequently transduced into biochemical and transcriptional signals to modulate tumor cell behavior is unclear.

Mobilization of a splicing factor through a nuclear kinase–kinase complex

The splicing of mRNA is dependent on serine-arginine (SR) proteins that are mobilized from membrane-free, nuclear speckles to the nucleoplasm by the Cdc2-like kinases (CLKs). This movement is critical for SR protein-dependent assembly of the macromolecular spliceosome. Although CLK1 facilitates such trafficking through the phosphorylation of serine-proline dipeptides in the prototype SR protein SRSF1, an unrelated enzyme known as SR protein kinase 1 (SRPK1) performs the same function but does not efficiently modify these dipeptides in SRSF1. We now show that the ability of SRPK1 to mobilize SRSF1 from speckles to the nucleoplasm is dependent on active CLK1. Diffusion from speckles is promoted by the formation of an SRPK1-CLK1 complex that facilitates dissociation of SRSF1 from CLK1 and enhances the phosphorylation of several serine-proline dipeptides in this SR protein. Down-regulation of either kinase blocks EGF-stimulated mobilization of nuclear SRSF1. These findings establish a signaling pathway that connects SRPKs to SR protein activation through the associated CLK family of kinases.

Biomaterials to model and measure epithelial cancers

The use of biomaterials has substantially contributed to both our understanding of tumorigenesis and our ability to identify and capture tumour cells in vitro and in vivo. Natural and synthetic biomaterials can be applied as models to recapitulate key features of the tumour microenvironment in vitro, including architectural, mechanical and biological functions. Engineered biomaterials can further mimic the spatial and temporal properties of the surrounding tumour niche to investigate the specific effects of the environment on disease progression, offering an alternative to animal models for the testing of cancer cell behaviour. Biomaterials can also be used to capture and detect cancer cells in vitro and in vivo to monitor tumour progression. In this Review, we discuss the natural and synthetic biomaterials that can be used to recreate specific features of tumour microenvironments. We examine how biomaterials can be applied to capture circulating tumour cells in blood samples for the early detection of metastasis. We highlight biomaterial-based strategies to investigate local regions adjacent to the tumour and survey potential applications of biomaterial-based devices for diagnosis and prognosis, such as the detection of cellular deformability and the non-invasive surveillance of tumour-adjacent stroma.This Review discusses how biomaterials can be used to recreate and understand the influence of specific tumour microenvironment properties on cancer progression and highlights materials-based strategies to capture, detect and assess metastatic cancer cells in vitro and in vivo.

Molecular and Cellular Mechanobiology of Cancer

Besides biochemical interactions between the primary tumor and its microenvironment, the mechanical involvement of tumor–stroma interactions has recently emerged as an important contributor for tumor progression. Indeed, solid tumors often initiate by a fibrotic state associated with increased matrix deposition and remodeling, an important contributor of tumor progression. Specifically in breast cancer patients, the presence of dense clusters of collagen fibrils, or fibrotic foci, associated with significant increased tissue rigidity, is a prognostic marker of distant metastasis and correlates with poor survival. Clinical and experimental data have shed light on the role of extracellular matrix stiffness during progression to invasive and metastatic tumors, especially regarding breast cancer. Here we describe how extracellular matrix stiffness contributes to tumorigenesis, through different roles on tumor cells or stromal cells, at the primary tumor and at metastatic sites. Eventually we discuss the latest and most promising therapeutic approaches targeting or taking advantage of this newly defined implication of mechanoregulation in cancer progression.