Matrix stiffness potently regulates cellular behavior 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 mechano-mediator 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 fiber 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.

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 pro-inflammatory 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.

The RNA helicases DDX5 and DDX17 are members of a large family of highly conserved proteins that are involved in gene-expression regulation; however, their in vivo targets and activities in biological processes such as cell differentiation, which requires reprogramming of gene-expression programs at multiple levels, are not well characterized. Here, we uncovered a mechanism by which DDX5 and DDX17 cooperate with heterogeneous nuclear ribonucleoprotein (hnRNP) H/F splicing factors to define epithelial- and myoblast-specific splicing subprograms. We then observed that downregulation of DDX5 and DDX17 protein expression during myogenesis and epithelial-to-mesenchymal transdifferentiation contributes to the switching of splicing programs during these processes. Remarkably, this downregulation is mediated by the production of miRNAs induced upon differentiation in a DDX5/DDX17-dependent manner. Since DDX5 and DDX17 also function as coregulators of master transcriptional regulators of differentiation, we propose to name these proteins "master orchestrators" of differentiation that dynamically orchestrate several layers of gene expression.

Highlights d High ECM stiffness activates LYN kinase to promote EMT and invasion d LYN directly phosphorylates TWIST1 to promote nuclear localization of TWIST1 d High ECM stiffness promotes ligand-independent EPHA2 signaling to activate LYN d EPHA2/LYN axis promotes breast tumor invasion and metastasis

Summary 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 serinearginine 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.

Breast cancer development is associated with increasing tissue stiffness over years. To more accurately mimic the onset of gradual matrix stiffening, which is not feasible with conventional static hydrogels, mammary epithelial cells (MECs) were cultured on methacrylated hyaluronic acid hydrogels whose stiffness can be dynamically modulated from “normal” (<150 Pascals) to “malignant” (>3,000 Pascals) via two-stage polymerization. MECs form and remain as spheroids, but begin to lose epithelial characteristics and gain mesenchymal morphology upon matrix stiffening. However, both the degree of matrix stiffening and culture time before stiffening play important roles in regulating this conversion as, in both cases, a subset of mammary spheroids remained insensitive to local matrix stiffness. This conversion depended neither on colony size nor cell density, and MECs did not exhibit “memory” of prior niche when serially cultured through cycles of compliant and stiff matrices. Instead, the transcription factor Twist1, transforming growth factor β (TGFβ), and YAP activation appeared to modulate stiffness-mediated signaling; when stiffness-mediated signals were blocked, collective MEC phenotypes were reduced in favor of single MECs migrating away from spheroids. These data indicate a more complex interplay of time-dependent stiffness signaling, spheroid structure, and soluble cues that regulates MEC plasticity than suggested by previous models.

Loss of apical-basal polarity and activation of Epithelial-Mesenchymal Transition (EMT) both contribute to carcinoma progression and metastasis. Here, we report that apical-basal polarity inhibits EMT to suppress metastatic dissemination. Using mouse and human epithelial 3D organoid cultures, we show that the PAR/atypical protein kinase C (aPKC) polarity complex inhibits EMT and invasion by promoting degradation of SNAIL family protein SNAI1. Under intact apical-basal polarity, aPKC kinases phosphorylate SNAI1 on S249, leading to protein degradation. Loss of apical-basal polarity prevents aPKC-mediated SNAI1 phosphorylation and stabilizes SNAI1 protein to promote EMT and invasion. In human breast tumour xenografts, inhibition of the PAR complex-mediated SNAI1 degradation mechanism promotes tumour invasion and metastasis. Analyses of human breast tissue samples reveal negative correlations between PAR3 and SNAI1 protein levels. Our results demonstrate that apical-basal polarity functions as a critical checkpoint of EMT to precisely control epithelial-mesenchymal plasticity during tumour metastasis.

The alternative splicing of human genes is dependent on SR proteins, a family of essential splicing factors whose name derives from a signature C-terminal domain rich in arginine-serine dipeptide repeats (RS domains). Although the SRPKs (SR-specific protein kinases) phosphorylate these repeats, RS domains also contain prolines with flanking serines that are phosphorylated by a second family of protein kinases known as the CLKs (Cdc2-like kinases). The role of specific serine-proline phosphorylation within the RS domain has been difficult to assign since CLKs also phosphorylate arginine-serine dipeptides and, thus, display overlapping residue specificities with the SRPKs. In this study, we address the effects of discrete serine-proline phosphorylation on the conformation and cellular function of the SR protein SRSF1. Using chemical tagging and dephosphorylation experiments, we show that modification of serine-proline dipeptides broadly amplifies the conformational ensemble of SRSF1. The induction of these new structural forms triggers SRSF1 mobilization in the nucleus and alters its binding mechanism to an exonic splicing enhancer in precursor mRNA. These physical events correlate with changes in the alternative splicing of over one hundred human genes based on a global splicing assay. Overall, these studies draw a direct causal relationship between a specific type of chemical modification in an SR protein and the regulation of alternative gene splicing programs.