Matrix Rigidity Mediates TGFβ1-Induced Epithelial-Myofibroblast Transition by Controlling Cytoskeletal Organization and MRTF-A Localization

Snail1-driven plasticity of epithelial and mesenchymal cells sustains cancer malignancy

The transcription factor Snail1 induces epithelial-to-mesenchymal transition (EMT) in tumor epithelial cells, a process associated with the emergence of stemness, invasion and cancer malignancy. Here, we review recent reports indicating that Snail1 also regulates mesenchymal plasticity and paracrine signaling and propose that Snail1 orchestrates the generation of cancer stem cells (CSCs) and cancer-associated fibroblasts (CAFs). Our view supports the current models for tumorigenesis that consider stemness and tumor microenvironment as retroactive actors for metastasis formation, revealing Snail1 as a regulator of these metastatic forces. This view offers new perspectives for understanding and targeting metastasis.

The extracellular matrix and transforming growth factor-β1: Tale of a strained relationship

Physiological tissue repair aims at restoring the mechano-protective properties of the extracellular matrix. Consequently, redundant regulatory mechanisms are in place ensuring that tissue remodeling terminates once matrix homeostasis is re-established. If these mechanisms fail, stromal cells become continuously activated, accumulate excessive amounts of stiff matrix, and fibrosis develops. In this mini-review, I develop the hypothesis that the mechanical state of the extracellular matrix and the pro-fibrotic transforming growth factor (TGF)-β1 cooperate to regulate the remodeling activities of stromal cells. TGF-β1 is stored in the matrix as part of a large latent complex and can be activated by cell contractile force that is transmitted by integrins. Matrix straining and stiffening lower the threshold for TGF-β1 activation by increasing the mechanical resistance to cell pulling. Different elements of this mechanism can be pharmacologically targeted to interrupt the mechanical positive feedback loop of fibrosis, including specific integrins and matrix protein interactions.

A Role of Myocardin Related Transcription Factor-A (MRTF-A) in Scleroderma Related Fibrosis

In scleroderma (systemic sclerosis, SSc), persistent activation of myofibroblast leads to severe skin and organ fibrosis resistant to therapy. Increased mechanical stiffness in the involved fibrotic tissues is a hallmark clinical feature and a cause of disabling symptoms. Myocardin Related Transcription Factor-A (MRTF-A) is a transcriptional co-activator that is sequestered in the cytoplasm and translocates to the nucleus under mechanical stress or growth factor stimulation. Our objective was to determine if MRTF-A is activated in the disease microenvironment to produce more extracellular matrix in progressive SSc. Immunohistochemistry studies demonstrate that nuclear translocation of MRTF-A in scleroderma tissues occurs in keratinocytes, endothelial cells, infiltrating inflammatory cells, and dermal fibroblasts, consistent with enhanced signaling in multiple cell lineages exposed to the stiff extracellular matrix. Inhibition of MRTF-A nuclear translocation or knockdown of MRTF-A synthesis abolishes the SSc myofibroblast enhanced basal contractility and synthesis of type I collagen and inhibits the matricellular profibrotic protein, connective tissue growth factor (CCN2/CTGF). In MRTF-A null mice, basal skin and lung stiffness was abnormally reduced and associated with altered fibrillar collagen. MRTF-A has a role in SSc fibrosis acting as a central regulator linking mechanical cues to adverse remodeling of the extracellular matrix.