Signaling Mechanisms of Myofibroblastic Activation: Outside-in and Inside-Out

Abstract

Myofibroblasts are central mediators of fibrosis. Typically derived from resident fibroblasts, myofibroblasts represent a heterogeneous population of cells that are principally defined by acquired contractile function and high synthetic ability to produce extracellular matrix (ECM). Current literature sheds new light on the critical role of ECM signaling coupled with mechanotransduction in driving myofibroblastic activation. In particular, transforming growth factor β1 (TGF-β1) and extra domain A containing fibronectin (EDA-FN) are thought to be the primary ECM signaling mediators that form and also induce positive feedback loops. The outside-in and inside-out signaling circuits are transmitted and integrated by TGF-β receptors and integrins at the cell membrane, ultimately perpetuating the abundance and activities of TGF-β1 and EDA-FN in the ECM. In this review, we highlight these conceptual advances in understanding myofibroblastic activation, in hope of revealing its therapeutic anti-fibrotic implications.

Keywords: EDA-FN; Extracellular matrix; Myofibroblast; Positive feedback loop; TGF-β1.

Figure 1.. Outside-in and inside-out signal transduction in myofibroblastic activation

Outside-in: Activated TGF-β1 (freed from LAP) binds to the TGF-βR complex stimulating intracellular signaling that promotes α-SMA production. In parallel, ECM-to-cell mechanical transduction through the integrin/focal adhesion (FA) pathway activates RhoA, leading to assembly of α-SMA stress fibers. Inside-out: TGF-β1-activated EDA-FN splicing increases the EDA-FN protein in the ECM; cell contraction executed by α-SMA stress fibers alters the LAP (gray) conformation via the integrin/LAP interaction releasing TGF-β1 (blue) from LAP. Positive feedback loops: 1. TGF-β1 stimulates the production of EDA-FN which in turn facilitates the ECM incorporation and activation of TGF-β1 via the LTBP-1/EDA-FN/integrin interaction network. 2. ECM-integrin mechano-transduction activates RhoA which enhances the assembly of α-SMA stress fibers that execute cell contraction and integrin mechano-transduction, further augmenting RhoA activation. 3. TGF-β1-stimulated stress fiber production ultimately enhances TGF-β1 release from the ECM via mechano-activation through the integrin/LAP interaction. 4. The assembly of G-actin (monomer α-SMA) into F-actin (stress fiber) allows for the escape of MRTF from G-actin sequestration in the cytosol and its nuclear translocation and SRF activation, which in turn propagates α-SMA and stress fiber production. Notes: 1. Latent TGF-β forms SLC with the LAP “straight jacket”, where it is trapped; SLC and LTBP forms LLC. 2. Green arrows indicate TGF-βR and integrin/FA mutual modulations. 3. Activated RhoA promotes MRTF nuclear translocation by enhancing F-actin assembly while reducing G-actin in the cytosol.