Rho-stimulated contractility drives the formation of stress fibers and focal adhesions.

Abstract

Abstract. Activated rhoA, a ras-related GTP-binding protein, stimulates the appearance of stress fibers, focal adhesions, and tyrosine phosphorylation in quiescent cells (Ridley, A.J., and A. Hall, 1992. Cell. 70:389-399). The pathway by which rho triggers these events has not been elucidated. Many of the agents that activate rho (e.g., vasopressin, endothelin, lysophosphatidic acid) stimulate the contractility of smooth muscle and other cells. We have investigated whether rho's induction of stress fibers, focal adhesions, and tyrosine phosphorylation is the result of its stimulation of contractility. We demonstrate that stimulation of fibroblasts with lysophosphatidic acid, which activates rho, induces myosin light chain phosphorylation. This precedes the formation of stress fibers and focal adhesions and is accompanied by increased contractility. Inhibition of contractility by several different mechanisms leads to inhibition of rho-induced stress fibers, focal adhesions, and tyrosine phosphorylation. In addition, when contractility is inhibited, integrins disperse from focal adhesions as stress fibers and focal adhesions disassemble. Conversely, upon stimulation of contractility, diffusely distributed integrins are aggregated into focal adhesions. These results suggest that activated rho stimulates contractility, driving the formation of stress fibers and focal adhesions and elevating tyrosine phosphorylation. A model is proposed to account for how contractility could promote these events.

Scaffold mediated regulation of MAPK signaling and cytoskeletal dynamics: A perspective

“…Signaling through its effectors Rho kinase (ROCK) and mDia, Rho stimulates actin stress fiber formation by stimulating the monomeric actin binding protein profilin, and stabilizes stress fibers by inhibiting the activity of the actin depolymerizing protein, cofilin [35][36][37][38][39][40]. Bundling of actin filaments with myosin to form stress fibers, itself stimulated by ROCK and myosin light chain kinase, stimulates cell contraction and stabilization of focal adhesions [41][42][43][44], points of integrin-extracellular matrix contact between the ventral surface of the cell and the underlying substrate. Rho-mediated contractility is important for retraction of the cell rear and translocation of the cell body [26].…”

“…The formation of focal adhesions from nascent adhesions is determined by the rigidity of the matrix as sensed by integrins, which in turn modulate cell contractility primarily through signaling cascades impinging on the actomyosin system [63]. Activation of Rho stimulates contractility and focal adhesion formation by promoting bundling of actin filaments and associated myosin motors into stress fibers [44]. Rho-ROCK signaling influences contraction through effects on both the myosin and actin: ROCK elevates myosin light chain phosphorylation by inhibiting myosin phosphatase and/or by directly phosphorylating myosin light chain (MLC) [41][42][43].…”

Signaling by integrin receptors

“…This kinase phosphorylates the myosin binding subunit (MBS) of myosin light chain phosphatase, thereby suppressing the activity of the enzyme . The resulting increase in myosin light chain phosphorylation is believed to induce a conformational change in myosin, thereby increasing its binding to actin ®laments and promote actomyosin contractility and formation of focal complexes and stress ®bers (Chirzanowska-Wondnicka et al, 1996). More recently, evidence has been presented that Rho kinase can also phosphorylate MLC at the same site phosphorylated by MLC kinase .…”

RhoA/ROCK-mediated switching between Cdc42- and Rac1-dependent protrusion in MTLn3 carcinoma cells

“…Initially, the role of RhoA during cell motility was thought to be restricted to the generation of contractile force and focal adhesion turnover needed for tail retraction [10,11]. This is achieved through its downstream effector, the serine/threonine kinase p160ROCK, which leads to myosin phosphorylation and actin-myosin contractility [12]. Based on the antagonism between Rho/ROCK and Rac, it was originally postulated that RhoA activity at the front of a migrating cell was incompatible with membrane protrusion [10,13].…”