Integrins αvβ5 and αvβ3 promote latent TGF-β1 activation by human cardiac fibroblast contraction

Abstract

AimsPathological tissue remodelling by myofibroblast contraction is a hallmark of cardiac fibrosis. Myofibroblasts differentiate from cardiac fibroblasts under the action of transforming growth factor-b1 (TGF-b1), which is secreted into the extracellular matrix as a large latent complex. Integrin-mediated traction forces activate TGF-b1 by inducing a conformational change in the latent complex. The mesenchymal integrins avb5 and avb3 are expressed in the heart, but their role in the activation of TGF-b1 remains elusive. Here, we test whether targeting avb5 and avb3 integrins reduces latent TGF-b1 activation by cardiac fibroblasts with the goal to prevent the formation of a-smooth muscle actin (a-SMA)-expressing cardiac myofibroblasts and their contribution to fibrosis. Methods and resultsUsing a porcine model of induced right ventricular fibrosis and pro-fibrotic culture conditions, we show that integrins avb5 and avb3 are up-regulated in myofibroblast-enriched fibrotic lesions and differentiated cultured human cardiac myofibroblasts. Both integrins autonomously contribute to latent TGF-b1 activation and myofibroblast differentiation, as demonstrated by function-blocking peptides and antibodies. Acute blocking of both integrins leads to significantly reduced TGF-b1 activation by cardiac fibroblast contraction and loss of a-SMA expression, which is restored by adding active TGF-b1. Manipulating integrin protein levels in overexpression and shRNA experiments reveals that both integrins can compensate for each other with respect to TGF-b1 activation and induction of a-SMA expression. ConclusionsIntegrins avb5 and avb3 both control myofibroblast differentiation by activating latent TGF-b1. Pharmacological targeting of mesenchymal integrins is a possible strategy to selectively block TGF-b1 activation by cardiac myofibroblasts and progression of fibrosis in the heart.--

The role of transforming growth factor (TGF)-β in the infarcted myocardium

“…Cell surface integrins interact with LAP and have been directly implicated in TGF-β activation in many tissues (39); however, whether integrins play an important role in activating TGF-β in the infarcted heart remains unknown. Although in vitro, αvβ5 and αvβ3 integrins contributed to latent TGF-β activation and subsequent cardiac myofibroblast differentiation (40); the in vivo significance of these interactions has not been directly tested. Proteases of various classes (including serine proteases, cathepsins, metalloproteinases, and cysteine proteases) are capable of activating TGF-β in vitro.…”

Mechanobiology of myofibroblast adhesion in fibrotic cardiac disease

“…Another β1 integrin that has been linked to cardiac fibrotic disease is α5β1 integrin, the classic fibronectin receptor. Expression of α5β1 is increased in fibrotic myocardium and signaling downstream of α5β1 promotes the expression of additional fibronectin in an example of positive feedback (Sarrazy et al, 2014;Wang et al, 2010). Secretion of fibronectin contributes to further MyoFB differentiation by initiating signaling through FAK that facilitates the formation of new integrin adhesions and increases matrix stiffness (Fig.…”

“…Intracellular force generated by actin stress fibers can be transmitted through these adhesions to the ECM to alter its organization. Certain growth factors, including TGF-β1, are secreted into the ECM in an inactive form, but αvβ3 integrins can activate such latent TGF-β1 trough ECM tension, thereby further promoting MyoFB differentiation and fibrosis (Hinz, 2013;Sarrazy et al, 2014;Wipff et al, 2007). MyoFB differentiation results in an increase in α-SMA stress fibers, which can strengthen and increase the activation of integrins in FAs, forming a positive mechanical feedback loop.…”

The Stressful Life of Cardiac Myofibroblasts

“…Vascular smooth muscle cells and pericytes are also α-SMA-positive and additional markers are required to make the distinction [46]. In normal adult tissue, smooth muscle cells express late smooth muscle [150] differentiation markers that are not expressed by myofibroblasts, including desmin, smooth muscle myosin heavy chain, h-caldesmon, and smoothelin [47]. However, in conditions of organ injury, fibrosis, and in cell culture, SMCs lose these late differentiation markers and attain a myofibroblastic and collagen synthesizing phenotype [48].…”

“…Latent TGF-β1 (TGF-β1 with it's associated propeptide LAP) is stored in the ECM by together with the latent TGF-β1 binding protein LTBP-1. Upon actin/myosin promoted myofibroblast contraction, interaction of integrins with RDG binding sites in the LAP activates TGF-β1 by inducing a putative conformation change in LAP [145,149,150]. Binding of the latent TGF-β1 storage protein LTBP-1 to other ECM proteins, including fibrillins and fibronectin makes latent TGF-β1 an integral component of the ECM [35,144,[151][152][153].…”