Myofibroblast differentiation during fibrosis: role of NAD(P)H oxidases

Abstract

Progression of fibrosis involves interstitial hypercellularity, matrix accumulation, and atrophy of epithelial structures, resulting in loss of normal function and ultimately organ failure. There is common agreement that the fibroblast/myofibroblast is the cell type most responsible for interstitial matrix accumulation and consequent structural deformations associated with fibrosis. During wound healing and progressive fibrotic events, fibroblasts transform into myofibroblasts acquiring smooth muscle features, most notably the expression of alpha-smooth muscle actin and synthesis of mesenchymal cell-related matrix proteins. In renal disease, glomerular mesangial cells also acquire a myofibroblast phenotype and synthesize the same matrix proteins. The origin of interstitial myofibroblasts during fibrosis is a matter of debate, where the cells are proposed to derive from resident fibroblasts, pericytes, perivascular adventitial, epithelial, and/or endothelial sources. Regardless of the origin of the cells, transforming growth factor-beta1 (TGF-β1) is the principal growth factor responsible for myofibroblast differentiation to a profibrotic phenotype and exerts its effects via Smad signaling pathways involving mitogen-activated protein kinase and Akt/protein kinase B. Additionally, reactive oxygen species (ROS) have important roles in progression of fibrosis. ROS are derived from a variety of enzyme sources, of which the nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase family has been identified as a major source of superoxide and hydrogen peroxide generation in the cardiovasculature and kidney during health and disease. Recent evidence indicates that the NAD(P)H oxidase homolog Nox4 is most accountable for ROS-induced fibroblast and mesangial cell activation, where it has an essential role in TGF-β1 signaling of fibroblast activation and differentiation into a profibrotic myofibroblast phenotype and matrix production. Information on the role of ROS in mesangial cell and fibroblast signaling is incomplete, and further research on myofibroblast differentiation during fibrosis is warranted.

Figure 1. Proposed origin(s) of interstitial myofibroblasts during fibrosis

Proposed origin(s) of interstitial myofibroblasts during fibrosis. Myofibroblasts responsible for interstitial expansion and structural damage during progressive organ injury have been proposed to be derived from one or more sources: (1) activation of resident fibroblasts or pericytes, (2) infiltration of circulating bone marrow-derived fibrocytes, (3) expansion of perivascular adventitial fibroblasts, (4) endothelial–mesenchymal (EndoMT) transition, and/or (5) epithelial-to-mesenchymal transition (EMT). Transforming growth factor-beta (TGF-β) released via paracrine or autocrine pathways induces myofibroblast differentiation expressed by the acquisition of an alpha-smooth muscle actin (α-SMA) phenotype and consequent synthesis of mesenchymal matrix proteins collagen type I (Col I), collagen type III (Col III), and fibronectin EIIIA (FN). Nox NAD(P)H oxidase (Nox4)-derived reactive oxygen species (ROS) have a central role in TGF-β signaling of myofibroblast differentiation.

Figure 2. Structure and molecular organization of the cardiorenal nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidases of the Nox family

The top right panel illustrates the topology and the enzymatic reaction catalyzed by the Nox enzymes. The other panels represent the molecular structure of the different isoforms of Nox oxidases predominantly expressed in the cardiorenal system, gp91phox/Nox2, Nox1, and Nox4. All cardiorenal Nox proteins form a complex with p22phox, but the cytosolic subunits differ from the Nox oxidase isoforms. FAD, flavin adenine dinucleotide; H2O2, hydrogen peroxide; O₂^•−, superoxide.

Figure 3

Pathways of Nox-dependent signal transductions implicated in mesangial cells (MCs) (left panel) and renal fibroblasts (right panel) activation. See text for detail. Ang II, angiotensin II; ERK, extracellular receptor kinase; H₂O₂, hydrogen peroxide; MEK, MAP kinase kinase; NFAT, nuclear factor of activated T cell; O₂^•−, superoxide; PKB, protein kinase B; ROS, reactive oxygen species; α-SMA, alpha-smooth muscle actin; TGF-β, transforming growth factor-beta.