Fibroblast Growth Factor 2 as an Antifibrotic: Antagonism of Myofibroblast Differentiation and Suppression of Pro-Fibrotic Gene Expression

Abstract

Fibrosis is a pathological condition that is characterized by the replacement of dead or damaged tissue with a nonfunctional, mechanically aberrant scar, and fibrotic pathologies account for nearly half of all deaths worldwide. The causes of fibrosis differ somewhat from tissue to tissue and pathology to pathology, but in general some of the cellular and molecular mechanisms remain constant regardless of the specific pathology in question. One of the common mechanisms underlying fibroses is the paradigm of the activated fibroblast, termed the "myofibroblast," a differentiated mesenchymal cell with demonstrated contractile activity and a high rate of collagen deposition. Fibroblast growth factor 2 (FGF2), one of the members of the mammalian fibroblast growth factor family, is a cytokine with demonstrated antifibrotic activity in non-human animal, human, and in vitro models. FGF2 is highly pleiotropic and its receptors are present on many different cell types throughout the body, lending a great deal of variety to the potential mechanisms of FGF2 effects on fibrosis. However, recent reports demonstrate that a substantial contribution to the antifibrotic effects of FGF2 comes from the inhibitory effects of FGF2 on connective tissue fibroblasts, activated myofibroblasts, and myofibroblast progenitors. FGF2 demonstrates effects antagonistic towards fibroblast activation and towards mesenchymal transition of potential myofibroblast-forming cells, as well as promotes a gene expression paradigm more reminiscent of regenerative healing, such as that which occurs in the fetal wound healing response, than fibrotic resolution. With a better understanding of the mechanisms by which FGF2 alters the wound healing cascade and results in a shift away from scar formation and towards functional tissue regeneration, we may be able to further address the critical need of therapy for varied fibrotic pathologies across myriad tissue types.

Keywords: FGF2; Fibroblast; Fibrosis; Myofibroblast; TGF-β.

Figure 1. Sources of wound myofibroblasts

Depending on the tissue, wound fibroblasts can be derived from various sources. Chemical signals in the wound site promote the migration of fibroblasts from neighboring tissues into the wound site. Additionally, mesenchymal progenitor cells from neighboring tissues, such as the bone marrow, can migrate to the wound site and differentiate into fibroblasts in order to take part in the wound healing process. Transdifferentiation can also contribute to the presence of fibroblasts at the wound site. For example, epithelial cells can undergo the epithelial-to-mesenchymal transition (EMT) whereby they gain enhanced motility characteristic of fibroblasts and can migrate to the wound site as well. In all of these cases, fibroblasts at the wound site can become activated to become myofibroblasts by chemical and physical factors, most notably transforming growth factor beta (TGF-β) signaling, where they contribute to wound contraction and deposition of extracellular matrix. Myofibroblasts can be characterized phenotypically by their large spread area and large, “supermature” focal adhesions, cytoplasmic stress fibers containing the actin isoform smooth muscle alpha actin (α-SMA), enhanced contractile activity, and high degree of synthesis and deposition of extracellular matrix molecules including collagen I and ED-A fibronectin.

Figure 2. FGF2/FGFR signaling

Schematic overview of FGF2/FGFR signaling. The extracellular FGF2 ligand binds its receptor, FGFR, in conjunction with cell surface heparin sulfate proteoglycans (HSPGs). This causes receptor dimerization and activation of intracellular tyrosine kinase (TK) domains. These tyrosine kinase domains can directly phosphorylate target signaling proteins such as JAK, leading to activation of JAK/STAT signaling, or PLCγ. Alternatively, the tyrosine kinase domains can phosphorylate adaptor proteins like FRS2α, leading to activation of myriad downstream pathways. Adaptor protein-mediated signal transduction through PI3K leads to activation of AKT and, subsequently, mTOR. In parallel, activation of RAS transduces a signal that activates mitogen-activated protein kinases (MAPKs). RAS can activate RAF, which leads to phosphorylation and activation of ERK and subsequent ERK signaling targets, or RAC, which leads to activation of the MAPKs p38 and JNK. The diversity of FGF2/FGFR signal effectors accounts for much of the complexity and the pleiotropy of FGF2 signaling.

Figure 3. TGFβ signaling

Schematic overview of canonical TGF-β/SMAD signaling. The extracellular TGF-β ligand binds to the TGF-βRII receptor, which subsequently phosphorylates TGF-βRI and propagates the growth factor signal. In the canonical signaling pathway, TGF-βRI phosphorylates serine residues in the C-terminal, MH2 region of SMAD2 and SMAD3. When phosphorylated, these residues are able to associate with SMAD4 and translocate to the nucleus, where the SMAD complex associates with other protein co-factors (not shown) at SMAD binding elements (SBEs) located in the promoters of particular genes, driving expression of these genes. The inhibitory protein SMAD7 inhibits the interaction between TGF-βRI and SMAD2, thus preventing SMAD2 phosphorylation and subsequent association with SMAD4, which is necessary for nuclear translocation and promotion of further TGF-β-mediated signal transduction.