The Extra Domain A of Fibronectin Activates Toll-like Receptor 4

Abstract

Cellular fibronectin, which contains an alternatively spliced exon encoding type III repeat extra domain A (EDA), is produced in response to tissue injury. Fragments of fibronectin have been implicated in physiological and pathological processes, especially tissue remodeling associated with inflammation. Because EDAcontaining fibronectin fragments produce cellular responses similar to those provoked by bacterial lipopolysaccharide (LPS), we examined the ability of recombinant EDA to activate Toll-like receptor 4 (TLR4), the signaling receptor stimulated by LPS. We found that recombinant EDA, but not other recombinant fibronectin domains, activates human TLR4 expressed in a cell type (HEK 293 cells) that normally lacks this Toll-like receptor. EDA stimulation of TLR4 was dependent upon co-expression of MD-2, a TLR4 accessory protein. Unlike LPS, the activity of EDA was heat-sensitive and persisted in the presence of the LPS-binding antibiotic polymyxin B and a potent LPS antagonist, E5564, which completely suppressed LPS activation of TLR4. These observations provided a mechanism by which EDA-containing fibronectin fragments promote expression of genes involved in the inflammatory response.

Increased macrophage activation mediated through toll‐like receptors in rheumatoid arthritis

“…We suggest that once inflammation within the joint causes joint destruction, a variety of molecules are released, such as ED-A of fibronectin, HSPs, and HMGB-1. Each of these molecules is highly expressed in the rheumatoid joint, and each is capable of activating TLR-2 and/or TLR-4 (22)(23)(24)(25)(26)(27)(28)(29). Therefore, once joint damage occurs, a selfperpetuating process, mediated by TLR ligation by endogenous ligands, may be established that promotes the chronic, progressive destruction mediated by the continued activation of macrophages.…”

“…Bacterial PG was identified in RA synovial tissue by a monoclonal antibody, particularly in macrophages and antigen-presenting cells (20,21). Additionally, endogenous mammalian TLR agonists, including fibrinogen, extra domain A (ED-A) of fibronectin, Hsp60 and Hsp70, low molecular weight fragments of hyaluronic acid, and high mobility group box chromosomal protein 1 (HMGB-1), a highly conserved nuclear protein that stabilizes nucleosome formation, are expressed in the RA joint, and each has been shown to activate NF-B through TLR-4 and/or TLR-2 (22)(23)(24)(25)(26)(27)(28)(29).…”

Toll-like receptors and their role in experimental models of microbial infection

“…Recognition of endogenous ligands by TLRs is also an area of active investigation due to its potential relevance to autoimmune diseases, noninfectious inflammatory disorders, and elimination of neoplastic or damaged cells. A number of putative endogenous TLR ligands have already been studied in vitro, including heat shock protein 60 (Hsp60), 78 fibronectin, 79 and the extra domain A of fibrinogen. 80 Recent data have also demonstrated that low-molecular weight soluble hyaluronan (sHA) fragments derived from the extracellular matrix at sites of inflammation are able to induce DC maturation in vitro as well as in vivo through TLR4.…”

Deletion of Apoptosis Inhibitor of Macrophage (AIM)/CD5L Attenuates the Inflammatory Response and Infarct Size in Acute Myocardial Infarction

“…TLR‐4 is activated by the recognition of not only pathogen‐associated molecular patterns, such as bacterial lipopolysaccharide (LPS),17 but also endogenous host‐derived ligands including heat shock protein 60 (HSP60), high‐mobility group box 1 (HMGB‐1), extradomain degradation products of the extracellular matrix (ECM), and free fatty acids (FFA) in innate immunity 14, 15, 16, 18, 19, 20, 21, 22…”