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

TLDR

Cellular fibronectin containing the extra domain A (EDA) is produced after tissue injury, and its fragments are implicated in tissue remodeling and inflammatory processes. The study investigated whether recombinant EDA can activate Toll‑like receptor 4 (TLR4). The authors expressed human TLR4 in HEK 293 cells and tested activation by recombinant EDA. Recombinant EDA, but not other fibronectin domains, activated TLR4 in a MD‑2‑dependent, heat‑sensitive manner that persisted in the presence of polymyxin B and the LPS antagonist E5564, revealing a distinct mechanism for EDA‑induced inflammatory gene expression.

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 EDA-containing 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.