Role of TLR4 Receptor Complex in the Regulation of the Innate Immune Response by Fibronectin.

Mingzhe Zheng,Anthony Ambesi,Paula J Mckeown-Longo

doi:10.3390/cells9010216

Abstract

Chronic inflammation and subsequent tissue fibrosis are associated with a biochemical and mechanical remodeling of the fibronectin matrix. Due to its conformational lability, fibronectin is considerably stretched by the contractile forces of the fibrotic microenvironment, resulting in the unfolding of its Type III domains. In earlier studies, we have shown that a peptide mimetic of a partially unfolded fibronectin Type III domain, FnIII-1c, functions as a Damage Associated Molecular Pattern (DAMP) molecule to induce activation of a toll-like receptor 4 (TLR4)/NF-κB pathway and the subsequent release of fibro-inflammatory cytokines from human dermal fibroblasts. In the current study, we evaluated the requirement of the canonical TLR4/MD2/CD14 receptor complex in the regulation of FnIII-1c induced cytokine release. Using dermal fibroblasts and human embryonic kidney (HEK) cells, we found that all the components of the TLR4/MD2/CD14 complex were required for the release of the fibro-inflammatory cytokine, interleukin 8 (IL-8) in response to both FnIII-1c and the canonical TLR4 ligand, lipopolysaccharide (LPS). However, FnIII-1c mediated IL-8 release was strictly dependent on membrane-associated CD14, while LPS could use soluble CD14. These findings demonstrate that LPS and FnIII-1c share a similar but not identical mechanism of TLR4 activation in human dermal fibroblasts.

Highlights

Chronic inflammation plays a significant role in many fibrotic diseases including cancer
To understand the potential impact of fibronectin strain on tumor progression, we have used a fibronectin peptide, FnIII-1c, which corresponds to a stable intermediate structure predicted to form during force induced unfolding of the first Type III domain of fibronectin [11]. We have identified this peptide as a Damage Associated Molecular Pattern molecule or DAMP which induces the expression of several fibro-inflammatory genes in human dermal fibroblasts [12,13,14], DAMPs are endogenous products of tissue damage which work through toll-like receptors (TLR) to activate innate immune responses [15]
Dependent expression of inflammatory and profibrotic genes. When this process becomes dysregulated, feed forward loops are created which lead to chronic inflammation and eventual tissue fibrosis [35,36,37]

Summary

Introduction

Chronic inflammation plays a significant role in many fibrotic diseases including cancer. The signaling networks between stromal and cancer cells are exceedingly complex and interdependent, occurring on a background of poorly understood mechanical signals which are activated in response to increasing tissue rigidity. The tumor microenvironment is characterized by fibrosis and inflammation which contributes to tissue rigidity and is considered integral to tumor growth and metastasis [1]. Fibronectin is an extracellular matrix (ECM) protein which is polymerized by adherent cells into a mechanically sensitive network of interacting fibers. Fibronectin is up-regulated in the stroma of solid tumors and has been shown to contribute to cancer cell growth, migration, invasion, survival and resistance to chemotherapy [2,3]. The molecular pathways regulated by the pathological remodeling of stromal fibronectin are not well understood.

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