The Role of Human HtrA1 in Arthritic Disease

Sandra Grau,Peter J Richards,Briedgeen Kerr,Clare Hughes,Bruce Caterson,Anwen S Williams,Uwe Junker,Simon A Jones,Tim Clausen,Michael Ehrmann

doi:10.1074/jbc.m500361200

Abstract

Human HtrA1 belongs to a widely conserved family of serine proteases involved in various aspects of protein quality control and cell fate. Although HtrA1 has been implicated in the pathology of several diseases, its precise biological functions remain to be established. Through identification of potential HtrA1 targets, studies presented herein propose that within the context of arthritis pathology HtrA1 contributes to cartilage degradation. Elevated synovial HtrA1 levels were detected in fluids obtained from rheumatoid and osteoarthritis patients, with synovial fibroblasts identified as a major source of secreted HtrA1. Mass spectrometry analysis of potential HtrA1 substrates within synovial fluids identified fibronectin as a candidate target, and treatment of fibronectin with recombinant HtrA1 led to the generation of fibronectin-degradation products that may be involved in cartilage catabolism. Consistently, treatment of synovial fibroblasts with HtrA1 or HtrA1-generated fibronectin fragments resulted in the specific induction of matrix metalloprotease 1 and matrix metalloprotease 3 expression, suggesting that HtrA1 contributes to the destruction of extracellular matrix through both direct and indirect mechanisms.

Highlights

Progressive degradation of components of the extracellular matrix plays an important role in the pathogenesis of arthritic diseases (9, 10)
It was suggested that several proteoglycans and glycoproteins in the extracellular matrix may serve as potential substrates for HtrA1 (20 –22) and that this protease may be pivotal in the onset of destructive joint pathology seen in arthritic disease
HtrA1 levels in arthritic patients were compared with those detected in synovial fluids taken from non-arthritic trauma patients

Summary

Introduction

Progressive degradation of components of the extracellular matrix plays an important role in the pathogenesis of arthritic diseases (9, 10). Effect of Fibronectin Fragments on MMP and TIMP Expression—To prepare fibronectin fragments, 10 ␮g of fibronectin was incubated with 5 ␮g of HtrA1 for 16 h at 37 °C in buffer in 50 mM Tris-HCl, pH 8.5, 150 mM NaCl. Subsequently, these samples were applied with and without 5 ␮M HtrA1 inhibitor to synovial fibroblasts for 24 h before determining MMP and TIMP mRNA levels by RT-PCR.

Results

Conclusion