The JAK/STAT pathway but not the Erk1/2 pathway mediates IL-6/sIL-6R downregulation of type II collagen, aggrecan core and link protein transcription in articular chondrocytes

Introduction

IL-6 regulates several functions, including immunological reactions in host defense or inflammation. Accumulating evidence suggests that IL-6 is implicated in rheumatoid arthritis and in osteoarthritis. IL-6 may contribute to the destructive changes of cartilage accompanying joint diseases, but its mechanism of action on the cartilage and the pathways whereby it controls their gene expression are unknown. However, until recently, the IL-6 signaling pathway for these cells has not been identified nor has the functional significance of their effects on the major extracellular matrix components been investigated.

Objectives

The aim of this study was to analyze the signaling pathways of IL-6/sIL-6R in articular chondrocytes, and their involvement in the control of matrix gene expression.

Methods

Bovine articular chondrocytes were treated by IL-6 (100–500 ng/ml) with or without IL-6sR (200–500 ng/ml) for increasing periods. Western blotting and gel retardation assays were performed with or without mitogen-activated protein kinase inhibitor (PD 98059) and STAT inhibitor (parthenolide). Collagen type II and aggrecan core and link protein mRNA levels were determined by northern blots.

Results

In the absence of sIL-6R, IL-6 (500 ng/ml) had a slight effect on JAK-1 and JAK-2 phosphorylation. Addition of sIL-6R at 500 ng/ml dramatically increased these effects, with a peak at 15 min. STAT1 and STAT3 were also found to be phosphorylated and translocated to the nucleus. Increased binding of protein factors to specific STAT responsive elements was observed. ERK1 and ERK2 were activated by IL-6/sIL-6R treatment, with a maximum at 30 min. Exposure of cells to IL-6/sIL-6R induced an inhibition of collagen type II, aggrecan core and link protein expression. This downregulation was abolished in the presence of STAT inhibitor but not in the presence of mitogen-activated protein kinase inhibitor, indicating that the STAT signaling pathway is implicated in the control of these two cartilage-specific matrix genes.

Conclusions

To exert a significant effect on chondrocytes, IL-6 requires the presence of its soluble receptor. The cytokine plays a role in the cartilage breakdown through a STAT-induced inhibition of collagen type II, aggrecan core and link protein expression.