Understanding the molecular basis for MMP-13 repression by IL-4

Abstract

Cartilage destruction in arthritic disease is characterised by irreversible collagenolysis, resulting in loss of efficient joint function. Of the enzymes capable of hydrolysing native collagen fibrils, matrix metalloproteinase-13 (MMP-13) is the major collagenolytic MMP in osteoarthritis (OA), making this MMP an important diseasemodifying target. Addition of interleukin-1 (IL-1) and oncostatin M (OSM) to bovine nasal cartilage in explant culture results in a synergistic loss of the collagen matrix, accompanied by a dramatic increase in the expression of collagenase enzymes. Interleukin-4 (IL-4) is able to ameliorate this collagen degradation by the strong repression of IL-1+OSM-induced MMP-13 expression. The aim of this work was to determine the mechanism by which IL-4 abolishes IL-1+OSM-induced MMP-13 expression. Work examining the effect of IL-4 on the methylation status of CpG residues within the MMP-13 promoter failed to identify a role for epigenetic modification in the mechanism of action of IL-4. Subsequent cell signalling studies demonstrated Akt activation by IL- 4. Therefore, genome-wide microarray analyses of cytokine stimulated cartilage and chondrocytes was used to identify candidate Akt-interacting proteins involved in the repressive effects of IL-4 on MMP-13. Trb1 was identified as a novel gene potentially involved in the repression of MMP-13 by IL-4 via Akt. Gene silencing experiments in chondrocytes confirmed that transfection with Trb1 specific siRNA resulted in the rescue of IL-4 mediated repression of IL-1+OSM-induced MMP-13 expression, indicating an anti-inflammatory role for Trb1. Trb1 belongs to family of three tribbles proteins and additional studies to investigate the roles of other tribbles family members in MMP regulation identified Trb3 as having a potentially pro-inflammatory role in MMP regulation in chondrocytes. Silencing of Trb3 was reproducibly shown to abolish IL-1+OSM-induced MMP-13 expression. The novel data presented in this thesis indicate that tribbles proteins act as key regulators of catabolic and anabolic responses in chondrocytes. From these findings it could be hypothesised that alterations in functional levels of specific tribbles proteins may protect against aberrant MMP gene expression in chondrocytes. The identification of this potentially important regulatory mechanism of signalling pathways important in MMP-13 gene expression in chondrocytes could be translated into a tractable therapy for arthritis once the mechanism has been unravelled.