Abstract

OBJECTIVE:

This study addresses the role of the NR4A orphan nuclear receptors in synoviocyte transformation, hyperplasia, and regulation of MMPs and TIMPs in models of inflammatory arthritis.

METHODS:

NR4A mRNA levels were measured in synovial tissues and primary synoviocytes by RT-qPCR. NR4A2 was stably over-expressed in normal synoviocytes and cell proliferation, survival, anchorage-independent growth, migration, and invasion were monitored in vitro. MMP and TIMP expression levels were analyzed by RT-qPCR and MMP-13 promoter activity was measured by reporter assays. Stable depletion of endogenous NR4A levels was achieved by lentiviral transduction of NR4A shRNA and effects on proliferation, migration, and MMP-13 expression were analyzed.

RESULTS:

NR4A2 is expressed at elevated levels in normal, OA, and RA synovial tissues and in primary RA synoviocytes. TNF-α rapidly and selectively induces expression of NR4A2 in synoviocytes. Ectopic expression of NR4A2 in normal synoviocytes significantly increases proliferation and survival, promotes anchorage-independent growth, and induces migration and invasion. MMP-13 gene expression is synergistically induced by NR4A2 and TNF-α, while expression of TIMP-2 is antagonized. NR4A2 directly transactivates the proximal MMP-13 promoter and a point mutation in the DNA-binding domain of NR4A2 abolishes transcriptional activation. Depletion of endogenous NR4A receptors with shRNA reduces synoviocyte proliferation, migration, and MMP-13 expression.

CONCLUSION:

The orphan nuclear receptor NR4A2 is a downstream mediator of TNF-α signaling in synovial tissue. NR4A2 transcriptional activity contributes to the hyperplastic and invasive phenotype of synoviocytes that leads to cartilage destruction, suggesting that this receptor may show promise as a therapeutic target in inflammatory arthritis.

Copyright © 2012 by the American College of Rheumatology.