Discrepancy between Jun/Fos Proto-Oncogene mRNA and Protein Expression in the Rheumatoid Arthritis Synovial Membrane

René Huber,Raimund W Kinne,Bruno Stuhlmüller,Elke Kunisch

doi:10.3390/j3020015

Abstract

Rheumatoid arthritis (RA) is a chronic inflammatory and destructive joint disease characterized by overexpression of pro-inflammatory/pro-destructive mediators, whose regulation has been the focus of our previous studies. Since the expression of these proteins commonly depends on AP-1, the expression of the AP-1-forming subunits cJun, JunB, JunD, and cFos was assessed in synovial membrane (SM) samples of RA, osteoarthritis (OA), joint trauma (JT), and normal controls (NC) using ELISA and qRT-PCR. With respect to an observed discrepancy between mRNA and protein levels, the expression of the mRNA stability-modifying factors AU-rich element RNA-binding protein (AUF)-1, tristetraprolin (TTP), and human antigen R (HuR) was measured. JunB and JunD protein expression was significantly higher in RA-SM compared to OA and/or NC. By contrast, jun/fos mRNA expression was significantly (cjun) or numerically decreased (junB, junD, cfos) in RA and OA compared to JT and/or NC. Remarkably, TTP and HuR were also affected by discrepancies between their mRNA and protein levels, since they were significantly decreased at the mRNA level in RA versus NC, but significantly or numerically increased at the protein level when compared to JT and NC. Discrepancies between the mRNA and protein expression for Jun/Fos and TTP/HuR suggest broad alterations of post-transcriptional processes in the RA-SM. In this context, increased levels of mRNA-destabilizing TTP may contribute to the low levels of jun/fos and ttp/hur mRNA, whereas abundant mRNA-stabilizing HuR may augment translation of the remaining mRNA into protein with potential consequences for the composition of the resulting AP-1 complexes and the expression of AP-1-dependent genes in RA.

Highlights

Rheumatoid arthritis (RA) is characterized by chronic inflammation and destruction of multiple joints, perpetuated by an invasive pannus tissue
The median mRNA expression levels of cjun mRNA were elevated in normal controls (NC)-synovial membrane (SM) samples in comparison to all other groups, though without significant differences
The junB levels were roughly comparable in RA, OA, and NC-SM, but numerically increased in joint trauma (JT) (2.5-3-fold; Figure 1B)

Summary

Introduction

Rheumatoid arthritis (RA) is characterized by chronic inflammation and destruction of multiple joints, perpetuated by an invasive pannus tissue. Activated fibroblast-like synoviocytes (FLS) are a major component of the pannus and contribute to joint destruction by secretion of pro-inflammatory cytokines and tissue-degrading enzymes [1]. In former studies, our group analyzed the expression and regulation of proteins involved in the inflammatory, destructive, and fibrotic processes observed in RA, e.g., cytokines [2,3,4], proteases [2,5], or collagens [6]. CJun and cFos are regarded as activating AP-1 subunits, whereas JunD and JunB mediate predominantly deactivating/inhibitory effects [12]. To expand a former study focusing on the junB expression in the synovial membrane (SM) of patients with rheumatic diseases [14], the expression of AP-1 subunits in the RA-SM was analyzed. Samples from osteoarthritis (OA) and joint trauma (JT) patients were used, as well as postmortem normal controls (NC)

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Conclusion