Presently, the use of gold salts in the treatment of RA is of little clinical interest in western countries due to the common occurrence of side effects and relatively low and unpredictable efficacy
. However, gold ions, dissolucytotically released from metallic gold implants, have been shown to be anti-inflammatory
[18, 23–26]. Together with recent findings of decreased articular pain and inflammation in veterinary use
[6, 8], it is of interest to analyse the mode of action of metallic gold which can be applied to specific points of inflammation without systemic effects.
The aetiology of RA remains unknown, however, many cell types such as macrophages, lymphocytes and fibroblasts, and cytokines have been implicated in the pathogenesis of RA
. A key role for macrophages has been suggested, in part by successful treatment with blockade of TNF-α. In inflamed tissue TNF-α production is widely performed by activated macrophages
. In RA, the number of macrophages in the synovial tissue correlates to the degree of joint erosion
, and increased number of macrophages are an early hallmark of active disease
. Some mechanisms have been put forward to explain the efficacy of gold salts in RA, including direct effects on synovial macrophages since gold salts are predominantly uptaken by macrophages
. However, the exact mode of action of gold in RA and other conditions having pathogenetic similarities is still unclear.
A limitation in the present study is the sole use of the human THP-1 cell line which shares many properties with human monocyte-derived macrophages
 but does not resemble monocytes-macrophages isolated from human donors. This has been taken into account interpreting the results in this study. Our results showed that THP-1 cells incorporate gold ions bio-released from metallic gold. Gold uptake had no effect on the viability of THP-1 cells indicating that anti-inflammatory effects might not be mediated via macrophage cell death although apoptotic processes cannot totally be excluded since only one apoptotic test was performed in this study. However, we found that uptake of gold modulated the gene expression profile in THP-1 cells, differentially regulating a large number of genes (1028 genes with a FC ≥ 2 and 156 genes with a FC ≥ 3).
Since we were most interested in the effect of gold on inflammation we analysed a cluster of genes involved in inflammation and found 34 genes differentially regulated. In this group we found decreased expression of the LTB gene and decreased protein secretion in cell culture supernatants of THP-1 cells after gold uptake. LTB is involved in chronic inflammation and autoimmunity and LTB antibodies have been tested for inhibiting LTB mediated inflammation
. O’Rourke et al. found high levels of LTB gene expression in RA synovium and showed a significant positive correlation between LTB synovial gene expression and pain VAS score. They conclude that LTB may play a role in RA disease pathogenesis by contributing to a more intense inflammatory reaction in the synovium
. Both EGR1 gene and protein expression was downregulated in our experiments. Several studies demonstrated the significant role of EGR1 in inflammation
. EGR1 protein is expressed in T-cells and is involved in the acute phase of the IL-4 transcription in response to T-cell receptor stimulation
. Interestingly, EGR1 is directly involved in TNF-α mediated upregulation of prostaglandin E2 leading to inflammation and arthritis
. These results suggest a possible role for gold in the treatment of RA by suppressing expression of LTB and EGR1.
FADS1 gene encodes for delta-5 desaturase, a key enzyme in polyunsaturated fatty acid metabolism catalyzing the production of pro-inflammatory arachidonic acid (AA) and eicosanoids, which are biologically active at very low concentrations
. Gold exposure of THP-1 cells revealed decreased expression of FADS1 gene and decreased protein secretion. Future studies have to clear a functional role of gold induced suppression of FADS1 gene in inhibiting inflammation.
Our inflammation gene cluster showed decreased CLEC5A expression in THP-1 cells after gold uptake. CLEC5A is a key regulator of synovial injury and bone erosion during autoimmune joint inflammation. Activation of CLEC5A leads to enhanced recruitment of inflammatory macrophages and neutrophils to the joint and promotes bone erosion. Functional blockade of CLEC5A reduces the clinical signs of autoimmune joint inflammation. These findings suggest that CLEC5A may be a therapeutic target for treatment of immune-mediated skeletal disorders
Surgical synovectomy to remove the inflammatory synovium can temporarily ameliorate rheumatoid inflammation and delay the progress of joint destruction. An efficient medically induced programmed cell death (apoptosis) in the rheumatoid synovium might play a role similar to synovectomy but without surgical tissue damage. Gene transfer of FASLG has increased the frequency of apoptotic cells in mouse and rabbit arthritic synovium. A previous study showed that repeated FASLG gene transfer could remove human inflammatory synovial tissue in situ and function as a ‘gene scalpel’ for molecular synovectomy to arrest inflammatory synovium at an early stage of RA
. The results obtained in our study showed that gold uptake induced FASLG gene expression in macrophages.
Interestingly, among the genes strongly regulated with a FC ≥ 3 after dissolucytosis of gold ions several are directly involved in the pathogenesis of RA.
HGF has been shown to inhibit osteoblast differentiation and plasma levels of HGF predict joint damage in RA suggesting that HGF plays a role in RA joint destruction
. Other studies are linking HGF to angiogenesis in RA
 and HGF is highly upregulated in synovial fluids of patients with RA
. Our data revealed strong downregulation of HGF after gold ion uptake implicating a potential new anti-inflammatory pathway of gold.
Expression of inhibitor of differentiation (ID) gene family is considered to be relevant to the pathogenesis of RA, because ID family genes have been shown to play a role in cell proliferation and angiogenesis and it was proposed that inhibition of expression and/or function of ID1 and 3 may potentially be of therapeutic value for conditions associated with pathological angiogenesis
. A previous study showed increased mRNA and immunohistochemistry staining of ID1 and 3 in the synovium of RA patients
 and interestingly, our data showed strong downregulation of ID1 and 3 by gold ion uptake implicating a mode of action for gold.
In a recent publication Midwood et al. revealed TNC as a novel endogenous activator of TLR4-mediated immunity that mediates persistent synovial inflammation and tissue destruction in arthritic joint disease
. Our array results showed strong downregulation of TNC as a result of gold ion uptake. Numerous recent studies highlight the important role of TNC in RA
[47–49] supporting our hypothesis that a potential anti-inflammatory effect gold is mediated by suppressing TNC production.
Matrix metalloproteinases are known to contribute to the development of RA
. MMP13 has been shown to be associated with synovitis in RA
 and recently, studies of leflunomide and tacrolimus, two active substances in the treatment of RA, were found to be partly active by suppressing expression of MMP13
[52, 53]. This is in line with our findings showing suppression of MMP13 expression by gold.
The anti-TNF-α antibody adalimumab is used in the treatment of RA and beside blocking TNF-α Adalimumab increases CD36 on human macrophages
. Our data revealed upregulation of CD36 in macrophages after gold uptake suggesting a possible anti-inflammatory effect of gold via CD36 upregulation.
The nuclear hormone receptors NR4A1 has been implicated in RA and apoptosis. The purine antimetabolite 6-Mercaptopurine (6-MP), which is widely used as an anti-neoplastic and anti-inflammatory drug, induces NR4A1 expression
. DeSilva et al. showed that NR4A1 overexpression in T cells attenuates the development and progression of collagen-induced arthritis, CIA, probably by promoting activation-induced T cell apoptosis and by inhibiting collagen type II specific antibody production
. Further studies are needed to show whether and how gold induced expression of NR4A1 in macrophages is involved in the anti-inflammatory effect of gold in RA.
Beside macrophages T- and B-lymphocytes play an important role in RA
 and it is of interest to further study the effect of gold ions on the gene expression profile of these cells. A limitation for such studies is that contact hypersensitivity to gold is common. The lymphocyte transformation test (LTT) has been studied in the diagnosis of contact hypersensitivity to gold showing significantly higher stimulation indexes for LTT
[58, 59] in patients sensitized to gold. Therefore patch testing should be performed in future studies on gold including lymphocytes from human donors.