Metallothionein in cultured human epithelial cells and synovial rheumatoid fibroblasts after in vitro treatment with auranofin

Transcriptional signature associated with early rheumatoid arthritis and healthy individuals at high risk to develop the disease

Background

Little is known regarding the mechanisms underlying the loss of tolerance in the early and preclinical stages of autoimmune diseases. The aim of this work was to identify the transcriptional profile and signaling pathways associated to non-treated early rheumatoid arthritis (RA) and subjects at high risk. Several biomarker candidates for early RA are proposed.

Methods

Whole blood total RNA was obtained from non-treated early RA patients with <1 year of evolution as well as from healthy first-degree relatives of patients with RA (FDR) classified as ACCP+ and ACCP- according to their antibodies serum levels against cyclic citrullinated peptides. Complementary RNA (cRNA) was synthetized and hybridized to high-density microarrays. Data was analyzed in Genespring Software and functional categories were assigned to a specific transcriptome identified in subjects with RA and FDR ACCP positive. Specific signaling pathways for genes associated to RA were identified. Gene expression was evaluated by qPCR. Receiver operating characteristic (ROC) analysis was used to evaluate these genes as biomarkers.

Results

A characteristic transcriptome of 551 induced genes and 4,402 repressed genes were identified in early RA patients. Bioinformatics analysis of the data identified a specific transcriptome in RA patients. Moreover, some overlapped transcriptional profiles between patients with RA and ACCP+ were identified, suggesting an up-regulated distinctive transcriptome from the preclinical stages up to progression to an early RA state. A total of 203 pathways have up-regulated genes that are shared between RA and ACCP+. Some of these genes show potential to be used as progression biomarkers for early RA with area under the curve of ROC > 0.92. These genes come from several functional categories associated to inflammation, Wnt signaling and type I interferon pathways.

Conclusion

The presence of a specific transcriptome in whole blood of RA patients suggests the activation of a specific inflammatory transcriptional signature in early RA development. The set of overexpressed genes in early RA patients that are shared with ACCP+ subjects but not with ACCP- subjects, can represent a transcriptional signature involved with the transition of a preclinical to a clinical RA stage. Some of these particular up-regulated and down-regulated genes are related to inflammatory processes and could be considered as biomarker candidates for disease progression in subjects at risk to develop RA.

Induction of Metallothionein

Metallothioneins (MTs) are cysteine-rich metal-binding proteins. These proteins play a pivotal role in heavy metal homeostasis and have been widely studied by biochemists, toxicologists, nutritional scientists, and molecular biologists. It is well established that MTs are inducible proteins. They are normally synthesized at low basal levels, but exposure to a wide variety of heavy metals and many organic compounds will dramatically increase synthesis of MTs. This paper summarizes MT induction by reviewing the chemicals that induce these proteins, the molecular mechanism involved in this induction, and the relationship between MT induction and biological function.

Megalin mediates renal uptake of heavy metal metallothionein complexes

Although several heavy metal toxins are delivered to the kidney on the carrier protein metallothionein (MT), uncertainty as to how MT enters proximal tubular cells limits treatment strategies. Prompted by reports that MT-I interferes with renal uptake of the megalin ligand β2-microglobulin in conscious rats, we tested the hypothesis that megalin binds MT and mediates its uptake. Three lines of evidence suggest that binding of MT to megalin is critical in renal proximal tubular uptake of MT-bound heavy metals. First, MT binds megalin, but not cubilin, in direct surface plasmon resonance studies. Binding of MT occurs at a single site with a Kd∼10−4and, as with other megalin ligands, depends on divalent cations. Second, antisera and various known megalin ligands inhibit the uptake of fluorescently labeled MT in model cell systems. Anti-megalin antisera, but not control sera, displace >90% bound MT from rat renal brush-border membranes. Megalin ligands including β2-microglobulin and also recombinant MT fragments compete for uptake by megalin-expressing rat yolk sac BN-16 cells. Third, megalin and fluorescently labeled MT colocalize in BN-16 cells, as shown by fluorescent microscopic techniques. Follow-up surface plasmon resonance and flow cytometry studies using overlapping MT peptides and recombinant MT fragments identify the hinge SCKKSCC region of MT as a critical site for megalin binding. These findings suggest that disruption of the SCKKSCC motif can inhibit proximal tubular MT uptake and thereby eliminate much of the renal accumulation and toxicity of heavy metals such as cadmium, gold, copper, and cisplatinum.

The stimulation of metallothionein synthesis in neuroblastoma IMR-32 by zinc and cadmium but not dexamethasone

Metallothioneins are a class of cysteine-rich and low molecular weight, metal-binding proteins that are inducible by a wide variety of agents, including metal ions, such as cadmium and zinc, glucocorticoid hormones, interferon, and tumor promoters. In an effort to delineate the regulation of the synthesis of the recently identified brain metallothionein-like protein, a study was undertaken to compare the induction of metallothionein in human neuroblastoma IMR-32 cells by zinc, cadmium, and dexamethasone using the human Chang liver cells as a control. Both cadmium (1 microM) and zinc (100 microM) significantly enhanced the incorporation of [35S]cysteine into metallothioneins isolated from both neuroblastoma and Chang liver cells. Dexamethasone in concentrations of 10 microM stimulated the synthesis of metallothionein in the Chang cells, whereas it had no effects on the synthesis of metallothionein in the neuroblastoma cells at concentrations ranging from 2.5--100 microM. The degree of stimulation of metallothionein synthesis in the Chang cells by cadmium and zinc was significantly higher than seen in neuroblastoma cells. The neuroblastoma IMR-32 exhibited less tolerance to the toxicity of both cadmium and zinc than the Chang cells, which may correlate with the inherent ability of these ions to induce metallothioneins in these dissimilar cells. The results of these studies are interpreted to indicate that the factors regulating the synthesis of metallothioneins in the Chang and neuroblastoma cells are not identical, suggesting also of the presence of dissimilar regulatory mechanisms in the liver and brain.

Characterization of the metal composition of metallothionein isoforms using reversed-phase high-performance liquid chromatography with atomic absorption spectrophotometric detection

Reversed-phase high-performance liquid chromatography (RP-HPLC) was used to separate metallothionein (MT) isoforms and on-line atomic absorption spectrophotometric (AAS) detection was used to quantitatively determine their metal content. With this coupled system (HPLC-AAS), it was possible to determine the zinc, cadmium and copper content of individual horse kidney MT isoforms. When rabbit liver MT and the purified isoforms (MT-1 and MT-2) were subjected to RP-HPLC and the zinc-containing peaks of the MT sample to MT-1 or MT-2. HPLC-AAS was used to identify zinc-induced MT in heat-treated cytosol from turkey hen liver, thereby demonstrating its application to the analysis of crude tissue extracts. A standard curve was established using turkey liver MT for the quantitative determination of the zinc content of MT isoforms. There was excellent linear correlation between the micrograms of zinc bound to MT injected onto the column (ranging from 0.34 to 3.43 micrograms of MT-bond zinc) and the integrated peak area of the atomic absorbance for zinc. Using this standard curve, it was possible to quantitate the amount of MT-bound zinc in cytosol extracts of cultured turkey embryo hepatocytes exposed to varying levels of supplemental zinc in the culture medium.

Metallothionein induction by nonsteroidal antiinflammatory drugs

In the present study we report on the effects of commonly used nonsteroidal antiinflammatory drugs on metallothionein (MT) and MT-I mRNA levels. A single dose of chloroquine (100 mg/kg), diclofenac (100 mg/kg), indomethacin (10 mg/kg), or piroxicam (100 mg/kg) was administered ip to C57B1 mice. After 18 h, MT levels were determined with a Cd-saturation radioassay. MT-I mRNA levels were measured by Northern Blot analyses using a probe containing the mouse MT-I gene. All drugs tested caused an increase in the MT content of the liver but not of the kidneys and lung. The lowest and highest effects were observed with chloroquine (8 times the control value) and diclofenac (18 times), respectively. In accordance with the stimulation of MT synthesis, increased accumulation of hepatic MT-I mRNA could be demonstrated. These results indicate that elevated MT levels may contribute to the effectiveness of nonsteroidal antiinflammatory drugs in the treatment of rheumatoid arthritis (RA).

Mechanisms of gold resistance

The efficacy of gold treatment in rheumatoid arthritis is hampered by toxicity, most prevalent in early phases, and unresponsiveness which can only be assessed after more than 6 months on therapy. There is some evidence that more severe disease increases the likelihood of drug resistance. No data indicate an altered pharmacokinetics in resistant individuals, although increased dosage of parenteral gold has been claimed effective in uncontrolled studies. Exposure to metals induces cell adaption and resistance in both prokaryotic and eukaryotic cell lines. In particular this has been shown for gold chloride, sodium aurothiomalate and auranofin. Auranofin induces increased synthesis of metallothionein, a low molecular weight cystein-rich peptide with metal-binding and -homeostatic properties. Thus there is experimental evidence suggesting cellular adaptation as a potential mechanism for gold resistance. However, with the possible exception of auranofin, the nature of the processes remains unknown.

Partial cross-resistance between auranofin and sodium aurothiomalate in cultured human cells

Cross-resistance between auranofin and sodium aurothiomalate was tested in two substrains of cultured human epithelial cells. The HEAF cells were previously made resistant to the otherwise lethal concentration of 2 mumol auranofin per litre of culture medium. The HEmyo cells were made resistant to the otherwise lethal effect of 300 mumol sodium aurothiomalate/l. After 4 days of continuous treatment with 2 mumol auranofin/l, 4.6% of the parent HE cells, 55.7% of the HEAF cells and 51.7% of the HEMyo cells continued to grow, compared with controls grown without drug supplement. After 4 days on 300 mumol sodium aurothiomalate/l, 17.6% of the HE cells, 101.0% of the HEMyo cells and 45.3% of the HEAF cells continued to grow. Thus, there was an almost complete cross-resistance to auranofin in the HEMyo cells, and a relative cross-resistance to sodium aurothiomalate in the HEAF cells. The cross-resistance was not found to be related to decreased cytosolic concentration of total gold in the cross-resistant lines.

Chemistry and biochemistry of metallothionein

A wealth of chemical, spectroscopic and structural data attest to the uniqueness of the metallothioneins as a group of novel bioinorganic structures. Their earmarking feature is the arrangement of "soft" metal ions in complexes with cysteine side chains to form discrete metal-thiolate clusters. In this review an account is given of the chemical characteristics of the 52 metallothioneins whose primary structures are now known completely or in part. Also included is an up-to-date summary of the spectroscopic properties and of the spatial structure models derived from X-ray diffraction crystallographic analysis and from two-dimensional nuclear magnetic resonance spectroscopy.