Mechanism of auranofin toxicity: Inhibition of tetrahydrobiopterin metabolism.

Auranofin is a gold-containing anti-inflammatory drug used for the treatment of rheumatoid arthritis. Recent studies indicate that auranofin targets cellular antioxidants including the thioredoxin system and the ubiquitin-proteasome system, which can result in cellular oxidative stress. In the present studies, we identified a novel mechanistic site of action of auranofin, namely tetrahydrobiopterin (BH4) biosynthesis. BH4 is an essential cofactor required for aromatic amino acid hydroxylases, nitric oxide synthase and alkylglycerol monooxygenase, enzymes that produce mediators important in regulating inflammation. It is synthesized de novo from GTP via the action of GTP cyclohydrolase I, 6-pyruvoyl-tetrahydrobiopterin synthase and sepiapterin reductase (SPR), which leads to the production of dihydrobiopterin (BH2). Further metabolism of BH2 to BH4 is mediated by dihydrofolate reductase (DHFR). We discovered that auranofin causes a rapid depletion of cellular BH4 in several different cell types; PC12 cells were most sensitive to auranofin followed by Jurkat cells, BeWo cells, HaCaT cells, SKNMC cells, RAW246.7 cells and CX-1 cells. This was due to the inhibition of both SPR and DHFR. BH4 is required for tyrosine hydroxylase and tryptophan hydroxylase, enzymes mediating the synthesis of dopamine and serotonin, respectively. In PC12 cells, blocking BH4 biosynthesis with auranofin correlated directly with inhibition of serotonin and dopamine production indicating that the drug targeted aromatic amino acid hydroxylase activity. Our findings provide novel insights into the metabolic consequences of treating inflammatory diseases or cancer cell growth and metastasis with auranofin.