Mizoribine, an inhibitor of inosine monophosphate dehydrogenase, inhibits interleukin-6 production by freshly prepared rheumatoid synovial cells

Abstract

Mizoribine, an immunosuppressive drug, has been used for treatment in organ transplantation, lupus nephritis, and rheumatoid arthritis (RA). On the basis of in vitro experiments, mizoribine has been postulated to be an inhibitor of inosine monophosphate (IMP) dehydrogenase, a pivotal enzyme in the formation of guanine ribonucleotides from IMP. To further characterize the mechanism of the antirheumatic action of this drug, we examined the effect of mizoribine on the production of interleukin (IL)-6, a major inflammatory cytokine in rheumatoid synovia, by freshly prepared rheumatoid synovial cells (RSC). Mizoribine (1.25–5 μg/ml) was able to inhibit the spontaneous production of IL-6 by fresh RSC in a dose–response fashion. The addition of guanosine monophosphate (GMP) reversed its inhibitory effects. In addition, mizoribine inhibited the enhanced production of IL-6 by the IL-1α and/or tumor necrosis factor α-stimulated RSC. Inhibition was also observed at the mRNA level, determined by Northern blot analysis. In contrast, mizoribine did not affect IL-8 production by these cells. These data suggest that mizoribine inhibits IL-6 production by fresh RSC, possibly owing to the depletion of intracellular GMP, and that this inhibitory effect of the drug on rheumatoid synovial cells may be related to its efficacy in RA.