Macrophage migration inhibitory factor (MIF) is a highly conserved regulatory cytokine, known to exert pro-inflammatory effects. MIF biological activity is primarily regulated via interaction with the CD74 receptor. MIF also exhibits catalytic tautomerase activity via a conserved N-terminal proline residue. Interestingly, this proline is located within the region of MIF that binds to CD74, and small molecules designed to dock and inhibit tautomerase activity have been recognized to interfere with receptor binding. MIF knockout mice are protected in mouse models of rheumatoid arthritis, cardiovascular disease, sepsis, inflammatory bowel disease and cancer. Anti-MIF antibodies also display efficacy in these models. The first small molecule MIF inhibitor, ISO-1, also shows biological activity in disease models.
Isothiocyanates are a new class of MIF inhibitors that have recently been discovered. Isothiocyanates are a class of phytochemicals, which have been shown to exhibit anti-cancer and anti-inflammatory activity. PEITC is able to covalently modify the N-terminal proline of MIF, resulting in the complete loss of catalytic tautomerase activity.
This study investigated a structure-activity relationship of isothiocyanate inhibition of MIF tautomerase activity. PEITC had an IC50 value of 1.55μM for MIF tautomerase activity in Jurkat T-lymphoma cells, and an LD50 value of 8.4μM. This was 10-fold more effective than ISO-1 at inhibiting MIF tautomerase activity. Increasing the alkyl chain length of isothiocyanates did not greatly influence inhibitory capacity, although PHITC had an IC50 value of 4.2μM compared to that of BITC, with an IC50 value of just 0.54μM. Chlorine, amino and hydroxyl constituents showed minimal effect on the inhibitory capacity, however their cytotoxicity was significantly reduced, with LD50 values of 59μM for OH-PEITC and 60μM for NH2-PEITC. Aliphatic isothiocyanates varied widely in their inhibitory capacity. AITC was the most potent inhibitor with an IC50 value of 0.25μM, while SFN was less effective with an IC50 value of 2.2μM. Both showed very low cytotoxicity, with LD50 values above 100μM. Both AITC and PEITC were shown to inhibit the immunoreactivity of cellular MIF with a monoclonal antibody in a concentration dependent manner, while ISO-1 was shown to have no effect.
Since cytokine inhibition therapy is widely considered to have great medicinal prospects, selective targeting of MIF with specific chemical inhibitors, such as isothiocyanates, might offer new therapeutic avenues for these disorders. This study lays the foundation for further drug design efforts.
