MIF tautomerase inhibitor 4-IPP alleviates HFD-induced obesity by restoring CD137-mediated browning of white adipocytes in mice

Macrophage migration inhibitory factor (MIF) is a cytokine that possesses multiple enzymatic activities, such as keto-enol tautomerase and thiol-oxidoreductase. We previously found that lack of MIF tautomerase activity significantly alleviated high fat diet (HFD)-induced obesity in mice. In this study, we investigated the regulatory mechanisms of MIF tautomerase in obesity. HFD-induced obese mouse model was established. Adipogenic differentiation was induced in mouse preadipocyte cell line 3T3-L1 and mouse adipose-derived mesenchymal stem cells (ADSCs) in vitro. We showed that MIF tautomerase inhibitors ISO-1 or 4-IPP dose-dependently promoted lipid degradation and mitochondrial thermogenesis by enhancing basal oxygen consumption rate and proton leak, accompanied by increased expression of browning markers (UCP1, PGC-1α, DIO2, CD137) in 3T3-L1 cells under adipogenic induction conditions. In HFD-induced obese mice, administration of 4-IPP (5, 10 mg/kg, i.p.) every 2 days for 12 weeks significantly ameliorated HFD-induced obesity, improved insulin sensitivity, and enhanced energy expenditure. In white adipocytes, 4-IPP (1, 5, 10 μM) dose-dependently promoted CD137 expression, and restored CD137-mediated activation of the PI3K/AKT signaling to improve lipid metabolism. CD137 deficiency abrogated the browning effect of 4-IPP in white adipocytes in vitro. CD137−/− mice exhibited increased susceptibility to HFD-induced obesity and almost abolished the anti-obesity effects of 4-IPP. Simulation of the protein interaction revealed a direct interaction between MIF and CD137: MIF competitively bound to CD137 on white adipocytes with the endogenous ligand of CD137, which was further confirmed by co-immunoprecipitation. Furthermore, 4-IPP and recombinant CD137 protein inhibited the tautomerase activity of MIF in vitro. In conclusion, MIF promotes obesity by binding CD137 through its tautomerase domain, suppressing CD137-mediated the activation of the PI3K/AKT signaling. MIF tautomerase inhibitors disrupt this interaction, restore CD137 function, and enhance adipocyte browning, offering a promising therapeutic strategy for obesity management.