Background Dietary intake of the soy isoflavone genistein is associated with reduced severity of asthma, but the mechanisms responsible for this effect are unknown.
Objective To determine whether genistein blocks eosinophil leukotriene C4 (LTC4) synthesis and to evaluate the mechanism of this effect, and to assess the impact of a 4-week period of soy isoflavone dietary supplementation on indices of eosinophilic inflammation in asthma patients.
Methods Human peripheral blood eosinophils were stimulated in the absence and presence of genistein, and LTC4 synthesis was measured. 5-lipoxygenase (5-LO) nuclear membrane translocation was assessed by confocal immunofluorescence microscopy. Mitogen-activated protein (MAP) kinase activation was determined by immunoblot. Human subjects with mild-to-moderate persistent asthma and minimal or no soy intake were given a soy isoflavone supplement (100 mg/day) for 4 weeks. The fraction of exhaled nitric oxide (FENO) and ex vivo eosinophil LTC4 production were assessed before and after the soy isoflavone treatment period.
Results Genistein inhibited eosinophil LTC4 synthesis (IC50 80 nm), blocked phosphorylation of p38 MAP kinase and its downstream target MAPKAP-2, and reduced translocation of 5-LO to the nuclear membrane. In patients with asthma, following 4 weeks of dietary soy isoflavone supplementation, ex vivo eosinophil LTC4 synthesis decreased by 33% (N=11, P=0.02) and FENO decreased by 18% (N=13, P=0.03).
Conclusion At physiologically relevant concentrations, genistein inhibits eosinophil LTC4 synthesis in vitro, probably by blocking p38- and MAPKAP-2-dependent activation of 5-LO. In asthma patients, dietary soy isoflavone supplementation reduces eosinophil LTC4 synthesis and eosinophilic airway inflammation. These results support a potential role for soy isoflavones in the treatment of asthma.