Kaempferol inhibits IL-4-induced STAT6 activation by specifically targeting JAK3.

IL-4 is involved in several human diseases including allergies, autoimmunity, and cancer. Its effects are mainly mediated through the transcription factor STAT6. Therefore, investigation of compounds that regulate STAT6 activation is of great interest for these diseases. Natural polyphenols are compounds reported to have therapeutic properties in diseases involving IL-4 and STAT6. The aim of this study was to investigate the effect of these compounds in the activation of this transcription factor. We found that in hemopoietic cells from human and mouse origin, some flavonoids were able to inhibit the activation of STAT6 by IL-4. To identify molecular mechanisms, we focused on kaempferol, the compound that showed the greatest inhibitory effect with the lowest cell toxicity. Treatment of cells with kaempferol did not affect activation of Src kinase by IL-4 but did prevent the phosphorylation of JAK1 and JAK3. Further enzymatic analysis demonstrated that kaempferol blocked the in vitro phosphorylation activity of JAK3 without affecting JAK1, suggesting that it specifically targeted JAK3 activity. Accordingly, kaempferol had no effect on STAT6 activation in nonhemopoietic cell lines lacking JAK3, supporting its selective inhibition of IL-4 responses through type I receptors expressing JAK3 but not type II lacking this kinase. The inhibitory effect of kaempferol was also observed in IL-2 but not IL-3-mediated responses and correlated with the inhibition of MLC proliferation. These findings reveal the potential use of kaempferol as a tool for selectively controlling cell responses to IL-4 and, in general, JAK3-dependent responses.