STAT3 and obesity due to melanocortin 4 receptor deficiency: Mechanistically, the excessive Kyn-induced aryl hydrocarbon receptor (AhR) overexpression, which then transcribed Signal transducer and activator of transcription 3 (STAT3) and activated the STAT3-IL6 signaling, by which Kyn mediated a systemic effect on the development of obesity and insulin resistance.