MTOR and obesity due to melanocortin 4 receptor deficiency: However, studies to date have consistently demonstrated lack of impaired mTOR signaling, and its downstream target ribosomal protein S6 kinase-1 (S6K1), in humans with obesity in the fasted state (Bak et al., 2016; Beals et al., 2016; Tran et al., 2018).