INS and obesity due to melanocortin 4 receptor deficiency: By considering the central role of TNF-α in obesity-induced insulin resistance in peripheral organs [26], we hypothesized an involvement of this cytokine in the onset of insulin resistance in HFF rats, but we found no variations of TNF-α levels between all the groups of rats in skeletal muscle and also in TNF-inducible transcription factor NFkB p65, a pathway normally antagonizing insulin signaling [28].