FOS and obesity due to melanocortin 4 receptor deficiency: The response to the glucose challenge at t = 0.6 in both LF and HF is characterized by the up-regulation of Jun and Fos, together forming the transcription factor AP-1, and Il1a and Il1b. This might indicate that AP-1 initiates several processes in the early response to glucose, possibly activated by Il1, and that this mechanism is not influenced by obesity or insulin resistance.