MAPK3 and obesity due to melanocortin 4 receptor deficiency: Inhibition of ERK1/2 pathway activation was found in the constructed mouse model of animal obesity, which led to downregulation of plasma APN content, but the expression of APN gene level was not affected, suggesting that ERK1/2 pathway may mediate the degradation of APN protein level, which is indeed the case by applying MG132.