SPP1 and obesity due to melanocortin 4 receptor deficiency: If OPN was associated in the development of adipose tissue inflammation and IR [117,200], obesity, and hepatic steatosis [201,202], there was an unaltered fatty acid oxidation and synthesis between wild-type and OPN knockout livers in obesity (along with the downregulation of Forkhead box O1 and PPAR gamma co-activator 1α), which usually enhance mitochondrion genesis, lipid degradation, and insulin sensitivity [142].