INS and obesity due to melanocortin 4 receptor deficiency: ↓↓ fasting blood glucose (−18.5%), plasma insulin (−25.3%), and insulin resistance (−33.9%), and markers of obesity-related fatty liver disease in high fat-fed mice↑↑ mRNA levels of the nuclear respiratory factor, medium-chain acyl-CoA decarboxylase, uncoupling protein 3, and peroxisome proliferator responsive element