title: Impaired coordination of nutrient intake and substrate oxidation in melanocortin-4 receptor knockout mice.
abstract: Mutations in the melanocortin-4 receptor (MC4R) are associated with obesity. The obesity syndrome observed in humans with MC4R haploinsufficiency is similar to that observed in MC4R knockout mice, including increased longitudinal growth, hyperphagia, and fasting hyperinsulinemia. For comparison with other commonly investigated models of obesity and insulin resistance, we have backcrossed Mc4r-/- mice into the C57BL/6J (B6) background. Female obese Mc4r-/- mice exhibit reduced energy expenditure and an attenuated increase in fatty acid (FA) oxidation after exposure to high-fat diets compared with obese Lepob/Lepob mice. The reduced energy expenditure and FA oxidation correlates with changes in hepatic gene expression. The expression of genes involved in FA oxidation increased in obese Lepob/Lepob mice compared with wild-type and obese Mc4r-/- mice. In contrast, a key lipogenic enzyme, FA synthase (FAS), is increased in obese Mc4r-/- mice compared with obese Lepob/Lepob mice. Hyperinsulinemia, increased FAS mRNA expression and hepatic steatosis appear to be secondary to obesity in B6 Mc4r-/- mice. However, Mc4r-/- mice in a mixed genetic background develop severe hepatic steatosis at an early age. This might suggest an important role of the MC4R in regulating liver FA metabolism that is masked on the B6 background. Interestingly, the 10- to 20-fold increase in liver triglyceride in the outbred strain of Mc4r-/- mice is not always associated with fasting hyperinsulinemia or increased FAS mRNA expression. This observation suggests that changes in liver secondary to triglyceride accumulation lead to hyperinsulinemia and increased hepatic FAS expression in Mc4r-/- mice.
authors.affiliation: Pennington Biomedical Research Center/Louisiana State University, Baton Rouge, Louisiana 70808, USA.
authors.forename: Diana C
authors.initials: DC
authors.lastname: Albarado
authors.forename: Jennifer
authors.initials: J
authors.lastname: McClaine
authors.forename: Jacqueline M
authors.initials: JM
authors.lastname: Stephens
authors.forename: Randall L
authors.initials: RL
authors.lastname: Mynatt
authors.forename: Jianping
authors.initials: J
authors.lastname: Ye
authors.forename: Anthony W
authors.initials: AW
authors.lastname: Bannon
authors.forename: William G
authors.initials: WG
authors.lastname: Richards
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Butler
mesh_terms: D000818:Animals; D001835:Body Weight / Q000502:physiology; D004041:Dietary Fats / Q000493:pharmacokinetics; D004435:Eating / Q000502:physiology*; D064429:Fatty Acid Synthases / Q000235:genetics; D005227:Fatty Acids / Q000378:metabolism; D005234:Fatty Liver / Q000378:metabolism / Q000503:physiopathology; D005260:Female; D015870:Gene Expression; D006946:Hyperinsulinism / Q000378:metabolism / Q000503:physiopathology; D007328:Insulin / Q000097:blood; D055504:Insulin Receptor Substrate Proteins; D047908:Intracellular Signaling Peptides and Proteins; D020738:Leptin / Q000235:genetics; D008055:Lipids / Q000097:blood; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009765:Obesity / Q000378:metabolism* / Q000503:physiopathology*; D010084:Oxidation-Reduction; D010750:Phosphoproteins / Q000378:metabolism; D044105:Receptor, Melanocortin, Type 4 / Q000235:genetics* / Q000378:metabolism*; D014280:Triglycerides / Q000378:metabolism
chemical_list: D004041:Dietary Fats; D005227:Fatty Acids; C526297:IRS2 protein, human; D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D047908:Intracellular Signaling Peptides and Proteins; C526298:Irs2 protein, mouse; D020738:Leptin; D008055:Lipids; D010750:Phosphoproteins; D044105:Receptor, Melanocortin, Type 4; D014280:Triglycerides; D064429:Fatty Acid Synthases