A metabolic defect promotes obesity in mice lacking melanocortin-4 receptors

Abstract

Melanocortin-4 receptor (Mc4r)-null mice exhibit late-onset obesity. To determine whether aberrant metabolism contributes to the obesity, food consumption by Mc4r-null mice was restricted to (pair-fed to) that consumed by wild-type (WT) mice. Pair-fed Mc4r-null females maintained body weights intermediate to that of WT and nonpair-fed Mc4r-null females, whereas pairfeeding normalized the body weights of Mc4r-null male mice. Fat pad and circulating leptin levels were elevated in both male and female pair-fed Mc4r-null mice compared with WT mice. Oxygen consumption of Mc4r-null mice with similar body weights as WT controls was reduced by 20%. Locomotor activity of young nonobese Mc4r-null males was significantly lower than that of WT males; however, locomotion of young nonobese females was normal. Core body temperature of Mc4r-null mice was normal, and they responded normally to cold exposure. Young nonobese Mc4r-null females were unable to induce uncoupling protein 1 (UCP1) in brown adipose tissue in response to peripheral leptin administration, whereas UCP1 mRNA was increased by 60% in the WT females. These results indicate that Mc4r deficiency enhances caloric efficiency, similar to that seen in the agouti obesity syndrome and in melanocortin-3 receptor-null mice.

Figure 1

Growth curves. (A) Weekly body weights of 12 WT, 14 PF Mc4r-null, and nine NPF Mc4r-null females. *, First week that body weight differed significantly between WT and NPF females (P < 0.05). #, First week that body weight differed significantly between WT and PF females (P < 0.05). +, First week that body weight differed significantly between PF and NPF females (P < 0.05). (B) Weekly body weights of 14 WT, nine PF Mc4r-null, and 12 NPF Mc4r-null males. †, First week that body weight of NPF Mc4r-null males differed significantly from WT or PF Mc4r-null males (P < 0.05).

Figure 2

Fat pad measurements. Adipose tissue from four fat depots (retroperitoneal, inguinal, reproductive, and scapular) were dissected and weighed. (A) Fat pad measurements from 12 WT, 14 PF Mc4r-null, and 10 NPF Mc4r-null female mice. (B) Fat pad measurements from14 WT, eight PF Mc4r-null, and 13 NPF Mc4r-null male mice. *, Significant difference from WT mice (P < 0.001). #, Significant difference between NPF Mc4r-null and PF mice (P < 0.001).

Figure 3

Food consumption. Food consumption was measured for 5 days and reported as a daily average. Measurements were made at the first age when body weights were significantly different and at 14 wk. (A) Five WT and five NPF Mc4r-null females. (B) Five WT and five NPF Mc4r-null males. *, Significant difference from WT mice (P < 0.05).

Figure 4

Ambulatory activity of Mc4r-null mice. Data are presented as meters traveled. The locomotion during the first hour represents activity in a novel environment; locomotion during 12-h light and dark cycles was recorded 24 h later. (A) Ambulatory activity of 4- to 8-wk-old WT (n = 10) and Mc4r-null (n = 11) females. (B) Ambulatory activity of 4- to 8-wk-old WT (n = 13) and Mc4r-null (n = 14) males. *, Significant difference between WT and Mc4r-null (P < 0.05).

Figure 5

Core body temperature and cold responsiveness. Core body temperature of individually housed mice was measured before and at 1, 4, 8, and 24 h after placing mice at 4°C. (A) Eight WT and seven Mc4r-null female mice. (B) Six WT and six Mc4r-null male mice.

Figure 6

Oxygen consumption. Mice (7.5–8.5 wk old) were allowed to acclimate in the testing chamber for 2.5 h, then oxygen consumption was measured for 2 h at the same time each day (≈12:00–14:00 h). Total consumption was calculated for each mouse as indicated in Materials and Methods. The data represent the means from six trials with four WT mice and seven trials with four Mc4r-null mice. *, Significant difference between WT and NPF Mc4r-null mice (P < 0.003).

Figure 7

Effect of leptin on UCP1 mRNA in BAT. Female WT (n = 6) and Mc4r-null (n = 6) mice were housed individually for 1 wk before treatment with leptin. The mice were injected daily with 20 μg/g body weight leptin or an equivalent volume of PBS vehicle for 3 days. Solution hybridization was used to measure UCP1 mRNA levels relative to total nucleic acid content. *, Significant increase relative to vehicle (P < 0.05).