Obesity and hyperleptinemia in metallothionein (-I and -II) null mice

Metallothionein (MT) has several putative roles in metal detoxification, in Zn and Cu homeostasis, in scavenging free radicals, and in the acute phase response. Mice of mixed 129/Ola and C57BL/6J background with targeted disruption of MT-I and MT-II genes are more sensitive to toxic metals and oxidative stress. We noted that these animals were larger than most strains of mice, and we systematically studied aspects of their physiology and biochemistry relating to energy metabolism. During the first 2 weeks after weaning, the growth rates of MT-null and C57BL/6J mice were similar, but the transgenic mice became significantly heavier at age 5-6 weeks. At age 14 weeks, the body weight and food intake of MT-null mice was 16 and 30% higher, respectively, compared with C57BL/6J mice. Most 22- to 39-week-old male MT-null mice were obese, as shown by increased fat accretion, elevated obese (ob) gene expression, and high plasma leptin levels, similar to those recorded in Zucker fatty (fa/fa) rats. Seven-week-old MT-null mice also had significantly higher levels of plasma leptin and elevated expression of ob, lipoprotein lipase, and CCAAT enhancer binding protein alpha genes as compared with age-matched C57BL/6J mice. These observations indicate that abnormal accretion of body fat and adipocyte maturation is initiated at 5-7 weeks of age, possibly coincident with sexual maturation. Targeted disruption of MT-I and MT-II genes seems to induce moderate obesity, providing a new obese animal model. A link between MT and the regulation of energy balance is implied.