Loss of μ-crystallin causes PPARγ activation and obesity in high-fat diet-fed mice

The thyroid hormone-binding protein μ-crystallin (CRYM) mediates thyroid hormone action by sequestering triiodothyronine in the cytoplasm and regulating the intracellular concentration of thyroid hormone. As thyroid hormone action is closely associated with glycolipid metabolism, it has been proposed that CRYM may contribute to this process by reserving or releasing triiodothyronine in the cytoplasm. We aimed to clarify the relationship between CRYM and glycolipid metabolism by comparing wild-type and CRYM knockout mice fed a high-fat diet. Each group was provided a high-fat diet for 10 weeks, and then their body weight and fasting blood glucose levels were measured. Although no difference in body weight was observed between the two groups with normal diet, the treatment with a high-fat diet was found to induce obesity in the knockout mice. The knockout group displayed increased dietary intake, white adipose tissue, fat cell hypertrophy, and hyperglycemia in the intraperitoneal glucose tolerance test. In CRYM knockout mice, liver fat deposits were more pronounced than in the control group. Enhanced levels of PPARγ, which is known to cause fatty liver, and ACC1, which is a target gene for thyroid hormone and is involved in the fat synthesis, were also detected in the livers of CRYM knockout mice. These observations suggest that CRYM deficiency leads to obesity and lipogenesis, possibly in part through increasing the food intake of mice fed a high-fat diet.