Adipocyte MTERF4 regulates non-shivering adaptive thermogenesis and sympathetic-dependent glucose homeostasis

In humans, low brown adipose tissue (BAT) mass and activity have been associated with increased adiposity and fasting glucose levels, suggesting that defective BAT-dependent thermogenesis could contribute to the development of obesity and/or type 2 diabetes. The thermogenic function of BAT relies on a vast network of mitochondria exclusively equipped with UCP1. Mitochondrial biogenesis is exquisitely regulated by a well-defined network of transcription factors that coordinate the expression of nuclear genes required for the formation of functional mitochondria. However, less is known about the mitochondrial factors that control the expression of the genes encoded by the mitochondrial genome. Here, we have studied the role of mitochondrial transcription termination factor-4 (MTERF4) in BAT by using a new mouse model devoid of MTERF4 specifically in adipocytes (MTERF4-FAT-KO mice). Lack of MTERF4 in BAT leads to reduced OxPhos mitochondrial protein levels and impaired assembly of OxPhos complexes I, III and IV due to deficient translation of mtDNA-encoded proteins. As a result, brown adipocytes lacking MTERF4 exhibit impaired respiratory capacity. MTERF4-FAT-KO mice show a blunted thermogenic response and are unable to maintain body temperature when exposed to cold. Despite impaired BAT function, MTERF4-FAT-KO mice do not develop obesity or insulin resistance. Still, MTERF4-FAT-KO mice became resistant to the insulin-sensitizing effects of β₃-specific adrenergic receptor agonists. Our results demonstrate that MTERF4 regulates mitochondrial protein translation and is essential for proper BAT thermogenic activity. Our study also supports the notion that pharmacological activation of BAT is a plausible therapeutic target for the treatment of insulin resistance.