Thyroid-Hormone-Induced Browning of White Adipose Tissue Does Not Contribute to Thermogenesis and Glucose Consumption

Abstract

Summary: Regulation of body temperature critically depends on thyroid hormone (TH). Recent studies revealed that TH induces browning of white adipose tissue, possibly contributing to the observed hyperthermia in hyperthyroid patients and potentially providing metabolic benefits. Here, we show that browning by TH requires TH-receptor β and occurs independently of the sympathetic nervous system. The beige fat, however, lacks sufficient adrenergic stimulation and is not metabolically activated despite high levels of uncoupling protein 1 (UCP1). Studies at different environmental temperatures reveal that TH instead causes hyperthermia by actions in skeletal muscle combined with a central body temperature set-point elevation. Consequently, the metabolic and thermogenic effects of systemic hyperthyroidism were maintained in UCP1 knockout mice, demonstrating that neither beige nor brown fat contributes to the TH-induced hyperthermia and elevated glucose consumption, and underlining that the mere presence of UCP1 is insufficient to draw conclusions on the therapeutic potential of browning agents. : Thyroid hormone induces browning of white fat, but it is unclear whether this contributes to thermogenesis. Here, Johann et al. show that thyroid-hormone-induced beige fat is metabolically inactive due to lack of central stimulation and that the metabolic and thermogenic effects of the hormone are independent of UCP1. Keywords: brown adipose tissue, beige adipose tissue, uncoupling protein 1, thyroid hormone receptor, body temperature, norepinephrine, sympathetic nervous system, metabolism, beta3-adrenergic receptor, pyrexia, hyperthermia, glucose tolerance