Thyroid hormones, including 3,5,3'-triiodothyronine (T3), cause a wide spectrum of genomic effects on cellular metabolism and bioenergetic regulation in various tissues. The non-genomic actions of T3 have been reported but are not yet completely understood. Acute T3 treatment significantly enhanced basal, maximal, ATP-linked, and proton-leak oxygen consumption rates (OCRs) of primary differentiated mouse brown adipocytes accompanied with increased protein abundances of uncoupling protein 1 (UCP1) and mitochondrial Ca2+ uniporter (MCU). T3 treatment depolarized the resting mitochondrial membrane potential (Ψm) but augmented oligomycin-induced hyperpolarization in brown adipocytes. Protein kinase B (AKT) and mammalian target of rapamycin (mTOR) were activated by T3, leading to the inhibition of autophagic degradation. Rapamycin, as an mTOR inhibitor, blocked T3-induced autophagic suppression and UCP1 upregulation. T3 increases intracellular Ca2+ concentration ([Ca2+]i) in brown adipocytes. Most of the T3 effects, including mTOR activation, UCP1 upregulation, and OCR increase, were abrogated by intracellular Ca2+ chelation with BAPTA-AM. Calmodulin inhibition with W7 or knockdown of MCU dampened T3-induced mitochondrial activation. Furthermore, edelfosine, a phospholipase C (PLC) inhibitor, prevented T3 from acting on [Ca2+]i, UCP1 abundance, Ψm, and OCR. We suggest that short-term exposure of T3 induces UCP1 upregulation and mitochondrial activation due to PLC-mediated [Ca2+]i elevation in brown adipocytes.
Keywords: Ca2+ signaling; brown adipose tissue (BAT); mitochondria; thyroid hormone; uncoupling protein 1 (UCP1).