Endocrine Abstracts

The endogenous decarboxylated thyroid hormone (TH) metabolite 3-Iodothyronamine (3-T1AM) exerts partially TH antagonistic effects in rodents and was shown to suppress the hypothalamic pituitary thyroid axis in rats after single dose application. Therefore, it might play a role in maintaining TH homoeostasis. Among the molecular targets of 3-T1AM are G protein-coupled receptors like the trace amine associated receptor 1 (TAAR1) and adrenergic receptors. Furthermore, 3-T1AM was shown to activate the cold-sensitive transient receptor potential melastatin 8 channel (TRPM8). By acting on these target structures 3-T1AM modulates cAMP and Ca²⁺ signaling in several cell lines in vitro. We recently obtained experimental evidence that 3-T1AM (1 μM for 1 – 3 h) interferes with the TH synthesis machinery and energy metabolism in the rat thyrocyte cell line PCCL3. To elucidate the mechanism underlying these effects we analyzed the activation of signaling pathways by 3-T1AM and well established receptor ligands in PCCL3 cells. Levels of cAMP were measured with a competitive assay and cytosolic free Ca²⁺ was analyzed with the calcium indicator fura-2/AM and fluorescence imaging. 3-T1AM (0.1 – 1 μM) incubation did neither modify basal nor TSH-stimulated cAMP concentration. Incubation of PCCL3 cells with phenylethylamine or norepinephrine/isoprenaline, endogenous ligands of TAAR1 and adrenergic receptors, respectively, left cAMP unaltered indicating absent expression or functional inertness of the receptors to these agents. In contrast, 3-T1AM induced a strong increase in cytosolic free calcium in PCCL3 cells within minutes, which was abolished by co-incubation with the TRPM8 blocker BCTC. In conclusion, Ca²⁺ signaling via TRPM8 rather than TAAR1- or adrenergic receptor-dependent cAMP signaling is considered as a candidate mechanism that might mediate the metabolic and functional effects of 3-T1AM in PCCL3 cells.