Functional and metabolic responses of thyrocytes to 3-Iodothyronamine (3-T1AM)

3-T₁AM, a decarboxylated thyroid hormone (TH) metabolite, is an endogenous iodinated compound detectable in human serum and most likely formed extrathyroidally. In rodents, pharmacological doses of 3-T₁AM administered i.p. rapidly lead to remarkable effects that are partially contrary to the classic effects of excess T3 e.g. bradycardia in mice, decreased metabolic rate and body temperature in mice and djungarian hamsters. Furthermore, 3-T₁AM (i.p.) decreased plasma concentration of TSH and T3 in rats, indicating its interference with the HPT axis. In vivo effects of 3-T₁AM suggest its involvement in fine tuning or antagonising TH action which led us to hypothesise 3-T₁AM mediated regulation at the thyroid gland level. Therefore, thyrocyte-specific functional and metabolic responses to 3-T₁AM were analysed. To this aim 3-T₁AM action on two highly differentiated thyrocyte cell lines was tested in vitro, firstly the human XTC.UC1 cell line which is derived from a Hürthle cell carcinoma and secondly the PCCl3 cell line which is derived from normal thyroid epithelium of Fisher rats. Uptake and metabolism of 3-T₁AM by XTC.UC1 cells was determined by LC-MS/MS measurement after liquid-liquid extraction of 3-T₁AM and its metabolites from cell lysates. An increase in 3-T₁AM and its deiodinated metabolite T₀AM was observed over time in lysates of cells incubated with 1µM 3-T₁AM. Effects on mitochondrial respiration were measured with the Seahorse Extracellular Flux Analyzer. A slight and dose dependent decrease of the spare respiratory capacity of XTC.UC1 cells was observed after acute treatment with 1 to 1000 nM 3-T₁AM. In PCCl3 cells mRNA levels of genes relevant for TH synthesis were decreased after 6h of incubation with 10 and 100 nM 3-T₁AM as determined by qPCR. In conclusion, experimental evidence is presented for cellular uptake and metabolism of 3-T₁AM, as well as mitochondrial action and influence on expression of genes contributing to TH synthesis.

Supported by DFG SPP 1629 Thyroid Trans Act.