Stimulation of Phospholipid Methylation and Thyroid Hormone Secretion by Thyrotropin

Synthesis of phosphatidylcholine (PC) by Sadenosylmethionine-dependent methylation of phosphatidylethanolamine has previously been associated with receptor-mediated histamine and pituitary hormone secretion. We investigated stimulation of phospholipid methylation by TSH and its possible role in thyroid hormone secretion. Rat hemithyroids were incubated in Krebs-Henseleit-glucose-BSA buffer and the effect of various treatments on the incorporation of [3H-methyl]L-methionine into PC and T4/T3 secretion was studied. TSH treatment elevated thyroid phosphatidylethanolamine methyltransferase activity, the incorporation of [3H-methyl]L-methionine into PC, and T4/T3 secretion. The increase in PC synthesis was linear up to 6 h in a dose-dependent fashion (half-maximal stimulation at 2.5 ng TSH/ml). Stimulation required protein synthesis, because cycloheximide inhibited the increasee in PC synthesis by 77%. Inhibitors of phospholipid methylation (100 nM adenosine + 10 μM L-homocysteine thiolactone + 10 μM erythro-9...