Increases in cytosolic Ca ++ down regulate thyrotropin receptor gene expression by a mechanism different from the cAMP signal

Abstract

Thyrotropin (TSH) receptor mRNA levels in rat FRTL-5 thyroid cells are decreased by treatment with the calcium ionophores, A23187 or ionomycin, as well as with TSH, choleratoxin, forskolin, and 8-bromo-cAMP. Down regulation is, in each case, associated with a decrease in [ 125I]TSH binding and a decreased ability of TSH to increase cAMP levels. The ionophore does not alter cAMP levels and ethylene glycol-bis-(β-aminoethyl ether) N, N′-tetraacetic acid (EGTA) in the medium prevents down regulation of TSH receptor mRNA levels by the ionophore, but not by TSH; the EGTA action is reversed by the simultaneous addition of Ca ++. Whereas down regulation by TSH and its cAMP signal requires the presence of insulin and/or serum in the medium; down regulation by a calcium ionophore is still evident in their absence. Down regulation of TSH receptor mRNA levels and receptor desensitization by TSH cAMP or an ionophore is lost in cells transfected with a full length TSH receptor cDNA devoid of regulatory elements, but able to reconstitute TSH receptor signal generation.