Defective thyrotropin receptor G-protein cyclic adenosine monophosphate signaling mechanism in the FTC human follicular thyroid cancer cell line

Abstract

Background. Several studies report the effect of thyrotropin (thyroid-stimulating hormone [TSH]) on weakly (FTC-133) and aggressively invasive (FTC-238) clones of a human follicular thyroid cancer cell line. Specifically, TSH induces fibronectin secretion by FTC-133, possibly as a result of increased cyclic adenosine monophosphate (cAMP), yet induces in vitro invasion through a protein kinase C-dependent mechanism. In normal thyrocytes, TSH activates cAMP through a stimulatory G-protein (Gs)-linked pathway. In the FTC model we studied the effect of TSH on adenylate cyclase activation. Methods. TSH receptor (TSH-R) mRNA was studied by reverse transciptase polymerase chain reaction. Fibronectin transcription was analyzed by Northern blot and densitometry. cAMP levels were determined by an enzyme immunoassay. Gs α. expression was determined by Western blot and a possible activating mutation at position 201 in Gs α sought by direct sequencing. Results. Reverse transcriptase polymerase chain reaction confirmed the presence of TSH-R mRNA in FTC-133 and FTC-238. TSH did not increase transcription of fibronectin mRNA. FTC-133 cells exhibited higher cAMP levels than did FTC-238 cells: 30.4 ± 8.0 versus 13.0 ± 3.5 femtomoles/10 4 cells (mean ± SD; p < 0.001, Mann-Whitney rank-sum test). TSH did not raise cAMP levels in either clone. Gs α expression is equal in both cell lines and is not increased by TSH; sequencing showed no position 201 mutations in Gs α. Conclusions. Prototypical TSH-Gs-cAMP signal transduction is not functional in FTC-133 or FTC-238. Our findings implicate perturbation in TSH-R.