DEFECTIVE THYROTROPIN RECEPTOR G-PROTEIN CYCLIC ADENOSINE-MONOPHOSPHATE SIGNALING MECHANISM IN THE FTC HUMAN FOLLICULAR THYROID-CANCER CELL-LINE

Background. Several studies report the effect of thyrotropin (thyroid- stimulating hormone [TSH]) on weakly (FTC-l33) and aggressively invasi ve (FTC-238) clones of a human follicular thyroid cancer cell line. Sp ecifically, TSH induces fibronectin secretion by FTC-133 possibly as a result of increased cyclic adenosine monophosphate (cAMP), yet induce s 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 runs analyzed by Northern blot and densitometry. cAMP l evels were determined by an enzyme immunonssay. Gs alpha expression wa s determined by Western blot and a possible activating mutation at pos ition 201 in Gs alpha sought by direct sequencing. Results. Reverse tr anscriptase polymerase chain reaction confirmed the presence of TSH-R mRNA in FTC-133 and FTC-238. TSH did not increase transcription of fib ronectin 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 (m ean +/- SD; p < 0.001, Mann-Whitney rank-sum test), TSH did not raise cAMP levels in either clone. Gs alpha expression is equal in both cell lines and is not increased by TSH; sequencing showed no position 201 mutations in Gs alpha. Conclusions. Prototypical TSH-Gs-cAMPP signal t ransduction is not functional in FTC-133 or FTC 238. Our findings impl icate perturbation in TSH-R.