CLONING AND FUNCTIONAL EXPRESSION OF A THYROTROPIN RECEPTOR CDNA FROM RAT FAT-CELLS

Thyrotropin receptor (TSH-R) has been thought to be thyroid-specific, but, by Northern blot analysis, we found that rat adipose tissue expressed TSH-R mRNAs in amounts approaching those in the thyroid, To investigate the function of TSH-R from adipose tissue, we screened a rat fat cell lambda gt11 cDNA library for TSH-R sequences using a P-32-labeled rat thyroid TSH-R cDNA as a probe, Among 10(6) plaques, we obtained four positive clones, Sequencing of these cDNAs has revealed that two of them (F alpha and F beta) contained both initiation and termination codons, Comparison of F alpha with the thyroid TSH-R cDNA sequence revealed that F alpha was almost identical to the thyroid TSH-R, except that nucleotides 1041 and 1277 were changed from A to G and from C to T, respectively, In contrast, we found that F beta contained 21 novel nucleotides between nucleotides 467 and 468 of the thyroid TSH-R cDNA, encoding an additional 7 amino acids, However, when we prepared mRNA from adipose tissue and transcribed it into cDNA, we failed to amplify the F beta type of TSH-R cDNA by polymerase chain reaction, suggesting that F beta mRNAs are rare in the tissue, We then ligated F cDNAs into pSG5 and transfected them with pSV(2)-neo into Chinese hamster ovary (CHO)-K1 cells. TSH stimulated cAMP formation in CHO-F alpha cells in a manner similar to that in CHO cells transfected with thyroid TSH-R cDNA, In contrast, no increase of cAMP was observed in CHO-F beta cells, IgG from patients with Graves' disease (n = 4) showed thyroid stimulating antibody activity only in CHO-F alpha cells (1288-4582%), In addition, CHO-F alpha cells and CHO cells transfected with thyroid TSH-R showed similar I-125-TSH binding activity, These results indicate that the fat cell expresses high levels of a TSH-R whose function is indistinguishable from that in the thyroid and suggest that the TSH-R autoantibody plays an important role in the pathogenesis of the extrathyroidal manifestations of Graves' disease.