Further studies on the covalent crosslinking of thyrotropin to its receptor: evidence that both the alpha and beta subunits of thyrotropin are crosslinked to the receptor

The thyrotropin receptor

This chapter has outlined the complex process required for thyroid growth and function. Both events are regulated by TSHR via a multiplicity of signals, with the aid of and requirement for a multiplicity of hormones that regulate the TSHR via receptor cross-talk: insulin, IGF-I, adrenergic receptors, and purinergic receptors. Cross-talk appears to regulate G-protein interactions or activities induced by TSH as well as TSHR gene expression. The TSHR structure and its mechanism of signal transduction is being rapidly unraveled in several laboratories, since the recent cloning of the receptor. In addition, the epitopes for autoantibodies against the receptor that can subvert the normal regulated synthesis and secretion of thyroid hormones, causing hyper- or hypofunction, have been defined. Studies of regulation of the TSHR minimal promotor have uncovered a better understanding of the mechanisms by which TSH regulates both growth and function of the thyroid cell. A key novel component of this phenomenon involves TSH AMP positive and negative regulation of the TSHR. Negative transcriptional regulation is a common feature of MHC class I genes in the thyroid. Subversion of negative regulation or too little negative regulation is suggested to result in autoimmune disease. Methimazole and iodide at autoregulatory levels may be important in reversing this process and returning thyroid function to normal. Their action appears to involve factors that react with the IREs on both the TSHR and the TG promoter. Too much negative regulation, as in the case of ras transformation, results in abnormal growth without function. TTF-1 is implicated as a critical autoregulatory component in both positive and negative regulation of the TSHR and appears to be the link between TSH, the TSHR, TSHR-mediated signals, TG and TPO biosynthesis, and thyroid hormone formation. Differentially regulated expression of the TSHR and TG by cAMP and insulin depend on differences in the specificity of the TTF-1 site, that is, the lack of Pax-8 interactions with the TSHR, and the IRE sites. Single-strand binding proteins will become important in determining how TSHR transcription is controlled mechanistically.

TSH binding site structures in human eye muscle fractions identified by using covalent-crosslinking

125I-labelled human TSH was crosslinked to the human thyroid and extraocular eye muscle membrane and cytosol fractions (which were obtained by centrifugation). Studying crosslinking of 125I-labelled TSH to the thyroid fractions, TSH binding sites' structures were demonstrated on the eye muscle membranes and in the cytosol fractions. The binding of 125I-labelled TSH was inhibited by the addition of 120 mIU/mL of unlabelled TSH (and not with 12 mIU/mL) which confirmed the presence of TSH binding sites structures (MW about 66,000 Da) on the eye muscle membrane and in its cytosol. Adding purified IgG fractions from the sera from controls and Graves' disease (with high titer of antibodies against TSH receptor) to the thyroid and eye muscle membranes and cytosol fractions, the binding of 125I-labelled human TSH was inhibited by molecular weight of about 66,000 Da in the cytosol fractions. The affinity constant of the binding sites in the human eye muscle cytosol and the number of TSH receptors were found to be 146 x 10(9) M-1 and 9.8 x 10(10) molecules/mg/mL by Scatchard analysis, respectively.

The thyroid stimulating hormone receptor in human disease

The initial step in the action of thyrotropin (TSH) is its binding to the TSH receptor. TSH receptor antibodies are detected in up to 90% of patients with Graves' disease. Serial measurements of TSH receptor antibodies in patients with Graves' hyperthyroidism are helpful in predicting relapse. The TSH receptor was purified using affinity chromatography on wheat germ lectin agarose and TSH-agarose. Using an immunoblotting technique to characterize the TSH receptor, it was found to be an oligomeric glycoprotein consisting of three noncovalently bound subunits of Mr approximately 70,000, approximately 50,000 and approximately 35,000 which on reduction yield a single subunit of Mr approximately 25,000.