New Ideas in Thyroxine-Binding Globulin Biology

Jacob Robbins

Genetics and Biochemistry Branch National Institute of Diabetes and Digestive and Kidney Diseases National Institutes of Health Bethesda, Maryland 20892-1587

Address correspondence and requests for reprints to: Jacob Robbins, M.D., Genetics and Biochemistry Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, 9000 Rockville Pike, Building 10, Room 6C 201A, Bethesda, Maryland 20892-1587.


    Introduction
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 Introduction
 References
 
For a long time, T4-binding globulin (TBG) has been more interesting as a protein molecule with a strong and specific affinity for the hormone rather than as an active participant in physiological or pathological processes. Its function as the principal carrier of thyroid hormone in human plasma has been relegated to the unexciting role of a buffer and its clinical importance to effects of inherited defects and other events that influence its ability to bind hormone and, thus, result in altered or misleading tests of thyroid function (1). Because repeated attempts to obtain TBG crystals suitable for x-ray diffraction analysis met with failure, interest in further exploration of its molecular properties lagged until its DNA was cloned (2). This led to the surprising discovery that TBG is a member of the large and ubiquitous SERPIN family. Many SERPINs are serine protease inhibitors, whence the family name is derived, with a great variety of important physiological functions. Among them are antithrombin, {alpha}-1 antitrypsin, and plasminogen activator inhibitors (3). SERPINs constitute as much as 10% of the proteins in human plasma. Their distinctive property is that they are converted from a stressed to a relaxed form when a specific peptide bond is cleaved by the serine protease. This results in release of a small, C-terminal peptide loop and a structural change in the remainder of the molecule that causes it to bind tightly to the protease and inhibit its further proteolytic action (4).

Other members of the SERPIN family, while sharing many molecular properties, lack protease inhibitory activity. Among them are the abundant egg protein, ovalbumin, and the much less abundant plasma proteins TBG and its sister hormone-binding protein, CBG, corticosteroid binding globulin (5). A molecular property that these proteins share is the structural response to limited proteolysis, and in the case of the hormone transporters, the obvious question is whether the change in structure can affect hormone binding. Pemberton et al. (6) found that exposure to elastase increased the thermal stability of both TBG and CBG, but affinity for the respective hormone was decreased only in the case of CBG. Modeling of the molecular structure of the cleaved TBG, based on its homology with {alpha}-1 antitrypsin (7, 8), then led to a detailed definition of the T4 binding site, an achievement that had not been possible by conventional methods.

In this issue of JCEM, Jirasakuldech et al. (9) now present convincing evidence that exposure of TBG to pancreatic elastase or to activated polymorphonuclear leukocytes does, in fact, bring about a major decrease in its affinity for T4. A similar result was reported earlier in abstracts published by the same group (10) and from two other laboratories (11, 12) using three different methods: dialysis, resin uptake, and ANS (analinonaphthalene sulfonic acid) competition. Schussler’s group also demonstrated that the limited proteolysis was more than simply an in vitro phenomenon. By studying patients afflicted by bacterial sepsis, they were also able to show the presence of the cleaved TBG molecule in serum by virtue of its altered size but persistent binding to anti-TBG immunoglobulin (9).

In the case of CBG, it is easy to imagine that release of cortisol at the site of an inflammatory process might have an important pathophysiological role. Hammond et al. (13, 14) showed that leukocytes activated by inflammation caused hormone release from CBG and that this event takes place at the leukocyte plasma membrane. Cortisol of course has known anti-inflammatory action. The potential role of increased T4 or T3 release from TBG is less obvious. Over 30 yr ago, Klebanoff (15) explored the antibacterial effect of iodine in a myeloperoxidase system. Although iodide was 200 times more effective than chloride, the much larger available concentration of chloride made the role of iodide less attractive. Subsequent experiments by Klebanoff and Green (16), however, demonstrated that activated leukocytes rapidly degrade T4 and T3 and greatly increase the iodide concentration within the cell. Others demonstrated an accelerated disappearance of T4 and T3 from plasma in pneumococcal infection in man (17) and in monkeys (18). It is tempting to postulate, as Jirasakuldech et al. (9) have done, that localized release of T4 from TBG by limited proteolysis might play a role in pathological, and even in physiological, processes. Very rough calculations suggest that release in a few hours of about half of the bound T4 in a relatively small volume of infected tissue might, if leukocytes can deiodinate T4 quickly, provide enough iodide and iodine for significant bactericidal activity. An important corollary is that this release would take place at specific anatomical sites in response to some initiating event, such as an infection or an inflammatory response to trauma. (See Ref. 19 in their report.) The authors also raise the interesting possibility that this phenomenon may explain the increased free to bound T4 ratio and other abnormalities seen in nonthyroidal illness, and the rapid fall in serum T4 that may occur during acute inflammation [their Refs. 26 and 20, and work from their own laboratory (Ref. 11 , still in press)].

A side issue in their experiments is the finding of a large band of protein with an apparent molecular mass of 27 kDa in immunoblotting of SDS-PAGE-separated proteins from both normal and sepsis serum. Their interpretation that this probably is due to reaction of an impure anti-TBG antiserum with apolipoprotein A-1 should alert readers to the need for more pure antisera when performing immunoassay of TBG in serum. A significant achievement in their experiments with sepsis serum, however, is the demonstration of a sizable amount of cleaved TBG with mass 49–50 kDa. Because the cleaved remnants of other SERPINs have been found to have new and unexpected biological activities (their Refs. 6, 24, and 25)—witness the antiangiogenic and tumor suppressive activity recently demonstrated for cleaved antithrombin III (19)—this raises the tantalizing possibility that TBG may also possess hidden biological effects.

Further speculation on potential pathophysiological effects of limited TBG proteolysis has been presented by Schussler in a recent review (20). These interesting experiments and ideas should serve to rescue TBG from the doldrums in the foreseeable future.

Received September 11, 2000.

Accepted September 13, 2000.


    References
 Top
 Introduction
 References
 

  1. Robbins J. 2000 Thyroid hormone transport proteins and the physiology of hormone binding. In: Braverman LE, Utiger RD, eds. The thyroid, ed 8. Philadelphia: Lippincott-Williams and Wilkins; 105–120.
  2. Flink IL, Bailey TJ, Gustafson A, et al. 1986 Complete amino acid sequence of human thyroxine-binding globulin deduced from cloned DNA: close homology to the serine antiproteases. Proc Natl Acad Sci USA. 83:7708–7712.[Abstract]
  3. Carrell RW, Pemberton PA, Boswell DR. 1987 The SERPINs: evolution and adaptation in a family of protease inhibitors. Cold Spring Harbor Symp Quant Biol. 52:527–535.[Medline]
  4. Huber R, Carrell RW. 1989 Implications of the three-dimensional structure of {alpha}-1 antitrypsin for structure and function of SERPINs. Biochemistry. 28:8951–8966.[Medline]
  5. Hammond GL, Smith CL, Goping IS, et al. 1987 Primary structure of human corticosteroid binding globulin deduced from hepatic and pulmonary cDNAs exhibits homology with serine protease inhibitors. Proc Natl Acad Sci USA. 84:5153.[Abstract]
  6. Pemberton PA, Stein PE, Pepys MB, et al. 1988 Hormone binding globulins undergo serpin conformational change in inflammation. Nature. 336:257–258.[CrossRef][Medline]
  7. Jarvis JA, Munro SLA, Craik DJ. 1992 Homology model of thyroxine-binding globulin and elucidation of the thyroid hormone binding site. Protein Eng. 5:61–67.[Abstract]
  8. Terry CJ, Blake CF. 1992 Comparison of the modeled thyroxine binding site in TBG with the experimentally determined site in transthyretin. Protein Eng. 5:505–510.[Abstract]
  9. Jirasakuldech B, Schussler GC, Yap MG, et al. 2000 A characteristic serpin cleavage product of thyroxine binding globulin appears in sepsis sera. J Clin Endocrinol Metab. 85:3996–3999.[Abstract/Free Full Text]
  10. Schussler GC, Yap MG, Josephson A, Drew H. Proteolysis of thyroxine binding globulin by human polymorphonuclear leulocytes releases thyroxine, a mechanism for site specific delivery. Proc of the 71st Annual Meeting of the American Thyroid Association, Portland, OR, 1998; p 114.
  11. Janssen OE, Golcher HM, Treske B, Heufleder AE. Characterization of thyroxine-binding globulin digested with human elastase. Proc of Meeting of the International Endocrine Society, San Francisco, CA, 1996; p 1012.
  12. Suda SA, Gettins PGW, Patston PA. Changes in the hormone binding site of thyroxine binding globulin. Proc of 2nd Internatl Symp Struct and Biol of the SERPINs, Cambridge, UK, 1999 (http://smokeroom.cimr.cam.ac.uk).
  13. Hammond GL, Smith CL, Underhill CM, Nguyen VTT. 1990 Interaction between corticosteroid binding globulin and activated leukocytes in vitro. Biochem Biophys Res Commun. 172:172–177.[Medline]
  14. Hammond GL, Smith CL, Paterson NAM, Sibbald WJ. 1990 A role for corticosteroid-binding globulin in delivery of cortisol to activated neutrophils. J Clin Endocrinol Metab. 71:34–39.[Abstract]
  15. Klebanoff SJ. 1968. Myeloperoxidase-halide-hydrogen peroxide antibacterial system. J Bacteriol. 95:2131–2138.
  16. Klebanoff SJ, Green WL. 1973 Degradation of thyroid hormones by phagocytosing human leukocytes. J Clin Invest. 52:60–62.[Medline]
  17. Gregerman RI, Solomon N. 1967 Acceleration of thyroxine and triiodothyronine turnover during bacterial pulmonary infections and fever: implications for the functional state of the thyroid during stress and senescence. J Clin Endocrinol. 27:93–105.[Medline]
  18. DeRubertis FR, Woeber KA. 1972 Evidence for enhanced cellular uptake and binding of thyroxine in vivo during acute infection with Diplococcus pneumoniae. J Clin Invest. 51:788–795.[Medline]
  19. O’Reilly MS, Pirie-Shepherd S, Lane WS, Folkman J. 1999 Antiangiogenic activity of the cleaved conformation of the serpin antithrombin. Science. 285:1926–1928.[Abstract/Free Full Text]
  20. Schussler GC. 2000 The thyroxine-binding proteins. Thyroid 10:141–149.




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