Clearance of normal and tumor-related thyroglobulin from the circulation of rats: role of the terminal sialic acid residues

Effect of Thyroglobulin Autoantibodies on the Metabolic Clearance of Serum Thyroglobulin

BACKGROUND

In order to establish whether thyroglobulin autoantibodies (TgAb) influence the metabolic clearance of thyroglobulin (Tg) in humans, serum Tg and TgAb were correlated shortly after radioiodine (¹³¹I) treatment.

METHODS

Samples were collected from 30 consecutive patients undergoing ¹³¹I activity for Graves' hyperthyroidism at the time of treatment and every 15 days thereafter, up to 90 days. Tg and TgAb were measured by immunometric assays (functional sensitivities: 0.1 ng/mL and 8 IU/mL).

RESULTS

Tg was detectable in all patients at day 0. Tg concentrations rose from a mean of 33.2 ng/mL [confidence interval (CI) 17.8-61.0 ng/mL] at day 0 to a mean of 214.6 ng/mL [CI 116.9-393.4 ng/mL] at day 30 and then steadily decreased, reaching the lowest concentration at day 90 (M = 10.9 ng/mL [CI 5.5-20.9 ng/mL]). Compared to their levels at day 0 (M = 23.6 IU/mL [CI 10.5-52.9 IU/mL]), TgAb remained stable through day 15 and then gradually increased up to a mean of 116.6 IU/mL [CI 51.9-262.2 IU/mL] at day 90. Patients were then split into two groups according to their TgAb status at day 0: undetectable (<8 IU/mL; 9 patients) or detectable (≥8 IU/mL; 21 patients) TgAb. Compared to the other cohort, patients with detectable TgAb showed significantly lower Tg concentrations at day 0 (M = 20.3 ng/mL [CI 10.1-40.2 ng/mL] vs. M = 101.8 ng/mL [CI 36.6-279.8 ng/mL]), similar at day 15, lower levels at day 30 (M = 146.5 ng/mL [CI 74.3-287.8 ng/mL] vs. M = 514.8 ng/mL [CI 187.8-1407.9 ng/mL]), at day 45 (M = 87.5 ng/mL [CI 43.1-176.6 ng/mL] vs. M = 337.9 ng/mL [CI 120.1-947.0 ng/mL]), at day 60 (M = 61.6 ng/mL [CI 31.0-121.4 ng/mL] vs. M = 255.8 ng/mL [CI 79.0-823.8 ng/mL]), and at day 75 (M = 24.5 ng/mL [CI 11.9-49.2 ng/mL] vs. M = 249.5 ng/mL [CI 63.5-971.1 ng/mL]), and similar levels at day 90. Patients with detectable TgAb showed a lower (M = 182.5 ng/mL [CI 92.0-361.0 ng/mL] vs. M = 514.8 ng/mL [CI 187.8-1407.9 ng/mL]) and an earlier (day 15 vs. day 30) peak of Tg. The mean Tg concentration was lower in patients with detectable TgAb than in those with undetectable TgAb (area under the curve: 17,340 ± 16,481 ng/mL vs. 36,883 ± 44,625 ng/mL; p = 0.02).

CONCLUSIONS

TgAb influence the changes in Tg concentrations observed immediately after ¹³¹I treatment, inducing lower levels and an earlier peak of Tg. These observations indicate that TgAb significantly influence the metabolic clearance of Tg, supporting the concept that their interference in the measurement of Tg is mainly due to an in vivo effect.

Significance of postoperative serum thyroglobulin levels in patients with papillary and follicular thyroid carcinomas

BACKGROUND AND OBJECTIVES

Although there are many factors that affect postoperative serum levels of thyroglobulin (Tg), such levels have been previously used to detect recurrence of papillary and follicular thyroid carcinomas. This study was conducted to elucidate the significance of postoperative levels of Tg in patients with clinical presentations of papillary thyroid carcinoma, follicular thyroid carcinomas, or both.

METHODS

To collect data pertaining to patients with thyroid cancer who were treated in Chang Gung Medical Center in Linkou, Taiwan, records relating to a total of 847 patients with pathologically verified papillary or follicular thyroid cancer, all of whom received total thyroidectomy and thyroid remnant ablation with radioactive iodide ((131)I), were studied. To evaluate the clinical significance of postoperative levels of Tg, the patients were categorized into three groups based on postoperative Tg level. Group A was classified as those demonstrating a 1-month postoperative Tg levels less than 1 ng/ml. Group B patients were classified as those displaying a 1-month postoperative Tg levels greater than or equal to 1 ng/ml, but less than 10 ng/ml. Group C patients were classified as those exhibiting a 1-month postoperative Tg levels great than or equal to 10 ng/ml.

RESULTS

Of the patients in group A, none presented with distant metastases at the time of diagnosis or during the follow-up period. In group B, 15 patients (3.5%) died of thyroid cancer. In this group, tumor size was an important factor in cancer-related mortality, diagnostic clinical class, and follow-up status. Of the 491 patients in group C, 49 (10.0%) patients died of thyroid cancer. Among the patients in group C, age, histopathologic type, stage of diagnosis, and follow-up Tg values were important factors. Among groups A, B, and C, there were 161 (95.8%), 253 (76.4%), and 129 (37.1%) patients, respectively, with disease-free status at the end of 1998.

CONCLUSIONS

Postoperative serum Tg levels can be used as a prognostic indicator in patients with papillary and follicular thyroid cancer. For patients with Tg levels greater than or equal to 10 ng/ml, Tg levels are a useful marker to predict prognosis.

Extrathyroidal release of thyroid hormones from thyroglobulin by J774 mouse macrophages

Thyroglobulin appears in the circulation of vertebrates at species-specific concentrations. We have observed that the clearance of thyroglobulin from the circulation occurs in the liver by macrophages. Here we show that the thyroid hormones T3 and T4 were released by incubation of mouse macrophages (J774) with thyroglobulin. Thyroid hormone release was a fast process, with an initial rate of approximately 20 pmol T4/mg per min and approximately 0.6 pmol T3/mg per min, indicating that macrophages preferentially release T4. The bulk of released thyroid hormones appeared after 5 min of incubation of macrophages with thyroglobulin, whereas degradation of the protein was detectable only after several hours. During internalization of thyroglobulin, endocytic vesicles and endosomes were reached at 5 min and lysosomes at 60 min. T4 release started extracellularly by secreted proteases and continued along the endocytic pathway of thyroglobulin, whereas T3 release occurred mainly intracellularly when thyroglobulin had reached the lysosomes. This shows that the release of both hormones occurred at distinct cellular sites. Our in vitro observations suggest that macrophages in situ represent an extrathyroidal source for thyroid hormones from circulating thyroglobulin.

Low-molecular-weight iodoproteins in the congenital goiters of cog/cog mice

Previously we described sedimentation and immunologic abnormalities of thyroglobulin (Tg) in a strain of mice with inherited congenital goiter and hypothyroidism (cog/cog). The goals of the present study were to determine the extent to which thyroid gland stimulation by TSH accounts for the abnormal properties of cog/cog Tg and to characterize further the abnormally small iodoproteins found in cog/cog mice. Cog/cog and control +/cog and BALB/c mice were fed with either normal or thyroid-hormone-containing diets and were injected with Na125I. Sucrose density gradient centrifugation of labeled thyroid extracts from cog/cog mice on normal diet showed that 82% of the iodine was in iodoproteins smaller than Tg, with sedimentation rates of 3-8S. No 12S and 19S peaks, characteristic of normal Tg, were present, but distinct and stable 12S and 19S peaks emerged after recentrifugation of the 12S and 19S areas. In contrast, in cog/cog mice treated with T4, a smaller (55%) amount of 3-8S iodoproteins and distinct 12S and 19S peaks were present. In both groups of mice, the labeled 3-8S iodoproteins were composed of three fractions: 15% precipitated by antirat Tg serum, 38% precipitated by antimouse albumin serum, and 47% not precipitated by either serum. The 3-8S iodoproteins contained labeled MIT and DIT and no T4. On sodium dodecyl sulfate polyacrylamide gel electrophoresis the 3-8S iodoprotein fraction that reacted with anti-Tg serum contained a distinct electrophoretic band at 49K. The 3-8S nonreactive iodoproteins resolved into several bands of lower molecular weight. We conclude that the 3-8S iodoproteins in cog/cog mice are heterogeneous and that TSH stimulation contributes to the production of these low-molecular-weight iodoproteins.

Cooperation between the polyomavirus middle-T-antigen gene and the human c-myc oncogene in a rat thyroid epithelial differentiated cell line: model of in vitro progression

Two rat thyroid epithelial differentiated cell lines, PC Cl 3 and PC myc, were infected with the polyoma murine leukemia virus (PyMLV) carrying the Middle-T-antigen gene of polyomavirus. After infection, both cell lines acquired the typical markers of neoplastic transformation; however, the PC myc cells showed a greater malignant phenotype. Furthermore, the thyroid differentiated functions were completely suppressed in PC myc cells transformed by PyMLV, whereas they were, at least partially, retained in PC Cl 3 cells transformed by PyMLV, and in particular, thyroglobulin synthesis and secretion were not affected at all. Since no differences in the expression of the middle-T-antigen gene were observed in the two PyMLV-transformed cell lines, the different properties shown by these two infected cell lines must be ascribed to the expression of the c-myc oncogene.

Cooperation between the polyomavirus middle-T-antigen gene and the human c-myc oncogene in a rat thyroid epithelial differentiated cell line: model of in vitro progression

Two rat thyroid epithelial differentiated cell lines, PC Cl 3 and PC myc, were infected with the polyoma murine leukemia virus (PyMLV) carrying the Middle-T-antigen gene of polyomavirus. After infection, both cell lines acquired the typical markers of neoplastic transformation; however, the PC myc cells showed a greater malignant phenotype. Furthermore, the thyroid differentiated functions were completely suppressed in PC myc cells transformed by PyMLV, whereas they were, at least partially, retained in PC Cl 3 cells transformed by PyMLV, and in particular, thyroglobulin synthesis and secretion were not affected at all. Since no differences in the expression of the middle-T-antigen gene were observed in the two PyMLV-transformed cell lines, the different properties shown by these two infected cell lines must be ascribed to the expression of the c-myc oncogene.

European interlaboratory comparison of serum thyroglobulin measurement

A European interlaboratory comparison of serum thyroglobulin measurements was performed after an initiative from the European Organization of Research and Therapy of Cancer. Fifty-two laboratories were addressed and 45 of these (83%) participated in the study by measuring serum thyroglobulin and its autoantibody in 5 thyroglobulin containing sera. Thyroglobulin antibodies were added to two of the sera. Two commercial kits were used by a large number of the laboratories (11 and 8, respectively). Each kit showed a reasonably low interlaboratory coefficient of variation at concentrations above 25 micrograms/l, but with discrepancy between the methods. The remaining miscellaneous methods (24) showed a variation above 65% in all samples. In all laboratories the addition of thyroglobulin antibodies resulted in false thyroglobulin measurements with either elevated or depressed levels. It is concluded that a reference calibrator for serum thyroglobulin is strongly needed as the first essential step towards interlaboratory standardization of serum thyroglobulin, thereby opening a possibility for multicentre studies of its value in the post-therapy follow-up of patients with differentiated thyroid carcinoma.

Degradation of circulating thyroglobulin

Reported half lives of rat Tg were different according to various investigators. In order to elucidate whether the derivatives of rat Tg in the peripheral circulation affect the results of kinetic studies of Tg, the present study was performed to investigate kinetics of rat Tg after separation of 19S Tg from its derivatives using gel-filtration. Radiolabeled Tg was obtained from thyroids of rats injected with 125I 24 hours before death, and subsequently purified by ammonium sulfate precipitation. The plasma samples obtained at varying time intervals after intravenous injection of 125I-rat Tg were fractionated on a Sephacryl S-300 column. As determined by sucrose density gradient, 99% of in vivo radiolabeled Tg was 19S. On gel-filtration, the injected labeled Tg and plasma samples obtained within two hours after injection showed a single peak in an identical area. A second peak in an area corresponding to a molecular weight of 60,000 to 70,000 appeared within six hours, and became as high as the first within 24 hours. In the second peak, 22.8 +/- 3.8% (mean +/- SE) of radioactivity was precipitated by anti-rat Tg antibody, and 14.4 +/- 1.7% (mean +/- SE) of radioactivity of its TCA precipitate was not extracted by n-butanol. Thus, the second peak could affect the results of Tg kinetic studies which utilize TCA precipitation, n-butanol extraction or RIA procedures. The half life of rat Tg in the present study was calculated from the disappearance curves of radioactivity of 19S Tg separated from other radioactive substances.(ABSTRACT TRUNCATED AT 250 WORDS)

A mos oncogene-containing retrovirus, myeloproliferative sarcoma virus, transforms rat thyroid epithelial cells and irreversibly blocks their differentiation pattern

Differentiated, cloned rat thyroid epithelial cells (424 cells) were infected with a wild-type and a temperature-sensitive strain of the myeloproliferative variant of the Moloney murine sarcoma virus. The thyroid cells were productively infected and transformed by both virus strains and displayed some of the typical properties of malignant cells, such as morphological changes, growth in soft agar, and in vivo tumorigenicity. The acquisition of the transformed phenotype by the virus-infected cells was accompanied by a loss of the typical differentiated features of the thyroid epithelium, such as thyroglobulin (TG) secretion, iodide uptake, and dependence for growth on six factors including thyrotropin, the physiological thyroid stimulator. TG mRNA could not be demonstrated in cells transformed by both viral strains, suggesting a block at the level of the TG gene transcription. While the transformed state of the cell clones infected with the temperature-sensitive strain could be reverted by shifting the cultures to the temperature nonpermissive for transformation (39 degrees C), no reversion of the differentiated functions took place after such a shift, showing that the v-mos oncogene irreversibly shuts off the differentiation of thyroid epithelial cells in vitro. These results demonstrate, for the first time, an oncogenic potential of the v-mos oncogene family towards differentiated epithelial cells in vitro.

Thyrotropin increases the iodine content of rat circulating thyroglobulin as measured by equilibrium density gradient centrifugation

In previous work we demonstrated that circulating thyroglobulin contains very little or no iodine. We have now characterized circulating thyroglobulin following administration of thyrotropin (TSH) to determine whether its iodine content remains low or increases after stimulation. The iodine content of circulating thyroglobulin was estimated from its density determined by equilibrium density gradient (isopycnic) centrifugation. TSH stimulated thyroglobulin from 182 +/- 28 ng/ml to 571 +/- 83 ng/ml at 8-14 h. Circulating thyroglobulin in the basal state had a density consistent with very little or no iodine. Its density increased following TSH to a maximum at 8-14 h which was nearly the same as the density of thyroglobulin extracted directly from the thyroid. To determine whether selective peripheral metabolism, based on the degree of iodination, could account for the density shift, purified rat thyroid thyroglobulin was injected into thyroidectomized rats. The density of thyroglobulin remained unchanged for 25 h during which time it was metabolized by more than 97%. Therefore, selective metabolism of thyroglobulin based on iodine content did not occur. We conclude that TSH causes a marked increase in the iodine content of circulating thyroglobulin. It is most likely that in the basal state circulating thyroglobulin comes from selective release of poorly iodinated molecules, while after TSH, it comes from release of previously synthesized, iodinated and stored molecules.

Dissociation between transformed and differentiated phenotype in rat thyroid epithelial cells after transformation with a temperature-sensitive mutant of the Kirsten murine sarcoma virus

Differentiated rat thyroid epithelial cells, infected in vitro with a temperature-sensitive mutant of the Kirsten murine sarcoma virus, expressed at the permissive temperature (33 degrees C) some phenotypic properties typical of transformed cells, including morphological features, colony formation in agar, and induction of tumors in newborn animals. Specific functional markers of these differentiated cells, i.e., synthesis/secretion of thyroglobulin, synthesis of thyroglobulin mRNA and iodide uptake, were blocked during growth at 33 degrees C. Normal morphology, failure to grow in agar, and the requirement of hormones for optimal growth were all restored after shifting to the temperature nonpermissive for transformation (39 degrees C), though the typical differentiated functions remained blocked. Infection with a leukemia helper virus clone (Moloney or Kirsten murine leukemia virus) did not lead to the loss of the differentiated phenotype of rat epithelial thyroid cells, thus demonstrating that the loss of the differentiated phenotype is caused by the sarcoma virus component. These results indicate that the expression of some of the phenotypic properties of transformed differentiated rat thyroid epithelial cells is under the direct control of the p21 thermosensitive activity, whereas the block in the expression of two typical differentiation markers of thyroid epithelial cells is irreversible and probably controlled by different mechanisms.

Dissociation between transformed and differentiated phenotype in rat thyroid epithelial cells after transformation with a temperature-sensitive mutant of the Kirsten murine sarcoma virus

Differentiated rat thyroid epithelial cells, infected in vitro with a temperature-sensitive mutant of the Kirsten murine sarcoma virus, expressed at the permissive temperature (33 degrees C) some phenotypic properties typical of transformed cells, including morphological features, colony formation in agar, and induction of tumors in newborn animals. Specific functional markers of these differentiated cells, i.e., synthesis/secretion of thyroglobulin, synthesis of thyroglobulin mRNA and iodide uptake, were blocked during growth at 33 degrees C. Normal morphology, failure to grow in agar, and the requirement of hormones for optimal growth were all restored after shifting to the temperature nonpermissive for transformation (39 degrees C), though the typical differentiated functions remained blocked. Infection with a leukemia helper virus clone (Moloney or Kirsten murine leukemia virus) did not lead to the loss of the differentiated phenotype of rat epithelial thyroid cells, thus demonstrating that the loss of the differentiated phenotype is caused by the sarcoma virus component. These results indicate that the expression of some of the phenotypic properties of transformed differentiated rat thyroid epithelial cells is under the direct control of the p21 thermosensitive activity, whereas the block in the expression of two typical differentiation markers of thyroid epithelial cells is irreversible and probably controlled by different mechanisms.

Serum thyroglobulin in patients undergoing subtotal thyroidectomy for toxic and nontoxic goiter

Twenty-five patients with nontoxic nodular goiters and six with toxic goiter were studied prior to subtotal thyroidectomy, with closely spaced blood sampling up to three weeks after surgery, and approximately one year after surgery. Serum thyroglobulin (Tg) was measured by a previously described radioimmunological method. The mean serum Tg was elevated in patients with nontoxic nodular and toxic goiters compared to sex and age matched control groups with pronounced increases during surgery. The disappearance curves of Tg in both groups had two exponentials, an initial steep slope with a half-life of 4.0 +/- 1.8 (SD) h in nontoxic goiter and 4.5 +/- 3.2 h in toxic goiter. This was followed by a more shallow slope with a half-life of 3.6 +/- 1.1 days in nontoxic goiter and 3.4 +/- 0.8 h in toxic goiter, the breaking point between the slopes lying approximately at 48-72 h. There was no significant difference between the half-life of Tg in patients with toxic or nontoxic goiters, respectively. There was a weak correlation between the weight of the removed thyroid tissue and the maximally obtained Tg concentration at the time of surgery, but no correlation with the levels before operation. Serum Tg was significantly lower three weeks after operation and approximately one year after surgery. In conclusion, different forms of the Tg molecules seem to be removed at different rates, independent on the type of goiter.