Variability in production and immunoreactivity of in-vitro secreted human thyroglobulin

We examined basal and bTSH-stimulated human thyroglobulin (hTg) secretion by autologous normal and abnormal (benign and malignant) human thyroid cell monolayers. Basal and bTSH-stimulated hTg secretion was highly variable and ranged from 50-700 ng/ml/10(5) cells over a 6 day period. All normal and benign 'non-functioning' adenoma cells demonstrated a dose and time related stimulation of hTg secretion in response to bTSH. Comparison of hTG secretion by autologous normal and abnormal cells showed that in six of eight pairs, the normal thyroid cells had a greater output of hTg than the benign adenoma cells in contrast to our previous studies using non-autologous cells. Malignant thyroid cell hTg production was less predictable than that obtained with normal and benign thyroid cells varying from absent to normal responses to bTSH. Characterization studies of the secreted hTg showed no difference between normal, benign and malignant thyroid cell hTg with reference to molecular weight. However, hTg secreted in vitro was non-iodinated and had a marked reduction (up to 200-fold) in immunoreactivity assessed by both polyclonal and monoclonal antibodies to hTg when compared to hTg standard prepared from intact thyroid tissue (which had 4.58 micrograms iodine/mg). This reduction in hTg immunoreactivity was greatest for hTg secreted by malignant thyroid cells. These data demonstrate the wide variability in the hTg secretory capacity of human thyroid cell monolayers and indicate, when compared to autologous normal cells, that abnormal human thyroid epithelial cells may be relatively deficient in their ability to secrete hTg in vitro. There were also qualitative differences in the immunoreactivity, and iodine content, of in-vitro secreted hTg. These observations suggest that there may be much greater heterogeneity in hTg secreted in vitro than previously realized, perhaps secondary to differences in iodine content and/or degree of glycosylation. Human thyroglobulin (hTg) is the major secretory protein of the thyroid cell (Van Herle et al., 1979). Intracellular hTg is the site of thyroid hormone iodination yet its extrathyroidal role, if any, remains unclear. While hTg is usually present in the peripheral circulation of normal individuals, in species of differing molecular weight (Feldt-Rasmussen et al., 1978), there have been few studies of in-vitro production of hTg by isolated human thyroid cells. Our interest in hTg is in its role as an antigen in thyroid autoimmune disease (De Bernardo et al., 1983; 1986).(ABSTRACT TRUNCATED AT 400 WORDS)