Identification and Characterization of Two Labeled Intermediates in the Biosynthesis of Rat Thyroglobulin

Transfection of TTF-1 gene induces thyroglobulin gene expression in undifferentiated FRT cells

The thyroglobulin gene, the substrate for thyroid hormone biosynthesis, is not expressed in the FRT cell line, which, even though it manifests the polarised epithelial phenotype, does not express any of the thyroid functional properties. Two transcription factors, TTF-1 and Pax-8, have been implicated in thyroid specific expression of the thyroglobulin gene. FRT cells contain Pax-8 but they lack TTF-1. In this paper, we show that transfection of TTF-1 expression vectors in FRT cells results in activation of thyroglobulin gene expression. If the expression vector encoded for TTF-1-ER, a fusion gene coding for the entire TTF-1 protein fused to the hormone-binding domain of the steroid receptor, under the control of the RSV promoter, thyroglobulin gene expression was controlled by estrogen. These data provide a direct demonstration that TTF-1 activates the chromosomal thyroglobulin promoter. Since transfection of TTF-1 expression vectors in non-thyroid cell types did not result in thyroglobulin gene expression, it is suggested that Pax-8, in addition, perhaps, to a specific cellular environment, might be required for thyroid specific expression of the thyroglobulin gene.

Vitamin A-deficiency impairs the normal mannosylation, conformation and iodination of thyroglobulin: a new etiological approach to endemic goitre

This study was undertaken in order to validate the hypothesis that vitamin A-deficiency alters the structure of thyroglobulin (Tg). For that purpose, four groups of 20 Sprague-Dawley rats were submitted during two months to varying dietary conditions, namely a control diet (C+), a vitamin A-deficient diet (A-), an iodine-deficient diet (I-) and a diet characterized by the association of both deficiencies (A-I-). Both the conventional parameters of thyroid function, the intracellular steps of Tg glycosylation and iodination were analyzed. In the A- and A-I- groups, blood levels of retinol fell to one tenth of the control mean and circulating concentrations of total and free T4 and T3 increased significantly. This biochemical hyperthyroidism contrasted with the maintenance of normal TSH plasma values, suggesting a generalized peripheral refractoriness to thyroid hormones. In both A- and A-I- groups, thyroid cytosol 3H-RPM (retinyl-phosphate-mannose) and 3H-mannose incorporation into the core of the 12S-Tg and 19S-Tg species were reduced by 40-50%. In contrast, cytosolic concentrations of 3H-DPM (dolichyl-phosphate-mannose) rose, suggesting that the N-glycosylation pathways are affected in opposite direction. The sedimentation coefficient in sucrose gradient of the purified dimeric 125I-19S-Tg after guanidine 6M and dithiothreitol denaturation showed that most of the A- Tg molecules were transformed into monomeric 12S species, implying alterations of both noncovalent and covalent bonds. Finally, the radiochromatogram of 125I-iodothyronines recovered after Tg pronase digestion revealed a significant increase in the mono- (MIT) and diiodothyronine (DIT) fractions in contrast with a significant decrease in the T3 and T4 hormonal compounds. These findings are consistent with the view that vitamin A-depletion impairs the endogenous RPM synthesis and, therefore, the normal Tg 0-mannosylation. The growing peptide is characterized by steric hindrance, leading to abnormal closure of disulphide bonds, reduced MIT-DIT coupling reactions and depressed generation of physiologically active thyroid hormones. pure iodine deficit (I-) induces no effects on the above-mentioned glycosylation reactions, but iodine shortage superimposed on preexisting vitamin A-deficit (A-I-) aggravates the Tg dysmaturation.(ABSTRACT TRUNCATED AT 400 WORDS)

Thyroglobulin messenger RNA: translation of a 33-S mRNA into a peptide immunologically related to thyroglobulin

Poly(UC)--Sepharose chromatography of the RNA extracted from a thyroid fraction sedimenting between 800 X g and 27000 X g allows the purification of two RNA fractions amounting each to 1% of the applied material. The first one is loosely bound to the column from which it is eluted at 25 degrees C. It is mainly composed of 16-S and 12-S RNA comprising no poly(A) sequences. This could correspond to mitochondrial rRNA. The second one, which is eluted at 50 degrees C, is poly(A)-rich and represents 33-S and 17--18-S RNA species. The 33-S RNA resists heating at 80 degrees C, suggesting that it is composed of one polynucleotide chain. When injected into Xenopus oocytes, the 33-S RNA specifically promotes the synthesis of a peptide with an apparent molecular weight of 185000 and an apparent sedimentation coefficient of 10-S. This peptide is immunologically related to thyroglobulin and could represent its main precursor. Under the conditions tested it does not polymerize spontaneously into 19-S thyroglobulin, suggesting that assembly of the molecule could require specific, post-translational alterations of the precursor and/or the presence of additional lighter subunits.