Action of TSH on nuclear ADP-ribosylation in dog thyroid slices

Influence of nicotinamide on the radiosensitivity of normal and goitrous thyroid in the rat

Radioiodine is used to treat thyroid cancer and hyperthyroidism. In order to reduce radiation hazard to the patient and to people in contact with the patient it would be desirable to obtain the same therapeutic effect with lower activities of the radioisotope. This could be achieved by the simultaneous administration of a compound that increases tissue radiosensitivity. In this study we analyzed the use of nicotinamide (NA) as a radiosensitizer to radioiodine, to increase 131I efficacy. NA administered during 30 days to Wistar rats failed to alter thyroid weight. The influence of NA on radiothyroidectomy induced by increasing doses of 131I was examined in otherwise nontreated rats. NA produced a significant increase in the ablation caused by radioiodine. Goiter was then induced by the administration of methylmercaptoimidazol (MMI) to rats, followed by the treatment with radioiodine with and without simultaneous administration of NA. Thyroid weight per 100 g of body weight ratio was not changed by NA alone; 131I administration caused a 25% decrease in goiter size, while 131I plus NA produced a reduction of the ratio of 46% (p < 0.01 vs. NA). No changes were observed in adenosine diphosphate (ADP)-ribosilation of thyroid nuclear protein in NA-treated rats. Thyroid blood flow (determined by 86Rb uptake) was increased by 84% by NA. In conclusion, nicotinamide has a significant radiosensitizing effect to 131I both in normal and goitrous rats. This action is because of an increase in thyroid blood flow, which probably enhances tissue oxgenation.

Relationship between nuclear ADP-ribosylation and RNA transcription in calf and human thyroid

ADP-ribosylation is a posttranslational modification of proteins that has been related to many cellular events, such as DNA replication and repair, cell proliferation and differentiation. The present studies were performed in order to explore the possible relationship between nuclear protein ADP-ribosylation and RNA transcription in the thyroid gland. Inhibition of RNA transcription by alpha-amanitin and actinomycin D caused a decrease in ADP-ribosylation of 27 and 17%, respectively. Nicotinamide caused a dose-related inhibition of ADP-ribosylation, which was highest at 2 mM (around 90%). At this dose nicotinamide inhibited total RNA transcription by 46%, while the activity due to RNA polymerase II decreased by 50% and that related to RNA polymerases I+III dropped by 24%. These results suggest that inhibition of total nuclear protein ADP-ribosylation is accompanied by a parallel decrease in RNA transcription. Since our previous work has shown that TSH stimulates both nuclear ADP-ribosylation and RNA transcription it may be concluded that these activities follow parallel changes within the thyroid. When the same activities were assayed in normal human and in glands bearing follicular adenoma, RNA polymerase II was increased 4 fold in the latter group, without change in nuclear ADP-ribosylation. These results would suggest that a mechanism, distinct from ADP-ribosylation, may also be involved in the regulation of RNA transcription. This latter might be altered under this pathologic condition.

Thyroglobulin gene activation by thyrotropin and cAMP in hormonally depleted FRTL-5 thyroid cells

Thyroglobulin gene control by cAMP and thyrotropin has been compared in FRTL-5 thyroid cells maintained in the absence of serum, insulin, cortisol, and thyrotropin for 7 days. Readdition of thyrotropin induces an increase in the expression of the thyroglobulin gene with little effect on total RNA synthesis. In contrast, addition of 8-bromo cyclic AMP and cholera toxin stimulate total RNA synthesis as well as thyroglobulin gene expression. The same non-selective effect of cyclic AMP on the thyroglobulin gene was observed when transcriptional activity was measured in permeabilized FRTL-5 cells.