Inhibition of peroxidase and catalase activities and modulation of hydrogen peroxide level by inositol phosphoglycan-like compounds.

Inositol phosphoglycan-like compounds are produced by the hydrolysis of the membrane bound glycosyl phosphoinositides. Besides being short term mediators of insulin action, they inhibit peroxidases and catalase, increasing the concentration of cellular hydrogen peroxide. Although high concentrations of hydrogen peroxide are toxic, moderate increases of its basal level are signals for different metabolic pathways. The inhibitor, localized in the cytosol of the cell, acts on peroxidases and catalase of the same tissue (homologous action) and of other tissues or organisms (heterologous action). The inositol phosphoglycan-like compound inhibits peroxidases with different prosthetic groups, i.e. containing iron such as: thyroid peroxidase, lactoperoxidase, horseradish peroxidase, soy bean peroxidase; and containing selenium such as glutathione peroxidase and 2-cys peroxiredoxin with no prosthetic group. Besides peroxidases, the inositol phosphoglycan-like compound inhibits catalase, another heme enzyme. The inhibition kinetics demonstrates a noncompetitive effect. The site of action is not the prosthetic group, given that the inhibitor does not produce any effect on the peak in the Soret region in the presence or absence of hydrogen peroxide. In conclusion, the inositol phosphoglycan-like compound is the general inhibitor of peroxidases and catalase involved in the modulation of hydrogen peroxide level that acts in different metabolic pathways as a signal transducer.

Long-term effect of norepinephrine on thyroglobulin gene expression in FRTL-5 cells.

Many types of evidence support a role of the sympathetic nervous system in the regulation of thyroid function, although there is no general consensus on the type of influence that catecholamines exert. Depending on the experimental approach, epinephrine and norepinephrine (NE) can stimulate, inhibit, or fail to act on thyroid function. The aim of this study was to determine the effect of NE on thyroglobulin (Tg) synthesis and gene expression in FRTL-5 cells. Tg content, measured by immunoprecipitation with a specific antibody, showed that NE caused a 45% inhibition of thyrotropin (TSH) effect. The content of Tg mRNA was analyzed by Northern blot, the relative inhibition in total Tg mRNA levels from NE-treated cells, compared to TSH alone, ran parallel with inhibition in Tg content, while total RNA did not change after incubation with NE. There was no alteration in Tg mRNA stability by NE. When plasmids harboring different sequences of Tg promoter fused to the CAT reporter gene were transfected into FRTL-5 cells, TSH treatment stimulated promoter activity while NE diminished this effect by 43%-55%. Northern blots were performed to analyze mRNA for thyroid transcription factors (TTF1, TTF2, Pax8), and no significant changes were observed with the different treatments. In conclusion these results suggest that NE inhibits Tg synthesis at the transcriptional level.

The protein kinase C pathway inhibits iodide uptake by calf thyroid cells via sodium potassium-adenosine triphosphatase.

The effect of the phorbol esther phorbol myristate acetate (PMA) on iodide uptake was studied in primary cultures of calf thyroid cells. PMA caused a dose- and time-dependent inhibition of thyrotropin (TSH), forskolin, and db-cAMP stimulation, indicating an effect distal to both TSH receptor and cAMP generation. No action was found on iodide efflux, indicating a selective inhibition of iodide uptake. This inhibition was observed even after 5 minutes of incubation, thus excluding a possible genomic action. Bisindolmaleimide (BS), a specific inhibitor of the protein kinase C (PKC) pathway, reverted the effect of PMA. A similar degree of inhibition of the Na+/K+ adenosine triphosphatase (ATPase) and iodide uptake by PMA was found, thus suggesting a link between both parameters. These results indicate that the PKC pathway inhibits thyroid iodide uptake by an action distal to cAMP generation and probably because of a decrease in Na+/K+-ATPase activity.

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.

Effect of the interaction of TSH and insulin on the stimulation of 2-deoxyglucose uptake in FRTL-5 cells.

Since thyroid glycogen stores are low, the uptake of glucose is very important in order to maintain cell function (house-keeping). Previous studies have shown that TSH and insulin, independently, are regulators of this parameter. Since their corresponding mechanisms of action are different, we investigated the possible effect of the interaction between TSH and insulin on the stimulation of 2-deoxyglucose (2-DOG) uptake, a non metabolizable derivative of glucose. Confluent FRTL-5 cells were submitted to different treatments, usually for 72 h. In one series of experiments the concentration of TSH was kept constant, at 1 U/l, and the addition of insulin, from 0.16 to 1.6 micromol/l caused a progressive synergic increase in DOG uptake. When insulin concentration was kept constant, increasing amounts of TSH, from 0.5 to 10 U/l), also caused a synergic stimulation of DOG uptake. The effect of insulin was mimicked by IGF-1 (1-10 nmol/l), while that of TSH was mimicked by forskolin. Timecourse studies showed that TSH had a peak at 3 h of incubation, while insulin caused a progressive increase for up to 72 h. At short incubation times, up to 6 h, an additive effect of TSH and insulin was observed, while at longer times the interaction was synergic. The present results suggest that the interaction between the cAMP and the tyrosine kinase pathways on DOG uptake would involve two different mechanisms. At early times the effects of both hormones are additive, while in longer periods it becomes synergic.

Presence of a soluble inhibitor of thyroid iodination in primary cultures of thyroid cells.

Monolayer cultures of thyroid cells lose their iodide organification capacity a few days before the disappearance of thyroid peroxidase (TPO) activity. The present studies were performed in order to clarify this point. The above mentioned difference was due to the presence of an inhibitor in the monolayer thyroid cells culture, given that total homogenate prepared from confluent cells caused a significant inhibition of activity of TPO from fresh tissue. The inhibitor was localized in the 105000g supernatant of the homogenate of the cell culture, but not in a similar preparation obtained from fresh thyroid. It is thermostable, dialyzable and has a molecular weight of less than 2 kDa. Addition of the inhibitor at the end of the reaction of tyrosine iodination failed to alter the results. This fact suggests that the compound does not destroy the iodinated product. The presence of the cytosolic inhibitor was observed in monolayer thyroid cell cultures of different species (bovine, porcine, rat and human) but not in free follicles cultures.

Hexachlorobenzene-induced alterations of rat hepatic microsomal membrane function.

The time and dose-dependent effects of the in vivo administration of hexachlorobenzene (HCB), on hepatic microsomal membrane functions, were studied in female Wistar rats. Administration of HCB (100 mg/100 g b.w.) resulted in time-dependent decreases in the activity of two membrane-bound enzymes: 5'nucleotidase and Na+/K+ ATPase. HCB was found to cause a significant rise in protein tyrosine kinase (PTK) activity during the early stages of intoxication (day 2), followed by a significant decrease at 10 days, returning to control levels after 20 days of treatment. A stimulatory effect of HCB on in vitro endogenous microsomal protein phosphorylation was observed from 2 days of intoxication up to 30 days of treatment, with an important stimulation of phosphorylation at 5 days. Administration of HCB (100 mg/100 g b.w.) for 10 days caused a 50% reduction in epidermal growth factor receptor (EGF-R) ligand binding. The effects of known specific inhibitors of protein phosphatases on endogenous protein phosphorylation were studied. HCB affected the labelling of several bands, as well as the 5'nucleotidase and PTK activities, in a dose-dependent manner. In conclusion, this study indicated that the in vivo administration of HCB results in a significant alteration of membrane function.

Long-term effect of norepinephrine on iodide uptake in FRTL-5 cells.

The sympathetic nervous system plays a role in the regulation of thyroid function. In FRTL-5 rat thyroid cells, norepinephrine (NE) acutely depresses intracellular I- by increasing I- efflux. The present study was undertaken to determine the effect of NE on iodide transport after a longer time period. NE inhibited the ability of thyrotropin (TSH) to induce iodide uptake by FRTL-5 cells after 48 or 72 hours, but not after 24 hours. The effect of NE was more evident with increasing concentrations of TSH. NE did not modify the rate of I- efflux. Inhibition was associated with a decrease in the Vmax and no change in the Km for iodide influx. To determine if this was a generalized effect of NE on thyroid cell membrane, the uptake of alpha-aminoisobutyric acid (a nonmetabolizable aminoacid) and of 2-deoxyglucose was measured. NE did not inhibit TSH stimulation of the uptake of the two compounds. NE inhibited the action of dibutyryl cAMP (dbcAMP) on iodide uptake in a similar manner to TSH, but did not alter the cyclic adenosine monophosphate (cAMP) levels increased by TSH. The effects of different adrenoreceptor agonists and antagonists demonstrated that norepinephrine acts through an alpha1-adrenergic receptor.

Role of cyclic 3'5' guanosine monophosphate and nitric oxide in the regulation of iodide uptake in calf thyroid cells.

Sodium nitroprusside (SNP) spontaneously produces nitric oxide (NO). In many cell types, this activates the soluble form of the enzyme guanylyl cyclase (GC), resulting in the elevation of cGMP. We herein report the role of NO and cGMP on iodide uptake in primary cultures of calf thyroid cells. Iodide uptake is the limiting step in thyroid hormone biosynthesis and a typical functional parameter. The effect of SNP on this parameter was thus determined. In cells treated with TSH for 72 h, addition of 5 mM SNP for the last 2 h caused a significant inhibition on iodide uptake, with no change in cells not treated with TSH. This action was mimicked by an analogue of cGMP, 8Br-cGMP, and blocked by reduced hemoglobin, thus suggesting that it is mediated by the GC-cGMP pathway. SNP also inhibited the stimulation caused by forskolin or analogues of cAMP, indicating that the effect takes place in this pathway, which would be distal to cAMP generation. The accumulation of radioiodine by thyroid cells is a consequence of the balance between influx and efflux. The studies demonstrate that SNP does not affect iodide efflux, thus revealing that it inhibits the influx.

Effects of iodide on thyroglobulin biosynthesis in FRTL-5 cells.

Iodide inhibits several thyroid parameters through an organic intermediate, and this process has been related to thyroid autoregulation. The aim of this study was to determine the effect of iodine on thyroglobulin (Tg) synthesis in the rat thyroid cell line FRTL-5. TSH stimulated amino acid incorporation into the cells by 400% and iodine had no effect on this parameter. No effect of TSH or iodide on [35S] methionine incorporation into protein was found under our experimental conditions (approximately 80% of total [35S]methionine incorporated was found in TCA-precipitable material). TSH caused an increase in Tg synthesis, after 1 h, while iodide partially blocked the effect of TSH (control 6.4% of TCA precipitable radioactivity; TSH 10.7%; iodide 8.4%). After 24 h, the protein released into the medium was measured. TSH stimulated total protein liberation and iodide inhibited this parameter. TSH stimulated total RNA content, and iodide caused an inhibition. Northern analysis did not show inhibition by iodide of TSH-stimulated Tg mRNA levels. The present results show an inhibitory effect of excess iodide on TSH-stimulated thyroglobulin biosynthesis in FRTL-5 cells.

Studies on the goiter inhibiting action of iodolactones.

The thyroid gland synthesizes 6-delta-iodolactone, a compound shown to inhibit goiter growth in vivo and cell proliferation in culture. The present studies were performed to characterize this effect further with the aim of exploring the possible therapeutic action of iodolactones. Prevention assay: rats were treated simultaneously with a goitrogen, methylmercaptoimidazole, and either 6-delta-iodo-lactone or 14-iodo-omega-lactone, a synthetic derivative, given either i.p. or p. o. Both compounds caused a significant decrease in thyroid weight irrespective of the route of administration, but oral administration was less effective. A dose-response relationship was observed, the minimal effective dose (i.p.) being 3 micrograms/day. Involution assay: goiter was first induced with methylmercaptoimidazole and then the iodolactones were injected. Both compounds caused a significant involution, which was dose-related. Acute (10 days) administration of the iodolactones did not produce significant changes in several serum parameters (total T3 and T4, cholesterol, total protein, urea and acetylcholinesterase). These results give further support to the potential therapeutic application of iodolactones.

Effect of iodoarachidonates on thyroid FRTL-5 cells growth.

Excess iodide inhibits several thyroid parameters, by a putative organic iodocompound. Different iodolipids, including iodinated derivatives of arachidonic acid (IAs), are produced by rat, calf and pig thyroid. The action of two iodolactones, one bearing the iodine atom at the position 6 (IL-d) and the other at position 14 (IL-w) on growth of FRTL-5 cells was studied. KI, IL-w and IL-d exert a dose-related inhibition on FRTL-5 cell proliferation. The first two compounds caused inhibition at 1 microM while IL-d was effective at 10 microM. This inhibitory action of iodolactones (ILs) was not altered by 1 mM methyl-mercaptoimidazol (MMI), indicating that they exert their effect per se. The action of ILw on cell growth was reversible. The growth-stimulating effect of 10 microM forskolin was inhibited by IAs, showing that one possible site of action lies at the cAMP pathway. The present results give further support to our hypothesis about the role of IAs in thyroid growth autoregulation.

Identification of a mutation in the coding sequence of the human thyroid peroxidase gene causing congenital goiter.

Thyroid peroxidase (TPO) is the key enzyme in the synthesis of thyroid hormones, and the TPO defects are believed to be the most prevalent causes of the inborn errors of thyroid metabolism. We investigated an adopted boy with iodide organification defect, who presented with florid hypothyroidism at the age of 4 mo, poorly complied with thyroxine treatment, and developed a compressive goiter necessitating partial resection at the age of 12 yr. Biochemical studies revealed the absence of TPO activity in the resected tissue. Genomic DNA studies identified a 4 base-pair insertion in the eighth exon of the TPO gene, and showed that the patient was homozygous for this frameshift mutation. The direct genetic diagnosis of this mutation can be made by digestion of polymerase chain reaction products with NaeI restriction enzyme. This will help assessing its prevalence among the heterogenous genetic group of TPO defects.

Further studies on the antigoitrogenic action of iodoarachidonates.

Previous studies have shown that iodoarachidonates (IAs) prevent goiter production in rats. In the present studies we show that both IL-d and IL-w (IAs bearing the iodine atom at the positions 6 and 14, respectively), cause a significant involution of preformed goiter. This effect was evident when IAs were administered either orally or via i.p., although the first one required larger doses to obtain the same degree of inhibition. No changes were observed in serum protein, urea, cholesterol, cholinesterase, T3 or T4. In vitro studies with FRTL-5 cells showed that both IAs inhibit iodide and alpha-AIB uptake, as well as ATPase activity.

Thyroid autoregulation: evidence for an action of iodoarachidonates and iodide at the cell membrane level.

Iodolipids are the possible mediators of excess iodide in thyroid autoregulation. Previous work from our laboratory has shown that 14-iodo-15-hydroxy-5,8,11 eicosatrienoic acid (I-HO-A) and its omega lactone (IL-w) mimic the inhibitory action of excess iodide upon several parameters of thyroid metabolism. The present experiments were performed in order to study the mechanism of the inhibitory effect of I-HO-A and IL-w on 2-deoxy-D-glucose (DOG) and aminoisobutyric acid (AIB) uptake by calf slices. I-HO-A, IL-w and KI 0.1 mM caused a 33, 31 and 25% inhibition, respectively, of AIB uptake. The presence of 0.1 mM methimazole (MMI) only reversed the effect of KI. The transport of DOG was inhibited by both compounds: I-HO-A caused a 62% decrease, while IL-w produced a 64% inhibition; and MMI failed to relieve their action. On the contrary, the 33% inhibition caused by KI disappeared when MMI was present. Taking into account that AIB and DOG transport across the membrane requires energy, supplied by Na-K-ATPase, changes in its activity were studied. TSH (10 mU/ml) produced a 74% increase in the enzyme activity which was significantly blocked by KI (82%), I-HO-A (100%) and IL-w (100%). Basal enzyme activity was impaired by IL-w (33%), but not by KI. These results were correlated with the decrease of DOG uptake produced by 1 mM ouabain. Tissue specificity effect of iodoarachidonates was demonstrated by the absence of action on DOG transport in kidney and liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential levels of thyroid peroxidase and thyroglobulin messenger ribonucleic acids in congenital goiter with defective thyroglobulin synthesis.

The biosynthesis of thyroid hormones requires iodide, thyroid peroxidase (TPO), thyroglobulin (Tg) and H2O2. We have studied two sisters with congenital large goiters and hypothyroidism. Perchlorate tests were negative. Serum T3 and T4 were decreased, TSH was increased and Tg was within the lower limit of normal. Biochemical and molecular studies were performed on goiter samples obtained after surgery. Tg content in both tissues was negligible. Paper chromatography of labeled iodocompounds showed a decrease in T4, and the presence of a pronase/pancreatin-resistant iodoprotein. TPO activity was normal in the tissues. Sephacryl S-300 gel filtration demonstrated labeled iodoalbumin-like protein and the absence of a Tg peak. Salting out studies of soluble protein fraction gave an abnormal pattern. Agarose gel electrophoresis showed the presence of an iodoalbumin-like protein and the absence of Tg in the tissues. This last finding was confirmed by immunoelectrophoresis. The Tg and TPO mRNAs levels were also analyzed. Dot-blot hybridization studies with pM5 (TPO cDNA) and phTgM2 (Tg cDNA) probes showed increased and decreased signals, respectively. The increase in TPO mRNA can be explained as a compensatory mechanism vis a vis an increase in serum TSH caused by decreased serum T3 and T4 due to the impairment in Tg mRNA. The Tg mRNA of both patients was further studied with four different probes covering 5' and 3' regions (phTgM1, phTgB1, phTgB2 and phTgB3). Hybridization was observed with all four probes, thus excluding a dramatic deletion defect. Northern transfer showed a clear signal of hybridization with the phTgB1 probe in the 8-9 Kb range. We may conclude that the biochemical and molecular abnormality of these patients is characterized by a decrease of Tg mRNA and of Tg translation.

Thyroid autoregulation. Inhibition of goiter growth and of cyclic AMP formation in rat thyroid by iodinated derivatives of arachidonic acid.

Thyroid autoregulation has been related to intraglandular content of an unknown putative iodocompund. Data from different laboratories have shown that the thyroid is capable of producing different iodolipids, including iodinated derivatives of arachidonic acid; such as 5-hydroxy-6-iodo-8, 11, 14-eicosatrienoic-delta-lactone (IL-delta). Previous results from our laboratory showed that a semi-purified preparation of iodinated arachidonic acid exerts an inhibitory action in vitro on calf thyroid. In the present studies three purified iodinated derivatives of arachidonic acid were synthesized: IL-delta; 14-iodo-15-hydroxy-5, 8, 11-eicosatrienoic acid (I-OH-A) and its corresponding omega-lactone (IL-omega). Their action on MMI-induced goiter was studied in rats. Administration of MMI to rats during 10 days increased thyroid weight by 124%. This effect was significantly inhibited by the simultaneous injection of 5 micrograms/day of I-OH-A (57% inhibition of MMI action), IL-W (39%), IL-delta (33%) and T3 (95%), while arachidonic acid was without action. No inhibition was found with 1.25 micrograms/day Kl, a dose equivalent to that which could be originated from total dehalogenation of the iodocompounds. These results support the idea that these iodocompounds have an intrinsic biologic activity and that there is a correlation between action and chemical structure. Serum TSH was increased around 15-20 fold after MMI administration. Chronic or acute injection of I-OH-A failed to alter TSH levels, indicating that this iodocompound exerts its action directly on the gland, without altering TSH concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid autoregulation. Inhibitory effects of iodinated derivatives of arachidonic acid on iodine metabolism.

Thyroid autoregulation has been linked to an organified iodocompound. Since several iodolipids are produced by the gland their possible role in thyroid autoregulation was examined. The following pure synthetic compounds were prepared: 1) 14-iodo-15-hydroxy-5,8,11-eicosatrienoic acid (I-OH-A); 2) its omega lactone (IL-omega); 3) 5-hydroxy-6-iodo-8,11,14-eicosatrienoic acid delta lactone (IL-delta). Their action on iodine metabolism was studied. Iodine uptake was measured in calf thyroid slices. At 10(-4)M I-OH-A caused a 64% decrease in the T/M ratio, while IL-omega inhibited it by 36% and IL-delta was without effect. At 10(-5)M the inhibition was 44% for I-OH-A and 19% for IL-omega, while T3 was without action. A possible isotopic dilution effect was excluded, and no change in iodine efflux was observed. The inhibition by I-OH-A of iodide uptake was observed after only 15 min preincubation. This compound also decreased 125I accumulation in rats. In calf thyroid slices, I-OH-A at 10(-4)M, inhibited PB125I formation by 80%, IL-omega by 62% and IL-delta by 37%. T3 and arachidonic acid were without action. I-OH-A also caused a dose-dependent inhibition of TSH-stimulated iodide organification. The present results demonstrate, for the first time, that iodinated derivatives of arachidonic acid inhibit thyroid function and mimic the effect of iodide on thyroid autoregulation.

The inhibition of PB125I formation in calf thyroid caused by 14-iodo-15-hydroxy-eicosatrienoic acid is due to decreased H2O2 availability.

Previous work from our laboratory has shown that 14-iodo-15-hydroxy-5,8,11-eicosatrienoic acid (I-HO-A) is a potent inhibitor of iodine organification in calf thyroid slices. The present studies were performed in order to clarify the mechanism of this action. Incubation of thyroid slices with 10(-4)M I-HO-A caused a 47 and 53% decrease in PB125I formation after 30 and 60 min incubation, respectively. In a series of experiments an inverse relationship between the degree of inhibition caused by I-HO-A and total iodine content and basal iodoprotein formation was observed. Chromatographic analysis of the labeled compounds showed a significant decrease in 125I incorporation into MIT, DIT, T3 and total iodolipid. The site of the inhibitory effect of I-HO-A was then sought. TPO was measured by three different methods. When TPO was solubilized from I-HO-A treated slices, no change in enzymatic activity was observed. Moreover, the same lack of action was found when solubilized TPO was incubated with I-HO-A. The production and release of H2O2 into the incubation medium was measured by chemiluminiscence technique. In control slices the values increased during the first 10 min and reached a plateau. Pretreatment of the slices with 10(-4)M KI caused a 51% inhibition, while the same concentration of I-HO-A produced a 59% inhibition. The possibility that I-HO-A might exert its action through a putative protein inhibitor was also explored. Incubation of slices with 10(-5)M I-HO-A caused a 46% decrease in PB125I formation and addition of actinomycin D or puromycin failed to alter this effect.(ABSTRACT TRUNCATED AT 250 WORDS)

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.