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J D Kieffer

Publications and source records attributed to J D Kieffer.

At least 37 records · Page 2Linked to original sources
PubMed1992

Cloning and in vitro expression of the human selenoprotein, type I iodothyronine deiodinase.

The type I 5' iodothyronine deiodinase (5' DI) catalyzes the deiodination of T4 to the biologically active hormone T3 and accounts for a significant fraction of its production. We have recently cloned the complementary DNA (cDNA) for the rat 5' DI, which contains the rare amino acid selenocysteine, and used this to screen human liver and kidney cDNA libraries to identify a human 5' DI cDNA clone. From these, we constructed a cDNA encoding a functional 5' DI. The 2222 base pair human 5' DI cDNA is approximately 200 nucleotides shorter than the 2.4-kilobase hybridizing band in Northern blots of human liver, kidney, and thyroid, because of missing 5' untranslated sequence and the poly A tail. The deduced amino acid sequence codes for a protein of 28.7 kilodaltons assuming the UGA codon at position 382 encodes selenocysteine, and is highly homologous (88% similarity) to the rat. We transiently expressed the 5' DI in COS-7 cells to establish that it encodes a functional enzyme and to study its kinetics. These show saturable deiodination of rT3 (Ka 0.52 +/- 0.04 mumol/L and Vmax 63.2 +/- 16.4 pmol min-1 mg-1). T4 and gold thioglucose are competitive inhibitors of rT3 deiodination. 6-n-Propylthiouracil (PTU) is an uncompetitive inhibitor (with rT3) and competitive inhibitor (with dithiothreitol) of rT3 deiodination. 6-n-Propylthiouracil inhibits T4 to T3 conversion. Labeling of COS-7 cells transiently transfected with the human 5' DI cDNA with bromoacetyl-125I-T3 demonstrates a 28-kilodalton protein. This indicates that in the human, as well as in the rat messenger RNA, the UGA encodes selenocysteine and translation terminates at the UAA codon at nucleotides 754 to 756. Reverse T3 and gold thioglucose (100 nmol/L) block bromoacetyl-125I-T3 labeling of the transiently expressed human and rat 5' DI proteins. These results demonstrate that the human 5' DI is a selenoprotein, analogous to the rat enzyme. Given the previously demonstrated critical role of the selenium atom in catalyzing deiodination by this protein, we conclude that this trace element is essential for normal thyroid hormone action in man.

Affinity Labels↗
PubMed1991

Recognition of UGA as a selenocysteine codon in type I deiodinase requires sequences in the 3' untranslated region.

Selenocysteine is incorporated cotranslationally at UGA codons, normally read as stop codons, in several bacterial proteins and in the mammalian proteins glutathione peroxidase (GPX), selenoprotein P and Type I iodothyronine 5' deiodinase (5'DI). Previous analyses in bacteria have suggested that a stem-loop structure involving the UGA codon and adjacent sequences is necessary and sufficient for selenocysteine incorporation into formate dehydrogenase and glycine reductase. We used the recently cloned 5'DI to investigate selenoprotein synthesis in eukaryotes. We show that successful incorporation of selenocysteine into this enzyme requires a specific 3' untranslated (3'ut) segment of about 200 nucleotides, which is found in both rat and human 5'DI messenger RNAs. These sequences are not required for expression of a cysteine-mutant deiodinase. Although there is little primary sequence similarity between the 3'ut regions of these mRNAs and those encoding GPX, the 3'ut sequences of rat GPX can substitute for the 5'DI sequences in directing selenocysteine insertion. Computer analyses predict similar stem-loop structures in the 3'ut regions of the 5'DI and GPX mRNAs. Limited mutations in these structures reduce or eliminate their capacity to permit 5'DI translation. These results identify a 'selenocysteine-insertion sequence' motif in the 3'ut region of these mRNAs that is essential for successful translation of 5'DI, presumably GPX, and possibly other eukaryotic selenocysteine-containing proteins.

Animals↗
PubMed1991

Selenocysteine confers the biochemical properties characteristic of the type I iodothyronine deiodinase.

The conversion of thyroxine to 3,5,3'-triiodothyronine (T3) is the first step in thyroid hormone action, and the Type I iodothyronine deiodinase supplies most of this extrathyroidal T3 in the rat. We found that the cDNA coding for this enzyme contains an in-frame UGA encoding the rare amino acid selenocysteine. Using site-directed mutagenesis, we have converted selenocysteine to cysteine and expressed the wild-type and cysteine mutant enzymes in JEG-3 cells by transient transfection. The kinetic properties of the transiently expressed wild-type enzyme are nearly identical to those reported for rat liver Type I deiodinase. Substitution of sulfur for selenium causes a 10-fold increase in the Km of the enzyme for the favored substrate 3,3',5'-triiodothyronine (rT3), a 100-fold decrease in the sensitivity of rT3 deiodination to competitive inhibition by gold and a 300-fold increase in the apparent Ki for uncompetitive inhibition by 6-n-propylthiouracil. These results demonstrate that selenium is responsible for the biochemical properties which characterize Type I iodothyronine monodeiodination.

Animals↗
PubMed1991

Evidence that cysteine, not selenocysteine, is in the catalytic site of type II iodothyronine deiodinase.

Recent cloning of the cDNA for Type I iodothyronine deiodinase revealed that the mRNA contains a UGA codon encoding the amino acid selenocysteine. Mutagenesis of the selenocysteine codon to a cysteine codon produced a protein with lower deiodinase activity. The presence or absence of selenocysteine in Type II deiodinase, which differs from the Type I enzyme in a number of parameters, has not been determined. Gold inhibits the activity of both the Type I deiodinase and the only other known eukaryotic selenocysteine-enzyme, glutathione peroxidase. Substitution of cysteine for selenocysteine in Type I deiodinase reduced its sensitivity to inhibition by gold 500-fold. We found that gold thioglucose was a competitive inhibitor with respect to the iodothyronine substrate of both deiodinases. However, the Type II enzyme from brown fat and pituitary was 100 to 1000-fold less sensitive to gold than was Type I activity in liver and pituitary, similar to the results with the cysteine-substituted Type I enzyme. This suggests that Type II deiodinase contains cysteine instead of selenocysteine in the active site.

Adipose Tissue, Brown↗
PubMed1991

Photoaffinity labeling of rat type I iodothyronine deiodinase.

The photoreactive compound p-nitrophenyl-2-diazo-3,3,3-trifluoropropionate (PAL) was coupled to [125I]rT3, T4, or T3 and incubated with liver and kidney microsomes of hypo-, hyper-, or euthyroid rats to identify the type I iodothyronine deiodinase. Various substrates or inhibitors of the enzyme, including rT3, T4, T3, 6-n-propylthiouracil (PTU), and iopanoic acid, were used as competitors to establish the specificity of protein labeling. The PAL derivatization enhanced the behavior of T4 and T3 as substrates for the type I enzyme. No specific labeling of microsomal proteins was observed with either rT3 or T4-PAL, presumably due to deiodination of the labeled compound. In contrast, T3-PAL labeled a 27-kDa band, the presence of which paralleled thyroid status. The labeling of only this protein was blocked by either substrates or enzyme inhibitors in a dose-dependent fashion, with a rank order of potency predicted by the activity of such compounds in type I enzyme assays. The specific nature of these competitions provides further evidence that this 27-kDa protein, identified in previous studies using N-bromoacetyl [125I]T3 or -T4, contains the active site of the rat type I deiodinase. This is in agreement with the mol wt of the rat type I deiodinase deduced from the recently identified cDNA coding for this protein.

Affinity Labels↗
PubMed1986

Modulation of myocardial L-triiodothyronine receptors in normal, hypothyroid, and hyperthyroid rats.

To characterize the nuclear receptor believed to mediate the thyroid hormones' actions on the heart, binding of L-[125I]T3 to extracts of myocardial cell nuclei from normal, propylthiouracil, and T4-treated rats has been investigated. Analysis of in vitro iodothyronine binding to this solubilized nuclear preparation revealed multiple saturable, specific binding sites for T3. High affinity binding for T3 (Kd = 4.2 +/- 1.0 X 10(-10) mol/L), and lower affinity (Kd approximately 10(-8) mol/L) binding activity appeared to be independent (Hill plot slope = 1). The mean maximal binding capacity of the high affinity binding site for T3 in euthyroid rats (84 +/- 8 fmol/mg DNA) was increased by approximately 50% in hypothyroidism (120 +/- 6 fmol/mg DNA) and unchanged in hyperthyroidism (88 +/- 25 fmol/mg DNA). The molecular weight of this T3 binding site is estimated to be 50,000-55,000 daltons. The properties of this solubilized binding activity from rat myocardial nuclei are consistent with its putative role as the biologic thyroid hormone receptor. The increase in binding capacity with hypothyroidism suggest regulation by thyroid hormone of its nuclear receptor in myocardium.

Animals↗
PubMed1986

Plasma eicosanoid levels in rats with nonketotic diabetes mellitus: effect of severity.

Nonketotic diabetes mellitus (DM) is associated with increased platelet production of thromboxane (TX) A2 and decreased endothelial production of prostacyclin (prostaglandin [PG]I2), but measurements of stable derivatives of these substances in the circulation have yielded discordant results. We studied the relationship between the severity of nonketotic DM and the plasma levels of 13,14-dihydro-15-keto-PGE2, 6-keto-PGF1 alpha, and TXB2 (stable derivatives of PGE2, PGI2, and TXA2, respectively) in rats, using three commonly employed doses of streptozotocin (40, 50, and 60 mg/kg body weight) to induce nonketotic DM of varying severity. Small differences in the severity of DM were associated with considerable differences in the plasma levels of 13,14-dihydro-15-keto-PGE2 and 6-keto-PGF1 alpha but not TXB2. Each eicosanoid responded differently to variations in the severity of DM. The plasma 13,14-dihydro-15-keto-PGE2 level was significantly lower than normal in the rats given 40 mg/kg streptozotocin, was unchanged from normal in the rats given 50 mg/kg and was significantly higher than normal in the rats given 60 mg/kg. The plasma 6-keto-PGF1 alpha level was significantly increased in rats given 40 mg/kg and 60 mg/kg, but was unchanged in those given 50 mg/kg. The plasma TXB2 level was not significantly different from normal in any one of the three groups of rats with nonketotic DM. The effect of severity on the plasma levels of the PGE2 and PGI2 derivatives is unexplained, but may reflect the origin of these derivatives from diverse organs and tissues, and the differing effects of abnormal metabolic factors (eg, fatty acids, glucose, insulin, pH) on the synthesis of these derivatives.(ABSTRACT TRUNCATED AT 250 WORDS)

6-Ketoprostaglandin F1 alpha↗
PubMed1986

Prostacyclin production by isolated rat adipocytes: evidence for cyclic adenosine 3',5'-monophosphate-dependent and independent mechanisms and for a selective effect of insulin.

The rat adipocyte contains two separate mechanisms for prostaglandin (PG) production. Norepinephrine stimulates prostacyclin (PGI2) and PGE2 production and triglyceride lipolysis in isolated rat adipocytes. In contrast, the vasoactive peptides angiotensin II, vasopressin, and bradykinin stimulate PGI2 production, but not PGE2 production or triglyceride lipolysis, in these cells. In this study, we characterized the two separate mechanisms of PG production with respect to the time course, the role of cAMP, the identity of the adrenergic receptor, and the effects of insulin and glucocorticoids. Angiotensin II stimulated PGI2 production rapidly (at 5 min) and independently of cAMP. beta-Adrenergic stimulation with isoproterenol produced a rapid 11-fold increase in the cAMP concentration and stimulated PGI2 production more slowly (at 120 min). The phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine (0.2 and 0.5 mM) and the adenylate cyclase activator forskolin (10 microM) also stimulated cAMP production rapidly and PGI2 production more slowly. 1-Methyl-3-isobutylxanthine (5.0 mM) further stimulated cAMP levels, but prevented the increase in PGI2 production and blunted the increase in glycerol release seen at lower concentrations. beta-Adrenergic blockade with propranolol or timolol completely inhibited the norepinephrine- or isoproterenol-stimulated production of PGI2 and triglyceride lipolysis, respectively. Insulin selectively inhibited isoproterenol-stimulated PGI2 production and triglyceride lipolysis at physiological concentrations, but had no effect on angiotensin II-stimulated PGI2 production. In contrast, dexamethasone inhibited PGI2 production induced by both isoproterenol and angiotensin II. We conclude that: angiotensin II stimulates PGI2 production rapidly and independently of cAMP, but isoproterenol stimulates PGI2 production more slowly, an effect that is cAMP dependent; insulin inhibits the cAMP-dependent beta-adrenergic stimulation of PGI2 production (and triglyceride lipolysis), but not the cAMP-independent angiotensin II-induced stimulation of PGI2 production (this suggests that the former effect is mediated by a decrease in cAMP levels in the adipocyte); and dexamethasone inhibits both mechanisms of PGI2 production. Both mechanisms of PGI2 production by rat adipocytes are exquisitely sensitive to hormonal regulation.

1-Methyl-3-isobutylxanthine↗
PubMed1985

Pure alpha-subunit producing tumor derived from a thyrotropic tumor: impaired regulation of alpha-subunit and its mRNA by thyroid hormone.

We have recently described a mouse pituitary tumor line which produces only the alpha-subunit of the glycoprotein hormones. This tumor line may be a useful animal model to study autonomous pituitary tumors which secrete only alpha-subunit. Our pure alpha-subunit producing tumor was derived from a thyrotropic tumor which secreted intact TSH as well as free alpha-subunit. Our current studies compare the regulation of alpha-subunit biosynthesis in a conventional thyrotropic tumor and the alpha-subunit producing tumor. Thyroxine or triiodothyronine administration to mice bearing the alpha-subunit producing tumor resulted in no change in plasma alpha-subunit concentration, and a 10-19% reduction in tumor alpha-subunit mRNA concentration that was not statistically significant. In contrast, thyroxine administration to mice bearing the thyrotropic tumor resulted in an 81% reduction in plasma alpha-subunit concentration, and a 75% reduction in tumor alpha-subunit mRNA concentration (P less than 0.01). Other studies using a cDNA specific for thyrotropin-beta (TSH beta) failed to detect TSH beta mRNA in the alpha-subunit producing tumor, while TSH beta mRNA was easily detected in the conventional thyrotropic tumor. We conclude that during the development of the alpha-subunit producing tumor from a thyrotropic tumor, loss of TSH beta mRNA was also associated with an impaired capacity for thyroid hormone to decrease concentrations of alpha-subunit mRNA.

Animals↗
PubMed1985

Pure alpha-subunit-secreting mouse pituitary tumor: inhibition of growth and secretion by combined treatment with thyroxine and bromocriptine.

We have recently documented the spontaneous development of a mouse pituitary tumor line (MGH 101A) secreting only the alpha-subunit of the glycoprotein hormones. Secretion of alpha-subunit by MGH 101A was not inhibited by either physiological or pharmacological levels of thyroid hormones. The present study demonstrates that combined treatment with bromocriptine and pharmacological doses of T4 for 1 month significantly decreased both tumor growth and alpha-subunit secretion of MGH 101A in thyroidectomized host mice. Either treatment alone was ineffective. The fall in plasma alpha-subunit concentrations after the combined treatment was probably due to the inhibition of tumor growth, since the decreases in plasma alpha-subunit levels and tumor weights were quantitatively similar, and there was no significant change in tumor steady state concentrations of alpha-subunit mRNA. In two of three experiments, the combined treatment resulted in a significant fall in tumor total DNA concentrations; this suggests that decreased tumor growth was at least in part due to inhibition of tumor cell replication. To compare the effects of the various treatments on the pure alpha-subunit-producing pituitary tumor with their effects on non-tumorous pituitary, we also measured secretion of TSH and PRL into plasma by the host pituitary. Treatment with T4 alone profoundly suppressed plasma concentrations of TSH and significantly reduced plasma PRL levels compared to levels in the thyroidectomized controls. Surprisingly, both thyroidectomized and euthyroid tumor hosts treated with bromocriptine alone showed no suppression of plasma PRL levels. However, combined treatment with bromocriptine and pharmacological doses of T4 resulted in plasma PRL concentrations significantly lower than those after treatment with T4 alone. Thus, the tumor and nontumorous host pituitaries differed in their responses to T4 alone. However, the two tissues were similar in their responses to bromocriptine alone and to combined treatment with bromocriptine and T4. We conclude that combined treatment with T4 and bromocriptine inhibited the growth and secretion of the pure alpha-subunit-secreting tumor MGH 101A. The data on PRL secretion by the host pituitary suggest that T4 may have acted by enhancing the function of dopamine receptors.

Animals↗
PubMed1984

Effects of propylthiouracil on D-galactosamine hepatotoxicity in the rat. Evidence for a non-thyroidal effect.

The cytoprotective effects of propylthiouracil (PTU) were studied in rats treated with the hepatotoxin D-galactosamine (D-GNH2). Five days of PTU pretreatment prior to D-GNH2 caused hypothyroidism and a significant reduction in liver injury as assessed by serum transaminase levels. When PTU was administered as a single dose with D-GNH2, significant decreases in transaminase also occurred at times when thyroid function was unchanged. Furthermore, aminopyrine oxidation showed significant impairment after D-GNH2 and was normalized by one dose of PTU. Further studies were carried out in thyroidectomized rats. PTU caused significant reductions in transaminase levels when given for 5 days pretreatment or as a single dose. Animals receiving pretreatment with PTU plus thyroxine (T4) also had significant decreases in serum transaminase. The antithyroid drug methimazole also had a hepatoprotective effect, while two other potent antithyroid compounds (2-thiouracil and 2-thiobarbituric acid) did not. These data suggest that PTU can protect against liver injury induced by D-GNH2, that the effect is independent of thyroid function, and that this effect is not common to all thiol-containing antithyroid drugs.

Aminopyridines↗
PubMed1984

Methimazole pharmacology in the rat: studies using a newly developed radioimmunoassay for methimazole.

Methimazole [1-methyl-2-mercaptoimidazole (MMI)] was given to normal male rats in their drinking water in concentrations ranging from 0.0001-0.05% for either 1 week or 1 month. Serum MMI levels in the rats ranged from 0.008-19.6 micrograms/ml, and were similar after 1 week and 1 month of treatment. Serum MMI was linearly related to the MMI concentration in the drinking water (r = 0.98, P less than 0.001). In contrast, intrathyroid MMI content plateaued with increasing MMI concentrations in the water, and was linearly related to the logarithm of the MMI concentration. At the highest MMI concentration (0.05%), thyroid MMI contents were similar in the 1-week and 1-month groups (approximately 1 X 10(-4) M). Surprisingly, at lower MMI concentrations, thyroid MMI content was significantly higher in the 1-week group than the 1-month group. Thyroid function was inhibited by MMI with similar depression of serum T4 or T3 after 1 week or 1 month of MMI treatment. Although the MMI concentration for 50% suppression of thyroid PBI was 0.003% in both groups, thyroid MMI content at this MMI concentration was 97 microM after 1 week but only 15 microM after 1 month. The continued thyroid-inhibiting activity of MMI at 1 month, despite a striking decrease in thyroid MMI content, may relate to intrathyroid iodide depletion, which was more severe after 1 month (thyroid 127I = 40 microM) than after 1 week (thyroid 127I = 140 microM) or in controls (470 microM). Rats were given 0.05% MMI for either 1 week or 1 month, and the drug was then withdrawn. In the 1-week group, serum MMI disappeared biexponentially, with a rapidly declining phase (t1/2 = 3.2 h) and a second, slower disappearance phase (t1/2 = 47.7 h). Similar findings were noted after 1 month of treatment. The disappearance of thyroid MMI was also biexponential after 1 week, but this variable could not be evaluated after 1 month because thyroid MMI fell rapidly to undetectable levels. There was a highly significant correlation in the 1-week group between the disappearance of MMI from the thyroid and the recovery of thyroid function as assessed by thyroid PBI (r = 0.81, P less than 0.01). Despite the very rapid disappearance of MMI from the thyroid after 1 month of treatment, the recovery time of thyroid PBI was significantly longer than after 1 week of treatment (2.1 days vs. 1.4 days for 50% recovery, P less than 0.01).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1983

The effect of somatostatin on the release of thyrotropin and its subunits from bovine anterior pituitary cells in vitro.

The effects of somatostatin (SRIF) on the production and release of TSH and its subunits have been investigated in bovine anterior pituitary monolayer cultures. SRIF caused a dose-dependent inhibition of TSH and subunit release by TRH, with a half-maximal effective dose of 3 X 10(-8) M. This effect was time dependent and was greater for TSH than for its subunits. However, the basal release and total production of TSH and its subunits over a 24-h period were not affected by SRIF. The effect of SRIF was additive to that of thyroid hormones in suppressing the release of TSH and its subunits by TRH. A combination of SRIF and thyroid hormone completely suppressed the release of TSH and its subunits by TRH. In contrast, thyroid hormones caused a dose-dependent inhibition of the total production, as well as the release, of TSH and its subunits induced by TRH. Furthermore, thyroid hormones produced a dose-dependent increase (r = 0.81; P less than 0.001) in the effectiveness of a single dose of SRIF in suppressing TSH release by TRH. Analysis of these data revealed that thyroid hormones interacted synergistically with the SRIF effect to suppress TRH-mediated TSH and subunit release.

Animals↗
PubMed1983

Changes in tissue concentrations of thyrotropin, free thyrotropin beta, and alpha-subunits after thyroxine administration: comparison of mouse hypothyroid pituitary and thyrotropic tumors.

We have previously demonstrated divergent changes in pituitary free TSH beta and alpha-subunit concentrations in hypothyroid mice during prolongation of experimental hypothyroidism and after T4 administration. This report compares the simultaneous responses of pituitary and thyrotropic tumor TSH, TSH beta, and alpha-subunit to 12 days of T4 administration in four groups of hypothyroid LAF1 mice bearing thyrotropic tumors. Half of each group received daily injections of T4 (10 micrograms/100 g BW, ip) for 12 days; the other half of each group received saline. Plasma concentrations of TSH, free TSH beta, and free alpha-subunit were suppressed by T4 administration to 0.1-1.5%, 2.0-3.9%, and 4.1-25% of control concentrations, respectively. Pituitary TSH and free TSH beta concentrations fell significantly with treatment to 24-43% and 10-28% of control concentrations, respectively. In contrast, pituitary alpha-subunit concentrations did not fall (they were 106-203% of control values), and a rise in the pituitary alpha-subunit concentration was statistically significant in one group (P less than 0.02). Thyrotropic tumor TSH and free TSH beta concentrations fell significantly with treatment to 9-31% and 8-35% of control concentrations, respectively. Tumor alpha-subunit concentrations did not fall (they were 87-195% of control values), and a 2-fold rise was statistically significant in one group (P less than 0.05). Both pituitary and tumor alpha-subunit to TSH beta molar ratios increased significantly with T4 administration in all groups. We conclude that (1) pituitary and thyrotropic tumor TSH beta concentrations fall after 12 days of T4 administration while alpha-subunit concentrations are unchanged or actually rise; (2) this divergent response is qualitatively similar in hypothyroid mouse pituitary and thyrotropic tumors; and (3) these data suggest differences between the regulation of intracellular TSH beta and alpha-subunit.

Animals↗
PubMed1983

Propylthiouracil (PTU) pharmacology in the rat. I. Serum and thyroid PTU measurements by radioimmunoassay.

We have developed a highly sensitive and specific RIA for propylthiouracil (PTU) which uses 125I-labeled PTU as the radioactive ligand. At a final antibody dilution of 1:10,000, the detection limit for PTU was 100 pg; cross-reactivity with circulating, urinary, and intrathyroid PTU metabolites was negligible. Using this assay, serum and thyroid PTU levels were determined after short term (1 week) and long term (1 month) PTU treatment at doses of 0.0001-0.05%. Serum PTU was a linear function of the PTU dose (r = 0.99; P less than 0.001), whereas thyroid PTU was a linear function of the logarithm of the PTU dose (r = 0.99; P less than 0.001). Serum PTU levels were higher after 1 month of treatment than after administration for 1 week, probably because steady state conditions were not achieved after 1 week. At several doses, thyroid PTU levels were also higher after 1 month of treatment, but the differences were not as striking as those seen in the serum levels. The pharmacokinetic data are consistent with a multicompartmental model for PTU distribution. The logarithmic relationship between thyroid PTU and PTU dose suggests a saturable uptake mechanism for PTU by the thyroid; inhibition of thyroid PTU uptake by PTU itself could also explain these observations.

Animals↗
PubMed1983

Propylthiouracil (PTU) pharmacology in the rat. II. Effects of PTU on thyroid function.

Using a sensitive and specific RIA for propylthiouracil (PTU), we examined the effects of short term (1 week) and long term (1 month) PTU treatment on thyroid function in the rat, and correlated changes in thyroid function with serum and thyroid PTU levels. After 1 week, dose-dependent decreases in thyroid PBI, serum T4, and serum T3 were observed, with concomitant elevations in the serum rT3 to T4 ratio and serum TSH. Fifty percent suppression of thyroid PBI occurred at a PTU concentration in the drinking water of 0.0005% (ED50), with concomitant serum and thyroid PTU levels of 0.3 micrograms/ml and 300 ng/thyroid, respectively. After 1 month of PTU, serum T4 values were lower than after 1 week of treatment for all PTU concentrations, but values for the other thyroid functional variables were similar to those in the 1 week group at comparable PTU dosage. The PTU dose-response curve for thyroid PBI was similar to that seen after 1 week of treatment, with an ED50 of 0.0004%. After discontinuation of PTU treatment, PTU disappeared from serum in a biexponential fashion, with an early rapid distribution phase (t 1/2 = approximately 4 h) and a second slower elimination phase (t 1/2 = approximately 2.6 days). In the thyroid, an initial increase in PTU content was seen up to 18 h after PTU withdrawal; thereafter, thyroid PTU declined linearly, with a t 1/2 of 1.4 days in both groups. After PTU withdrawal, thyroid PBI recovered with a t 1/2 of 1.09 days after 1 week on PTU, but recovery was prolonged (t 1/2 = 2.8 days) after 1 month of treatment. Log thyroid PTU and log thyroid PBI were linearly related after PTU withdrawal (r = 0.97; P less than 0.001) after 1 week but not after 1 month. Serum T4 and serum T3 remained below control values for 2 days, but then rapidly normalized, with T3 values rising transiently above the control value. This rebound occurred at a time when PTU was still present within the thyroid, before thyroid PBI had returned to baseline. These data indicate a close inverse relationship between PTU dose and both thyroid hormone biosynthesis and peripheral T4 deiodination. In addition, short and long term PTU treatments have quantitatively similar effects on thyroid function, although recovery of thyroid function is prolonged after long term treatment. The biexponential disappearance of PTU from the serum is compatible with a two-compartment model of PTU distribution. The early increase in thyroid PTU after drug withdrawal is suggestive of an inhibitory effect of PTU upon its own uptake by the thyroid, whereas the faster disappearance of PTU from the thyroid than from serum is consistent with intrathyroid drug metabolism.

Animals↗
PubMed1983

Mouse pituitary tumor line secreting only the alpha-subunit of the glycoprotein hormones: development from a thyrotropic tumor.

This report describes the conversion of a murine pituitary thyrotropic tumor (MGH 101) to a pure alpha-subunit-secreting tumor (MGH 101A) during a 6-yr period of serial transplantation. MGH 101 was a thyrotropic tumor originating from a hypothyroid mouse pituitary, growing only in hypothyroid hosts, and secreting large quantities of intact TSH and free alpha-subunit. Between the fourth and ninth transplantation generations, tumor TSH secretion declined progressively by at least 500-fold, to undetectable levels. In contrast, tumor secretion of free alpha-subunit decreased only 10-fold, and has since remained stable for nine transplantation generations. During the conversion to pure alpha-subunit secretion, MGH 101A exhibited growth in euthyroid as well as hypothyroid hosts, and increased its growth rate 2 to 3-fold. In contrast, the conventional thyrotropic tumor TtT 97 has maintained its secretion of both intact TSH and free alpha-subunit, its dependence on a hypothyroid environment, and its slower growth rate for nine generations. Gel chromatography of the media from tumor cell cultures confirmed that MGH 101A secreted only the free alpha-subunit, whereas TtT 97 secreted immunoactive TSH, TSH beta, and free alpha-subunit which eluted as separate peaks. We conclude that a dependent thyrotropic tumor has spontaneously developed into a pure alpha-subunit-secreting tumor which is independent of host thyroid function for its growth and alpha-subunit production.

Animals↗