Inhibitory effects of calcium channel blockers on thyroid hormone uptake in neonatal rat cardiomyocytes

The effects of the Ca2+ channel blockers verapamil, nifedipine, and diltiazem on triiodothyronine (T3) and thyroxine (T4) uptake were tested in cultured cardiomyocytes from 2-day-old rats. Experiments were performed at 37 degrees C in medium with 0.5% BSA for [125I]T3 (100 pM) or 0.1% BSA for [125I]T4 (350 pM). The 15-min uptake of [125I]T3 was 0.124 +/- 0.013 fmol/pM free T3 (n = 6); [125I]T4 uptake was 0.032 +/- 0.003 fmol/pM free T4 (n = 12). Neither T3 nor T4 uptake was affected by 1% DMSO (diluent for nifedipine and verapamil). Uptake of [125I]T3 but not of [125I]T4 was dose dependently reduced by incubation with 1-100 microM verapamil (49-87%, P < 0.05) or nifedipine (53-81%, P < 0.05). The relative decline in [125I]T3 uptake after 4 h of incubation with 10 microM verapamil or nifedipine was less than after 15 min or 1 h, indicating that the major inhibitory effect of the Ca2+ channel blockers occurred at the level of the plasma membrane. The reduction of nuclear [125I]T3 binding by 10 microM verapamil or nifedipine was proportional to the reduction of cellular [125I]T3 uptake. Diltiazem (1-100 microM) had no dose-dependent effect on [125I]T3 uptake but reduced [125I]T4 uptake by 45% (P < 0.05) at each concentration tested. Neither the presence of 20 mM K+ nor the presence of low Ca2+ in the medium affected [125I]T3 uptake. In conclusion, the inhibitory effects of Ca2+ channel blockers on T3 uptake in cardiomyocytes are not secondary to their effects on Ca2+ influx but, rather, reflect interference with the putative T3 carrier in the plasma membrane.

Thyroid hormone transport by the heterodimeric human system L amino acid transporter

Transport of thyroid hormone across the cell membrane is required for thyroid hormone action and metabolism. We have investigated the possible transport of iodothyronines by the human system L amino acid transporter, a protein consisting of the human 4F2 heavy chain and the human LAT1 light chain. Xenopus oocytes were injected with the cRNAs coding for human 4F2 heavy chain and/or human LAT1 light chain, and after 2 d were incubated at 25 C with 0.01-10 microM [(125)I]T(4), [(125)I]T(3), [(125)I]rT(3), or [(125)I]3,3'-diiodothyronine or with 10-100 microM [(3)H]arginine, [(3)H]leucine, [(3)H]phenylalanine, [(3)H]tyrosine, or [(3)H]tryptophan. Injection of human 4F2 heavy chain cRNA alone stimulated the uptake of leucine and arginine due to dimerization of human 4F2 heavy chain with an endogenous Xenopus light chain, but did not affect the uptake of other ligands. Injection of human LAT1 light chain cRNA alone did not stimulate the uptake of any ligand. Coinjection of cRNAs for human 4F2 heavy chain and human LAT1 light chain stimulated the uptake of phenylalanine > tyrosine > leucine > tryptophan (100 microM) and of 3,3'-diiodothyronine > rT(3) approximately T(3) > T(4) (10 nM), which in all cases was Na(+) independent. Saturation analysis provided apparent Michaelis constant (K(m)) values of 7.9 microM for T(4), 0.8 microM for T(3), 12.5 microM for rT(3), 7.9 microM for 3,3'-diiodothyronine, 46 microM for leucine, and 19 microM for tryptophan. Uptake of leucine, tyrosine, and tryptophan (10 microM) was inhibited by the different iodothyronines (10 microM), in particular T(3). Vice versa, uptake of 0.1 microM T(3) was almost completely blocked by coincubation with 100 microM leucine, tryptophan, tyrosine, or phenylalanine. Our results demonstrate stereospecific Na(+)-independent transport of iodothyronines by the human heterodimeric system L amino acid transporter.

Thyroid hormone uptake in cultured rat anterior pituitary cells: effects of energy status and bilirubin

Transport of thyroxine (T(4)) into the liver is inhibited in fasting and by bilirubin, a compound often accumulating in the serum of critically ill patients. We tested the effects of chronic and acute energy deprivation, bilirubin and its precursor biliverdin on the 15-min uptake of [(125)I]tri-iodothyronine ([(125)I]T(3)) and [(125)I]T(4) and on TSH release in rat anterior pituitary cells maintained in primary culture for 3 days. When cells were cultured and incubated in medium without glucose and glutamine to induce chronic energy deprivation, the ATP content was reduced by 45% (P<0. 05) and [(125)I]T(3) uptake by 13% (NS), but TSH release was unaltered. Preincubation (30 min) and incubation (15 min) with 10 microM oligomycin reduced ATP content by 51% (P<0.05) and 53% (P<0. 05) under energy-rich and energy-poor culture conditions respectively; [(125)I]T(3) uptake was reduced by 66% (P<0.05) and 64% (P<0.05). Neither bilirubin nor biliverdin (both 1-200 microM) affected uptake of [(125)I]T(3) or [(125)I]T(4). Bilirubin (1-50 microM) did not alter basal or TRH-induced TSH release. In conclusion, the absence of inhibitory effects of chronic energy deprivation and bilirubin on thyroid hormone uptake by pituitary cells supports the view that the transport is regulated differently than that in the liver.

Identification of thyroid hormone transporters

Thyroid hormone action and metabolism are intracellular events that require transport of the hormone across the plasma membrane. We tested the possible involvement of the Na+/taurocholate cotransporting polypeptide (Ntcp) and organic anion transporting polypeptide (oatp1) in the hepatic uptake of the prohormone T4, the active hormone T3, and the metabolites rT3 and 3,3'-T2. Xenopus laevis oocytes were injected with 2.3 ng Ntcp or oatp1 cRNA and, after 2-3 days, incubated for 1 h at 25 degrees C with usually 0.1 microM 125I-labeled ligand. Uninjected oocytes showed marked uptake of iodothyronines and this was further increased by Ntcp and oatp1 cRNA, i.e., 1.9- and 2.8-fold for T4, 1.7- and 1.7-fold for T3, 1.8- and 6.0-fold for rT3, and 1.3- and 1.4-fold for 3,3'-T2, respectively. Mostly due to much lower uptake by uninjected oocytes, Ntcp and oatp1 cRNA induced larger, 12- to 76-fold increases in uptake of iodothyronine sulfates. The Ntcp cRNA-induced iodothyronine uptake was completely inhibited in Na+-deplete medium, whereas the oatp1 cRNA-induced uptake was not affected. These results suggest that hepatic uptake of thyroid hormones and their metabolites is mediated at least in part by Ntcp and oatp1.

Uptake of thyroid hormones in neonatal rat cardiac myocytes

The uptake and metabolism of T3 and T4 were investigated in cardiomyocytes isolated from 2-day-old rats. Myocytes (2-5 x 10(5) cells/well) were cultured for 1 day in medium with 5% horse serum-5% FCS and subsequently for 4 days without serum; in some cases myocytes were cultured with serum throughout the culture period. Experiments were performed at 37 C in medium with 0.5% BSA for measurement of [125I]T3 (200,000 cpm; 200 pM) uptake and with 0.1% BSA for measurement of [125I]T4 (200,000 cpm; 350 pM) uptake. Uptake of [125I]T3, expressed as femtomoles per picomolar concentration of free hormone, with any incubation time between 15 min and 24 h was at least 2-fold higher than that of [125I]T4. Neither T3 nor T4 was deiodinated within 24 h. This was observed in cells cultured in the absence or presence of serum. After 15 min of incubation, [125I]T3 uptake was 0.048 +/- 0.002 fmol/pM free T3 (n = 9), and [125I]T4 uptake was 0.018 +/- 0.003 fmol/pM free T4 (n = 9). Although [125I]T3 uptake was reduced by 31-40% (P < 0.05) by coincubation with 100 nM to 10 microM unlabeled T3, that of [125I]T4 was not affected by 1 nM to 10 microM unlabeled T4, nor was [125I]T3 uptake reduced by 10 microM unlabeled T4. Preincubation (30 min) and incubation (15 min) with 10 microM oligomycin reduced cellular ATP by 56% (P < 0.05) and [125I]T3 uptake by 73% (P < 0.05), but had no effect on [125I]T4 uptake. Similarly, [125I]T3 uptake, but not [125I]T4 uptake, was dependent on temperature and partly dependent on the Na+ gradient, as shown by the inhibitory effect of 10 microM monensin (27%; P < 0.05). The effect of aromatic amino acids (2 mM) on [125I]T3 uptake increased in the order phenylalanine < tyrosine < tryptophan. It is concluded that T3 is taken up in neonatal cardiomyocytes by an energy-dependent carrier-mediated mechanism that is also partly dependent on the Na+ gradient. Such a transport mechanism for T4 is not present in the neonatal heart, but it may appear later during development.

Effects of interleukin-1 beta on thyrotropin secretion and thyroid hormone uptake in cultured rat anterior pituitary cells

The effects of interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF alpha) on basal and TRH-induced TSH release, and the effects of IL-1 beta on the uptake of [125I]T3 and [125I]T4 and on nuclear binding of [125I]T3 were examined. Furthermore, the release of other anterior pituitary hormones in the presence of IL-1 beta was measured. Anterior pituitary cells from male Wistar rats were cultured for 3 days in medium containing 10% FCS. Incubation were performed at 37 C in medium with 0.5% BSA for measurement of [125I]T3 uptake and with 0.1% BSA for measurement of [125I]T4 uptake. Exposure to IL-1 beta (1 pM-1 nM) or TNF alpha (100 pM) for 2-4 h resulted in a significant decline in TSH release, which was almost 50% (P < 0.05) for 1 nM IL-1 beta and 24% (P < 0.05) for 100 pM TNF alpha. Measurement of other anterior pituitary hormones (FSH, LH, PRL, and ACTH) in the same incubation medium showed that IL-1 beta did not alter their release. When the effects of IL-1 beta (1 pM-1 nM) and TNF alpha (100 pM) on TRH-induced TSH release were measured in short term experiments, the inhibitory effects had disappeared. The addition of 1-100 nM octreotide, a somatostatin analog, resulted in a decrease in TRH-induced TSH release up to 33% of the control value (P < 0.05). Exposure to dexamethasone (1 nM to 1 microM) affected basal and TRH-induced TSH release similar to the effect of IL-1 beta. The 15-min uptake of [125I]T3 and [125I]T4, expressed as femtomoles per pM free hormone, was not affected by the presence of IL-1 beta (1-100 pM). When IL-1 beta (100 pM) was present during 3 days of culture, TSH release was reduced to 88 +/- 2% of the control value (P < 0.05). This effect was not associated with an altered [125I]T3 uptake (15 min to 4 h) or with any change in nuclear T3 binding. We conclude that 1) IL-1 beta decreases TSH release by a direct action on the pituitary; 2) this effect is not due to elevated thyroid hormone uptake or increase T3 nuclear occupancy; 3) IL-1 beta does not affect TRH-induced TSH release or the release of other anterior pituitary hormones; and 4) TNF alpha affects basal and TRH-induced TSH release in the same way as IL-1 beta.

Uptake of 3,3',5,5'-tetraiodothyroacetic acid and 3,3',5'-triiodothyronine in cultured rat anterior pituitary cells and their effects on thyrotropin secretion

We compared the uptake, metabolism, and biological effects of tetraiodothyroacetic acid (Tetrac) and rT3 in anterior pituitary cells with those of T4 and T3. Cells were isolated from adult male Wistar rats and cultured for 3 days in medium with 10% fetal calf serum. Uptake was measured at 37 C in medium with 0.1% BSA for [125I]Tetrac (200,000 cpm; 240 pM) and [125I]T4 (100,000 cpm; 175 pM) or with 0.5% BSA for [125I]rT3 (100,000 cpm; 250 pM) and [125I]T3 (50,000 cpm; 50 pM). The free fraction of Tetrac was 1% that of T4 (in medium with 0.1 and with 0.5% BSA), and the free fraction of rT3 was half that of T3. Uptake of the four tracers increased sharply up to 1 h of incubation and then leveled off. Expressed as femtomoles per pM free hormone, uptake at equilibrium was 1.16 +/- 0.16 (n = 6) for Tetrac, 0.15 +/- 0.01 (n = 6) for T4, 0.023 +/- 0.003 (n = 6) for rT3, and 0.21 +/- 0.02 (n = 6) for T3. Cell-associated radioactivity after incubation for 24 h with [125I]Tetrac was represented for 15% by [125I]Triac; after incubation with [125I]T4 for 15-20% by [125I]T3, after incubation with [125I]rT3 for 6% by [125I]3,3'-T2, while [125I]T3 was still for 98% [125I]T3. Exposure of cells for 2 h to 100 nM TRH stimulated TSH release by 90-135%. Tetrac was effective in reducing this response at a free concentration of 0.05 pM, but rT3 was effective only at a free concentration of 16 nM. A free Tetrac concentration of 5 pM was equally effective as 50 pM free T4 in reducing the TSH response to TRH. In human serum, Tetrac was exclusively bound to T4-binding prealbumin. The free Tetrac fraction was 0.001% in control subjects and rose 2- to 12-fold in patients with nonthyroidal illness. As uptake of [125I]Tetrac in the pituitary was higher than that of T4 and T3, and it was more potent than T4 in reducing TSH release, Tetrac may be of potential significance for the regulation of TSH secretion in vivo.

Effects of a furan fatty acid and indoxyl sulfate on thyroid hormone uptake in cultured anterior pituitary cells

A furan fatty acid, 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) and indoxyl sulfate (Indox) accumulate in serum of uremic patients and inhibit the active uptake of thyroxine (T4) into hepatocytes. We tested the effects of CMPF and Indox on the uptake of [125I]triiodothyronine (T3) and [125I]T4 and thyroid-stimulating hormone (TSH) release in anterior pituitary cells. Pituitary cells (500,000/well) were cultured for 3 days in medium with 10% fetal calf serum. Experiments were performed at 37 degrees C in the same medium with 0.5% bovine serum albumin (BSA; [125I]T3 uptake and TSH secretion) or 0.1% BSA ([125I]T4 uptake). The 15-min uptake of [125I]T3 amounted to 0.074 +/- 0.003 fmol/pM free T3 (n = 23) and that of [125I]T4 to 0.033 +/- 0.002 fmol/pM free T4 (n = 32). Preincubation (30 min) and incubation (15 min) with CMPF (20-200 microM) did not alter the uptake of [125I]T3 but reduced [125I]T4 uptake by 27% (P < 0.05) at the highest concentration tested. Indox (40-400 microM) did not affect the uptake of [125I]T3 or [125I]T4. CMPF (40 microM) and Indox (80 microM) did not directly affect the basal or thyrotropin-releasing hormone (TRH)-induced TSH release nor interfere with the effect of 10 nM T3 on TRH-induced TSH release. In conclusion, the absence of inhibitory effects of CMPF or Indox on thyroid hormone uptake by pituitary cells suggests that the transport mechanism is regulated differently compared with that in hepatocytes and underscores the significance of the thyroid hormone carriers for the intracellular availability of T3.

Uptake of triiodothyroacetic acid and its effect on thyrotropin secretion in cultured anterior pituitary cells

The uptake of [125I]triiodothyroacetic acid ([125I]Triac) in anterior pituitary cells was investigated and compared with that of [125I]T3. Furthermore, the effects of Triac, T3, and T4 on TSH release were compared. Cells isolated from adult male Wistar rats were cultured for 3 days in medium with 10% fetal calf serum. Uptake was measured at 37 C with [125I]Triac (100,000 cpm; 120 pM) or [125I]T3 (50,000 cpm; 50 pM) in medium with 0.5% BSA. In this medium, the ratio of the free fractions of Triac, T3, and T4 was 1:8:1. Exposure of cells to 100 nM TRH for 2 h stimulated TSH release by 80-110% (P < 0.001). Comparing total hormone levels (1 nM to 1 microM), Triac and T3 were equally effective in reducing this response, and both were 10-fold more effective than T4. The time course (15 min to 4 h) of [125I]Triac uptake was similar to that of [125I]T3, showing equilibrium after 1 h. Unlabeled Triac (1 microM) reduced the uptake of [125I]Triac and [125I]T3 at all time intervals. Expressed per pM free hormone, the cellular and nuclear uptake of [125I]Triac were twice those of [125I]T3. The 15-min uptake of [125I]Triac was reduced by incubation with 10 nM unlabeled Triac (35%; P < 0.001). Maximum inhibition (56%; P < 0.001) was found with 10 microM Triac. A similar effect was seen with 10 microM T3, T4, or 3,3',5,5'-tetraiodothyroacetic acid. Preincubation (30 min) and incubation (15 min) with 10 microM oligomycin reduced the cellular ATP content by 51% (P < 0.001), [125I]T3 uptake by 77% (P < 0.001), and [125I]Triac uptake by only 25% (P < 0.001). The temperature dependence of [125I]Triac and [125I]T3 uptake was the same. Preincubation and incubation with 10 microM monensin (reduces the Na+ gradient) or 10 microM monodansylcadaverine (inhibits receptor-mediated endocytosis) reduced 15-min [125I] Triac uptake by 15% (P < 0.005) and 19% (P < 0.005), respectively. The data show that 1) Triac, on the basis of the free hormone concentration, is more potent than T3 or T4 in suppressing TSH secretion; and 2) the rapid uptake of [125I]Triac by the anterior pituitary occurs by a carrier-mediated mechanism that is only partially dependent on ATP or the Na+ gradient.

Uptake of thyroxine in cultured anterior pituitary cells of euthyroid rats

The uptake of [125I]T4 was investigated in cultured anterior pituitary cells isolated from adult fed Wistar rats and cultured for 3 days in medium containing 10% fetal calf serum. Experiments were performed with [125I]T4 (10(5) to 2 x 10(6) cpm; 0.35-7 nM) in medium containing 0.5% or 0.1% BSA. The uptake of [125I]T4 increased with time and showed equilibrium after around 1 h of incubation. The presence of 10 microM unlabeled T4 during incubation decreased the uptake of [125I]T4 by 65-70% at all time intervals. After 24 h of incubation, 1.5% iodide and 3.2% conjugates were detected in the medium, whereas around 20% of cellular radioactivity represented [125I]T3. The 15-min uptake of [125I]T4 was significantly reduced by simultaneous incubation with 100 nM T4 (by 24%; P < 0.05), 100 nM T3 (by 38%; P < 0.001), or 10 microM rT3 (by 32%; P < 0.001), whereas 10 microM tetraiodothyroacetic acid (Tetrac) had no effect. Furthermore, preincubation (30 min) and incubation (15 min) with 10 microM monodansylcadaverine, oligomycin, or monensin reduced the uptake of [125I]T4 by 30%, 50%, and 40%, respectively (all P < 0.001). Substitution of Na+ in the buffer by K+ diminished the uptake of [125I]T4 by 39% (P < 0.005); 2 mM phenylalanine, tyrosine, or tryptophan reduced [125I]T4 uptake by 18% (P < 0.05), 18% (P = NS), and 33% (P < 0.005), respectively. Our data suggest that the pituitary contains a specific carrier-mediated energy-requiring mechanism for [125I]T4 uptake that is partly dependent on the Na+ gradient. In addition, part of [125I]T4 uptake in the pituitary might occur through an amino acid transport system. When expressed per pM of free hormone, the 15-min uptake of [125I]T4 was approximately as high as that of [125I]T3. Because the reduction of [125I]T4 uptake by T4, T3, monodansylcadaverine, oligomycin, and monensin was roughly the same as the previously reported reduction of [125I]T3 uptake by the same compounds, it is further suggested that T4 and T3 share a common carrier in cultured anterior pituitary cells.

Ki-67 expression and BrdUrd incorporation as markers of proliferative activity in human prostate tumour models

The validity of the use of the monoclonal antibodies Ki-67 and anti-BrdUrd to evaluate proliferative activity of human prostate tumour models was studied. Growth of the transplantable PC-82 and PC-EW prostate tumours, as assessed by tumour volume measurements, was significantly correlated with the proliferative activity as reflected by BrdUrd incorporation into DNA (r = 0.64 and r = 0.78, respectively). The proliferative activity of PC-82 tumours detected by Ki-67 antigen expression paralleled the pattern observed with BrdUrd (r = 0.51) and a significant correlation (r = 0.60) between the results obtained with both markers was found. In growing PC-82 and PC-EW tumours only small variations in the Ki-67 and BrdUrd indices were observed. In contrast, Ki-67 expression in regressing PC-82 tumours varied considerably (2.7 +/- 2.2%). The BrdUrd index in regressing PC-82 tumours showed less variation (1.3 +/- 0.2%), but part of the BrdUrd-positive cells were found in the stromal (murine) part of the regressing tissue. It is concluded that the Ki-67 and BrdUrd proliferation markers are reliable parameters to monitor changes in growth of prostate tumour lines, but that in slow growing or regressing tumours Ki-67 and BrdUrd data should be interpreted with caution.

Assessment of the critical level of androgen for growth response of transplantable human prostatic carcinoma (PC-82) in nude mice

The androgen dependent prostatic carcinoma of human origin, PC-82, was used as a model system to investigate the effect of various levels of androgen on the growth of prostatic tumor tissue. Plasma testosterone levels in mice were correlated to tumor growth and intratumor concentrations of testosterone and 5 alpha-dihydrotestosterone. PC-82 tumor burden remained stable at plasma testosterone levels of 0.8 nmol/l., whereas tumor growth occurred at higher levels and tumor regression was observed at lower plasma levels. This critical level of testosterone corresponded with intratumor testosterone and 5 alpha-dihydrotestosterone concentrations of six to 10 and three to four pmol/gm. tissue, respectively, which are significantly above the levels found in castrated non-supplemented animals (3.1 and 1.4 pmol/gm. respectively). This indicates that remaining concentrations of dihydrotestosterone, which amount to two to three times the castrate level, are not stimulatory for tumor growth in the model of the androgen dependent PC-82 tumor.

Pharmacia Award 1990. The biological significance of low testosterone levels and of adrenal androgens in transplantable prostate cancer lines

The transplantable androgen-dependent human prostate tumor models PC-82 and PC-EW were used to study whether low levels of testosterone and androgens of adrenal origin were capable of stimulating the growth of prostatic carcinoma cells in these tumor models. At all circulating plasma testosterone levels applied in this study, much lower levels of dihydrotestosterone were found in PC-EW tumor tissue than in PC-82 tumor tissue. Nevertheless, both prostate tumor models had a similar threshold level of dihydrotestosterone for growth stimulation, i.e. 3-4 pmol/g tissue. This critical androgen level for tumor growth amounted to 2-3 times the tissue level found in castrated animals. At this threshold level approximately 2% of the cells showed proliferative activity, as assessed by bromodeoxyuridine incorporation into DNA. The adrenal androgen dehydroepiandrosterone did not stimulate PC-82 tumor growth, whereas androstenedione did induce a moderate increase in tumor volume. It is concluded that the adrenal androgen androstenedione exerts a stimulating effect on prostatic cancer cells when its conversion results in intra-tissue testosterone and dihydrotestosterone levels exceeding the threshold level for tumor growth.

Effects of low testosterone levels and of adrenal androgens on growth of prostate tumor models in nude mice

Two transplantable, androgen dependent prostate tumor models of human origin, PC-82 and PC-EW, were used to study the effect of low androgen levels and adrenal androgens on prostate tumor cell proliferation. Tumor load of the PC-82 and PC-EW tumors could be maintained constant when plasma testosterone levels were 0.8 and 0.9 nmol/l, respectively, corresponding with an intratissue 5 alpha-dihydrotestosterone level of 3-4 pmol/g tissue. This critical androgen level for prostate tumor growth stimulation amounted to 2-3 times the castration level and proved to be similar for both tumor models. Relatively high levels of androstenedione resulted in physiological levels of plasma testosterone causing androgen concentrations in PC-82 tumor tissue exceeding the critical level for tumor growth. These results indicate that submaximal suppression of androgens can stop tumor growth in these prostate tumor models.