Interaction between thyroid hormones and erythrocyte membranes: competitive inhibition of binding 131 I-L-triiodothyronine and 131 I-L-thyroxine by their analogs

Tetrac as an anti-angiogenic agent in cancer

The thyroid hormones T3 and T4 have emerged as pro-angiogenic hormones with important implications for cancer management. Endogenous circulating hormone levels may help stimulate cancer progression and limit the effectiveness of anticancer therapy, though clinical data remain inconclusive. The capacity of thyroid hormones to modulate angiogenesis is mediated through non-canonical mechanisms initiated at the cell surface receptor integrin αvβ3. This integrin is predominantly expressed on tumour cells, proliferating endothelial cells and tumour stroma-associated cells, emphasising its potential relevance in angiogenesis and tumour biology. Thyroid hormone/integrin αvβ3 signalling results in the activation of intracellular pathways that are commonly associated with angiogenesis and are mediated through classical pro-angiogenic molecules such as vascular endothelial growth factor. The naturally occurring T4 analogue tetrac blocks the pro-angiogenic actions of thyroid hormones at the integrin receptor, in addition to agonist-independent anti-angiogenic effects. Tetrac reduces endothelial cell proliferation, migration and tube formation through a reduction in the transcription of vascular growth factors/growth factor receptors, hypoxia-inducible factor-1α, pro-angiogenic cytokines and a number of other pro-angiogenic genes, while at the same time stimulating the expression of endogenous angiogenesis inhibitors. It further modulates vascular growth factor activity by disrupting the crosstalk between integrin αvβ3 and adjacent growth factor receptors. Moreover, tetrac disrupts thyroid hormone-stimulated tumour recruitment, differentiation and the pro-angiogenic signalling of tumour stroma-associated mesenchymal stem cells. Tetrac affects tumour-associated angiogenesis via multiple mechanisms and interferes with other cancer cell survival pathways. In conjunction with its low toxicity and high tissue selectivity, tetrac is a promising candidate for clinical application.

The kinetics of thyroid hormone transporters and their role in non-thyroidal illness and starvation

Kinetic tracer studies show that thyroid hormones are transported into target tissues by stereospecific, high-affinity, low-capacity transporters, both in animals and humans. The K(d) of binding to the transporter varies within the nanomolar range. The different thyroid hormones (T(4), T(3), and rT(3)) are transported via different transporters, except in the pituitary, where they share the same transporter. The molecular mass of the transport proteins varies between 52 and 65kDa. The transport mechanisms are dependent on the energy charge of the cell and -- often -- the sodium gradient over the plasma membrane. A relationship exists with the transport systems of the aromatic amino acids. In non-thyroidal illness and starvation T(4) transport into T(3)-producing tissues is decreased, resulting in a low plasma T(3) concentration, by some considered to be an energy saving mechanism in situations of stress.

Cell-surface receptor for thyroid hormone and tumor cell proliferation

Integrin αVβ3 is a structural protein of the plasma membrane that transduces signals from extracellular matrix proteins and has recently been shown to contain a novel receptor for thyroid hormone. Thyroid hormone signals are converted by αVβ3 into mitogen-activated protein kinase (MAPK) (ERK1/2) activation and downstream intracellular events in the cell nucleus. The latter include post-translational modification of the nuclear thyroid hormone receptor (TRβ1) and complex cellular or tissue responses, such as hormone-induced angiogenesis via basic fibroblast growth factor release. The integrin receptor for thyroid hormone has been shown to mediate proliferative effects of the hormone on certain tumor cell lines, including murine glioma/glioblastoma cells and human breast cancer (MCF-7) cells. More than one mechanism may account for this hormonal action, but in vitro studies indicate a direct hormonal action on cellular proliferation. Other possible mechanisms involve indirect actions via the release of tumor growth factors and effects on cell migration. In the intact organism, support of tumor growth by thyroid hormone is postulated to include angiogenesis. Crosstalk between the integrin thyroid hormone receptor and the epidermal growth factor receptor on the plasma membrane may be another mechanism by which thyroid hormone may modify tumor cell growth. Tetraiodothyroacetic acid (tetrac) is an iodothyronine analog that has no agonist activity at the integrin receptor, but inhibits binding of l-thyroxine and 3,5,3´-triiodo-l-thyronine to the receptor, preventing MAPK activation and consequent actions downstream of MAPK. In vitro studies and a preliminary in vivo experiment indicate that tetrac blocks the action of thyroid hormone on tumor cell proliferation. Both unmodified tetrac and tetrac reformulated as a nanoparticle that does not gain access to the cell interior are under investigation in animal models as anticancer agents. Also under study is the susceptibility of other human cancer cell lines to induction of proliferation by physiological concentrations of thyroid hormone.

Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability

Although it was originally believed that thyroid hormones enter target cells by passive diffusion, it is now clear that cellular uptake is effected by carrier-mediated processes. Two stereospecific binding sites for each T4 and T3 have been detected in cell membranes and on intact cells from humans and other species. The apparent Michaelis-Menten values of the high-affinity, low-capacity binding sites for T4 and T3 are in the nanomolar range, whereas the apparent Michaelis- Menten values of the low-affinity, high-capacity binding sites are usually in the lower micromolar range. Cellular uptake of T4 and T3 by the high-affinity sites is energy, temperature, and often Na+ dependent and represents the translocation of thyroid hormone over the plasma membrane. Uptake by the low-affinity sites is not dependent on energy, temperature, and Na+ and represents binding of thyroid hormone to proteins associated with the plasma membrane. In rat erythrocytes and hepatocytes, T3 plasma membrane carriers have been tentatively identified as proteins with apparent molecular masses of 52 and 55 kDa. In different cells, such as rat erythrocytes, pituitary cells, astrocytes, and mouse neuroblastoma cells, uptake of T4 and T3 appears to be mediated largely by system L or T amino acid transporters. Efflux of T3 from different cell types is saturable, but saturable efflux of T4 has not yet been demonstrated. Saturable uptake of T4 and T3 in the brain occurs both via the blood-brain barrier and the choroid plexus-cerebrospinal fluid barrier. Thyroid hormone uptake in the intact rat and human liver is ATP dependent and rate limiting for subsequent iodothyronine metabolism. In starvation and nonthyroidal illness in man, T4 uptake in the liver is decreased, resulting in lowered plasma T3 production. Inhibition of liver T4 uptake in these conditions is explained by liver ATP depletion and increased concentrations of circulating inhibitors, such as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indoxyl sulfate, nonesterified fatty acids, and bilirubin. Recently, several organic anion transporters and L type amino acid transporters have been shown to facilitate plasma membrane transport of thyroid hormone. Future research should be directed to elucidate which of these and possible other transporters are of physiological significance, and how they are regulated at the molecular level.

Binding of thyroid hormones to human hemoglobin and localization of the binding site

Radiolabeled thyroid hormones were allowed to bind to erythrocyte cytosol and the complex was fractionated by Sephadex G-100 or by high-performance liquid chromatography (HPLC). On Sephadex G-100, four radioactive peaks (P1-P4) were obtained, whereas HPLC gave only three radioactive peaks (P1-P3). Chromatographic studies with human adult Hb and non-Hb cytosol protein fractions, which had been reacted with radiolabeled thyroid hormones, and immune precipitation with specific antisera for the hormones, confirmed that the first peak of Sephadex G-100 radioactivity was a mixture of Hb and non-Hb proteins, while the second peak was Hb. The third peak was free 125I and the fourth peak was unbound 125I-T3 or 125I-T4. The third peak of HPLC was confirmed to be a mixture of free 125I and unbound radiolabeled thyroid hormones. Scatchard analysis of the interaction between T4 and apo-Hb, and the alpha- and beta-chains of human Hb suggested the presence of the specific binding site(s) for the hormone. Interaction between T4 and synthesized peptides, which constitute the heme pocket of the beta-chain of Hb (beta 61-75, beta 71-85, beta 81-95), indicated that the T4 binding site of Hb resides within the heme-binding cavity. It is concluded that human erythrocyte cytosol does not contain "receptor" for thyroid hormones and cannot be a model for studying functions of cytosol "receptor" for the hormones; rather, it contains binding protein with large binding capacity, including Hb and non-Hb proteins, which possibly constitute a large reservoir for the hormone in blood.

Solubilization and purification of a membrane-associated 3,3',5-tri-iodo-L-thyronine-binding protein from rat erythrocytes

3,3',5-Tri-iodo-L-thyronine (L-T3) binding sites from rat erythrocyte membranes were solubilized in an active form by using the zwitterionic detergent CHAPS or the anionic detergent lauroylsarcosine. The binding protein was successively purified by Sephadex G-200 and affinity chromatography. The purified material retained its binding activity and exhibited high affinity and specificity compared with those displayed in the original membrane. Yield was about 10% of the starting activity. The specific binding activity was enriched by approx. 100-fold, which represents a purity of only 0.1%. Analysis of the purified preparation on SDS/PAGE showed two major protein bands (Mr 64,000 and Mr 50,000), but these could not represent the binding protein since the purity obtained was low. However, affinity-labelling experiments with N-bromoacetyl-L-[125I]T3 in intact membranes showed that two proteins (also with Mr values of 64,000 and 50,000) bound the hormone specifically, suggesting a co-migration of hormone receptors and contaminants on gel electrophoresis.

Action of the thyroid hormone at the level of the plasma membrane

In this presentation, I present evidence indicating a direct action of thyroid hormone at the level of the plasma membrane. Characteristically, the plasma membrane-mediated effects of thyroid hormones are prompt in onset, independent of new protein synthesis, and are associated with changes in the transmembrane transport of ions and substrates. The presence of specific binding sites for thyroid hormone in plasma membrane of various tissues and species, although inconclusive in itself, provides additional support for the direct action of thyroid hormone on the plasma membrane. A model for the mechanism of action of thyroid hormone at the plasma membrane level to increase sugar uptake by rat thymocytes is delineated, and the physiological role of the plasma membrane-mediated action of thyroid hormone is discussed.

Immunofluorescent detection and localization of thyroxine in blood of adult amphibians

Indirect immunofluorescent staining was used to detect and localize thyroxine (T4) in blood smears from different species of adult amphibians, namely, Rana pipiens, Rana catesbeiana, Bufo marinus, Xenopus laevis, and Notopthalmus viridescens. Fluorescence, indicative of T4, was observed in both plasma and erythrocytes (RBC) from all individuals of the five species studied. It was weak and diffuse in the plasma and in the cytoplasm of the RBC but was intense in the nuclei (especially the nuclear perimeter) of these cells. The finding of intracellular T4 suggests that thyroid hormone may be of some physiological importance in adult amphibians.

In vitro effects of thyroid hormones on red blood cell Ca++-dependent ATPase activity

Thyroid hormones (TH) have been shown to exert a direct stimulatory effect on the Ca++-dependent ATPase from human and other mammalian erythrocytes. In this in vitro system, T4 has been shown to be more effective than T3. In the present study, TH effects on Ca++-dependent ATPase were investigated, using rabbit and human erythrocyte membranes, after preincubation with 10(-10) M T4, in the presence or in the absence of exogenous calmodulin (CaM) (5.10(-12) M to 5.10(-9) M). Ca++-dependent ATPase activity was measured as inorganic phosphate (Pi) release from 1 mM ATP. The results showed that basal Ca++-dependent ATPase activity in rabbits was moderately increased by T4 (1.44 +/- 0.05 vs 1.32 +/- 0.04 mumol Pi/mg protein/90 min, mean +/- SE; p less than 0.05). The time course of Pi release did not show any stimulatory effect of T4 during the first hour of incubation. The effect of T4 became apparent, however, 1 h after the addition of ATP (delta T4: 15%). With human membranes, T4 induced a relative stimulation of the Ca++-dependent ATPase of 8-10% (p less than 0.05) in experimental conditions where the enzyme was not maximally stimulated by CaM (delta CaM over basal activity: 5-40%). In conditions of high CaM stimulation (delta CaM: 50-320%), T4 had no effect. These results confirm that Ca++-dependent ATPase activity is increased by T4. The effect of T4 is small, and appears as a late event during incubation with ATP. Stimulation by T4 is expressed in states of low enzyme activation by CaM.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunofluorescent detection and localization of thyroxine in blood of Rana catesbeiana from early larval through metamorphic stages

Indirect immunofluorescent staining was used to detect and localize thyroxine (T4) in blood smears from individual Rana catesbeiana tadpoles in almost every stage of larval development. Earlier radioimmunoassays revealed a surge of T4 in the blood plasma during metamorphic stages, but plasma T4 concentrations in earlier stages were either very low or below the minimal detectable limits of the assays. With the present immunofluorescent method, T4 was found in plasma of tadpoles throughout the entire larval period from early limb bud stages to the end of metamorphosis. Moreover, T4 was also found in association with cytoplasm and nuclei of red blood cells, particularly nuclei of the new population of adult red cells differentiating during metamorphic stages. In conclusion, thyroid hormone is present in both blood plasma and erythrocytes of R. catesbeiana from early through late stages of larval development.

Evidence that the uptake of tri-iodo-L-thyronine by human erythrocytes is carrier-mediated but not energy-dependent

We investigated 3,3',5-tri-iodo-l-thyronine transport by human erythrocytes and by ;ghosts' prepared from these cells. Uptake of tri-iodothyronine by erythrocytes at 37 degrees C was time-dependent with a maximum reached after 60min. Tracer analysis after incubation for 1min revealed only one saturable binding site, with K(m) 128+/-19nm (mean+/-s.e.m.; n=7) and V(max.) 4.6+/-0.7pmol of tri-iodothyronine/min per 6x10(7) cells. After 10min incubation K(m) 100+/-16nm (n=10) was found with V(max.) 7.7+/-1.2pmol of tri-iodothyronine/10min per 6x10(7) cells. At 0 degrees C the uptake system is still active, with K(m) 132+/-26nm and V(max.) 1.8+/-0.3pmol of tri-iodothyronine/10min per 6x10(7) cells. The V(max.) with intact cells is 5-fold greater than the V(max.) with membranes derived from the same amount of cells when uptake studies are performed in media with similar free tri-iodothyronine concentrations. This indicates that at least 80% of tri-iodothyronine taken up by the intact erythrocytes enters the cell. This saturable uptake system can be inhibited by X-ray-contrast agents in a dose-dependent fashion. (+/-)-Propranolol, but not atenolol, has the same effect, indicating that the membrane-stabilizing properties of (+/-)-propranolol are involved. Furthermore, there is no inhibition by ouabain or vanadate, which indicates that tri-iodothyronine uptake is not dependent on the activity of Na(+)+K(+)-dependent adenosine triphosphatase. We have prepared erythrocyte ;ghosts', resealed after 2.5min with 0mm-, 2mm- or 4mm-ATP inside. Inclusion of ATP and integrity of the membrane of the erythrocyte ;ghosts' were verified on the basis of an ATP-concentration-dependent functioning of the Ca(2+) pump. No difference was found in the uptake of tri-iodothyronine by erythrocyte ;ghosts' with or without ATP included, indicating that uptake of tri-iodothyronine is not ATP-dependent. The following conclusions are drawn. (1) Tri-iodothyronine enters human erythrocytes. (2) There is only one saturable uptake system present for tri-iodothyronine, which is neither energy (i.e. ATP)-dependent nor influenced by the absence of an Na(+) gradient across the plasma membrane. This mode of uptake of tri-iodothyronine by human erythrocytes is in sharp contrast with that of rat hepatocytes, which uptake system is energy-dependent and ouabain-sensitive [Krenning, Docter, Bernard, Visser & Hennemann (1978) FEBS Lett.91, 113-116; Krenning, Docter, Bernard, Visser & Hennemann (1980) FEBS Lett.119, 279-282]. (3) X-ray-contrast agents inhibit tri-iodothyronine uptake by erythrocytes in a similar fashion to that by which they inhibit the uptake of tri-iodothyronine by rat hepatocytes [Krenning, Docter, Bernard, Visser & Hennemann (1982) FEBS Lett.140, 229-233].

Studies on the mechanism of thyroid hormone stimulation in vitro of human red cell Ca2+-ATPase activity

The stimulation in vitro of human red blood cell Ca2+-ATPase activity by thyroxine (T4) and triiodothyronine (T3) in physiological concentrations is shown to depend upon binding of iodothyronines to red cell membranes. Calmodulin enhances the activity of thyroid hormone in this model system but there is no direct interaction of calmodulin and hormone.

Binding of thyroid hormone by human erythrocyte cytosol proteins

Gel filtration (G-100, 0.01 M Tris, pH 7.4) of post-100,000 x g supernatant from lysate of washed human erythrocytes (RBC) revealed 3 fractions (R-1, R-2, R-3) which bound labeled T3 and T4. Major peak R-2 emerged with the mehoglobin fraction (A560 nm) and binding by this fraction was partially dissociable; the dissociable site bound D-T4, but not tetraidothyroacetic acid or reverse T3. Non-dissociable binding characterized peaks R-1 and R-3. R-1, R-2, and R-3 were pronase-digestible and R-1 binding was acid-unstable (pH 6.8 vs. 7.4). Evidence developed herein and elsewhere indicates that hemoglobin, itself, accounts for the binding within fraction R-2. Intact RBCs maintained for 72 hr at 4C in buffer enriched with T3 or T4 showed progressive incorporation with time of iodothyronines into the hemoglobin fraction.