Influence of calmodulin antagonists and calcium channel blockers on triiodothyronine uptake by rat hepatoma and myoblast cell lines

Transport of thyroxine and 3,3',5-triiodothyronine in human umbilical vein endothelial cells

The prerequisite for the uptake of thyroid hormone (TH) in peripheral tissues is the exit of TH from the bloodstream. The first step in this process is transport across the endothelium. Little is known about this important step in TH physiology. Therefore, we aimed to characterize the TH transport processes across the endothelium using human umbilical vein endothelial cells as a model. Transport studies showed rapid uptake of 1 nm [(125)I]T(3) and [(125)I]T(4) in these cells. The apparent Michaelis constant value for [(125)I]T(3) uptake was about 1 microm, and the IC(50) for T(4) inhibition of T(3) uptake was about 3 microm. The aromatic amino acids phenylalanine, tyrosine, and tryptophan and the L-type amino acid transporter-specific ligand 2-aminobicyclo-(2, 2, 1)-heptane-2-carboxylic acid did not inhibit [(125)I]T(3) or [(125)I]T(4) uptake. Verapamil was capable of reversibly reducing transport of [(125)I]T(3) and [(125)I]T(4). Human umbilical vein endothelial cells incubated with the affinity label BrAcT(3) resulted in a labeling of multiple proteins, which are probably protein disulfide isomerase related. Extrapolating our findings to the endothelial lining of blood vessels suggests that T(3) and T(4) uptake is mediated by the same transport system. Because TH transport characteristics do not correspond to known TH transporters, further studies are required to identify the TH transporter protein(s) at the molecular level. Possible candidates may be widely expressed Na(+)-independent transporter proteins.

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.

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.

Role of calmodulin in the differentiation/activation of microglial cells

In the present work the role of calmodulin (CaM) in regulating lipopolysaccharide (LPS)-induced microglial activation and in the spontaneous microglial differentiation has been investigated. We used pure rat microglial cell cultures to examine the effects of W13, a specific inhibitor of CaM, on microglial activation produced by LPS and the effect of CaM inhibition on microglial proliferation induced by the macrophage colony-stimulating factor (M-CSF). Microglial morphological transformation, inducible nitric oxide synthase (iNOS) activity and proliferating cell nuclear antigen (PCNA) immunostaining were determinate. Results show that CaM does not participate in the microglial increase of iNOS produced by LPS. In contrast, it is involved in spontaneous microglial ramification and in the activation of proliferation from quiescence. Multiple second-messenger pathways are involved in the transduction of signals initiated by LPS. The study of these mechanisms may allow us to extend our knowledge of the signals controlling the expression of these mediators.

Thyroid hormone export in rat FRTL-5 thyroid cells and mouse NIH-3T3 cells is carrier-mediated, verapamil-sensitive, and stereospecific

Export of L-T3 out of the cell is one factor governing the cellular T3 content and response. We previously observed in liver-derived cells that T3 export was inhibited by verapamil, suggesting that it is due to either ATP-binding cassette/multidrug resistance (MDR1/mdr1b) or multidrug resistance-related (MRP1/mrp1) proteins. To test this hypothesis we measured T3 export in FRTL-5, NIH-3T3, and rat hepatoma (HTC) cells that varied in expression of these proteins. FRTL-5 and NIH-3T3 cells were found to contain a T3 efflux mechanism that is verapamil inhibitable, saturable, and stereospecific. By contrast, T3 efflux in HTC cells was slow and unaffected by verapamil. Neither FRTL-5 nor NIH-3T3 cells express mdrlb, but all three cell types express mrpl, as assessed by immunoblotting. Overexpression of MDR1 in NIH-3T3 cells did not enhance verapamil-inhibitable T3 efflux. Photoaffinity labeling of FRTL-5 and NIH-3T3 cells with [125I]L-T3 revealed a labeled 90- to 100-kDa protein that was not present in HTC cells. Verapamil and excess nonradioactive L-T3, but not D-T3, inhibited labeling of this protein. The lack of correlation between T3 efflux and MDR1 and mrpl expression and the finding of a photoaffinity-labeled putative transport protein smaller than MDR1 or mrp1 protein (approximately 170 kDa) suggest that a novel protein is involved in the transport of T3 out of cells.

3-Amino-5-phenoxythiophenes: syntheses and structure-function studies of a novel class of inhibitors of cellular L-triiodothyronine uptake

A series of substituted 3-amino-5-phenoxythiophenes was synthesized starting from malodinitrile and carbon disulfide. The resulting dicyanoketenedithiolate reacts via Thorpe-Dieckmann cyclization with halogen methanes bearing electron-withdrawing groups to give thiophene-2-thiolates, which can be transformed into 3-amino-5-(methylsulfonyl)thiophene-4-carbonitriles. Replacement of the methylsulfonyl groups by substituted phenolates provides the substituted 3-amino-5-phenoxythiophenes. Some of the derivatives show a considerable inhibitory potency for the L-T3 uptake in inhibition studies on human HepG2 hepatoma cells with maximum values of about 60% at a dose of 10(-5) M for the most potent 2-benzoyl derivatives. The structure of the phenoxythiophenes fits well into a general concept derived for other classes of L-T3 uptake inhibitors, which postulates an angular and perpendicular orientation of the ring systems in these compounds as a prerequisite for an inhibitory potency. Docking studies for the phenoxythiophenes with transthyretin as a receptor model show their preferred attack at the L-T4/L-T3 binding channel.

Inhibition of thyroid hormone uptake by calcium antagonists of the dihydropyridine class

A series of substituted 4-phenyl-1,4-dihydropyridines 2a-m was tested for their inhibitory effects on L-triiodothyronine (L-T3) uptake by human HepG2 hepatoma cells. The most potent compounds were the nitro-substituted derivatives 2,6-dimethyl-4-(4'-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylic acid 3-ethyl ester 5-methyl ester (2m) and the well-known calcium antagonists nitrendipine (2k) and nifedipine (2j) with an uptake inhibition between 80.5 and 85.8% at an application dose of 10(-5) M. On the basis of a theoretical conformational analysis (ab initio MO theory, molecular mechanics, molecular dynamics) of the dihydropyridine derivatives, a unifying stereochemical concept was derived postulating an angular arrangement of the two rings where the phenyl ring of the calcium antagonists, which corresponds to the outer phenyl ring of the thyroid hormones, is bisecting the dihydropyridine ring as a prerequisite for inhibitory potency. This model includes also inhibitors of the N-phenylanthranilic acid type. The interaction of the calcium antagonists with transthyretin (TTR) is discussed in relation to thyroid hormones. The influence of hydrophobicity was estimated by the experimental determination of the 1-octanol/water partition coefficients.

Potentiation by thyroxine of interferon-gamma-induced antiviral state requires PKA and PKC activities

Added to HeLa cells previously exposed to recombinant human interferon (IFN)-gamma for 20 h, thyroid hormone [L-thyroxine (T4)] in physiological concentrations potentiates the antiviral action of IFN-gamma by more than 100-fold in 4 h. We examined protein kinase activities for their contributions to the mechanism of this posttranslational effect of thyroid hormone. Added concurrently with thyroid hormone, the protein kinase C (PKC) inhibitor CGP-41251 (5 nM) blocked T4 potentiation of IFN-gamma action. Coincubated with CGP-41251, phorbol 12-myristate 13-acetate (PMA) reversed the effect of the inhibitor on thyroid hormone action. U-73122 (10 nM), a phospholipase C inhibitor, also blocked hormone potentiation. KT-5720 (500 nM), a protein kinase A (PKA) inhibitor, completely inhibited the T4 effect, whereas 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) restored hormone action in the presence of KT-5720. In the absence of T4, 8-BrcAMP and PMA, added together to cells in the 4-h paradigm, fully reproduced hormone potentiation of the antiviral effect of IFN-gamma. Incubated individually with IFN-gamma-treated cells, the two agonists had no potentiating action. Thyroid hormone apparently must activate both PKA and PKC in the nongenomic pathway of IFN-gamma action to enhance antiviral activity in HeLa cells.

Thyroid hormone export regulates cellular hormone content and response

Actions of thyroid hormones (THs) are determined by intracellular free hormone concentration. Here we report that enhanced TH extrusion via a saturable, cold-sensitive mechanism lowers intracellular TH and causes TH resistance in hepatoma cells. Since these cells overexpress multidrug resistance P-glycoproteins and TH extrusion and resistance are blunted by verapamil, P-glycoproteins may mediate this resistance. Verapamil-inhibitable TH efflux was also found in primary hepatocytes, cardiocytes, and fibroblasts. These findings demonstrate that TH extrusion can modulate TH availability and action in mammalian cells.

Preferential inhibition of cytoplasmic T3 binding is associated with reduced nuclear binding in cultured cells

Previous studies from our laboratory have suggested that the nonsteroidal antiinflammatory drug, diclofenac (DCF), is a more potent competitor for T3 binding sites in cytoplasm than for those in the nucleus. In the present study we have examined the competitive potency for DCF and its effect on nuclear binding of T3 in cultured cells. DCF was a weak competitor for T3 binding sites in cytosol and nuclear extracts prepared from HepG2 cells with a potency of 21 and 295 microM, respectively. When expressed relative to T3, DCF was 135-fold more potent in cytosol than in nuclear extract. In intact cells, T3 was bound by nuclei with an affinity, Kd of 0.22 +/- 0.07 nM whereas in nuclear extract the affinity was 0.60 +/- 0.21 nM. DCF was a competitive inhibitor in both preparations but reduced the apparent affinity 4-fold in intact cells but only 2-fold in nuclear extract. In whole-cell experiments, DCF increased the rate of dissociation of T3 from cells prelabeled with hormone for 30 min. When these prelabeled cells were incubated with DCF, 0.1 mM, cell-associated T3 was significantly lower at 30 and 60 min than in cells reincubated without the drug. These data show that cellular transport mechanisms precede nuclear binding by T3 and suggest that there is a critical role for nonnuclear binding proteins in thyroid hormone action.

Potentiation by thyroxine of interferon-gamma-induced HLA-DR expression is protein kinase A- and C-dependent

L-Thyroxine (T4) and 3,3',5-L-triiodothyronine (T3) potentiate the antiviral state induced by interferon-gamma(IFN-gamma) in homologous cells by a mechanism that is dependent upon calcium/phospholipid-dependent protein kinase (PKC). L-T4 and T3 also potentiate induction by IFN-gamma of MHC class II HLA-DR antigen expression in HeLa cells. In the present studies of HLA-DR expression, the PKC inhibitor staurosporine (0.1-1 nM) enhanced the expression of HLA-DR when the inhibitor was added simultaneously with IFN-gamma, 100 IU/ml. In the presence of IFN-gamma and 10(-7) M T4, the same concentrations of staurosporine inhibited potentiation of HLA-DR expression by thyroid hormone. A more specific PKC inhibitor, CGP41251 (0.5-5 nM), similarly enhanced HLA-DR expression in the presence of IFN-gamma but inhibited thyroid hormone potentiation of antigen expression. Both actions of CGP41251 were suppressed when cells were also treated with phorbol 12-myristate 13-acetate (PMA). A phospholipase C inhibitor, U73122 (1-1000 nM), did not alter the potentiating ability of T4, although it inhibited in a concentration-dependent manner the expression of HLA-DR induced by IFN-gamma. The potentiating effect of T4 was much more sensitive to a cyclic AMP-dependent protein kinase (PKA) inhibitor,KT5720 (1-1000nM), than was the induction of HLA-DR by IFN-gamma. The inhibitory effects of KT5720 were reversed by concurrent 8-bromo-cAMP treatment. The calmodulin antagonist W-7 (5-50 microM) did not alter IFN-gamma induction of HLA-DR in either the presence or absence of T4. HLA-DR expression in HeLa cells appears to be under PKC-associated inhibition; IFN-gamma reverses this inhibition to promote the appearance of the DR antigen. In contrast, potentiation by T4 of induction of HLA-DR by IFN-gamma requires activation of PKC. PKA is involved both in DR induction by IFN-gamma and in potentiation of the latter by T4. Thus, PKA and PKC have discrete roles in IFN-gamma-induced MHC class II antigen expression and its modulation by thyroid hormone.