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Biomedical subjects

S Y Cheng

Publications and source records attributed to S Y Cheng.

At least 127 records · Page 7Linked to original sources
PubMed1985

Solubilization and characterization of a membrane 3, 3', 5-triiodo-L-thyronine binding protein from rat pituitary tumor GH3 cells.

To understand the mechanism by which T3 enters cells and carries out its biological functions membrane binding sites for 3, 3', 5-triiodo-L-thyronine were solubilized from rat pituitary tumor GH3 cells by detergents. Among three detergents tested, CHAPS is the best in preserving hormonal binding affinity and specificity. Least square analysis of the binding data show one class of binding site with a Kd of (6.35 +/- 1.27) nM and Bmax of (0.84 +/- 0.056) pmoles/50 micrograms protein. Hormone binding activity is lost by heating, pronase digestion and in the absence of NaCl. The pH optimum for binding is 7.0 and the binding activity is enhanced by dithiothreitol. The solubilization of membrane-associated thyroid hormone binding proteins will facilitate further characterization and exploration of their biological functions.

Animals↗
PubMed1985

Transverse motion of spin-labeled 3,3',5-triiodo-L-thyronine in phospholipid bilayers.

Using electron spin resonance stop-flow technique, the transverse motion (flip-flop) of 3-([alpha-carboxy-4-(4-hydroxy-3-iodophenoxy)-3,5- diiodophenethyl]carbamoyl)-2,2,5,5-tetramethyl-3-pyrrolin (T3-SL) in dipalmitoyl L-alpha-phosphosphatidylcholine (DPPC) membranes was evaluated. At 22 degrees C, the electron spin resonance spectra of T3-SL in DPPC vesicles were compared before and after the addition of sodium ascorbate, a membrane impermeable reducing agent. The addition of ascorbate reduces the signal amplitude by 67% in 3 min but yields no further reduction for at least 60 min. These results indicate that T3-SL does not flip-flop at any appreciable rate in the membranes. This finding suggests that once partitioned into the membrane, T3 remains in the outer half of the lipid bilayer, thus reducing the possibility that T3 enters the cell by passive diffusion.

Ascorbic Acid↗
PubMed1984

Molecular dynamics of 3,3',5-triiodo-L-thyronine in model membranes: a spin label study.

A spin-labeled derivative of 3,3',5-triiodo-L-thyronine, 3-[( alpha-carboxy-4-(4-hydroxy-3-iodophenoxy)-3,5-diiodophenethyl++ +] carbamoyl)-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy (SL-T3) has been synthesized. Evaluation of its binding to nuclei after incubation with rat pituitary tumor GH3 cells at 37 degrees C showed that it bound to nuclei with a 18% potency of that of T3. The dynamic interaction of SL-T3 with multilamellar vesicles prepared from dimyristoylphosphatidylcholine (DMPC) was investigated using electron spin resonance techniques. At 31 degrees C, the lateral diffusion constant of SL-T3 in DMPC membranes was found to be 3.0 X 10(-8) cm2/s as determined by the ESR line-broadening method. The temperature dependency of the ESR spectrum of SL-T3 in DMPC multilamellar vesicles showed a break at 23.5 degrees C, which is close to the main phase-transition temperature, 23.7 degrees C, of DMPC membranes. This suggests that the motion of the probe reflects the motion of phospholipids in DMPC membranes, and that the probe itself does not perturb the membrane structure. SL-T3 appears to be a useful probe for studying the motion of thyroid hormone in the plasma membrane of responsive cells.

Animals↗
PubMed1983

Characterization of binding and uptake of 3,3',5-triido-L-thyronine in cultured mouse fibroblasts.

The binding and internalization of 3,3'-[125I] 5-triiodo-L-thyronine ([125I]T3) was studied in cultured Swiss 3T3-4 mouse fibroblasts. At 0 C, the binding of T3 to cells is saturable, reversible, and stereospecific. These results together with those of earlier fluorescence studies using rhodamine-labeled T3 demonstrate the presence of specific plasma membrane T3 receptors. At 37 C, the uptake of T3 reached a steady state after 1 h, and approximately 57 fmol T3 were specifically taken up by 10(6) cells. In other cell lines, 7, 19, and 201 fmol T3 were specifically taken up by Chinese hamster ovary cells (subclone 10001), Kirsten sarcoma virus-transformed NIH 3T3 mouse fibroblasts, and nontransformed NIH 3T3 mouse fibroblasts, respectively. Incorporation of T3 into nuclei followed similar kinetics and accounted for approximately 9% of the total cellular uptake. Equilibrium binding studies of T3 to isolated nuclei showed one class of binding sites with an apparent association constant of 5 X 10(9) M-1 and a binding capacity of 16 fmol/100 micrograms DNA. At 37 C, the internalization of T3 was nearly totally blocked by antimycin A or rotenone, inhibitors of oxidative phosphorylation. These results indicate that the uptake of T3 is an energy-dependent process. In the presence of bacitracin or monodansylcadaverine, substances that inhibit the receptor-mediated endocytosis of alpha 2-macroglobulin, the cellular uptake of T3 as well as the nuclear incorporation of T3 were inhibited in a concentration-dependent manner. The half-maximal inhibitory concentrations for the cellular uptake of T3 were 90 and 660 microM for monodansylcadaverine and bacitracin, respectively; for nuclear incorporation, they were 70 and 350 microM for monodansylcadaverine and bacitracin, respectively. These results indicate that receptor-mediated endocytotic uptake of T3 is a physiologically significant pathway.

Animals↗
PubMed1983

Structural similarities in the plasma membrane 3,3',5-triiodo-L-thyronine receptors from human, rat and mouse cultured cells. Analysis by affinity labeling.

Affinity labeling of Swiss 3T3-4 mouse fibroblasts with 0.3 nM N-bromoacetyl-3,[125I]3',5-triiodo-L-thyronine (BrAc[125I]T3) at 4 C for 0.5 h showed three labeled protein bands with apparent molecular masses of 55, 53 and 33 kilodaltons (kDal) in a ratio of 85:8:7. Only the labeling of 55-kDal protein was selectively reduced to approximately 50% by 15 microM unlabeled T3. Affinity labeling of the purified plasma membranes under identical conditions yielded similar results. One-dimensional peptide mapping by Staphylococcus aureus V8 or elastase digestion of the 55-kDal proteins from cells or plasma membranes gave identical peptide fragments. Thus the 55-kDal protein is a membrane-associated protein. Furthermore, affinity labeling of human epithelioid carcinoma A431 cells and purified plasma membranes gave similar results as those of Swiss 3T3 cells. Peptide mapping of elastase or S. auerus digestion of 55-kDal proteins from A431, Swiss 3T3 and GH3 rat pituitary tumor cells gave identical patterns. These results indicate that plasma membrane T3 receptors from three species have structural similarity in the hormone binding domains. Thus, the plasma membrane T3 receptor probably are highly conserved.

Affinity Labels↗
PubMed1982

Inhibition of the nuclear entry of 3,3',5'-triiodo-L-thyronine by monodansylcadaverine in GH3 cells.

We have recently reported that the entry of 3,3',5-triiodo-L-thyronine (T3) into mouse 3T3 fibroblasts occurs by receptor-mediated endocytosis (Cheng, S. Y., Maxfield, F. R., Robbins, J., Willingham, M. C., and Pastan, I. (1980) Proc. Natl. Acad. Sci. U. S. A. 77: 3425-3429). In this communication, we evaluated the functional significance of this mode of entry using GH3 cells, a growth hormone-producing cell line which has a high number of T3 nuclear receptors. T3-specific, saturable membrane uptake systems were demonstrated in GH3 cells. Monodansylcadaverine, an inhibitor of alpha 2-macroglobulin, epidermal growth factor, vesicular stomatitis virus, and T3 uptake in fibroblasts, blocked virtually all of the cellular uptake of T3, with a half-maximal concentration of 29 microM. Concomitant with the inhibition of the cellular uptake of T3, the accumulation of T3 into nuclei was reduced. The inhibitory effect of monodansylcadaverine on the reduction of T3 incorporation into nuclei was not due to the inhibition of binding to nuclear receptors, but probably was due to a decrease in the cytoplasmic availability of T3 as a result of inhibition of cellular entry. These results indicate that the uptake of T3 by receptor-mediated endocytosis is a physiologically significant process.

Animals↗
PubMed1981

A spin label study of the thyroid hormone-binding sites in human plasma thyroxine transport proteins.

The binding site topographies of the three thyroid hormone-transporting proteins in human serum--prealbumin, thyroxine binding globulin, human serum albumin--have been studied with the aid of five spin-labeled analogs of L-thyroxine in which the distance between the phenolic hydroxyl and the nitroxide nitrogen ranged from 17 to 23 A. In the presence of prealbumin, the electron spin resonance spectrum of 3-([alpha-carboxy-4-(4-hydroxy-3,5-diiodophenoxy)-3,5-diiodophenethyl]-carbamoyl)-2,2,5,5-tetramethyl-3-pyrrolinl-yloxy-ethyl ester revealed the presence of a highly immobilized spin label. As the chain length between the thyroxyl moiety and the pyrroline ring was increased, the mobility of the nitroxide group in the prealbumin-bound labels increased. If the spin labels bind in an extended conformation, the thyroxine-binding site was estimated to be approximately 21 A in depth. This finding is consistent with the known crystal structure of prealbumin and suggests that the solution and crystal conformations of the protein are very similar. In contrast to prealbumin, the thyroxine-binding site on thyroxine-binding globulin was found to be more open and possibly deeper. Human serum albumin has two binding sites for thyroxine, one of which has a higher affinity and is deep enough to accommodate a molecule that is 23 A in length. The lower affinity site is somewhat shallower and probably wider, as thyroxine spin labels bound to this site exhibited greater mobility.

Binding Sites↗
PubMed1981

Binding and mobility of the cell surface receptors for 3,3',5-triiodo-L-thyronine.

A fluorescent derivative of the thyroid hormone 3,3'-triiodo-L-thyronine binds to cultured mouse fibroblasts; such binding is saturable. Video intensification fluorescence microscopy indicates that binding occurs at the plasma membrane. Diffusion coefficients, obtained by fluorescence photobleaching recovery, are consistent with binding to a protein receptor on the cell surface.

Animals↗
PubMed1980

Affinity labeling of rat liver thyroid hormone nuclear receptor.

The thyroid hormone receptor from rat liver nuclei has been covalently labeled with the N-bromoacetyl derivatives of L-thyroxine (T4) and 3,3',5-triiodo-L-thyronine (T3). Displacement binding studies showed that, in the presence of 100-fold molar excess of unlabeled N-bromoacetyl-T3 or T4, binding of [125I]T3 or [125I]T4 was nearly totally inhibited. Heat inactivation of the receptor (55 degrees C for 15 min) resulted in parallel losses in the binding of T3 (95%) and N-bromoacetyl-T3 (93%). These results indicated that T3 and T4 and their bromoacetyl derivatives compete for the same binding site. The nuclear receptor showed identical behavior in high-pressure liquid chromatography (HPLC) whether bound to T3 or T4 or covalently labeled with their bromoacetyl derivatives. HPLC provided a single-step 100-fold purification of the nuclear receptor. Na-DodSO4 gel electrophoresis of the nuclear receptor labeled with N-bromoacetyl derivatives of [125I]T3 or [125I]T4 showed one major radioactive component with a molecular weight of 56,000. Furthermore, in the absence of denaturant, the nuclear receptor either bound to [125I]T3 or covalently labeled with N-bromoacetyl-[125I]T3 showed identical mobility. These results suggested that the nuclear receptor is a single polypeptide chain and binds either T3 or T4. Nuclear receptors covalently linked with N-bromoacetyl derivatives of [125I]T3 or [125I]T4 may be useful as a marker for the preparative purification of receptor.

Affinity Labels↗
PubMed1980

Receptor-mediated uptake of 3,3',5-triiodo-L-thyronine by cultured fibroblasts.

Using video intensification fluorescence microscopy and tetramethylrhodamine (Rho)-labeled 3,3',5-triiodo-L-thyronine (T3), we studied the uptake of T3 by cultured mouse fibroblasts. After incubation of cells with Rho-T3 for 30 min at 37 degrees C the fluorescent hormone was concentrated in many small bright accumulations. With a 1000-fold excess of unlabeled T3, only weak background fluorescence was seen. Furthermore, when cells were incubated with Rho or Rho-thyronine only background fluorescence was detected. These results indicate that the cellular uptake of Rho-T3 occurred through a T3-specific receptor-mediated process. Most of these accumulations underwent saltatory motion in living cells, indicating that the T3 was contained within endocytic vesicles. When cultured cells were incubated with Rho-T3 for 60 min at 4 degrees C, only diffuse fluorescence was observed, Rho-T3 became concentrated in vesicles upon warming of the cells to either 23 degrees C or 37 degrees C. Simultaneous incubation of cells with fluorescein-labeled alpha 2-macroglobulin and Rho-T3 showed that Rho-T3 was internalized in the same vesicles as alpha 2-macroglobulin. Furthermore, as previously reported for alpha 2-macroglobulin in the presence of methylamine, dansylcadaverine, or bacitracin, clustering and internalization were inhibited but the overall fluorescence intensity of the cells did not appear to be affected. Because it has been previously shown that receptor-mediated endocytosis of alpha 2-macroglobulin occurs through clustering of ligands in coated pits on the cell surface, these results indicate that Rho-T3 follows the same pathway. Thus it has now been demonstrated that a low-molecular weight hormone enters cells by this pathway.

Animals↗
PubMed1977

Characterization of human thyroxine-binding globulin. Evidence for a single polypeptide chain.

Thyroxine-binding globulin (TBG) was purified from fresh human plasma by affinity, anion exchange, and gel filtration chromatography. The protein gave a single band in overloaded analytical disc gel electrophoresis. The molecular weight was 54,000 and E1%/1 cm at 280 nm, corrected for thyroxine (T4) absorbance, was 6.17. Six preparations of TBG contained from 0.09 to 0.64 mol of T4/mol; the TBG used in this study contained 0.19 mol of T4 and was able to bind an additional 0.85 mol. The carbohydrate composition was determined and accounted for 23% of the molecular weight. Four lines of chemical and physical evidence failed to demonstrate subunits. These included quantitative COOH-terminal amino acid analysis, peptide mapping and amino acid composition, treatment with sodium dodecyl sulfate, and denaturation of the reduced, alkylated protein with guanidine. From these data, we conclude that TBG is a single polypeptide chain.

Amino Acids↗
PubMed1977

Affinity labeling of human serum prealbumin with N-bromoacetyl-L-thyroxine.

Affinity labeling of human serum prealbumin with N-bromoacetyl-L-thyroxine (BrAcT4) was used to investigate the binding domain for L-thyroxine (T4) on prealbumin. Fluorescence titration with 8-anilinonaphthalene-1-sulfonate revealed a strong and a weak binding site for BrAcT4 (K1 = 1 X 10(8) M-1; K2 = 1 X 10(6) M-1). The reaction of BrAcT4 with prealbumin to form a covalent bond was inhibited in the presence of T4 and binding of T4 to prealbumin was nearly abolished after affinity labeling with BrAcT4. Affinity labeling with 2 mol of BrAcT4/mol of prealbumin resulted in covalent binding of 1 mol of ligand. Acid hydrolysis of affinity-labeled prealbumin gave Nepsilon-carboxymethyllysine and iminodiacetic acid, the latter being derived from the NH2-terminal glycine. A combination of analytical procedures, including tryptic digestion after maleylation, cyanogen bromide cleavage, digestion with yeast protease C, and sequential Edman degradations, revealed that the Nepsilon-carboxymethyllysine was derived from lysine-9 and lysine-15 and that the affinity label had distributed itself among glycine-1, lysine-9, and lysine-15 in a ratio of 29:63:8.

Affinity Labels↗