Receptor最新文献

Typical procedures for the isolation of triiodothyronine (T3) receptors from mammalian nuclei involve extraction of nuclei with buffers containing divalent cations and 0.40M KCl. However, when applied to tadpole erythrocyte (RBC) nuclei, this method gave low yields of relatively unstable T3 receptors. The use of EDTA (10 mM) and 0.4M KCl in a sucrose-Tris buffer resulted in the extraction of 90% of the specifically bound [125I]-T3 from RBC nuclei. It was also found that 5 mM thiol reagent (DTT, GSH, or beta-mercaptoethanol) was required for maximal stability of the receptor. Fractionation of labeled RBC nuclear extracts on a Sephadex G-100 column yielded only one peak of specific T3 binding activity. The T3 receptor peak eluted at the same position as bovine serum albumin (BSA), with an estimated mol wt of 68 kDa. Specific T3 binding activity was destroyed by protease digestion but not by DNAse or RNAse. Scatchard analysis of the fractions from the receptor peak supported the existence of one class of T3 binding sites, with an estimated Kd (about 7 pM) comparable to the Kd reported for the intact RBC. Using the same methods, T3 receptors from the nuclei of chick embryo RBCs were also isolated, again with a Kd (7 pM) similar to that for the intact RBC. The chick receptor also eluted from the Sephadex G-100 column at the same position as BSA. The estimated mol wt of the T3 receptors from both sources is comparable to those reported for T3 receptors from other sources. The results show that T3 receptors derived from both tadpole and chick RBC nuclei could be isolated in a soluble and stable form with no apparent change in Kd.

A simple technique to isolate rat renal preglomerular vessels is described. Kidneys were pressed against a 0.3 mm stainless steel grid. The whole vascular tree, including the interlobar, arcuate, and interlobular arteries, as well as the afferent arterioles, remained on the grid surface from where they were recovered. Extensive washing yielded a highly pure preparation of renal microvessels. Radioligand binding experiments were performed to characterize 125I-[Sar1,Ile8]-ANG II binding sites in preglomerular microvessel membranes. Equilibrium saturation binding experiments revealed the presence of one group of high affinity receptors (Kd = 1.22 +/- 0.171 nM; Bmax = 209 +/- 14 fmol/mg protein). Competitive inhibition experiments with two highly specific nonpeptide ANG II antagonists, losartan (DuP 753), which is specific for the AT1 receptor subtype, and PD123319, which is specific for the AT2 subtype, demonstrated that the large majority of, if not all, ANG II receptors in rat renal preglomerular vessels correspond to the AT1 subtype.

Target tissues contain microsomal receptors for steroid hormones. High-affinity, low-capacity binding sites for steroids are located in the endoplasmic reticulum and do not bind DNA. Studies by diverse groups on the nature and function of these receptors using biochemical and morphological approaches are discussed. The findings indicate that microsomes can be the site of receptor synthesis. Microsomes can also play a role in the control of receptor recycling and/or receptor processing after the complexes exit the nucleus of target cells. Moreover, microsomal receptors may be involved in posttranscriptional actions of steroid hormones.

Nuclei were isolated from porcine endometrium of castrated pigs either unexposed or exposed to estradiol in vivo by two techniques, one of which included a hypotonic step. Aliquots were analyzed for estradiol content. Receptor was extracted from buffered, Surfynol-stabilized suspensions by either (a) KCl alone, (b) in combination with dithiothreitol, or (c) by dithiothreitol with polypentosanesulfate and addition of KCl. The yields rose from a-->c. The same proportional gains with increasing extractant efficacies were obtained from nuclei of unstimulated and estradiol-treated cells. Receptor recovery with extractant "c" rose linearly over the range of 9-80 x 10(6) nuclei/mL and was independent of the technique used for isolation. Nuclear fractions isolated using steroid-free solutions contained more estrogen receptor than estradiol; the numerical excess in control nuclei persisted in the nuclei of stimulated cells featuring a stoichiometric rise of ligand and receptor contents. The increase of receptor contents in nuclei isolated from hormone-stimulated cells coincided with a decline in the cytoplasmic fractions. An excess of hormone over receptor was seen only when nuclei were isolated from untreated cells with media containing 10 nM estradiol. Our data strengthen earlier notions of an estradiol-promoted receptor translocation into the nucleus and are not compatible with the ligand-filling hypothesis of preexisting nuclear binding sites.

Steroid and thyroid hormone receptor superfamily members are ligand potentiated transcription factors. Recent evidence indicates that one aspect of steroid receptor action is an interaction with other trans-acting factors, such as the glucocorticoid receptor with the AP1 transcription factor, for example. Using a structural approach to identify domains of the glucocorticoid receptor responsible for interactions with affiliated transacting factors and DNA, we have identified a putative helix-turn-zipper motif that is conserved in all steroid, thyroid hormone, retinoic acid, and vitamin-D3 receptors. This structural motif is also conserved among new members of the family, the peroxisome proliferator-activated receptors and the retinoid-X receptors. This structural domain is characterized by a pair of amino acids (I,L,V)P that is conserved in all superfamily members. Additional characteristics include six heptad repeats of hydrophobic amino acids, four of which form a canonical leucine zipper in the rat glucocorticoid receptor. Although this leucine repeat is not absolutely conserved among superfamily members, the periodicity of hydrophobic residues is conserved throughout. Based on sequence analyses from the GenEMBL and SwissProt databases using the Genetics Computer Group and MacVector sequence analysis software packages, and the Brookhaven structural database, we present evidence for a novel structural domain, a helix-turn-zipper that is conserved in all superfamily members, and may function in transactivation of cognate genes.

Endometrium was collected by curettage from castrated pigs, either untreated or exposed to estradiol in vivo by intrauterine injection, and processed for electron microscopy. The resin LR Gold was used for embedding, and sections were floated on droplets of 10 nm diameter gold particles, coated with the immunoglobulin-G1 (IgG1) fraction or its Fab2 fragment of a monospecific polyclonal antiserum raised in goats against the C-terminal half of the estradiol receptor. On average, only one gold particle per microns 2 became attached in the cytoplasmic area of untreated cells, whereas four were found over the nuclear area. These figures rose to 2-3/microns 2 and 15-26/microns 2, respectively, within 10 min after exposure to estradiol. The labeling intensities of nuclei in cell clusters and of coprocessed nuclei released from cells ruptured during curettage were identical in all situations. Nuclear pores were frequently tagged after estradiol treatment. The proportions of tagging densities in nuclei of untreated and estradiol-exposed cells corresponded to those of receptor contents measured in extracts of isolated nuclei by ligand binding. This correlation was not seen for the cytoplasmic compartment of untreated cells, the scarce tagging of which is interpreted by hidden antigenic determinants. Our morphological analyses support the conclusions drawn from biochemical data (Sierralta et al., 1992) of an estradiol-promoted translocation of receptor from the cytoplasm into the nucleus.

Infusion of 17-beta-estradiol for 3 h caused a more than 100% increase in the number of PRL receptors (PRL) in the microsomal liver fraction of male rats without affecting the hormone-receptor affinity. A small but significant increase in PRL receptors also was found in testes but not kidneys. Testosterone infusion to female virgin rats for a similar time period resulted in some nonsignificant decreases in PRL receptors in the liver but not the mammary gland or kidneys. Testosterone infusion to males or 17-beta-estradiol infusion to females did not change the number or affinity of PRL and growth hormone receptors. The level of serum PRL was not influenced during infusion with both hormones.

Incubation of cultured rat aortic smooth muscle cells (A-10) with activators of cyclic nucleotides resulted in transiently increased activity of extractable topoisomerase I or topoisomerase II. ANF, which induces cGMP accumulation, potentiated camptothecin-induced, topoisomerase I linked DNA strand breakage and increased the specific activity of extractable topoisomerase I (maximum activity 5-15 min after treatment), but had no effect on topoisomerase II activity. These effects are similar to those reported for AVP and phorbol esters, activators of protein kinase C. Forskolin and isoproterenol, which induce cAMP accumulation, activated extractable topoisomerase II (maximum 5-15 min after treatment), but not topoisomerase I. Permeable cyclic nucleotide analogs dBcAMP and 8BrcGMP selectively activated extractable topoisomerase II and topoisomerase I activities, respectively. Activation of topoisomerase I by either AVP or PdBu was attenuated by cotreatment with 8BrcGMP or dBcAMP, and activation of topoisomerase II by dBcAMP was attenuated by cotreatment with AVP or PdBU, suggesting that elements of the protein kinase C and the cyclic nucleotide linked signal-transduction pathways can interact to modify nuclear enzymic activity. IBMX, which elevates intracellular cAMP and cGMP, increased the extractable activities of both topoisomerase I and topoisomerase II. Thus, topoisomerase activity in cells may be governed in part by cyclic nucleotide levels.

Increases in membrane lipid unsaturation and drug-induced increases in membrane fluidity have been shown to be associated with increases in insulin receptor concentration in animals, cultured cells, and liposomes. In the current study, we have examined the effect of increased membrane fatty acid unsaturation on insulin action. Friend Erythroleukemia cells were grown with exogenous polyunsaturated fatty acids for three days. After growth in medium supplemented with fatty acids, the unsaturation index of the phospholipids increased from 1.08 to 1.92, and this was associated with a significant decrease in anisotropy, as measured by fluorescence polarization. When measured at 15 degrees C, insulin receptor number rose from 9000 to 22,000 per cell with increased fatty acid unsaturation. The affinity for insulin in the polyunsaturated fatty acid treated cells decreased, however, resulting in similar amounts of insulin binding at low insulin concentrations but more binding at high insulin concentrations when compared to control cells. In contrast, binding of IGF-I was not influenced by increased membrane fatty acid unsaturation. When measured at 37 degrees C there were no changes in binding of insulin or IGF-I. Internalization of insulin was identical in control cells and in cells with increased membrane fatty acid unsaturation. Thymidine incorporation, an insulin-dependent function in these cell, was measured in control and fatty acid treated cells. In control cells, insulin increased thymidine incorporation by 80%, with an ED50 of about 5 nM. In cells treated with polyunsaturated fatty acids, the insulin stimulated thymidine incorporation was slightly higher and the ED50 was about 0.2 nM. In contrast, there was no increase in the sensitivity or responsiveness of fatty acid treated cells to IGF-I. We conclude that increased membrane fatty acid unsaturation greatly influences insulin binding and biological sensitivity, but not that of IGF-I. At low insulin levels, there was a greater insulin bioeffectiveness, despite the same or lower insulin binding, suggesting more efficient coupling of the insulin-effector complex.