Ah,sweet mystery of death! Galectins and control of cell fate

Control of cell death is critical in eukaryotic development, immune system homeostasis, and control of tumorigenesis. The galectin family of lectins is implicated in all of these processes. Other families of molecules function as death receptors or death effectors, but galectins are uniquely capable of acting both extracellularly and intracellularly to control cell death. Extracellularly, galectins cross-link glycan ligands to transduce signals that lead directly to death or that influence other signals regulating cell fate. Intracellular expression of galectins can modulate other signals controlling cell viability. Individual galectins can act on multiple cell types, and multiple galectins can act on the same cell. Understanding how galectins regulate cell viability and function will broaden our knowledge of the roles of galectins in basic biological processes and facilitate development of therapeutic applications for galectins in autoimmunity, transplant-related disease, and cancer.     

Photoaffinity labeling of the Ah receptor

A series of halodibenzo-p-dioxins with the photolabile aryl azide functional group were synthesized and screened as potential photoaffinity labels for the Ah receptor, and 2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin was selected for radiosynthesis with 125I (specific activity 2176 Ci/mmol, equilibrium dissociation constant, KD = 0.76 nM). Following incubation of this 125I-labeled photoaffinity ligand with the protamine sulfate-precipitated fraction of C57BL/6J mouse liver cytosol, and irradiation with long wavelength ultraviolet light, the radiolabeled macromolecules were precipitated with acetone and analyzed by denaturing gel electrophoresis and autoradiography. Among the labeled products, two peptides with apparent molecular masses of 95,000 and 70,000 daltons had the following properties: 1) they were selectively labeled at low ligand concentrations; 2) they were labeled in approximately a 1:1 ratio; 3) co-incubation with receptor agonists inhibited the photoaffinity labeling of both peptides to a similar extent, and structure activity relationship for inhibition of labeling by these agonists corresponded to that for their binding affinity to the Ah receptor; 4) upon nondenaturing chromatographic separation of photoaffinity labeled cytosol on high performance liquid chromatography size exclusion and anion exchange columns, the 95- and 70-kDa peptides coelute; 5) the migration of these peptides upon denaturing electrophoresis is the same in the presence or absence of a thiol reducing agent; and 6) proteolysis of the 95- and 70-kDa peptides produces a similar pattern of cleavage peptides. The simplest structure of the Ah receptor in mouse liver cytosol, appears to be a dimer composed of two noncovalently linked subunits of apparent molecular masses of 95 and 70 kDa, which have homologous structure and similar ligand binding sites, but other possibilities are discussed.     

Intracellular location of the Ah receptor

The subcellular distribution of the Ah receptor from the mouse hepatoma line, Hepa-1, was investigated following cytochalasin B treatment and cell enucleation. Probing the resultant cytoplast and nucleoplast fractions with radiolabelled tetrachlorodibenzo-p-dioxin (TCDD) revealed the presence of a specifically bound peak of receptor only in the cytoplast fraction. However, the quantity of receptor recovered in these experiments was only 10–12% of the expected value. We therefore undertook an investigation to determine the fate of the Ah receptor in the presence of cytochalasin B. Incubation of Hepa-1 cells with this compound resulted in a rapid loss or inactivation of cytosolic binding activity with a concomitant decrease in the amount of receptor partitioned into the nucleus at all time periods examined. Control experiments indicated that cytochalasin B did not compete with TCDD for binding to the Ah receptor and furthermore, that its mechanism of action could not be attributed to a non-specific effect on all cytosolic proteins. The results obtained are discussed in relation to the proposed models for induction by the estrogen and glucocorticoid binding receptors.     

向日葵生物技术研究进展

本文在广泛收集向日葵生物技术研究文献的基础上,对向日葵茎尖培养、幼胚培养、子房培养和原生质体培养的结果做了综述,并对向日葵基因工程做了简要介绍,提出向日葵生物技术发展方向。     

Carbon Metabolism Alterations in Sunflower Plants Infected with the Sunflower Chlorotic Mottle Virus

Sunflower chlorotic mottle virus (SuCMoV) causes chlorotic mottling symptoms and important growth reductions and yield losses in sunflower (Helianthus annuus L., cv. Contiflor 7). This paper describes the effects of SuCMoV on some aspects of carbon metabolism of sunflower plants. After symptoms became evident, CO₂ fixation rates decreased, nevertheless, soluble sugars and starch increased in infected leaves. High H₂O₂ accumulation, lipid peroxidation and chlorophyll degradation were, like the other changes, observed only after symptom expression. Increased soluble carbohydrate accumulation was not related to changes in α‐amylase (EC 3.2.1.1) activity, nor in the activities of enzymes associated with sugar import and hydrolysis such as invertase (EC 3.2.1.26) and sucrose synthase (EC 2.4.1.13), suggesting it did not derive from starch hydrolysis nor increased sugar import. Rather, it may derive from recycling of cell components associated with the development of oxidative damage. The physiological alterations caused by this virus share many common features with the development of senescence.     

内葵杂3号染色体核型分析

对内蒙古地区的栽培品种内葵杂3号三交种和单交种了进行了核型分析。其结果为:内葵杂3号三交种和单交种的染色体数均为2n=34,各具一对随体染色体。三交种第2对染色体具随体且为近中部着丝粒染色体,其余为中部着丝粒染色体,染色体相对长度变异范围4.105%～7.703%,核型公式为2n=2x=34=32m+2sm(2sat),核型类型属于1A型;单交种均为中部着丝粒染色体,第4对染色体具随体,染色体相对长度变异范围3.661%～8.128%,其核型公式为:2n=2x=34=34m(2sat),核型类型属于1B型。     

Characterization of the Ah receptor from human placental tissue

The rate of thermal inactivation of the unliganded human Ah receptor, studied by sucrose density gradient centrifugation, with respect to loss of ligand binding ability, was found to be greater than those of most rodents at 20°C, but the temperature coefficient of the rate constant was much smaller than for the rodent species. This implies that the unliganded human Ah receptor would be thermally more stable than the rodent analogs at physiological temperatures. The liganded form of the human Ah receptor was found to be less stable with respect to ligand release than the rodent receptors. These differences in behavior between human and rodent Ah receptors underline the difficulties in using rodent data in the development of receptor-based models of dioxin toxicity. Attempts to develop an alternative to sucrose density gradient centrifugation, comparable with the hydroxylapatite adsorption method used to assay rodent hepatic Ah receptor, were unsuccessful.     

Ca2+-dependent proteolysis of the Ah receptor

We previously reported (J. Biol. Chem. (1986) 261, 6352-6465) that the photoaffinity ligand for the Ah receptor, [125I]-2-azido-3-iodo-7,8-dibromodibenzo-p-dioxin, upon incubation with the liver cytosol fraction from C57BL/6 mice, labeled in a 1:1 ratio two peptides that had apparent molecular masses of 95 and 70 kDa and similar proteolytic fragmentation patterns. In the cytosolic fraction of Hepa 1 cells, a cloned murine hepatoma cell line, the product of photoaffinity labeling is almost exclusively a 95-kDa peptide which is rapidly hydrolyzed by a Ca2+-dependent proteinase to a 70-kDa peptide as well as other fragments. Thus, the ligand binding unit of the Ah receptor in C57BL/6 mouse liver and Hepa 1 cell is a 95-kDa peptide, and the 70-kDa fragment is a proteolytic artifact. The Ca2+-dependent proteinase which hydrolyzes the 95-kDa peptide has the properties of calpain II: (i) an absolute requirement for Ca2+, with maximal activity at 0.5 to 1.0 mM Ca2+; (ii) a pH optimum of 7.5 to 8.0; (iii) inhibition by EDTA, iodoacetamide, leupeptin and L-trans-epoxysuccinylleucylamido(4-guanidino)butane, but not by soybean trypsin inhibitor, aprotinin, or phenylmethanesufonyl fluoride. Upon chromatographic separation of the liver cytosol of C57BL/6 mice on DEAE-Sephacel, Ca2+-dependent proteinase activity (using casein or the labeled 95-kDa peptide as substrates) elutes with 0.25 M NaCl, and a specific proteinase inhibitor elutes with 0.15 M NaCl. Ca2+-dependent proteinase activity that hydrolyzes the 95-kDa peptide is found in the liver cytosols of several mammalian species.     

广义向日葵方程的周期解

本文利用拓扑度理论,研究了一类广义向日葵方程周期解的存在性,获得了一些充分条件.