The vascular endothelium as a target tissue in acute cadmium toxicity

Sensitivity of tissues to the acute toxicity of cadmium is reviewed. It is concluded that the initial effect of acute cadmium administration is on the integrity and permeability of the vascular endothelium; other necrotic changes occur secondarily to this effect. In a sensitive tissue, not all of the endothelial cells are susceptible to cadmium. Furthermore, after necrosis of the sensitive cells, the resistant cells proliferate and result in regeneration of the vasculature and subsequent acquired resistance to the metal. It is found that sex hormones are probably important in determining susceptibility and response of tissues to cadmium. The role of metallothionein in these phenomena remains to be elucidated.     

The human placenta: a model for tenascin expression

Summary Tenascin is a large glycoprotein of the extracellular matrix. Previous reports have demonstrated that it is associated with epithelial-mesenchymal interfaces and is expressed during embryonic and tumour development, wound healing, cell proliferation and it may be involved in immunomodulation. The human placenta shows numerous features related to these aspects. We have investigated the presence of tenascin in the human placenta throughout pregnancy by immunohistochemistry. We used monoclonal (mAb) and polyclonal (pAb) antibodies to tenascin, a mAb to fibrin, a pAb to fibrinogen, and the mAb Ki-67 as proliferation marker. Tenascin was highly expressed in the mesenchymal villi which are considered the basis of growth and differentiation of the villous trees. Moreover, fibrinoid deposits at the surfaces of the villous trees were always separated from the fetal stroma by tenascin. The stroma of villi encased in fibrinoid was also positive for tenascin. This glycoprotein was also expressed in the villous stroma directly apposed to cell islands and cell columns. In the proximal portions of both epithelial structures, cytotrophoblast was Ki-67 positive. These data show that tenascin is expressed during the development of the placenta, particularly in the mesenchymal villi, cell islands and cell columns. These structures are considered to be the proliferating units of the villous trees. Tenascin underlying fibrinoid deposits suggests that it also participates in repair mechanisms. Thus, in the human placenta tenascin expression can be correlated with villous growth, cell proliferation, and fibrinoid deposition. Its role in immunoprotection of fetal tissues in areas where syncytiotrophoblast as barrier is missing or damaged is discussed.     

The human placenta: a model for tenascin expression.

Tenascin is a large glycoprotein of the extracellular matrix. Previous reports have demonstrated that it is associated with epithelial-mesenchymal interfaces and is expressed during embryonic and tumour development, wound healing, cell proliferation and it may be involved in immunomodulation. The human placenta shows numerous features related to these aspects. We have investigated the presence of tenascin in the human placenta throughout pregnancy by immunohistochemistry. We used monoclonal (mAb) and polyclonal (pAb) antibodies to tenascin, a mAb to fibrin, a pAb to fibrinogen, and the mAb Ki-67 as proliferation marker. Tenascin was highly expressed in the mesenchymal villi which are considered the basis of growth and differentiation of the villous trees. Moreover, fibrinoid deposits at the surfaces of the villous trees were always separated from the fetal stroma by tenascin. The stroma of villi encased in fibrinoid was also positive for tenascin. This glycoprotein was also expressed in the villous stroma directly apposed to cell islands and cell columns. In the proximal portions of both epithelial structures, cytotrophoblast was Ki-67 positive. These data show that tenascin is expressed during the development of the placenta, particularly in the mesenchymal villi, cell islands and cell columns. These structures are considered to be the proliferating units of the villous trees. Tenascin underlying fibrinoid deposits suggests that it also participates in repair mechanisms. Thus, in the human placenta tenascin expression can be correlated with villous growth, cell proliferation, and fibrinoid deposition. Its role in immunoprotection of fetal tissues in areas where syncytiotrophoblast as barrier is missing or damaged is discussed.     

The distal nephron in the chick embryo as a target tissue for 1-alpha-25-dihydroxycholecalciferol

A histological and ultrastructural study as well as an autoradiographic analysis after injection of tritiated 1,25-dihydroxycholecalciferol (1,25-DHCC) were conducted on the distal convoluted tubules of chick embryos. Distal tubules in the embryo were shown to have the same spatial distribution as described for the adult kidney; they presented a convoluted portion located in the vicinity of the central intralobular veins and straight portions irradiating from this region towards the periphery. The epithelium in these tubules was well differentiated; its cells had numerous interdigitating folds in their lateral boundaries which were especially numerous at the basal ends. This device greatly increased the membrane surface available for interchange and was interpreted as an expression of active water and/or mineral transport. Nuclear concentration of radioactivity was found 2 h after injection of tritiated 1,25-DHCC in both the pars convoluta and the pars recta of the distal tubules. This concentration could be blocked by the previous administration of large amounts of nonradioactive 1,25-DHCC. These facts were interpreted as indicating that distal convoluted tubules in the chick embryo are functionally differentiated and contain target cells for 1,25-DHCC.     

The thymus as a target for mycobacterial infections

Mycobacterial infections are among the major health threats worldwide. Ability to fight these infections depends on the host's immune response, particularly on macrophages and T lymphocytes produced by the thymus. Using the mouse as a model, and two different routes of infection (aerogenic or intravenous), we show that the thymus is consistently colonized by Mycobacterium tuberculosis, Mycobacterium avium or Mycobacterium bovis BCG. When compared to organs such as the liver and spleen, the bacterial load reaches a plateau at later time-points after infection. Moreover, in contrast with organs such as the spleen and the lung no granuloma were found in the thymus of mice infected with M. tuberculosis or M. avium. Since T cell differentiation depends, to a large extent, on the antigens encountered within the thymus, infection of this organ might alter the host's immune response to infection. Therefore, from now on, the thymus should be considered in studies addressing the immune response to mycobacterial infection.     

The bacterial ribosome as a target for antibiotics

Many clinically useful antibiotics exert their antimicrobial effects by blocking protein synthesis on the bacterial ribosome. The structure of the ribosome has recently been determined by X-ray crystallography, revealing the molecular details of the antibiotic-binding sites. The crystal data explain many earlier biochemical and genetic observations, including how drugs exercise their inhibitory effects, how some drugs in combination enhance or impede each other's binding, and how alterations to ribosomal components confer resistance. The crystal structures also provide insight as to how existing drugs might be derivatized (or novel drugs created) to improve binding and circumvent resistance.     

The lung as a target organ for thyroxine

The isolated perfused in situ rat lung preparation was used to investigate the chronic effect of thyroxine on the intermediary metabolism in the mammalian lung. Treatment with thyroxine caused stimulation of the rate of glucose utilization (91 +/- 11 mumol/g dry weight/hr versus 54 +/- 5 mumol/g dry weight/hr). The increase in the rate of glucose uptake was not accompanied by a similar increase in lactate output. Alanine and pyruvate release were also similar in both groups. The implication is that oxidative metabolism of glucose was increased. This study provides the first unequivocal evidence that the mammalian lung is a target organ for thyroxine.     

Activated vitronectin as a target for anticancer therapy with human antibodies

The formation of a provisional extracellular matrix represents an important step during tumor growth and angiogenesis. Proteins that participate in this process become activated and undergo conformational changes that expose biologically active cryptic sites. Activated matrix proteins express epitopes not found on their native counterparts. We hypothesized that these epitopes may have a restricted tissue distribution, rendering them suitable targets for therapeutic human monoclonal antibodies (huMabs). In this study, we exploited phage antibody display technology and subtractive phage selection to generate human monoclonal antibody fragments that discriminate between the activated and native conformation of the extracellular matrix protein vitronectin. One of the selected antibody fragments, scFv VN18, was used to construct a fully human IgG/ monoclonal antibody with an affinity of 9.3 nM. In immunohistochemical analysis, scFv and huMab VN18 recognized activated vitronectin in tumor tissues, whereas hardly any activated vitronectin was detectable in normal tissues. Iodine 123–radiolabeled huMabVN18 was shown to target to Rous sarcoma virus-induced tumors in chickens, an animal model in which the epitope for huMab VN18 is exposed during tumor development. Our results establish activated vitronectin as a potential target for tumor therapy in humans.Supported in part by the Dutch Cancer Society (grant UU1999-2114) and the Vanderes Foundation, The Hague, The Netherlands.     

The human placenta exhibits a unique transcriptomic void

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The thioredoxin system as a target for mercury compounds

BackgroundMercury interaction with selenium in vivo has been recognized for >50 years. Several researchers attempted to use selenium to mitigate the detrimental effects of mercurial compounds but the results were controversial. Selenium pools in living organisms are quite low and the high affinity of mercury to bind selenols pointed out selenoproteins as possible targets of toxicity. Such was the case of the selenoenzyme thioredoxin reductase (TrxR) which is an integrant part of the thioredoxin system. Given the important role of this redox system for cellular functioning and the high affinity of mercury for TrxR's active site, this interaction can be key to understand the mechanism by which Hg causes cell death.Scope of the reviewThis review discusses the current state of knowledge concerning the interaction between mercury compounds and the thioredoxin system, its implications for the development of toxicity and the effects of selenium co-exposure.Major conclusionsThe mechanism of toxicity of mercurials is a complex chain of events starting with inhibition of the selenoenzyme, TrxR. Selenium supplementation protects TrxR from the toxicity of inorganic forms of mercury (i.e., Hg(II)) to a certain extent, but not from methylmercury.When TrxR is inhibited, thioredoxin is reduced by alternative mechanisms involving glutathione and glutaredoxin and only when this pathway is hampered does cell death occur.General significanceUnderstanding the molecular mechanism of mercury toxicity and the mechanisms of enzymatic compensation allows the design of mitigation strategies and, since TxrR and Trx exist in the plasma, puts forward the possibility for future use of changes in activity/expression of these enzymes as biomarkers of mercury toxicity, thus refining the risk assessment process.     

The EGFR as a target for viral oncoproteins.

The epidermal growth factor receptor (EGFR) is a potent stimulator of the mitogen-activated protein kinase (MAPK) signaling pathway. Chronic stimulation of the EGFR and of multiple steps in the MAPK signaling pathway is involved in the development of cancer. Several tumor viruses encode proteins that induce EGFR expression or stimulate EGFR-mediated signaling and are thus likely to play an important role in the transformation of virus-infected cells.     

The GABAA receptor as a target for photochromic molecules

Photochromic ligands, molecules that can be induced to change their physical properties through applied light, are currently the topic of much chemical biology research. This specialized class of small organic structures are, surprisingly to many, fairly common in nature. At the core of a number of natural biological processes lies a small molecule that changes shape or some other measurable property in response to light absorption. For instance, conformational changes invoked by reversible photoisomerization of a retinoid small molecule found in the photoreceptors of the human eye leads to vision. In plants, photoisomerization of a cinnamate moiety leads to altered gene expression. The photosensitive molecule can be viewed simply as a nanosensor of light, much like a photosensitive electrical component might be added to a circuit to sense day versus night to turn an electrical circuit on or off. Synthetic organic chemists and chemical biologists have been, for at least the last 15 years, trying to either mimic or exploit the native photochromism found in nature. Here, we describe the design process to develop a photochromic molecule to be used in neurobiology.     

The flavivirus protease as a target for drug discovery

Many flaviviruses are significant human pathogens causing considerable disease burdens, including encephalitis and hemorrhagic fever, in the regions in which they are endemic. A paucity of treatments for flaviviral infections has driven interest in drug development targeting proteins essential to flavivirus replication, such as the viral protease. During viral replication, the flavivirus genome is translated as a single polyprotein precursor, which must be cleaved into individual proteins by a complex of the viral protease, NS3, and its cofactor, NS2B. Because this cleavage is an obligate step of the viral life-cycle, the flavivirus protease is an attractive target for antiviral drug development. In this review, we will survey recent drug development studies targeting the NS3 active site, as well as studies targeting an NS2B/NS3 interaction site determined from flavivirus protease crystal structures.     

Towards a rational modeling for the human placenta

An attempt is made to provide a general framework sufficient of handling analytically typical questions concerned with a physiological functioning of a human placenta. The principles of mechanics and thermodynamics, as they apply to a continuum theory of mixtures, are the essential tools in constructing the model. In general terms, it is assumed that each point of the placenta space is simultaneously occupied by placenta tissue, fetal blood, and maternal blood. These three constituents are allowed to interact mechanically, chemically, and thermally. One of the basic features of the model is its capability of providing quantitative information on the deformation of the placenta tissue and the pressure distribution of the fetal and maternal blood. Effects of inertia, deformation, and changes of shape and weight of the placenta during gestation are also taken into account.     

The ribosome as a drug target

The elucidation of the crystal structure of the ribosome and its subunits has dramatically increased our understanding of this organelle and the molecular interactions that determine its functional capabilities. Two recent publications, one on the structure of the bacterial ribosome at 3.5A resolution and one on the identification of functionally relevant sites within the small subunit rRNA, illustrate the importance of interdisciplinary approaches in exploiting the ribosome as a drug target.     

Glutathione S-transferase pi as a target for tricyclic antidepressants in human brain

GST pi, the main glutathione S-transferase isoform present in the human brain, was isolated from various regions of the brain and the in vitro effect of tricyclic antidepressants on its activity was studied. The results indicated that amitripyline and doxepin - derivatives of dibenzcycloheptadiene, as well as imipramine and clomipramine - derivatives of dibenzazepine, inhibit the activity of GST pi from frontal and parietal cortex, hippocampus and brain stem. All these tricyclics are noncompetitive inhibitors of the enzyme with respect to reduced glutathione and noncompetitive (amitripyline, doxepin) or uncompetitive (imipramine, clomipramine) with respect to the electrophilic substrate. Their inhibitory effect is reversible and it depends on the chemical structure of the tricyclic antidepressants rather than on the brain localization of the enzyme. We conclude that the interaction between GST pi and the drugs may reduce their availability in the brain and thus affect their therapeutic activity. On the other hand, tricyclic antidepressants may decrease the efficiency of the enzymatic barrier formed by GST and increase the exposure of brain to toxic electrophiles. Reactive electrophiles not inactivated by GST may contribute in adverse effects caused by these drugs.     

The human placenta: a novel source of inhibin

Human placental extracts contain inhibin bioactivity and immunoactivity giving dose response curves parallel to a human follicular fluid inhibin standard. Inhibin bioactivity in vitro was neutralised by preincubation of extracts with antisera raised to pure bovine inhibin. Umbilical cord blood from term infants contained immunoactivity. Human placental inhibin differs from human ovarian inhibin in terms of its biological: immunological ratio.     

Heparin binding lectin of human placenta as a tool for histochemical ligand localization and isolation

Biotinylated heparin has been used to detect the presence of specific binding sites in sections of human placenta, which has prompted demonstration of expression of lectin activity for this proteoglycan. Purification of this lectin from full-term placenta facilitates the synthesis of its biotinylated derivative, using biotin-amidocaproyl hydrazide, without affecting its activity. It also enables immunization to obtain antibodies. The labeled lectin is shown to bind specifically to nuclear and cytoplasmic locations in various cell types of human placenta, nuclear expression of lectin binding sites being more pronounced at the full-term stage than after 8 weeks of development. The structurally related histone H2B exhibits obvious differences in its binding pattern. The presence of ligands accessible to the lectin whose binding activity can be inhibited by addition of an excess of heparin correlates in most instances with the level of lectin expression detected immunohistochemically. Biochemical information on the nature of the glycohistochemically inferred lectin-specific ligand(s) is obtained by affinity chromatography on resin-immobilized lectin. It leads to isolation of a proteoglycan with similar electrophoretic mobility in agarose-polyacrylamide gel electrophoresis relative to the independently purified heparan sulfate-containing fibronectin binding proteoglycan from human placenta. Both fractions inhibit binding of heparin to the lectin and contain immunologically detected co-purified lectin, emphasizing their ligand properties. Application of labeled tissue lectins in conjunction with lectin-specific antibodies is proposed to obtain valuable insights into the expression of the receptor as well as the ligand part of protein-carbohydrate recognition.