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Biomedical subjects

M Tavassoli

Publications and source records attributed to M Tavassoli.

At least 127 records · Page 7Linked to original sources
PubMed1987

Transendothelial transport (transcytosis) of iron-transferrin complex in the rat liver.

To determine the transport pathway of iron-transferrin complex (Fe-TF) into the hepatocyte, we labeled Fe-TF with colloidal gold and perfused rat liver through the portal vein with this probe under different conditions. The tissue was then studied by transmission electron microscopy. At a cold temperature (approximately 4 degrees C), the probe bound to the luminal surface of sinusoidal endothelium without internalization. The binding was limited to the endothelium and there was no binding to Kupffer cells or hepatocytes. The binding was inhibitable in the presence of excess soluble Fe-TF, indicating the specificity of the bindings. At 37 degrees C the probe was internalized via a system of coated pits and vesicles. Morphometric analyses of the temporal sequence of events suggested that the probe was transported, in part, across the endothelium via a system of tubules and vesicles and externalized on the abluminal side via a system of coated pits. The probe was then taken up by hepatocytes. Only minimal uptake was noted when gold-labeled bovine serum albumin was used either at the low temperature or at 37 degrees C. The findings suggest that the liver uptake of Fe-TF complex by hepatocytes is a transendothelial phenomenon (transcytosis).

Animals↗
PubMed1987

In vitro homing of hemopoietic stem cells is mediated by a recognition system with galactosyl and mannosyl specificities.

We synthesized a number of neoglycoprotein probes by covalently linking three biologically relevant sugars (mannose, galactose, and fucose) to a protein molecule so as to retain the pyranose (ring) form of sugars necessary for their interaction with lectins. In the presence of galactosyl and mannosyl but not fucosyl probes, the production of CFU-S [colony-forming unit(s) in spleen] and total cells was halted in murine long-term marrow cultures. Cobblestone areas disappeared in these cultures, indicating the inhibition of binding of hemopoietic cells to the stroma. Electron microscopy revealed no alterations of the stroma, and the probes did not have direct cytotoxic or inhibitory effects on the growth of CFU-S or CFU-C [colony-forming unit(s) in culture]. Stroma grown for 5 weeks in the presence of the probes could subsequently support the growth of hemopoietic progenitor cells when the probes were removed from the medium. Conversely, the proliferative capacity of CFU-S in the supernate, grown in the presence of the probes, was retained upon grafting to control stroma. Galactosyl and mannosyl but not fucosyl probes differentially agglutinated CFU-S in whole-marrow-cell suspensions, suggesting the presence of membrane lectins with specificity for these sugars on the surface of CFU-S. We conclude that the binding of CFU-S to marrow stroma (homing) is mediated by a recognition system with galactosyl and mannosyl specificities.

Animals↗
PubMed1987

Transferrin-mediated cellular iron uptake.

The basic model for cellular uptake of iron relies on the iron-chelating protein transferrin (Tf), which is capable of binding iron under one set of conditions and releasing it under another set of conditions. Tf has specific membrane receptors on the surface of the cells that require iron. Tf-receptor binding is followed by internalization through a system of coated pits and vesicles. The rapid decline of pH of these vesicles leads to release and sequestration of iron by the cell. Apotransferrin-receptor complex returns to the cell surface, where, under neutral pH conditions, apotransferrin is dissociated. Other models for cellular uptake of iron include extraction of iron from Tf on the cell surface without internalization, uptake by adsorptive mechanism, and fluid-phase endocytosis. Recent advances in cellular and molecular biology, gene cloning, and monoclonal antibody technique have elucidated many features of these processes at a molecular level. These advances are reviewed and prospects for future work discussed.

Animals↗
PubMed1987

Nonspecific adsorption of transferrin to K562 cell membrane.

In the absence of albumin, incubation of K562 cells with radiolabeled transferrin revealed a binding component with high affinity and one with low affinity. The component with low affinity was eliminated in the presence of albumin, indicating it results from non-specific adsorption of ligand to cell membrane. The lack of specificity was further confirmed by demonstration that this component could be dissociated by pronase but not by acid. The high affinity component was responsible for most or all of the iron uptake by the cell. However, in certain cell densities of the culture, some iron could be taken up via other mechanisms. Therefore, the mechanism of low affinity binding could contribute to iron uptake by the cell in certain proliferative states.

Absorption↗
PubMed1987

Interaction of murine granulocyte-macrophage progenitors and supporting stroma involves a recognition mechanism with galactosyl and mannosyl specificities.

To study the molecular basis of "homing" of granulocyte-macrophage progenitors (CFU-C), we synthesized probes by covalent linking of sugars to bovine serum albumin. Long-term marrow cultures were established in the presence and absence of these probes. In the presence of galactosyl and mannosyl probes, total cell and CFU-C production in the supernate as well as the adherent layer were halted, and cobblestones (representing CFU-C bound to stroma) disappeared. Fucosyl probe and diffusible sugars had no effect on these parameters. These studies suggested membrane lectins with specificity for galactosyl and mannosyl residues may be responsible for the binding of CFU-C to supporting stroma. To determine if CFU-C possesses homing receptors with these specificities, we induced agglutination in marrow cell suspensions with these neoglycoprotein probes. Selective agglutination was observed only by galactosyl and mannosyl probes. The results suggest that CFU-C homing receptors are membrane lectins with specificity for galactosyl and mannosyl residues.

Animals↗
PubMed1987

Structural alterations of marrow during inflammation.

In response to infections and inflammations, bone marrow reacts to mobilize its granulocyte reserve. Three sets of factors are involved in this mobilization. The structure of the sinus wall is altered and adventitial cells retract to permit interaction of migrating cells with the endothelium. During the maturation process, granulocytes lose their binding potential to the supporting stroma, but their motility, chemotactic ability, and deformability increase. Consequently, they move toward the sinus endothelium with which they interact to enter the circulation. Soluble factors are also involved in granulocyte mobilization. The best characterized of these factors is C3e, an acidic fragment of the alpha chain of C3 with MW of 10-12 KD and ability to bind to granulocyte membrane. Other soluble factors may also be involved, but due to lack of adequate methodology, this area has been relatively underexplored.

Animals↗
PubMed1987

Homing receptors for hemopoietic stem cells are lectins with galactosyl and mannosyl specificities.

Binding of hemopoietic stem cells to stroma in vitro is inhibited in the presence of galactosyl and mannosyl neoglycoproteins. This indicates that an interaction between membrane lectins and sugar-bearing membrane molecules is responsible for the binding (homing) of stem cells to stroma. Agglutination studies locate these lectins, designated as homing receptors, to the membrane of CFU-S. Thus, homing receptors for hemopoietic stem cells are lectins with galactosyl and mannosyl specificities.

Animals↗
PubMed1986

Liver endothelium desialates ceruloplasmin.

Our previous work indicated that ceruloplasmin (CP) is transported through liver endothelium via a receptor-mediated mechanism and is subsequently externalized on the abluminal side. In the present work 125I or 3H-labeled ceruloplasmin was chased through the endothelium and the supernate was subjected to affinity chromatography using RCA120 column. Evidence was obtained that liver endothelium completely desialates CP in the course of its transport. The results substantiated the view that desialated CP is subsequently removed by hepatocytes through asialoglycoprotein receptors.

Animals↗
PubMed1986

Albumin inhibition of transferrin low-affinity binding to K562 cells.

Several reports have suggested that variations of albumin concentration in the incubation medium can modulate the magnitude of transferrin binding to the cells. We have investigated this problem further using K562 cells. In the absence of human serum albumin, transferrin binding demonstrated a non-saturable curve which, upon Scatchard analysis, showed two components with high and low affinities. In the presence of 0.5% human serum albumin, the low-affinity but not the high-affinity component was totally inhibited and, thus, the binding showed a saturation plateau at transferrin concentration of 6 micrograms/ml. Increasing concentrations of human serum albumin in the incubation medium led to progressive inhibition of transferrin binding, reaching a plateau at 0.2% human serum albumin. At this concentration transferrin binding was about 12 ng/10(6) cells, corresponding to the saturation plateau for high-affinity binding. Low-affinity transferrin binding in the absence of human serum albumin could readily be displaced by subsequent addition of albumin. Similar inhibition was obtained by another serum protein, ceruloplasmin, suggesting that this inhibition is not unique to albumin and may be a common property of all proteins. Incubation at 37 degrees C with 59Fe-labeled transferrin indicated that all iron uptake occurs through high-affinity binding. We conclude that the reported variations in magnitude of transferrin binding by the cell due to variations in albumin concentration are the result of inhibition of low-affinity binding of transferrin by albumin.

Cell Line↗
PubMed1986

Recovery of transferrin receptors on hepatocytes membrane after collagenase perfusion.

Hepatocyte cell suspensions obtained by collagenase perfusion method did not have transferrin (TF) receptors. However, after incubation at 37 degrees, they appeared to gain TF receptors, the number of which was the function of incubation time at 37 degrees. It is suggested that hepatocyte TF receptors are collagenase-sensitive. This study can explain previous observations that hepatocyte isolated with collagenase treatment of the tissue do not bind TF at 4 degrees but take up TF at 37 degrees.

Animals↗
PubMed1986

Mapping of the rat liver endothelial membrane with lectins and glycosylated ferritins.

We explored the luminal surface of liver sinus endothelium for the presence of lectin receptors and lectinlike substances capable of interacting with specific sugars. We used ferritin-conjugated lectins and glycosylated ferritins as probes. Incubation of small blocks of rat liver with these probes led to the binding of concanavalin A (on A), Ricinus communis (RCA), wheat germ agglutinin (WGA), phytohemagglutinin (PHA) and mannosyl ferritins to the luminal surface of endothelium. Ulex europaeus agglutinin I (UEA), fucosyl, galactosyl, and chitobiosyl-ferritins did not bind. The binding was patchy and sparse in the case of Con A and mannosyl-ferritins but uniform for others. Binding density did not correlate with hemagglutinability of lectins, suggesting that the difference in the hemagglutinability of these lectins did not account for the difference in their binding densities. Bindings were all completely inhibited in the presence of excess specific sugar inhibitors, indicating the specificity of binding. The distribution of binding was segregated on the endothelial membrane, being heaviest on luminal pits. To define the functional significance of this segregated distribution, sinus endothelium was compared to portal-vein endothelium in which endothelial fenestrations are also seen; and these fenestrations as well as pits may be covered by a thin diaphragm. Of interest was the total absence of binding to the diaphragm. The significance of these findings is discussed.

Animals↗
PubMed1986

Expression of lectin receptors on the surface of granulocyte-macrophage progenitor cells (CFU-gm).

It has been emphasized that specific bindings between membrane glycoproteins and membrane lectin-like substances are important in cell-to-cell interactions. We explored the surface of granulocyte-macrophage precursor cells (CFU-gm) by the differential agglutination technique. Enrichment of CFU-gm in the agglutinated fraction, containing the cells which have lectin receptors, from marrow treated with soybean agglutinin (SBA), peanut agglutinin (PNA) and concanavalin A (Con A), suggests the presence of reactive galactosyl and mannosyl residues on the surface of CFU-gm. On the other hand, wheat germ agglutinin (WGA), phytohemagglutinin (PHA) and ulex europaeus agglutinin (UEA), which bind to reactive N-acetylglucosamine, N-acetylgalactosamine and fucose, respectively, did not specifically agglutinate CFU-gm. Thus, reactive groups containing galactosyl and mannosyl structures on the surface of CFU-gm may possibly play a role in the process of cell-to-cell interactions between CFU-gm and marrow stromal cells.

Animals↗
PubMed1986

Medical problems of space flight.

Several consistent medical problems have been encountered by astronauts during space flights. These include vestibular dysfunction, weight loss, increase in height, upward fluid shift, anemia, cardiovascular deconditioning, muscle atrophy, and bone loss. Almost all of these alterations can be attributed to the absence of gravitational force. Most are adaptive in nature and therefore reversible, but readaptation after returning to earth may cause further problems (e.g., in the case of vestibular dysfunction). The most recalcitrant and disturbing of all these problems is the relentless bone loss associated with negative calcium balance. This problem appears to be irreversible, and critical demineralization can occur after two years in a weightless state. Unless its mechanism is elucidated and preventive measures are taken, the bone loss may prove to be the medically limiting factor for the duration of space flight.

Anemia↗
PubMed1986

Double labeling of transferrin: tritium labeling of sialic acid and 125I or 59Fe labeling of the protein moiety.

A method is described in which the glycoprotein transferrin was double labeled. Its sialic acid residues were labeled with 3H through a consecutive oxidation-reduction technique utilizing tritiated NaBH4. Its protein moiety was labeled with either 125I or 59Fe. Incubation of this double-labeled molecule at 4 degrees C with K562 cells gave overlapping curves, indicating identical patterns of binding for all labels. At 37 degrees C, 3H and 125I demonstrated identical patterns while 59Fe was cummulatively retained. This method can be used to follow the fate of other glycoproteins and their possible desialation in vivo.

Biological Transport, Active↗
PubMed1986

Liver endothelium mediates the uptake of iron-transferrin complex by hepatocytes.

We have previously shown that in the liver, transferrin (TF) receptors are limited to endothelial cells, and hepatocytes and Kupffer cells do not have TF receptors. To study the transport of iron into hepatocytes, we fractionated liver cell suspensions into endothelium and hepatocyte fractions. At 4 degrees C liver (but not umbilical cord) endothelium bound Fe-TF with a saturable kinetics. At 37 degrees C, the endothelial uptake was followed by its gradual release. Transendothelial transport of TF was visually demonstrated by perfusion of liver using colloidal gold-labeled TF. The released Fe-TF acquired the potential for binding to fresh target hepatocytes and binding was not inhibited by excess cold TF but was inhibitable by asialofetuin, suggesting galactosyl receptors and not TF receptors as a recognition mechanism. Isoelectrofocusing of the supernate after preincubation for 90 min at 37 degrees C with endothelial cells, demonstrated the presence of a newly generated band which co-migrated with asialotransferrin. We conclude that Fe-TF is initially removed by liver endothelium where it is modified probably by desialation to expose the galactosyl residues of the glycoproteins. The modified molecule is subsequently released and recognized by hepatocytes through a TF receptor-independent mechanism which may involve galactosyl receptors of hepatocytes. The findings indicate a key role for endothelium in the transport of Fe-TF into the liver and may suggest a physiological function for galactosyl receptors on hepatocyte surface.

Animals↗