Cell surface enzymes: effects on mitotic activity and cell adhesion

IL-1beta, IFN-gamma and TNF-alpha increase vulnerability of pancreatic beta cells to autoimmune destruction

In the pathogenesis of type-1 diabetes insulin-producing beta-cells are destroyed by cellular autoimmune processes. The locality of beta-cell destruction is the inflamed pancreatic islet. During insulitis cytokines released from islet-infiltrating mononuclear cells affect beta-cells at several levels. We investigated whether cytokine-induced beta-cell destruction is associated with changes in the expression of the surface receptors intercellular adhesion molecule (ICAM)-1 and Fas. Islets from diabetes-prone and congenic diabetes-resistant BB rats were exposed to interleukin (IL)-1beta alone or in combination with interferon (IFN)-gamma plus tumour necrosis factor (TNF)-alpha. Cytokines decreased islet insulin content, suppressed glucose stimulated insulin secretion and generated enhanced amounts of nitric oxide and DNA-strand breaks. While no membrane alterations of IL-1beta treated islets cells were detectable, the cytokine combination caused damage of cell membranes. Independent of diabetes susceptibility IL-1beta treated islet beta-cells expressed a significantly increased amount of ICAM-1 on their surfaces which was not further increased by IFN-gamma+TNF-alpha. However, IL-1beta induced Fas expression was significantly enhanced only on beta-cells from diabetes-prone BB rats. From these results we suggest that IL-1beta mediates the major stimulus for ICAM-1 induction which is possibly a necessary but not sufficient step in the process of beta-cell destruction. Obviously, the additional enhancement of Fas expression on the surface of beta-cells is important for destruction. The combined action of all three cytokines induced the expression of Fas on the beta-cell surface independent of diabetes susceptibility, indicating that such a strong stimulus in vitro may induce processes different from the precise mechanisms of beta-cell destruction in vivo.

Biological and pathobiological aspects of the glycocalyx of the small intestinal epithelium. A review

The literature on the glycocalyx of small intestinal epithelium is reviewed. The structure, general and barrier functions, synthesis, and degradation of the glycocalyx, and pathobiological aspects of the glycocalyx in relation to its barrier function are mentioned. Topics for future research are indicated.

Substrate-effected release of surface-located protein kinase from intact cells

Protein kinase activity that is independent of cAMP has been reported to exist on the surface of intact HeLa cells. Here we report that the protein kinase activity can be released by the use of casein or phosvitin within a short period of time. The discharge of the enzyme occurs from intact cells since (i) the cells do not release intracellular material and (ii) the cultures continue to grow within any morphological alteration. As shown with phosvitin, the release of protein kinase depends on substrate concentration, incubation time, and temperature. The degree of inducible release or surface protein kinase is inversely related to cell density. Four incubations with phosvitin (1 mg/ml) are sufficient to liberate most of the enzyme, thus greatly reducing the capacity of the cells to phosphorylate cellular substrates at the surface. Within approximately 24 hr after protein kinase removal, cultures have restored their surface protein kinase. Cultured cells of different origin (rat liver, mouse cerebellum, and human lung) exhibited phosvitin-induced protein kinase release from intact cells. The possible significance of these observations with respect to extracellular protein phosphorylation is discussed.

Approach to the mechanism of Zajdela's hepatoma cell growth inhibition induced by concanavalin A and phytohaemagglutinin

Cell growth of tumour ascites cells was inhibited by concanavalin A, phytohaemagglutinin and Ricinus lectin at 2-100 micrograms/ml. As expected, the Ricinus lectin inhibited the protein synthesis estimated by leucine incorporation and decreased thymidine incorporation, whereas concanavalin A and phytohaemagglutinin stimulate the uptake and the incorporation of both leucine and thymidine, and thus, synthesis of protein and DNA. These results suggest that different mechanisms are involved in the hepatoma cell growth inhibition by the lectins. This difference was not related to the kinetic characteristics of the lectin interactions with the cells which represent a first and necessary step. It was showed that concanavalin A and phytohaemagglutinin as well as chloroquine inhibited the 14C-labelled asialofetuin degradation. We can conclude that Ricinus lectin present a toxic effect whereas both concanavalin A and phytohaemagglutinin show an anti-protease activity.

The site of incorporation of sialic acid residues into glycoproteins and the subsequent fates of these molecules in various rat and mouse cell types as shown by radioautography after injection of [3H]N-acetylmannosamine. I. Observations in hepatocytes

To study the site of incorporation of sialic acid residues into glycoproteins in hepatocytes, we gave 40-g rats and 15-g Swiss albino mice a single intravenous injection of [3H]N-acetylmannosamine (8 mCi) and then sacrificed them after 2 and 10 min. To trace the subsequent migration of the labeled glycoproteins, we injected 40-g rats with 4 mCi of [3H]N-acetylmannosamine and sacrificed them after 20 and 30 min, 1, 4, and 24 h, and 3 and 9 d. Concurrent biochemical experiments were carried out to test the specificity of injected [3H]N-acetylmannosamine as a precursor for sialic acid residues of glycoproteins. In radioautographs from rats and mice sacrificed 10 min after injection, grain counts showed that over 69% of the silver grains occurred over the Golgi region. The majority of these grains were localized over the trans face of the Golgi stack, as well as over associated secretory vesicles and possibly GERL. In rats, the proportion of grains over the Golgi region decreased with time to 37% at 1 h, 11% at 4 h, and 6% at 24 h. Meanwhile, the proportion of grains over the plasma membrane increased from 4% at 10 min to 29% at 1 h and over 55% at 4 and 24 h; two-thirds of these grains lay over the sinusoidal membrane, and the remainder were equally divided over the lateral and bile canalicular membranes. Many silver grains also appeared over lysosomes at the 4- and 24-h time intervals, accounting for 15-17% of the total. At 3 and 9 d after injection, light microscope radioautographs revealed a grain distribution similar to that seen at 24 h, with a progressive decrease in the intensity of labeling such that by 9 d only a very light reaction remained. Because our biochemical findings indicated that [3H]N-acetylmannosamine is a fairly specific precursor for the sialic acid residues of glycoproteins (and perhaps glycolipids), the interpretation of these results is that sialic acid is incorporated into these molecules in the Golgi apparatus and that the latter then migrate to secretion products, to the plasma membrane, and to lysosomes in a process of continuous renewal. It is possible that some of the label seen in lysosomes at later time intervals may have been derived from the plasma membrane or from material arising outside the cells.

The role of bacterial surface structures in pathogenesis

Modern research has revealed that the true surfaces of animal cells consist of polysaccharide chains that are linked to proteins hydrophobically anchored in the membrane and protrude to form a dense glycocalyx. It has become increasingly clear that most pathogenic bacteria must position themselves at the surface of their "target" cell in order to exert their toxic or otherwise deleterious effects. The true surface of most pathogenic bacteria has also been recently shown to consist of a protruding mass of polysaccharide chains--the bacterial glycocalyx--that is composed of teichoic acids in many gram-positive species and of acid polysaccharides in many gram-negative organisms. Through this bacterial glycocalyx certain cell surface proteins and organized protein structures (e.g., pili) are known to project, so that the bacterial surface is a mosaic of polysaccharides and proteins; both of these types of molecules have been implicated in instances of specific pathogenic adhesion. Besides their role in specific adhesion to target cells, these surface components interpose a highly charged, and often very extensive, barrier that can prevent the penetration of antibodies and antibiotics to their target sites in the bacterial cell. They may also frustrate mucociliary clearance, phagocytosis, and other clearance mechanisms of the host. We will discuss the chemical and physical nature of these bacterial surface components that mediate pathogenic adhesion and counteract host defense mechanisms sufficiently to allow infections to become established.

Growth regulation of lens epithelial cells. Chemically-modified sepharose as a suitable substratum for studying cell-substratum interactions

Suitable substrata for studying cell-substratum interactions were prepared by melting of unsubstituted and substituted agarose beads (Sepharose). The reductive alkylation procedure proved to be useful in introducing various ligands into the gel matrix. The resulting bonds remained stable during sterilisation and melting. The introduction of charges into the gel matrix facilitated cell adhesion to the substratum. But only positively-charged agarose substrata were as effective as glass surfaces in supporting cell attachment. The correlation found between cell attachment and the stimulation of DNA synthesis demonstrates that lens epithelial cells are strongly anchorage dependent.

Sialyltransferase activities of aging diploid fibroblasts

Sialyltransferase activity and cell-cell adhesion rates of aging WI-38 cells were studied to determine the possible basis for a previously described decrease in membrane bound sialic acid and loss of proliferation of senescent cells. Ectosialyltransferase was demonstrated on the surface of both young and old WI-38 cells. The sialyltransferase assays consist of an enzyme source which is either the surface of intact cells (ectoenzyme) or a Triton X-100 cell homogenate, the nucleotide sialic acid donor (cytidine monophosphate-N-acetylneuraminic acid), and an asialo-acceptor which may be endogenous to the enzyme preparation or may be added exogenously. When sialyltransferase activity is measured in the absence of exogenous acceptors, there is a greater amount of sialic acid transferred by odl cells. However, when exogenous acceptors are provided, the amount of transfer is stimulated to a greater extent in young cells equalizing the amount of sialic acid incorporated into young and old cells. This suggests that there are fewer asialoglycoproteins and that acceptor concentration is a limiting factor in assays of young cell sialyltransferase. The end result of this may be the previously described decreased amount of membrane-bound sialic acid of old cells. A change in the adhesiveness of old cells is described which may be related to the altered cell surface.