Kenneth Olden

Carbohydrate moieties of glycoproteins. A re-evaluation of their function.

Abstract Glycosylation-deficient mutants and inhibitors of glycosulation have been used to investigate the biological function of the carbohydrate moieties of glycoproteins. These and other experimental findings are reviewed and critically evaluated in the present treatise. An hypothesis is proposed to explain the biological significance of the covalent attachment of carbohydrate to protein. It is proposed that the carbohydrate acts as a chemical ‘tag’ which, upon interaction with a specific intracellular membrane receptor, directs glycoproteins to specific cellular organelles following synthesis on the rough endoplasmic reticulum. It is further proposed that secretion of extracellular products is a more general phenomenon which appears not to be absolutely carbohydrate dependent. Also, data are presented which support the view that the carbohydrate moeity is required for the proteolytic or conformational stabilization of the protein component of glycoproteins, but not for the mediation of protein specific biological activity. A model is presented which suggests, for the first time, that the localization function of carbohydrates is not restricted to lysosomal enzymes.

Role of carbohydrates in protein secretion and turnover: Effects of tunicamycin on the major cell surface glycoprotein of chick embryo fibroblasts

Using tunicamycin, we have investigated the role of glycosylation in the biosynthesis, processing and turnover of CSP, the major cell surface glycoprotein of chick embryo fibroblasts (CEF). This antibiotic specifically inhibits glycosylation mediated by dolichol pyrophosphate and consequently inhibits the glycosylation of asparaginyl residues of glycoproteins. Tunicamycin inhibited the incorporation of 3H-mannose into CSP by 92--98% and 14C-glucosamine by 84--96%, whereas total protein synthesis was decreased by only 15--45%. Tunicamycin treatment decreased total amounts of CSP by approximately 50--65%, with equal decreases in CSP occurring on the cell surface and in culture medium, whereas intracellular pools of CSP were not substantially affected. In contrast to CSP, three other membrane-associated proteins of apparent molecular weights 75,000, 95,000 and 150,000 daltons were found in increased amounts. Procollagen secretion was not inhibited by tunicamycin. Both procollagen and CSP secretion into culture medium were also not increased in AD6, a glycosylation-deficient, mutant mouse 3T3 cell line compared to wild-type cells. We examined the mechanism of the decrease in CSP after tunicamycin treatment. The rate of CSP biosynthesis as measured by pulse-labeling with 14C-leucine was not altered. Tunicamycin had only a slight effect on the initial times and rates of CSP appearance on the cell surface; some apparent intracellular redistribution of CSP was detected by immunofluorescence. The major effect of tunicamycin treatment was to accelerate the rate of degradation of CSP 2--3 fold. This increase is sufficient to account for the observed decreases after tunicamycin treatment. Our results suggest that carbohydrates may not be essential for CSP or procollagen synthesis, intracellular processing and secretion, but that carbohydrates may help stabilize CSP against proteolytic degradation.

Fibronectin and integrins in invasion and metastasis

SummaryThe adhesive glycoprotein fibronectin and integrin receptors appear to play important roles in the progression of metastatic disease. Fibronectin is a multifunctional extracellular glycoprotein that has at least two independent cell adhesion regions with different receptor specificities. The cell adhesive region in the central portion of fibronectin is comprised of at least two minimal amino acid sequences - an Arg-Gly-Asp (RGD) sequence and a Pro-His-Ser-Arg-Asn (PHSRN) sequence - which function in synergy. Another cell adhesive region is located near the carboxy-terminus in the alternatively spliced IIICS module. The critical minimal sequences for this region are Leu-Asp-Val (LDV) and Arg-Glu-Asp-Val (REDV) which function in an additive rather than synergistic fashion. Integrins are heterodimeric, transmembrane cell adhesion receptors for fibronectin and other extracellular matrix molecules. Several different integrins bind to fibronectin. The α5β1 fibronectin-specific integrin binds to the central RGD/PHSRN site. The α4β1 integrin binds to the IIICS site. Fibronectin-integrin interactions are important in tumor cell migration, invasion, and metastasis. In addition to promoting cell adhesion to the extracellular matrix, these proteins may also function in chemotaxis and control of proliferation. Peptide and antibody inhibitors of fibronectin and integrin functions have been shown to be effective inhibitors of metastasis, and are potentially important reagents for the study and control of cancer.

Investigation of the biological effects of anti-cell adhesive synthetic peptides that inhibit experimental metastasis of B16-F10 murine melanoma cells.

The experimental metastasis of B16-F10 murine melanoma cells is blocked by the anti-cell adhesive pentapeptide Gly-Arg-Gly-Asp-Ser (GRGDS) derived from the central cell-binding domain of fibronectin. In this report, we show that peptide treatment substantially extends the survival time for mice injected intravenously with B16-F10 cells (8/8 vs. 0/8 mice alive at 150 d), thereby demonstrating the potential efficacy of GRGDS treatment in protection against metastatic colonization. We have also examined the specificity of GRGDS activity by testing a series of related homologues for their effects on experimental metastasis. The overall profile of the relative inhibitory activities of these peptides closely matched their previously established capacity to disrupt adhesion in vitro. Lung retention studies with radiolabeled B16-F10 cells revealed an accelerated rate of cell loss from the lung 0-6 h after coinjection with the active peptide GRGDS. This early effect of GRGDS was consistent with its short circulatory half-life, which was found to be 8 min. Taken together, these results suggest that peptide-mediated inhibition of pulmonary colonization is due to interference with B16-F10 cell adhesion to structures in the target organ. Possible peptide interference in tumor cell-blood cell interactions was examined in order to assess (a) possible biological side-effects of peptide treatment and (b) whether such interactions might be an alternative mechanism for GRGDS-mediated inhibition of pulmonary colonization. GRGDS was found to retain full inhibitory activity when coinjected with B16-F10 cells into mice in which platelet function was impaired by acetylsalicylic acid treatment or into thrombocytopenic mice treated with antiplatelet serum (76-93% inhibition of colony formation). These data suggest that platelet involvement in the effects of the peptide is minimal. Similarly, GRGDS was also found to be a potent inhibitor of experimental metastasis in natural killer (NK) cell-deficient beige mice (86% inhibition), thereby discounting the possibility that GRGDS artifactually enhanced NK cell activity. We conclude as a result of these studies that cell-binding fibronectin peptides are specific inhibitors of experimental metastasis that prolong survival, that they appear to function by blocking the adhesion of B16-F10 cells to structures in the target organ, and that they do not appear to act through side effects on certain metastasis-related blood cell functions. In the future, derivatives of fibronectin peptides may be potentially useful prophylactic agents for interfering with the process of metastasis.

Function of glycoprotein glycans

Abstract Recent results, obtained simultaneously in several laboratories, are reviewed which strengthen earlier notions that the glycan moieties of some glycoproteins play important roles in (1) maintenance of protein conformation and solubility, (2) proteolytic processing and stabilization of the polypeptide against uncontrolled proteolysis, (3) mediation of biological activity, (4) intracellular sorting and externalization of glycoproteins, and (5) embryonic development and differentiation.

Mechanism of the decrease in the major cell surface protein of chick embryo fibroblasts after transformation

Abstract The major cell surface glycoprotein of cultured chick embryo fibroblasts (CSP, a LETS protein) is substantially decreased after neoplastic transformation. We investigated the regulation of this glycoprotein by determining the kinetics of CSP biosynthesis, transit to the cell surface, and degradation before and after transformation by Rous sarcoma virus. CSP synthesis, as measured by immunoprecipitation after pulse-labeling with 14 C-leucine, is decreased 3–6 fold after transformation by the Bryan high titer, Schmidt-Ruppin and temperature-sensitive ts68 and T5 strains of Rous sarcoma virus. Steady state quantities of CSP in intracellular pools are also decreased 4–5 fold after transformation. However, the rate at which newly synthesized CSP is processed and exported to the cell surface is similar before and after transformation. Degradation and release of CSP from cells were measured after labeling for 24 hr. The half-life of CSP on normal cells is 36 hr and is decreased to 16–26 hr after transformation. The absolute amount of intact CSP released into the culture medium is decreased 3 fold after transformation; these amounts, however, represent losses of approximately 20 and 40% of the total CSP synthesized by normal and transformed cells, respectively. These results indicate that the major mechanism for the decrease in CSP after transformation is reduction in its biosynthesis, although increased degradation and loss from the cell surface also contribute significantly. These changes can account for the observed 5–6 fold decreases in cell-associated CSP after transformation of chick embryo fibroblasts.

TRANSFORMATION‐SENSITIVE CELL SURFACE PROTEIN: ISOLATION, CHARACTERIZATION, AND ROLE IN CELLULAR MORPHOLOGY AND ADHESION

Cell surface protein is the major cell surface glycoprotein of chick embryo fibroblasts. We have isolated and purified this glycoprotein and find that it is an adhesive protein that increases cell-cell and cell-substratum adhesiveness in a variety cellular adhesion assays. Transformation of chick fibroblasts results in decreased quantities of CSP due primarily to a fivefold reduction in CSP biosynthesis, although increased proteolytic degradation and shedding from the cell surface also contribute. The decreased biosynthesis is apparently due to a fivefold reduction in translatable mRNA for CSP. Reconstitution of isolated purified CSP on 14 transformed cell lines from several species results in reversion to a more normal fibroblastic morphology, adhesiveness, cell surface architecture, microfilament bundle organization, motility, and alignment at confluence. Cell surface protein does not restore growth control. The effects of CSP appear to be due to at least two actions, increased cell-substratum adhesion plus altered cell-cell interactions. Untransformed chick cells treated with affinity-purified antibodies to CSP develop the rounded morphology characteristic of many transformed cells that are deficient in CSP (LETS protein). Cell surface protein is found primarily in fibrillar aggregates on the cell surface. These CSP fibrils are relatively immobile and do not affect the mobility of other cell surface components. However, CSP can be eventually redistributed to caplike structures with anti-CSP. Isolated CSP consists of highly asymmetric disulfide-linked dimers and multimers. The interchain disulfide bridges are confined to a short terminal fragment that is readily removed by trypsin. Cell surface protein and cold-insoluble globulin have similar compositions but differ in solubility and amino termini. Cell surface protein contains primarily asparagine-linked oligosaccharides that appear to be responsible for CSPs concanavalin A receptor activity. Inhibition of CSPs glycosylation by treatment with tunicamycin results in decreased CSP due to marked increases in its degradation rate, without inhibition of synthesis or secretion. Studies of this major cell surface glycoprotein have provided insight into the biochemical mechanisms of cellular adhesion, morphology, and social interaction and provide an approach to analyze the dynamics and regulation of protein synthesis, glycosylation, secretion, and turnover.

Role of Fibronectin in Adhesion, Migration, and Metastasis

AbstractAdhesive macromolecules of the extracellular matrix regulate cellular migration, differentiation, and growth. They also contribute actively to the pathogenesis of diseases characterized by aberrant adhesion, including cancer. One of the most well characterized adhesion factors is the glycoprotein fibronectin; biochemical analyses of this molecule have substantially improved our understanding of how cells adhere, migrate, and invade, and it is now thought that fibronectin plays a key role in the tissue remodeling and cell migration events that occur during normal embryonic development and adult wound healing. In this review, we describe how recent studies, focusing on the basic question of precisely how a cell adheres to fibronectin at the molecular level, have led to insights into the mechanisms of cellular migration by normal and malignant cells and to the development of a novel, synthetic inhibitor of experimental metastasis. We also critically discuss the future prospects for antiadhesive agent...

Environmental health and genomics: visions and implications

The relationship between genes and the environment can be compared to a loaded gun and its trigger. A loaded gun by itself causes no harm; it is only when the trigger is pulled that the potential for harm is released. Genetic susceptibility creates an analogous situation, where the loaded gun is one or a combination of susceptibility genes (alleles) and the trigger is an environmental exposure. The key objective of the Environmental Genome Project is to identify alleles that confer susceptibility to the adverse effects of environmental agents. Here we discuss the goals of the Environmental Genome Project, its implications and, in particular, its potential effect on our ability to assess human disease risk in the future.

Direct detection of antigens in sodium dodecyl sulfate-polyacrylamide gels

Abstract We describe a simple immunochemical technique for the detection of specific antigens by antibody binding in polyacrylamide gels. Proteins are solubilized in sodium dodecyl sulfate and separated by electrophoresis in SDS-slab gels. Following fixation and removal of SDS, gel strips are incubated with normal or immune sera. After washing out unbound antibody, the gel strips are either fixed and stained with Coomassie blue or exposed to anti-immunoglobulin conjugated to horseradish peroxidase. The region(s) of antibody-antigen binding are determined from densitometric scans of the Coomassie blue-stained gels versus controls or by treatment of the gels with diaminobenzadine to localize the peroxidase. We have used this technique successfully with antibodies against fibroblast myosin, bovine serum albumin, goat immunogolbulin, the 220,000-dalton fibroblast cell-surface protein, and chicken gizzard filamin. Lectin-binding proteins can also be detected by substituting lectins for the immunoglobulins.