Effects of tunicamycin, N-methyl-1-deoxynojirimycin, and manno-1-deoxynojirimycin on the biosynthesis of lactosaminoglycans in F9 teratocarcinoma cells

Molecular cloning, chromosomal mapping and tissue-specific expression of a novel human alpha1,2-mannosidase gene involved in N-glycan maturation

Class I alpha1,2-mannosidases play an essential role in the elaboration of complex and hybrid N -glycans in mammalian cells. Using degenerate primers based on amino acid sequences conserved in all members of this enzyme family for RT-PCR, two distinct PCR products were obtained from placenta and lymphocyte cDNAs. One of these was related to the previously cloned human and murine alpha1, 2-mannosidase IA whereas the other was very similar to murine alpha1, 2-mannosidase IB. Northern blot analysis of human tissues with these two alpha1,2-mannosidase probes revealed very different patterns of tissue-specific expression. Similar tissue-specific expression of alpha1,2-mannosidase IA and IB was also observed on Northern blots of adult mouse tissues. A human placenta cDNA library was screened and PCR of brain, placenta, and lymphocyte cDNAs was performed in order to isolate the human alpha1,2-mannosidase IB cDNA. This cDNA encodes a type II membrane protein of 73 kDa that is 94% identical in amino acid sequence to the murine alpha1,2-mannosidase IB (Herscovics et al., 1994, J. Biol. Chem., 269, 9864-9871). A truncated soluble form of the human alpha1,2-mannosidase IB lacking its N -terminal transmembrane domain was expressed as a secreted protein in Pichia pastoris . The recombinant enzyme was incubated with [3H]Man9GlcNAc and [3H]Man8GlcNAc (isomer B), and high performance liquid chromatography analysis of the products showed that [3H]Man9GlcNAc was readily converted to [3H]Man6GlcNAc and much more slowly to [3H]Man5GlcNAc, whereas [3H]Man8GlcNAc was rapidly trimmed to [3H]Man5GlcNAc. The human alpha1,2-mannosidase IB gene was isolated from a P1 human genomic library and shown to be at least 60 kb in size and to contain at least 13 exons. The gene was localized by fluorescence in situ hybridization to human chromosome 1p13, a region that undergoes many aberrations in various types of human cancers. These results show that there are at least two Class I alpha1,2-mannosidases in the human and murine genomes with very distinct transcriptional regulation in different tissues.

Comparative study of high-mannose-type oligosaccharides in membrane glycoproteins of rat hepatocytes and different rat hepatoma cell lines

A comparative study was undertaken to characterize the oligosaccharides released by endo-beta-N-acetylglucosaminidase H (endo H) from the membrane glycoproteins of rat hepatocytes and three different Morris hepatoma cell lines (NA-MH 7777, HTC and MH1C1). It is shown that the membrane glycoproteins of hepatocytes and hepatoma cells contain markedly different quantities and forms of high-mannose-type carbohydrate chains. After radiolabelling of the cells with D-[2-3H]mannose, in the absence and presence of 1 mM 1,5-dideoxy-1,5-imino-D-mannitol (1-deoxymannojirimycin), high-mannose-type oligosaccharides were released from delipidated membrane glycoproteins by enzymic digestion with endo H. The carbohydrate chains were converted to their corresponding oligosaccharide alditols by reduction with sodium borohydride, then further analysed by HPLC using an APS-2 Hypersil column. In the absence of 1-deoxymannojirimycin, up to 10% of the radiolabelled oligosaccharides were released by endo H-treatment of the membrane glycoprotein fraction from rat hepatocytes. In contrast, the quantity of radiolabelled high-mannose-type carbohydrate chains released by endo H-treatment from tumour-cell membrane glycoproteins of hepatoma cell lines NA-MH 7777 (31.5%). MH1C1-MH 7795 (37.2%) and HTC-MH 7288c (48%) was increased up to fivefold. The formation of higher-mannosylated structures after oligosaccharide analysis was observed in all hepatoma cell lines, with Man8GlcNAcOH as the major component, whereas in hepatocytes Man5GlcNAcOH was the predominant high-mannose-type structure. In contrast, in the presence of the Golgi alpha-D-mannosidase I inhibitor, 1-deoxymannojirimycin, no significant differences were observed between the distribution of high-mannose-type oligosaccharides in the membrane glycoproteins of hepatocytes and hepatoma cells. However, in the presence of this inhibitor, the proportion of radiolabelled glycans sensitive to deglycosylation by endo H was greatly increased (> 85%) in all the cell lines investigated, the predominant structures being Man8-9-GlcNAcOH. This study shows that an increased content of high-mannose-type sugar chains is a general characteristic of membrane-bound glycoproteins for malignant transformed hepatocytes.

Anti-invasive activities of experimental chemotherapeutic agents

We have discussed a number of agents that affect invasion and we have grouped them according to their most probable targets. This strategy is based on the following hypothesis. Invasion is the result of cellular responses to extracellular signals. Candidate signals are components of the extracellular matrix, which are rendered inactive by the flavonoid (+)-catechin (see Section III). Signals are recognized by receptors on the plasma membrane, possibly glycoproteins, that may lose their recognition function through alteration of the oligosaccharide side chains by inhibitors of protein glycosylation (see Section IV) and possibly also by alkyllysophospholipids (see Section V). Synthetic oligopeptides reflecting sequences from cell-binding domains of extracellular matrix molecules are also effective tools for blocking specific receptors (see Section VI). GTP-binding proteins (G proteins) act as signal transducers and can be inactivated by pertussis toxin (see Section VII). An intriguing aspect of both alkyllysophospholipids and pertussis toxin is that they can either inhibit the invasion of constitutively invasive cells or induce invasion of constitutively noninvasive cells. Without doubt, cellular responses implicated in invasion are many-fold. Discussed here are cell motility and directional migration with inhibition through dipyridamole and its analogs and through microtubule inhibitors, respectively (see Section VIII). Alternative hypotheses and alternative strategies for the dissection of the invasion process do exist, and alternative cellular and molecular mechanisms of action may explain the anti-invasive activity of the agents discussed earlier. The latter are mentioned in each section. It is the authors' opinion that the possibilities for exploiting the battery of anti-invasive agents have by no means been exhausted. Introducing researchers to experiments that may lead to an understanding of the mechanisms of invasion and metastasis and to new rationales for cancer treatment has been the purpose of our review.