An assessment of the effects of swainsonine on survival of mice injected with B16-F10 melanoma cells

Bitter sweetness of complexity

Glycosylation of proteins, lipids and mucins has gained increasing complexity in the course of evolution. Metazoans and mammals exhibit extensively exploited pathways of N-glycan biosynthesis, with unique features that are not found in plants or protozoans.Paralleling the complexity of N-glycan structure, their impact on regulatory processes has become very diverse and has evolved into a multidimensional lattice imprinting modes of cellular communication. Processes that are regulated by N-glycans are cellular adhesion and motility, growth factor and cytokine signalling, metabolic homeostasis, and binding of certain pathogens. Consequently, alterations in N-glycan biosynthesis interfere with cellular proliferation and differentiation and may produce disturbances in embryonic development, trigger inflammatory processes, favour tumour development and enhance the metastastic dissemination of primary tumours. Particular N-glycans that have been causally related to these pathological scenarios are the complex-type N-glycans, branching from oligomannosidic core structures into β-glycosidic linkages, connected to acetylated glucosamine and galactose, and yield extended lactosamine chains of variable length. These N-acetyllactosamines are preferred building blocks for further modification by fucosylation, sialylation, and sulphation, thus creating binding sites for different galectins or selectins. The focus of this review will be on the b1,6-N -acetylglucosaminyltransferase-V/GnT-V/MGAT5, a phylogenically conserved enzyme that is required for the synthesis of β1,6-branched complex-type oligosaccharides in the medial Golgi compartment, and its implications in metabolism and cancer progression.

N-acetylglucosaminyltransferase V (Mgat5)-mediated N-glycosylation negatively regulates Th1 cytokine production by T cells

The differentiation of naive CD4(+) T cells into either proinflammatory Th1 or proallergic Th2 cells strongly influences autoimmunity, allergy, and tumor immune surveillance. We previously demonstrated that beta1,6GlcNAc-branched complex-type (N-acetylglucosaminyltransferase V (Mgat5)) N-glycans on TCR are bound to galectins, an interaction that reduces TCR signaling by opposing agonist-induced TCR clustering at the immune synapse. Mgat5(-/-) mice display late-onset spontaneous autoimmune disease and enhanced resistance to tumor progression and metastasis. In this study we examined the role of beta1,6GlcNAc N-glycan expression in Th1/Th2 cytokine production and differentiation. beta1,6GlcNAc N-glycan expression is enhanced by TCR stimulation independent of cell division and declines at the end of the stimulation cycle. Anti-CD3-activated splenocytes and naive T cells from Mgat5(-/-) mice produce more IFN-gamma and less IL-4 compared with wild-type cells, the latter resulting in the loss of IL-4-dependent down-regulation of IL-4Ralpha. Swainsonine, an inhibitor of Golgi alpha-mannosidase II, blocked beta1,6GlcNAc N-glycan expression and caused a similar increase in IFN-gamma production by T cells from humans and mice, but no additional enhancement in Mgat5(-/-) T cells. Mgat5 deficiency did not alter IFN-gamma/IL-4 production by polarized Th1 cells, but caused an approximately 10-fold increase in IFN-gamma production by polarized Th2 cells. These data indicate that negative regulation of TCR signaling by beta1,6GlcNAc N-glycans promotes development of Th2 over Th1 responses, enhances polarization of Th2 cells, and suggests a mechanism for the increased autoimmune disease susceptibility observed in Mgat5(-/-) mice.

Biomedical applications of poisonous plant research

Research designed to isolate and identify the bioactive compounds responsible for the toxicity of plants to livestock that graze them has been extremely successful. The knowledge gained has been used to design management techniques to prevent economic losses, predict potential outbreaks of poisoning, and treat affected animals. The availability of these compounds in pure form has now provided scientists with tools to develop animal models for human diseases, study modes of action at the molecular level, and apply such knowledge to the development of potential drug candidates for the treatment of a number of genetic and infectious conditions. These advances are illustrated by specific examples of biomedical applications of the toxins of Veratrum californicum (western false hellebore), Lupinus species (lupines), and Astragalus and Oxytropis species (locoweeds).

Limits of stimulation of proliferation and differentiation of bone marrow cells of mice treated with swainsonine

The limits of stimulation of the immunomodulatory alkaloid swainsonine (8alphabeta-indolizidine-1alpha,2alpha,8beta-triol) were studied in inbred C57BL/6 mice for potential support of intense high dose cancer chemotherapy and/or radiation because of its attractive pharmacologic profile on the hematopoietic system. Specifically, the effects of swainsonine on bone marrow cellularity and on in vitro progenitor cell proliferation to total colony forming units (CFU) and differentiation to different lineages were studied as a function of number of days post drug administration. The lineages evaluated were colony forming units-granulocyte-macrophage (CFU-GM), erythroid-burst forming units (BFU-e) and CFU-granulocyte-erythrocyte-monocyte-megakaryocyte (CFU-GEMM or CFU-Mix). Groups of mice were treated with swainsonine or plain vehicle, phosphate buffered saline for 10 consecutive days. The effects of these agents on the hematopoietic system were studied up to 60 days following their discontinuation. The magnitude of the effects of swainsonine on bone marrow system gradually declined with increasing duration of days following its discontinuation. Nevertheless, its residual stimulatory effects on bone marrow cellularity, total CFU, CFU-GM, BFU-e and CFU-Mix continued to be significant (P<0.0001) up to 45, 50, 50, 55 and 50 days, respectively, compared to those of diluent buffer or untreated controls. Since cancer chemotherapeutic agents or radiation are normally given in schedules and/or cycles, these results strongly suggest that swainsonine effects are sustained long enough to potentially support and facilitate hematopoietic recovery during anti-cancer cytotoxic treatment.

Primers of glycosaminoglycan biosynthesis from Peruvian rain forest plants

We have developed a rapid, high throughput screening assay for compounds that alter the assembly of glycosaminoglycan chains in Chinese hamster ovary cells. The assay uses autoradiography to measure the binding of newly synthesized [35S]proteoglycans and [35S]glycosaminoglycans to a positively charged membrane. Screening over 1000 extracts from a random plant collection obtained from the Amazon rain forest yielded five plants that stimulated glycosaminoglycan assembly in both wild-type cells and a mutant cell line defective in xylosyltransferase (the first committed enzyme involved in glycosaminoglycan biosynthesis). Fractionation of an extract of Maieta guianensis by silica gel and reverse-phase chromatography yielded two pure compounds with stimulatory activity. Spectroscopic analysis by NMR and mass spectrometry revealed that the active principles were xylosides of dimethylated ellagic acid. One of the compounds also contained a galloyl group at C-3 of the xylose moiety. These findings suggest that plants and other natural products may be a source of agents that can potentially alter glycosaminoglycan and proteoglycan formation in animal cells.

Application of chemical immunomodulators to the treatment of cancer and AIDS

A number of potential advantages, development of promising new agents, and the discovery of synergy with cytokines or cell products continue to spur research into the application of chemical immunomodulators for the treatment of cancer and AIDS. In preclinical in vitro and in vivo systems, chemical immunomodulators definitely modulate the immune system and have therapeutic efficacy. Although clinical trials have shown the ability of these agents to modulate the human immune system, thus far chemical immunomodulators have generally not fulfilled the therapeutic promise generated in animal models for the treatment of human diseases. While the discrepancy in results between animal models and human trials is obvious, the basis is not apparent. Species differences in elimination kinetics, presentation of active drug at the site of action, and the development of tachyphylaxis have been postulated as reasons for the minimal activity of these agents in humans. In addition, the use of investigational techniques established for cytotoxic agents may not be appropriate for immunomodulators. As with any immunomodulator, determining an optimal immunostimulatory dose and schedule and applying the therapy to patients with minimal tumor burden would perhaps be more appropriate than use of a maximally tolerated dose in patients with advanced disease. A dose-immunological effect relationship has recently been demonstrated for levamisole at doses higher than those used for many years in levamisole trials (99). While research and clinical investigation have identified several potentially useful chemical immunomodulators, the elementary understanding of the biochemical mechanisms involved in immunoregulation remains basic. Future research must elucidate these mechanisms, particularly in humans, to maximize the benefits of chemical immunomodulators as single agents or combined with cytotoxic chemotherapeutic agents, surgery, radiation therapy, other immunomodulators, and antiviral agents.

Accumulation of pentamannose oligosaccharides in human mononuclear leukocytes by action of swainsonine, an inhibitor of glycoprotein processing

Swainsonine, a known inhibitor of the alpha-mannosidase II involved in processing of asparagine-linked glycoproteins, causes accumulation of hybrid-type oligosaccharide-containing glycoproteins in mammalian cells. Swainsonine augments lymphokine-activated, killer-cell induction at suboptimal doses of interleukin-2; the amount needed to increase LAK activity is 100-1000 fold higher than required to completely inhibit mannosidase II. Human mononuclear lymphocytes, when treated with these relatively high (58 microM) concentrations of swainsonine showed a 3-4 fold increase in D-[3H]mannose incorporation into the glycan as compared to glycans of untreated cells. Analysis indicated accumulation of high-mannose type, free oligosaccharides in the soluble fractions of the cell. Chromatographic analysis of glycan obtained by D-[2-3H]mannose labeling of human mononuclear lymphocytes showed synthesis of a new oligosaccharide, at 58 microM of swainsonine, that contained 36% of the total radioactivity incorporated into the glycan (oligosaccharide pool). This oligosaccharide fraction was resistant to hydrolysis by endoglycosidase H, endoglycosidase F, O and N-glycanase, but was susceptible to cleavage by Jack bean alpha-mannosidase and was bound > 90% to concanavalin A-Sepharose. A similar chromatographic elution profile was obtained from glycans labeled with D-[2-3H]mannose from mouse B16F10 melanoma and baby hamster kidney cells subsequent to swainsonine treatment. Methylation analysis of free oligosaccharides obtained from MNL revealed the presence of a pentamannose. These results indicate the accumulation of a free high-mannose oligosaccharide rather than expected hybrid-type structure on treatment of cells with relatively high concentrations of swainsonine.

The potential importance of swainsonine in therapy for cancers and immunology

Swainsonine, an indolizidine alkaloid, was initially used in biomedical research as a tool to investigate the biosynthesis and function of asparagine-linked 'complex' type oligosaccharide moieties of glycoproteins. Recently, swainsonine has generated interest in its potential use as an anticancer agent with reports that it (i) inhibits tumor growth and metastasis, (ii) augments natural killer (NK) and macrophage-mediated tumor cell killing, and (iii) stimulates bone marrow cell proliferation. The antineoplastic activity of swainsonine can be explained at least in part by augmentation of immune effector mechanisms. The potential application of swainsonine as an anticancer agent is discussed.