Inhibition of topoisomerase I by heparin

Selective inhibition of bacterial and human topoisomerases by N-arylacyl O-sulfonated aminoglycoside derivatives

Numerous therapeutic applications have been proposed for molecules that bind heparin-binding proteins. Development of such compounds has primarily focused on optimizing the degree and orientation of anionic groups on a scaffold, but utility of these polyanions has been diminished by their typically large size and non-specific interactions with many proteins. In this study N-arylacyl O-sulfonated aminoglycosides were synthesized and evaluated for their ability to selectively inhibit structurally similar bacterial and human topoisomerases. It is demonstrated that the structure of the aminoglycoside and of the N-arylacyl moiety imparts selective inhibition of different topoisomerases and alters mechanism. The results here outline a strategy that will be applicable to identifying small, structurally defined oligosaccharides that bind heparin-binding proteins with a high degree of selectivity.

Inhibition of DNA topoisomerases I and II, and growth inhibition of human cancer cell lines by a marine microalgal polysaccharide

We have previously reported purification of an extracellular polysaccharide GA3P, D-galactan sulfate associated with L-(+)-lactic acid, produced by a toxic marine microalga Dinoflagellate Gymnodinium sp. A(3) (GA3), and induction thereby of apoptosis on human myeloid leukemia K562 cells. In the present report, we show that the GA3P is a potent inhibitor of DNA topoisomerase (topo) I and topo II, irrespective of the presence or absence of the lactate group. Dextran sulfate also showed similar level of inhibition of topo I and topo II. We also demonstrated that, unlike camptothecin (CPT) or teniposide (VM-26), the inhibition of topo I or topo II by the polysaccharide does not involve accumulation of DNA-topo I/II cleavable complexes, clearly showing that they are not topo poisons but catalytic inhibitors with dual activity. Furthermore, the polysaccharide, when added to the reaction mixture with CPT or VM-26, inhibited stabilization of cleavable complex induced by the latter compounds. In addition, when added to the reaction mixture after the formation of the cleavable complexes by topo poisons, CPT for topo I and VM-26 for topo II, either GA3P or dextran sulfate diminished the amount of the complexes already accumulated, i.e. reversal of the reaction. These results suggest that the polysaccharides bind to the enzymes with high affinities, and that, as for topo I/II inhibition, the GA3P shares a common mechanism with dextran sulfate. As examined in vitro with a human cancer cell line panel, GA3P exhibited significant cytotoxicity against a variety of cancer cells. These findings show that the polysaccharide GA3P would prove to be a potential anticancer chemotherapeutic agent with dual activity of topo I and topo II catalytic inhibition.

Actions of heparin that may affect the malignant process

BACKGROUND

Heparin has many actions that may affect the malignant process, especially metastasis.

METHODS

The author conducted an extensive review of the available medical literature about heparin activity that may apply to important factors involved in the malignant process.

RESULTS

Thrombin is generated by tumors, and the resultant fibrin formation impedes natural killer cell activity. Microthrombi arrest tumor cells in capillaries. Heparin prevents the formation of thrombin and neutralizes its activity. Angiogenesis has an important role in metastasis; heparin minimizes angiogenesis via the inhibition of vascular endothelial growth factor, tissue factor, and platelet activating factor. It decreases tumor cell adhesion to vascular endothelium as it inhibits selectin and chemokine actions, and it also decreases the replication and activity of some oncogenic viruses. Matrix metalloproteinases, serine proteases, and heparanases have an important role in metastasis. Heparin decreases their activation and limits their effects. It competitively inhibits tumor cell attachment to heparan sulfate proteoglycans. It blocks the oncogenic action of ornithine decarboxylase and enhances the antineoplastic effect of transforming growth factor-beta. Heparin inhibits activator protein-1, which is the nuclear target of many oncogenic signal transduction pathways, and it potently inhibits casein kinase II, which has carcinogenic activity. Platelet-derived growth factor, which has oncogenic effects, is also inhibited by heparin, as are reverse transcriptase, telomerase, and topoisomerase prooncogenic actions.

CONCLUSIONS

These various heparin actions justify clinical investigation of its possible beneficial effect on malignant disease.

Inhibition of DNA topoisomerase I activity by heparan sulfate and modulation by basic fibroblast growth factor

Eukaryotic DNA topoisomerase I catalyzes changes in the superhelical state of duplex DNA by transiently breaking single strands thereby allowing relaxation of both positively and negatively supercoiled DNA. Topoisomerase I is a nuclear enzyme localized at active sites of transcription, and abnormal levels of the enzyme have been observed in a variety of neoplasms. Because the enzyme binds heparin and, given the presence of heparan sulfate within the nuclei of mammalian cells, we sought to investigate the interaction between topoisomerase I and sulfated glycosaminoglycans isolated from normal and neoplastic human liver. The results demonstrated that low concentrations (approximately 100 nM) of heparan sulfate from normal liver but not from its malignant counterpart effectively blocked relaxation of supercoiled DNA driven by either purified holoenzyme or topoisomerase I activity present in nuclear extracts of three malignant cell lines. Heparin acted at even lower (approximately 10 nM) concentrations. Moreover, we show that basic fibroblast growth factor could interfere with this heparan sulfate/heparin-driven inhibition and that both basic fibroblast growth factor and heparin-binding sites co-localized in the nuclei of U937 leukemic cells. Our results suggest that DNA topoisomerase I activity may be modulated in vivo by specific heparan sulfate moieties present in normal cells but markedly reduced or absent in their transformed counterparts.

Characterization of mitochondrial DNA topoisomerase I from Neurospora crassa

DNA topoisomerase I isolated from the lower eukaryote Neurospora crassa mitochondria was characterized. Molar mass of the enzyme in the native state is 120 kDa and 60-65 kDa when denatured. The pH optimum of the enzyme is 7.8 and the KCl optimum concentration is 40 mmol/L. This topoisomerase is independent of ATP and Mg2+. N-Ethylmaleimide, 4-chloromercuribenzoate, SDS, guanidinium chloride, polyethylene glycol, heparin and ethidium bromide inhibit its activity, while novobiocin, nalidixic acid, Triton X-100 and chloroquine do not. Polyamines and histone H1 stimulate the topoisomerase activity. We classify this DNA topoisomerase as type I and eukaryotic. Conversion of the topoisomerase to a nonspecific endonuclease at increased temperature is proposed.

Localization of heparan sulfate glycosaminoglycan and proteoglycan core protein in aged brain and Alzheimer's disease

Two monoclonal antibodies, one which recognizes a glycosaminoglycan epitope present in heparan sulfate glycosaminoglycan and another which recognizes the core protein of a basement membrane heparan sulfate proteoglycan, were used to study the distribution and localization of these components in Alzheimer's disease and control brain. The cytoplasm of neurons, and occasional neurofibrillary tangles, senile plaques and astrocytes were immunopositive for the heparan sulfate glycosaminoglycan antibody in control brains. In Alzheimer's tissue, however, the number and intensity of these elements was more extensive than in control brains. In addition, within the Alzheimer's brains studied, the nuclei of select neurons and a small number of microglia were also immunopositive for heparan sulfate glycosaminoglycan in contrast to controls, where nuclei and neuroglia were immuno-negative. Some senile plaques in Alzheimer's tissue also contained strong heparan sulfate glycosaminoglycan-positive neurites which were not seen in controls. In Alzheimer's tissue, double labeling for heparan sulfate glycosaminoglycans and the beta-amyloid protein in adjacent sections revealed that, in general, heparan sulfate glycosaminoglycan- and beta-amyloid protein-immunopositive plaques were co-localized. Occasionally, however, beta-amyloid-positive plaques were seen without heparan sulfate glycosaminoglycan immunoreactivity and vice versa. Heparan sulfate glycosaminoglycan immunoreactivity and Tau immunoreactivity co-localized in many neurofibrillary tangles; however a small number of heparan sulfate glycosaminoglycan-positive neurofibrillary tangles did not co-localize with Tau-positive neurofibrillary tangles. In contrast, the heparan sulfate proteoglycan antibody immunostained only the walls of blood vessels and a few senile plaques in Alzheimer's brains and primarily blood vessels in control brains. Heparan sulfate glycosaminoglycan immunostaining was present within neurons, glia, neurofibrillary tangles and senile plaques in Alzheimer's tissue. These results suggest that heparan sulfate-like molecules play an important role in the pathogenesis of the characteristic lesions of Alzheimer's disease and could serve as a marker reflecting early pathological changes.

Comparative study on Lewis lung tumour lines with 'low' and 'high' metastatic capacity. III. Glycosaminoglycan synthesis, transport and degradation in cell lines

The glycosaminoglycans (GAGs) of low (LM) and highly metastatic (HM) cell lines of the Lewis lung tumour (3LL) were compared using [3H]glucosamine labelling techniques. The GAGs isolated from nuclei, cytoplasm, pericellular fractions and medium were analysed by cellulose acetate electrophoresis and by digestion with specific enzymes, and the following conclusions were drawn. 1. Increased cellular uptake and incorporation of [3H]glucosamine into glycoconjugates of the cytoplasm was a typical feature of the highly metastatic cell line after a 48-h labelling. However, there was no elevated radioactivity in glycolipids. 2. Radioactivity of the purified GAGs was two and three times higher in nuclear and cytoplasmic fractions of HM cells than in those of LM cells. There was much less difference between the two cell lines in the pericellular fractions. 3. A definite change from chondroitin sulphate to dermatan sulphate dominancy was recorded in each GAG fraction. Higher heparan sulphate labelling was observed in the cytoplasmic and pericellular GAGs of HM cultures. 4. In the post-labelling period about three times more GAG was present in the extracellular compartment of the HM cultures compared with the LM cultures. 5. In the LM cultures the total GAG-associated radioactivity decreased by 73 per cent in the 48-h chase period whereas in the HM cultures it decreased by only 30 per cent. This indicates a higher rate of GAG degradation in the LM cultures.

Characterization of a mammalian mutant with a camptothecin-resistant DNA topoisomerase I

DNA topoisomerase I was purified to near homogeneity from a clonal line of human lymphoblastic leukemia cells, RPMI 8402, that is resistant to camptothecin, a cytotoxic alkaloid from Camptotheca acuminata, and compared with that of the parent wild-type cells. As assayed by relaxation of the supercoiled plasmid DNA and by formation of enzyme-linked DNA breaks, the purified enzyme from the resistant cells was shown to be greater than 125-fold as resistant to camptothecin as the wild-type enzyme, comparable to a cellular resistance index of about 300. Therefore, the cellular resistance appears to be due to the resistance of the enzyme. The amount of the immunoreactive enzyme protein in whole extract appeared to be reduced to less than half that of the wild-type enzyme. These results establish that DNA topoisomerase I is the cellular target of camptothecin and that DNA topoisomerase I is essential for the survival of mammalian cells.

Mechanism of inhibition of mammalian DNA topoisomerase I by heparin

We have previously shown that heparin is a potent inhibitor of a mammalian DNA topoisomerase I. We have now investigated the mechanism of its inhibition. This was carried out first by scrutinizing the structural features of heparin molecules responsible for the inhibition. Commercial heparin preparation was fractionated by antithrombin III-Sepharose into non-adsorbed, low-affinity and high-affinity fractions, of which only the high-affinity fraction of heparin is known to contain a specific oligosaccharide sequence responsible for the binding to antithrombin III. These fractions all exhibited essentially similar inhibitory activities. Furthermore, when chemically sulphated to an extent comparable with or higher than heparin, otherwise inactive glycosaminoglycans such as heparan sulphate, chondroitin 4-sulphate, dermatan sulphate and neutral polysaccharides such as dextran and amylose were converted into potent inhibitors. Sulphated dermatan sulphate, one of the model compounds, was further shown to bind competitively to the same sites on the enzyme as heparin. These observations strongly suggested that topoisomerase inhibition by heparin is attributable primarily, if not entirely, to the highly sulphated polyanionic nature of the molecules. In a second series of experiments we examined whether heparin inhibits only one or both of the topoisomerase reactions, i.e. nicking and re-joining. It was demonstrated that both reactions were inhibited by heparin, but the nicking reaction was more severely affected than was the re-joining reaction.

A unique heparan sulfate in the nuclei of hepatocytes: structural changes with the growth state of the cells

Growing and confluent cultures of a rat hepatocyte cell line were labeled with 35SO4(2-) and the heparan sulfate in the culture medium, the pericellular matrix, the nucleus, the nuclear outer membrane, and the remaining cytoplasmic pool was purified by DEAE-cellulose chromatography. The heparan sulfate in all pools from the confluent cells was bound more strongly on the DEAE-cellulose column than the corresponding pools from the growing cells. Gel filtration of each pool before and after beta-elimination showed that the heparan sulfate from the nuclear and nuclear membrane pools was composed of primarily free chains, whereas the heparan sulfate in all of the other pools was a mixture of proteoglycans and free chains. The heparan sulfate in each pool was cleaved with nitrous acid to obtain mixtures of di- and tetrasaccharides. Analysis of these mixtures showed that the structural features of the heparan sulfates in each pool were different and were altered significantly when the growing cells became confluent. The nuclear-plus-nuclear membrane pools represented 6.5% and 5.4% of the total cell-associated heparan sulfate in the growing cells and the confluent cells, respectively. The structural features of the heparan sulfate in the two nuclear pools were very similar to each other, but were markedly different from those of the heparan sulfate from the other pools or from any previously described heparan sulfate or heparin. The most unusual aspect of these structures was the high content of beta-D-glucuronosyl(2-SO4)----D-glucosamine-N,O-(SO4)2 disaccharide units in these sequences. The mode of biosynthesis and delivery of these unusual sequences to the nucleus and the potential significance of these observations are discussed.

Regulation of polyamine-responsive protein kinase by certain highly specific polyamines and charged carbohydrates

Polyamine-responsive protein kinase, a cyclic nucleotide-independent protein kinase from the cytosol of Morris hepatoma 3924A, was stimulated 8-9 fold by several different polymers of polylysine, polyornithine and random copolymers of lysine-alanine; spermidine, spermine, and mixtures of spermine and spermidine stimulated 2, 3, and 5 fold, respectively. The protein kinase was not stimulated by poly-carboxybenzyl-lysine, random copolymer of lysine-tyrosine, polyhistidine, polymethionine, polyglutamic acid, polyaspartic acid, dipeptide (Lys-Lys), lysine, ornithine, and putresine. The polyamine stimulation of the protein kinase was prevented by certain specific charged carbohydrates: heparin, chondroitin sulfates A, B, and C, dextran sulfate and hyaluronic acid. It was not prevented by noncharged carbohydrates: dextran, glycogen, starch, sucrose, etc; or by sulfate salts: ammonium sulfate, potassium sulfate, sodium thiosulfate, etc. The inhibition was reversed by increased polylysine. Heparin was non-competitive inhibitor of Mg2+-ATP. It would appear that this enzyme is regulated by certain highly specific molecules with certain sizes and charges; plus charge is stimulatory, negative charge prevents the stimulation.