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Sialyltransferases biosynthesize sialyl-glycoconjugates involved in many biological and pathological processes. We investigated and characterized synthetic flavonoid derivatives as sialyltransferase inhibitors. We first examined 54 compounds by solid-phase enzyme assay using beta-galactoside alpha2,6-sialyltransferase 1 (ST6Gal I) and beta-galactoside alpha2,3-sialyltransferase. Several compounds inhibited sialyltransferase enzyme activity regardless of sialyl-linkage reactions. Among them, two compounds showed inhibitory activity against ST6Gal I with IC(50) values less than 10 microM. Three characteristic features of flavonoids were determined by structure-inhibitory activity relationships. First, a double bond between C2-C3 linkages is required for the activity. Second, increasing hydrophilic properties on the B-ring markedly augmented the inhibitory effect. Third, a hydrophobic functional group introduced on the hydroxyl groups of the A-ring enhanced the inhibitory activity. Kinetic analysis using human ST6Gal I indicated a mixed inhibition mechanism of the compounds. In conclusion, the flavonoids identified could be applied for control of cellular expression of sialic acid.

The function of gangliosides, sialic acid-containing glycolipids, on the secretion and assembly of apoB-containing lipoproteins is poorly understood. Here, we report that the GD3 synthase is involved in apoB secretion in retinoic acid (RA)-treated Chang liver cells via transcriptional induction of microsomal triglyceride transfer protein (MTP). The overexpression of GD3 synthase in Chang liver cells increases the expression of the MTP gene, but GM3 synthase-transfected cells did not. The levels of GM3 and GD3 gangliosides in each of the transfected cells were increased in the cell extract as well as the medium. In addition, GD3 synthase-transfected cells showed an increased secretion of triglyceride-enriched apoB. In contrast, the triglyceride content in GM3 synthase-transfected cells was relatively lower. Treatment with small interfering RNAs (siRNAs) and GD3 antibody decreased apoB secretion. These results indicate that plasma membrane associated GD3 play important roles in apoB secretion, and that an enhancement in GD3 levels might be a risk factor for the development of atherosclerosis by increasing the secretion of triglyceride-enriched apoB containing lipoproteins.

Elevated expression of fucosylated glycoconjugates and fucosyltransferases (Fuc-Ts) is found in various tumor cells and has been correlated with aspects of tumor progression such as cell adhesion and metastasis. Thus, fucosyltransferase inhibitors are potentially useful as anti-tumor agents. In the present study, three known spirocyclic drimanes (1, 2, and 3) were isolated from the culture broth of the fungus Stachybotrys cylindrospora. Compound 1 (stachybotrydial) exhibits potent inhibitory activity against alpha1,3-fucosyltransferase (Fuc-TV) during screening, while compounds 2 and 3 show no such inhibitory activity. Kinetic analysis indicates that compound 1 is an uncompetitive inhibitor with respect to GDP-fucose and a noncompetitive inhibitor with respect to N-acetyllactosamine with Ki values of 10.7 and 9.7 microM, respectively. In addition, all three compounds also possess inhibitory activity against sialyltransferase (ST) but not against beta1,4-galactosyltransferase. These observations provide novel chemical structure information that will help in the design of novel Fuc-T and ST inhibitors.

A growing number of reports demonstrate that hypersialylation, which is observed in certain pathological processes, such as oncogenic transformation, tumor metastasis, and invasion, is associated with enhanced sialyltransferase (ST) activity. There is therefore a need for the development of ST inhibitors to modulate ST activity and thus alleviate the disease processes caused by STs. In the present study, soyasaponin I had been discovered to be a potent and specific ST inhibitor by screening strategy from 7500 samples including micribial extracts and natural products. Kinetic analysis shows that it is a CMP-Neu5Ac competitive inhibitor with for ST3Gal I with an inhibition constant (K(i)) of 2.1 microM. In addition, it is only active against ST, but not against the other tested glycosyltransferases and glycosidases. Our study is the first report to discover ST inhibitor by screening method and also to provide the new chemical structure information that should be useful in the development of other novel ST inhibitors., (Copyright 2001 Academic Press.)

The effects of nucleotides, nucleotide sugars and nucleotide dialdehydes on the activity and kinetics of cytidine 5'-monophospho-N-acetylneuraminic acid:lactosylceramide (alpha 2-->3) sialyltransferase (SAT-1) in microsomes derived from embryonic chick brain were investigated. Although under physiological conditions this enzyme utilizes a CMP-sugar as substrate, it was found that UDP-dialdehyde was an effective inhibitor of SAT-1 activity. CMP-dialdehyde was only slightly more efficient at inhibiting SAT-1 activity. Similar findings were found for the inhibitory effects of UDP versus CMP. In addition, two UDP-sugars (UDP-Gal and UDP-GalNAc) were also slightly inhibitory. Kinetic analyses demonstrate that both UDP- and CMP-dialdehydes are competitive inhibitors of SAT-1 activity. The data suggests that the substrate specificity of microsomal SAT-1 resides more in the sugar moiety, rather than in the nucleotide portion of the substrate.

The sialic acid nucleoside conjugate KI-8110 has been shown to inhibit the formation of hepatic metastases from human colorectal cancer cell lines in a nude mouse intrasplenic injection model. The compound does not inhibit sialyltransferases from either human colorectal tumor cells or human liver. Transport of CMP-sialic acid into endoplasmic reticulum and Golgi vesicles is inhibited and can account for the reduction in surface sialic acid found on treated cell lines. Only a 50% inhibition of CMP-sialic acid transport could be achieved suggesting the presence of more than one transport protein with differing specificities.