Initiation of chondroitin sulphate synthesis by beta-D-galactosides. Substrates for galactosyltransferase II

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.

Synthesis and glycosaminoglycan priming activity of three disaccharides related to the linkage region tetrasaccharide of proteoglycans

To test if disaccharides might serve as primers of oligosaccharide synthesis in animal cells, we synthesized 2-naphthyl O-(beta-D-galactopyranosyl)-(1 --> 4)-beta-D-xylopyranoside, 2-naphthyl O-(beta-D-galactopyranosyl)-(1 --> 3)-beta-D-galactopyranoside, and 2-naphthyl O-(beta-D-glucopyranosyluronic acid)-(1 --> 3)-beta-D-galactopyranoside. These three disaccharides are related to subunits of the linkage tetrasaccharide of heparan sulfate and chondroitin sulfate chains in animal cell proteoglycans. The disaccharides were synthesized with coupling efficiencies of 40-70% using thioglycosides or by activating the monosaccharides with trichloroacetimidate. The structures of these compounds were confirmed by 1H NMR, 13C NMR and elemental analysis. The ability of these disaccharides to prime glycosaminoglycan chains was examined in a Chinese hamster ovary cell mutant, p gsA 745, which lacks xylosyltransferase. The missing enzyme renders the cells dependent on exogenous primers for making glycosaminoglycan chains. 2-Naphthyl O-(beta-D-galactopyranosyl)-(1 --> 3)-beta-D-galactopyranoside and 2-naphthyl O-(beta-D-glucopyranosyluronic acid)-(1 --> 3)-beta-D-galactopyranoside did not stimulate glycosaminoglycan synthesis, but 2-naphthyl O-(beta-D-galactopyranosyl)-(1 --> 4)-beta-D-xylopyranoside at high concentration primed chains. The peracetylated derivative (2-naphthyl O-(2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)-(1 --> 4)- 2,3-di-O-acetyl-beta-D-xylopyranoside) primed chains at lower concentration (100 microM), suggesting that cells took up the compound and removed the acetyl groups apparently in the compartment where glycosaminoglycan synthesis occurs.

Control of proteoglycan biosynthesis. Further studies on the effect of serum on cultured bovine articular cartilage

Proteoglycan synthesis in explant cultures of adult bovine articular cartilage is stimulated in a dose-dependent manner when the tissue is cultured in the presence of foetal-calf serum. The stimulation of proteoglycan synthesis is paralleled by a similar increase in DNA synthesis; however, when DNA synthesis is inhibited by hydroxyurea the stimulation of proteoglycan synthesis by serum remains essentially the same. The apparent half-life of the pool of proteoglycan core protein precursor was measured in freshly isolated tissue as well as in tissue cultured for 7 days in the presence and in the absence of foetal-calf serum; under all conditions the half-life was the same, suggesting that this value is independent of the net rate of proteoglycan synthesis. In the presence of actinomycin D, an inhibitor of RNA synthesis, there was a difference in the apparent half-life of the available pool of mRNA coding for proteoglycan core protein: 8.5 h for tissue maintained in the presence of serum and 3.8 h for tissue cultured in the absence of serum. It is suggested that proteoglycan synthesis is stimulated by serum factors at the level of DNA-dependent RNA synthesis. Concomitant with an increase in the rate of proteoglycan synthesis induced by the presence of serum in the culture medium, an increase in the concentrations of several glycosyltransferases involved in chondroitin sulphate synthesis was also observed.

The relation of RNA synthesis to chondroitin sulphate biosynthesis in cultured bovine cartilage

Addition of actinomycin D (or cordycepin, an alternative inhibitor of RNA synthesis) to cartilage cultures resulted in a first-order decrease in the rate of incorporation of [35S]sulphate into proteoglycan (half-life = 7.5 +/- 1.1 h). Addition of 1.0 mM-benzyl beta-D-xyloside relieved the initial inhibition of glycosaminoglycan synthesis induced by actinomycin D; however, after a lag of about 10 h the rate of xyloside-initiated glycosaminoglycan synthesis also decreased with apparent first-order kinetics (half-life = 7.1 +/- 1.8 h), which paralleled the decrease in the rate of core-protein-initiated glycosaminoglycan synthesis. The hydrodynamic size of the proteoglycans formed in the presence of actinomycin D remained essentially constant (Kav. 0.21-0.23), whereas the constituent glycosaminoglycan chains were larger than those formed by control cultures, which suggested that the core protein was substituted with fewer but larger glycosaminoglycan chains. Proteoglycans formed in the presence of beta-D-xyloside were significantly smaller (Kav. approximately 0.33) than those synthesized by control cultures, and were further diminished in size after exposure of cultures to actinomycin D. Glycosaminoglycan chains synthesized by these same cultures on to both core-protein and xyloside acceptors were also smaller than those of control cultures. The decrease in synthesis observed after exposure to actinomycin D was not reflected by any significant decrease in the activities of several glycosyltransferases involved in chondroitin sulphate synthesis (galactosyltransferase-I, galactosyltransferase-II, N-acetylgalactosaminyltransferase and glucuronosyltransferase-II).