A series of aryl and alkyl O-beta-D-xylosides and their analogues with S, NH or CH2 in the glycosidic linkage were prepared and examined for their ability to act as artificial chain initiators of chondroitin (dermatan) sulphate synthesis in embryonic chick cartilage, foetal rat skin and 6-week-old-rat aorta under conditions where normal protein-core synthesis was inhibited by cycloheximide. For all these tissues in culture, phenyl O-beta-D-xyloside and phenyl beta-D-thioxyloside were clearly more effective than the corresponding N-xyloside and homo-C-xyloside. Introduction of a carboxy group to the para position of their aglycone yielded derivatives with far lower initiator activity. In a concentration range lower than 0.1 mM, the effectiveness of alkyl beta-D-thioxyloside was greatly influenced by the carbon number (n) of the alkyl group and was at a maximum at n = 7 or 8 for the cartilage, at n = 5 for the skin and at n = 4 for the aorta. In the beta-xyloside-treated cartilages, the average length of newly formed chondroitin sulphate chains reflected the chain-initiator activity of added xyloside, i.e. the higher the initiator activity, the shorter the average chain length. In the skin and aorta, none of the drugs could relieve the inhibition of heparan sulphate synthesis caused by cycloheximide. Fertilized hens' eggs were each injected on day 9 with 9.2 mumol of beta-xyloside and the skeletal systems of embryos were examined a week later. The embryos treated with beta-xylosides of relatively high initiator activity showed a 30-40% decrease in the overall growth rate of skeletons, whereas those treated with beta-xylosides of low initiator activity showed little or no decrease in the growth rate. The results are consistent with the notion that the observed change in skeletal morphology results mainly, if not completely, from beta-xyloside-induced synthesis of core-protein-free chondroitin sulphate, and further suggest that a procedure employing a series of beta-xyloside homologues with various initiator activities will furnish an easily applied criterion on which to test the specificity of xyloside action on biological processes.

The monoblastic cell line U-937 was cultured in the presence of C-ethyl beta-D-xyloside (E-xyl), hexyl beta-D-thioxyloside (HX-xyl), p-nitrophenyl beta-D-xyloside, phenyl beta-D-xyloside or phenyl alpha-D-xyloside. All of the beta-D-xylosides inhibited proliferation, but HX-xyl was by far the most efficient, and had a maximum effect at 1 mM concentration. The inhibitory effect of HX-xyl could be reversed; after washing, the HX-xyl-treated cells proliferated with a pattern similar to that of control cells. For more detailed analysis of the effects of beta-D-xylosides on cell proliferation and chondroitin sulphate (CS)/chondroitin sulphate proteoglycan (CSPG) structure, a comparison between the effects of E-xyl and HX-xyl was made. Treating the cells with 1 mM-HX-xyl resulted in a large increase in CS synthesis, whereas 1 mM-E-xyl had only minor effects on the rate of PG/glycosaminoglycan synthesis. Sepharose CL-6B gel chromatography of medium and cell fractions from 35S-labelled cells revealed that HX-xyl treatment resulted in the expression of only free CS chains, whereas E-xyl exposure leads to the synthesis of both large and small CSPGs, as well as some free CS chains. The expression of elevated levels of free CS chains was clearly correlated to the inhibition of proliferation. The proliferation of U-937-4, a clone of U-937 synthesizing ten times more CSPG/CS than the parent line, was equally inhibited by HX-xyl treatment. With this clone, however, there was no stimulation of CS synthesis after xyloside exposure, indicating that the elevated level of CS evident after xyloside treatment of the parent cell line is not causing the inhibition of proliferation. Furthermore, the biosynthesis of hyaluronate was shown not to be implicated in the xyloside-induced decrease in proliferation. The inhibition of proliferation observed in the presence of 1 mM-HX-xyl did not lead to differentiation of the cells into macrophage-like cells, as is observed when the cells are cultured in the presence of phorbol esters, agents also known to inhibit proliferation of U-937 cells.

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