Inhibition of scavenger receptor-mediated modified low-density lipoprotein endocytosis in cultured bovine aortic endothelial cells by the glycoprotein processing inhibitor castanospermine

Abstract

Castanospermine (1,6,7,8-tetrahydroxyoctahydroindolizidine) is a plant alkaloid that inhibits alpha-glucosidases, including the glycoprotein processing glucosidase I. When endothelial cells were grown for 48 h, or longer, in the presence of this alkaloid, they produced scavenger receptors for modified low-density lipoproteins (LDL) that had mostly Glc3Man7-9(GlcNAc)2 structures rather than the usual complex types of oligosaccharides. Furthermore, growth in the presence of castanospermine resulted in a substantial inhibition in degradation of endocytosed 125I-acetylated LDL, as well as a dose-dependent inhibition of 125I-acetylated LDL binding to these cells. Scatchard analysis of binding curves indicated that the diminished binding was due to a decrease in the number of scavenger receptor molecules at the cell surface rather than to a change in the affinity of the receptors for their ligand. Since castanospermine-treated cells had the same total number of cellular receptor molecules as did controls cells, it seemed likely that castanospermine caused an alteration in receptor targeting, rather than an inhibition in receptor synthesis or a stimulation in receptor degradation. Density gradient fractionation of cell homogenates showed that castanospermine-treated cells did have a much greater percentage of scavenger LDL receptor molecules in the endoplasmic reticulum-Golgi fraction and fewer receptors in the plasma membrane fraction, whereas normal cells showed the opposite distribution.

Twisting integrin receptors increases endothelin-1 gene expression in endothelial cells

“…3). Endothelial cells express abundant scavenger receptors and HLA class I molecules on their surface (10,44).…”

Indolizidine and quinolizidine alkaloids

“…Lactam reduction and global deprotection completed the synthesis of ( + )-castanospermine (36). Similar late-stage transformations on diol (42) afforded 6,7-di-epi-castanospermine (44), another minor alkaloid of Castanuspermum australe. An alternative approach to 6,7-diepi-castanospermine from the same research group33 exploited intramolecular dipolar cycloaddition of azidodiene (45), which took place with complete diastereoselectivity to afford the bicyclic aziridine (46).…”

“…Treating this compound with di-t-butyl carbonate induced regio-and stereoselective cleavage of the aziridine ring to give pyrrolidine (47). Protection of the alcohol group followed by asymmetric osmylation in the presence of (DHQD),-PHAL yielded diol (48), which was transformed as illustrated into the target alkaloid (44).…”

“…In this way an impressive range of castanospermine analogues [(49), X = C1, F, CN, OBn, NHR (R = Ac, Me, allyl, Bun, Bu', Bus, Bn, CH,CH,OMe), N(Ac) Bn], often accompanied by pyrrolizidine by-products, was obtained after final hydrolysis of the acetate groups. Epoxide (51), made from mesylate [(49, X = OMS] and sodium hydroxide in DMF, was converted by acidic hydrolysis into 6,7-di-epi-castanospermine (44). In an alternative strategy, the monobenzoate (49) (X = PhCO,) was transformed via readily prepared 1,8-O-isopropylidene and cyclohexylidene derivatives into the 7-@MOM ethers (52 a).…”

“…The highly oxygenated castanospermine analogues (-)-( 63), (+)-( 64 Incubation of aortic endothelial cells with castanospermine altered the distribution of scavenger receptors for low-density lipoproteins, a finding that has implications for the deposition of cholesterol on arterial walls. 44 The alkaloid also affects processing of the oligosaccharide chains in lipoprotein lipase from 3T3-Ll a d i p o ~y t e s . ~~, ~~ Subcutaneous administration of castanospermine to rats that had undergone heart transplantation suppressed the expression of glycoproteins implicated in allograft rejecti~n.~' The alkaloid's ability to inhibit glucosidases involved in glycoprotein processing has been exploited in studies on T-cell activation48 and the development and differentiation of oligodendrocyte progenitor cells.4g…”