Effects of brefeldin A on the synthesis of chondroitin 4-sulfate by cultures of mouse mastocytoma cells

Spatiotemporal diversity and regulation of glycosaminoglycans in cell homeostasis and human disease

Glycosaminoglycans (GAGs) are long, linear polysaccharides that are ubiquitously expressed on the cell surface and in the extracellular matrix of all animal cells. These complex carbohydrates play important roles in many cellular processes and have been implicated in many disease states, including cancer, inflammation, and genetic disorders. GAGs are among the most complex molecules in biology with enormous information content and extensive structural and functional heterogeneity. GAG biosynthesis is a nontemplate-driven process facilitated by a large group of biosynthetic enzymes that have been extensively characterized over the past few decades. Interestingly, the expression of the enzymes and the consequent structure and function of the polysaccharide chains can vary temporally and spatially during development and under certain pathophysiological conditions, suggesting their assembly is tightly regulated in cells. Due to their many key roles in cell homeostasis and disease, there is much interest in targeting the assembly and function of GAGs as a therapeutic approach. Recent advances in genomics and GAG analytical techniques have pushed the field and generated new perspectives on the regulation of mammalian glycosylation. This review highlights the spatiotemporal diversity of GAGs and the mechanisms guiding their assembly and function in human biology and disease.

PAPST1 regulates sulfation of heparan sulfate proteoglycans in epithelial MDCK II cells

Proteoglycan (PG) sulfation depends on activated nucleotide sulfate, 3'-phosphoadenosine-5'-phosphosulfate (PAPS). Transporters in the Golgi membrane translocate PAPS from the cytoplasm into the organelle lumen where PG sulfation occurs. Silencing of PAPS transporter (PAPST) 1 in epithelial MDCK cells reduced PAPS uptake into Golgi vesicles. Surprisingly, at the same time sulfation of heparan sulfate (HS) was stimulated. The effect was pathway specific in polarized epithelial cells. Basolaterally secreted proteoglycans (PGs) displayed an altered HS sulfation pattern and increased growth factor binding capacity. In contrast, the sulfation pattern of apically secreted PGs was unchanged while the secretion was reduced. Regulation of PAPST1 allows epithelial cells to prioritize between PG sulfation in the apical and basolateral secretory routes at the level of the Golgi apparatus. This provides sulfation patterns that ensure PG functions at the extracellular level, such as growth factor binding.

In vitro synthesis of sulfated glycosaminoglycans coupled to inter-compartmental Golgi transport

We have used isolated rat liver Golgi membranes to reconstitute the synthesis of sulfated glycosaminoglycans (GAGs) onto the membrane-permeable, external acceptor xyloside. Biosynthetic labeling of GAGs with [35S]sulfate in vitro is shown to have an absolute requirement for ATP and cytosolic proteins and is inhibited by dismantling the Golgi apparatus with okadaic acid or under mitotic conditions suggesting that inter-compartmental transport between Golgi cisternae is a prerequisite for the successful completion of the initiation, polymerization, and sulfation of GAGs. Accordingly, we show that in vitro synthesis of 35S-GAGs utilizes the same machinery employed in Golgi transport events in terms of vesicle budding (ADP-ribosylation factor and coatomer), docking (Rabs), targeting (SNAREs), and fusion (N-ethylmaleimide-sensitive factor). This provides compelling evidence that GAGs synthesis is linked to Golgi membrane traffic and suggests that it can be used as a complementation-independent method to study membrane transport in Golgi preparations from any source. We have applied this system to show that intra-Golgi traffic requires the function of the Golgi target-SNARE, syntaxin 5.

Biosynthesis of the proteoglycan decorin -- identification of intermediates in galactosaminoglycan assembly

Biosynthesis of decorin was investigated by incubating a rat fibroblast cell line with various radiolabelled protein and galactosaminoglycan precursors. The following cell-associated and distinct intermediates were isolated and identified: a pool of non-glycosylated core protein, two pools of decorin with incomplete chains, one with three sulphated disaccharide repeats and another with five or more sulphated disaccharide repeats, as well as decorin with mature chains. Results of pulse/chase experiments indicated that these pools represented discrete stages in chain growth. Treatment with brefeldin A, which blocks transport from the endoplasmic reticulum to the Golgi, resulted in accumulation of decorin with an incomplete chain containing six or seven largely unsulphated disaccharide repeats. During recovery from drug treatment, 4-sulfation reappeared earlier than 6-sulfation. The results suggest that the galactosaminoglycan assembly-line consists of separate multienzyme complexes that build only a limited section of the chain. Furthermore, brefeldin A causes segregation of compartments involved in separate stages of the assembly line. In an earlier report [Moses, J., Oldberg. A., Cheng, F. & Fransson, L.-A. (1997) Eur. J. Biochem. 248, 521-526] we took advantage of such segregation to identify and characterize a transient 2-phosphorylation of xylose in the linkage region.

Mouse mastocytoma cells synthesize undersulfated heparin and chondroitin sulfate in the presence of brefeldin A

In order to study the subcellular localization and organization of the enzymes involved in the glycosylation of the hybrid proteoglycan serglycin, mouse mastocytoma cells were metabolically labeled with [35S]sulfate or [3H]glucosamine in the absence or presence of brefeldin A. This drug is known to induce a disassembly of the proximal part of the Golgi complex, resulting in a redistribution of cis-, medial-, and trans-Golgi resident enzymes back to the endoplasmic reticulum, and to block the anterograde transport of proteins to the trans-Golgi network. Although the total incorporation of [3H]glucosamine into glycosaminoglycan chains was reduced to about 25% in brefeldin A-treated cells compared to control cells, both control cells and cells treated with brefeldin A synthesized heparin as well as chondroitin sulfate chains. Therefore, enzymes involved in the biosynthesis of both types of glycosaminoglycan chains seem to be present proximal to the trans-Golgi network in these cells. Chondroitin sulfate and heparin synthesized in cells exposed to brefeldin A were undersulfated, as demonstrated by ion-exchange chromatography, compositional analyses of disaccharides, as well as by a lower [35S]sulfate/[3H]glucosamine ratio compared to controls. In heparin biosynthesis, both N- and O-sulfation reactions were impaired, with a larger relative decrease in 2-O-sulfation than in 6-O-sulfation. Despite undersulfation, the heparin chains synthesized in the presence of brefeldin A were larger (30 kDa) than the heparin synthesized by control cells (20 kDa). The reduced [3H]glucosamine incorporation in brefeldin A-treated cells was partly due to decreased number of glycosaminoglycan chains synthesized, but also to the biosynthesis of chondroitin sulfate chains of smaller molecular size (8 versus 15 kDa in control cells). Brefeldin A had no effect on the glycosaminoglycan synthesis when used in a cell-free, microsomal fraction, indicating that brefeldin A does not interfere directly with the enzymes involved in the biosynthesis of glycosaminoglycans.

Mast cell procarboxypeptidase A. Molecular modeling and biochemical characterization of its processing within secretory granules

Previously, we characterized murine mast cell procarboxypeptidase A (MC-proCPA) as an inactive zymogen. To investigate the mechanisms for this lack of enzymatic activity and the processing of the zymogen to the active form, we now have performed molecular modeling of the tertiary structure of murine MC-proCPA based on the x-ray crystallographic structures of porcine pancreatic procarboxypeptidases A and B. Our model predicts that MC-proCPA retains a high degree of structural similarity to its pancreatic homologues. The globular propeptide physically blocks access to the fully formed active site of the catalytic domain and contains a salt bridge to the substrate-binding region that precludes docking of even small substrates. Based on consideration of the predicted tertiary structure and charge field characteristics of the model, the activation site (between GluA94 and Ile1) appears to be highly exposed even after MC-proCPA binds to secretory granule proteoglycans. Based on the steady-state levels of MC-proCPA versus MC-CPA, cycloheximide inhibition of protein synthesis, and brefeldin A blockage of protein sorting, we show that MC-proCPA is processed rapidly in murine mast cell line KiSV-MC14 with a half-life of 26 +/- 5 min (mean +/- S.D., n = 3), and the processing occurs within the secretory granules. The enzyme responsible for this processing may be a thiol protease since treatment of the KiSV-MC14 with 200 microM E-64d, a selective thiol-protease inhibitor, increases MC-proCPA by 2.7 +/- 0.2-fold (mean +/- S.D., n = 3) within 6 h of application.

Sulphation of proteochondroitin and 4-methylumbelliferyl beta-D-xyloside-chondroitin formed by mouse mastocytoma cells cultured in sulphate-deficient medium

Mouse mastocytoma cells were cultured in medium containing [3H]GlcN and concentrations of [35S]sulphate varying from 0.01 to 0.5 mM. Intracellular [35S]sulphate incorporation increased severalfold from the lowest concentrations, reaching a maximum at 0.1-0.2 mM, whereas incorporation of [3H]hexosamine remained constant at all sulphate concentrations. Proteo[3H]-chondroitin [35S]sulphate was isolated and incubated with chondroitin ABC lyase, yielding 35S-labelled and/or 3H-labelled delta Di-0S and delta Di-4S disaccharide products. The increasing percentage of delta Di-4S was consistent with the increasing sulphate incorporation at each higher [35S]sulphate concentration. Examination of proteochondroitin [35S]sulphate size by Sepharose CL-6B chromatography indicated a range consistent with various numbers of glycosaminoglycan chains on the protease-resistant serglycin core protein. Alkali-cleaved chondroitin [35S]sulphate products indicated similar size distributions at all sulphate concentrations with no indication of preferential sulphation being related to smaller or larger size. DEAE-cellulose chromatography of [3H]chondroitin [35S]sulphate glycosaminoglycans indicated a random undersulphation as [35S]sulphate concentration was lowered. Addition of 4-methylumbelliferyl beta-D-xyloside to the cultures resulted in a 2-2.5-fold stimulation of [3H]chondroitin [35S]sulphate synthesis with formation of beta-xyloside-[3H]chondroitin [35S]sulphate which was much smaller, as estimated by Sepharose CL-6B chromatography, than the decreased amount of [3H]chondroitin [35S]sulphate derived from proteo[3H]chondroitin [35S]sulphate. Much higher concentrations of sulphate were necessary to produce sulphation of the beta-xyloside-[3H]chondroitin comparable with that of proteo[3H]-chondroitin, as indicated by chondroitin ABC lyase products and DEAE-cellulose chromatography. The specific radioactivities of the [3H]GalN in the proteo[3H]chondroitin [35S]sulphate and beta-xyloside-[3H]chondroitin [35S]sulphate were calculated from the 3H and 35S c.p.m. of isolated dual-labelled delta Di-4S from each, and indicated that the presence of the beta-xyloside resulted in a dilution of the [3H]GlcN by endogenous GlcN that was 4 times higher than that of cultures lacking the beta-xyloside. The higher sulphate concentrations needed for sulphation of beta-xyloside-chondroitin suggests that the membrane-bound nature of the proteochondroitin acceptor in juxtaposition to a chondroitin sulphate-synthesizing enzyme complex effectively reduces the apparent Km for adenosine 3'-phosphate 5'-phosphosulphate.