Proteoglycans in normal and neoplastic monocytes

35S proteoglycans produced by normal and neoplastic (U-937) monocytes after a 20-h pulse with [35S]sulfate in vitro have been isolated and compared. Both cell types produce exclusively chondroitin sulfate proteoglycan (CSPG), which are released into the medium and are not contained within the cells. The neoplastic cell-derived molecules were much larger in molecular size, due to the substitution of galactosaminoglycan chains, with an approximate Mr of 60,000. The corresponding chains in monocyte CSPG had an Mr of approx. 20,000. The latter chains were also found to be more sulfated than their neoplastic counterparts.     

Cell density-dependent expression of chondroitin sulfate proteoglycan in cultured human monocytes

Monocytes were isolated and established in vitro at different cell densities. The incorporation of [35S]sulfate into macromolecules in monocytes (day 1 in culture) and monocyte-derived macrophages (day 5 in culture) was found to increase with decreasing cell density in approximately the same way in both day 1 and day 5 cell cultures. [35S]Sulfate was found to be incorporated almost exclusively into chondroitin sulfate proteoglycan (CSPG) in both high and low density monocyte and monocyte-derived macrophage cultures. The molecular size of the [35S]CSPGs produced by the high and low cell density cultures were not found to differ as judged by gel chromatography elution patterns. The molecular size and the structure of the glycosaminoglycan chains were found to be almost similar in high and low density day 1 and day 5 cultures. Only a small degree of proteoglycan degradation could be observed in both high and low density cultures. Furthermore, cell density-dependent differences in CSPG biosynthesis could be observed already 2 h after the establishment of the cultures, indicating that a process of "down-regulation" in high density cultures was already in operation. The glycosaminoglycan synthesis in high cell density day 1 cultures could be increased slightly following exposure to 0.5 mM benzyl-beta-D-xyloside, but not to the same level as that observed in untreated low cell density cultures. By contrast, the expression of 35S-macromolecules by cells cultured at high cell density for 5 days could be increased by xyloside treatment almost to the same level as that observed in the low density cultures.     

Effect of benzyl beta-D-xyloside on the biosynthesis of chondroitin sulphate proteoglycan in cultured human monocytes.

Monocytes isolated from human blood differentiate into macrophage-like cells when maintained in vitro for 3-5 days on plastic or glass culture dishes. In the process the cells display characteristic morphological changes, and in addition, a transition in glycosaminoglycan biosynthesis, from the production of chondroitin 4-sulphate to the formation of a polysaccharide containing 20% 4,6-disulphated disaccharide units [Kolset, Kjellén, Seljelid & Lindahl (1983) Biochem. J. 210, 661-667]. Cells were incubated with inorganic [35S]sulphate on day 1 or day 6 in culture, in the presence or absence of benzyl beta-D-xyloside, and labelled polysaccharide was isolated from the culture medium. In the presence of xyloside, the secretion of proteoglycans (90% galactosaminoglycan) was inhibited in a dose-dependent fashion and replaced by release of single polysaccharide chains, the size of which decreased with increasing dose of xyloside. The single polysaccharide chains produced on day 6 in the presence of 0.5 mM-xyloside showed the same proportion of disulphated disaccharide units as did the corresponding control material. Day-1 polysaccharide contained negligible amounts of this component, irrespective of the presence or absence of xyloside. It is concluded that the regulatory mechanism that induces 'oversulphation' during the differentiation process operates independently of any association between the polysaccharide chains and the core protein. Moreover, cells maintained in the presence of 0.5 mM-xyloside throughout a 6-day culture period showed the same morphological change, indicative of differentiation into macrophage-like cells, as did untreated control cells. The xyloside did not significantly affect the cytotoxicity of the monocytes, or of the differentiated macrophage-like cells, toward tumour cells.     

Effect ofin vitro differentiation on proteoglycan structure in cultured human monocytes

Parellel toin vitro differentiation of human monocytes into macrophage-like cells, the cells change their synthesis of glycosaminoglycans from chondroitin 4-sulfate to highly sulfated chondroitin sulfate, containing 4,6-disulfatedN-acetylgalactosamine units [Kolsetet al. (1983) Biochem J 210:661–67]. After exposure of monocyte cultures to [³⁵S]sulfate for 24h either from the onset of cultivation, prior to differentiation, or from day 4, after differentiation,³⁵S-macromolecules from medium and cell-layer were isolated and characterized. The cell-layer of day 5 cultures contained both proteoglycans and free polysaccharide chains, while the³⁵S-macromolecules present in the cell-layer of day 1 cultures and in medium of both monocytes and macrophage-like cells were almost exclusively of proteoglycan nature. Proteoglycans produced by macrophage-like cells were of larger size than the monocyte proteoglycans, most likely due to an increased polysaccharide chain length. These proteoglycans, in contrast to the monocyte-derived species, also showed affinity for fibronectin at physiological ionic strength.     

Structure and interactions of proteoglycans in the extracellular matrix produced by cultured human fibroblasts.

Subconfluent cultures of human embryonic skin fibroblasts were labelled with [35S]sulphate for 3 days, after which cell-free extracellular matrix was isolated. A chondroitin sulphate proteoglycan (CSPG) and a heparan sulphate proteoglycan (HSPG) were purified from the matrix. Chromatography on Sepharose CL-2B gave peak Kav. values of 0.35 and 0.38 respectively for the CSPG and the HSPG. The polysaccharide chains released from the two PGs were of similar size (Kav. 0.50 on Sepharose CL-4B). Approx. 50% of the CSPG showed affinity for hyaluronic acid (HA). However, it differed immunologically from the HA-aggregating CSPG of human articular cartilage, and had a larger core protein (apparent molecular mass 290 kDa) than had the cartilage PG. Neither metabolically [35S]sulphate-labelled PGs, isolated from the medium of fibroblast cultures, nor chemically 3H-labelled polysaccharides (HA, CS, HS and heparin) were incorporated into the extracellular matrix when added to unlabelled cell cultures. These results indicate that the matrix PGs are not derived from the PGs present in the medium and that an interation between polysaccharide chains and matrix components is not sufficient for incorporation of PGs into the matrix. Incubation of cell-free 35S-labelled matrix with unlabelled polysaccharides did not lead to the release of any 35S-labelled material, supporting this conclusion. Furthermore, so-called 'link proteins' were not present in the fibroblast cultures, indicating that the CSPGs were anchored in the matrix in a manner different from the link-stabilized association of CSPG with HA in chondrocyte matrix. The identification of a proteinase, secreted by fibroblasts in culture, that after activation with heparin has the ability to release 35S-labelled PGs from the matrix may also indicate that the core proteins are important for the association of the PGs to the matrix.     

Modulation of the morphology and glycosaminoglycan biosynthesis of human monocytes, induced by culture substrates

Monocytes were isolated from human blood and cultured in vitro on plastic culture dishes or on fibronectin-coated dishes. After 5 days in vitro, the cells on plastic dishes displayed marked morphological changes compared with day 1, with an epithelioid appearance resembling that of foreign-body cells. This transition was inhibited in cells cultured on fibronectin-coated dishes. 35S-labelled polysaccharides were isolated from the culture media after 24h incubation periods with inorganic [35S]sulphate. The cells cultured for 5 days on a plastic substrate synthesized, and secreted into the medium, an oversulphated galactosaminoglycan previously shown to contain 4,6-di-O-sulphated N-acetylgalactosamine units [Kolset, Kjellén, Seljelid & Lindahl (1983) Biochem. J. 210, 661-667]. In contrast, 35S-labelled polysaccharide produced by cells cultured on plastic for 1 day only, or on fibronectin for either 1 or 5 days, contained only minor amounts of such disulphated sugar units. These findings indicate that the formation of oversulphated chondroitin sulphate is coupled to the conversion of monocytes into epithelioid cells. Furthermore, they suggest that the overall process is induced by contact with artificial substrates, and that it may be regarded as the equivalent of a foreign-body reaction in vivo.     

Modulation of the expression of chondroitin sulfate proteoglycan in stimulated human monocytes

Proteoglycan biosynthesis was studied in human monocytes and monocyte-derived macrophages (MDM) after exposure to typical activators of the monocyte/macrophage system: interferon-gamma (IFN-gamma), lipopolysaccharide (LPS), and phorbol 12-myristate 13-acetate (PMA). By morphological examination, both monocytes and MDM were stimulated by these activators. Treatment with IFN-gamma resulted in a slight decrease in the expression of [35S]chondroitin sulfate proteoglycan (CSPG) in both monocytes and MDM, whereas LPS treatment increased the [35S]CSPG expression 1.8 and 2.2 times, respectively. PMA, in contrast, decreased the CSPG expression 0.4 times in monocytes, whereas MDM were stimulated to increase the biosynthesis 1.9 times. An increase in the sulfate density of the chondroitin sulfate chains was evident following differentiation of monocytes into MDM due to the expression of disulfated disaccharide units of the chondroitin sulfate E type (CS-E). However, monocytes exposed to PMA did also express disaccharides of the chondroitin sulfate E type. Furthermore, the expression of CS-E in MDM was increased 2 times following PMA treatment. An inactive phorbol ester, phorbol 12,13-diacetate, did not affect the expression of CS-E in either monocytes or MDM when compared with control cultures, suggesting that protein kinase C-dependent signal pathways may be involved in the regulation of sulfation of CSPG. Exposure to LPS or IFN-gamma did not lead to any changes in the sulfation of the chondroitin sulfate chains.     

Isolation of 35S- and 3H-labelled proteoglycans from cultures of human embryonic skin fibroblasts.

35SO42- - and [3H]-leucine-labelled proteoglycans were isolated from the medium of a fibroblast culture, from an EDTA extract of the monolayer, and from consecutive dithiothreitol and guanidine hydrochloride extracts of the cells. Proteoglycans of different sizes were isolated from the extracts by gel chromatography on Sepharose 4B. In the medium and the EDTA extract the largest proteoglycans contained only 35S-labelled galactosaminoglycan, whereas all other fractions contained in addition heparan [35S-labelled galactosaminoglycan, whereas all other fractions contained in addition heparin [35S]sulphate. The galactosaminoglycan-containing proteoglycans of the various extracts were separated into a larger component, containing chondroitin sulphate-like side chains, and a smaller component, containing dermatan sulphate. The larger proteoglycan of the medium showed reversible association-dissociation behaviour when chromatographed on Sepharose CL2B in phosphate-buffered saline and 4M-guanidine hydrochloride respectively. This property remained after removal of extraneous proteins by CsCl-density-gradient centrifugation in guanidine hydrochloride. The association was markedly increased by the addition of high-molecular-weight hyaluronic acid.     

Degradation of heparin proteoglycan in cultured mouse mastocytoma cells.

Pulse-labelling of mouse mastocytoma cell cultures, established from ascites fluid, with inorganic [35S]sulphate for 1 h yielded labelled heparin proteoglycan containing polysaccharide chains of Mr 60,000-100,000. After chase incubation for 24 h most of the 35S appeared in intracellular polysaccharide fragments similar in size to commercially available heparin, Mr 5000-25,000, as indicated by gel chromatography. Products isolated from cultures after 6 h of chase incubation consisted of partially degraded free polysaccharide chains and, in addition, residual proteoglycans that were of smaller size than the proteoglycans initially pulse-labelled. The polysaccharide chains released by alkali treatment from the residual chase-incubated proteoglycans were of the same size as the chains derived from proteoglycans after 1 h of pulse labelling. These results suggest that the intracellular degradation of heparin proteoglycan to polysaccharide fragments is initiated by release of intact polysaccharide chains, probably by action of a peptidase, and is pursued through cleavage of these chains by an endoglycosidase. An endoglucuronidase with stringent substrate specificity [Thunberg, Bäckström, Wasteson, Ogren & Lindahl (1982) J. Biol. Chem. 257, 10278-10282] has previously been implicated in the latter step. Cultures of more purified mastocytoma cells (essentially devoid of macrophages) did not metabolize [35S]heparin proteoglycan to polysaccharide fragments, but instead accumulated free intact polysaccharide chains, i.e. the postulated intermediate of the complete degradation pathway. When such purified cells were co-cultured with adherent mouse peritoneal cells, presumably macrophages, formation of polysaccharide fragments was observed. It is tentatively proposed that the expression of endoglucuronidase activity by the mast cells depends on collaboration between these cells and macrophages.     

Macromolecules released into the culture medium during the vegetative cell cycle of the unicellular green alga Chlamydomonas reinhardii.

The culture medium of growing Chlamydomonas reinhardii cells contains hydroxyproline-rich glycoproteins, which are mainly liberated during release of the zoospores from the mother-cell wall. Pulse-labelling studies with [3H]proline and [35S]methionine have been performed in order to detect the protein components released by synchronously growing cells at different stages of the cell cycle. When either [3H]proline or [35S]methionine were applied during the phase of cell growth, radioactive label appeared in the released macromolecules after a lag period of 40 min, whereas incorporation into the insoluble part of the cell wall was delayed only by 20 min. When applied at the end of the growth phase, e.g. 13 h after beginning of the illumination period, the radioactive amino acids were incorporated into the cell wall, but radioactive labelling of macromolecules released into the culture medium could not be detected before the zoospores were liberated from the mother-cell wall. Maximal incorporation of [3H]proline and [35S]methionine into the insoluble part of the cell wall was observed during cell division, but essentially no radioactively-labelled macromolecules were released into the culture medium during this time period. Analysis of the macromolecules, which were liberated during cell enlargement, by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed distinct radioactive bands, which were differentially labelled with [3H]proline and [35S]methionine. Among the macromolecules released into the culture medium during cell growth, a component of an apparent Mr 35 000 was preferentially labelled with [3H]proline. This component was also detected after labelling with [35S]methionine, but components of an apparently higher Mr were more prominent after labelling with [35S]methionine. Macromolecules released during the cell-enlargement period of synchronously growing cultures in the presence of [3H]proline contained radioactively-labelled hydroxyproline in addition to proline. These results show that, during cell-wall growth, specific protein components are released into the culture medium and that at least one of these components contains large amounts of proline and hydroxyproline. At least some of these macromolecules seem to be constituents of the cell wall, because during pulse-chase experiments radioactively-labelled macromolecules appeared in the culture medium mainly during the time period when the specific radioactivity of the insoluble inner-cell-wall layer decreased.     

Synthesis and secretion of sulphated glycoproteins by rabbit oviduct explants in vitro

The ability of rabbit oviduct explants to incorporate radiolabelled precursors into specific secretory products was investigated. Ampullary and isthmic oviduct segments were cultured in the presence of [3H]glucosamine or [35S]sodium sulphate. Medium samples were analysed for the presence of secreted, labelled macromolecules. Explants incorporated the [3H]glucosamine and secreted labelled glycoproteins in vitro. SDS gel electrophoresis and subsequent fluorographic analysis of culture medium demonstrated a differential secretion of glycoproteins between the ampulla and the isthmus. Although ampullary tissue secreted a greater amount of labelled glycoproteins during the sampling period, the major secretory constituent of Mr approximately 66,000 was common to both oviduct segments. Tissue incubated with [35S]sodium sulphate also secreted a labelled glycoprotein or subunit of Mr approximately 66,000. The results indicate that rabbit oviduct explants are capable of synthesis and secretion of specific sulphated glycoproteins in vitro and that there is a difference in the type and amount of secretion produced between the two oviduct segments.     

Rapid catabolism of tyrosine-O-sulphated proteins and the formation of free tyrosine O-sulphate as an end product in rat embryo fibroblasts.

Rat embryo fibroblasts, line 3Y1, were prelabelled for 24 h with [35S]sulphate and incubated in fresh medium without [35S]sulphate. A rapid efflux of the overall 35S-labelled compounds from the cells into the medium was observed. After 9 h of incubation, about 50% of the total 35S radioactivity appeared in the medium and up to 84.3% did so at the end of a 48 h incubation. Determination of [35S]sulphated macromolecules present in both the cell-associated and the incubation-medium fractions at different time points during incubation indicated that the majority of the 35S-labelled compounds released from the cells were low-Mr products derived from digestion of the [35S]sulphated macromolecules. Further analysis for tyrosine-O-[35S]sulphated proteins, which constituted only a small fraction of the overall [35S]sulphated macromolecules, showed that, after 9 h of incubation, there was a 65% decrease in the cell-associated fraction, and only 16.4% remained after 48 h. During that time, an amount equivalent to 20.7% of the cell-associated tyrosine-O-[35S]sulphated proteins originally present was released into the medium. Free tyrosine O-[35S]sulphate was generated in the cells and excreted into the incubation medium. Its rate of increase with time, however, was slow, and could account for only 12.4% of the tyrosine-O-[35S]sulphated proteins catabolized at the end of the 48 h incubation.     

The effects of cycloheximide on the biosynthesis and secretion of proteoglycans by chondrocytes in culture.

Proteoglycans synthesized by rat chondrosarcoma cells in culture are secreted into the culture medium through a pericellular matrix. The appearance of [35S]sulphate in secreted proteoglycan after a 5 min pulse was rapid (half-time, t 1/2 less than 10 min), but that of [3H]serine into proteoglycan measured after a 15 min pulse was much slower (t 1/2 120 min). The incorporation of [3H]serine into secreted protein was immediately inhibited by 1 mM-cycloheximide, but the incorporation of [35S]sulphate into proteoglycans was only inhibited gradually(t 1/2 79 min), suggesting the presence of a large intracellular pool of proteoglycan that did not carry sulphated glycosaminoglycans. Cultures were pulsed with [3H]serine and [35S]sulphate and chased for up to 6 h in the presence of 1 mM-cycloheximide. Analysis showed that cycloheximide-chased cells secreted less than 50% of the [3H]serine in proteoglycan of control cultures and the rate of incorporation into secreted proteoglycan was decreased (from t 1/2 120 min to t 1/2 80 min). Under these conditions cycloheximide interfered with the flow of proteoglycan protein core along the route of intracellular synthesis leading to secretion, as well as inhibiting further protein core synthesis. The results suggested that the newly synthesized protein core of proteoglycan passes through an intracellular pool for about 70-90 min before the chondroitin sulphate chains are synthesized on it, and it is then rapidly secreted from the cell. Proteoglycan produced by cultures incubated in the presence of cycloheximide and labelled with [35S]sulphate showed an increase with time of both the average proteoglycan size and the length of the constituent chondroitin sulphate chain. However, the proportion of synthesized proteoglycans able to form stable aggregates did not alter.     

Enhanced channelling of sulphate through a rapidly exchangeable sulphate pool in response to stimulated glycosaminoglycan synthesis in pancreatic epithelial cells.

The ability of cells to decorate glycosaminoglycans (GAGs) with sulphate in highly specific patterns is important to extracellular matrix biogenesis and placing appropriate glycosulphated ligands on the cell surface. We have examined sulphate metabolism in two pancreatic duct epithelial cell lines - PANC-1 and CFPAC-1 (derived from a cystic fibrosis patient) with a view to understanding how pancreatic cells utilise intracellular sulphate. [35S]Sulphate uptake was rapid and reached near steady state levels within 10 min. However, the intracellular specific activity of [35S]sulphate for PANC-1 and CFPAC-1 reached only 35 and 10%, respectively, of the medium specific activity at 10 min. Therefore, sulphate appears to reside within two compartments; a rapidly exchangeable sulphate pool (RESP) and a slowly exchangeable sulphate pool (SESP). Reducing chloride in the medium, increased the specific activity of [35S]sulphate within cells and increased the size of the inorganic sulphate pool, suggesting that the RESP was enlarged. Sulphate pools were not different in size between the two cell lines in physiological NaCl. Increasing the size of the sulphate pool had no effect on [35S]sulphate:[3H]glucosamine ratios incorporated into glycosaminoglycans (GAGs); however, stimulating the synthesis of GAGs with 4-methylumbelliferyl-beta-d-xyloside, stably elevated [35S]:[3H] ratios. This was due to higher [35S]sulphate incorporation. [35S]Cysteine contributed less than 0.1% of the cells' sulphate requirements. We conclude that in the face of elevated demand for sulphate, pancreatic cells appear to channel a greater proportion through the RESP.     

Selective inhibition of proteoglycan and hyaluronate synthesis in chondrocyte cultures by cyclofenil diphenol, a non-steroidal weak oestrogen.

Cyclofenil diphenol, a weak non-steroidal oestrogen, binds to albumin. In the presence of concentrations of albumin just sufficient to keep cyclofenil diphenol in solution, the compound inhibited the synthesis of [35S]proteoglycans, [3H]glycoproteins, [3H]hyaluronate and [3H]proteins in primary cultures of chondrocytes from the Swarm rat chondrosarcoma in a dose-dependent manner. When excess albumin was present, conditions were found (90 micrograms of cyclofenil diphenol and 4 mg of albumin per ml of culture medium) which completely inhibited [35S]proteoglycan and [3H]hyaluronate synthesis but had little effect on [3H]protein or [3H]glycoprotein synthesis. The time of onset of inhibition of [35S]proteoglycan synthesis by cyclofenil diphenol was very rapid (t1/2 less than 25 min) and incompatible with an action mediated through suppression of proteoglycan core protein synthesis. Cyclofenil diphenol inhibited the synthesis of [35S]chondroitin sulphate chains onto p-nitrophenyl beta-D-xyloside in the cultures. Cyclofenil diphenol had little effect on the secretion from chondrocytes of [35S]proteoglycans synthesized immediately prior to treatment. Chondrocyte cultures treated with cyclofenil diphenol recovered their biosynthetic activities almost completely within 3 h of removing the compound from the culture medium. Cyclofenil diphenol had a similar inhibitory action on the synthesis of [35S]proteoglycans in secondary cultures of human dermal fibroblasts from both normal subjects and patients with systemic sclerosis. It is proposed that cyclofenil diphenol inhibits the synthesis of [35S]proteoglycans by interfering with the formation of the glycosaminoglycan side chains of these molecules in the Golgi apparatus of cells. The action may be due to disturbance of Golgi membrane organization by the compound.     

Effects of platelet-derived growth factor and transforming growth factor-beta 1 on the synthesis of a large versican-like chondroitin sulfate proteoglycan by arterial smooth muscle cells

Platelet-derived growth factor (PDGF) and transforming growth factor-beta 1 (TGF-beta 1) increase [35S]sulfate incorporation into proteoglycan (PG) by monkey arterial smooth muscle cells but have opposite effects on cell proliferation. The combination of these two growth regulatory peptides has an additive effect on PG synthesis but no effects on cell proliferation. The time course of sulfate incorporation after stimulation indicates that both growth factors cause maximal incorporation of sulfate into glycosaminoglycan chains by 12-18 h. The PG that is most affected is a large CSPG (Mr approximately 1.2 x 10(6)) which can be immunoprecipitated by an antibody against versican, a large CSPG synthesized by human skin fibroblasts. The hydrodynamic size of this molecule increases after PDGF and TGF-beta 1 stimulation, but the size of the core glycoprotein (Mr approximately 450,000) remains the same. Treatment with either growth factor leads to an increase in the amount of core glycoprotein for this PG. This increase correlates with an increase in the steady state level of mRNA identified by hybridization to a cDNA encoding versican. The two growth factors also increase the glycosaminoglycan chain length of this PG accounting for the greater hydrodynamic size of the molecule after stimulation. In contrast, PDGF and not TGF-beta 1 changes the composition of the glycosaminoglycan chains attached to this PG by doubling the ratio of chondroitin 6-sulfate to chondroitin 4-sulfate. These results indicate that although both of these growth factors increase the net synthesis of a large versican like CSPG, they differ in their effects on the structure of the glycosaminoglycan chains. These post-translational modifications may relate to the growth state of the cells.     

Aortic endothelial cells synthesize a large chondroitin sulphate proteoglycan capable of binding to hyaluronate.

Confluent cultures of mouse aortic endothelial (END-D) were incubated with either [35S]methionine or 35SO4 2-, and the radiolabelled proteoglycans in media and cell layers were analysed for their hyaluronate-binding activity. The proteoglycan subfraction which bound to hyaluronate accounted for about 18% (media) and 10% (cell layers) of the total 35S radioactivity of each proteoglycan fraction. The bound proteoglycan molecules could be dissociated from the aggregates either by digestion with hyaluronate lyase or by treatment with hyaluronate decasaccharides. Digestion of [methionine-35S]proteoglycans with chondroitinase and/or heparitinase, followed by SDS/polyacrylamide-gel electrophoresis, indicated that the medium and cell layer contain at least three chondroitin sulphate proteoglycans, one dermatan sulphate proteoglycan, and two heparan sulphate proteoglycans which differ from one another in the size of core molecules. Among these, only the hydrodynamically large chondroitin sulphate species with an Mr 550,000 core molecule was shown to bind to hyaluronate. A very similar chondroitin sulphate proteoglycan capable of binding to hyaluronate was also found in cultures of calf pulmonary arterial endothelial cells (A.T.C.C. CCL 209). These observations, together with the known effects of hyaluronate on various cellular activities, suggest the existence of possible specialized functions of this proteoglycan subspecies in cellular processes characteristic of vascular development and diseases.     

Biosynthesis and regulation of rat alpha 1-inhibitor3, a negative acute-phase reactant of the macroglobulin family.

The biosynthesis of rat alpha 1-inhibitor3, a negative acute-phase reactant specifically found in rodents, was studied in vitro in a cell-free translation system from rabbit reticulocytes, in rat hepatocyte primary cultures and in vivo by immunocytochemistry using normal and turpentine-injected rats. By sucrose-gradient centrifugation and subsequent translation of the fractionated RNA in vitro it was found that the mRNA coding for alpha 1-inhibitor3 exhibited a size of about 28S. For the alpha 1-inhibitor3 translated in vitro an apparent Mr of 155,000 was determined. A continuous decrease in the level of alpha 1-inhibitor3 in serum during experimental inflammation induced by turpentine injection was demonstrated by means of quantitative 'rocket' immunoelectrophoresis. This result agrees with the observation by immunocytochemistry of a drastic decrease in alpha 1-inhibitor3 levels in hepatocytes 24 h after turpentine injection. At that time alpha 1-inhibitor3 is mainly located in the Golgi apparatus, whereas it is also present in the membranes of the rough and smooth endoplasmic reticulum when normal liver is used. All hepatocytes, but no other hepatic cells, contain alpha 1-inhibitor3. When hepatocyte primary cultures were labelled with [35S]methionine and alpha 1-inhibitor3 was immunoprecipitated from the hepatocyte medium and the supernatant of homogenized cells, two different forms of alpha 1-inhibitor3 were found. The intracellular form of alpha 1-inhibitor3, with an apparent Mr of 173,000, is characterized by oligosaccharide side chains of the high-mannose type. The form of alpha 1-inhibitor3 in the medium exhibited an Mr of 186,000 and carried carbohydrate side chains of the complex type. After labelling hepatocytes with radioactive sugars, [3H]mannose was found in both forms of alpha 1-inhibitor3, whereas [3H]fucose and [3H]galactose were incorporated only into the form found in the medium. In the presence of tunicamycin an unglycosylated alpha 1-inhibitor3 with an apparent Mr of 154,000 was found in cells and in the medium. In a pulse-chase experiment it was shown that inhibition of glycosylation by tunicamycin resulted in a marked delay of secretion of alpha 1-inhibitor3. Thus the oligosaccharide side chains of alpha 1-inhibitor3 play an important role during its transport into the medium.     

Basement-membrane heparan sulphate with high affinity for antithrombin synthesized by normal and transformed mouse mammary epithelial cells.

Basement-membrane proteoglycans, biosynthetically labelled with [35S]sulphate, were isolated from normal and transformed mouse mammary epithelial cells. Proteoglycans synthesized by normal cells contained mainly heparan sulphate and, in addition, small amounts of chondroitin sulphate chains, whereas transformed cells synthesized a relatively higher proportion of chondroitin sulphate. Polysaccharide chains from transformed cells were of lower average Mr and of lower anionic charge density compared with chains isolated from the untransformed counterparts, confirming results reported previously [David & Van den Berghe (1983) J. Biol. Chem. 258, 7338-7344]. A large proportion of the chains isolated from normal cells bound with high affinity to immobilized antithrombin, and the presence of 3-O-sulphated glucosamine residues, previously identified as unique markers for the antithrombin-binding region of heparin [Lindahl, Bäckström, Thunberg & Leder (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 6551-6555], could be demonstrated. A significantly lower proportion of the chains derived from transformed cells bound with high affinity to antithrombin, and a corresponding decrease in the amount of incorporated 3-O-sulphate was observed.     

Temporal regulation of hyaluronan and proteoglycan metabolism by human bone cells in vitro

Osteoblasts elaborate a dynamic extracellular matrix that is constructed and mineralized as bone is formed. This matrix is primarily composed of collagen, along with noncollagenous proteins which include glycoproteins and proteoglycans. After various times in culture, human bone cells were labeled with [35S]sulfate, [3H] leucine/proline, or [3H]glucosamine and the metabolism of hyaluronan and four distinct species of proteoglycans (PGs) was assayed in the medium, cell layer, and intracellular pools. These cells produce hyaluronan (Mr approximately 1,400,000; a chondroitin sulfate PG (CSPG), Mr approximately 600,000; a heparan sulfate PG (HSPG), Mr approximately 400,000; and two dermatan sulfate PGs with Mr approximately 270,000 (biglycan, PG I) and Mr approximately 135,000 (decorin, PG II) that distribute between the medium and cell layer. Two days following subculture, 12 h [35S]sulfate steady-state labeling yielded a composition of 24, 27, 31, and 18% for total CSPG, HSPG, biglycan, and decorin, respectively. While HSPG and decorin levels and distribution between medium and cell layer remained relatively constant during steady-state labeling at different times in culture, CSPG and biglycan levels increased dramatically at late stages of growth, and their distribution changed throughout culture. These results were independent of cell density, media depletion, and labeling pool effects. In contrast, hyaluronan synthesis was uncoupled from PG synthesis and apparently density-dependent. Pulse chase labeling at different stages of culture showed that the CSPG and decorin behaved as secretory PGs. Both HSPG and biglycan underwent catabolism, with HSPG possessing a t1/2 of 8 h and biglycan a t1/2 of 4 h. While the rate of HSPG turnover did not appreciably change between early and late culture, that of biglycan decreased. The mRNA for decorin was constant, while that of biglycan changed during culture. These results suggest that each PG possesses a distinct pattern of cellular and temporal distribution that may reflect specific stages in matrix formation and maturation.