Regulation of the synthesis of mucin glycoproteins in swine trachea explants

Swine tracheal epithelium has been cultured as explants in a chemically defined medium for periods of up to 2 wk. The viability of the explants was shown by the preservation of the ultrastructural features of cells in the epithelial layer and by the active incorporation of radioactive glucosamine and sulfate into secreted mucin glycoproteins. The rate of secretion of mucin glycoprotein was about 0.035 mg per cm2 per d. After initial 24 h lag period was shown to be due to the equilibration of intracellular mucin glycoprotein pools with radioactive precursors. The rate of secretion of glycoprotein showed a linear dependence on the area of the explant, and maximal incorporation was observed at 200 microM glucosamine. A higher concentration of 35SO4, 1000 microM, was required for maximal incorporation of the precursor. Insulin at 0.1 to 1 microgram/ml increased the rate of secretion twofold, whereas 0.1 to 100 micrograms/ml of hydrocortisone and 0.1 to 100 micrograms/ml of epinephrine significantly decreased the rate of secretion. Vitamin A had little or no effect of normal trachea explants at low concentrations, and, at higher concentrations, 10(-5) M, it decreased the secretion of mucin glycoproteins. Vitamin A, at a concentration of 10(-9) M, increased the rate of synthesis of glycoprotein at least fourfold in trachea explants from vitamin A-deficient rats. Mucus secretions collected from the surface of swine trachea and from the culture medium of trachea explants were purified. The mucus was solubilized by reduction and carboxymethylation, and the high molecular weight mucin glycoproteins were purified by chromatography on Sepharose CL-6B columns under dissociating conditions in 2 M guanidine HCl. The mucin glycoproteins purified from swine trachea and from the culture medium of trachea explants were virtually indistinguishable. They showed the same properties when examined by gel electrophoresis and immunoprecipitation. The purified glycoproteins contained about 25% protein, and serine, threonine, and proline were the principal amino acids present. More than 80% of the carbohydride chains in both samples were released by treatment with alkaline borohydride. Nearly the same molar ratio of N-acetylgalactosamine, N-acetylglucosamine, galactose, fucose, sulfate, and sialic acid was found in both preparations.     

The effect of chronic ethanol consumption on temperature-dependent physical properties of liver mitochondrial membranes

We have reported that the monovalent ionophore monensin causes undersulfated chondroitin sulfate biosynthesis in cultured chondrocytes. In order to clarify the mechanism of this diminished sulfation, we have measured the rate of incorporation of sulfate into chondrocytes and assayed the cellular ATP levels. We have also measured sulfatase activity, the incorporation of ³⁵SO₄ into 3′-phosphoadenosine 5′-phospho[³⁵S]sulfate and endogenous sulfotransferase activity in the cell-free extracts. We find that: (1) The incorporation of ³⁵SO₄ into the free sulfate pool in chondrocytes was not inhibited by monensin. (2) The ATP levels of monensin-treated chondrocytes were the same as control cells. (3) There was no sulfatase activity in both control and monensin-treated chondrocytes. (4) Enzymatic analyses revealed that ³⁵SO₄ incorporation into 3′-phosphoadenosine 5′-phospho[³⁵S]sulfate and subsequent sulfotransferase activity were not inhibited in the presence of monensin. At present the most tenable hypothesis to account for monensin causing undersulfated chondroitin sulfate synthesis is that the ionophore impairs the access of proteoglycans to the sulfotransferases in the luminal walls of the Golgi structures.     

Influence of monovalent cation transport on anabolism of glycosphingolipids in cultured human fibroblasts

We have reported [Saito, M., Saito, M., & Rosenberg, A. (1984) Biochemistry 23, 1043-1046] that the monovalent cationic ionophore monensin reduced the incorporation of labeled galactose into oligosaccharidyl glycosphingolipids (globotriaosylceramide, globotetraosylceramide, and gangliosides) and induced a cellular accumulation of glucosyl- and lactosylceramide in cultured diploid human fibroblasts. We have undertaken further studies on the effects of monensin and made comparison with the effects of related monovalent cation transporters on plasma membrane glycosphingolipid anabolism in human fibroblasts. Our results demonstrate that ionic flux can markedly influence glycosphingolipid synthesis, and they indicate that, like glycoprotein, the sites of glycosylation of the initial, precursor glycosphingolipids are different from the sites of higher glycosylation. At a concentration of 10(-7) M, monensin induced the maximum inhibition of incorporation of labeled galactose into polyglycosyl sphingolipids: globotriaosylceramide, globotetraosylceramide, and gangliosides; increased incorporation of labeled galactose into glucosyl- and lactosylceramide was clearly evident, and their content rose measurably in the cell at concentrations of monensin as low as 10(-8) M. These effects of monensin were reversible. Incorporation of labeled galactose into higher glycosylated neutral glycosphingolipids and gangliosides slowly resumed, and the accumulated glycosylceramide diminished after removal of monensin from the culture medium. Ouabain (plasma membrane Na+,K+-ATPase inhibitor) and A23187 (Ca2+ ionophore) also caused a rapid increase in incorporation of labeled hexose into glucosylceramide and decreased its incorporation into higher neutral glycosphingolipids and into gangliosides.(ABSTRACT TRUNCATED AT 250 WORDS)     

Increasing the intra-Golgi pH of cultured LS174T goblet-differentiated cells mimics the decreased mucin sulfation and increased Thomsen-Friedenreich antigen (Gal beta1-3GalNac alpha-) expression seen in colon cancer.

Mucins in ulcerative colitis and colon cancer share common properties of reduced sulfation and increased oncofetal carbohydrate antigen expression. It has previously been shown that there is no simple correlation between these changes and the activity of the relevant glycosyl-, sialyl-, and sulfo-transferases. We examined mucin sulfation and expression of oncofetal Thomsen-Friedenreich (TF) antigen (galactosyl beta1-3N-acetylgalactosamine alpha-) in the goblet cell-differentiated human colon cancer cell line LS174T following treatment with bafilomycin A(1, )which raises intra-Golgi pH, or monensin, which disrupts medial-trans Golgi transport. Cells were dual-labeled with sodium [(35)S]-sulfate and D-[6-(3)H(N)]-glucosamine hydrochloride, or labeled with L-[U-(14)C]-threonine alone. Mucin was purified using Sepharose CL-4B gel filtration. Mucin sulfo-Lewis(a) and TF antigen expression were assessed using the F2 anti-sulfo-Lewis(a) monoclonal antibody and peanut agglutinin binding respectively. Bafilomycin (0.01 microM; 48 h) reduced total mucin sulfation, expressed relative to incorporation of glucosamine, to 0.50 +/- 0.04 d.p.m. [(35)S]-sulfate per d.p.m. [(3)H]-glucosamine compared to control, 0.84 +/- 0.05 (p < 0.001, n = 16). This was accompanied by 50.3 +/- 8.0% increased expression of TF antigen (p < 0.01) and 50.1 +/- 5.5% decreased expression of sulfo-Lewis(a) (p < 0.01). The reduced sulfate:glucosamine ratio was largely due to increased incorporation of glucosamine into newly synthesized mucin rather than reduction in total sulfate incorporation. In contrast, monensin only reduced total mucin glycosylation at concentrations > 0.1 microM and had no significant effect on mucin sulfation or TF expression. Intra-Golgi alkalinization affects mucin glycosylation, resulting in decreased mucin sulfation and increased expression of TF antigen, changes that mimic those seen in cancerous and premalignant human colonic epithelium.     

Regulation of heparan sulphate metabolism by adenosine 3′:5′-cyclic monophosphate in hepatocytes in culture

Freshly isolated rat hepatocytes maintained as monolayers in a serum-free medium synthesize sulphated glycosaminoglycans, most of which behave as heparan sulphate and are mainly distributed into intracellular compartments. Cyclic AMP, dibutyryl cyclic AMP, glucagon, noradrenaline, prostaglandin E(1), and theophylline, all drugs and hormones known to increase intracellular cyclic AMP concentrations, decreased the incorporation of (35)SO(4) (2-) into heparan sulphate of intra-, extra- and peri-cellular pools. The inhibition mediated by dibutyryl cyclic AMP was dose-dependent and observed as early as 2h after exposure to the drug. In the presence of 1mm-dibutyryl cyclic AMP, incorporation of (35)SO(4) (2-) or [(14)C]glucosamine into heparan sulphate was decreased to 40-50%, suggesting that dibutyryl cyclic AMP interfered with the synthesis of heparan sulphate. This was further supported by pulse-chase experiments, where dibutyryl cyclic AMP had no effect on the degradation of sulphated glycosaminoglycans. Heparan sulphates synthesized and secreted into the extracellular pool in the presence of dibutyryl cyclic AMP were smaller in size, whereas the degree of sulphation and molecular size of the heparan sulphate chains released by beta-elimination from these proteoglycans were not different from control values. In the presence of 1mm-cycloheximide, (35)SO(4) (2-) incorporation was decreased to 5%. Addition of p-nitrophenyl beta-d-xyloside, an artificial acceptor of glycosaminoglycan chain synthesis, enhanced this incorporation to 18%. Dibutyryl cyclic AMP did not have any inhibitory effect on the synthesis of chains initiated on p-nitrophenyl beta-d-xylosides. Incorporation of [(3)H]serine into heparan sulphate was not affected by dibutyryl cyclic AMP, whereas the degree of substitution of serine residues with heparan sulphate chains was less in heparan sulphate synthesized in the presence of dibutyryl cyclic AMP, suggesting that cyclic AMP exerts its effect on the metabolism of sulphated glycosaminoglycans by affecting the transfer of xylose on to the protein core.     

Effects of monensin on the synthesis, transport, and intracellular degradation of proteoglycans in rat ovarian granulosa cells in culture

Rat ovarian granulosa cells, isolated from immature female rats 48 h after stimulation with 5 IU of pregnant mare's serum gonadotropin, were maintained in culture. The effects of monensin, a monovalent cationic ionophore, on various aspects of proteoglycan metabolism were studied by metabolically labeling cultures with [35S]sulfate, [3H]glucosamine, or [3H]glucose. Monensin inhibited post-translational modification of both heparan sulfate (HS) proteoglycans and dermatan sulfate (DS) proteoglycans, resulting in decreased synthesis of completed proteoglycans [( 35S]sulfate incorporation decreased to 10% of control by 30 microM monensin, with an ED50 approximately 1 microM). Proteoglycans synthesized in the presence of monensin showed undersulfation of both DS and HS glycosaminoglycans and altered N-linked and O-linked oligosaccharides, suggesting that the processing of all sugar moieties is closely associated. Monensin caused a decrease in the endogenous sugar supply to the UDP-N-acetylhexosamine pool as indicated by an increased 3H incorporation into DS chains [( 3H]glucosamine as precursor) in spite of the decrease in glycosaminoglycan synthesis. Monensin reduced and delayed transport of both secretory and membrane-associated proteoglycans from the Golgi complex to the cell surface. It took 2-4 min for newly labeled proteoglycans to reach the main transport process inhibited by monensin. Monensin at 30 microM did not prevent internalization of cell surface 35S-labeled proteoglycans but almost completely inhibited their intracellular degradation to free [35S]sulfate (ED50 approximately 1 microM), resulting in intracellular accumulation of both DS and HS proteoglycans. Pulse-chase experiments demonstrated that one of the intracellular degradation pathways involving proteolysis of both DS and HS proteoglycans and limited endoglycosidic cleavage of HS continued to operate in the presence of monensin. These results suggest that the intracellular degradation of proteoglycans involve both acidic and nonacidic compartments with monensin inhibiting those processes that normally occur in such acidic compartments as endosomes or lysosomes by raising their pH.     

Effects of the monovalent ionophore monensin on the intracellular transport and processing of pro-opiomelanocortin in cultured intermediate lobe cells of the rat pituitary

Detailed studies on the effects of the ionophore monensin upon synthesis, maturation, and intracellular transport of pro-opiomelanocortin in cultures of rat pituitary intermediate lobe cells have been carried out. When added at concentrations larger than 5 X 10(-8) M monensin significantly inhibited protein synthesis by cultured intermediate lobe cells. Pro-opiomelanocortin synthesis was also reduced proportionally to the overall rate of protein synthesis. During pulse-chase experiments, monensin when added at a concentration of 10(-5) M at the beginning of the chase incubation completely inhibited the proteolytic processing of pro-opiomelanocortin. Using a subcellular fractionation procedure of intermediate lobe cell extracts on Percoll gradients, we were able to show that after the addition of monensin (10(-5) M), labeled pro-opiomelanocortin molecules synthesized during a 15-min pulse-incubation were recovered intact after a 2-h chase, in the fractions of the density gradient corresponding to the rough endoplasmic reticulum and Golgi elements. No maturation products or precursor molecules entered the granule fractions as observed in nontreated cells. Taken together these results strongly suggest that monensin blocks the intracellular transport of newly synthesized pro-opiomelanocortin molecules at the Golgi level and that inhibition of proteolytic processing is due to the failure of the prohormone to enter the cell compartment (probably the secretion granules) where maturation proteases are located.     

Proteoglycan synthesis in normal and Lowe syndrome fibroblasts

Lowe (oculocerebrorenal) syndrome (LS) is an X-linked disorder characterized by congenital cataracts, generalized hypotonia, mental retardation, and renal Fanconi syndrome. The basic defect remains unknown, but the possibility that fibroblasts express reduced sulfation of glycosaminoglycans has been studied in several laboratories. A mechanism involving overproduction of an enzyme (nucleotide pyrophosphatase) active against adenosine 3'-phosphate, 5'-phosphosulfate (PAPS) has been postulated. Decreased synthesis of normally sulfated glycosaminoglycans was also reported. We measured the synthesis of proteoglycans and glycosaminoglycans by incorporation of [3H]glucosamine and Na2(35)SO4 into cultured fibroblasts from four LS patients and related it directly to the synthesis in six normal fibroblast cultures. We found that the rate of synthesis varied greatly among the normal cultures (cv, 30%), but not significantly between LS and the normal. The LS fibroblasts' ability to sulfate glycosaminoglycans was assayed as the amount of 3H-glycosaminoglycan eluting at low ionic strength on anion exchange chromatography, the amount of non-sulfated disaccharide present in chondroitinase digests of labeled proteoglycans, and the ratio of 35S to 3H incorporation into proteoglycans. Each parameter suggested that the LS cells were synthesizing normally sulfated glycosaminoglycans (e.g. % delta Di-0S, 21 +/- 6 in normal; 27 +/- 6 in LS). The cells' ability to sulfate glycosaminoglycans was tested under conditions of markedly stimulated glycosaminoglycan synthesis, by treating the cultures with a beta-D-xyloside. LS and normal cells responded to the treatment by elevating the rate of synthesis of normally sulfated glycosaminoglycans (3.5-6-fold in normal, 3-7-fold in LS). Nucleotide pyrophosphatase activities were found to be elevated in each of our four LS cell strains as in the previous studies, excluding genetic heterogeneity as an explanation for our findings. We conclude that LS fibroblasts do not express defects in sulfation of glycosaminoglycans or in synthesis of proteoglycans.     

Synthesis and secretion of proteoglycans by cultured chondrocytes : Effects of monensin, colchicine and β- -xyloside

Chondrocytes, isolated from elastic ear cartilage of young rabbits, were grown in monolayer cultures in Ham's F-12 medium. Synthesis and secretion of macromolecules were monitored by labelling with radioactive precursors and the effect of monensin and other experimental agents was investigated. Monensin caused an inhibition of the incorporation of precursors into macromolecular material and a moderate intracellular accumulation when used in higher concentrations. The effect was more pronounced for 35SO4 than for 3H-labelled glucose or proline. p-Nitrophenyl-beta-D-xyloside alleviated this inhibition to some extent, but there was a concomitant increase in the amount of intracellular labelled material. Colchicine and monensin together caused a severe inhibition of the incorporation of 35SO4 and a marked shift of the label to the intracellular compartment. Colchicine also increased the sensitivity of the cells to monensin, lowering the minimal effective concentration about one order of magnitude. The latter results are consistent with the idea that cytoplasmic microtubules have a stabilizing function on the secretory pathways and, that their removal by colchicine, causing a 'randomizing' of the Golgi complex, makes these pathways more vulnerable to monensin.     

Monensin inhibits the secretion of α-amylase but not polysaccharide slime from seedling tissues ofZea mays

Summary The effect of monensin on polysaccharide slime secretion by root tips of corn (Zea mays) was studied. Various treatment times and ionophore concentrations were tested: none resulted in inhibition of slime secretion. Because monensin changes the pH of the medium, its effect was also monitored in strongly buffered media and at different pH's. Even in such media, monensin did not inhibit slime secretion. We also measured the effect of the drug after a pulse with [³H]fucose or a pulse followed by a chase. The amount of labeled slimed secreted was not altered by the ionophore. However, 10M monensin affected the development of root tips and drastically reduced their growth. We showed that monensin inhibits the secretion of -amylase by the scutellum of the same plantlet. The importance of the nature of the secretory compound in relation to monensin inhibition of its secretion is discussed.Abbrevations Hepes N-2-hydroxyethylpiperazine-N-2-ethane-sul-fonic acid - Mes 2-(N-morpholino)ethane-sulfonic acid     

Mechanism for the decreased biosynthesis of cartilage proteoglycan in the scorbutic guinea pig

Our previous work showed that vitamin C deficiency caused about a 70-80% decrease in the incorporation of [35S]sulfate into proteoglycan of guinea pig costal cartilage, coordinately with a decrease in collagen synthesis (Bird, T. A., Spanheimer, R. G., and Peterkofsky, B. (1986) Arch. Biochem. Biophys. 246, 42-51). We examined the mechanism for decreased proteoglycan synthesis by labeling normal and scorbutic cartilage in vitro with radioactive precursors. Proteoglycan monomers from scorbutic tissue were of a slightly smaller average hydrodynamic size than normal but there was no difference in the size of the glycosaminoglycan chains isolated after papain digestion. The type of glycosaminoglycans synthesized and the degree of sulfation were unaffected as determined by chondroitinase ABC digestion and duel labeling with [35S]sulfate and [3H]glucosamine. Conversion of [3H]glucosamine to [3H]galactosamine also was unimpaired. There was about a 40% decrease in core protein synthesis, measured by [14C]serine incorporation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Nevertheless, decreased incorporation of [35S]sulfate into scorbutic tissue persisted in the presence of p-nitrophenyl-beta-D-xyloside and cycloheximide, which indicated that the site of the scorbutic defect was beyond core protein synthesis and xylosylation. Galactosyltransferase activity in scorbutic cartilage decreased to about one-third the levels in control samples in parallel with the decreases in proteoglycan and collagen synthesis. Our results suggest that the step catalyzed by this enzyme activity, the addition of galactose to xylose prior to chondroitin sulfate chain elongation, is the major site of the scorbutic defect in proteoglycan synthesis. Decreased enzyme activity may be related to increased cortisol levels in scorbutic serum.     

Effect of monensin on the sulfation of heparan sulfate proteoglycan from endothelial cells.

Monensin is a monovalent metal ionophore that affects the intracellular translocation of secretory proteins at the level of trans-Golgi cisternae. Exposure of endothelial cells to monensin results in the synthesis of heparan sulfate and chondroitin sulfate with a lower degree of sulfation. The inhibition is dose dependent and affects the ratio [35S]-sulfate/[3H]-hexosamine of heparan sulfate from both cells and medium, with no changes in their molecular weight. By the use of several degradative enzymes (heparitinases, glycuronidase, and sulfatases) the fine structure of the heparan sulfate synthesized by control and monensin-treated cells was investigated. The results have shown that among the six heparan sulfate disaccharides there is a specific decrease of the ones bearing a sulfate ester at the 6-position of the glucosamine moiety. All other biosynthetic steps were not affected by monensin. The results are indicative that monensin affects the hexosamine C-6 sulfation, and that this sterification is the last step of the heparan sulfate biosynthesis and should occur at the trans-Golgi compartment.     

The effects of monensin on secretion of very-low-density lipoprotein and metabolism of asialofetuin by cultured rat hepatocytes.

Primary cultures of rat hepatocytes were used to study secretion of very-low-density lipoproteins and metabolism of asialofetuin. The ionophore monensin inhibited both secretion of very-low-density lipoproteins and binding and degradation of asialofetuin in a concentration-dependent manner. Secretion as well as receptor binding were markedly decreased after 15 min treatment with monensin. The inhibitory effect of the ionophore was fully reversible, and no effect on protein synthesis was observed at concentrations up to 50 microM. The secretion of apoproteins (B-small, B-large and E) and that of albumin were inhibited to the same extent as was triacylglycerol secretion. Secretion of very-low-density lipoproteins was more sensitive to low concentrations of monensin than was the metabolism of asialofetuin. Maximum inhibition of very-low-density-lipoprotein secretion was obtained at 5-10 microM-monensin, whereas 25 microM was required to obtain maximum inhibition of binding and degradation of asialofetuin. The number of surface receptors for asialofetuin decreased to about half when the cells were exposed to 25 microM-monensin. It is possible that monensin inhibits endo- and exo-cytosis via a similar mechanism, e.g. by disturbing proton gradients. Since secretion of very-low-density lipoproteins was more sensitive to low concentrations of monensin, it is likely that monensin independently inhibits endocytic and secretory functions in cultured hepatocytes.     

Enhanced synthesis and extracellular accumulation of hyaluronic acid during stimulation of quiescent human fibroblasts by mouse epidermal growth factor

The effect of mouse epidermal growth factor (mEGF) on the synthesis of glycosaminoglycans and glycoproteins by human fibroblasts has been studied. The addition of physiological concentrations (10^?9 M) of mEGF to quiescent cultures preincubated in the absence of serum was found to elicit an increased incorporation of ³H-glucosamine into the glycosaminoglycans and glycoproteins of both the cellular and extracellular fractions. Although the growth response to the factor, as measured by DNA replication, was minimal under these conditions as compared with the effect of serum, the mEGF-induced incorporation of glucosamine into these cellular constituents and into the extracellular glycoproteins was comparable to that elicited by serum shift-up. Serum, however, caused a significantly larger incorporation of glucoasmine into extracellular, acid-soluble glycosaminoglycans, which were shown to contain hyaluronic acid as the major component. As previously demonstrated, the growth response to mEGF can be enhanced several fold by an mEGF-binding arginine esterase, which is normally associated with the factor in vivo, and by ascorbate. The esterase was found to increase markedly the mEGF-induced incorporation of glucosamine into extracellular hyaluronic acid, while the addition of ascorbic acid did not significantly alter glucosamine incorporation.     

Lipoprotein synthesis and secretion by cultured rat hepatocytes. Parallel inhibition of secretion of VLDL, HDL and albumin by monensin

The biosynthesis and secretion of very-low-density lipoproteins (VLDL) and high-density lipoproteins (HDL) by cultured normal rat hepatocytes was investigated with particular emphasis on its modification by monensin. This acidic ionophore coordinately inhibited the rates of secretion of the several VLDL apolipoproteins and the VLDL lipids, suggesting an effect late in the process of biosynthesis and secretion, probably at the stage of exiting from the Golgi apparatus. The secretion of immunoreactive albumin into the medium was comparably inhibited, implying that the pathway and mechanisms involved in albumin secretion may be closely similar to those for VLDL synthesis and secretion. Secretion of phospholipids and of apolipoproteins E and A-I in the HDL fraction increased progressively with time over 18 h in control incubations but was strongly inhibited by monensin. During extended incubation with monensin at high concentrations (10 microM), there was a net release to the medium of a number of hepatocyte proteins, including some that comigrated with apolipoprotein A-I and apolipoprotein C, making it appear that monensin increased the secretion of these apolipoproteins. However, using labeled amino acids, it was shown by autoradiography and by immunoprecipitation that secretion of newly-synthesized, radioactive apolipoprotein A-I and apolipoprotein C was actually inhibited by monensin. These results are compatible with the conclusion that HDL synthesis and secretion may occur by mechanisms closely related to those for synthesis and secretion of albumin and VLDL.     

Stimulation by glucocorticoids of the differentiated phenotype of chondrocytes and the proliferation of rabbit costal chondrocytes in culture

Hydrocortisone stimulated glycosaminoglycan (GAG) synthesis, a characteristic of the cartilage phenotype, of rabbit costal chondrocytes in confluent quiescent culture, as judged by the incorporations of [35S]sulfate and [3H]glucosamine. Hydrocortisone also stimulated incorporation of [3H]serine into proteoglycan. The stimulation of GAG synthesis by hydrocortisone was dose-dependent and maximal at a physiological concentration of 10(-7) M. Hydrocortisone also stimulated GAG synthesis in cultures in the log-phase of growth. In this case, its maximal effect was observed at a concentration of 10(-6) M. The magnitude of the increase of GAG synthesis in response to hydrocortisone was larger in confluent culture than in log-phase cultures. Hydrocortisone stimulated DNA synthesis dose-dependently, and its effect was observable at a physiological concentration. However, no stimulation of DNA synthesis by hydrocortisone was observed in serum-free medium, in contrast to that of GAG synthesis. Hydrocortisone also increased protein synthesis and the cell number. Dexamethasone also stimulated the syntheses of both GAG and DNA. These results show that glucocorticoids stimulated both the differentiated phenotype of chondrocytes and the proliferation of rabbit costal chondrocytes in culture. Moreover, the effect of glucocorticoids was primarily on the differentiated phenotype of chondrocytes and its effect on proliferation was permissive.     

Monensin blocks the first meiotic cell division of the Xenopus oocyte: Effect at the nuclear membrane level

The carboxylic ionophore monensin inhibits the meiotic maturation of the Xenopus oocyte. When oocytes are exposed to high concentrations of monensin (10 μM), both progesterone and MPF-induced (maturation-promoting factor-induced) maturations are blocked. Lower doses of monensin (1–10 μM) do not inhibit the formation or amplification of MPF activity in the oocyte cytoplasm; however, breakdown of the nuclear envelope does not occur. These observations show that monensin, which is known to abolish intracellular proton gradients, interferes with the mechanism of the breakdown of the nuclear envelope induced by MPF.     

Incorporation of [3H]glucosamine into glycosaminoglycans in different cell culture conditions by rabbit aortic smooth muscle cells

This study sought to elucidate the optimal cell culture conditions for studies concerned with the incorporation of [³H]glucosamine into glycosaminoglycans by rabbit aortic smooth muscle cells. The incorporation of radioactivity into extracellular sulphated glycosaminoglycans was linear for at least 72 h and that into pericellular sulphated glycosaminoglycans for up to 24 h. The incorporation of radiolabel into hyaluronic acid was linear only up to 12 h. In the exponential growth phase the incorporation of [³H]glucosamine into sulphated glycosaminoglycans and hyaluronic acid proved to be less marked than in the stationary growth phase, but the highest values were nevertheless obtained immediately after trypsinisation. When studied in the stationary growth phase, cell density and incorporation of [³H]glucosamine were positively correlated in the case of hyaluronic acid, but in the case of sulphated glycosaminoglycans there was a negative correlation. The serum concentration of the incubation medium and the incorporation of radioactivity into hyaluronic acid were positively related. With sulphated glycosaminoglycans this was the case only after a 7-day preincubation in the different serum concentrations. when incorporation was studied without preincubation, the incorporation of radioactivity into sulphated glycosaminoglycans proved to be negatively associated with the serum concentration of the medium. The environmental pH of the cells was associated with the incorporation of radioactivity into hyaluronic acid and sulphated glycosaminoglycans in that between pH values 6.8 and 7.9 the incorporation of radioactivity increased when the pH of the medium was raised.     

Stimulation by concanavalin A of cartilage-matrix proteoglycan synthesis in chondrocyte cultures

The effect of concanavalin A on proteoglycan synthesis by rabbit costal and articular chondrocytes was examined. Chondrocytes were seeded at low density and grown to confluency in medium supplemented with 10% fetal bovine serum, and then the serum concentration was reduced to 0.3%. At the low serum concentration, chondrocytes adopted a fibroblastic morphology. Addition of concanavalin A to the culture medium induced a morphologic alteration of the fibroblastic cells to spherical chondrocytes and increased by 3- to 4-fold incorporation of [35S]sulfate and [3H]glucosamine into large chondroitin sulfate proteoglycan that was characteristically found in cartilage. The stimulation of incorporation of labeled precursors reflected real increases in proteoglycan synthesis, as chemical analyses showed a 4-fold increase in the accumulation of macromolecules containing hexuronic acid in concanavalin A-maintained cultures. Furthermore, the effect of concanavalin A on [35S]sulfate incorporation into proteoglycans was greater than that of various growth factors or hormones. However, concanavalin A had smaller effects on [35S]sulfate incorporation into small proteoglycans and [3H]glucosamine incorporation into hyaluronic acid and chondroitinase AC-resistant glycosaminoglycans. Since other lectins tested, such as wheat germ agglutinin, lentil lectin, and phytohemagglutinin, had little effect on [35S]sulfate incorporation into proteoglycans, the concanavalin A action on chondrocytes seems specific. Although concanavalin A decreased [3H]thymidine incorporation in chondrocytes, the stimulation of proteoglycan synthesis could be observed in chondrocytes exposed to the inhibitor of DNA synthesis, cytosine arabinoside. These results indicate that concanavalin A is a potent modulator of proteoglycan synthesis by chondrocytes.     

The phytoestrogens daidzein and genistein enhance the insulin-stimulated sulfate uptake in articular chondrocytes

Clinical observations have suggested a relationship between osteoarthritis and a changed estrogen metabolism in menopausal women. Phytoestrogens have been shown to ameliorate various menopausal symptoms. Proteoglycans (PG) consisting of low and high sulfated glycosaminoglycans (GAG) are the main components of articular cartilage matrix, and their synthesis is increased by insulin in growth plate cartilage. We have investigated whether GAG synthesis and sodium [35S]sulfate incorporation in female bovine articular chondrocytes are affected by daidzein, genistein, and/or insulin. For comparative purposes, estradiol incubations were performed. Articular chondrocytes were cultured in monolayers at 5% O2 and 5% CO2 in medium containing serum for 7 days followed by the addition of 10(-11) M-10(-4) M daidzein, genistein, 17beta-estradiol, or 5 microg/ml insulin in a serum-free culture phase of 2 days. Photometrically analyzed GAG synthesis was significantly suppressed by high doses (10(-5) M-10(-4) M) of daidzein, genistein, and 17beta-estradiol. Although insulin raised the sodium [35S]sulfate uptake significantly, different concentrations of daidzein, genistein, or 17beta-estradiol showed no significant effects. However, the stimulating effect of insulin on sulfate incorporation was enhanced significantly after preincubation of cells with 10(-11) M-10(-5) M daidzein or 10(-9) M-10(-5) M genistein but not by 17beta-estradiol. In view of the risks of long-term estrogen replacement therapy, further experiments should clarify the potential benefit of phytoestrogens and insulin in articular cartilage metabolism.