Monensin inhibits the expression of sucrase-isomaltase in Caco-2 cells at the mRNA level

Using L-[35S]methionine labeling, SDS-PAGE and Northern blot analysis of sucrase-isomaltase mRNA, two different concentrations of monensin were used to delineate in Caco-2 cells the effect of the drug on the conversion of the high mannose to the complex form of sucrase-isomaltase from its dual effect on the biosynthesis of the enzyme and on the rate of glucose consumption. At 0.1 microM the drug has no effect on the rate of glucose consumption and, although it inhibits the conversion of the high mannose to the complex form of the enzyme, it has no effect on the level of sucrase-isomaltase mRNA and on the amount of neosynthesized enzyme. At 1 microM, in addition to its inhibiting effect on the maturation of the enzyme, monensin provokes concomitantly an increase in the rate of glucose consumption and a decrease in the level of sucrase-isomaltase mRNA and in the amount of neosynthesized enzyme. All these effects are reversible within 48 h after removal of the drug.     

Biosynthesis of mouse Thy-1 antigen

The biosynthesis and the maturation of Thy-1 antigen of mouse thymocytes have been studied by using a xenogeneic rabbit anti-mouse Thy-1 antibody. The earliest form of Thy-1 detected after a 5-min pulse with [35S]methionine and [35S]cysteine had an apparent m.w. of 26,500. During chase, this band converted to a molecular ratio (Mr) = 25,000 polypeptide, probably derived from the latter by trimming of glucose or mannose residues from the three high-mannose glycan units of Thy-1. Mature Thy-1 molecules were detected at the cell surface after a 15-min chase. At least one of the three N-linked oligosaccharide units was shown to be in the high mannose form at the cell surface, as indicated by its susceptibility to endo-beta-N-acetylglucosaminidase H digestion. Treatment of the early and late forms of Thy-1 antigen with endo-beta-N-acetylglucosaminidase F generated a single polypeptide of Mr = 13,500. The same precursor was obtained when cells were labeled in the presence of tunicamycin. This indicates the absence of O-linked glycan in the mature cell surface antigen. Finally, the resistance of Thy-1 antigen to trypsin digestion when associated with membranes confirmed that this molecule has no cytoplasmically oriented portion.     

Evidence for biosynthesis of lactase-phlorizin hydrolase as a single-chain high-molecular weight precursor

Precursor forms of lactase-phlorizin hydrolase, sucrase-isomaltase and aminopeptidase N were studied by pulse-labelling of organ-cultured human intestinal biopsies. After labelling the biopsies were fractionated by the Ca2+-precipitation method and the enzymes isolated by immunoprecipitation. The results indicate that the lactase-phlorizin hydrolase is synthesized as a Mr 245 000 polypeptide, which is intracellularly cleaved into its mature Mr 160 000 form. Sucrase-isomaltase is shown to be synthesized as a single chain precursor (Mr 245 000 and 265 000) while the precursor of aminopeptidase N is shown to be of apparently the same size as the mature enzyme (Mr 140 000 and 160 000).     

Enterocytic differentiation of a subpopulation of the human colon tumor cell line HT-29 selected for growth in sugar-free medium and its inhibition by glucose

In order to study the effect of glucose on the differentiation of cultured human colon cancer cells, a subpopulation of HT-29 cells was selected for its capacity to grow in the total absence of sugar. These cells (Glc-cells) exhibit, after confluency, an enterocytic differentiation, in contrast to cells grown with glucose (Glc+ cells), which always remain undifferentiated. The differentiation is characterized by a polarization of the cell layer with apical brush borders and tight junctions, and by the presence of sucrase-isomaltase. The differentiation of Glc- cells is reversible: the addition of glucose to postconfluent cultures of Glc- cells results in an inhibiting effect on the expression of sucrase-isomaltase; switching growing cultures of Glc- cells to the Glc+ medium for several passages results in a progressive reversion to the undifferentiated state, which is completed after seven passages. The dedifferentiation process is associated with a parallel, passage-related, increase in the rates of glucose consumption and lactic acid production, and decreases of intracellular glycogen content, which return to the values of the undifferentiated original Glc+ cells. The values of these metabolic parameters are correlated, at each passage, with the degree of dedifferentiation of the cells. When these dedifferentiated cells, after having been cultured in Glc+ medium for 20 passages, are switched back to the Glc- medium, they readily grow without mortality, and reexpress the same enterocytic differentiation as the parent Glc- cells. These results show that the capacity of this subpopulation to grow and differentiate in the absence of sugar is a stable characteristic. They further suggest that glucose metabolism interferes with the program of differentiation of HT-29 cells.     

Biosynthesis of alpha-galactosidase A in cultured Chang liver cells

An investigation of the structure and biosynthesis of alpha-galactosidase A (alpha-D-galactoside glycohydrolase, EC 3.2.1.22) and its N-linked oligosaccharide chains was undertaken by metabolic labeling of Chang liver cells with [2-3H]mannose, immunoprecipitation of the activity, and examination of the resulting immunoprecipitates. From cells pulse labeled for 3 h, two radioactive bands with Mr = 58,000 and 49,000 were detected by SDS-gel electrophoresis; following a 20-h chase, only the Mr = 49,000 band was observed. Examination of the oligosaccharide fraction derived from pulse-labeled enzyme revealed that 18% of the asparagine-linked oligosaccharides were complex and 82% were high-mannose type. After a 20-h chase, 48% of the oligosaccharides were complex and 52% were high mannose. The high-mannose oligosaccharides of alpha-galactosidase A immunoprecipitated from both pulsed and pulse-chased cells had the same mobilities as Man8-9GlcNAc on thin-layer chromatography and Bio-Gel P-4. Two fractions of complex glycopeptides derived from the alpha-galactosidase A of pulsed and pulse-chased cells had the same migration on Bio-Gel P-4 as glucose oligomers containing 14 and 19-39 glucose units. Based on their apparent size and their behavior on concanavalin A-Sepharose, the complex oligosaccharides are believed to be composed of tri- and/or tetraantennary structures.     

Biosynthesis and glycosylation of the human complement regulatory protein decay-accelerating factor

The biosynthesis and oligosaccharide structure of the human complement regulatory glycoprotein decay-accelerating factor (DAF) were studied in erythrocytes and cell lines. Initial information relative to carbohydrate moieties of DAF was obtained by enzymatic digestions. The 74,000 Mr erythrocyte DAF was lowered 3000 by endoglycosidase F, whereas endoglycosidase H had no effect, indicating one N-linked complex-type unit. Treatment with endo-alpha-N-acetylgalactosaminidase to remove O-linked oligosaccharides resulted in a 48,000 Mr molecule (67% of the Mr shift being due to sialic acid), which decreased to 45,000 Mr after sequential endoglycosidase F treatment. To additionally define the oligosaccharide structure and identify precursors in biosynthetic pathways, DAF was studied in the HL-60 cell line differentiated by vitamin D toward monocytes. Pulse-chase experiments with [35S]methionine revealed a precursor species of 43,000 Mr that underwent an early post-translational modification to a 46,000 Mr intermediate, and subsequently was chased into a mature species of 80,000 Mr that aligned with 125I surface-labeled DAF from these cells. All three forms of DAF were approximately 3000 lower in Mr in the presence of tunicamycin. The two lower Mr DAF species were sensitive to endoglycosidases F and H but not to neuraminidase or endo-alpha-N-acetylgalactosaminidase. In summary, DAF is synthesized as a 43,000 Mr precursor species containing one N-linked high-mannose unit. Before entering the central region of the Golgi, it is converted to a 46,000 Mr species by an as yet unknown post-translational modification. The 46,000 Mr form is converted to the 74,000 Mr (erythrocyte) or 80,000 Mr (leukocyte) membrane form of DAF by the addition of multiple, sialylated O-linked oligosaccharide chains (responsible for the large electrophoretic mobility shift) and conversion of the single N-linked high-mannose unit to a complex-type structure. The cell-specific Mr variation between red and white blood cells arises during this post-translational modification from the 46,000 Mr biosynthetic intermediate to the mature DAF species expressed on the cell surface.     

Biosynthesis of placental alkaline phosphatase and its post-translational modification by glycophospholipid for membrane-anchoring

The biosynthesis and post-translational modification of placental alkaline phosphatase were studied in human choriocarcinoma cells, JEG-3. Pulse-chase experiments with [35S]methionine demonstrated that placental alkaline phosphatase was synthesized as a major precursor form with Mr 63,000, which was then converted to a mature form with Mr 66,000, by processing of its N-linked oligosaccharides from the high-mannose type to the complex type. In addition, the two forms of the protein were found to be modified by a glycophospholipid, components of which were characterized by metabolic incorporation into placental alkaline phosphatase of 3H-labeled compounds such as myo-inositol, palmitic acid, stearic acid, mannose, glucosamine, and ethanolamine. When placental alkaline phosphatase labeled with these compounds was treated with phosphatidylinositol-specific phospholipase C or papain, the phospholipase C removed only the 3H-labeled fatty acids, whereas papain, that is known to cleave the C-terminal region, released all the radioactive glycolipid components including [3H]ethanolamine. More detailed analysis with shorter pulse-chase experiments demonstrated that placental alkaline phosphatase was primarily synthesized as a form with Mr 64,500 which was not yet labeled with [3H]palmitic acid. This form was converted by papain digestion to the above-mentioned major precursor with Mr 63,000. Taken together, these results suggest that placental alkaline phosphatase is initially synthesized as the precursor with Mr 64,500, which is immediately converted to the intermediate form with Mr 63,000 by simultaneously occurring proteolysis of the C terminus and replacement by the glycophospholipid, and finally to the mature form with Mr 66,000 by terminal glycosylation of its N-linked oligosaccharides. The glycophospholipid thus attached is considered to function as the membrane-anchoring domain of placental alkaline phosphatase.     

Proteins of the kidney microvillar membrane. Effects of monensin, vinblastine, swainsonine and glucosamine on the processing and assembly of endopeptidase-24.11 and dipeptidyl peptidase IV in pig kidney slices.

The effects of various inhibitors were studied on the biogenesis of endopeptidase-24.11 (EC 3.4.24.11) and dipeptidyl peptidase IV (EC 3.4.14.5) in slices of renal cortex, from piglets of the Yucatan strain, maintained in organ culture. These microvillar peptidases were synthesized within membrane compartments and underwent glycosylation to yield high-mannose and complex forms [the preceding paper, Stewart & Kenny (1984) Biochem. J. 224, 549-558]. Monensin caused very gross ultrastructural changes in the proximal-tubular cells, resulting from distension of the Golgi sacs. It blocked the processing of the high-mannose to the complex glycosylated forms of the peptidases and prevented their assembly in the microvillar membrane. Swainsonine, an inhibitor of alpha-mannosidase II, generated new 'hybrid' forms of the proteins, intermediate in Mr between the high-mannose and the complex forms, but did not prevent assembly of the hybrid forms in microvilli. Vinblastine, an agent that affects microtubules, delayed, but did not abolish, either the processing or the transport to microvilli. Glucosamine interfered with the initial glycosylation reactions and generated heterogeneous sets of partially glycosylated polypeptides of lower Mr than the high-mannose forms. These results are discussed in relation to the site and mechanism of glycosylation and the involvement of the Golgi complex and microtubules in the biogenesis of these membrane peptidases.     

Mechanism of biosynthesis of soluble and membrane-bound forms of dopamine beta-hydroxylase in PC12 pheochromocytoma cells

Dopamine beta-hydroxylase was present as 2 subunit forms (apparent Mr = 77,000 and 73,000) in the PC12 pheochromocytoma cell line as detected by immunoprecipitation from [35S]methionine-labeled cultures, and analyzed by sodium dodecyl sulfate gel electrophoresis and fluorography. The Mr = 77,000 form was present in a crude membrane fraction, while the Mr = 73,000 form was soluble. Both forms appeared to be present in approximately equal amounts, and both were glycosylated. Treatment of PC12 cells with tunicamycin, a potent inhibitor of core glycosylation in the endoplasmic reticulum, completely inhibited the appearance of the Mr = 77,000 and Mr = 73,000 forms, and 2 new immunoreactive polypeptides were obtained (apparent Mr = 67,000 and 63,000). Pulse-chase experiments suggested that the Mr = 77,000 form is initially synthesized (by 5 min) and a portion is converted in 15-90 min to the Mr = 73,000 form. Thereafter, the ratio between forms remains relatively constant, at least for several hours. Translation of mRNA from bovine and rat adrenals, and immunoprecipitation, indicated that dopamine beta-hydroxylase is initially synthesized as a single polypeptide (apparent Mr = 67,000). The subcellular site of biosynthesis of dopamine beta-hydroxylase was determined by isolation of mRNA from free and membrane-bound polysomes from bovine adrenal medulla. Translation in a cell free system and immunoprecipitation localized the synthesis of dopamine beta-hydroxylase on membrane-bound polysomes. These experiments suggest that both soluble and membrane-bound forms of dopamine beta-hydroxylase are synthesized and core glycosylated in the endoplasmic reticulum, and that there probably is a precursor-product relationship between the Mr = 77,000 and the Mr = 73,000 subunit forms of dopamine beta-hydroxylase.     

Reversible forskolin-induced impairment of sucrase-isomaltase mRNA levels, biosynthesis, and transport to the brush border membrane in Caco-2 cells

Hybridization analysis of mRNA with a cDNA probe for human sucrase-isomaltase, pulse-chase experiments with L-[35S]-methionine followed by SDS-PAGE, and immunofluorescence detection of sucrase-isomaltase were used to analyze the level(s) at which forskolin interferes with the expression of the enzyme in Caco-2 cells in culture. Three effects are observed in Caco-2 cells treated with forskolin: 1) a marked decrease in the level of sucrase-isomaltase mRNA, 2) a marked decrease in the biosynthesis of the enzyme without any alteration of its stability, and 3) an almost total inhibition of its transport to the brush border membrane. All three effects are reversible when the drug is removed from the culture medium, though this reversibility is asynchronous: transport to the brush border membrane resumes after 24 h, sucrase-isomaltase mRNA levels are back to the normal after 5 days, whereas the biosynthesis of the enzyme, although increasing progressively, remains lower than in control cells, even 10 days after removal of the drug. The possibility that some effects are directly dependent on cAMP and others a consequence of changes in glucose metabolism is discussed.     

Biosynthetic and glycosylation studies of cell surface platelet-derived growth factor receptors

The cell surface pool of metabolically labeled platelet-derived growth factor (PDGF) receptors in BALB/c3T3 fibroblasts was studied using an antiphosphotyrosine antibody. Exposure of intact cells to PDGF stimulates autophosphorylation of surface PDGF receptors and allowed immunoaffinity purification of only PDGF-activated receptors. Pulse-chase experiments demonstrated appearance of newly synthesized receptors in a surface activatable pool within 30-45 min of synthesis. In the absence of exogenous PDGF, the apparent half-life of this pool was 2 h. The presence of both N- and O-linked oligosaccharide chains on cell surface PDGF receptors was demonstrated. Enzymatic removal of the N-linked oligosaccharide chains reduced the receptor's apparent Mr by approximately 40 kDa and removal of O-linked oligosaccharide caused approximately a 7-kDa reduction. Activation of receptor tyrosine autophosphorylation by PDGF did not require either processing of high-mannose N-linked oligosaccharides to complex forms or the presence of sialic acid on receptor oligosaccharide chains. Tryptic cleavage of PDGF-activated surface receptors in intact cells yielded two discrete phosphotyrosine-containing fragments of 107 and 85 kDa. Cleveland digest patterns from each fragment indicate that both are derived from the intact PDGF receptor. These data indicate that PDGF receptors are synthesized and turn over rapidly in the absence of ligand. Partial characterization of the extracellular domain oligosaccharide contribution to receptor function and trypsin susceptibility is provided.     

Acyl-CoA: cholesterol acyltransferase in HT 29 cell subpopulations. Defect of activity in the undifferentiated cells

The ACAT activity was studied on different subpopulations deriving from HT29 cells, a human colon carcinoma cell line. Grown on standard medium (25 mM glucose), about 95% of these cells are undifferentiated (G + cells). From this heterogeneous population, differentiated cells were selected by glucose deprivation and grown either on medium without glucose (G - cells) or in standard medium containing 25 mM glucose (G-Rev cells). The G- and G-Rev cells have the features of differentiated small intestine cells. The two types of differentiated cells (G- and G-Rev) exhibited similar ACAT activities and the kinetic characteristics of the enzyme were also similar. A time-course study showed increasing activity during the exponential phase and a decrease just after confluency. It was possible to stimulate the enzyme by micellar or lipoprotein cholesterol. In contrast, the ACAT activity was hardly detectable in undifferentiated G + cells. In addition, all the experimental conditions known to stimulate ACAT activity, and confirmed in the differentiated HT29 cells, were inefficient in the undifferentiated G + cells. Therefore, the different models derived from HT29 cells provide the opportunity to study cholesterol esterification as well as the consequences of its aberrances in intestinal cells.     

The processing of asparagine-linked oligosaccharides in HT-29 cells is a function of their state of enterocytic differentiation. An accumulation of Man9,8-GlcNAc2-Asn species is indicative of an impaired N-glycan trimming in undifferentiated cells

Studies on the regulation of the enterocytic differentiation of the human colon cancer cell line HT-29, which is differentiated in the absence (Glc-) but not in the presence of glucose (Glc+), have recently shown that the post-translational processing of sucrase-isomaltase and particularly its glycosylation vary as a function of cell differentiation (Trugnan G., Rousset, M., Chantret, I., Barbat, A., and Zweibaum, A. (1987) J. Cell Biol. 104, 1199-1205). Other studies indicate that in undifferentiated HT-29 Glc+ cells there is an accumulation of UDP-N-acetylhexosamine, which is involved in the glycosylation process (Wice, B. M., Trugnan, G., Pinto, M., Rousset, M., Chevalier, G., Dussaulx, E., Lacroix, B., and Zweibaum, A. (1985) J. Biol. Chem. 260, 139-146). The purpose of the present work is to investigate whether an overall alteration of protein glycosylation is associated with the inability of HT-29 cells to differentiate. At least three alterations are detected: (i) after a 10-min pulse, the incorporation of D-[2-3H]mannose in undifferentiated cells is severely reduced, compared to differentiated cells. (ii) After a 24-h period of labeling with D-[2-3H]mannose, undifferentiated cells accumulate more than 60% of the radioactivity in the high mannose glycopeptides, whereas differentiated HT-29 Glc- cells accumulate only 38%. (iii) The analysis of the high mannose oligosaccharides transferred "en bloc" from the lipid precursor shows that Man9,8-GlcNAc2 species accumulate in undifferentiated cells, whereas no such accumulation can be detected in differentiated cells. This glycosylation pattern is consistent with an impairment of the trimming of high mannose into complex glycans. It is concluded that N-glycan processing is correlated with the state of enterocytic differentiation of HT-29 cells.     

Biosynthesis and processing of the type II insulin-like growth factor receptor in H-35 hepatoma cells

The biosynthesis and post-translational processing of the insulin-like growth factor II (IGF-II) receptor has been studied in H-35 hepatoma cells using a specific polyclonal anti-receptor immunoglobulin preparation. Cells were pulse-labeled with [35S]methionine followed by incubation with excess unlabeled methionine (chase). Gel electrophoresis of the immunoadsorbed receptors shows that the receptor is first synthesized as a 245-kDa precursor which is transformed to the mature 250-kDa form with a half-time of about 2 h. The 245-kDa precursor could also be labeled biosynthetically with [3H]mannose, only one-half of which was ultimately found associated with the 250-kDa product. Neuraminidase converts the 250-kDa receptor species to a 245-kDa form. Whereas the 250-kDa receptor is insensitive to detectable cleavage by endoglycosidase H, digestion of the 245-kDa species with this enzyme produces a 232-kDa form. A similar 232-kDa receptor species accumulates in H-35 cells incubated with tunicamycin (2 micrograms/ml). This tunicamycin-induced aglyco-receptor is not further processed to the 250-kDa form. Monensin (50 nM) blocks receptor processing at the 245-kDa stage. Endoglycosidase H treatment of the monensin-induced 245-kDa species indicates that this is a mixture of partially processed precursors having equivalent Mr. No evidence was obtained for the presence of O-linked oligosaccharides on the IGF-II receptor. The IGF-II binding activity of the three different biosynthetic forms of the receptor was assessed by affinity cross-linking of 125I-IGF-II to the receptors using disuccinimidyl suberate. Both the mature 250-kDa receptor and the neuraminidase-digested 245-kDa form specifically bound 125-I-IGF-II. However, the 232-kDa aglyco-receptor had no detectable IGF-II binding activity using this method. In summary, these studies show: 1) that the H-35 cell IGF-II receptor is synthesized first as a 245-kDa precursor having 4-6 high-mannose oligosaccharide side chains, 2) processing of the receptor oligosaccharides by mannose removal and terminal sialylation converts the 245-kDa precursor to the 250-kDa mature product which has been previously identified as the functional receptor in the plasma membrane, 3) the apparent molecular mass of the receptor in the absence of N-glycosylation is 232-kDa, and 4) glycosylation of the IGF-II receptor is required for the acquisition of IGF-II binding activity.     

The elastin associated glycoprotein gp115. Synthesis and secretion by chick cells in culture

Synthesis of gp115 by aorta smooth muscle cells and tendon fibroblasts isolated from chick embryos was investigated. gp115 was specifically immunoprecipitated by both polyclonal and monoclonal antibodies from cell lysates and culture medium of matrix free cells metabolically labeled with [3H]leucine and [35S]methionine. The component of gp115 isolated from the cell lysate had an apparent Mr in reduced sodium dodecyl sulfate polyacrylamide gels lower (105,000) than the protein isolated from the culture medium (Mr = 115,000). In immunoblot experiments, the latter corresponded in apparent Mr to the form isolated from chick tissues. gp115 was glycosylated in vitro; it was labeled with [3H]fucose, and when cells were cultured and labeled in the presence of tunicamycin, a lower Mr form with an apparent Mr = 90,000 was immunoprecipitated in both the cell lysate and the culture medium. In pulse-chase experiments, the intracellular and the extracellular forms were clearly suggestive of a direct precursor-product relationship in the absence of intermediate forms. The kinetics of secretion appeared very slow compared with that of other proteins of the extracellular matrix investigated in the same system; about 50-70% of gp115 in the form of the Mr = 105,000 species was still cell-associated after 4 h, whereas the half-time for secretion of fibronectin, type VI collagen, and tropoelastin was about 60 min, 3 h, and 60 min, respectively. Newly synthesized and processed cell-associated gp115 migrated in both reduced and non-reduced gels as a monomer. On the contrary, the secreted protein was present in the culture medium as large aggregates that did not enter the gel in the absence of reducing agents.     

Multiple cysteine proteinase forms during the life cycle of Dictyostelium discoideum revealed by electrophoretic analysis.

Proteinases of the cellular slime mould Dictyostelium discoideum have been analysed using electrophoresis on polyacrylamide gels containing gelatin (gelatin/PAGE). Multiple proteinase forms were apparent in vegetative myxamoebae, but the presence of individual enzyme forms depended on the manner in which the cells were grown. Axenic cells had a characteristic A-pattern of proteinases consisting of six bands, the most active enzymes having apparent Mr values of 51,000 and 45,000 (these have been named ddCP51 and ddCP45, respectively). Some of the proteinases were also present in the medium, the major extracellular form was ddCP42, a 42,000-Mr enzyme. Cells grown in association with bacteria had a distinct B-pattern with three main enzymes that had apparent Mr values of 48,000, 43,000 and 38,000. All of the A- and B-pattern proteinases were most active at acid pH in the presence of dithiothreitol and were inhibited by various agents such as trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane (E64), leupeptin and chymostatin, which inactivate cysteine proteinases. One of the enzymes, ddCP30, was identified as cysteine proteinase B which had been purified and characterized previously [North, M.J. & Whyte, A. (1984) J. Gen. Microbiol. 130, 123-134]. During starvation of axenic cells in shaken suspensions some of the vegetative proteinases disappeared, ddCP42 was released from the cells and one new enzyme with an apparent Mr of 48,000 appeared. Addition of cyclic AMP had little effect on these changes. When the axenically grown myxamoebae underwent development on filters, similar changes in band pattern were observed and the aggregation stage was characterized by the presence of three cysteine proteinase bands (apparent Mr values of 48,000, 45,000 and 43,000). Proteinases, especially ddCP42, were released from the cells and could be collected from the buffer-saturated pads which supported the filters. The results demonstrate that cysteine proteinases are present throughout growth and development of D. discoideum and that the forms present are subject to nutritional and developmental regulation.     

Multiple forms of human dopamine beta-hydroxylase in SH-SY5Y neuroblastoma cells

Dopamine beta-hydroxylase exists as three forms in human neuroblastoma (SH-SY5Y) cells. The membrane-bound form of the hydroxylase contains three different species with apparent relative molecular weights of 73,000, 77,000, and 82,000. The intracellular soluble form of dopamine beta-hydroxylase was present as a single species with an apparent molecular weight of 73,000. Pulse-chase experiments showed that membranous dopamine beta-hydroxylase contains two subunit forms of 73,000 and 77,000 after short chase times. The soluble hydroxylase was synthesized as a single species of 73,000 at approximately the same rate as the lower molecular weight species of the membranous enzyme. A constitutively secreted third form of the enzyme with an intermediate apparent molecular weight also incorporated [35S]sulfate, whereas no significant amount of [35S]sulfate was observed in the cellular forms of the enzyme. The [35S]sulfate was incorporated on N-linked oligosaccharides. Approximately 12% of the enzyme is released constitutively within 1 h. These results demonstrate that neuronal cells have the ability to constitutively secrete a specific form of dopamine beta-hydroxylase which may contribute to the levels of this enzyme found in plasma.     

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.     

Folding of intestinal brush border enzymes. Evidence that high-mannose glycosylation is an essential early event.

A polyvalent antiserum which precipitates the native, folded, but not the denatured molecular forms of pig intestinal aminopeptidase N (EC 3.4.11.2) and sucrase-isomaltase (EC 3.2.1.48, EC 3.2.1.10) was used to determine the kinetics of polypeptide folding of the two newly synthesized brush border enzymes. In pulse-labeled mucosal explants, complete synthesis of the polypeptide chains of aminopeptidase N and sucrase-isomaltase required about 2 and 4 min, respectively, whereas maximal antiserum precipitation was acquired with half-times of 4-5 and 8 min, respectively. Fructose, which induces a defective cotranslational high-mannose glycosylation, increased the half-time of polypeptide folding to about 12 min for aminopeptidase N as well as for sucrase-isomaltase. Short-pulse experiments suggested that fructose exerts its effect by slowing the rate of glycosylation, making this partially a posttranslational process. In the presence of fructose, not only the malglycosylated forms but also the electrophoretically normal, high-mannose-glycosylated form of the brush border enzymes were retained in the endoplasmic reticulum and proteolytically degraded. The results obtained demonstrate an intimate interrelationship between glycosylation and polypeptide folding in the synthesis of membrane glycoproteins and, more specifically, indicate that the timing of these two early biosynthetic events is essential for correct polypeptide folding.     

Expression of enzymatically active rat dipeptidyl peptidase IV in Chinese hamster ovary cells after transfection

Dipeptidyl peptidase IV (DPPIV) is a cell surface membrane glycoprotein expressed in many tissues. We have subcloned the coding region of a full-length cDNA for DPPIV into the inducible eukaryotic expression vector pMSG. The resulting construct was used to transfect Chinese hamster ovary (CHO) cells. Stable transformants were found to express DPPIV, and the expression is enhanced by dexamethasone. Metabolic labeling of the transfected cells with [35S]Met followed by immunoprecipitation revealed the presence of two specific products of apparent Mr 100,000 (100-kDa form) and 110,000 (110-kDa form), respectively. Pulse-chase experiments demonstrated that the 100-kDa form can be chased into the 110-kDa form, suggesting the 100-kDa form is the precursor of the 110-kDa form. Further studies with endo H treatment demonstrated that the carbohydrate structures are of the high-mannose type, and of the complex type for the 100- and 110-kDa forms, respectively. The 110-kDa form is present at the cell surface as shown by its accessibility to cell surface iodination. The DPPIV expressed on the cell surface is resistant to digestion by relatively high concentrations of trypsin. Studies also demonstrated that the surface DPPIV is fairly stable with a half-life for turnover of about 40 h. Furthermore, the DPPIV produced in the transfected cells displays specific dipeptidyl peptidase activity. The stably transfected cells that express enzymatically active DPPIV in an inducible manner will provide an excellent system for further biochemical, functional, and cell biological characterizations of DPPIV.