Isolation of plasma membranes from human skin fibroblasts

The biosynthesis of polyunsaturated fatty acids by rat sertoli cells

1. The biosynthesis of polyunsaturated fatty acids (PUFA) of the n-6 and n-3 series was investigated in cultured Sertoli cells. 18:2n-6, 18:3n-6, 20:2n-6, 18:3n-3 and 20:3n-3 were added individually at a concentration of 20 mumol to culture media. 2. Maximum incorporation of 20- and 22-carbon PUFA into membrane lipids was observed after 72 hr of incubation with all the exogenous substrates used. 3. As reported in other cell systems, the delta 6 desaturation was the first rate-limiting step; the major factor regulating this activity was the concentration of linoleic acid or alpha-linolenic acid in the medium. 4. Our data show that the delta 5-desaturation represents a second regulatory step in PUFA biosynthesis. 5. The sum of n-6 and n-3 PUFA of the 22 carbon chain length constantly represented between 11 and 12% of total fatty acids, regardless of the exogenous substrate used. 6. Our kinetic studies of the incorporation of PUFA of the n-6 and n-3 series did not permit detection of a delta 8 desaturase activity.

Metabolic utilization of linoleate and alpha-linolenate in cultured Sertoli cells

1. The capacity of cultured Sertoli cells to synthesize long-chain polyunsatured fatty acids (PUFA) from the essential fatty acid (EFA) precursors 18:2 n-6 and 18:3 n-3 was tested, and the concentrations of each EFA required to obtain maximal incorporation into membrane lipids were determined. 2. The two EFA were added to the culture medium as free fatty acids complexed to albumin in a molar ratio of 12:1. 3. When the substrates were added individually, the maximal levels of biosynthesis were obtained with 0.7 micrograms/ml of 18:2 n-6 and 2 micrograms/ml of 18:3 n-3. 4. When the two EFA were added together, clear alterations in the behavior of the desaturases with regard to the n-6 and n-3 fatty acids were observed. 5. It was found that a concentration of 0.35 micrograms/ml of each EFA represented the "ideal" required level in order to ensure optimal incorporation of 22-carbon PUFA into the membrane lipids. 6. These results provide the first data on the definition of EFA requirements for Sertoli cells in culture.

Identification and characterization of parathyroid hormone receptors on dog kidney, human kidney, chick bone and human dermal fibroblast. A comparative study of functional and structural properties

To evaluate the functional and structural characteristics of the parathyroid hormone (PTH) receptors on different tissues and the possible heterogeneity in structure and function, PTH receptors on dog kidney membrane, human kidney membrane, chick bone cell membrane and human dermal fibroblast membrane were evaluated. The results showed that human kidney plasma membrane, canine kidney plasma membrane and chick bone cell membrane possess one single class of PTH receptor with a Kd (dissociation constant) of 1-5 nM and an IC50 also of 1-5 nM. The number of binding sites was 800 fmol per mg of protein for chick bone cell particulate membrane, 1-5 pmol per mg of protein for human kidney plasma membrane and 2.2 pmol per mg of protein for dog kidney plasma membrane. Photoaffinity labelling identified a major binding component with a molecular mass of 70 kDa in all three types of membrane. The plasma membrane fraction from human dermal fibroblast contained two different binding sites for PTH with high (Kd = 2 nM) and low (Kd = 580 nM) affinities respectively. The IC50 for the adenylate cyclase is about 2 nM, which is similar to the Kd of the high-affinity site. Photoaffinity labelling also demonstrated a major binding component with a molecular weight of 70 kDa. We conclude that structural and functional similarity exists among the PTH receptors present on chick bone cell membrane, dog kidney membrane and human kidney membrane. The human dermal fibroblast possesses two different binding sites, one of which is coupled to adenylate cyclase.

Membrane-associated chondroitin sulfate proteoglycans of human lung fibroblasts

Cultured human fetal lung fibroblasts produce some chondroitin sulfate proteoglycans that are extracted as an aggregate in chaotropic buffers containing 4 M guanidinium chloride. The aggregated proteoglycans are excluded from Sepharose CL4B and 2B, but become included, eluting with a Kav value of 0.53 from Sepharose CL4B, when Triton X-100 is included in the buffer. Conversely, some of the detergent-extractable chondroitin sulfate proteoglycans can be incorporated into liposomes, suggesting the existence of a hydrophobic membrane-intercalated chondroitin sulfate proteoglycan fraction. Purified preparations of hydrophobic chondroitin sulfate proteoglycans contain two major core protein forms of 90 and 52 kD. A monoclonal antibody (F58-7D8) obtained from the fusion of myeloma cells with spleen cells of BALB/c mice that were immunized with hydrophobic proteoglycans recognized the 90- but not the 52-kD core protein. The epitope that is recognized by the antibody is exposed at the surface of cultured human lung fibroblasts and at the surface of several stromal cells in vivo, but also at the surface of Kupffer cells and of epidermal cells. The core proteins of these small membrane-associated chondroitin sulfate proteoglycans are probably distinct from those previously identified in human fibroblasts by biochemical, immunological, and molecular biological approaches.

Parathyroid hormone receptors in human dermal fibroblasts: structural and functional characterization

Previous studies have established the presence of parathyroid hormone (PTH)-sensitive adenylate cyclase activity in cultured human skin fibroblasts. The present study was undertaken to identify and quantitate PTH receptors directly in such cells. Human dermal fibroblast cell line CRL 1564 was found to possess specific binding sites for [125I]PTH(1-34). These sites bound PTH selectively; bovine and human PTH(1-34) and PTH(1-84) competed for [125I]PTH(1-34) binding sites, whereas the unrelated peptides calcitonin, insulin, AVP, angiotensin II, and ACTH(1-24) were inactive even at micromolar concentrations. Competitive binding experiments demonstrated the presence of binding site heterogeneity. These data fit a "two-site" model (p less than 0.001) in which one binding component has high affinity (Kd = 2.5 ng/ml = 0.6 nM) and low capacity (10(4) sites/cell) while the other has low affinity (Kd = 5.9 micrograms/ml = 1.5 microM) and high capacity (greater than 10(7) sites/cell). Similar high- and low-affinity [125I]bPTH(1-34) binding sites were seen also in CRL 1564 membranes containing a PTH-responsive adenylate cyclase. The Kd of the high-affinity sites was identical to the concentration of unlabeled bPTH(1-34) (4.2 ng/ml = 1.0 nM) required to half-maximally elevated cyclic AMP in CRL 1564 cells. Affinity labeling of specific PTH binding sites revealed the presence of multiple components with Mrs of 85, 70, 40, 33, and 23 kD on SDS-PAGE. Competition experiments did not disclose structurally discrete high- and low-affinity sites. Thus, structurally homologous PTH receptors in human skin fibroblasts apparently can assume two affinity states: (i) a high-affinity state coupled to adenylate cyclase and (ii) a low-affinity state that may represent uncoupled receptors.

Biochemical and genetic analysis of the OKa blood group antigen

The monoclonal antibody TRA-1-85 recognizes a cell surface antigen which is expressed by all human cell types tested, including red blood cells (RBCs), but not by mouse cells. All the human RBCs tested were TRA-1-85 positive except those with the rare phenotype Ok(a-). Oka is a blood group antigen of very high frequency and only three unrelated Ok(a-) people are known. The red cells of all three propositi were negative with the TRA-1-85 antibody. To confirm the relationship between the TRA-1-85 antibody and anti-Oka, the immune antibody found in the serum of Ok(a-) individuals, Western blot analysis was used: the TRA-1-85 antibody and anti-Oka gave identical but complex patterns of reactivity in Western blot analysis of human cell lysates or membranes. This suggests that the anti-Oka and TRA-1-85 antibodies recognize the same cell-surface determinant and implies that Oka is not restricted in its expression to the surface of RBCs but is expressed on white blood cells (WBCs) of Ok(a+) individuals and all human cell lines tested to date. WBCs from one of the Ok(a-) propositi were tested and found to be negative with the TRA-1-85 antibody. Finally, the species specificity of the TRA-1-85 antibody has been exploited by the use of somatic cell hybrids and DNA transfection techniques to examine the genetic control of the Oka antigen defined by the TRA-1-85 antibody. We report that the determinant is controlled by a single gene OK present on human chromosome 19.

5'-Nucleotidase in skin fibroblasts from patients with Duchenne muscular dystrophy

The 5'-nucleotidase of plasma membranes of cultured skin fibroblasts from patients with Duchenne muscular dystrophy had a reduced affinity for its substrate, 5'-AMP. The Arrhenius plot of the temperature dependence of this enzyme activity was normal. There was no difference between patients and controls in the specific 5'-nucleotidase activity in the whole cell homogenates.

Differential effects of 1-deoxynojirimycin on the intracellular transport of secretory glycoproteins of human hepatoma cells in culture

To investigate our earlier hypothesis that carbohydrates play a regulatory role in the intracellular transport of secretory glycoproteins, we used 1-deoxynojirimycin (DNJ), and inhibitor of glucosidase I and II of the rough endoplasmic reticulum (RER), to modify the structure of N-linked glycan moieties of secretory glycoproteins of human hepatoma (Hep G2) cells in culture. Using a pulse-chase protocol, we found that treatment of Hep G2 cultures with 1.25 mM DNJ markedly reduced the rate of secretion of alpha 1-protease inhibitor, ceruloplasmin, and alpha 2-macroglobulin, but had no effect on the export of fibronectin, alpha-fetoprotein and transferrin, nor on albumin which lacks carbohydrate. For example, 50% of newly synthesized alpha 1-protease inhibitor, the glycoprotein most dramatically affected, was secreted by 27 min in control cultures versus 110 min in DNJ-treated cultures. Percoll gradient cell fractionation analyses revealed that DNJ inhibited transport of the affected secretory glycoproteins in the RER segment of the ER/Golgi pathway. For example, 50% of newly synthesized alpha 1-protease inhibitor was lost from the RER fraction by 10 min in untreated cells, but 70 min was required for the transport of a similar amount of protein in DNJ-treated cells. DNJ treatment also inhibited the rate at which the N-linked glycan moieties of the affected glycoproteins became resistant to endo H in the Golgi. Since the glycan moiety of secreted forms of the affected glycoproteins were fully processed to the complex structure, suggesting escape from DNJ inhibition, we concluded that removal of terminal glucose residues from the glycan chain of secretory glycoproteins is required for their transport from the RER to the Golgi. We suggest that the oligosaccharide moieties on alpha 1-protease inhibitor, ceruloplasmin and alpha 2-macroglobulin form part of the binding site for a receptor which regulates transport of these glycoproteins.

Heparan sulfate proteoglycans of human lung fibroblasts. Occurrence of distinct membrane, matrix and secreted forms

Human lung fibroblasts (HLF) were labeled with 35SO2-4 for 48 h and extracted with a guanidinium chloride buffer. A fraction of the extracted heparan sulfate proteoglycan (HSPG) appeared micelle-associated. In the absence of detergent these HSPG eluted in the void volume of Sepharose CL2B columns. In the presence of detergent these HSPG were included in Sepharose CL2B (Kav = 0.55) and 4B (Kav = 0.3) columns. This type of HSPG was specifically associated with isolated HLF cell membranes, suggesting that it may represent a fraction of integral membrane proteoglycans. Most of the HSPG in the HLF monolayers, however, eluted in the included volume of Sepharose CL2B (Kav = 0.4) and CL4B (Kav = 0.1) columns in the absence of detergent. This type of HSPG was not affected by detergent and was specifically retained in 'extracellular matrix' preparations. The medium of HLF monolayers contained HSPG of similar Mr as the membrane-associated HSPG. Of these three distinct HSPG fractions only the membrane-associated form could be incorporated in liposomes, confirming that the HSPG in this fraction may be integral membrane components.

Lysosomal membrane dynamics: structure and interorganellar movement of a major lysosomal membrane glycoprotein

The biochemistry and intracellular transit of an integral membrane glycoprotein of chicken fibroblast lysosomes were studied with monoclonal antibody techniques. The glycoprotein had an apparent molecular weight of 95,000-105,000. Structural analysis involving metabolic labeling with [35S]methionine and cleavage with glycosidases revealed the presence of numerous oligosaccharide chains N-linked to a core polypeptide of apparent molecular weight 48,000. A primary localization of the glycoprotein to lysosomes was demonstrated by the coincidence of antibody binding sites with regions of acridine orange uptake, electron immunocytochemical labeling on the inner surface of lysosome-like vacuolar membranes, and preferential association of the glycoprotein with lysosome-enriched subcellular fractions from Percoll gradients. In addition, small quantities of the glycoprotein were detected on endocytic vesicle and plasma membranes. To study the intracellular pathway of the glycoprotein, we used a monoclonal antibody whose binding to the glycoprotein at the cell surface had no effect on the number or subcellular distribution of antigen molecules. Incubation of chicken fibroblasts with monoclonal antibody at 37 degrees C led to the rapid uptake and subsequent delivery of antibody to lysosomes, where antibody was degraded. This process continued undiminished for many hours on cells continuously exposed to the antibody and was not blocked by the addition of cycloheximide. The rate at which antigen sites were replenished in the plasma membrane of cells prelabeled with antibody (t1/2 = 2 min) was essentially equivalent to the rate of internalization of antibody bound to cell surfaces. These results suggest that there is a continuous and rapid exchange of this glycoprotein between plasma membrane and the membranes of endosomes and/or lysosomes.

Transport competence of plasma membrane vesicles from cultured human fibroblasts

We obtained plasma membranes from cultured human skin fibroblasts. The preparation was enriched 10-fold with about 40 percent yield. There was minimal contamination with other cell membranes. Various observations indicated vesicular conformation of a portion of the plasma membranes, notably by electron microscopy and from the effect of osmotic pressure on the distribution of solutes between mass and medium at equilibrium. Other studies indicated that these fibroblast plasma membrane vesicles retained mediated transport processes for a variety of substrates. The evidence included: stereospecific and temperature-dependent uptake of glucose; dependence of L-alanine uptake on sodium ion and an inward-directed transmembrane Na+ gradient; stimulation of L-alanine uptake, with overshoot, by enhancement of the interior-negative transmembrane potential; concentration dependent uptake of methotrexate with apparent competitive inhibition by folinic acid; stimulation of L-lysine uptake by trans-L-arginine. These findings indicate that human fibroblast plasma membrane vesicles could be used to study membrane transport processes and, perhaps, expression of mutant genes that cause inborn errors of transport.

Subcellular fractions in rubella immunoassays

Rubella virus-infected cells were fractionated by differential and sucrose gradient centrifugations. Rubella virus antigens distributed into all fractions but particulate material in the 100,000 x g pellet was shown to be enriched about two-fold for rubella virus antigen. Similarly, sucrose gradient fractions for rough endoplasmic reticulum and smooth cellular membranes were enriched for rubella virus antigens. The 100,000 x g pellet and the isolated cellular membranes proved to be useful when different fractions were used in solid-phase immunoassays for rubella virus-specific IgG or IgM. These fractions were equal in quality of the semipurified rubella virus preparations in the IgG assays but inferior to those in the IgM assays. However, simultaneous use of 35/25% sucrose fractions from infected and non-infected cells reveals non-specific binding of IgM to the antigens and renders the IgM tests more specific for rubella virus.

Characterization of plasma membranes and rough endoplasmic reticulum of synovial cells cultured from rheumatoid arthritis patients

[35S]methionine-labelled plasma membrane (PM) and rough endoplasmic reticulum (RER) proteins were analysed from synovial cell cultures derived from rheumatoid arthritis (RA) and reference patients. Pure RER was prepared by a modification of CsCl-containing sucrose-cushion method. The PM proteins were resolved into 30-35 bands using sodium dodecyl sulphate-electrophoresis in polyacrylamide gradient gels. The major polypeptides had apparent molecular masses of 170, 140, 110, 60, 40, 33, 23 and 14 kilodaltons. The major RER polypeptides had the apparent molecular masses of 170, 74, 58 and 35 kD. No reproducible differences were demonstrated in the polypeptide patterns or in the specific activities of the PM and ER (endoplasmic reticulum) marker enzymes between cells from RA and reference patients. The results reflect similarities of membrane structures in RA and reference cells, although differences have been reported in their metabolism.

Order measurements in plasma membranes from Duchenne dystrophy fibroblasts

Plasma membranes have been isolated using different methods from Duchenne dystrophy and control human skin fibroblasts. Fluorescence techniques were utilized to resolve the rotational properties and the degree of hindered rotation of the fluorescent probe, 1,6-diphenyl-1,3,5-hexatriene in the membranes. Under specific conditions of fibroblast processing and membrane fractionation, plasma membranes from Duchenne fibroblasts showed significantly less order (0.0125 greater than P less than 0.0025) and less hindrance to probe rotation than membranes from control fibroblasts. The order differences did not seem to be the result of heterogeneity in the membrane environment sampled by the probe. The frequency dependence of the fluorescence lifetime for diphenylhexatriene indicated no measurable contribution by a short lifetime component. Analysis of diphenylhexatriene rotation in the plasma membranes using the 'wobbling-in-cone' theory suggested that both the angle of probe rotation (theta c) and the rotational rate (Dw) were important parameters in understanding the variations between Duchenne and control membranes at 16, 22 and 30 degrees C. Electron spin resonance studies with 5'-doxylstearic acid at 25 degrees C confirmed our fluorescence results. The segmental motion exhibited by the spin label revealed less order in the Duchenne membranes.

Deficient guanine nucleotide regulatory unit activity in cultured fibroblast membranes from patients with pseudohypoparathyroidism type I. a cause of impaired synthesis of 3',5'-cyclic AMP by intact and broken cells

Deficient activity of the guanine nucleotide regulatory protein (G unit), an integral component of the membrane-bound adenylate cyclase complex, has been implicated as the biochemical lesion in many patients with pseudohypoparathyroidism (PHP) type I. In addition to renal resistance to parathyroid hormone in this disorder, there is decreased responsiveness of diverse tissues to hormones that act via 3',5'-cyclic AMP (cAMP). To assess whether a deficiency of G units could account for impaired adenylate cyclase activity, we studied cAMP production in intact cultured fibroblasts and fibroblast plasma membranes from five patients with PHP in response to several activators of adenylate cyclase. The number of G units in PHP fibroblast membranes, measured by cholera toxin-dependent [(32)P]ADP ribosylation of G-unit peptides, as well as the G-unit activity, determined by the ability of detergent extracts to reconstitute adenylate cyclase activity in G-unit-deficient S49 CYC(-) membranes, were found to be markedly reduced compared with control membranes (43 and 40%, respectively), The activation of fibroblast membrane adenylate cyclase by effectors that act directly through the G unit (guanosine triphosphate, guanosine 5'-0-[3-thiotriphosphate] [GTP-gamma-S], NaF) was significantly greater in control membranes than in membranes from patients with PHP. Moreover, we found that hormone (prostaglandin E(1)) stimulated adenylate cyclase activity was also greater in control membranes than in PHP membranes. Neither the apparent affinity of membrane adenylate cyclase for GTP-gamma-S (apparent K(m) =5 X 10(-8) M) nor the rate of enzyme activation by GTP-gamma-S was significantly different in fibroblast membranes from control subjects and patients with PHP. In contrast to the notable differences in hormone and G-unit-activated adenylate cyclase shown in fibroblast membranes from PHP patients and control subjects, the intrinsic catalytic activity of membranes, as determined by forskolin-stimulated adenylate cyclase, was not significantly different in the two groups. Intact fibroblasts derived from patients with PHP accumulated significantly (P 0.001) less cAMP (46+/-21 pmol cAMP/mcg DNA, n = 5) than cells from normal individuals (170+/-51 pmol cAMP/mcg DNA, n = 11) when stimulated with PGE(1). PGE(1)-stimulated accumulation of cAMP by intact fibroblast monolayers correlated closely with PGE(1) plus GTP-activated membrane adenylate cyclase activity in both patients and controls (r = 0.97, P < 0.001). Our data show that, in patients with PHP, (a) fibroblast membranes show a decreased complement of G units, (b) membrane catalytic activity is normal, but adenylate cyclase activity is reduced when stimulated by hormone or by effectors which activate the G unit, (c) the ability of cells to accumulate cAMP in response to hormone stimulation is reduced, and (d) reduced membrane adenylate cyclase activity correlates well with impaired cellular cAMP synthesis. These results, taken together, indicate that a deficiency of G-unit activity can impair synthesis of cAMP by both intact and broken cells, and may explain the resistance of multiple tissues to hormones that act via cAMP observed in PHP.

The purification of plasma membranes from WI-38 fibroblasts: effects of ageing on their composition

A three-step method for the purification of plasma membranes from WI-38 fibroblasts was developed thus allowing the recovery of 36--44% of the plasma membrane. Except in the case of galactosyltransferase, the activity of the contaminating enzymes was very low. Morphological observations confirm the presence of a homogeneous population of vesicles. Preparations obtained from young and old cell cultures were compared for their enzymatic and protein contents. With ageing the activity of 5-nucleotidase significantly increases whereas that of alkaline phosphodiesterase I decreases. Out of the 26 components detected after sodium dodecyl sulphate polyacrylamide gel electrophoresis, four decreased but only one increased. Cellular ageing seems to fulfil a specific and localized effect on the plasma membrane.

Isolation and characterization of the plasma membranes of cultured lymphoblasts from patients with cystic fibrosis and normal individuals

Plasma membranes were isolated from two cystic fibrosis lymphoblastoid cell lines and two age and sex-matched normal cell lines after controlled cell disruption. The preparations were enriched approx. 20-fold in plasma membrane markers and were essentially free of markers for other organelles except the Golgi-localized enzyme, galactosyltransferase. Sodium dodecyl sulphate polyacrylamide gel electrophoresis showed virtually identical polypeptide profiles after staining with Coomassie blue. Membranes from normal cells contained 0.49--0.67 mumol/mg of total phospholipid/mg membrane protein and those from cystic fibrosis cells, 0.53-0.67 mumol. The cholesterol content was 0.35-0.41 ummol/mg protein for normal, and 0.43--0.49 mumol for cystic fibrosis cell membranes. The proportions of individual phospholipid classes and their fatty acid compositions as well as that of the total membrane lipid also did not differ significantly in the two cell types. The bulk lipid fluidity as reflected by the fluorescence polarization of beta-parinaric acid and diphenylhexatriene was also not changed. The carbohydrate composition of cell surface glycopeptides released with papain was not consistently different with the normal and cystic fibrosis cells. The quantity of fucose in intact plasma membranes was also quantitated enzymatically and found to be very similar in both cell types.

(Mg2+ + Ca2+)-ATPase activity in plasma membrane enriched preparations of human skin fibroblasts: decreased activity in fibroblasts derived from cystic fibrosis patients

Plasma membrane enriched preparations obtained from cultured human skin fibroblasts by differential centrifugation and sucrose density centrifugation techniques were found to contain a Mg2+-dependent Ca2+-stimulated ATPase activity. The specific activity of the (Mg2+ + Ca2+)-ATPase present was 4-5-fold higher than that present in crude membrane preparations and 80-100-fold higher than that present in homogenates. The (Mg2+ + Ca2+)-ATPase activity of both crude and plasma membrane enriched preparations of cultured fibroblasts from cystic fibrosis patients was significantly reduced compared to that activity observed in age-matched controls. This study corroborates our previous observations made in crude homogenate preparations of fibroblasts and indicates another cell type where (Mg2+ + Ca2+)-ATPase activity may be altered in cystic fibrosis.

Plasma membrane lipids of human diploid fibroblasts from normal individuals and patients with cystic fibrosis

The lipid composition of isolated plasma membranes of human skin fibroblasts is described for the first time. Plasma membranes from a number of strains of fibroblasts from patients with cystic fibrosis and matched normals were isolated by a recently described procedure and analysed for major phospholipid classes, cholesterol and fatty acids. No differences in the quantities of these compounds were detected between cells of the two different origins. The fetal calf serum used to supplement the growth medium contained relatively more palmitoleate and oleate but less stearate than the membranes. There were also no consistent differences between cystic fibrosis and normal membranes in terms of the fatty acid compositions of their individual phospholipid classes. Consistent with this lack of chemical change in the lipids of membranes of cystic fibrosis cells, the degree of fluorescence polarization of diphenylhexatriene, an index of fluidity, was also unchanged.