Analysis of the mitogenic effect of fetuin preparations on arterial smooth muscle cells: the role of contaminant platelet-derived growth factor

Characterization of intact N- and O-linked glycopeptides using higher energy collisional dissociation

Simultaneous elucidation of the glycan structure and the glycosylation site are needed to reveal the biological function of protein glycosylation. In this study, we employed a recent type of fragmentation termed higher energy collisional dissociation (HCD) to examine fragmentation patterns of intact glycopeptides generated from a mixture of standard glycosylated proteins. The normalized collisional energy (NCE) value for HCD was varied from 30 to 60% to evaluate the optimal conditions for the fragmentation of peptide backbones and glycoconjugates. Our results indicated that HCD with lower NCE values preferentially fragmented the sugar chains attached to the peptides to generate a ladder of neutral loss of monosaccharides, thereby enabling the putative glycan structure characterization. In addition, detection of the oxonium ions enabled unambiguous differentiation of glycopeptides from non-glycopeptides. In contrast, HCD with higher NCE values preferentially fragmented the peptide backbone and, thus, provided information needed for confident peptide identification. We evaluated the HCD approach with alternating NCE parameters for confident characterization of intact N- and O-linked glycopeptides in a single liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. In addition, we applied a novel data analysis pipeline, so-called GlycoFinder, to form a basis for automated data analysis. Overall, 38 unique intact glycopeptides corresponding to eight glycosylation sites (six N-linked and two O-linked sites) were confidently identified from a standard protein mixture. This approach provided concurrent characterization of both the peptide and the glycan, thereby enabling comprehensive structural characterization of glycoproteins in a single LC-MS/MS analysis.

The effect of platelet-derived growth factor and adipogenic hormones on the expression of CCAAT/enhancer-binding proteins in 3T3-L1 cells in serum-free conditions

In the absence of serum and serum substitutes, insulin at high doses together with platelet-derived growth factor BB (PDGF BB), corticosterone, and 3-isobutyl-1-methyl-xanthine is required to stimulate differentiation of 3T3-L1 preadipocytes. Under these conditions the differentiating cells express the CCAAT/enhancer-binding proteins (C/EBP) C/EBPdelta, C/EBPbeta, and C/EBP alpha with a similar sequence as described for differentiating cells in the presence of serum. The major differences detected under serum-free conditions are as follows: (a) PDGF BB is the major stimulator of the expression of the C/EBPbeta isoform liver activator protein (LAP). (b) The expression of LAP is also increased in preconfluent, proliferating cells due to the treatment with PDGF BB. (c) A small protein of 20 kDa (p20C/EBPbeta is detected with the anti-C/EBPbeta antibody. It is synthesized at high levels in such cells, which subsequently express high levels of the differentiation markers C/EBP alpha and glycerol-3-phosphate dehydrogenase. (d) Treatment of cells with fibroblast growth factor-2 (bFGF) in addition to adipogenic hormones results in differentiation and C/EBP alpha expression only to a very moderate extent as compared to treatment with PDGF BB but leads to a strong expression of both C/EBPbeta and C/EBPdelta.

Ligation of the alpha 2-macroglobulin signalling receptor on macrophages induces protein phosphorylation and an increase in cytosolic pH

We have recently described an alpha 2-macroglobulin (alpha 2M) signalling receptor which is distinct from the low-density lipoprotein-related protein/alpha 2M receptor (LRP/alpha 2MR). Ligation of the macrophage signalling receptor by alpha 2M-methylamine stimulates production of several second messengers and involves a pertussis toxin-insensitive G-protein. We now report that binding of alpha 2M-methylamine, or the cloned M(r) = 20,000 receptor-binding fragment from rat alpha 1M, to macrophage alpha 2M signalling receptors induces protein phosphorylation. By use of a monoclonal antibody to phospholipase C gamma l (PLC gamma l) we were able to identify it as one target for protein phosphorylation. Phosphorylation was time and concentration dependent, being optimal at about 60 s of incubation and a 100-200 nM ligand concentration. By use of a second monoclonal antibody directed against phosphotyrosine, we were able to demonstrate that at least a portion of the label was incorporated into one or more tyrosine residues. PLC gamma l phosphorylation was then studied in membrane preparations at 4 degrees C in order to minimize serine or threonine modification. Preincubation of macrophage membranes with genistein, a tyrosine kinase inhibitor, drastically reduced phosphorylation of PLC gamma l. Receptor-associated protein, which blocks alpha 2M binding to LRP/alpha 2MR but not to the alpha 2M signalling receptor, had no effect on alpha 2M-methylamine-induced tyrosine phosphorylation of PLC gamma l. Binding of lactoferrin to LRP/alpha 2MR failed to induce phosphorylation of PLC gamma l, further supporting the hypothesis that the alpha 2M signalling receptor and LRP/alpha 2MR are distinct entities. Growth factors which induce tyrosine phosphorylation typically cause a rise in cytosolic pH. Binding of a2M-methylamine to macrophages also gradually increased the intracellular pH in a concentration-dependent manner, being optimal at a 200 nM ligand concentration. The increase in pH was amiloride sensitive. We propose that receptor-recognized forms of a2M may function like growth factors with regard to macrophage regulation.

GRASP: a novel heparin-binding serum glycoprotein that mediates oligodendrocyte-substratum adhesion

Cell-substratum adhesion plays a crucial part in the cascade of events that control growth or turn on and consummate a differentiation program. We are investigating the molecular basis of oligodendrocyte (OLG) cytodifferentiation, employing pure cultures of OLGs isolated from postmyelination brains. We have shown that such OLGs will regenerate in vitro and reenact the ontogenic development of myelin, but to do so they need a signal. Adherence to a polylysine surface in the presence of 20% horse serum generates such a signal. Among the events that are turned on upon OLG adhesion is the phosphorylation of myelin basic protein; no such phosphorylation takes place in the non-adhered cell. We postulated that horse serum provides an adhesion molecule. Laminin, fibronectin, collagen and native vitronectin failed to replace horse serum. Hence, we set out to fractionate horse serum by screening with an adhesion assay. We report here the identification, purification and partial characterization of a novel, heparin-binding horse serum glycoprotein that we have termed Glycine-Rich Adhesion Serum Protein--GRASP--to stress the fact that this protein has a high content of glycine and functions, in vitro, as an adhesion molecule for OLGs. There is 61% similarity at the N-terminus between GRASP and histidine-rich glycoprotein precursor (HRGP), an alpha 2-glycoprotein from human plasma. However, our data suggest that GRASP is not the horse serum homolog of HRGP. First, the two Gps are functionally distinct: HRGP does not promote the adhesion of OLGs. Second, the amino acid compositions differ significantly, e.g., GRASP is not histidine- but rather glycine-rich. Third, the region of sequence similarity between GRASP and HRGP is conserved throughout the cystatin superfamily. Fourth, anti-Gp55 polyclonal Abs recognize a similar set of polypeptides--save for slight differences in M(r)-in human serum as in horse serum, indicating that HRGP and GRASP are two distinct but related proteins and are both present in human and horse sera. GRASP is a dimer trimer of seemingly identical subunits of M(r) approximately 55,000 ; the native protein has an M(r) x 10(-3) approximately 120-140, of which 24-27% is contributed by carbohydrate. Using GRASP as a substratum allows the growth of OLGs in serum-free medium. GRASP is as good an effector of myelin basic protein phosphorylation as 20% horse serum. We conjecture that the mechanism of GRASP function features: 1) exposure of a cryptic sequence--after a change in conformation induced upon binding to polylysine--with affinity for an OLG signal-transducing receptor; and 2) interaction of its heparin-binding domain with OLG surface heparin sulfate proteoglycans and/or the aforementioned receptor.

A serum-free defined medium capable of supporting growth of four established human prostatic carcinoma cell lines

This paper describes a serum-free defined medium (Gc) that was initially designed to support growth of the human prostatic carcinoma cell line LNCaP. Our studies indicate that this medium formulation is capable of supporting short-term, long-term, and clonal growth of the LNCaP cell line. Component deletion experiments have shown that the three most critical components for LNCaP short-term growth are insulin, triiodothyronine (T3), and fetuin. Additionally, this medium was found to support short-term and clonal growth of three other human prostatic carcinoma cell lines, DU 145, PC-3, and ALVA-31. The availability of such a medium should aid in the distinction of the regulatory factors involved in growth and differentiation of malignant prostatic epithelium.

Fetuin--an old friend revisited

Bovine fetuin, the first fetal protein to be described, has recently been shown to be a species homologue of a well known human plasma protein--alpha 2HS glycoprotein (alpha 2HS). The fetuins are now known to be members of the cystatin superfamily. The structural properties of the six fetuins that have been fully sequenced are compared. Despite the structural homology of these proteins, their described properties in the literature make them appear to be quite different. The diverse in vitro properties claimed for fetuin/alpha 2HS are reviewed. In vivo, fetuins are involved in the acute phase response. In development, in all species studied so far, fetuins are present in a specific cell population that forms the developing neocortex. The possible functional significance of this distribution is discussed.

The nucleotide and deduced amino acid structures of sheep and pig fetuin. Common structural features of the mammalian fetuin family

This study was initiated to gain further insight into the structural features of the mammalian fetuin family. The cDNA structures of sheep and pig fetuin were determined. The cDNA insert encoding sheep (pig) fetuin comprised 1550 (1470) nucleotides, including 54 (46) nucleotides encoding a signal peptide of 18 (15) residues and 1038 (1041) nucleotides encoding the 346 (347) amino acids of the mature plasma protein. The predicted amino-terminal sequence of the mature pig fetuin was confirmed by the amino-terminal sequence of the purified protein. However, two alternative sheep amino-terminal sequences were found in fetuin purified from the plasma of a single sheep fetus; the minor product was the one predicted by comparison with other fetuin sequences while the major product was two amino acids longer. Comparison of the deduced amino acid sequences of sheep and pig fetuin showed an extensive sequence identity between them (75%) and with other proteins of the mammalian fetuin family, i.e. human alpha 2-HS glycoprotein, and bovine and rat fetuins. Twelve cysteine residues were found at invariant positions in all fetuin sequences, suggesting strongly that the arrangement of disulphide bridges identified in human alpha 2-HS glycoprotein is common to the members of the family. Further sequence comparisons revealed that the structures of mammalian fetuins are organised in three domains: two cystatin-like domains (D1 and D2) and a complex carboxyl-terminal domain (D3). The proposed three-domain structure of the protein is reflected in the organisation of the rat fetuin structural gene which has recently been published.

Fetuin and alpha-2HS glycoprotein induce alkaline phosphatase in epiphyseal growth plate chondrocytes

A previously described chondrocyte alkaline phosphatase induction factor (CAP-IF) for chicken epiphyseal growth plate chondrocytes has been purified to SDS-PAGE homogeneity from fetal bovine serum by ammonium sulfate precipitation and by dye-ligand affinity (Affi-Gel Blue and Reactive Green-19 agarose) and hydroxyapatite column chromatographies. As determined by immunoprecipitation of [35S]methionine-labeled cellular proteins after 3 day treatment, this highly purified CAP-IF increases the level of AP and certain other membrane proteins 2- to 3-fold over control values. The pure protein of apparent 64.5 kDa molecular weight has been identified as fetuin by N-terminal amino acid sequencing. This was confirmed by the finding that high alkaline phosphatase (AP)-inducing activity is present in fetuin prepared by the Spiro method. However, fetuins prepared by the Pedersen or Deutsch procedures are inactive. At least half of the CAP-IF activity of fetuin was irreversibly destroyed by treatment with EDTA and addition of Zn2+ did not reactivate the EDTA-treated fetuin. Ascorbate synergistically enhanced the effect of fetuin on chondrocyte AP activity by over 8-fold during 3 day exposure. Because of the very high homology between fetuin and the A-chain of alpha 2-HS glycoprotein, we also tested and found that alpha 2HS glycoproteins from human serum and bovine bone are both strong AP inducers. Our findings suggest that the AP-inducing activity resides in a labile, cystatin/Zn(2+)-binding domain common to these related serum glycoproteins. These proteins appear to play a role in enhancing AP expression in normal growth plate cartilage differentiation.

Separation of growth-promoting activity for human muscle cells from fetuin

Whether the growth-promoting activity of Pedersen fetuin is due to fetuin itself or to a contaminant(s) has been a long-standing puzzle. The possibility that the growth-promoting activity of Pedersen fetuin for human muscle satellite cells (HMSC) could be caused by some other component of fetal bovine serum (FBS) that remained in the fetuin as a contaminant has been investigated. One liter of FBS was first precipitated with 50% saturated ammonium sulfate, which leaves the serum albumin in solution, and then with 25% polyethylene glycol, which leaves the fetuin in solution, to generate a fraction 50 PEG 2x that was enriched 11-fold in growth-promoting activity for HMSC, with 68% recovery of total activity. Further purification with FPLC anion exchange chromatography achieved 99-fold enrichment of the activity with 30% overall recovery. The activity is heat labile and pH sensitive, suggesting that it is of protein nature, and the size of the activity is above 70 kDa. SDS-PAGE of the most active fractions shows that they are virtually free of fetuin. Thus, although the active fractions are not homogeneous, these studies demonstrate that the growth-promoting activity for HMSC can be fully separated from fetuin.

PDGF-stimulated fibroblast proliferation is enhanced synergistically by receptor-recognized alpha 2-macroglobulin

alpha-Macroglobulins derived from plasma or secreted by macrophages are platelet-derived growth factor (PDGF) binding proteins that compete with cell-surface receptors on fibroblasts for PDGF binding. alpha 2-Macroglobulin (alpha 2M) derived from bovine plasma was tested for its ability to modulate the PDGF-induced proliferation of primary passage rat lung fibroblasts (RLFs) and a human skin fibroblast cell line (CRL 1508). Fibroblasts were grown in 10% fetal bovine serum (FBS) for 24 hr, then washed with serum-free medium before adding serum-free defined medium (SFDM) containing insulin and transferrin. To this medium were added varying concentrations of human plasma-derived AB-PDGF and alpha 2 M, alone or in combination. Receptor-recognized alpha 2M was prepared by treatment with methylamine. Both native alpha 2M and the alpha 2M-methylamine (alpha 2M-MA) were tested for growth promoting activity in the absence or presence of PDGF. After 3 days, a concentration-dependent growth curve of fibroblast proliferation was demonstrated for PDGF alone, with near maximal stimulation reached at 15-20 ng/ml PDGF. alpha 2M and alpha 2M-MA alone had no effect on cell proliferation. However, alpha 2M-MA concentrations above 32 micrograms/ml synergistically enhanced PDGF-stimulated proliferation greater than 100% in the presence of 15 ng/ml PDGF. Native alpha 2M enhanced PDGF-stimulated growth 80-100% above PDGF controls only at low concentrations (32-64 micrograms/ml alpha 2M). High concentrations of native alpha 2M (128-256 micrograms/ml) either had no effect on growth or were inhibitory to PDGF-stimulated growth, depending on the cell type tested. Rat lung fibroblasts were shown to secrete a factor(s) that inhibited the trypsin-binding capacity of native alpha 2M. We further demonstrated that early passage RLFs possess specific cell-surface receptors for [125I]-PDGF and [125I]-alpha 2M-MA, and preincubation of RLFs with alpha 2M-MA increased the specific binding of [125I]-PDGF to the cell surface of these fibroblasts. Considered together, these data support the view that receptor-recognized alpha 2M synergistically enhances the proliferative capacity of PDGF. We postulate that receptor-recognized alpha Ms enhance PDGF-stimulated growth by increasing the local concentration of PDGF at the cell surface, where the PDGF could be released in close proximity to its own receptors.

Novel action of carnitine: inhibition of aggregation of dispersed cells elicited by clusterin in vitro

A novel effect of carnitine and O-acylcarnitine derivatives has been described. The presence of these compounds has been shown to inhibit the aggregation of erythrocytes otherwise elicited by the addition of clusterin or fetuin. The specificity of carnitine action has been investigated by comparing influences of chemically related compounds. The concentrations required for inhibition by approximately 50% of aggregation of erythrocytes by clusterin under in vitro conditions defined were determined to be 1.5 mM for L(-) or D(+) enantiomers of carnitine; 0.5 mM for decanoyl(-)- or (+)-carnitine; 0.13 mM for lauroyl(-)- or (+)-carnitine, and 0.05 mM for myristoyl(-)- or (+)-carnitine. In contrast, concentrations up to 12.5 mM of dimethylcarnitine, deoxycarnitine, acetylcholine, acetyl-beta-methylcholine, or inositol had no detectable inhibitory effect on aggregation elicited by clusterin. Clusterin addition also resulted in the aggregation of three other cell types examined (guinea pig spermatozoa, a cell line derived from testes of neonatal mice called TM4 cells, and Sertoli cells from testes of 20 day-old rats). As in the case with erythrocytes, the presence of carnitine inhibited aggregation of spermatozoa, TM4 cells, and Sertoli cells in suspension. We consider possible mechanisms by which carnitine inhibits aggregation of erythrocytes and other populations of dispersed cells incubated in the presence of clusterin.

Activation of single-channel currents in mouse fibroblasts by platelet-derived growth factor

A nonselective cation channel that we characterized in the mouse L-cell membrane becomes quiescent with serum deprivation (arrested cell growth) and rapidly active upon readdition of serum or, specifically, platelet-derived growth factor (PDGF). Using the patch-clamp technique, we find that the predominant channel in the LMTK- cell line is a bursting nonselective cation channel (the NS channel). In cell-attached and inside-out patches, the channel has a conductance of 28 pS; equal selectivity for Na+, K+, and Cs+; and no anion or divalent cation permeability. The channel open probability is voltage insensitive and in inside-out patches does not correlate with intracellular calcium (0.5 nM to 50 microM). When cultures are rendered quiescent by incubation in serum-free medium, channel open probability is virtually 0 as compared to 0.26 (+/- 0.17) in exponentially growing cultures. If mitogenesis is initiated by readdition of serum to quiescent cells while maintaining cell-attached recording, there is a rapid (15-30 s) activation of the channel (n = 12). The open probability of the patch increases (greater than 0.75) for 2-3 min and then decreases. We have attempted applications of several growth factors (fibroblast-derived growth factor, epidermal growth factor, insulin, bombesin, alpha-thrombin, and vasopressin, individually or in combination) but find that only PDGF (5-100 ng/ml; n = 9) produces channel activation. This activation should provide a Na+ entry pathway parallel to that of the Na/H exchanger.

Growth and transparency in the lens, an epithelial tissue, stimulated by pulses of PDGF

The rat lens undergoes dramatic growth during early postnatal development. Lens weight increased by a factor of 23 in 26 days. Growth rate per day oscillated between 0 and 87 percent. A new culture system was designed to study the oscillations in growth during development. Lens growth and transparency in vitro required pulsatile delivery of platelet-derived growth factor (PDGF) in HL-1 serum-free medium. Continuous delivery of HL-1 medium with PDGF or pulsatile delivery of HL-1 medium without PDGF resulted in lens opacity and no growth. These results provide direct evidence that PDGF stimulates an epithelial tissue and that oscillations in growth occur during normal development of the rat lens.

Modification of MCDB 110 medium to support prolonged growth and consistent high cloning efficiency of diploid human fibroblasts

In preparation for studies on the growth factor requirements of normal and transformed human fibroblasts, we have developed a serum-free medium that supports vigorous long-term serial subculture of diploid human fibroblasts and allows them to form large-sized colonies with high efficiency (40 to 60%) when plated at cloning density (2 to 5 cells/cm2). This medium, which is a modification of Ham's MCDB 110 base medium with its serum replacement supplements, is relatively easy to prepare and the cost of the serum replacements is approximately the same as that of fetal bovine serum supplied at 10%. The ingredients of "Supplement B" of MCDB 110 medium were added in an ethanol solution, rather than in the form of liposomes, and were combined with bovine serum albumin (0.5%), a lipid carrier. Gelatin and fetuin were included as attachment factors instead of polylysine. Bioassays indicated that none of the ingredients in the medium were contaminated with either epidermal growth factor or platelet-derived growth factor. In this modified serum-free medium, which we have designated McM+SR1, diploid human fibroblasts grew for 21 days at the same rate as in the base medium, McM, supplemented with 10% FBS (i.e., 21 population doublings). During the next 20 days, they underwent 15 population doublings which was 75% of the rate of cells growing in the medium containing serum.

Transformation of diploid human fibroblasts by DNA transfection with the v-sis oncogene

The simian sarcoma virus (SSV) oncogene (v-sis) has a high degree of homology to the cellular gene coding for the B peptide of human platelet-derived growth factor (PDGF), a potent fibroblast mitogen. The cellular homolog of v-sis is activated in some mesenchymal human tumors and cell lines derived from them. To determine the phenotype produced by v-sis in diploid human fibroblasts, we constructed plasmids containing the SSV provirus and drug-resistance markers and transfected them into early-passage human cells. Fibroblasts that had integrated the plasmid were selected for drug resistance and shown to contain and express the v-sis oncogene by DNA and RNA hybridization. The v-sis-expressing cells grew to higher saturation densities than control cells transfected with the vector plasmid alone and formed large, well defined foci. This allowed selection of transfectants directly for focus formation. The v-sis transformed cells continued to grow well in the absence of serum, whereas age-matched, vector-transfected control cells ceased replicating under these conditions so that the final difference in density between the two populations was tenfold. Incorporation of thymidine in serum-free medium by the v-sis-transformed cells was independent of exogenous PDGF. In contrast, PDGF increased thymidine incorporation in such medium by the control cells to the level found in the v-sis-transformed cells with or without added PDGF. These results suggest that expression of the v-sis oncogene in diploid human fibroblasts causes sufficient endogenous synthesis of the B chain of PDGF to allow transformants to grow to abnormally high cell densities. When individual v-sis-transformed cells were grown on a background of normal cells, this higher cell density at confluence could be visualized as a focus.

Fetuin: a serum component associated with rat Sertoli and spermatogenic cells in coculture

Cocultures of rat Sertoli-spermatogenic cells plated in a culture medium supplemented with 10% fetal bovine serum for 6-12 h and then maintained in serum free, hormone/growth factor-supplemented medium accumulated an acidic glycoprotein of molecular weight of 68,000 dalton (68 kD) and isoelectric point range of about 4.2-3.5. Anion exchange chromatography has allowed the partial purification of this protein, which consists of a major protein band of 68 kD and two minor, low molecular weight components. A rabbit antiserum raised against the 68 kD component also crossreacts with the two low molecular weight components, thus suggesting that these two minor components are antigenically related to the 68 kD protein. The 68 kD protein has been identified as fetuin, the major component of fetal bovine serum, based on similar molecular weight, isoelectric point, immunoreactivity and trypsin inhibitory activity. Labeling experiments with [14C]amino acid mixture show that 68 kD protein is not synthesized by cocultured rat Sertoli and spermatogenic cells. Immunocytochemistry and Western blot approaches carried out under various experimental conditions support the view that the fetuin-68 kD protein is taken up from serum by both Sertoli cells and pachytene spermatocytes. Because fetuin 1) behaves as a carrier protein for growth factors, 2) has protease inhibitory activity, 3) is preferentially internalized by Sertoli cells and pachytene spermatocytes and 4) fetal bovine serum-supplemented medium impairs spermatogenic cell viability, there is a need to further define appropriate conditions for optimizing long-term viability and differentiation of spermatogenic cells in vitro.