Recycling of the asialoglycoprotein receptor in isolated rat hepatocytes. ATP depletion blocks receptor recycling but not a single round of endocytosis

Continuous endocytosis of 125I-asialo-orosomucoid (ASOR) mediated by the galactosyl receptor in rat hepatocytes is a cyclic process. 125I-ASOR-receptor complexes are internalized, processed, and the ligand is degraded while the receptor is returned to the cell surface for reutilization. Since a true cycle has a thermodynamic requirement for the input of external energy, we examined the effects of changes in intracellular ATP levels on the function of the receptor cycle. Hepatocytes were depleted of ATP to various extents prior to endocytosis by incubating cells at 15 degrees C in the presence of 2 mM NaF and 0-20 mM NaN3. A luciferase-luciferin bioluminescence assay was used to quantitate the amount of cellular ATP. ATP-depleted cells were allowed to bind 125I-ASOR at 0 degrees C, washed through discontinuous Percoll gradients, and only viable cells were isolated and incubated at 37 degrees C to initiate a synchronous single round of endocytosis. The extent of internalization of this surface-bound 125I-ASOR was unaffected by an ATP depletion to less than 1% of the control level. The rate of internalization of surface-bound ligand was unaffected until the ATP levels decreased to 30% or less; at greater than 98% ATP depletion the initial rate decreased by a maximum of 55% and the kinetics became biphasic. In contrast, continuous endocytosis in the presence of excess ASOR was inhibited by only a 25% decline in cellular ATP content and demonstrated a very sharp threshold response to changing ATP levels. Continuous endocytosis, which requires receptor recycling, was completely inhibited when the total cellular ATP level decreased by only 40%. We conclude that the internalization phase of endocytosis is not dependent on ATP but that the processing and/or externalization phases of the complete receptor cycle are either directly or indirectly dependent on ATP and very sensitive to changes in cellular ATP content.     

Protease mitogenic response of chick embryo fibroblasts and receptor binding/processing of human alpha-thrombin

Quiescent cultures of chick embryo fibroblasts incubated with human alpha-thrombin (14-219 pM) incorporated [methyl-3H]thymidine proportional to concentration. Inactivated forms of this protease (e.g. active-site-conjugated alpha-thrombin or its hirudin complex) had no mitogenic activity and did not compete with 124I-alpha-thrombin for binding to specific plasma membrane receptors. The noncoagulant but esterolytic active forms, gamma- and nitro-alpha-thrombins, were weakly mitogenic and correspondingly competed weakly for binding. Trypsin competed equally as well as native thrombin for binding, whereas chymotrypsin, elastase, and human urokinase competed with 80-fold less affinity. Plasma, arginine-specific proteases associated with nerve or epidermal growth factors, insulin, and insulin-like growth factors did not compete for binding. These data demonstrate that (a) functional catalytic residues of the thrombin active site are necessary for mitogenic activity and for specific binding; (b) regions adjacent to the active site, i.e. the high affinity protein recognition site, appear to enhance binding; and (c) the receptor can discriminate between other proteases and binds those which are also mitogens for the avian cells. The characteristics of 125I-alpha-thrombin binding were determined, and it was found to be (i) proportional to cell number; (ii) optimal at pH 6.8; (iii) 70-90% specific; (iv) at equilibrium after 60 min of incubation at 22-24 degrees C or 180 min at 0-4 degrees C (the rate constants for association, i.e. ka, at 22 and 4 degrees C were 18 and 1.1 x 10(7) M-1 min-1, respectively); and (v) essentially nondissociable. Nondissociable thrombin that bound during incubation at 0-4 degrees C was distributed equally between trypsin-sensitive and insensitive compartments. Thrombin associated with the former was released into the media when the cells were incubated at 0-4 degrees C with hirudin or hydroxylamine, or transferred to the insensitive compartment when incubated at 22 degrees C. Finally, confluent cultures of fibroblasts bind 2-3 x 10(4) 125I-alpha-thrombin molecules/cell with an apparent binding constant, i.e. Kd, of 0.7 nM (a true Kd could not be determined because of the irreversible nature of thrombin binding). The binding capacity per cell and the apparent Kd value increased proportionally to an increase in culture density.     

HGF基因真核表达质粒的构建及其细胞表达

目的:构建真核表达质粒p EGFP-N1-HGF并观察干细胞生长因子(hepatocyte growth factor,HGF)在人皮肤成纤维中的表达。方法:从扩增人的间充质干细胞中提取全长RNA为模板,设计合成引物,用RT-PCR法扩增出HGF基因片段,然后将HGF基因片段连接于真核表达质粒p EGFP-N1中。双酶切鉴定及测序分析后,用脂质体包裹转染体外培养的人的皮肤成纤维细胞,通过荧光显微镜观察其转染效果,酶联免疫吸附测定法(enzyme-linked immunosorbent assay,ELISA)检测HGF蛋白的表达。结果:经酶切鉴定及测序验证p EGFP-N1-HGF构建正确,经转染的人皮肤纤维细胞中观察到较强的绿色荧光并且可以表达一定量的HGF。结论:成功构建了真核表达质粒p EGFP-N1-HGF,为基因治疗脱发疾病提供实验基础。     

Binding uptake and degradation of antithrombin III X protease complexes by cultured corneal endothelial cells

Interaction of 125I-labeled human antithrombin III (125I-AT III) X protease complexes with bovine corneal endothelial cells has been studied in tissue culture. 125I-AT III does not bind to endothelial cells, but its complexes with either thrombin or trypsin bind specifically to the cultures. The binding of 125I-AT III X protease complexes is not via the moiety of the free antithrombin III (AT III) or the free protease, since neither AT III nor thrombin compete on the binding of 125I-AT III X thrombin complexes. Only unlabeled AT III X thrombin complexes compete on the binding of the iodinated ligand. 125I-AT III X trypsin complexes bind with a KD of 1.4 X 10(-7) M to high affinity-binding sites present on the cell surface of corneal endothelial cells. Saturation of binding to the cell surface is observed at a concentration of 2.5 X 10(-7) M 125I-AT III X trypsin complexes and the number of binding sites per cell is about 4 X 10(4). The cell surface binding reaches a maximum by 15 min and then decreases with time. The cells, when incubated at 37 degrees C, appear to internalize the bound complexes by adsorptive endocytosis which proceeds at a rate of 0.5-0.8 pmole/1 X 10(6) cells/h. The internalization process of 125I-AT III X protease complexes is saturated at a concentration of 2.5 X 10(-7) M. Since the cells release 125I-labeled material into the extracellular media which cannot be precipitated by trichloroacetic acid (TCA), it probably represents degradation of 125I-AT III X protease complexes into small fragments at a linear rate of about 0.5 pmole/1 X 10(6) cells/h. The described process of AT III X protease complexes binding, internalization and subsequent degradation by corneal endothelial cells may represent a clearing mechanism for extracellular AT III X protease complexes formed under pathological conditions.     

The perturbation of thrombin binding to human platelets by trypsin and neuraminidase

The initial step in the interaction of thrombin with human platelets in binding of the enzyme to the platelet surface. The effects of digestion of isolated platelets with trypsin and neuraminidase on aggregation, release of serotonin and binding of thrombin have been examined.Trypsin is a powerful inducer of platelet aggregation as well as the release reaction. The aggregation effect of trypsin may be blocked with disodium ehtylenediaminetatraacetate (EDTA). Further, in the presence of EDTA, trypsin-induced release of [¹⁴C]serotonin is 15–20% lower compared to controls and the initial lag period is prolonged. Conditions were developed under which trypsin did neither aggregate nor release serotonin from platelets. Even under these conditions, trypsin caused a profound loss in the thrombin binding capacity of platelets. Thus, the trypsin-induced fall in the thrombin binding capacity and the platelet response are dissociated. This loss in the thrombin binding by trypsin is due to a lower number of binding sites available on the platelet surface and is not due to an altered affinity.Neuraminidase did not induce platelet aggregation or the release reaction. The ability of platelets to bind thrombin was also unimpaired by prior digestion with neuraminidase. Thus, the sialic acid at the platelet surface is not essential in the function of thrombin recognition by the receptor. This moiety may nontheless be a constituent of a glycoprotein which might act as the thrombin receptor.     

Binding and degradation of platelet thrombospondin by cultured fibroblasts

Thrombospondin was purified from human platelets and labeled with 125I, and its metabolism was quantified in cell cultures of human embryonic lung fibroblasts. 125I-Thrombospondin bound to the cell layer. The binding reached an apparent steady state within 45 min. Trichloroacetic acid-soluble radioactivity was detected in the medium after 30 min of incubation; the rate of degradation of 125I-thrombospondin was linear for several hours thereafter. Degradation of 125I-thrombospondin was saturable. The apparent Km and Vmax for degradation at 37 degrees C were 6 X 10(-8) M and 1.4 X 10(5) molecules per cell per minute, respectively. Degradation was inhibited by chloroquine or by lowering the temperature to 4 degrees C. Experiments in which cultures were incubated with thrombospondin for 45 min and then incubated in medium containing no thrombospondin revealed two fractions of bound thrombospondin. One fraction was localized by indirect immunofluorescence to punctate structures; these structures were lost coincident with the rapid degradation of 50-80% of bound 125I- thrombospondin. The second fraction was localized to a trypsin- sensitive, fibrillar, extracellular matrix. 125I-Thrombospondin in the matrix was slowly degraded over a period of hours. Binding of 125I- thrombospondin to the extracellular matrix was not saturable and indeed was enhanced at thrombospondin concentrations greater than 3 X 10(-8) M. The ability of 125I-thrombospondin to bind to extracellular matrix was diminished tenfold by limited proteolytic cleavage with trypsin. Degradation of trypsinized 125I-thrombospondin was also diminished, although to a lesser extent than matrix binding. Heparin inhibited both degradation and matrix binding. These results suggest that thrombospondin may play a transitory role in matrix formation and/or organization and that specific receptors on the cell surface are responsible for the selective removal of thrombospondin from the extracellular fluid and matrix.     

Correlation between trypsin binding to a specific receptor and prostacyclin production in cultured vascular endothelial cells

The correlation between the binding and processing of trypsin and its effect on prostacyclin (PGI2) production in cultured adult bovine aortic endothelial (ABAE) cells was studied. ABAE cells demonstrated an ability to produce PGI2 in a dose-response manner to trypsin at the range of 0.1-2.0 micrograms/ml with a saturation at a concentration of 1 microgram/ml. Likewise, 125I-trypsin binding to the cultured cells increased in a dose-response way and reached saturation at a concentration of about 1 microgram/ml; 125I-trypsin was bound to a specific high-affinity cell-surface receptor with a dissociation constant (Kd) of 1.5 X 10(-8) M and each of the confluent ABAE cells has about 1.2 X 10(5) such receptors sites. The cell-surface receptor for trypsin displayed specific characteristics and an excess amount of unlabeled trypsin successfully abolished 125I-trypsin binding while thrombin in excess failed to compete for 125I-trypsin binding. Only a small fraction of the cell-surface-bound 125I-trypsin was internalized and subsequently degraded by ABAE cells as compared to the process of 125I-trypsin internalization by human skin fibroblasts (HSF). This study demonstrated that the stimulatory effect of trypsin on prostacyclin production and release by ABAE cells might be mediated by a specific cell-surface receptor for trypsin on these cells distinct from the thrombin receptor.     

Factors affecting the ability of isolated Plasmodium knowlesi merozoites to attach to and invade erythrocytes

Plasmodium knowlesi merozoites were prepared by the polycarbonate sieving method of Dennis, Mitchell, Butcher & Cohen (1975). Merozoite function was assayed by their attachment to and invasion of rhesus erythrocytes at 37 degrees C. The early merozoites from the culture chamber were the most invasive, although maximum numbers of merozoites appeared later. Merozoites were most stable when incubated at room temperature (23 degrees C). At 37 and 0 degrees C invasiveness rapidly declined to zero. Attachment was rapidly lost at 37 degrees C but was retained at 0 degrees C. Attachment was unchanged in the pH range 6.8--7.9 but invasion was reduced at pH 7.9. The presence of L-fucose, D-galactose, D-glucose, D-mannose, N-acetyl-D-galactosamine or N-acetyl-D-glucosamine did not reduce invasion. Attachment and invasion were greatly reduced or abolished by the presence of 2.5 mM EDTA or EGTA, by lactoperoxidase-catalysed iodination of the merozoites, or by treatment of the merozoites with trypsin at a concentration of 1 micrograms/ml or greater for 10 min at 23 degrees C.     

Fibronectin-independent attachment of human gingival fibroblasts to interstitial and basement membrane collagens

We have studied the ability of human gingival fibroblasts (HGF) to attach to different interstitial (types I, II and III) and basement membrane (types IV and V) collagens. HGF cells were plated onto collagen-coated Petri dishes under various conditions and the percentage of cells attaching to the collagen was determined. HGF were found to attach to all the different types of native collagens, but attached poorly to the corresponding denatured collagens. When plated in the presence of 15% fetal bovine serum (FBS) or fibronectin-depleted FBS, similar percentages (approximately 85%) of cells attached to both interstitial and basement membrane collagens, demonstrating an attachment mechanism that is independent of plasma fibronectin. That the attachment in the presence of serum was also independent of cellular fibronectin was shown by the inability of fibronectin antibodies to block attachment to any of the collagen types. HGF were also capable of attaching to all of the collagen types in the complete absence of serum. In previous studies, investigators using cell lines have suggested that cell attachment in the absence of serum is non-physiological. However, the serum-free attachment of HGF to collagen was found to be dependent on cellular protein synthesis indicating that this attachment mechanism has biological significance.     

Adipocyte insulin receptor. Generation of a cryptic domain of the alpha-subunit during internalization of hormone-receptor complexes.

The dynamics of the internalization of photoaffinity-labelled insulin-receptor complexes was investigated in isolated rat adipocytes by using tryptic proteolysis to probe both the orientation and cellular location of the labelled complexes. In cells that were labelled at 16 degrees C and not prewarmed, 150 micrograms of trypsin/ml rapidly degraded the labelled 125 kDa insulin-receptor subunit into a major proteolytic fragment of 70 kDa and minor amounts of 90- and 50-kDa fragments. With milder trypsin treatment conditions (100 micrograms of trypsin/ml, 15 s at 37 degrees C), the 90 kDa peptide (different from the 90 kDa beta-subunit of the insulin receptor) appeared as a major intermediate proteolytic product, but this species was rapidly and completely converted into the 70- and 50-kDa fragments with continued exposure to trypsin, such that it did not accumulate to appreciable amounts in cells that were not prewarmed before trypsin exposure. By contrast, trypsin treatment of cells prewarmed to 37 degrees C for various times showed that: first, a proportion of the labelled 125 kDa receptors was internalized (became trypsin-insensitive); secondly, the 90 kDa tryptic peptide was formed in large amounts, with proportionate decreases occurring in the amounts of the 70- and 50-kDa tryptic peptides. The increased accumulation of the 90 kDa tryptic peptide from cells preincubated at 37 degrees C, but not at 16 degrees C, indicated that trypsin cleavage sites within the 90 kDa segment of the insulin-receptor alpha-subunit that were exposed at 16 degrees C were made inaccessible by incubation at 37 degrees C, a finding that is consistent with generation of a cryptic domain of the receptor subunit. The tryptic generation of the 90 kDa peptide at 37 degrees C was rapid, becoming half-maximal in 4.4 +/- 0.6 min and maximal in 15-20 min, preceded the intracellular accumulation of labelled receptors (half-maximal in 12.6 +/- 0.7 min and maximal in 30-40 min), was highly correlated with receptor internalization, and was not observed in cultured IM-9 lymphocytes, a cell line in which photolabelled insulin receptors are primarily lost by shedding into the incubation media. These results show that, in adipocytes incubated at 37 degrees C, rapid masking of a previously (at 16 degrees C) accessible domain of the insulin-receptor alpha-subunit occurs and that this dynamic process happens at an early stage in the internalization of insulin-receptor complexes.     

Isolation of a proliferation inhibitor factor from uterine myomatosis fibroblasts

In this work, we report the isolation of a factor from the culture supernatant of confluent fibroblasts from human cervix with the diagnosis of uterine myomatosis. This factor possesses the capacity to inhibit the proliferation of normal fibroblasts. The proliferation inhibitor factor (PIF) was purified from the culture supernatant by precipitation with 80% ammonium sulfate, and by molecular sieve chromatography. Our results indicate that PIF is a protein of 23 kDa, which is highly sensitive to trypsin treatment, and is thermolabile, since temperatures equal to, or above, 60 degrees C eliminate the protein activity in 15 to 20 min. Western blot analyses identified no cross reactions of the purified PIF with TGF-alpha, TNFalpha, IFNgamma, or IL-1beta, suggesting that PIF is a new protein belonging to the group of factors secreted by fibroblasts able to inhibit cellular proliferation.     

Receptor-bound thrombin is not internalized through coated pits in mouse embryo cells

The localization of thrombin receptors on mouse embryo (ME) cells was examined using electron microscope (EM) immunocytological techniques. ME cells were fixed with formaldehyde, prior to thrombin binding, and thrombin visualized on cell surfaces using affinity-purified antithrombin rabbit antibody and colloidal gold labeled anti-rabbit IgG. Colloidal gold particles were found in clusters on the surface of cells incubated with thrombin. There were approximately seven particles per cluster observed in thin sections with cluster diameters ranging from 70 to 200 nm. These clusters were not observed on cells incubated without thrombin. The total number of particles present on cells incubated with and without thrombin indicate that the colloidal gold labeling is approximately 98% specific for thrombin. Only four colloidal gold particles out of approximately 1,200 were associated with coated pits. Thus the thrombin receptor clusters do not appear to associate with coated membrane regions. To determine whether receptor-bound thrombin was internalized by receptor-mediated endocytosis, ME cells were incubated with 125I-thrombin and examined using EM autoradiography and the trypsin sensitivity of 125I-thrombin which was associated with the cells. In two types of experiments, where thrombin was incubated with cells at 4 degrees C and the temperature increased to 37 degrees C and where initial incubation was at 37 degrees C, the receptor-directed specific internalization proceeded at approximately the same rate as nonspecific internalization. These studies indicate that thrombin that binds to its receptors on ME cells is not rapidly internalized by receptor-mediated endocytosis.     

31P-NMR studies of the freeze-tolerant larvae of the gall fly, Eurosta solidaginis

31P NMR was applied to an examination of the freeze-tolerant larvae of the gall fly, Eurosta solidaginis. Resonances from sugar phosphates, inorganic phosphate, adenylates and arginine phosphate were identified. Two peaks of Pi were identified corresponding to intracellular and extracellular Pi. Anoxia produced an expected decrease in peak intensities of ATP and arginine phosphate while the peak of intracellular Pi was enhanced and shifted to indicate intracellular acidification during anoxia. Spectra of whole larvae were monitored over a temperature range from -30 degrees to +25 degrees C. No abrupt alterations in the spectra were seen at the point of extracellular freezing which occurs at about -8 degrees C but temperature had dramatic effects upon the peak intensities of ATP and arginine phosphate. A reversible increase/decrease in peak intensities, relative to Pi, was observed as temperature was raised/lowered. At 15 degrees and -20 degrees C, the beta peak of ATP was 64% and 2% of the peak intensity of Pi while that of arginine phosphate was 78% and 11%, respectively. This temperature effect was not an artifact of instrumentation (as model solutions containing Pi, ATP and arginine phosphate did not show this effect) or a result of changes in the total amounts of these compounds in the cell with temperature. Rather it is apparent that these molecules become restricted in their rotational movement as temperature is lowered perhaps via binding to subcellular components. Changes in the amounts of freely soluble ATP and arginine phosphate with temperature could have important implications for metabolism and its control. Analysis of the effect of temperature on the chemical shift of Pi was also used to determine pH in the intracellular and extracellular compartments. Temperature change had no effect on extracellular (hemolymph) pH which remained constant at 6.1-6.3. Intracellular pH varied with temperature, however, from pH 6.8 at 15 degrees C to pH 7.3 at -12 degrees C with a change, delta pH/delta 0, of -0.0185 degrees C consistent with alphastat regulation.     

Involvement of cellular calcium incorporated from the medium in production of inositol trisphosphate in A-431 cells

The property of intensive 45Ca2+ uptake by A-431 human epidermoidal carcinoma cells was indicated to be an influx, not binding to the cell surface, since the two apparent dissociation constants (Kd) between 45Ca2+ and cells were almost the same when measured in either the presence or absence of 1 mM [ethylenebis (oxyethylenenitrilo)]tetraacetic acid (EGTA); these constants were approximately 5-10 x 10(-6) and 1 x 10(-4) M, respectively, which are much higher than the chelating constant of EGTA for Ca2+ (approximately 10(-11) M). Furthermore, addition of A23187, a calcium ionophore, rapidly released the 45Ca2+ incorporated into cells at both 37 degrees C and 0 degrees C. The 45Ca2+ associated with the cells was slowly released or exchanged when cells were incubated in medium depleted of Ca2+, or in that containing 1 mM non-radioactive Ca2+. The ability of A-431 cells to respond to extracellular ATP by elevating their level of intracellular calcium ions, as well as by producing inositol trisphosphate (InsP3), was suppressed in cells depleted of cellular calcium. These data suggest that calcium ions are extensively incorporated or exchanged with those outside the cells, maintained as stored calcium, and involved in production of InsP3, when A-431 cells are stimulated by ATP to trigger the signal transduction system.     

Induction of hepatocyte growth factor expression by maleic acid in human fibroblasts through MAPK activation

Carboxylic acids have various biological activities and play critical roles in cellular metabolic pathways such as the tricarboxylic acid (TCA) cycle. It has been shown that some carboxylic acids induce cell proliferation and production of cytokines or growth factors. However, there have been no reports on effects of carboxylic acids on hepatocyte growth factor (HGF) expression. In this study, we found that only maleic acid among various carboxylic acids examined markedly induced HGF production from human dermal fibroblasts. Maleic acid also induced HGF production from human lung fibroblasts and neuroblastoma cells. The stimulatory effect was accompanied by upregulation of HGF gene expression. Increase in phosphorylation of extracellular signal-regulated protein kinase (ERK) and c-Jun N-terminal kinase (JNK) but not in phosphorylation of p38 was observed from 6 h and up to 24 h after maleic acid addition. The ERK kinase inhibitor PD98059 and the JNK inhibitor SP600125 potently inhibited maleic acid-induced HGF production, while the p38 inhibitor SB203580 did not significantly inhibit the production. The protein synthesis inhibitor cycloheximide completely inhibited upregulation of HGF mRNA induced by maleic acid but superinduced HGF mRNA expression upregulated by 12-O-tetradecanoylphorbol 13-acetate (TPA). These results suggest that maleic acid indirectly induced HGF expression from human dermal fibroblasts through activation of ERK and JNK and that de novo protein synthesis is required for maleic acid-induced upregulation of HGF mRNA.     

The release of radioactive nucleic acids and mucoproteins by trypsin and ethylenediaminetetra-acetate treatment of baby-hamster cells in tissue culture

Monolayers of baby-hamster kidney cells were grown on glass in tissue culture and harvested with trypsin or EDTA in order to investigate the cell surface macromolecules removed by these cell-disaggregating agents. The release of nucleic acids from the cells during the harvesting procedure was monitored by labelling the cellular RNA with [5-(3)H]uridine and the cellular DNA with [2-(14)C]thymidine. Treatment of the cells with EDTA was found to cause an increase in the permeability of the plasma membrane with 7.6% of the cellular RNA, but less than 1% of the cellular DNA, being released. Moreover, 61% of the cells harvested with EDTA were permeable to Trypan Blue. With crude trypsin, lysis of the cell occurred with the release of similar amounts of RNA and DNA amounting to about 11% of the total cellular nucleic acid. In contrast, crystalline trypsin released only 1% of the cellular nucleic acids. Since virtually all the cells (99%) after harvesting in crystalline trypsin were impermeable to Trypan Blue, this method was suitable for obtaining cell surface macromolecules without contamination by intracellular damage. [1-(14)C]Glucosamine was incorporated by the cells only into bound hexosamines and sialic acids. [By monitoring the release of radioactivity in high-molecular-weight material in such experiments a measure of the release of macromolecules containing amino sugars was obtained.] Of the total macromolecules containing amino sugars in the cells 33%, 24% and 13% were released when the cells were harvested with crude trypsin, crystalline trypsin or EDTA respectively. Crystalline trypsin also released 39% of the total sialic acid of the cell, whereas less than 1% of the cellular sialic acid was present in the EDTA-treated fraction. It is concluded that the macromolecules containing amino sugars released with crude trypsin and EDTA are likely to be heavily contaminated with intracellular material. However, the macromolecules released by crystalline trypsin appear to come from the cell surface.     

Degradation of serum amyloid A and apolipoproteins by serum proteases

We have investigated the protease activity, present in human serum, that digests the serum amyloid A (SAA) protein. SAA radiolabeled with 125I was incubated at 37 degrees C with serum and plasma and analyzed for degradation products by alkaline urea-polyacrylamide gel electrophoresis and gel filtration chromatography. Serum initially digested SAA to intermediates of 3000-5000 in molecular weight, and these were further degraded to smaller peptides with prolonged incubation. SAA was not degraded by plasma anticoagulated with ethylenediaminetetraacetic acid (EDTA) or heparin. Recalcification of plasma anticoagulated with EDTA led to the generation of protease activity against SAA whereas EDTA plasma defibrinated with thrombin was inactive. We employed both nonselective and selective protease inhibitors and synthetic substrates for kallikrein and plasmin to further characterize the serum protease. These studies demonstrated that degradation of SAA is not directly attributable to enzymes involved in coagulation, kinin formation, or fibrinolysis, but the unidentified protease may be activated by one of the clotting factors. The specificity of the SAA degradation was demonstrated in experiments with three of the well-characterized apolipoproteins. Apolipoproteins A-I, C-I, and C-III-1, which also associate with the plasma high-density lipoproteins, were not degraded by serum although they were good substrates for purified thrombin and plasmin.     

Mechanisms of cell adhesion: early-forming junctions between aggregating fibroblasts.

When cultured fibroblasts (16C) are mildly dissociated with EGTA or trypsin/EDTA, they aggregate rapidly. The formation of aggregates has been found to involve junctions of the gap and adhaerens types which are seen by electron microscopy within minutes of allowing cells to come together. The process of adhesion between freshly dissociated, transformed 16C fibroblasts is therefore organized and establishes its usefulness as a model for studying cellular interactions in relation to supracellular organization.     

Extracellular matrix organization modulates fibroblast growth and growth factor responsiveness

To learn more about the relationship between extracellular matrix organization, cell shape, and cell growth control, we studied DNA synthesis by fibroblasts in collagen gels that were either attached to culture dishes or floating in culture medium during gel contraction. After 4 days of contraction, the collagen density (initially 1.5 mg/ml) reached 22 mg/ml in attached gels and 55 mg/ml in floating gels. After contraction, attached collagen gels were well organized; collagen fibrils were aligned in the plane of cell spreading; and fibroblasts had an elongated, bipolar morphology. Floating collagen gels, however, were unorganized; collagen fibrils were arranged randomly; and fibroblasts had a stellate morphology. DNA synthesis by fibroblasts in contracted collagen gels was suppressed if the gels were floating in medium but not if the gels were attached, and inhibition was independent of the extent of gel contraction. Therefore, growth of fibroblasts in contracted collagen gels could be regulated by differences in extracellular matrix organization and cell shape independently of extracellular matrix density. We also compared the responses of fibroblasts in contracted collagen gels and monolayer culture to peptide growth factors including fibroblast growth factor, platelet-derived growth factor, transforming growth factor-beta, and interleukin 1. Cells in floating collagen gels were generally unresponsive to any of the growth factors. Cells in attached collagen gels and monolayer culture were affected similarly by fibroblast growth factor but not by the others. Our results indicate that extracellular matrix organization influenced not only cell growth, but also fibroblast responsiveness to peptide growth factors.