Protease nexin-1 complexes and inhibits T cell serine proteinase-1

The T cell serine proteinase-1 (TSP-1) which most probably is involved in cell killing by cytotoxic T cells is inhibited by protease nexin-1 (PN-1), an extravascular serine protease inhibitor. The inhibition is irreversible and correlates with formation of SDS-stable complexes between the two proteins. Two distinct species of complexes (91 and 122 kDa) are observed upon SDS-PAGE analysis of the reacted proteins, indicating that PN-1 is capable of complexing and inhibiting both subunits of the homodimeric TSP-1 molecule. Heparin (2 micrograms/ml) increases the association rate constant from 4.2 x 10(4) M-1 sec-1 to 4.8 x 10(5) M-1 sec-1. These observations suggest that PN-1 may function as a major extravascular inhibitor of TSP-1 released from cytotoxic T lymphocytes.

Binding of protease nexin-1 to the fibroblast surface alters its target proteinase specificity

Protease nexin-1 is a protein proteinase inhibitor that is secreted by a variety of cultured cells and rapidly forms complexes with thrombin, urokinase, and plasmin; the complexes then bind back to the cells and are internalized and degraded. In fibroblast cultures, protease nexin-1 is localized to the extracellular matrix. Here we report that protease nexin-1, which is bound to the surface of fibroblasts, forms complexes with thrombin, but not urokinase or plasmin. Experiments were conducted to determine directly if protease nexin-1 binding to the fibroblast surface alters its proteinase specificity. To do this, cell surface protease nexin-1 was inhibited using anti-protease nexin-1 monoclonal antibodies that stoichiometrically block its ability to form complexes with target proteinases. Then, purified protease nexin-1 was added to these cells; the cell-bound molecule formed complexes with thrombin, but not urokinase or plasmin. Similar experiments showed that protease nexin-1 bound to preparations of fibroblast extracellular matrix also formed complexes with thrombin, but not urokinase or plasmin. Components of the extracellular matrix other than heparin-like glycosaminoglycans are required for this regulation since heparin did not block the formation of complexes between protease nexin-1 and urokinase or plasmin. These results suggest that protease nexin-1 is primarily a thrombin inhibitor in interstitial fluids where much of it would be bound to cell surfaces.

Proteolytic regulation of neurite outgrowth from neuroblastoma cells by thrombin and protease nexin-1

This review summarizes studies on the reciprocal regulation of neuroblastoma neurite outgrowth by thrombin and protease nexin-1 (PN-1). PN-1 recently was shown to possess the same deduced amino acid sequence as the glial-derived neurite-promoting factor. The neurite outgrowth activity of PN-1 depends on its ability to inhibit thrombin. Thrombin not only blocks the neurite outgrowth activity of PN-1, but it also brings about neurite retraction in the presence of PN-1. Thrombin also produces neurite retraction in the absence of PN-1 and other regulatory factors. This suggests that its activity is due to a direct action on cells. The neurite retraction by thrombin depends on its proteolytic activity. It does not occur with the other serine proteases that have been tested, indicating that it is a specific effect and is not due to a general proteolytic effect that could detach neurites from the culture dish. Serum brings about neurite retraction in certain neuroblastoma cells and primary neuronal cultures; most of this activity is due to residual thrombin in the serum. Together, these results suggest that PN-1 and thrombin (or a thrombin-like protease) play a role in regulation of neurite outgrowth.

Thrombin modulates and reverses neuroblastoma neurite outgrowth

Previous studies have shown that neuroblastoma cells and several types of primary neuronal cells in culture rapidly extend neurites when switched from serum-containing to serum-free medium. The present studies on cloned neuroblastoma cells show that thrombin blocked this spontaneous differentiation at 2 nM with a half-maximal potency of 50 pM. This required the catalytic activity of thrombin and was reversed upon thrombin removal. Thrombin also caused cells in serum-free medium to retract their neurites at equally low concentrations. Two other serine proteases, urokinase and plasmin, did not block or reverse neurite extension even at 100-fold higher concentrations. A specific assay for thrombin indicated that thrombin detected in serum-containing medium from neuroblastoma cultures was derived from serum and that it was likely responsible for much of the known capacity of serum to maintain neuroblastoma cells in a nondifferentiated state. This was supported by the finding that heparin addition reduced the thrombin concentration in serum-containing medium and stimulated neurite outgrowth from neuroblastoma cells in serum-containing medium. Studies on the ability of thrombin to modulate neurite outgrowth by other agents showed that it blocked and reversed the neurite outgrowth activity of two thrombin inhibitors: protease nexin-1 (which is identical to glial-derived neurite-promoting factor) and hirudin. Thrombin, however, did not block the neurite-promoting activity of dibutyryl cAMP or prostaglandin E1. These results suggest a specific role for thrombin in control of neurite outgrowth.

Monoclonal antibodies to protease nexin 1 that differentially block its inhibition of target proteases

Protease nexin 1 (PN-1) is a protease inhibitor secreted by cultured fibroblasts that forms complexes with certain serine proteases; the complexes bind back to the cells and are internalized and degraded. In the present studies, a panel of PN-1 monoclonal antibodies (mAbs) was isolated; none showed detectable cross-reactivity with four related plasma protease inhibitors. Four purified mAbs (mAbp1, mAbp6, mAbp9, and mAbp18) were tested for their ability to block the formation of complexes between PN-1 and target proteases. mAbp1, as well as a rabbit polyclonal anti-PN-1 IgG preparation, did not block formation of 125I-thrombin-PN-1 complexes. mAbp6, mAbp9, and mAbp18 blocked the formation of 125I-thrombin-PN-1 and 125I-urokinase-PN-1 complexes at stoichiometric concentrations of mAb and PN-1. Studies on their ability to block formation of 125I-trypsin-PN-1 complexes showed that mAbp18 also blocked this reaction at stoichiometric concentrations with PN-1 whereas mAbp6 and mAbp9 blocked less effectively. Thus, mAbp18 appears to bind at or close to the reactive center of PN-1. The blocking mAbs should be useful in studies to probe physiological functions of PN-1.

Purification of a form of protease nexin 1 that binds heparin with a low affinity

A form of protease nexin 1 (PN-1) that binds heparin with a low affinity (L-PN-1) was purified and studies since altered interactions with glycosaminoglycans could affect its inhibition of certain serine proteases. Purification of L-PN-1 and PN-1 was achieved by fractionating serum-free conditioned culture medium from human fibroblasts over dextran sulfate-Sepharose followed by immunoaffinity fractionation over a PN-1 monoclonal antibody-Sepharose column. The first step separated L-PN-1 from PN-1, and the second step resulted in apparently homogeneous L-PN-1 and PN-1. Comparisons of the two proteins showed that they could not be distinguished by the following properties: (a) molecular weight; (b) proteases complexed; (c) molecular weights of protease-L-PN-1 and protease-PN-1 complexes; (d) CNBr peptide maps; and (e) immunological cross-reactivity. Studies on activities that depend on the heparin binding domain revealed that heparin equally accelerated the rate of formation of 125I-thrombin-L-PN-1 and 125I-thrombin-PN-1 complexes even when the ratio of heparin to L-PN-1 or PN-1 was varied from 0.01 to 100. A functional difference, however, between L-PN-1 and PN-1 was observed in studies on the ability of the fibroblast surface to accelerate their reactions. Fixed fibroblasts accelerated the formation of 125I-thrombin-L-PN-1 complexes 2-fold, whereas they accelerated the formation of 125I-thrombin-PN-1 complexes 5-fold. The availability of purified L-PN-1 will permit studies on its functional relationship to PN-1.

Functional and structural similarities between protease nexin I and C1 inhibitor

Protease nexin I is a proteinase inhibitor that is secreted by human fibroblasts and forms stable complexes with certain serine proteinases; the complexes then bind to the fibroblasts and are rapidly internalized and degraded. In this report, we show that this inhibitor, which is present in very low concentrations in plasma, has functional and structural similarities to C1 inhibitor, an abundant proteinase inhibitor in plasma. Both inhibitors complex and inactivate certain proteinases that previously were known to rapidly react only with C1 inhibitor. Kinetic inhibition studies show that protease nexin I inhibits Factor XIIa and plasma kallikrein with second-order rate constants of 2.3 x 10(3) and 2.5 x 10(5) M-1 s-1, respectively, which are similar to the rate constants for inhibition of these proteinases by C1 inhibitor. Protease nexin I inhibits C1s about one-tenth as rapidly as does C1 inhibitor. Alignment of the amino acid sequences of protease nexin I and C1 inhibitor shows that these proteins have similarity at their reactive centers (from sites P7 to P1). The remaining regions of the two proteins share much less similarity. In contrast to protease nexin I, C1 inhibitor is not secreted by human fibroblasts. Although 125I-C1s-protease nexin I complexes readily bind to human fibroblasts, binding of 125I-C1s-C1 inhibitor complexes or other 125I-proteinase-C1-inhibitor complexes to these cells is not detectable. Thus, protease nexin I and C1 inhibitor may control some common regulatory proteinases in the extravascular and vascular compartments, respectively.

Effects of fibroblasts and endothelial cells on inactivation of target proteases by protease nexin-1, heparin cofactor II, and C1-inhibitor

Previous studies have shown that glycosaminoglycans in the extracellular matrix accelerate the inactivation of target proteases by certain protease inhibitors. It has been suggested that the ability of the matrix of certain cells to accelerate some inhibitors but not others might reflect the site of action of the inhibitors. Previous studies showed that fibroblasts accelerate the inactivation of thrombin by protease nexin-1, an inhibitor that appears to function at the surface of cells in extravascular tissues. The present experiments showed that endothelial cells also accelerate this reaction. The accelerative activity was accounted for by the extracellular matrix and was mostly due to heparan sulfate. Fibroblasts but not endothelial cells accelerated the inactivation of thrombin by heparin cofactor II, an abundant inhibitor in plasma. This is consistent with previous suggestions that heparin cofactor II inactivates thrombin when plasma is exposed to fibroblasts and smooth muscle cells. Neither fibroblasts nor endothelial cells accelerated the inactivation of C1s by plasma C1-inhibitor.

Localization of protease nexin-1 on the fibroblast extracellular matrix

Protease nexin-1 (PN-1) is a protease inhibitor that is secreted by fibroblasts and several other cultured cells. PN-1 forms complexes with certain serine proteases in the extracellular environment including thrombin, urokinase, and plasmin. The complexes then bind to the cells and are rapidly internalized and degraded. This report demonstrates that PN-1 is present on the surface of fibroblasts, bound to the extracellular matrix. Immunofluorescent studies showed that PN-1 colocalized with fibronectin on both intact cells and in preparations of extracellular matrix made from these cells. In contrast, PN-1 did not colocalize with the epidermal growth factor receptor, a plasma membrane marker. An enzyme-lined immunosorbent assay was developed which showed that the extracellular matrix contained at least 60-80% of the cellular immunoreactive PN-1. Extraction of the matrix with 2 M NaCl removed PN-1 in a form which reacted with 125I-thrombin to form complexes which were immunoprecipitated by anti-PN-1 IgG and were of identical size as complexes made from soluble PN-1 and 125I-thrombin. These data indicate that in addition to its role as a soluble protease inhibitor, PN-1 is also a component of the extracellular matrix and might control its proteolysis.

Glycosaminoglycans on fibroblasts accelerate thrombin inhibition by protease nexin-1

Protease nexin-1 (PN-1) is a proteinase inhibitor that is secreted by human fibroblasts in culture. PN-1 inhibits certain regulatory serine proteinases by forming a covalent complex with the catalytic-site serine residue; the complex then binds to the cell surface and is internalized and degraded. The fibroblast surface was recently shown to accelerate the rate of complex-formation between PN-1 and thrombin. The present paper demonstrates that the accelerative activity is primarily due to cell-surface heparan sulphate, with a much smaller contribution from chondroitin sulphate. This conclusion is supported by the effects of purified glycosaminoglycans on the second-order rate constant for the inhibition of thrombin by PN-1. Also, treatment of 35SO4(2-)-labelled cells with heparitin sulphate lyase or chondroitin sulphate ABC lyase demonstrated two discrete pools of 35S-labelled glycosaminoglycans; subsequent treatment of plasma membranes with these glycosidases showed that heparitin sulphate lyase treatment abolished about 80% of the accelerative activity and chondroitin sulphate ABC lyase removed the remaining 20%. These results show that two components are responsible for the acceleration of PN-1-thrombin complex-formation by human fibroblasts. Although dermatan sulphate is also present on fibroblasts, it did not accelerate the inhibition of thrombin by PN-1.

Purification of protease nexin II from human fibroblasts

Normal human fibroblasts secrete a protein named protease nexin II (PN II) which previously was shown to form sodium dodecyl sulfate (SDS)-stable complexes with epidermal growth factor-binding protein (EGF-BP). These complexes then bind to the same cells and are rapidly internalized and degraded (Knauer, D.J., and Cunningham, D.D. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 2310-2314). Here we describe a procedure for purifying PN II to apparent homogeneity from serum-free culture medium conditioned by human fibroblasts. The first step employed dextran sulfate-Sepharose affinity chromatography. Further purification was achieved by ion-exchange chromatography on DEAE-Sepharose followed by gel filtration on Sephacryl S-400. Sequence analysis of purified PN II identified 33 amino-terminal amino acids; a computer search of several protein sequence data banks failed to reveal homologies with other reported amino acid sequences. Purified PN II had an apparent Mr of 106,000 and an isoelectric point of approximately 7.2. It retained full activity after incubation in the presence of 0.05% SDS or at a pH of 1.5. PN II formed SDS-stable complexes with EGF-BP, the gamma subunit of 7 S nerve growth factor, and trypsin with estimated Mr of 120,000, 120,000, and 110,000, respectively. PN II was metabolically labeled with [35S]methionine and purified; the metabolically labeled protein formed complexes with EGF-BP. Complexes between purified PN II and EGF-BP bound to human fibroblasts. These results show that the purified protein possesses the properties previously attributed to PN II in cell culture medium.

Modulation of thrombin-stimulated lipid responses in cultured fibroblasts. Evidence for two coupling mechanisms

Treatment of cultured fibroblasts with thrombin results in the stimulation of cell division and lipid metabolism. Proteolytically active alpha-thrombin rapidly stimulates (a) release of arachidonic acid, (b) generation of inositol phosphates, and (c) increase in cellular diacylglycerol levels. Pretreatment of the fibroblasts with chymotrypsin before alpha-thrombin prevented the first two responses, (a) and (b), and reduced response c. Treatment of fibroblasts with gamma-thrombin, a proteolytic derivative of alpha-thrombin, produced a response indistinguishable from the alpha-thrombin treatment when preceded by chymotrypsin. These data support a model, similar to one for platelets [McGowan, E. B., & Detwiler, T. C. (1986) J. Biol. Chem. 261, 739-746], that fibroblasts possess two coupling mechanisms for the stimulation of lipid metabolism by thrombin. Similar to platelets, one mechanism, R1, mediates the stimulated release of arachidonic acid and is capable of activating Ni, a GTP-binding protein. R1 is inactivated by chymotrypsin and does not respond to gamma-thrombin. The other mechanism, R2, responds to gamma-thrombin and is not activated by chymotrypsin. In contrast to the mechanisms proposed for platelets, we demonstrate that the phospholipase C responsible for the hydrolysis of phosphoinositides is not activated by R2 but is activated via R1. Importantly, stimulation of either mechanism results in the elevation of cellular diacylglycerol. This indicates that the stimulated elevation of diacylglycerol, or those events dependent upon the elevation of diacylglycerol, is not a reliable indicator for establishing the hydrolysis of phosphoinositides.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of thrombin-stimulated arachidonic acid release and metabolism to mitogenesis and phosphatidylinositol synthesis

Thrombin and certain prostaglandins are both capable of stimulating the proliferation of cultured cells. Since thrombin stimulates the release and metabolism of arachidonic acid, the precursor of prostaglandins, we examined the relationship between this release and metabolism and the stimulation of cell division in cultured fibroblasts. We also examined the role of prostaglandin synthesis in thrombin-stimulated phosphatidylinositol synthesis. The data in this report demonstrate that the release and metabolism of arachidonic acid are not necessary for thrombin-stimulated cell division. The presence of a low concentration of chymotrypsin prevented thrombin-stimulated arachidonic acid release and metabolism without affecting the stimulation of cell division. Furthermore, thrombin-stimulated cell division occurred in the presence of indomethacin concentrations that prevented cyclooxygenase-mediated metabolism of arachidonic acid. The following experiments showed that thrombin-stimulated phosphatidylinositol synthesis was brought about by a cyclooxygenase-mediated metabolite(s) of arachidonic acid. Indomethacin inhibited the cyclooxygenase-mediated metabolism of arachidonic acid without affecting the thrombin-stimulated release of arachidonic acid. Indomethacin also inhibited thrombin-stimulated phosphatidylinositol synthesis. The dose dependence of this inhibition paralleled the inhibition by indomethacin of cyclooxygenase-mediated metabolism of arachidonic acid. In addition, prostaglandin F2 alpha stimulated phosphatidylinositol synthesis in the presence of indomethacin concentrations which prevented thrombin-stimulated phosphatidylinositol synthesis.

Purification of a proteinase inhibitor from bovine serum with C1-inhibitor activity

This report describes the purification of a novel proteinase inhibitor from bovine serum. This protein was purified to apparent homogeneity employing affinity binding to sulfated dextran and precipitation by ammonium sulfate, followed by sequential chromatography on DEAE-cellulose, heparin-Sepharose and Sephacryl S-200. Quantitative enzyme-linked immunosorbent assays revealed that the concentration of this inhibitor is approximately 3 microM in bovine serum. The inhibitor is a single polypeptide chain with an estimated Mr of 83,000 as determined by SDS-polyacrylamide gel electrophoresis. An aspartic acid was found at the amino terminus of the protein; N-terminal amino acid sequence data indicated that there was no significant homology with other reported amino acid sequences. This bovine inhibitor covalently complexed the human proteinases C1-r, C1-s, factor XIIa and plasma kallikrein, which are also complexed and inactivated by human C1-inhibitor. In addition, the bovine inhibitor complexed and inactivated bovine chymotrypsin, a feature which functionally distinguishes it from human C1-inhibitor. Although the bovine inhibitor appears functionally very similar to C1-inhibitor, we found no evidence for structural homology with the human counterpart.

Selective radiolabeling of cell surface proteins to a high specific activity

A procedure was developed for selective radiolabeling of membrane proteins on cells to higher specific activities than possible with available techniques. Cell surface amino groups were derivatized with 125I-(hydroxyphenyl)propionyl groups via 125I-sulfosuccinimidyl (hydroxyphenyl)propionate (125I-sulfo-SHPP). This reagent preferentially labeled membrane proteins exposed at the cell surface of erythrocytes as assessed by the degree of radiolabel incorporation into erythrocyte ghost proteins and hemoglobin. Comparison with the lactoperoxidase-[125I]iodide labeling technique revealed that 125I-sulfo-SHPP labeled cell surface proteins to a much higher specific activity and hemoglobin to a much lower specific activity. Additionally, this reagent was used for selective radiolabeling of membrane proteins on the cytoplasmic face of the plasma membrane by blocking exofacial amino groups with uniodinated sulfo-SHPP, lysing the cells, and then incubating them with 125I-sulfo-SHPP. Exclusive labeling of either side of the plasma membrane was demonstrated by the labeling of some marker proteins with well-defined spatial orientations on erythrocytes. Transmembrane proteins such as the epidermal growth factor receptor on cultured cells could also be labeled differentially from either side of the plasma membrane.

Binding sites for elastase on cultured human fibroblasts that do not mediate internalization

The proteolytic actions of elastases have been implicated in extracellular matrix damage, which is characteristic of a variety of pathological conditions including emphysema and rheumatoid arthritis. In order to elucidate the molecular events involved in elastase interaction with connective tissue cells, the present study was designed to investigate the association of elastase with human fibroblasts at 4 degrees C. Elastase bound saturably to binding sites that were present on the surface of these cells. Analysis of cell-bound elastase by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed the presence of a high molecular weight complex (Mr 54,000) that was not formed with elastase whose catalytic site serine was derivatized with a diisopropylphosphate group. The complex did not represent elastase bound to either protease nexin or contaminating serum. The cellular component with which elastase formed a complex could not be detected in the cell culture medium. Unexpectedly, elastase that had been pre-bound at 4 degrees C was not internalized after cells were warmed to 37 degrees C. The elastase binding site described in this report is therefore distinct from high affinity binding sites involved in receptor-mediated endocytosis and intracellular degradation.

Human fibroblasts accelerate the inhibition of thrombin by protease nexin

Protease nexin (PN) is a protein protease inhibitor secreted by human fibroblasts in culture that complexes and inhibits certain regulatory serine proteases. The PN-protease complexes then bind to these cells and are rapidly internalized and degraded. This report shows that the fibroblast surface accelerates the formation of PN-thrombin complexes. In contrast, it did not accelerate the formation of complexes between thrombin and antithrombin III, a closely related protease inhibitor found in plasma. These results support a role for PN in the regulation of certain proteases in the extravascular compartment at and near the surface of tissue cells. The activity that accelerated PN-thrombin complex formation was membrane-associated, since fixed cells, purified membranes, and extracellular matrix preparations all contained this activity. The ability of cells to accelerate the reaction between PN and thrombin was inhibited by protamine, suggesting that the activity was similar to that of heparin. Heparitinase digestion of plasma membranes prior to assay reduced the activity by about 80%, suggesting that heparan sulfate may account for most of the accelerative activity.

A simple two-step purification of protease nexin

This paper describes a simple purification procedure for protease nexin, a serine proteinase inhibitor secreted by cultured human fibroblasts that regulates proteinase activity at and near the cell surface. The first step in the procedure takes advantage of the high-affinity binding of protease nexin to dextran sulphate-Sepharose. This step eliminates the need for prior concentration of the serum-free fibroblast-conditioned medium, since protease nexin binds to the resin in the presence of physiological saline. The use of dextran sulphate also provides an affinity resin with considerably less variability than the heparin-based resins previously used. Final purification to homogeneity involves a combination of DEAE-Sepharose in-line with dextran sulphate-Sepharose to simultaneously purify and concentrate the protein. Purified protease nexin is shown by Ouchterlony analysis and peptide mapping to be immunologically and structurally distinct from antithrombin III and heparin cofactor II, two plasma proteinase inhibitors with similar properties.

Interactions of serine proteases with cultured fibroblasts

This review summarizes the mechanisms by which several serine proteases, particularly urokinase, thrombin, and elastase, interact with cultured fibroblasts. Many of these studies were prompted by findings that interactions of these proteases with cells and the extracellular matrix are important in a number of physiologic and pathologic processes. Two main pathways have been identified for specific interactions of these proteases with fibroblasts. One involves surface binding sites for the free protease that appear to bind only one particular protease. An unusual feature collectively shared by the binding sites for urokinase, thrombin, and elastase is that the bound protease is not detectably internalized by the fibroblasts. The other pathway by which serine proteases interact with fibroblasts involves proteins named protease nexins (PNs). Three PNs have been identified. They are secreted by fibroblasts and inhibit certain serine proteases by forming a covalent complex with the protease catalytic site serine. The complexes then bind back to the fibroblasts via the PN portion of the complex and are internalized and degraded. Recent studies showing that the fibroblast surface and extracellular matrix accelerate the inactivation of thrombin by PN-1 support the hypothesis that the PNs control protease activity at and near the cell surface. The PNs differ from plasma protease inhibitors in their molecular properties, absence in plasma, site of synthesis, and site of clearance of the inhibitor:protease complexes.

Effects of EGF and thrombin on inositol-containing phospholipids of cultured fibroblasts: stimulation of phosphatidylinositol synthesis by thrombin but not EGF

The effects of growth factors on inositol-containing phospholipids were investigated to test the hypothesis that alterations in their metabolism are involved in mitogenic stimulation. Thrombin and EGF stimulated comparable increases in the synthesis (30-50%) and degradation (20-40%) of phosphatidylinositol 4-monophosphate (DPI) and phosphatidylinositol 4,5-bisphosphate (TPI) in a cell line which is mitogenically responsive to both growth factors. The increases in synthesis were time and dose dependent in a manner which was consistent with their involvement in mitogenesis; the increases were observed only under conditions where a mitogenic response occurred. While it has been suggested that an increased synthesis of phosphatidylinositol (PI) is coupled to the stimulation of DPI and TPI synthesis, we found that thrombin stimulated an early synthesis PI but EGF did not. To further evaluate the involvement of PI in thrombin-stimulated cell division we determined the time and dose dependence of the stimulated PI synthesis and found that it also occurred in a manner which was consistent with its involvement in thrombin-stimulated cell division. Furthermore, the stimulated PI synthesis was not observed with nonmitogenic proteases or in cell lines which were not responsive to thrombin. These results demonstrate that the metabolism of DPI and TPI appears closely related to the mitogenic response generated by EGF and thrombin. However, an early stimulation of PI synthesis is not coupled to this metabolism and is not necessary for mitogenic stimulation by EGF. Thus, a stimulation of PI synthesis is not a valid measure of alterations in inositol-containing phospholipids and what has been termed the "PI response."

Intracellular processing of epidermal growth factor and its effect on ligand-receptor interactions

When normal human fibroblasts are brought to a steady state with 125I-labeled epidermal growth factor (125I-EGF), greater than 90% of the radioactivity is intracellular. We investigated this material to determine whether the 125I-EGF is intact or degraded. Our results show that 125I-EGF is rapidly processed after internalization and can be resolved into four peaks by native gel electrophoresis. These different forms were isolated and tested for their ability to bind to cell-surface EGF receptors. The first processed form was fully capable of binding to EGF receptors, but the second processed form could not. The third form was a collection of small degradation products. We calculated that at steady state about 60% of internalized "125I-EGF" was in a form still able to bind to EGF receptors. We then investigated the ability of different reported inhibitors of EGF "degradation" to block the processing of EGF. Although inhibitors of cathepsin B (leupeptin, antipain, N alpha-p-tosyl-L-lysine chloromethyl ketone, and chymostatin) were able to inhibit the release of monoiodotyrosine from treated cells in a time- and concentration-dependent manner, they had little effect on the processing step that apparently inactivates 125I-EGF. In contrast, agents that raised intravesicular pH, such as methylamine and monensin, inhibited the initial steps in EGF processing as well as the later steps. Low temperatures inhibited the transfer of 125I-EGF to the lysosomes and inhibited the conversion of EGF to a nonbindable form, but had little effect on the initial processing. We conclude that the intracellular processing of EGF is a multistep process that is initiated prior to lysosomal fusion, involves cathepsin B activity, and requires an acidic pH. In addition, many of the protease inhibitors that have been utilized to investigate the role of EGF degradation in mitogenesis do not block the conversion of EGF to a form that is apparently unable to interact with its receptor.