Identification of determinants involved in binding of tissue-type plasminogen activator-plasminogen activator inhibitor type 1 complexes to HepG2 cells

Extracellular matrix remodeling in hepatocellular carcinoma: effects of soil on seed?

Extracellular matrix plays two-edged roles, inhibitor and promoter, in the carcinogenesis and progression of hepatocellular carcinoma. On the one hand, extracellular matrix provides the survival signals, and controls the proliferation, differentiation and metastasis of hepatocellular carcinoma. On the other hand, hepatocarcinoma cells create a permissive soil by extracellular matrix remodeling, result in high proliferation, low differentiation, apoptosis block, invasion and metastasis. These malignant phenotypes are related with the change of the capsule around hepatocarcinoma cells that composed by collagens I and IV, the cell-extracellular matrix interaction induced by laminin and its receptor-integrins, and the degradation of ECM which is regulated by proteolytic enzymes and their inhibitor. Thus, normalization of ECM may represent a novel therapeutic strategy for hepatocarcinoma cells.

RECOMBINANT INTERFERON-γ SECRETED BY CHINESE HAMSTER OVARY-320 CELLS CULTIVATED IN SUSPENSION IN PROTEIN-FREE MEDIA IS PROTECTED AGAINST EXTRACELLULAR PROTEOLYSIS BY THE EXPRESSION OF NATURAL PROTEASE INHIBITORS AND BY THE ADDITION OF PLANT PROTEIN HYDROLYSATES TO THE CULTURE MEDIUM

Biosafety requirements increasingly restrict the cultivation of mammalian cells producing therapeutic glycoproteins to conditions that are devoid of any compound of animal origin. On cultivation in serum-free media, the proteases inhibitors, usually found in serum, cannot protect secreted recombinant proteins against unwanted endogenous proteolysis. Chinese hamster ovary (CHO) cells, secreting recombinant human interferon-gamma (CHO-320 cell line) and cultivated in suspension in an original protein-free medium, expressed at least two members of the matrix metalloproteinases (MMP), either at the cell surface (proMMP-14 and MMP-14) or secreted (proMMP-9). In addition, tissue- and urinary-type plasminogen activators were also secreted in such culture conditions. At the cell surface, dipeptidyl peptidase IV and tripeptidyl peptidase II (TPPII) activities were also detected, and their activities decreased during time course of batch cultures. The proteolytic activities of these proteins were counterbalanced by (1) their expression as zymogens (proMMP-9, proMMP-14), (2) the expression of their natural inhibitors, tissue inhibitors of metalloproteinases-1 and -2 and plasminogen activator inhibitor-1 (PAI-1), or (3) the addition of plant protein hydrolysates to the culture medium, acting as a nonspecific source of TPPII inhibitors. This study points out that, even in protein-free media, recombinant proteins secreted by CHO cells are actively protected against physiological and unwanted extracellular proteolysis either by endogenous or by exogenous inhibitors.

The putative tumor suppressor LRP1B, a novel member of the low density lipoprotein (LDL) receptor family, exhibits both overlapping and distinct properties with the LDL receptor-related protein

The low density lipoprotein receptor-related protein-deleted in tumor (LRP1B, initially referred to as LRP-DIT) was cloned and characterized as a candidate tumor suppressor. It is a new member of the low density lipoprotein receptor gene family. Its overall domain structure and large size (approximately 600 kDa) are similar to LRP and suggest that it is a multifunctional cell surface receptor. Herein, we characterize a series of ligands for the receptor using cell lines that stably express it as a domain IV minireceptor (mLRP1B4). Ligands of LRP including receptor-associated protein, urokinase plasminogen activator, tissue-type plasminogen activator, and plasminogen activator inhibitor type-1 each demonstrate binding, internalization, and degradation via mLRP1B4. Interestingly, the kinetics of ligand endocytosis is distinctly different from that of LRP, with LRP1B exhibiting a markedly diminished internalization rate. In addition, tissue expression analysis reveals that the LRP1B gene is expressed in brain, thyroid, and salivary gland. These studies thus extend the physiological roles of members of the LDL receptor family.

Identification of a major cyclic AMP-dependent protein kinase A phosphorylation site within the cytoplasmic tail of the low-density lipoprotein receptor-related protein: implication for receptor-mediated endocytosis

The low-density lipoprotein (LDL) receptor-related protein (LRP) is a multiligand endocytic receptor that belongs to the LDL receptor family. Recently, studies have revealed new roles of LDL receptor family members as transducers of extracellular signals. Our previous studies have demonstrated LRP phosphorylation within its cytoplasmic tail, but the nature of LRP phosphorylation and its potential function was unknown. In the present study using both in vivo and in vitro analysis, we found that LRP phosphorylation is mediated by the cAMP-dependent protein kinase A (PKA). Using site-directed mutagenesis and LRP minireceptor constructs, we further identified the predominant LRP phosphorylation site at serine 76 of its cytoplasmic tail. Finally, we demonstrated that mutations of serine 76, which abolish LRP phosphorylation by PKA, result in a decrease in the initial endocytosis rate of LRP and a lower efficiency in delivery of ligand for degradation. Thus, the role of PKA phosphorylation of LRP in receptor-mediated endocytosis may provide a mechanism by which the endocytic function of LRP can be regulated by external signals.

The YXXL motif, but not the two NPXY motifs, serves as the dominant endocytosis signal for low density lipoprotein receptor-related protein

All members of the low density lipoprotein (LDL) receptor family contain at least one copy of the NPXY sequence within their cytoplasmic tails. For the LDL receptor, it has been demonstrated that the NPXY motif serves as a signal for rapid endocytosis through coated pits. Thus, it is generally believed that the NPXY sequences function as endocytosis signals for all the LDL receptor family members. The primary aim of this study is to define the endocytosis signal(s) within the cytoplasmic tail of LDL receptor-related protein (LRP). By using LRP minireceptors, which mimic the function and trafficking of full-length endogenous LRP, we demonstrate that the YXXL motif, but not the two NPXY motifs, serves as the dominant signal for LRP endocytosis. We also found that the distal di-leucine motif within the LRP tail contributes to its endocytosis, and its function is independent of the YXXL motif. Although the proximal NPXY motif and the proximal di-leucine motif each play a limited role in LRP endocytosis in the context of the full-length tail, these motifs were functional within the truncated receptor tail. In addition, we show that LRP minireceptor mutants defective in endocytosis signal(s) accumulate at the cell surface and are less efficient in delivery of ligand for degradation.

Purification of two low-molecular-mass serine proteinase inhibitors from chicken liver

Two serine proteinase inhibitors, designated clTI-1 and clTI-2 were purified from livers of chickens to apparent homogeneity by a combination of ethanol-acetone fractionation, gel filtration and ion-exchange chromatography on CM-cellulose and Mono S columns. The inhibitor clTI-1 is a single polypeptide chain, low-molecular-mass protein (Mr about 6200), very stable to heat and ethanol. It inhibits chicken, porcine and bovine trypsins as well as human plasmin. The second protein, clTI-2 of Mr 17,000 was shown to be a very effective inhibitor of both trypsins and human cathepsin G. Since both inhibitors are sensitive to arginine modification with phenylglyoxal it is assumed that this amino acid residue is present at the P1 position of the reactive site peptide bond. The N-terminal amino acid sequence of 28 residues of clTI-2 (SVDVSKYPSTVSKDGRTLVACPRILSPV) revealed a high homology of this protein to the third domain of the chicken ovoinhibitor, whereas, the clTI-1 (APPAAEKYYSLPPGAPRYYSPVV) has some sequence identity to a fragment of the human inter-alpha-trypsin inhibitor.

Low density lipoprotein receptor-related protein (LRP) expression varies among Hep G2 cell lines

The multiligand receptor, low density lipoprotein receptor-related protein (LRP), is implicated in processes such as atherosclerosis and fibrinolysis through its mediation of the catabolism of lipoproteins, proteases, and protease inhibitor complexes. The hepatoma cell line Hep G2 expresses LRP and has been used widely to investigate the catabolism of LRP ligands including tissue-type plasminogen activator (tPA). However, the mechanism and degree by which tPA interacts with Hep G2 has been reported with some inconsistencies which may reflect variation in their level of LRP expression. To address this possibility we characterized, antigenically and functionally, LRP expression in high and low passage Hep G2 cells both from the parental line (ATCC sourced) and a cloned subline, a16. The LRP contribution to 125I-tPA binding varied from 65% for high passage a16 cells, to 20% for low passage parent cells as quantified by inhibition in the presence of 39-kD receptor associated protein (RAP) which prevents binding of all known LRP ligands. The same trend in LRP expression among Hep G2 sublines was further evident in their ability to degrade 125I-tPA and survive Pseudomonas exotoxin A challenge. These results imply wide variability in basal LRP expression among Hep G2 lines dependent on cell lineage and long-term culture conditions.

Reversible modification of tissue-type plasminogen activator by methylphosphonate esters

In spite of their rapid aqueous hydrolysis, 4-nitrophenyl 4-X-phenacyl methylphosphonates (X = H, (PMN) CH3, CH3O, Cl and NO2) inactivate many serine proteases of the pancreatic and blood coagulation systems efficiently. The rate constants, K/Ki, for the inactivation of tissue-type plasminogen activator enzyme (t-PA) are 470-750 M-1 S-1 with PMN, 4-CH3-PMN, and 4-CH3O-PMN in pH 7.8, 0.05 M Tris buffer at 7.0 +/- 0.5 degrees C, but t-PA cannot be inhibited with the 4-Cl and NO2 derivatives due to rapid competing hydrolysis. Enzyme activity returns from each enzyme-adduct at a characteristic rate, due to a self-catalyzed intramolecular reactivation process. The rate constants for spontaneous reactivation of t-PA from the adducts formed with the three inhibitors are K = 0.25-12.3 x 10(-2) min-1 at pH 7.4 and 25.0 +/- 0.1 degrees C and pH-dependent with an apparent pK approximately 8.3. The recovery of t-PA activity from the adducts in 40% human plasma buffered at pH 7.4 is the same or twice that in plain buffer. The presence of fibrin has a slight effect on inactivation but not on reactivation. The modulation of enzyme activity by reversible generation of the phosphonylated adducts has potential for medical application.

Urokinase binding and catabolism by Hep G2 cells is plasminogen activator inhibitor-1 dependent, analogous to interactions of tissue-type plasminogen activator with these cells

The adherent human hepatoma cell line Hep G2 exhibits receptor mediated endocytosis and catabolism of tissue-type plasminogen activator.plasminogen activator inhibitor type-1 (t-PA.PAI-1) complexes formed when exogenous t-PA combines with endogenous PAI-1 in the extracellular matrix. To determine whether the other major PA, urokinase (u-PA), which also complexes with PAI-1, is metabolised via the same mechanism, 125I-labelled high (hmw) and low (lmw) molecular weight forms of u-PA were incubated with Hep G2 cells at 4 degrees C for 2 hr in the absence and presence of a 100-fold excess of unlabelled ligand in order to detect specific binding. Both hmw and lmw 125I-u-PA formed complexes with PAI-1 and these bound specifically and with high affinity (apparent Kd 3.9 and 4.1 nM, with Bmax 78 x 10(3) and 83 x 10(3) binding sites/cell respectively). Binding by each form of radiolabelled u-PA was inhibited in a dose-dependent fashion by unlabelled t-PA, hmw-u-PA, lmw-u-PA, and by monoclonal anti-PAI-1 antibody. At 37 degrees C, bound hmw and lmw 125I-u-PA.PAI-1 complexes were internalised and degraded rapidly. These findings indicate that the specificity of the previously described receptor which mediates PAI-1 dependent catabolism of t-PA by Hep G2 cells extends to complexes of u-PA with this inhibitor.

Receptor-mediated endocytosis of plasminogen activators and activator/inhibitor complexes

Recent findings have elucidated the mechanism for clearance from the extracellular space of the two types of plasminogen activators, urokinase-type plasminogen activator (u-PA) and tissue-type plasminogen activator (t-PA), and their type-1 inhibitor (PAI-1). Activator/PAI-1 complexes and uncomplexed t-PA bind to the multi-ligand receptors alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein (alpha 2MR) and epithelial glycoprotein 330 (gp330). These receptors mediate endocytosis and degradation of u-PA/PAI-1 complex bound to the glycosyl phosphatidyl inositol-anchored urokinase receptor (u-PAR) on cell surfaces, and participate, in cooperation with other receptors, in hepatic clearance of activator/PAI-1 complexes and uncomplexed t-PA from blood plasma. The alpha 2MR- and gp330-mediated endocytosis of a ligand (u-PA/PAI-1 complex) initially bound to another receptor (u-PAR) is a novel kind of interaction between membrane receptors. Binding to alpha 2MR and gp330 is a novel kind of molecular recognition of serine proteinases and serpins.

Isolation and characterization of the mannose receptor from human liver potentially involved in the plasma clearance of tissue-type plasminogen activator

Various studies have shown that mannose receptors rapidly eliminate glycoproteins and microorganisms bearing high mannose-type carbohydrate chains from the blood circulation. The purpose of this study was to characterize the mannose receptor in the liver, which in vivo is involved in the rapid clearance of tissue-type plasminogen activator from the circulation. Human liver membranes were solubilized in Triton X-100, and the solution was applied to a tissue-type plasminogen activator Sepharose column. Bound proteins were eluted with ethylenediaminetetraacetate (10 mmol/L). A second, similar purification step rendered a single liver protein of 175,000 daltons. A combination of ligand blotting and a chromogenic assay for tissue-type plasminogen activator demonstrated that the identified liver protein is a mannose receptor because it bound tissue-type plasminogen activator, this tissue-type plasminogen activator binding being fully inhibited by 0.2 mol/L D-mannose. Western-blot analysis revealed that the isolated liver protein is immunologically identical to the human mannose receptor from placenta. Treatment of the liver protein and the placenta mannose receptor with trypsin yielded the same pattern of proteolytic degradation products as identified on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. We conclude that the physiologically relevant mannose receptor for tissue-type plasminogen activator clearance isolated from human liver is immunologically and structurally similar to or identical with the human mannose receptor isolated from placenta.