inhibitor that is rapidly converted into an inactive form at 37°C with a half life of ..... W.P., Davis, G.L., Kingsley, D., Chiu, A.T., and Reilly, T.M. (1990) Gene.
Vol. 39, No. 2, May 1996
BIOCHEMISTRY and MOLECULAR BIOLOGY INTERNATIONAL Pages235-242
HIGH-LEVEL EXPRESSION OF ACTIVE HUMAN PLASMINOGEN A C T I V A T O R I N H I B I T O R T Y P E 1 ( P A l - l ) IN E. C O L I
Aiwu Zhou, Yi Pei, Haihong Wu. Xueym Dang and Xianxiu Xu* Department of Biochemistry and National Laboratory of Pharmaceutical Biotechnology, Nanjing University, Natying 210008, P. R~ China Received January 5, 1996
SUMMARY: Plasminogen activator inhibitor 1 (PAl-l) is an important regulator of plasminogen activation, which inhibits both tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA). The DNA sequence encoding mature PAl-1 protein was inserted into an inducible expression vector. This gene was highly expressed to produce a soluble active protein in E. coil cells. The amount of the recombinant protein was up to 20% of total cellular protein. By efficient purification with a yield of about 15-20%, the recombinant protein could be purified to homogeneity with its specific activity up to 6.1x 104 (uPA) IU/mg. Its inhibitm2¢ activity declined during incubation at 37°C with a lialflife of about 2 hr. Key words: Plasminogen activator inhibitor 1 (PAl- 1), Gene expression Introduction Plasminogen activators play a central role in the regulation of the fibrinolytic system in blood. Their activity is controlled by specific inhibitors, PAls (1). Four different PAls have been identified: the endothelial cell type PAl (PAI-I), the placenta-type (PAI-2), the urinalNtype (PAI-3) and protease nexin I (PAI-4). PAI- 1 is the main physiological inlfibitor in normal plasma (2). As a member of superfamily of SERPINS (selqme protease inhibitors), it inhibits both tPA and uPA by forming SDS-stable complex with them (3). This inhibitor is produced as a 50kDa glycoprotein by a wide variety of cells in an active form, but it is a relatively labile inhibitor that is rapidly converted into an inactive form at 37°C with a half life of approximately 1-3 hr (4,5). The co~ffonnation of PAl-1 is commonly refen'ed to as latent PAIl, because inhibitory activity can be revealed by treatment with denaturants (6) or negatively
Abbreviations used: PA(s), plasminogen activator(s): PAl(s), plasminogen activator inhibitor (s); uPA, m'okinase-type plasminogen activator; tPA, tissue-type plasminogen activator; PBS: phosphate buffered saline; BSA, bovine serum albumin; SDS-PAGE, sodium dodecyl sulphate-polyac12¢lamide gel electrophoresis; DTT, dithiotlueitol. * To whom all con-espondence should be addressed.
1039-9712/96/020235--08505.00/0 235
Copyright © 1996 by Academic Press Australia. All rights of reproduction in any form reserved.
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BIOCHEMISTRY and MOLECULAR BIOLOGY INTERNATIONAL
charged phospholipids (7). Some clinical conditions suffering from coronary artery disease, venous tln'ombosis, obesity or being in the postoperative phase are associated with increased levels of PAI-t (8-14). Recent studies have shown that elevated PAI-1 levels are associated with a higher incidence of atheroscleosis and coronary heart disease (15). Now more attention has been paid to the roles of PAs and PAIs in tumor growth and metastasis (16, 17). The cloning and sequencing of cDNA coding for human PAl-1 has been described (18). The eDNA encodes a precursor polypeptide of 402 amino acid residues including a signal peptide of 21 amino acid residues. Also the natural human PAI-1 has been isolated from a variety of cell lines including HT1080 fibrosarcoma cells and endothelial cells (19, 20). However, the purified protein exists chiefly in an inactive or latent form, Low expression levels and latent protein have also hampered efforts to produce recombinant PAI-I in sufficient quantities for biological studies. Tiffs report describes the high level expression of nnglycosylated 381-residue PAl-1 as a soluble active protein in E. coli and the procedures for its purification. Materials and Methods Materials Restriction endonuclease, T4 DNA ligase and Calf Intestinal Alkaline Phosphatase were purchased from Promega Corp. and New England Biolabs. These en,zymes were used in according with the instructions of the suppliers. The PCR kit and T7 DNA sequencing kit were purchased from Promega Corp. The clu'omogenic plasmin substrate S2444 (Pyr-Ghi-Gly-A1g-pNA) was purchased from Sigma. Q-sepharose was product of Phalanacia. Rabbit antiserum to human PAl- 1 was kindly bestowed by Doctor Cao Qiangrong. Bacteria and plasmids E. coli BL21 (DE3)~ which contains an integrated copy of the T7 RNA polymerase gene, was used as the host cells for expression plasmids. E. coli DH5c~ was used for cloning plasmids. The PAI-1 expression plasmid construction made use of plasmid pET 11c (a generous gift from Dr. Lin Xiaozhong, Shanghai Institute of Biochemistry, Shanghai, China), and the eDNA sequence encoding PAI-1 was obtained from the plasmid pUC-PAI-1 (kindly bestowed by Dr. Cao Qiangrong, Department of Biology, Naniing Teacher's University, Nanjing, China). Two PCR primers were synthesized to reconstruct a full-length eDNA sequence encoding mature PAI-1. The 5'-primer (28mer: 5'-GCGAATTCA TATGGTGCACCATCCCCCA-3') can anneal pal"tially to the 5'-terminus of PAI- 1 gene. The 3'-p12mer (17mer: 5'-GC~CCCATGAGCTCCTT-3') can anneal wholly to the eDNA sequence between 381-398. The template DNA was pUC-PAI-I. After 30 cycles of amplification, the production was recovered and digested with Nde I and Sac I . Then this fi'agment and the large fragment of PAI- 1 eDNA which was digested with Sac I and Bgl II, were ligated and cloned into the Nde I and BamH I sites ofplasmid pETI le. Tlie construct was produced by standard techniques. The procedure of the construction was illustrated in Fig. 1. Production of PAl-1 Transfonlaants harbom~g the expression plasmid pET1 lc-PAl- 1 were incubated in LB medium supplemented with ampicillin (50 ug/ml) at 37°C ovemiglit and then the cultures were inoculated in fresh LB medium with ampicillin (25 ug/ml) at a ratio of 1:20. An hour late, expression of recombinant PAI-I was induced with IPTG (0.2 mM). After 1.5 hi- of incubation the cells were hax~cested, sonicated and centrifuged to obtain a clarified supentatant as starting material.
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SDS-PAGE and Western blotting Electrophoresis was perfolaned under reduced conditions (21). The separated proteins were electrophoreticaUy transferred to nitrocellulose membrane (Schleicher & Schuell Corp.), which was then blocked for 1 hr with 1% non-fat milk solution in TBS (Tris-buffered saline) and incubated 2 hr at 37°C with the anti-PAI-1 polyclunal antibodies at the concentration of I:500 in 1% nonfat milk solution in TBS. The membrane was washed with TBS and treated with 1:3000 dilution of Horseradish Peroxidaseconjugated goat anti-rabbit innnnnoglobulin G ( BRL Corp.) for l hi" at 37°C. The band was visualized by incubating the membrane for 3 minutes in the substrate solution of 6 mg diaminobenzidine tetrahydrockloride in 10 ml of t0 mM Tfis-C1 buffer, pH7.5. PAI-I activity assay Inhibitopy activity was measured by determining the inhibition of uPA activity using a synthetic substrate S-2444. The assay was performed in a manner similar to that described by Wun (22). One inhibitol3, unit was defined as the activity to inhibit one Ploug unit ofurokinase. Protein determination: Protein concentrations were measured by the method of Low~2¢ et al (23) using BSA as a standard.
Results Construction and Characterization of PAI-1 expression plasmid
For expression of
PAI-I ha E. coil, the DNA sequence encoding the signal peptide of PAI-1 was removed by PCR technique and the modified PAI-I gene was cloned into expression plasmid pET1 lc which contains the lac operator next to a T7 phage promoter. The procedure of the construclion was illustrated in Fig. 1. The recombinant e:~pression plasmid (pETllc-PAI-I) was confuaned to be correct by partial DNA sequencing (data not shown).
Productivity of recombinant PAl-1 in E. coil The transformants containing the plasmid pET1 Ic-PAI-1 were cultivated and assayed by SDS-PAGE to estimate the amount and size of the recombinant PAl-1 (rPAl-1 ) produced in the total cell lysates (Fig. 2.A). rPAlI was detected on Coomassie Brilliant Blue R-250-stained gel as a distinct band with the mobility about 42 kDa which was consistent with the predicted Mr for translation product of the PAl-l expression plasmid. No heavily stained protein bands in the IVh"region of the PAl-1 were observed in the uninduced control indicating that the T7 promoter was fully repressed. The expression level ofrPAl-1 was measured by densitometfic scanning of Coomassie Brilliant Blue R-250-stained gel and the amount of expressed PAl- 1 was about 20% of the total cellular proteins. The potypeptide band corresponding to 42 kDa was confirmed by immunoblotting with the polyclonal anti-human PAl-l antibody (Fig. 2.B). When the cell lysate fiom expression cultures was tested for PAl-I activity, the activity is evident. The total activities were about 5.0× l05 (uPA) IU/g wet cells. On the coutraw, no activity was seen in the lysate of control ceils containing plasmid pET I l c. No stinaulation of rPAI-1 was obsecved following treatment of cell lysate fiom expression cultures with 4M guanidine/HC1. This result indicated that the rPA]-1 expressed in BL21 (DE3) is an active rather than a latent form.
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Vol. 39, No. 2, 1996
A
BIOCHEMISTRY and MOLECULAR BIOLOGY INTERNATIONAL
PCR primerl (28mer): 5'-GCGA.ATTCATATGGTGCACCATCCCCCA-Y PCR primer2 ( 17mer): Y-TTCCTCGAGTACCCCGG-5' PAI-1 gene in pUC18: ~p_~rtmerI Sac I
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i
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i
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+1
+381
Natural mqmspaltclvlNalvfgeg SAVHHPP...VMEP rPM-1 m SAVHHPP...VMEP Fig. 1. The construction of expression plasmid pET 1 l c-PAl-1 (A) and the comparison of amino terminal residues of PAI-1 with those of natural PAI-I (B). Abbreviations: P, T7 promoter; O, lac operator; T, T7 terminator; SD, Shine-Delgarno sequence; ATG, translational start codon; TGA: translational termination codon. V1, pUC18 vector; V2, pETI lc vector.
Purification of active rPAI-1
Since the active rPAI- 1 was liable to lose its activity hi
room temperature, the following steps were performed at 4°C. The starting material was first fi'actionated by 65% saturated ammonium sulfate. Then the precipitate including active rPAl- I was dissolved in 10 mM PBS (pH7.5), dialyzed against the same buffer at 4°C overnight before loaded onto a DE52 column. After completely washed with Buffer A (10 mM PBS pH7.5, containing 30 mM NaC1, 0.1 mM DTT and 0.01% Tween 80), rPAI-1 was eluted with Buffer B (10 mM PBS, pHT.5, containing 60 mM NaC1, 0.1 mM DTT and 0.01% Tween 80). We found that the rPAI-1 activity emerged in the form of a major peak followed by a small shoulder (Fig. 3.A), but by SDS-PAGE, the rPA1-1 in two peaks had no apparent difference in their molecular weight. Because the major peak contained most of the activity and less contaminant, the major peak was pooled, diluted 1:1 with 20 mM Tris-Cl buffer (pH7.5) containing 0.2 mM DTT and 0.01% Tween 80, and then applied onto a Q-sepharose column. The bomld protein was washed with a 200 ml linear gradient in the range 0 -0.4 M NaC1 in 10 mM Tris-C1 buffer, pH7.5, containing 0.1 mM DTT and 0.01% Tween 80 (Fig. 3.B). The fractions eluted near 0.20 M NaC1 with PA[-I activity were pooled and stored at -70°C. In this way the rPAl-1 can be purified to homogeneity (Fig. 2.C). The four-step ptmfication procedure for rPAI- 1 is summmized in Table 1. 238
Vol. 39, No. 2, 1996
BIOCHEMISTRY and MOLECULAR BIOLOGY INTERNATIONAL
A k~a
1
2
B 3
4
5
C 6
kDa 7 97-64--
97-64--
8
9
10
43--
43--
29-
29-
17--
17--
Fig. 2. SDS-PAGE. (A) Coomassie Brilliant Blue R-250 staining gel. (B) Western blotting. (C) Characterization o f purified rPAI-I by silver staining SDS-PAGE gel. Lane 1, 7: molecular mass standards, with masses indicated in kDa; Lane 2, 5 : p E T 1 lc in E. coli BL21 (DE3); Lane 3 : p E T l l c - P A I - 1 in BL21 (DE3)uninduced. Lane 4, 6: p E T I lc-PAI-1 in BL21 (DE3) induced, with arrow indicating the band identified as rPAI-1. Lane 8: rPAI-1 fraction in the major peak from DE52 clu'omatography (see Fig. 3.A). Lane 9, 10: rPAI-1 fractions fiom Q-sepharose chromatography (see Fig. 3.B).
Characterization of purified rPAI-1
The pm-ified rPAI-1 was subjected to SDS-
PAGE. There was only one band in the gel stained by Coomassie Bl~liant Blue R-250 as well as by silver (Fig. 2.C). Isoelectfic focusing assay of PAl-1 also showed a single band and the isoelectfic point (pI) was determined to be 5.5 (data not shown). The specific activity was about 6.1 x 104 (uPA) IU/mg. The rPAI- 1 activity declined with a half life of approximately 2 Ill" when incubated at 37°C which was similar to the native active PAI-1 (5). Discussion There have been earlier reports of PAl- 1 being produced in both prokaryotic system and eukaryotic system (4,5, 24-26), but these did not encompass high-level expression of active PAI-I in E. coll. The gene was expressed at a level of 10-15% of the total cell protein when laid downstream of E. coil trp promoter, but most of them were ill latent form (4). In kpk promoter expression system and E. coli tac promoter expression system, the recombinam protein was produced in an active fonn, but the level of expression was only about 3-7% ot' the total cell protein (5, 26).
hi this repolt we described the production, purification and
characterization of active recombinant PAl-1 fi'om proka2cotic E. coli cells.
Expression of
rPAI-1 is achieved in bacteria by constructing a plasmid containing the modified PAl-1 gene
239
Vol. 39, No. 2, 1996
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BIOCHEMISTRYond MOLECULAR BIOLOGY INTERNATIONAL
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