SP-40,40 Is a Component of Plasminogen Activator Inhibitor-1-Binding Protein and Stabilizes Plasminogen Activator Inhibityor-1 Activity.

Abstract

SP-40,40 is an 80-kDa glycoprotein consisting of two 40-kDa subunits (a-chain and b-chain). It is a multi-functional protein which modulates the membrane attack complex of the complement, [1][2][3] causes cell aggregation, 4) accelerates immune complex formation 5) and binds to b-endorphin.6)The induction of SP-40,40 synthesis in the hippocampi of patients with Alzheimer's disease has been reported, 7,8) and we have also reported the incorporation of SP-40,40 into senile plaque.9) It was also reported that SP-40,40 existed in serum as a minor component of high density lipoproteins. [10][11][12] From its structural similarity, SP-40,40 is thought to be a human counterpart of ram clusterin, which causes cell aggregation, 13) rat SGP-2, which participates in spermatogenesis, 14) rat TRPM-2, which is induced during programmed cell death (apoptosis), 15) quail T64, which is induced by src gene expression, 16) bovine GPIII, which is purified from the chromaffin granule membrane of adrenal medula 17) and canine gp80, which is a marker protein of apical secretion. 18)Plasminogen activator inhibitor-1 (PAI-1) is an unstable protein which binds to plasminogen activator (PA) and inhibits its activity. 19,20) It was reported that S-protein (vitronectin) bound to PAI-1 and stabilized its activity. 21,22) Declerck et al. purified PAI-1-binding protein (PAI-1-BP), and it was shown to consist of S-protein and unidentified 40-kDa protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing conditions. 23) From the functional similarity of S-protein and SP-40,40, e.g., inhibition of the complement system 1-3) and binding to b-endorphin, 6) we supposed that the unidentified 40-kDa protein might be SP-40,40. In this paper, I describe that PAI-1-BP consists of S-protein and SP-40,40, and that SP-40,40 stabilizes PAI-1 activity as well as S-protein does. MATERIALS AND METHODSMaterials Plasminogen, urine plasminogen activator (uPA) and fibrinogen were purchased from Sigma. Fibrinogen was cleaved with BrCN in 70% formic acid and dialyzed against distilled water as described by Verheijen et al. 24) S-2251 was obtained from Kabi. Recombinant tissue plasminogen activator (tPA) was kindly gifted from Yamanouchi Co., Ltd. Outdated human plasma was obtained from the Japanese Red Cross Tokyo Blood Center, Musashino, Tokyo.Purification of PAI-1 PAI-1 was purified from the culture medium of HT1080 (human fibrosarcoma cell) according to Andreasen et al.19) The cells were maintained as a monolayer culture in Dulbecco-modified Eagle's medium (Gibco) supplemented with 10% fetal bovine serum (Gibco) and 1 mM dexamethasone (Sigma). After the cells reached 80% confluence, the medium was changed to serum-free medium containing 1 mM dexamethasone, and the culture was continued for 3 d. PAI-1 (500 mg) was purified from 1 l of the serum-free conditioned medium using a Con A-Sepharose (20 ml, Pharmacia) column. Purified PAI-1 was activated in 12 M urea at 37°C for 30 min and dialyzed against PBS at 4°C overnight. Purific...