Autoactivation of avian urokinase-type plasminogen activator (uPA). A novel mode of initiation of the uPA/plasmin cascade

The autoactivation of human single-chain urokinase-type plasminogen activator (uPA)

Most serine proteases are synthesized as inactive zymogens that are activated by cleavage by another protease in a tightly regulated mechanism. The urokinase-type plasminogen activator (uPA) and plasmin cleave and activate each other, constituting a positive feedback loop. How this mutual activation cycle begins has remained a mystery. We used hydrogen deuterium exchange mass spectrometry to characterize the dynamic differences between the inactive single-chain uPA (scuPA) and its active form two-chain uPA (tcuPA). The results show that the C-terminal β-barrel and the area around the new N terminus have significantly reduced dynamics in tcuPA as compared with scuPA. We also show that the zymogen scuPA is inactive but can, upon storage, become active in the absence of external proteases. In addition to plasmin, the tcuPA can activate scuPA by cleavage at K158, a process called autoactivation. Unexpectedly, tcuPA can cleave at position 158 even when this site is mutated. TcuPA can also cleave scuPA after K135 or K136 in the disordered linker, which generates the soluble protease domain of uPA. Plasmin cleaves scuPA exclusively after K158 and at a faster rate than tcuPA. We propose a mechanism by which the uPA receptor dimerization could promote autoactivation of scuPA on cell surfaces. These results resolve long-standing controversies in the literature surrounding the mechanism of uPA activation.

Development of a Mammalian suspension culture for expression of active recombinant human urokinase-type plasminogen activator

The development of specific catalytic inhibitors for the serine protease urokinase-type plasminogen activator (uPA) has been hindered due to difficulties in producing sufficient amounts of active recombinant uPA that is catalytically equivalent to native uPA. The purpose of this study was to develop an efficient system for the expression of recombinant human uPA that exhibits comparable proteolytic activity to that of the native protein. Since post-translational modifications (e.g. glycosylations) of uPA are necessary for efficient proteolytic activity, we have used a mammalian cell line [Chinese hamster ovary (CHO)-S] to express recombinant human uPA. CHO-S cells were selected to stably express full-length recombinant human uPA containing a hexahistidine tag at its C-terminus to permit purification by nickel-based affinity chromatography. Secretion of recombinant uPA into the culture media was confirmed by immunoblotting and the presence of an N-linked glycosylation was confirmed by PNGase sensitivity. Enzymatic activity of purified recombinant uPA was demonstrated using zymography and quantitatively compared to native uPA by kinetic analysis using an uPA-specific substrate. Native uPA and the recombinant uPA demonstrated comparable K(m) values (55.7 and 39 muM, respectively). Furthermore, inhibition studies using benzamidine resulted in a K(i) of 195 muM for native uPA, while recombinant uPA had a K(i) of 112 muM. These data indicate that recombinant human uPA expressed by CHO-S cells is functionally comparable to native uPA.

Species identification in cell culture: a two-pronged molecular approach

Species identification of cell lines and detection of cross-contamination are crucial for scientific research accuracy and reproducibility. Whereas short tandem repeat profiling offers a solution for a limited number of species, primarily human and mouse, the standard method for species identification of cell lines is enzyme polymorphism. Isoezymology, however, has its own drawbacks; it is cumbersome and the data interpretation is often difficult. Furthermore, the detection sensitivity for cross-contamination is low; it requires large amounts of the contaminant present and cross-contamination within closely related species may go undetected. In this paper, we describe a two-pronged molecular approach that addresses these issues by targeting the mitochondrial genome. First, we developed a multiplex PCR-based assay to rapidly identify the most common cell culture species and quickly detect cross-contaminations among these species. Second, for speciation and identification of a wider variety of cell lines, we amplified and sequenced a 648-bp region, often described as the "barcode region" by using a universal primer mix targeted at conserved sequences of the cytochrome C oxidase I gene (COI). This method was challenged with a panel of 67 cell lines from 45 diverse species. Implementation of these assays will accurately determine the species of cell lines and will reduce the problems of misidentification and cross-contamination that plague research efforts.

N-terminal processing is essential for release of epithin, a mouse type II membrane serine protease

Epithin was originally identified as a mouse type II membrane serine protease. Its human orthologue membrane type-serine protease 1 (MT-SP1)/matriptase has been reported to be localized on the plasma membrane. In addition, soluble forms of matriptase were isolated from human breast milk and breast cancer cell-conditioned medium. In this paper, we report a processing mechanism that appears to be required for the release of epithin. CHO-K1 or COS7 cells transfected with single full-length epithin cDNA generated two different-sized proteins in cell lysates, 110 and 92 kDa. The 92-kDa epithin was found to be an N-terminally truncated form of the 110-kDa epithin, and it was the only form detected in the culture medium. The 92-kDa epithin was also found on the cell surface, where it was anchored by the N-terminal fragment. The results of in vivo cell labeling experiments indicate that the 110-kDa epithin is rapidly processed to the 92-kDa epithin. Using site-directed mutagenesis experiments, we identified Gly(149) of the GSVIA sequence in epithin as required for the processing and release of the protein. These results suggest that N-terminal processing of epithin at Gly(149) is a necessary prerequisite step for release of the protein.

p300-mediated acetylation of human transcriptional coactivator PC4 is inhibited by phosphorylation

The human positive coactivator 4 (PC4) acts as a general coactivator for activator-dependent transcription, the activity of which is regulated negatively by phosphorylation. We report here that PC4 can be acetylated specifically by another coactivator, p300. Interestingly, phosphorylation of PC4 by casein kinase II inhibits the p300-mediated acetylation. Mass spectral analysis revealed that there are at least two lysine residues acetylated in PC4, as a result of which its DNA binding activity is stimulated.

Activation of proPHBSP, the zymogen of a plasma hyaluronan binding serine protease, by an intermolecular autocatalytic mechanism

The hyaluronic acid binding serine protease (PHBSP), an enzyme with the ability to activate the coagulation factor FVII and the plasminogen activator precursors and to inactivate factor VIII and factor V, could be isolated from human plasma in the presence of 6M urea as a single-chain zymogen, whereas under native conditions only its activated two-chain form was obtained. The total yield of proenzyme (proPHBSP) was 5-6 mg/l, corresponding to a concentration of at least 80-100nM in plasma. Upon removal of urea, even in the absence of charged surfaces a rapid development of amidolytic activity was observed that correlated with the appearance of the two-chain enzyme. The highest activation rate was observed at pH 6. ProPHBSP processing was concentration-dependent following a second order kinetic and was accelerated by catalytic amounts of active PHBSP, indicating an intermolecular autocatalytic activation. Charged macromolecules like poly-L-lysine, heparin, and dextran sulfate strongly accelerated the autoactivation, suggesting that in vivo proPHBSP activation might be a surface-bound process. The intrinsic activity of the proenzyme was determined to be 0.25-0.3%, most likely due to traces of PHBSP. The presence of physiological concentrations of known plasma inhibitors of PHBSP, like alpha2 antiplasmin and C1 esterase inhibitor, but not antithrombin III/heparin, slowed down zymogen processing. Our in vitro data suggest that the autoactivation of proPHBSP during plasma fractionation is induced by the removal of inhibitors of PHBSP and is accelerated by charged surfaces of the chromatographic resins.