Cloning, expression, purification, and biological activity of recombinant native and variant human alpha 1-antichymotrypsins

Proteomic analysis of multiple organ dysfunction induced by rhabdomyolysis

Rhabdomyolysis (RM) leads to dysfunction in the core organs of kidney, lung and heart, which is an important reason for the high mortality and disability rate of this disease. However, there is a lack of systematic research on the characteristics of rhabdomyolysis-induced injury in various organs and the underlying pathogenetic mechanisms, and especially the interaction between organs. We established a rhabdomyolysis model, observed the structural and functional changes in kidney, heart, and lung. It is observed that rhabdomyolysis results in significant damage in kidney, lung and heart of rats, among which the pathological damage of kidney and lung was significant, and of heart was relatively light. Meanwhile, we analyzed the differentially expressed proteins (DEPs) in the kidney, heart and lung between the RM group and the sham group based on liquid chromatography-tandem mass spectrometry (LC-MS/MS). In our study, Serpina3n was significantly up-regulated in the kidney, heart and lung. Serpina3n is a secreted protein and specifically inhibits a variety of proteases and participates in multiple physiological processes such as complement activation, inflammatory responses, apoptosis pathways, and extracellular matrix metabolism. It is inferred that Serpina3n may play an important role in multiple organ damage caused by rhabdomyolysis and could be used as a potential biomarker. This study comprehensively describes the functional and structural changes of kidney, heart and lung in rats after rhabdomyolysis, analyzes the DEPs of kidney, heart and lung, and determines the key role of Serpina3n in multiple organ injury caused by rhabdomyolysis. SIGNIFICANCE: This study comprehensively describes the functional and structural changes of kidney, heart and lung in rats after rhabdomyolysis, analyzes the DEPs of kidney, heart and lung, and determines the key role of Serpina3n in multiple organ injury caused by rhabdomyolysis.

Immune-related SERPINA3 as a biomarker involved in diabetic nephropathy renal tubular injury

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease and has become a serious medical issue globally. Although it is known to be associated with glomerular injury, tubular injury has been found to participate in DN in recent years. However, mechanisms of diabetic renal tubular injury remain unclear. Here, we investigated the differentially expressed genes in the renal tubules of patients with DN by analyzing three RNA-seq datasets downloaded from the Gene Expression Omnibus database. Gene set enrichment analysis and weighted gene co-expression network analysis showed that DN is highly correlated with the immune system. The immune-related gene SERPINA3 was screened out with lasso regression and Kaplan–Meier survival analyses. Considering that SERPINA3 is an inhibitor of mast cell chymase, we examined the expression level of SERPINA3 and chymase in human renal tubular biopsies and found that SERPINA3 was upregulated in DN tubules, which is consistent with the results of the differential expression analysis. Besides, the infiltration and degranulation rates of mast cells are augmented in DN. By summarizing the biological function of SERPINA3, chymase, and mast cells in DN based on our results and those of previous studies, we speculated that SERPINA3 is a protective immune-related molecule that prevents renal tubular injury by inhibiting the proliferation and activation of mast cells and downregulating the activity of chymase.

Serum Proteomics in COVID-19 Patients: Altered Coagulation and Complement Status as a Function of IL-6 Level

Over 5 million people around the world have tested positive for the beta coronavirus SARS-CoV-2 as of May 29, 2020, a third of which are in the United States alone. These infections are associated with the development of a disease known as COVID-19, which is characterized by several symptoms, including persistent dry cough, shortness of breath, chills, muscle pain, headache, loss of taste or smell, and gastrointestinal distress. COVID-19 has been characterized by elevated mortality (over 100 thousand people have already died in the US alone), mostly due to thromboinflammatory complications that impair lung perfusion and systemic oxygenation in the most severe cases. While the levels of pro-inflammatory cytokines such as interleukin-6 (IL-6) have been associated with the severity of the disease, little is known about the impact of IL-6 levels on the proteome of COVID-19 patients. The present study provides the first proteomics analysis of sera from COVID-19 patients, stratified by circulating levels of IL-6, and correlated to markers of inflammation and renal function. As a function of IL-6 levels, we identified significant dysregulation in serum levels of various coagulation factors, accompanied by increased levels of antifibrinolytic components, including several serine protease inhibitors (SERPINs). These were accompanied by up-regulation of the complement cascade and antimicrobial enzymes, especially in subjects with the highest levels of IL-6, which is consistent with an exacerbation of the acute phase response in these subjects. Although our results are observational, they highlight a clear increase in the levels of inhibitory components of the fibrinolytic cascade in severe COVID-19 disease, providing potential clues related to the etiology of coagulopathic complications in COVID-19 and paving the way for potential therapeutic interventions, such as the use of pro-fibrinolytic agents. Raw data for this study are available through ProteomeXchange with identifier PXD020601.

Identification of diagnostic upper gastrointestinal cancer tissue type-specific urinary biomarkers

Several potential urinary biomarkers exhibiting an association with upper gastrointestinal tumour growth have been previously identified, of which S100A6, S100A9, rabenosyn-5 and programmed cell death 6-interacting protein (PDCD6IP) were further validated and found to be upregulated in malignant tumours. The cancer cohort from our previous study was subclassified to assess whether distinct molecular markers can be identified for each individual cancer type using a similar approach. Urine samples from patients with cancers of the stomach, oesophagus, oesophagogastric junction or pancreas were analysed by surface-enhanced laser desorption/ionization-time-of-flight mass spectrometry using both CM10 and IMAC30 (Cu²⁺-complexed) chip types and LC-MS/MS-based mass spectrometry after chromatographic enrichment. This was followed by protein identification, pattern matching and validation by western blotting. We found 8 m/z peaks with statistical significance for the four cancer types investigated, of which m/z 2447 and 2577 were identified by pattern matching as fragments of cathepsin-B (CTSB) and cystatin-B (CSTB); both molecules are indicative of pancreatic cancer. Additionally, we observed a potential association of upregulated α-1-antichymotrypsin with pancreatic and gastric cancers, of PDCD6IP, vitelline membrane outer layer protein 1 homolog (VMO1) and triosephosphate isomerase (TPI1) with oesophagogastric junctional cancers, and of complement C4-A, prostatic acid phosphatase, azurocidin and histone-H1 with oesophageal cancer. Furthermore, the potential pancreatic cancer biomarkers CSTB and CTSB were validated independently by western blotting. Therefore, the present study identified two new potential urinary biomarkers that appear to be associated with pancreatic cancer. This may provide a simple, non-invasive screening test for use in the clinical setting.

Plasma proteome changes in cord blood samples from preterm infants

OBJECTIVE

In the presented study, we aimed to systematically analyze plasma proteomes in cord blood samples from preterm infants stratified by their gestational age to identify proteins and related malfunctioning pathways at birth, possibly contributing to the complications observed among preterm infants.

STUDY DESIGN

Preterm newborns were enrolled of three subgroups with different gestation age: newborns born ≤26 (group 1), between 27 and 28 (group 2) and between 29 and 30 (group 3) weeks of gestation, respectively, and compared to the control group of healthy, full-term newborns in respect to their plasma proteome composition.

RESULT

Preterm delivery is associated with multiple protein abundance changes in plasma related to a plethora of processes, including inflammation and immunomodulation, coagulation, and complement activation as some key features.

CONCLUSION

Plasma proteome analysis revealed numerous gestation-age-dependent protein abundance differences between term and preterm infants, which highlight key dysregulated pathways and potential new protein treatment targets.

An RNA Aptamer Inhibits a Mutation-Induced Inactivating Misfolding of a Serpin

Most serpins are fast and specific inhibitors of extracellular serine proteases controlling biological processes such as blood coagulation, fibrinolysis, tissue remodeling, and inflammation. The inhibitory activity of serpins is based on a conserved metastable structure and their conversion to a more stable state during reaction with the target protease. However, the metastable state also makes serpins vulnerable to mutations, resulting in disease caused by inactive and misfolded monomeric or polymeric forms ("serpinopathy"). Misfolding can occur either intracellularly (type-I serpinopathies) or extracellularly (type-II serpinopathies). We have isolated a 2'-fluoropyrimidine-modified RNA aptamer, which inhibits a mutation-induced inactivating misfolding of the serpin α1-antichymotrypsin. It is the first agent able to stabilize a type-II mutation of a serpin without interfering with the inhibitory mechanism, thereby presenting a solution for the long-standing challenge of preventing pathogenic misfolding without compromising the inhibitory function.

Mineralocorticoid receptors are present in skeletal muscle and represent a potential therapeutic target

Early treatment with heart failure drugs lisinopril and spironolactone improves skeletal muscle pathology in Duchenne muscular dystrophy (DMD) mouse models. The angiotensin converting enzyme inhibitor lisinopril and mineralocorticoid receptor (MR) antagonist spironolactone indirectly and directly target MR. The presence and function of MR in skeletal muscle have not been explored. MR mRNA and protein are present in all tested skeletal muscles from both wild-type mice and DMD mouse models. MR expression is cell autonomous in both undifferentiated myoblasts and differentiated myotubes from mouse and human skeletal muscle cultures. To test for MR function in skeletal muscle, global gene expression analysis was conducted on human myotubes treated with MR agonist (aldosterone; EC50 1.3 nM) or antagonist (spironolactone; IC50 1.6 nM), and 53 gene expression differences were identified. Five differences were conserved in quadriceps muscles from dystrophic mice treated with spironolactone plus lisinopril (IC50 0.1 nM) compared with untreated controls. Genes down-regulated more than 2-fold by MR antagonism included FOS, ANKRD1, and GADD45B, with known roles in skeletal muscle, in addition to NPR3 and SERPINA3, bona fide targets of MR in other tissues. MR is a novel drug target in skeletal muscle and use of clinically safe antagonists may be beneficial for muscle diseases.

Tomato cystatin SlCYS8 as a stabilizing fusion partner for human serpin expression in plants

Studies have reported the usefulness of fusion proteins to bolster recombinant protein yields in plants. Here, we assess the potential of tomato SlCYS8, a Cys protease inhibitor of the cystatin protein superfamily, as a stabilizing fusion partner for human alpha-1-antichymotrypsin (α1ACT) targeted to the plant cell secretory pathway. Using the model expression platform Nicotiana benthamiana, we show that the cystatin imparts a strong stabilizing effect when expressed as a translational fusion with α1ACT, allowing impressive accumulation yields of over 2 mg/g of fresh weight tissue for the human serpin, a 25-fold improvement on the yield of α1ACT expressed alone. Natural and synthetic peptide linkers inserted between SlCYS8 and α1ACT have differential effects on protease inhibitory potency of the two protein partners in vitro. They also have a differential impact on the yield of α1ACT, dependent on the extent to which the hybrid protein may remain intact in the plant cell environment. The stabilizing effect of SlCYS8 does not involve Cys protease inhibition and can be partly reproduced in the cytosol, where peptide linkers are less susceptible to degradation. The effect of SlCYS8 on α1ACT yields could be explained by: (i) an improved translation of the human protein coding sequence; and/or (ii) an overall stabilization of its tertiary structure preventing proteolytic degradation and/or polymerization. These findings suggest the potential of plant cystatins as stabilizing fusion partners for recombinant proteins in plant systems. They also underline the need for an empirical assessment of peptide linker functions in plant cell environments.

Nuclear α1-antichymotrypsin promotes chromatin condensation and inhibits proliferation of human hepatocellular carcinoma cells

BACKGROUND & AIMS

α1-Antichymotrypsin (α1-ACT), a member of the serpin family (SERPINA3), is an acute-phase protein secreted by hepatocytes in response to cytokines such as oncostatin M. α1-ACT is a protease inhibitor thought to limit tissue damage produced by excessive inflammation-associated proteolysis. However, α1-ACT also is detected in the nuclei of cells, where its activities are unknown. Expression of α1-ACT is down-regulated in human hepatocellular carcinoma (HCC) tissues and cells; we examined its roles in liver regeneration and HCC proliferation.

METHODS

We measured levels of α1-ACT messenger RNA in human HCC samples and healthy liver tissue. We reduced levels of α1-ACT using targeted RNA interference in human HCC (HepG2) and mouse hepatocyte (AML12) cell lines, and overexpressed α1-ACT from lentiviral vectors in Huh7 (HCC) cells and adeno-associated viral vectors in livers of mice. We assessed proliferation, differentiation, and chromatin compaction in cultured cells, and liver regeneration and tumor formation in mice.

RESULTS

Reducing levels of α1-ACT promoted proliferation of HCC cells in vitro. Oncostatin M up-regulated α1-ACT expression and nuclear translocation, which inhibited HCC cell proliferation and activated differentiation of mouse hepatocytes. We identified amino acids required for α1-ACT nuclear localization, and found that α1-ACT inhibits cell-cycle progression and anchorage-independent proliferation of HCC cells. HCC cells that overexpressed α1-ACT formed smaller tumors in mice than HCC cells that did not express the protein. α1-ACT was observed to self-associate and polymerize in the nuclei of cells; nuclear α1-ACT strongly bound chromatin to promote a condensed state that could prevent cell proliferation.

CONCLUSIONS

α1-ACT localizes to the nuclei of hepatic cells to control chromatin condensation and proliferation. Overexpression of α1-ACT slows the growth of HCC xenograft tumors in nude mice.

A limited role for gene duplications in the evolution of platypus venom

Gene duplication followed by adaptive selection is believed to be the primary driver of venom evolution. However, to date, no studies have evaluated the importance of gene duplications for venom evolution using a genomic approach. The availability of a sequenced genome and a venom gland transcriptome for the enigmatic platypus provides a unique opportunity to explore the role that gene duplication plays in venom evolution. Here, we identify gene duplication events and correlate them with expressed transcripts in an in-season venom gland. Gene duplicates (1,508) were identified. These duplicated pairs (421), including genes that have undergone multiple rounds of gene duplications, were expressed in the venom gland. The majority of these genes are involved in metabolism and protein synthesis not toxin functions. Twelve secretory genes including serine proteases, metalloproteinases, and protease inhibitors likely to produce symptoms of envenomation such as vasodilation and pain were detected. Only 16 of 107 platypus genes with high similarity to known toxins evolved through gene duplication. Platypus venom C-type natriuretic peptides and nerve growth factor do not possess lineage-specific gene duplicates. Extensive duplications, believed to increase the potency of toxic content and promote toxin diversification, were not found. This is the first study to take a genome-wide approach in order to examine the impact of gene duplication on venom evolution. Our findings support the idea that adaptive selection acts on gene duplicates to drive the independent evolution and functional diversification of similar venom genes in venomous species. However, gene duplications alone do not explain the "venome" of the platypus. Other mechanisms, such as alternative splicing and mutation, may be important in venom innovation.

Production of recombinant serpins in Escherichia coli

Serpins represent a diverse family of proteins that are found in a wide range of organisms and cellular locations. In order to study them, most need to be produced recombinantly, as isolation from their source is not always possible. Due to their relatively uncomplicated structure (single domain, few posttranslational modifications), the serpins are usually amenable to expression in Escherichia coli, which offers a fast and cost-effective solution for the generation of large amounts of protein. This chapter outlines the general procedures used in the expression and subsequent purification of serpins in E. coli, with a particular focus on the methods used for antitrypsin, the archetypal member of the family.

A companion protease inhibitor for the protection of cytosol-targeted recombinant proteins in plants

We reported earlier the potential of tomato cathepsin D inhibitor (SlCDI) as an in-built stabilizing agent for the protection of recombinant proteins in transgenic plant leaf crude extracts (Plant Biotechnol J.4, 359-368). Here we document the potential of SlCDI for the in situ protection of proteins in potato leaves. Total protein assays with control and SlCDI-expressing potato lines indicated a positive impact of slcdi transgene expression on leaf protein content, with a mean relative increase of 35%-40% depending on the light regime. Out of approximately 700 proteins detected on 2-D gels, only 20 exhibited a significantly altered level on a protein-specific basis, whereas most proteins were up-regulated on a leaf fresh weight basis, albeit at variable rates. Quantitative reverse trancriptase-PCR assays for rubisco activase showed similar transcript levels in leaves of test and control lines despite protein levels increased by two- to threefold in SlCDI-expressing lines. These observations, along with the unrelated biological functions assigned to MS-identified proteins up-regulated in leaves and protease assays showing slightly increased proteasome activity in protein extracts of SlCDI-expressing lines, suggest a general, proteasome-independent protein stabilizing effect of SlCDI in planta. Transient expression assays with human alpha(1)-antichymotrypsin also showed a stabilizing effect for SlCDI on heterologous proteins, leading to net levels of the human protein increased by approximately 2.5-fold in SlCDI-expressing plants. These data illustrate, overall, the potential of SlCDI as an in vivo protein-stabilizing agent in transgenic plant systems, useful to improve protein levels and recombinant protein accumulation.

Effect of reactive center loop structure of antichymotrypsin on inhibition of duodenase activity

Interaction between duodenase (a granase family member) from bovine duodenal mucosa and recombinant antichymotrypsin (rACT) and its P1 variants has been studied. Association rate constants (k(a)) were 11, 6.8, and 17 mM(-1).sec(-1) for rACT, ACT L358M, and ACT L358R, respectively. Natural antitrypsin (AT) compared to ACT was a 20 times more effective duodenase inhibitor (in terms of k(a)). Duodenase interacted with P1 variants of ACT via a suicide mechanism with stoichiometry of the process SI = 1.2. The nature of the P1 residue of the inhibitor did not influence the interaction if other residues did not meet conformational requirements of the duodenase substrate-binding pocket. Also, interaction of duodenase with ACT variants containing residues from AT reaction center loop (rACT P2-P3', rACT P3-P4', rACT P4-P3', and rACT P6-P4') was studied. The inhibition type ([E](0) = 1.10(-7) M, 25 degrees C) was revealed to be reversible-like, and efficacy of inhibition decreased with increase in the substituted part of the reactive center loop. Constants of inhibition (K(i)) were measured. Efficacy of interaction between the enzyme (duodenase) and inhibitor depends on topochemical correspondence between a substrate-binding pocket of the enzyme and substrate structure.

Targeting and post-translational processing of human alpha1-antichymotrypsin in BY-2 tobacco cultured cells

The post-translational processing of human alpha(1)-antichymotrypsin (AACT) in Bright Yellow-2 (BY-2) tobacco cells was assessed in relation to the cellular compartment targeted for accumulation. As determined by pulse-chase labelling experiments and immunofluorescence microscopy, AACT sent to the vacuole or the endoplasmic reticulum (ER) was found mainly in the culture medium, similar to a secreted form targeted to the apoplast. Unexpectedly, AACT expressed in the cytosol was found in the nucleus under a stable, non-glycosylated form, in contrast with secreted variants undergoing multiple post-translational modifications during their transit through the secretory pathway. All secreted forms of AACT were N-glycosylated, with the presence of complex glycans as observed naturally on human AACT. Proteolytic trimming was also observed for all secreted variants, both during their intracellular transit and after their secretion in the culture medium. Overall, the targeting of human AACT to different compartments of BY-2 tobacco cells led to the production of two protein products: (i) a stable, non-glycosylated protein accumulated in the nucleus; and (ii) a heterogeneous mixture of secreted variants resulting from post-translational N-glycosylation and proteolytic processing. Overall, these data suggest that AACT is sensitive to resident proteases in the ER, the Golgi and/or the apoplast, and that the production of intact AACT in the plant secretory pathway will require innovative approaches to protect its structural integrity in vivo. Studies are now needed to assess the activity of the different AACT variants, and to identify the molecular determinants for the nuclear localization of AACT expressed in the cytosol.

Nucleocytoplasmic transit of human alpha1-antichymotrypsin in tobacco leaf epidermal cells

Recently, we have observed a nuclear localization for human alpha(1)-antichymotrypsin (AACT) expressed in the cytosol of transgenic Bright Yellow-2 (BY-2) tobacco cultured cells (see accompanying paper: Benchabane, M., Saint-Jore-Dupas, C., Bardor, M., Faye, L., Michaud, D. and Gomord, V. (2008a) Targeting and post-translational processing of human alpha(1)-antichymotrypsin in BY-2 tobacco cultured cells. Plant Biotechnol. J. doi: 10.1111/j.1467-7652.2008.00382.x). In the present article, we assess whether the intrinsic DNA-binding activity of AACT can explain its nuclear localization, and whether this same activity has an impact on its protease inhibitory potency and stability in planta. An engineered form of AACT with no DNA-binding activity, rAACTDeltaK, was compared with the wild-type polypeptide, rAACT, in terms of chymotrypsin inhibitory potency, stability in planta and distribution in tobacco cells. In accordance with available data reporting distinct sites for protease inhibition and DNA binding, rAACT and rAACTDeltaK showed similar antichymotrypsin activity, similar to the activity of native AACT purified from human plasma. As observed for AACT in BY-2 tobacco cells, a green fluorescent protein (GFP)-AACT fusion transiently expressed in the cytosol of tobacco leaf epidermal cells was detected mainly in the nucleus by confocal laser microscopy. By contrast, rAACTDeltaK expressed as a GFP fusion showed a balanced distribution between the cytosol and the nucleus, similar to the distribution pattern of free GFP exhibiting no DNA-binding affinity. In line with immunodetection data showing higher accumulation levels for GFP-AACT in tobacco leaf cells, rAACTDeltaK was more susceptible than rAACT to tryptic digestion in the presence of DNA. Overall, these observations suggest the following: (i) a retention effect of DNA on AACT in the nucleus; and (ii) a stabilizing effect of the AACT-DNA interaction on rAACT challenged with non-target proteases, which, possibly, may be useful in protecting this protein in plant expression platforms.

Short-lived protease serpin complexes: partial disruption of the rat trypsin active site

Serpins inhibit serine proteases by mechanically disrupting the protease active site. The protease first reacts with the serpin's reactive center loop (RCL) to form an acylenzyme. Then the RCL inserts into a beta-sheet in the body of the serpin, translocating the attached protease approximately 70 A and deforming the protease active site, thereby trapping the acylenzyme. Loop insertion (approximately 1 s(-1)) is an order of magnitude slower than hydrolysis of a typical substrate acylenzyme (approximately 50 s(-1)), indicating that the protease is inhibited during translocation. We have previously trapped a partially translocated covalent complex of rat trypsin and alpha1-proteinase inhibitor (EpartI*) resulting from attractive interactions between cationic dyes and anionic rat trypsin. Here, using single pair Förster resonance energy transfer, we demonstrate that EpartI* is a metastable complex that can dissociate to free protease and cleaved serpin (I*) as well as convert to the canonical fully translocated complex EfullI*. The partitioning between these two pathways is pH dependent, with conversion favored at low pH and dissociation favored at high pH. The short lifetime of EpartI* (approximately 3 h at pH 7.4) and the pH dependence of EpartI* dissociation suggest that, unlike in EfullI*, the catalytic triad is intact in EpartI*. These results also demonstrate that interactions between target proteases and the body of the serpin can hinder protease translocation leading to short-lived covalent complexes.

Multiple domains of endopin 2A for serpin cross-class inhibition of papain

The serpin endopin 2A inhibits the cysteine protease papain in cross-class inhibition. This study demonstrates the novel finding that both the non-RSL NH(2)-domain and the RSL domain with P1-P1' residues participate in endopin 2A inhibition. Production of a chimeric mutant of endopin 2A with replacement of its NH(2)-domain with that of endopin 1 resulted in less effective inhibition of papain, indicated by its lower k(ass) association rate constant compared to wild-type endopin 2A. This chimeric mutant formed complexes with papain, but at lower levels compared to that with wild-type endopin 2A. Papain degradation of a portion of the chimeric mutant suggested a role for the NH(2)-domain in regulating relative amounts of endopin 2A that enter the substrate pathway compared to the serpin inhibitory pathway. Furthermore, site-directed mutagenesis demonstrated that the RSL domain with intact P1-P1' residues was necessary for inhibition. These findings indicate that the NH(2)-domain and the RSL region both participate in endopin 2A inhibition of papain.

Conformational distributions of protease-serpin complexes: a partially translocated complex

Serpins regulate serine proteases by forming metastable covalent complexes with their targets. The protease docks with the serpin and cleaves the serpin's reactive center loop (RCL) forming an acylenzyme intermediate. Cleavage triggers insertion of the RCL into beta sheet A, translocating the attached protease approximately 70 A and disrupting the protease active site, trapping the acylenzyme intermediate. Using single-pair Förster resonance energy transfer (spFRET), we have measured the conformational distributions of trypsin and alpha(1)-proteinase inhibitor (alpha(1)PI) covalent complexes. Bovine trypsin (BTryp) complexes display a single set of conformations consistent with the full translocation of BTryp (E(full)I*). However, the range of spFRET efficiencies is large, suggesting that the region around the trypsin label is mobile. Most complexes between alpha(1)PI variants and the more stable rat trypsin (RTryp) also show a single set of conformations, but the conformational distribution is narrower, indicating less disruption of RTryp. Surprisingly, RTryp complexes containing alpha(1)PI labeled at Cys232 with a cationic fluorophore display two equally populated conformations, E(full)I* and a conformation in which RTryp is only partially translocated (E(part)I*). Destabilizing the RTryp active site, by substituting Ala for Ile16, increases the E(full)I* population. Thus, interactions between anionic RTryp and cationic dyes likely exert a restraining force on alpha(1)PI, increasing the energy needed to translocate trypsin, and this force can be counteracted by active site destabilization. These results highlight the role of protease stability in determining the conformational distributions of protease-serpin covalent complexes and show that full translocation is not required for the formation of metastable complexes.

Inhibitory Activity of the Drosophila melanogaster Serpin Necrotic Is Dependent on Lysine Residues in the D-helix

Necrotic is a member of the serine protease inhibitor or serpin superfamily. It is a potent inhibitor of elastase and chymotrypsin type proteases and is responsible for regulating the anti-fungal response in Drosophila melanogaster. Necrotic contains three basic lysine residues within the D-helix that are homologous to those found in the heparin-binding domain of antithrombin and heparin co-factor II. We show here that substitution of all three lysine residues for glutamines caused cellular necrosis and premature death in Drosophila in keeping with a loss of function phenotype. The lysine to glutamine substitutions had no effect on the overall structure of recombinant Necrotic protein but abolished the formation of stable complexes with target proteases. Individual substitutions with either glutamine or alanine demonstrated that lysine 68 was the most critical residue for inhibitory activity. Despite the homology to other serpins, Necrotic did not bind, nor was it activated by sulfated glycans. These data demonstrate a critical role for basic residues within the D-helix (and lysine 68 in particular) in the inhibitory mechanism of the serpin Necrotic.

Viscous drag as the source of active site perturbation during protease translocation: insights into how inhibitory processes are controlled by serpin metastability

The native form of serine protease inhibitors (serpins) is kinetically trapped in a metastable state, which is thought to play a central role in the inhibitory mechanism. The initial binding complex between a serpin and a target protease undergoes a conformational change that forces the protease to translocate toward the opposite pole. Although structural determination of the final stable complex revealed a detailed mechanism of keeping the bound protease in an inactive conformation, it has remained unknown how the serpin exquisitely translocates a target protease with an acyl-linkage unhydrolyzed. We previously suggested that the acyl-linkage hydrolysis is strongly suppressed by active site perturbation during the protease translocation. Here, we address what induces the transient perturbation and how the serpin metastability contributes to the perturbation. Inhibitory activity of alpha1-antitrypsin (alpha1AT) toward elastase showed negative correlations with medium viscosity and Stokes radius of elastase moiety, indicating that viscous drag directly affects the protease translocation. Stopped-flow measurements revealed that the change in the inhibitory activity is primarily caused by the change in the translocation rate. The native stability of alpha1AT cavity mutants showed a negative correlation with the translocation rate but a positive correlation with the acyl-linkage hydrolysis rate, suggesting that the two kinetic steps are not independent but closely related. The degree of active site perturbation was probed by amino acid nucleophiles, supporting the view that the changes in the acyl-linkage hydrolysis rate are due to different perturbation states. These results suggest that the active site perturbation is caused by local imbalance between a pulling force driving protease translocation and a counteracting viscous drag force. The structural architecture of serpin metastability seems to be designed to ensure the active site perturbation by providing a sufficient pulling force, so the undesirable hydrolytic activity of protease is strongly suppressed during the translocation.

Characterization of recombinant human protein C inhibitor expressed in Escherichia coli

The serine protease inhibitor (serpin) protein C inhibitor (PCI; also named plasminogen activator inhibitor-3) regulates serine proteases in hemostasis, fibrinolysis, and reproduction. The biochemical activity of PCI is not fully defined partly due to the lack of a convenient expression system for active rPCI. Using pET-15b plasmid, Ni(2+)-chelate and heparin-Sepharose affinity chromatography steps, we describe here the expression, purification and characterization of wild-type recombinant (wt-rPCI) and two inactive mutants, R354A (P1 residue) and T341R (P14 residue), expressed in Escherichia coli. Wild-type rPCI, but not the two mutants, formed a stable bimolecular complex with thrombin, activated protein C and urokinase. In the absence of heparin, wt-rPCI-thrombin, -activated protein C, and -urokinase inhibition rates were 56.7, 3.4, and 2.3 x 10(4) M(-1) min(-1), respectively, and the inhibition rates were accelerated 25-, 71-, and 265-fold in the presence of 10 mug/mL heparin for each respective inhibition reaction. The stoichiometry of inhibition (SI) for wt-rPCI-thrombin was 2.0, which is comparable to plasma-derived PCI. The present report describes for the first time the expression and characterization of recombinant PCI in a bacterial expression system and demonstrates the feasibility of using this system to obtain adequate amounts of biologically active rPCI for future structure-function studies.

Tumor–host interactions contribute to the elevated expression level of α1-antichymotrypsin in metastatic breast tumor xenografts

We investigated alpha1-antichymotrypsin (ACT) gene expression in xenograft tumors generated by two isogenic human breast cancer cell lines derived from the same parent, MDA-MB-435, which display opposite metastatic behaviors. Microarray and real-time PCR experiments showed an overexpression of this serine protease inhibitor in the metastatic tumors (M-4A4T) and its derived metastases (M4-Mets) compared with the weakly metastatic tumors (NM-2C5T), and its release into the blood was confirmed by western-blotting. However, functional assays in vivo using genetically engineered tumor cells demonstrated that ACT up-regulation was not, by itself, responsible for the metastatic phenotype. We also made observations that ACT gene regulation was sensitive to tumor-host interactions: inoculation of these lines into the mouse mammary gland greatly increased ACT production and accentuated the intrinsic difference observed when they are cultured in vitro. Sensitivity of tumor cells to their environment was further analyzed by in vitro experiments, which demonstrated that a purified ECM environment and soluble components from normal host mammary cells were both able to significantly promote ACT expression. In addition, we took advantage of the xenogeneic nature of the model to measure ACT expression by the host cells (mouse) and the tumor cells (human) within the neoplasm using species-specific primers in real-time PCR experiments. It was found that the presence of tumor cells, irrespective of their metastatic capabilities, induced local ACT production by host cells at the primary and secondary tumor sites. Thus, this work indicates that there is a specific association of ACT overexpression with the metastatic phenotype in our breast cancer metastasis model. Moreover, because of the xenogeneic nature of our system, we were able to provide evidence of tumor-host reciprocal regulation of ACT production.

Demonstration of GTG as an alternative initiation codon for the serpin endopin 2B-2

This study demonstrates GTG as a novel, alternative initiation codon for translation of bovine endopin 2B-2, a serpin protease inhibitor. Molecular cDNA cloning revealed the endopin 2B-1 and endopin 2B-2 isoforms that are predicted to inhibit papain and elastase. Notably, GTG was demonstrated as the initiation codon for endopin 2B-2, whereas endopin 2B-1 possesses ATG as its initiation codon. GTG mediated in vitro translation of 46kDa endopin 2B-2. GTG also mediated translation of EGFP by in vitro translation and by expression in mammalian cells. Notably, mutagenesis of GTG to GTC resulted in the absence of EGFP expression in cells. GTG produced a lower level of protein expression compared to ATG. The use of GTG as an initiation codon to direct translation of endopin 2B, as well as the heterologous protein EGFP, demonstrates the role of GTG in the regulation of mRNA translation in mammalian cells. Significantly, further analyses of mammalian genomes based on GTG as an alternative initiation codon may predict new candidate gene products expressed by mammalian and human genomes.

Kinetic mechanism of protease inhibition by alpha1-antitrypsin

The native form of serine protease inhibitor (serpin) is kinetically trapped in a metastable state. Metastability in these proteins is critical to inhibit target protease by forming a stable covalent complex. Despite recent determination of the crystal structures of a Michaelis protease-serpin complex as well as a stable covalent complex, details on the kinetic mechanism remain unsolved. In this report, we examined the reaction mechanism of alpha1-antitrypsin toward elastase by a combination of stopped-flow experiments via fluorescence resonance energy transfer and rapid-quench studies. The results suggest a non-covalent complex intermediate other than Michaelis complex as an intermediate before the cleavage of P1-P1' scissile bond, whose formation is the rate-determining step of the overall reaction. This rate-limiting step represents rearrangement of the reactive site loop, and is regulated by a salt bridge between E354 and R196. The ionic interaction is unique to alpha1-antitrypsin, which suggests that protease inhibition mechanisms are varied among serpins.

Identification and activity of a lower eukaryotic serine proteinase inhibitor (serpin) from Cyanea capillata: analysis of a jellyfish serpin, jellypin

Delineating the phylogenetic relationships among members of a protein family can provide a high degree of insight into the evolution of domain structure and function relationships. To identify an early metazoan member of the high molecular weight serine proteinase inhibitor (serpin) superfamily, we initiated a cDNA library screen of the cnidarian, Cyanea capillata. We identified one serpin cDNA encoding for a full-length serpin, jellypin. Phylogenetic analysis using the deduced amino acid sequence showed that jellypin was most similar to the platyhelminthe Echinococcus multiocularis serpin and the clade P serpins, suggesting that this serpin evolved approximately 1000 million years ago (MYA). Modeling of jellypin showed that it contained all the functional elements of an inhibitory serpin. In vitro biochemical analysis confirmed that jellypin was an inhibitor of the S1 clan SA family of serine proteinases. Analysis of the interactions between the human serine proteinases, chymotrypsin, cathepsin G, and elastase, showed that jellypin inhibited these enzymes in the classical serpin manner, forming a SDS stable enzyme/inhibitor complex. These data suggest that the coevolution of serpin structure and inhibitory function date back to at least early metazoan evolution, approximately 1000 MYA.

Mutants of plasminogen activator inhibitor-1 designed to inhibit neutrophil elastase and cathepsin G are more effective in vivo than their endogenous inhibitors

Neutrophil elastase and cathepsin G are abundant intracellular neutrophil proteinases that have an important role in destroying ingested particles. However, when neutrophils degranulate, these proteinases are released and can cause irreparable damage by degrading host connective tissue proteins. Despite abundant endogenous inhibitors, these proteinases are protected from inhibition because of their ability to bind to anionic surfaces. Plasminogen activator inhibitor type-1 (PAI-1), which is not an inhibitor of these proteinases, possesses properties that could make it an effective inhibitor of neutrophil proteinases if its specificity could be redirected. PAI-1 efficiently inhibits surface-sequestered proteinases, and it efficiently mediates rapid cellular clearance of PAI-1-proteinase complexes. Therefore, we examined whether PAI-1 could be engineered to inhibit and clear neutrophil elastase and cathepsin G. By introducing specific mutations in the reactive center loop of wild-type PAI-1, we generated PAI-1 mutants that are effective inhibitors of both proteinases. Kinetic analysis shows that the inhibition of neutrophil proteinases by these PAI-1 mutants is not affected by the sequestration of neutrophil elastase and cathepsin G onto surfaces. In addition, complexes of these proteinases and PAI-1 mutants are endocytosed and degraded by lung epithelial cells more efficiently than either the neutrophil proteinases alone or in complex with their physiological inhibitors, alpha1-proteinase inhibitor and alpha1-antichymotrypsin. Finally, the PAI-1 mutants were more effective in reducing the neutrophil elastase and cathepsin G activities in an in vivo model of lung inflammation than were their physiological inhibitors.

Development of recombinant inhibitors specific to human kallikrein 2 using phage-display selected substrates

The reactive site loop of serpins undoubtedly defines in part their ability to inhibit a particular enzyme. Exchanges in the reactive loop of serpins might reassign the targets and modify the serpin-protease interaction kinetics. Based on this concept, we have developed a procedure to change the specificity of known serpins. First, reactive loops are very good substrates for the target enzymes. Therefore, we have used the phage-display technology to select from a pentapeptide phage library the best substrates for the human prostate kallikrein hK2 [Cloutier, S.M., Chagas, J.R., Mach, J.P., Gygi, C.M., Leisinger, H.J. & Deperthes, D. (2002) Eur. J. Biochem. 269, 2747-2754]. Selected substrates were then transplanted into the reactive site loop of alpha1-antichymotrypsin to generate new variants of this serpin, able to inhibit the serine protease. Thus, we have developed some highly specific alpha1-antichymotrypsin variants toward human kallikrein 2 which also show high reactivity. These inhibitors might be useful to help elucidate the importance of hK2 in prostate cancer progression.

The SANT2 domain of the murine tumor cell DnaJ-like protein 1 human homologue interacts with alpha1-antichymotrypsin and kinetically interferes with its serpin inhibitory activity

The murine tumor cell DnaJ-like protein 1 or MTJ1/ERdj1 is a membrane J-domain protein enriched in microsomal and nuclear fractions. We previously showed that its lumenal J-domain stimulates the ATPase activity of the molecular chaperone BiP/GRP78 (Chevalier, M., Rhee, H., Elguindi, E. C., and Blond, S. Y. (2000) J. Biol. Chem. 275, 19620-19627). MTJ1/ERdj1 also contains a large carboxyl-terminal cytosolic extension composed of two tryptophan-mediated repeats or SANT domains for which the function(s) is unknown. Here we describe the cloning of the human homologue HTJ1 and its interaction with alpha(1)-antichymotrypsin (ACT), a member of the serine proteinase inhibitor (serpin) family. The interaction was initially identified in a two-hybrid screening and further confirmed in vitro by dot blots, native electrophoresis, and fluorescence studies. The second SANT domain of HTJ1 (SANT2) was found to be sufficient for binding to ACT, both in yeast and in vitro. Single tryptophan-alanine substitutions at two strictly conserved residues significantly (Trp-497) or totally (Trp-520) abolished the interaction with ACT. SANT2 binds to human ACT with an intrinsic affinity equal to 0.5 nm. Preincubation of ACT with nearly stoichiometric concentrations of SANT2 wild-type but not SANT2: W520A results in an apparent loss of ACT inhibitory activity toward chymotrypsin. Kinetic analysis indicates that the formation of the covalent inhibitory complex ACT-chymotrypsin is significantly delayed in the presence of SANT2 with no change on the catalytic efficiency of the enzyme. This work demonstrates for the first time that the SANT2 domain of MTJ1/HTJ1/ERdj1 mediates stable and high affinity protein-protein interactions.

The Selective Inhibition of Serpin Aggregation by the Molecular Chaperone, α-Crystallin, Indicates a Nucleation-dependent Specificity

Small heat shock proteins (sHsps) are a ubiquitous family of molecular chaperones that prevent the misfolding and aggregation of proteins. However, specific details about their substrate specificity and mechanism of chaperone action are lacking. alpha1-Antichymotrypsin (ACT) and alpha1-antitrypsin (alpha1-AT) are two closely related members of the serpin superfamily that aggregate through nucleation-dependent and nucleation-independent pathways, respectively. The sHsp alpha-crystallin was unable to prevent the nucleation-independent aggregation of alpha1-AT, whereas alpha-crystallin inhibited ACT aggregation in a dose-dependent manner. This selective inhibition of ACT aggregation coincided with the formation of a stable high molecular weight alpha-crystallin-ACT complex with a stoichiometry of 1 on a molar subunit basis. The kinetics of this interaction occur at the same rate as the loss of ACT monomer, suggesting that the monomeric species is bound by the chaperone. 4,4'-Dianilino-1,1'-binaphthyl-5,5'-disulfonic acid (Bis-ANS) binding and far-UV circular dichroism data suggest that alpha-crystallin interacts specifically with a non-native conformation of ACT. The finding that alpha-crystallin does not interact with alpha1-AT under these conditions suggests that alpha-crystallin displays a specificity for proteins that aggregate through a nucleation-dependent pathway, implying that the dynamic nature of both the chaperone and its substrate protein is a crucial factor in the chaperone action of alpha-crystallin and other sHsps.

Sufficiency of the reactive site loop of maspin for induction of cell-matrix adhesion and inhibition of cell invasion. Conversion of ovalbumin to a maspin-like molecule

Maspin, an ov-serpin, inhibits tumor invasion and induces cell adhesion to extracellular matrix molecules. Here, we use maspin/ovalbumin chimeric proteins and the maspin reactive site loop (RSL) peptide to characterize the role of the RSL in maspin-mediated functions. Replacement of the RSL plus the C-terminal region or the RSL alone of maspin with that of ovalbumin resulted in the loss of the stimulatory effect on adhesion of corneal stromal cells to type I collagen, fibronectin, and laminin and of mammary carcinoma MDA-MB-231 cells to fibronectin. Maspin with ovalbumin as the C-terminal region retained activity, suggesting the maspin C-terminal polypeptide is not required. An R340Q mutant retained full maspin activity; however, an R340A mutant lost activity. This indicates the arginine side chain at the putative P1 site forms a hydrogen bond and not an ionic bond. The RSL peptide (P10-P5', amino acids 330-345) alone induced cell-matrix adhesion of mammary carcinoma cells and corneal stromal cells and inhibited invasion of the carcinoma cells. Substitution of the RSL of ovalbumin with that of maspin converted inactive ovalbumin into a fully active molecule. Maspin bound specifically to the surface of the mammary carcinoma cells with a kd of 367 +/- 67 nM and 32.0 +/- 2.2 x 10(6) binding sites/cell. The maspin RSL peptide inhibited binding, suggesting the RSL is involved in maspin binding to cells. Sufficiency of the maspin RSL for activity suggests the mechanism by which maspin regulates cell-matrix adhesion and tumor cell invasion does not involve the serpin mechanism of protease inhibition.

Nucleation of alpha 1-antichymotrypsin polymerization

Alpha(1)-antichymotrypsin is an acute phase plasma protein and a member of the serpin superfamily. We show here that wildtype alpha(1)-antichymotrypsin forms polymers between the reactive center loop of one molecule and the beta-sheet A of a second at a rate that is dependent on protein concentration and the temperature of the reaction. The rate of polymerization was accelerated by seeding with polymers of alpha(1)-antichymotrypsin and a complex of alpha(1)-antichymotrypsin with an exogenous reactive loop peptide but not with reactive loop cleaved alpha(1)-antichymotrypsin or with polymers of other members of the serpin superfamily. Sonication of alpha(1)-antichymotrypsin polymers markedly increased the efficacy of seeding such that polymers were able to form under physiological conditions. Taken together, these data provide the first demonstration that serpin polymerization can result from seeding. This mechanism is analogous to the fibrillization of the Abeta(1-42) peptide and may be important in the deposition of alpha(1)-antichymotrypsin in the plaques of Alzheimer's disease.

Comparative trajectories of active and S195A inactive trypsin upon binding to serpins

Serpins inhibit proteinases through a complicated multistep mechanism. The precise nature of these steps and the order by which they occur are still debated. We compared the fate of active and S195A inactive rat trypsin upon binding to alpha(1)-antitrypsin and P(1)-Arg-antichymotrypsin using stopped-flow kinetics with fluorescence resonance energy transfer detection and time-resolved fluorescence resonance energy transfer. We show that inhibition of active trypsin by these serpins leads to two irreversible complexes, one being compatible with the full insertion of the serpin-reactive site loop but not the other one. Binding of inactive trypsin to serpins triggers a large multistep reversible rearrangement leading to the migration of the proteinase to an intermediate position. Binding of inactive trypsin, unlike that of active trypsin, does not perturb the rhodamine fluorescence at position 150 on the helix F of the serpin. Thus, inactive proteinases do not migrate past helix F and do not trigger full serpin loop insertion.

The novel serpin endopin 2 demonstrates cross-class inhibition of papain and elastase: localization of endopin 2 to regulated secretory vesicles of neuroendocrine chromaffin cells

This study demonstrates that endopin 2 is a unique secretory vesicle serpin that displays cross-class inhibition of cysteine and serine proteases, indicated by effective inhibition of papain and elastase, respectively. Homology of the reactive site loop (RSL) domain of endopin 2, notably at P1-P1' residues, with other serpins that inhibit cysteine and serine proteases predicted that endopin 2 may inhibit similar proteases. Recombinant N-His-tagged endopin 2 inhibited papain and elastase with second-order rate constants (k(ass)) of 1.4 x 10(6) and 1.7 x 10(5) M(-1) s(-1), respectively. Endopin 2 formed SDS-stable complexes with papain and elastase, a characteristic property of serpins. Interactions of the RSL domain of endopin 2 with papain and elastase were indicated by cleavage of endopin 2 near the predicted P1-P1' residues by these proteases. Endopin 2 did not inhibit the cysteine protease cathepsin B, or the serine proteases chymotrypsin, trypsin, plasmin, and furin. Endopin 2 in neuroendocrine chromaffin cells was colocalized with the secretory vesicle component (Met)enkephalin by confocal immunonfluorescence microscopy, and was present in isolated secretory vesicles (chromaffin granules) from chromaffin cells as a glycoprotein of 72-73 kDa. Moreover, regulated secretion of endopin 2 from chromaffin cells was induced by nicotine and KCl depolarization. Overall, these results demonstrate that the serpin endopin 2 possesses dual specificity for inhibiting both papain-like cysteine and elastase-like serine proteases. These findings demonstrate that endopin 2 inhibitory functions may occur in the regulated secretory pathway.

The effects of reactive site location on the inhibitory properties of the serpin alpha(1)-antichymotrypsin

The large size of the serpin reactive site loop (RSL) suggests that the role of the RSL in protease inhibition is more complex than that of presenting the reactive site (P1 residue) to the protease. This study examines the effect on inhibition of relocating the reactive site (Leu-358) of the serpin alpha(1)-antichymotrypsin either one residue closer (P2) or further (P1') from the base of the RSL (Glu-342). alpha(1)-Antichymotrypsin variants were produced by mutation within the P4-P2' region; the sequence ITLLSA was changed to ITLSSA to relocate the reactive site to P2 (Leu-357) and to ITITLS to relocate it to P1' (Leu-359). Inhibition of the chymotrypsin-like proteases human chymase and chymotrypsin and the non-target protease human neutrophil elastase (HNE) were analyzed. The P2 variant inhibited chymase and chymotrypsin but not HNE. Relative to P1, interaction at P2 was characterized by greater complex stability, lower inhibition rate constants, and increased stoichiometry of inhibition values. In contrast, the P1' variant inhibited HNE (stoichiometry of inhibition = 4) but not chymase or chymotrypsin. However, inhibition of HNE was by interaction with Ile-357, the P2 residue. The P1' site was recognized by all proteases as a cleavage site. Covalent-complexes resistant to SDS-PAGE were observed in all inhibitory reactions, consistent with the trapping of the protease as a serpin-acyl protease complex. The complete loss in inhibitory activity associated with lengthening the Glu-342-reactive site distance by a single residue and the enhanced stability of complexes associated with shortening this distance by a single residue are compatible with the distorted-protease model of inhibition requiring full insertion of the RSL into the body of the serpin and translocation of the linked protease to the pole opposite from that of encounter.

Mast cell chymase modifies cell-matrix interactions and inhibits mitogen-induced proliferation of human airway smooth muscle cells

The hallmarks of chronic, severe asthma include prominent airway inflammation and airway smooth muscle (ASM) hypertrophy and hyperplasia. One of the factors that contribute to the injury and repair process within the airway is activation of proteases and turnover of extracellular matrix components. Mast cells, which are present in increased numbers in the asthmatic airway, are a rich source of the neutral protease chymase, which can degrade several basement membrane components. Recent data suggest that proteases also play a critical role in regulating the expression of CD44, the primary receptor for the matrix glycosaminoglycan hyaluronan. In this study we investigated the effects of chymase treatment on human ASM cell function. We found that chymase degraded the smooth muscle cell pericellular matrix. This was accompanied by an increased release of fibronectin and soluble CD44, but not soluble ICAM-1 or soluble hyaluronan, into the conditioned medium. In addition, chymase inhibited T cell adhesion to ASM and dramatically reduced epidermal growth factor-induced smooth muscle cell proliferation. These data suggest that the local release of mast cell chymase may have profound effects on ASM cell function and airway remodeling.

Novel secretory vesicle serpins, endopin 1 and endopin 2: endogenous protease inhibitors with distinct target protease specificities

Secretory vesicles of neuroendocrine cells possess multiple proteases for proteolytic processing of proteins into biologically active peptide components, such as peptide hormones and neurotransmitters. The importance of proteases within secretory vesicles predicts the presence of endogenous protease inhibitors in this subcellular compartment. Notably, serpins represent a diverse class of endogenous protease inhibitors that possess selective target protease specificities, defined by the reactive site loop domains (RSL). In the search for endogenous serpins in model secretory vesicles of neuroendocrine chromaffin cells, the presence of serpins related to alpha1-antichymotrypsin (ACT) was detected by Western blots with anti-ACT. Molecular cloning revealed the primary structures of two unique serpins, endopin 1 and endopin 2, that possess homology to ACT. Of particular interest was the observation that distinct RSL domains of these new serpins predicted that endopin 1 would inhibit trypsin-like serine proteases cleaving at basic residues, and endopin 2 would inhibit both elastase and papain that represent serine and cysteine proteases, respectively. Endopin 1 showed selective inhibition of trypsin, but did not inhibit chymotrypsin, elastase, or subtilisin. Endopin 2 demonstrated cross-class inhibition of the cysteine protease papain and the serine protease elastase. Endopin 2 did not inhibit chymotrypsin, trypsin, plasmin, thrombin, furin, or cathepsin B. Endopin 1 and endopin 2 each formed SDS-stable complexes with target proteases, a characteristic property of serpins. In neuroendocrine chromaffin cells from adrenal medulla, endopin 1 and endopin 2 were both localized to secretory vesicles. Moreover, the inhibitory activity of endopin 2 was optimized under reducing conditions, which required reduced Cys-374; this property is consistent with the presence of endogenous reducing agents in secretory vesicles in vivo. These new findings demonstrate the presence of unique secretory vesicle serpins, endopin 1 and endopin 2, which possess distinct target protease selectivities. Endopin 1 inhibits trypsin-like proteases; endopin 2 possesses cross-class inhibition for inhibition of papain-like cysteine proteases and elastase-like serine proteases. It will be of interest in future studies to define the endogenous protease targets of these two novel secretory vesicle serpins.

Structural similarity of the covalent complexes formed between the serpin plasminogen activator inhibitor-1 and the arginine-specific proteinases trypsin, LMW u-PA, HMW u-PA, and t-PA: use of site-specific fluorescent probes of local environment

We have used two fluorescent probes, NBD and dansyl, attached site-specifically to the serpin plasminogen activator inhibitor-1 (PAI-1) to address the question of whether a common mechanism of proteinase translocation and full insertion of the reactive center loop is used by PAI-1 when it forms covalent SDS-stable complexes with four arginine-specific proteinases, which differ markedly in size and domain composition. Single-cysteine residues were incorporated at position 119 or 302 as sites for specific reporter labeling. These are positions approximately 30 A apart that allow discrimination between different types of complex structure. Fluorescent derivatives were prepared for each of these variants using both NBD and dansyl as reporters of local perturbations. Spectra of native and cleaved forms also allowed discrimination between direct proteinase-induced changes and effects solely due to conformational change within the serpin. Covalent complexes of these derivatized PAI-1 species were made with the proteinases trypsin, LMW u-PA, HMW u-PA, and t-PA. Whereas only minor perturbations of either NBD and dansyl were found for almost all complexes when label was at position 119, major perturbations in both wavelength maximum (blue shifts) and quantum yield (both increases and decreases) were found for all complexes for both NBD and dansyl at position 302. This is consistent with all four complexes having similar location of the proteinase catalytic domain and hence with all four using the same mechanism of full-loop insertion with consequent distortion of the proteinase wedged in at the bottom of the serpin.

Concerted regulation of inhibitory activity of alpha 1-antitrypsin by the native strain distributed throughout the molecule

The native forms of common globular proteins are in their most stable state but the native forms of plasma serpins (serine protease inhibitors) show high energy state interactions. The high energy state strain of alpha(1)-antitrypsin, a prototype serpin, is distributed throughout the whole molecule, but the strain that regulates the function directly appears to be localized in the region where the reactive site loop is inserted during complex formation with a target protease. To examine the functional role of the strain at other regions of alpha(1)-antitrypsin, we increased the stability of the molecule greatly via combining various stabilizing single amino acid substitutions that did not affect the activity individually. The results showed that a substantial increase of stability, over 13 kcal mol(-1), affected the inhibitory activity with a correlation of 11% activity loss per kcal mol(-1). Addition of an activity affecting single residue substitution in the loop insertion region to these very stable substitutions caused a further activity decrease. The results suggest that the native strain of alpha(1)-antitrypsin distributed throughout the molecule regulates the inhibitory function in a concerted manner.

Kinetic dissection of alpha 1-antitrypsin inhibition mechanism

Serpins (serine protease inhibitors) inhibit target proteases by forming a stable covalent complex in which the cleaved reactive site loop of the serpin is inserted into beta-sheet A of the serpin with concomitant translocation of the protease to the opposite of the initial binding site. Despite recent determination of the crystal structures of a Michaelis protease-serpin complex as well as a stable covalent complex, details on the kinetic mechanism remain unsolved mainly due to difficulties in measuring kinetic parameters of acylation, protease translocation, and deacylation steps. To address the problem, we applied a mathematical model developed on the basis of a suicide inhibition mechanism to the stopped-flow kinetics of fluorescence resonance energy transfer during complex formation between alpha(1)-antitrypsin, a prototype serpin, and proteases. Compared with the hydrolysis of a peptide substrate, acylation of the protease by alpha(1)-antitrypsin is facilitated, whereas deacylation of the acyl intermediate is strongly suppressed during the protease translocation. The results from nucleophile susceptibility of the acyl intermediate suggest strongly that the active site of the protease is already perturbed during translocation.

Inhibition of prostate-specific antigen (PSA) by alpha(1)-antichymotrypsin: salt-dependent activation mediated by a conformational change

Prostate-specific antigen (PSA) and its SDS-stable complex with the serine proteinase inhibitor (serpin) alpha(1)-antichymotrypsin (ACT), which is the dominant form of PSA in serum, are in widespread use as markers for the diagnosis of prostate cancer, and there is increasing evidence for the involvement of PSA proteinase activity itself in the development of prostate and other cancers. However, both the formation and degradation of the PSA-ACT complex, denoted PSA*ACT* to indicate substantial changes in the structure of both proteins on complex formation, have been incompletely studied. Here we determine rate and equilibrium constants for the steps involved in PSA*ACT* formation and demonstrate that (a) the effects of added NaCl, polyamines, and Zn(2+) on this process parallel their effects on PSA catalytic activity [Hsieh, M.-C., and Cooperman, B. S. (2000) Biochim. Biophys. Acta 1481, 75-87], (b) the effect of added NaCl in dramatically increasing the rate of ACT inhibition of PSA correlates with salt-induced changes in PSA conformation, and (c) the PSA*ACT* complex is subject to proteolysis by human neutrophil elastase. Possible clinical implications of these findings are considered.

Heterogeneity in serpin-protease complexes as demonstrated by differences in the mechanism of complex breakdown

Serpins trap their target proteases in the form of an acyl-enzyme complex. The trap is kinetic, however, and thus serpin-protease complexes ultimately break down, releasing a cleaved inactive serpin and an active protease. The rates of this deacylation process vary greatly depending on the serpin-protease pair with half-lives ranging from minutes to months. The reasons for the diversity in breakdown rates are not clearly understood. In the current study, pH and solvent isotope effects were utilized to probe the mechanism of breakdown for an extremely stable complex and several unstable complexes. Two different patterns for the pH dependence of k(bkdn), the first-order rate constant of breakdown, were found. The stable complex, which breaks down at neutral pH with a half-life of approximately 2 weeks, exhibited a pH-k(bkdn) profile consistent with solvent-hydroxide ion mediated ester hydrolysis. There was no evidence for the participation of the catalytic machinery in the breakdown of this complex, suggesting extensive distortion of the active site. The unstable complexes, which break down with half-lives ranging from minutes to hours, exhibited a bell-shaped pH profile for k(bkdn), typical of the pH-rate profiles of free serine proteases. In the low to neutral pH range k(bkdn) increased with increasing pH in a manner characteristic of His57-mediated catalysis. In the alkaline pH range a decrease in k(bkdn) was observed, consistent with the titration of the Ile16-Asp194 salt bridge (chymotrypsinogen numbering). The alkaline pH dependence was not exhibited in pH-rate profiles of free or substrate-bound HNE, indicating that the salt bridge was significantly destabilized in the complexed protease. These results indicate that breakdown is catalytically mediated in the unstable complexes although, most likely, the protease is not in its native conformation and the catalytic machinery functions inefficiently. However, a mechanism in which breakdown is determined by the equilibrium between distorted and undistorted forms of the complexed protease cannot be completely dismissed. Overall, the results of this study suggest that the protease structure in unstable complexes is distorted to a lesser extent than in stable complexes.

Proteomic analysis of normal and malignant prostate tissue to identify novel proteins lost in cancer

BACKGROUND

Alterations of important protein pathways, including loss of prostate secretory granules, and disruption of the prostatic secretory pathway have been identified as early events in malignancy. In this study, proteomics was used to map the differences in protein expression between normal and malignant prostate tissues and to identify and analyze differentially expressed proteins in human prostate tissue with particular regard to the proteins lost in malignancy.

METHODS

Small quantities of normal and malignant prostate tissue were taken fresh from 34 radical prostatectomy cases. After histological examination, proteins were solubilized from selected tissues and separated using two-dimensional electrophoresis. Using image analysis, the proteome of normal and malignant tissues were mapped and differentially expressed proteins (present in normal and absent in malignant tissue) were identified and subsequently analyzed using peptide mass finger printing and N-terminal sequencing. Western blotting and immunohistochemistry were performed to examine expression profiles and tissue localization of candidate proteins.

RESULTS

Comparison of protein maps of normal and malignant prostate were used to identify 20 proteins which were lost in malignant transformation, including prostate specific antigen (PSA), alpha-1 antichymotrypsin (ACT), haptoglobin, and lactoylglutathione lyase. Three of the 20 had not previously been reported in human prostate tissue (Ubiquitin-like NEDD8, calponin, and a follistatin-related protein). Western blotting confirmed differences in the expression profiles of NEDD8 and calponin, and immunohistochemistry demonstrated differences in the cellular localization of these two proteins in normal and malignant prostate glands.

CONCLUSIONS

The expression of NEDD8, calponin, and the follistatin-related protein in normal prostate tissues is a novel finding and the role of these important functional proteins in normal prostate and their loss or reduced expression in prostate malignancy warrants further investigations.

Crosslinking of alpha 2-antiplasmin to fibrin

Human alpha 2-antiplasmin (alpha 2AP) is the primary inhibitor of plasmin-mediated fibrinolysis and is an efficient substrate of activated factor XIII (FXIIIa). Among 452 amino acid residues in alpha 2AP, Gln2 is believed to be the sole FXIIIa-reactive site that participates in crosslinking alpha 2AP to fibrin. We studied the effect of mutating Gln2 on the ability of FXIIIa to catalyze crosslinking of alpha 2AP to fibrin. By FXIIIa catalysis, [14C]methylamine was incorporated into a Q2A-alpha 2AP mutant in which Gln2 (Q) was replaced by Ala (A), thereby indicating that wildtype alpha 2AP has more than one FXIIIa-reactive site. To identify the FXIIIa-reactive sites in alpha 2AP, wildtype alpha 2AP and Q2A-alpha 2AP were labeled with 5-(biotinamido)pentylamine by FXIIIa. Each labeled alpha 2AP was digested with trypsin and applied to an avidin affinity column to capture labeled peptides. Edman sequencing and mass analysis of each labeled peptide showed that out of 35 Gln residues in wildtype alpha 2AP, four were labeled with the following order of efficiency: Gln2 > Gln21 > Gln419 > Gln447. Q2A-alpha 2AP was also labeled at the three minor sites, Gln21 > Gln419 > Gln447. Q2A-alpha 2AP became crosslinked to fibirin(ogen) by FXIIIa catalysis at approximately one-tenth the rate of wt-alpha 2AP. These results demonstrate that alpha 2AP has one primary (Gln2) and three minor substrate sites for FXIIIa and that the three minor sites identified in this study can also participate in crosslink formation between alpha 2AP and fibrin, but at a much lower efficiency than the Gln2 site.

Cavities of alpha(1)-antitrypsin that play structural and functional roles

The native form of inhibitory serine protease inhibitors (serpins) is strained, which is critical for their inhibitory activity. Previous studies on stabilizing mutations of alpha(1)-antitrypsin, a prototype of serpins, indicated that cavities provide a structural basis for the native strain of the molecule. We have systematically mapped the cavities of alpha(1)-antitrypsin that play such structural and functional roles by designing cavity-filling mutations at residues that line the walls of the cavities. Results show that energetically unfavorable cavities are distributed throughout the alpha(1)-antitrypsin molecule, and the cavity-filling mutations stabilized the native conformation at 8 out of 10 target sites. The stabilization effect of the individual cavity-filling mutations of alpha(1)-antitrypsin varied (0.2-1.9 kcal/mol for each additional methylene group) and appeared to depend largely on the structural flexibility of the cavity environment. Cavity-filling mutations that decreased inhibitory activity of alpha(1)-antitrypsin were localized in the loop regions that interact with beta-sheet A distal from the reactive center loop. The results are consistent with the notion that beta-sheet A and the structure around it mobilize when alpha(1)-antitrypsin forms a complex with a target protease.

Formation of the covalent chymotrypsin.antichymotrypsin complex involves no large-scale movement of the enzyme

alpha(1)-Antichymotrypsin is a member of the serine proteinase inhibitor, or serpin, family that typically forms very long-lived, enzymatically inactive 1:1 complexes (denoted E*I*) with its target proteinases. Serpins share a conserved tertiary structure, in which an exposed region of amino acid residues (called the reactive center loop or RCL) acts as bait for a target proteinase. Within E*I*, the two proteins are linked covalently as a result of nucleophilic attack by Ser(195) of the serine proteinase on the P1 residue within the RCL of the serpin. This species is formally similar to the acyl enzyme species normally seen as an intermediate in serpin proteinase catalysis. However, its subsequent hydrolysis is extremely slow as a result of structural changes within the enzyme leading to distortion of the active site. There is at present an ongoing debate concerning the structure of the E*I* complex; in particular, as to whether the enzyme, bound to P1, maintains its original position at the top of the serpin molecule or instead translocates across the entire length of the serpin, with concomitant insertion of RCL residues P1-P14 within beta-sheet A and a large separation of the enzyme and RCL residue P1'. We report time-resolved fluorescence energy transfer and rapid mixing/quench studies that support the former model. Our results indicate that the distance between residue P1' in alpha(1)-antichymotrypsin and the amino terminus of chymotrypsin actually decreases on conversion of the encounter complex E.I to E*I*. These results led us to formulate a comprehensive mechanism that accounted both for our results and for those of others supporting the two different E*I* structures. In this mechanism, partial insertion of the RCL, with no large perturbation of the P1' enzyme distance, is followed by covalent acyl enzyme formation. Full insertion can subsequently take place, in a reversible fashion, with the position of equilibrium between the partially and fully inserted complexes depending on the particular serpin-proteinase pair under consideration.