Reactive Center Loop (RCL) Peptides Derived from Serpins Display Independent Coagulation and Immune Modulating Activities

Serpins regulate coagulation and inflammation, binding serine proteases in suicide-inhibitory complexes. Target proteases cleave the serpin reactive center loop scissile P1–P1′ bond, resulting in serpin-protease suicide-inhibitory complexes. This inhibition requires a near full-length serpin sequence. Myxomavirus Serp-1 inhibits thrombolytic and thrombotic proteases, whereas mammalian neuroserpin (NSP) inhibits only thrombolytic proteases. Both serpins markedly reduce arterial inflammation and plaque in rodent models after single dose infusion. In contrast, Serp-1 but not NSP improves survival in a lethal murine gammaherpesvirus68 (MHV68) infection in interferon γ-receptor-deficient mice (IFNγR^−/−). Serp-1 has also been successfully tested in a Phase 2a clinical trial. We postulated that proteolytic cleavage of the reactive center loop produces active peptide derivatives with expanded function. Eight peptides encompassing predicted protease cleavage sites for Serp-1 and NSP were synthesized and tested for inhibitory function in vitro and in vivo. In engrafted aorta, selected peptides containing Arg or Arg-Asn, not Arg-Met, with a 0 or +1 charge, significantly reduced plaque. Conversely, S-6 a hydrophobic peptide of NSP, lacking Arg or Arg-Asn with −4 charge, induced early thrombosis and mortality. S-1 and S-6 also significantly reduced CD11b⁺ monocyte counts in mouse splenocytes. S-1 peptide had increased efficacy in plasminogen activator inhibitor-1 serpin-deficient transplants. Plaque reduction correlated with mononuclear cell activation. In a separate study, Serp-1 peptide S-7 improved survival in the MHV68 vasculitis model, whereas an inverse S-7 peptide was inactive. Reactive center peptides derived from Serp-1 and NSP with suitable charge and hydrophobicity have the potential to extend immunomodulatory functions of serpins.     

Identification of myxomaviral serpin reactive site loop sequences that regulate innate immune responses

The thrombolytic serine protease cascade is intricately involved in activation of innate immune responses. The urokinase-type plasminogen activator and receptor form complexes that aid inflammatory cell invasion at sites of arterial injury. Plasminogen activator inhibitor-1 is a mammalian serpin that binds and regulates the urokinase receptor complex. Serp-1, a myxomaviral serpin, also targets the urokinase receptor, displaying profound anti-inflammatory and anti-atherogenic activity in a wide range of animal models. Serp-1 reactive center site mutations, mimicking known mammalian and viral serpins, were constructed in order to define sequences responsible for regulation of inflammation. Thrombosis, inflammation, and plaque growth were assessed after treatment with Serp-1, Serp-1 chimeras, plasminogen activator inhibitor-1, or unrelated viral serpins in plasminogen activator inhibitor or urokinase receptor-deficient mouse aortic transplants. Altering the P1-P1' Arg-Asn sequence compromised Serp-1 protease-inhibitory activity and anti-inflammatory activity in animal models; P1-P1' Ala-Ala mutants were inactive, P1 Met increased remodeling, and P1' Thr increased thrombosis. Substitution of Serp-1 P2-P7 with Ala6 allowed for inhibition of urokinase but lost plasmin inhibition, unexpectedly inducing a diametrically opposed, proinflammatory response with mononuclear cell activation, thrombosis, and aneurysm formation (p < 0.03). Other serpins did not reproduce Serp-1 activity; plasminogen activator inhibitor-1 increased thrombosis (p < 0.0001), and unrelated viral serpin, CrmA, increased inflammation. Deficiency of urokinase receptor in mouse transplants blocked Serp-1 and chimera activity, in some cases increasing inflammation. In summary, 1) Serp-1 anti-inflammatory activity is highly dependent upon the reactive center loop sequence, and 2) plasmin inhibition is central to anti-inflammatory activity.     

In Vivo Anti-HIV Activity of the Heparin-Activated Serine Protease Inhibitor Antithrombin III Encapsulated in Lymph-Targeting Immunoliposomes

Endogenous serine protease inhibitors (serpins) are anti-inflammatory mediators with multiple biologic functions. Several serpins have been reported to modulate HIV pathogenesis, or exhibit potent anti-HIV activity in vitro, but the efficacy of serpins as therapeutic agents for HIV in vivo has not yet been demonstrated. In the present study, we show that heparin-activated antithrombin III (hep-ATIII), a member of the serpin family, significantly inhibits lentiviral replication in a non-human primate model. We further demonstrate greater than one log₁₀ reduction in plasma viremia in the nonhuman primate system by loading of hep-ATIII into anti-HLA-DR immunoliposomes, which target tissue reservoirs of viral replication. We also demonstrate the utility of hep-ATIIII as a potential salvage agent for HIV strains resistant to standard anti-retroviral treatment. Finally, we applied gene-expression arrays to analyze hep-ATIII-induced host cell interactomes and found that downstream of hep-ATIII, two independent gene networks were modulated by host factors prostaglandin synthetase-2, ERK1/2 and NFκB. Ultimately, understanding how serpins, such as hep-ATIII, regulate host responses during HIV infection may reveal new avenues for therapeutic intervention.     

Crystal structure of the apoptotic suppressor CrmA in its cleaved form

BACKGROUND: Cowpox virus expresses the serpin CrmA (cytokine response modifier A) in order to avoid inflammatory and apoptotic responses of infected host cells. The targets of CrmA are members of the caspase family of proteases that either initiate the extrinsic pathway of apoptosis (caspases 8 and 10) or trigger activation of the pro-inflammatory cytokines interleukin-1beta and interleukin-18 (caspase 1). RESULTS: We have determined the structure of a cleaved form of CrmA to 2.26 A resolution. CrmA has the typical fold of a cleaved serpin, even though it lacks the N-terminal half of the A helix, the entire D helix, and a portion of the E helix that are present in all other known serpins. The reactive-site loop of CrmA was mutated to contain the optimal substrate recognition sequence for caspase 3; however, the mutation only marginally increased the ability of CrmA to inhibit caspase 3. Superposition of the reactive-site loop of alpha1-proteinase inhibitor on the cleaved CrmA structure provides a model for virgin CrmA that can be docked to caspase 1, but not to caspase 3. CONCLUSIONS: CrmA exemplifies viral economy, selective pressure having resulted in a 'minimal' serpin that lacks the regions not needed for structural integrity or inhibitory activity. The docking model provides an explanation for the selectivity of CrmA. Our demonstration that engineering optimal substrate recognition sequences into the CrmA reactive-site loop fails to generate a good caspase 3 inhibitor is consistent with the docking model.     

Serp-1, a viral anti-inflammatory serpin,regulates cellular serine proteinase and serpin responses to vascular injury

Complex DNA viruses have tapped into cellular serpin responses that act as key regulatory steps in coagulation and inflammatory cascades. Serp-1 is one such viral serpin that effectively protects virus-infected tissues from host inflammatory responses. When given as purified protein, Serp-1 markedly inhibits vascular monocyte invasion and plaque growth in animal models. We have investigated mechanisms of viral serpin inhibition of vascular inflammatory responses. In vascular injury models, Serp-1 altered early cellular plasminogen activator (tissue plasminogen activator), inhibitor (PAI-1), and receptor (urokinase-type plasminogen activator) expression (p < 0.01). Serp-1, but not a reactive center loop mutant, up-regulated PAI-1 serpin expression in human endothelial cells. Treatment of endothelial cells with antibody to urokinase-type plasminogen activator and vitronectin blocked Serp-1-induced changes. Significantly, Serp-1 blocked intimal hyperplasia (p < 0.0001) after aortic allograft transplant (p < 0.0001) in PAI-1-deficient mice. Serp-1 also blocked plaque growth after aortic isograft transplant and after wire-induced injury (p < 0.05) in PAI-1-deficient mice indicating that increase in PAI-1 expression is not required for Serp-1 to block vasculopathy development. Serp-1 did not inhibit plaque growth in uPAR-deficient mice after aortic allograft transplant. We conclude that the poxviral serpin, Serp-1, attenuates vascular inflammatory responses to injury through a pathway mediated by native uPA receptors and vitronectin.     

Serine and cysteine proteases are translocated to similar extents upon formation of covalent complexes with serpins. Fluorescence perturbation and fluorescence resonance energy transfer mapping of the protease binding site in CrmA complexes with granzyme B and caspase-1

CrmA is a "cross-class" serpin family inhibitor of the proapoptotic serine protease, granzyme B, as well as cysteine proteases of the caspase family. To determine whether crmA inhibits these structurally diverse proteases by a common conformational trapping mechanism, we mapped the position of the protease in crmA complexes with granzyme B or caspase-1 by fluorescence perturbation and fluorescence resonance energy transfer (FRET) analyses of site-specific fluorophore-labeled crmAs. A reactive loop P6 NBD label underwent similar large fluorescence enhancements (>200%) either upon reactive loop cleavage by AspN protease or complex formation with granzyme B or caspase-1, consistent with the insertion of the cleaved reactive loop into sheet A in both types of crmA-protease complexes. NBD labels on the noninserting part of the reactive loop docking site for protease (P1' residue) or midway between the two ends of sheet A (helix F residue 101) showed no significant perturbations due to protease complexation. By contrast, labels at positions 68 and 261, lying at the end of sheet A most distal from the reactive loop, showed marked perturbations distinct from those induced by AspN cleavage and thus ascribable to granzyme B or caspase-1 proximity in the complexes. Substantial FRET between protease tryptophans and 5-dimethylaminonaphthalene-1-sulfonyl-labeled crmAs occurred in protease complexes with crmAs labeled at the 68 and 261 positions, but not the P1' position. These results suggest that granzyme B and caspase-1 are inhibited by crmA by a common mechanism involving full reactive loop insertion into sheet A and translocation of the protease to the distal end of the sheet as previously found for inhibition of other serine proteases by serpins.     

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.     

93-kDa twin-domain serine protease inhibitor (Serpin) has a regulatory function on the beetle Toll proteolytic signaling cascade

Serpins are protease inhibitors that play essential roles in the down-regulation of extracellular proteolytic cascades. The core serpin domain is highly conserved, and typical serpins are encoded with a molecular size of 35–50 kDa. Here, we describe a novel 93-kDa protein that contains two complete, tandemly arrayed serpin domains. This twin serpin, SPN93, was isolated from the larval hemolymph of the large beetle Tenebrio molitor. The N-terminal serpin domain of SPN93 forms a covalent complex with the Spätzle-processing enzyme, a terminal serine protease of the Toll signaling cascade, whereas the C-terminal serpin domain of SPN93 forms complexes with a modular serine protease and the Spätzle-processing enzyme-activating enzyme, which are two different enzymes of the cascade. Consequently, SPN93 inhibited β-1,3-glucan-mediated Toll proteolytic cascade activation in an in vitro system. Site-specific proteolysis of SPN93 at the N-terminal serpin domain was observed after activation of the Toll proteolytic cascade in vivo, and down-regulation of SPN93 by RNAi sensitized β-1,3-glucan-mediated larval death. Therefore, SPN93 is the first serpin that contains twin tandemly arrayed and functionally active serpin domains that have a regulatory role in the larval Toll proteolytic signaling cascade.     

Myxomavirus Anti-Inflammatory Chemokine Binding Protein Reduces the Increased Plaque Growth Induced by Chronic Porphyromonas gingivalis Oral Infection after Balloon Angioplasty Aortic Injury in Mice

Thrombotic occlusion of inflammatory plaque in coronary arteries causes myocardial infarction. Treatment with emergent balloon angioplasty (BA) and stent implant improves survival, but restenosis (regrowth) can occur. Periodontal bacteremia is closely associated with inflammation and native arterial atherosclerosis, with potential to increase restenosis. Two virus-derived anti-inflammatory proteins, M-T7 and Serp-1, reduce inflammation and plaque growth after BA and transplant in animal models through separate pathways. M-T7 is a broad spectrum C, CC and CXC chemokine-binding protein. Serp-1 is a serine protease inhibitor (serpin) inhibiting thrombotic and thrombolytic pathways. Serp-1 also reduces arterial inflammation and improves survival in a mouse herpes virus (MHV68) model of lethal vasculitis. In addition, Serp-1 demonstrated safety and efficacy in patients with unstable coronary disease and stent implant, reducing markers of myocardial damage. We investigate here the effects of Porphyromonas gingivalis, a periodontal pathogen, on restenosis after BA and the effects of blocking chemokine and protease pathways with M-T7 and Serp-1. ApoE^−/− mice had aortic BA and oral P. gingivalis infection. Arterial plaque growth was examined at 24 weeks with and without anti-inflammatory protein treatment. Dental plaques from mice infected with P. gingivalis tested positive for infection. Neither Serp-1 nor M-T7 treatment reduced infection, but IgG antibody levels in mice treated with Serp-1 and M-T7 were reduced. P. gingivalis significantly increased monocyte invasion and arterial plaque growth after BA (P<0.025). Monocyte invasion and plaque growth were blocked by M-T7 treatment (P<0.023), whereas Serp-1 produced only a trend toward reductions. Both proteins modified expression of TLR4 and MyD88. In conclusion, aortic plaque growth in ApoE^−/− mice increased after angioplasty in mice with chronic oral P. gingivalis infection. Blockade of chemokines, but not serine proteases significantly reduced arterial plaque growth, suggesting a central role for chemokine-mediated inflammation after BA in P. gingivalis infected mice.     

Inhibitory mechanism of a cross-class serpin,the squamous cell carcinoma antigen 1

The squamous cell carcinoma antigen (SCCA) 1 and its homologous molecule, SCCA2, belong to the ovalbumin-serpin family. Although SCCA2 inhibits serine proteinases such as cathepsin G and mast cell chymase, SCCA1 targets cysteine proteinases such as cathepsin S, K, L, and papain. SCCA1 is therefore called a cross-class serpin. The inhibitory mechanism of the standard serpins is well characterized; those use a suicide substrate-like inhibitory mechanism during which an acyl-enzyme intermediate by a covalent bond is formed, and this complex is stable against hydrolysis. However, the inhibitory mechanism of cross-class serpins remains unresolved. In this article, we analyzed the inhibitory mechanism of SCCA1 on a cysteine proteinase, papain. SCCA1 interacted with papain at its reactive site loop, which was then cleaved, as the standard serpins. However, gel-filtration analyses showed that SCCA1 did not form a covalent complex with papain, in contrast to other serpins. Interaction with SCCA1 severely impaired the proteinase activity of papain, probably by inducing conformational change. The decreased, but still existing, proteinase activity of papain was completely inhibited by SCCA1 according to the suicide substrate-like inhibitory mechanism; however, papain recovered its proteinase activity with the compromised level, when all of intact SCCA1 was cleaved. These results suggest that the inhibitory mechanism of SCCA1 is unique among the serpin superfamily in that SCCA1 performs its inhibitory activity in two ways, contributing the suicide substrate-like mechanism without formation of a covalent complex and causing irreversible impairment of the catalytic activity of a proteinase.     

The expanding superfamily of serpins: searching for the real targets

The serpin superfamily of proteins has expanded rapidly in recent years as represented by the ovalbumin-type serpin subfamily. PCR methods have been used to identify new ovalbumin-type serpins, including leupin or SCCA-2, a close relative of SCCA. Although in vitro inhibition of proteases by these serpins has been shown, and we have evidence that leupin can protect cells against apoptosis, the exact role of these serpins is not well understood. Finding relevant targets is a major challenge in serpin biology, and we have investigated the yeast-2-hybrid trap for identification of new interactions. Preliminary studies suggest that serpins are at the upper limit for bait size, but this technique may be useful for identifying interactions where full-length serpin protein is not required.     

Miropin,a Novel Bacterial Serpin from the Periodontopathogen Tannerella forsythia,Inhibits a Broad Range of Proteases by Using Different Peptide Bonds within the Reactive Center Loop

All prokaryotic genes encoding putative serpins identified to date are found in environmental and commensal microorganisms, and only very few prokaryotic serpins have been investigated from a mechanistic standpoint. Herein, we characterized a novel serpin (miropin) from the human pathogen Tannerella forsythia, a bacterium implicated in initiation and progression of human periodontitis. In contrast to other serpins, miropin efficiently inhibited a broad range of proteases (neutrophil and pancreatic elastases, cathepsin G, subtilisin, and trypsin) with a stoichiometry of inhibition of around 3 and second-order association rate constants that ranged from 2.7 × 10⁴ (cathepsin G) to 7.1 × 10⁵ m⁻¹s⁻¹ (subtilisin). Inhibition was associated with the formation of complexes that were stable during SDS-PAGE. The unusually broad specificity of miropin for target proteases is achieved through different active sites within the reactive center loop upstream of the P1-P1′ site, which was predicted from an alignment of the primary structure of miropin with those of well studied human and prokaryotic serpins. Thus, miropin is unique among inhibitory serpins, and it has apparently evolved the ability to inhibit a multitude of proteases at the expense of a high stoichiometry of inhibition and a low association rate constant. These characteristics suggest that miropin arose as an adaptation to the highly proteolytic environment of subgingival plaque, which is exposed continually to an array of host proteases in the inflammatory exudate. In such an environment, miropin may function as an important virulence factor by protecting bacterium from the destructive activity of neutrophil serine proteases. Alternatively, it may act as a housekeeping protein that regulates the activity of endogenous T. forsythia serine proteases.     

Nucleocytoplasmic distribution of the ovalbumin serpin PI-9 requires a nonconventional nuclear import pathway and the export factor Crm1.

Proteinase inhibitor 9 (PI-9) is a human serpin present in the cytoplasm of cytotoxic lymphocytes and epithelial cells. It inhibits the cytotoxic lymphocyte granule proteinase granzyme B (graB) and is thought to protect cytotoxic lymphocytes and bystander cells from graB-mediated apoptosis. Following uptake into cells, graB promotes DNA degradation, rapidly translocating to the nucleus, where it binds a nuclear component. PI-9 should therefore be found in cytotoxic lymphocyte and bystander cell nuclei to ensure complete protection against graB. Here we demonstrate by microscopy and subcellular fractionation experiments that PI-9 is present in the nuclei of human cytotoxic cells, endothelial cells, and epithelial cells. We also show that the related serpins, PI-6, monocyte neutrophil elastase inhibitor (MNEI), PI-8, plasminogen activator inhibitor 2 (PAI-2), and the viral serpin CrmA exhibit similar nucleocytoplasmic distributions. Because these serpins lack classical nuclear localization signals and are small enough to diffuse through nuclear pores, we investigated whether import occurs actively or passively. Large (approximately 70 kDa) chimeric proteins comprising PI-9, PI-6, PI-8, MNEI, or PAI-2 fused to green fluorescent protein (GFP) show similar nucleocytoplasmic distributions to the parent proteins, indicating that nuclear import is active. By contrast, CrmA-GFP is excluded from nuclei, indicating that CrmA is not actively imported. In vitro nuclear transport assays show that PI-9 accumulates at a rate above that of passive diffusion, that it requires cytosolic factors but not ATP, and that it does not bind an intranuclear component. Furthermore, PI-9 is exported from nuclei via a leptomycin B-sensitive pathway, implying involvement of the export factor Crm1p. We conclude that the nucleocytoplasmic distribution of PI-9 and related serpins involves a nonconventional nuclear import pathway and Crm1p.     

Tsetse GmmSRPN10 Has Anti-complement Activity and Is Important for Successful Establishment of Trypanosome Infections in the Fly Midgut

The complement cascade in mammalian blood can damage the alimentary tract of haematophagous arthropods. As such, these animals have evolved their own repertoire of complement-inactivating factors, which are inadvertently exploited by blood-borne pathogens to escape complement lysis. Unlike the bloodstream stages, the procyclic (insect) stage of Trypanosoma brucei is highly susceptible to complement killing, which is puzzling considering that a tsetse takes a bloodmeal every 2–4 days. In this study, we identified four tsetse (Glossina morsitans morsitans) serine protease inhibitors (serpins) from a midgut expressed sequence tag (EST) library (GmmSRPN3, GmmSRPN5, GmmSRPN9 and GmmSRPN10) and investigated their role in modulating the establishment of a T. brucei infection in the midgut. Although not having evolved in a common blood-feeding ancestor, all four serpins have an active site sharing remarkable homology with the human complement C1-inhibitor serpin, SerpinG1. RNAi knockdown of individual GmmSRPN9 and GmmSRPN10 genes resulted in a significant decreased rate of infection by procyclic form T. brucei. Furthermore, recombinant GmmSRPN10 was both able to inhibit the activity of human complement-cascade serine proteases, C1s and Factor D, and to protect the in vitro killing of procyclic trypanosomes when incubated with complement-activated human serum. Thus, the secretion of serpins, which may be part of a bloodmeal complement inactivation system in tsetse, is used by procyclic trypanosomes to evade an influx of fresh trypanolytic complement with each bloodmeal. This highlights another facet of the complicated relationship between T. brucei and its tsetse vector, where the parasite takes advantage of tsetse physiology to further its chances of propagation and transmission.     

Crystal structure of viral serpin crmA provides insights into its mechanism of cysteine proteinase inhibition

CrmA is an unusual viral serpin that inhibits both cysteine and serine proteinases involved in the regulation of host inflammatory and apoptosis processes. It differs from other members of the serpin superfamily by having a reactive center loop that is one residue shorter, and by its apparent inability to form SDS-stable covalent complexes with cysteine proteinases. To obtain insight into the inhibitory mechanism of crmA, we determined the crystal structure of reactive center loop-cleaved crmA to 2.9 A resolution. The structure, which is the first of a viral serpin, suggests that crmA can inhibit cysteine proteinases by a mechanism analogous to that used by other serpins against serine proteinases. However, one striking difference from other serpins, which may be significant for in vivo function, is an additional highly charged antiparallel strand for b sheet A, whose sequence and length are unique to crmA.     

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.     

The amplified mouse squamous cell carcinoma antigen gene locus contains a serpin (Serpinb3b) that inhibits both papain-like cysteine and trypsin-like serine proteinases

The clade B serpins occupy a unique niche among a larger superfamily by predominantly regulating intracellular proteolysis. In humans, there are 13 family members that map to serpin gene clusters at either 6p25 or 18q21. While most of these serpins display a unique inhibitory profile and appear to be well conserved in mammals, the clade B loci of several species show evidence of relatively recent genomic amplification events. However, it is not clear whether these serpin gene amplification events yield paralogs with functional redundancy or, through selective pressure, inhibitors with more diverse biochemical activities. A recent comparative genomic analysis of the mouse clade B cluster at 1D found nearly complete conservation of gene number, order, and orientation relative to those of 18q21 in humans. The only exception was the squamous cell carcinoma antigen (SCCA) locus. The human SCCA locus contains two genes, SERPINB3 (SCCA1) and SERPINB4 (SCCA2), whereas the mouse locus contains four serpins and three pseudogenes. At least two of these genes encoded functional, dual cross-class proteinase inhibitors. Mouse Serpinb3a was shown previously to inhibit both chymotrypsin-like serine and papain-like cysteine proteinases. We now report that mouse Serpinb3b extends the inhibitory repertoire of the mouse SCCA locus to include a second cross-class inhibitor with activity against both papain-like cysteine and trypsin-like serine proteinases. These findings confirmed that the genomic expansion of the clade B serpins in the mouse was associated with a functional diversification of inhibitory activity.     

Docosahexaenoic Acid Induces Apoptosis in MCF-7 Cells In Vitro and In Vivo via Reactive Oxygen Species Formation and Caspase 8 Activation

Background

The present study sought to further investigate the in vitro and in vivo anticancer effects of a representative omega-3 fatty acid, docosahexaenoic acid (DHA), with a focus on assessing the induction of oxidative stress and apoptosis as an important mechanism for its anticancer actions.

Methodology/Principal Findings

In vitro studies showed that DHA strongly reduces the viability and DNA synthesis of MCF-7 human breast cancer cells in culture, and also promotes cell death via apoptosis. Mechanistically, accumulation of reactive oxygen species and activation of caspase 8 contribute critically to the induction of apoptotic cell death. Co-presence of antioxidants or selective inhibition or knockdown of caspase 8 each effectively abrogates the cytotoxic effect of DHA. Using athymic nude mice as an in vivo model, we found that feeding animals the 5% fish oil-supplemented diet for 6 weeks significantly reduces the growth of MCF-7 human breast cancer cells in vivo through inhibition of cancer cell proliferation as well as promotion of cell death. Using 3-nitrotyrosine as a parameter, we confirmed that the fish oil-supplemented diet significantly increases oxidative stress in tumor cells in vivo. Analysis of fatty acid content in plasma and tissues showed that feeding animals a 5% fish oil diet increases the levels of DHA and eicosapentaenoic acid in both normal and tumorous mammary tissues by 329% and 300%, respectively.

Conclusions/Significance

DHA can strongly induce apoptosis in human MCF-7 breast cancer cells both in vitro and in vivo. The induction of apoptosis in these cells is selectively mediated via caspase 8 activation. These observations call for further studies to assess the effectiveness of fish oil as a dietary supplement in the prevention and treatment of human breast cancer.