Topology of catalytic portion of prostaglandin I(2) synthase: identification by molecular modeling-guided site-specific antibodies

The Protective Effects of Up-Regulating Prostacyclin Biosynthesis on Neuron Survival in Hippocampus

Cellular arachidonic acid (AA), an unsaturated fatty acid found ubiquitously in plasma membranes, is metabolized to different prostanoids, such as prostacyclin (PGI₂) and prostaglandin E₂ (PGE₂), by the three-step reactions coupling the upstream cyclooxygenase (COX) isoforms (COX-1 and COX-2) with the corresponding individual downstream synthases. While the vascular actions of these prostanoids are well-characterized, their specific roles in the hippocampus, a major brain area for memory, are poorly understood. The major obstacle for its understanding in the brain was to mimic the biosynthesis of each prostanoid. To solve the problem, we utilized Single-Chain Hybrid Enzyme Complexes (SCHECs), which could successfully control cellular AA metabolites to the desired PGI₂ or PGE₂. Our in vitro studies suggested that neurons with higher PGI₂ content and lower PGE₂ content exhibited survival protection and resistance to Amyloid-β-induced neurotoxicity. Further extending to an in vivo model, the hybrid of PGI₂-producing transgenic mice and Alzheimer's disease (AD) mice showed restored long-term memory. These findings suggested that the vascular prostanoids, PGI₂ and PGE₂, exerted significant regulatory influences on neuronal protection (by PGI₂), or damage (by PGE₂) in the hippocampus, and raised a concern that the wide uses of aspirin in cardiovascular diseases may exert negative impacts on neurodegenerative protection. Graphic Abstract Our study intended to understand the crosstalk of prostanoids in the hippocampus, a major brain area impacted in AD, by using hybrid enzymes to redirect the synthesis of prostanoids to PGE₂ and PGI₂, respectively. Our data indicated that during inflammation, the vascular mediators, PGI₂ and PGE₂, exerted significant regulatory influences on neuronal protection (by PGI₂), or damage (by PGE₂) in the hippocampus. These findings also raised a concern that the widely uses of non-steroidal anti-inflammatory drugs in cardiovascular diseases may exert negative impacts on neurodegenerative protection.

Prostaglandin E2 produced by inducible COX-2 and mPGES-1 promoting cancer cell proliferation in vitro and in vivo

AIM

Many cancers originate and flourish in a prolonged inflammatory environment. Our aim is to understand the mechanisms of how the pathway of prostaglandin E2 (PGE2) biosynthesis and signaling can promote cancer growth in inflammatory environment at cellular and animal model levels.

MAIN METHODS

In this study, a chronic inflammation pathway was mimicked with a stable cell line that over-expressed a novel human enzyme consisting of cyclooxygenase isoform-2 (COX-2) linked to microsomal (PGE2 synthase-1 (mPGES-1)) for the overproduction of pathogenic PGE2. This PGE2-producing cell line was co-cultured and co-implanted with three human cancer cell lines including prostate, lung, and colon cancers in vitro and in vivo, respectively.

KEY FINDINGS

Increases in cell doubling rates for the three cancer cell types in the presence of the PGE2-producing cell line were clearly observed. In addition, one of the four human PGE2 subtype receptors, EP1, was used as a model to identify PGE2-signaling involved in promoting the cancer cell growth. This finding was further proven in vivo by co-implanting the PGE2-producing cells line and the EP1-positive cancer cells into the immune deficient mice, after that, it was observed that the PGE2-producing cells promoted all three types of cancer formation in the mice.

SIGNIFICANCE

This study clearly demonstrated that the human COX-2 linked to mPGES-1 is a pathway that, when mediated by the EP, is linked to promoting cancer growth in a chronic inflammatory environment. The identified pathway could be used as a novel target for developing and advancing anti-inflammation and anti-cancer interventions.

Identification of Tumorigenesis from the Specific Coupling of Cyclooxygenase-2 with Microsomal Prostaglandin E2 Synthase-1 in vivo

A newly created hybrid enzyme (COX-2-10aa-mPGES-1), which mimics the specific biosynthesis of the inflammatory PGE2 through COX-2's coupling to mPGES-1, was stably expressed in HEK293 cells. The stable cell line, which consistently expresses the superior triple catalytic (Trip-Cat) activities from COX-2 and mPGES-1, was able to directly convert arachidonic acid into the pathogenic PGE2 and distinguish it from other PGE2 synthesizing pathways, as confirmed by enzyme immunoassay, LC/MS analysis and a specific [14C]-AA (arachidonic acid) metabolite analysis approach. A competitive assay confirmed that the endogenous cPGES and mPGES-2 in the HEK293 cells had little involvement in the presence of the expressed COX-2-10aa-mPGES-1 for the synthesis of pathogenic PGE2. Furthermore, subcutaneous injection of the stable cell lines into nu/nu mice revealed 100% (10 out 10) occurrence of tumor mass formation beginning on Day 7 and a continuous progression of the masses to the maximal size which required sacrificing the mice. In contrast, only 10% occurrence of tumor masses, though smaller and with slower growth rates, were observed for the group of vector-transfected HEK293 control cells expressing only endogenous cPGES and/or mPGES-2. The PGE2 produced from multiple pathways by the HEK293 cells co-expressing the individual wild type COX-2 and mPGES-1, and in the presence of endogenous cPGES and mPGES-2, showed also a significantly increased tumor occurrence rate to 30%, which confirmed that the sole coupling of COX-2 to mPGES-1 is a powerful tumor-advancing factor. This result implies that the engineered COX-2-10aa-mPGES-1 could be a promising molecule as a drug developing target against the pathway of COX-2 coupled to mPGES-1 to treat inflammatory diseases and cancers.

Cell signalling by reactive lipid species: new concepts and molecular mechanisms

The process of lipid peroxidation is widespread in biology and is mediated through both enzymatic and non-enzymatic pathways. A significant proportion of the oxidized lipid products are electrophilic in nature, the RLS (reactive lipid species), and react with cellular nucleophiles such as the amino acids cysteine, lysine and histidine. Cell signalling by electrophiles appears to be limited to the modification of cysteine residues in proteins, whereas non-specific toxic effects involve modification of other nucleophiles. RLS have been found to participate in several physiological pathways including resolution of inflammation, cell death and induction of cellular antioxidants through the modification of specific signalling proteins. The covalent modification of proteins endows some unique features to this signalling mechanism which we have termed the ‘covalent advantage’. For example, covalent modification of signalling proteins allows for the accumulation of a signal over time. The activation of cell signalling pathways by electrophiles is hierarchical and depends on a complex interaction of factors such as the intrinsic chemical reactivity of the electrophile, the intracellular domain to which it is exposed and steric factors. This introduces the concept of electrophilic signalling domains in which the production of the lipid electrophile is in close proximity to the thiol-containing signalling protein. In addition, we propose that the role of glutathione and associated enzymes is to insulate the signalling domain from uncontrolled electrophilic stress. The persistence of the signal is in turn regulated by the proteasomal pathway which may itself be subject to redox regulation by RLS. Cell death mediated by RLS is associated with bioenergetic dysfunction, and the damaged proteins are probably removed by the lysosome-autophagy pathway.

Engineering of a novel hybrid enzyme: an anti-inflammatory drug target with triple catalytic activities directly converting arachidonic acid into the inflammatory prostaglandin E2

Cyclooxygenase isoform-2 (COX-2) and microsomal prostaglandin E(2) synthase-1 (mPGES-1) are inducible enzymes that become up-regulated in inflammation and some cancers. It has been demonstrated that their coupling reaction of converting arachidonic acid (AA) into prostaglandin (PG) E(2) (PGE(2)) is responsible for inflammation and cancers. Understanding their coupling reactions at the molecular and cellular levels is a key step toward uncovering the pathological processes in inflammation. In this paper, we describe a structure-based enzyme engineering which produced a novel hybrid enzyme that mimics the coupling reactions of the inducible COX-2 and mPGES-1 in the native ER membrane. Based on the hypothesized membrane topologies and structures, the C-terminus of COX-2 was linked to the N-terminus of mPGES-1 through a transmembrane linker to form a hybrid enzyme, COX-2-10aa-mPGES-1. The engineered hybrid enzyme expressed in HEK293 cells exhibited strong triple-catalytic functions in the continuous conversion of AA into PGG(2) (catalytic-step 1), PGH(2) (catalytic-step 2) and PGE(2) (catalytic-step 3), a pro-inflammatory mediator. In addition, the hybrid enzyme was also able to directly convert dihomo-gamma-linolenic acid (DGLA) into PGG(1), PGH(1) and then PGE(1) (an anti-inflammatory mediator). The hybrid enzyme retained similar K(d) and V(max) values to that of the parent enzymes, suggesting that the configuration between COX-2 and mPGES-1 (through the transmembrane domain) could mimic the native conformation and membrane topologies of COX-2 and mPGES-1 in the cells. The results indicated that the quick coupling reaction between the native COX-2 and mPGES-1 (in converting AA into PGE(2)) occurred in a way so that both enzymes are localized near each other in a face-to-face orientation, where the COX-2 C-terminus faces the mPGES-1 N-terminus in the ER membrane. The COX-2-10aa-mPGES-1 hybrid enzyme engineering may be a novel approach in creating inflammation cell and animal models, which are particularly valuable targets for the next generation of NSAID screening.

An active triple-catalytic hybrid enzyme engineered by linking cyclo-oxygenase isoform-1 to prostacyclin synthase that can constantly biosynthesize prostacyclin, the vascular protector

It remains a challenge to achieve the stable and long-term expression (in human cell lines) of a previously engineered hybrid enzyme [triple-catalytic (Trip-cat) enzyme-2; Ruan KH, Deng H & So SP (2006) Biochemistry45, 14003-14011], which links cyclo-oxygenase isoform-2 (COX-2) to prostacyclin (PGI(2)) synthase (PGIS) for the direct conversion of arachidonic acid into PGI(2) through the enzyme's Trip-cat functions. The stable upregulation of the biosynthesis of the vascular protector, PGI(2), in cells is an ideal model for the prevention and treatment of thromboxane A(2) (TXA(2))-mediated thrombosis and vasoconstriction, both of which cause stroke, myocardial infarction, and hypertension. Here, we report another case of engineering of the Trip-cat enzyme, in which human cyclo-oxygenase isoform-1, which has a different C-terminal sequence from COX-2, was linked to PGI(2) synthase and called Trip-cat enzyme-1. Transient expression of recombinant Trip-cat enzyme-1 in HEK293 cells led to 3-5-fold higher expression capacity and better PGI(2)-synthesizing activity as compared to that of the previously engineered Trip-cat enzyme-2. Furthermore, an HEK293 cell line that can stably express the active new Trip-cat enzyme-1 and constantly synthesize the bioactive PGI(2) was established by a screening approach. In addition, the stable HEK293 cell line, with constant production of PGI(2), revealed strong antiplatelet aggregation properties through its unique dual functions (increasing PGI(2) production while decreasing TXA(2) production) in TXA(2) synthase-rich plasma. This study has optimized engineering of the active Trip-cat enzyme, allowing it to become the first to stably upregulate PGI(2) biosynthesis in a human cell line, which provides a basis for developing a PGI(2)-producing therapeutic cell line for use against vascular diseases.

Large-scale expression, purification, and characterization of an engineered prostacyclin-synthesizing enzyme with therapeutic potential

Recently, we reported that a novel hybrid enzyme (TriCat enzyme), engineered by linking human cyclooxygenase-2 (COX-2) with prostacyclin (PGI(2)) synthase (PGIS) together through a transmembrane domain, was able to directly integrate the triple catalytic (TripCat) functions of COX-2 and PGIS and effectively convert arachidonic acid (AA) into the vascular protector, PGI(2) [K.H. Ruan, H. Deng, S.P. So, Biochemistry 45 (2006) 14003-14011]. In order to confirm the important biological activity and evaluate its therapeutic potential, it is critical to characterize the properties of the enzyme using the purified protein. The TriCat enzyme cDNA was subcloned into a baculovirus vector and its protein was expressed in Sf-9 cells in large-scale with a high-yield ( approximately 4% of the total membrane protein), as confirmed by Western blot and protein staining. The Sf-9 cells' membrane fraction, rich in TriCat enzyme, exhibited strong TriCat functions (K(m)=3 microM and K(cat)=100 molecules/min) for the TriCat enzyme and was 3-folds faster in converting AA to PGI(2) than the combination of the individual COX-2 and PGIS. Another superiority of the TriCat enzyme is its dual effect on platelet aggregation: it completely inhibited platelet aggregation at the low concentration of 2 microg/ml and then displayed the ability to reverse the initially aggregated platelets to their non-aggregated state. Furthermore, multiple substrate-binding sites were confirmed in the single protein by high-resolution NMR spectroscopy, using partially purified TriCat enzyme. These studies have clearly demonstrated that the isolated TriCat enzyme protein functions in the selective biosynthesis of the vascular protector, PGI(2), and revealed its potential for anti-thrombosis therapeutics.

A phage antibody to the active site of human placental alkaline phosphatase with higher affinity to the enzyme–substrate complex

Selection of specific antibodies from large repertoires is of importance in generating antibodies to specific structural determinants and in studying structure-function relationships. Alkaline phosphatase (AP) has several isozymes with various degrees of homology and a range of common synthetic substrates. We have previously reported the generation of isozyme specific anti-enzyme antibodies to an oncofetal antigen, placental alkaline phosphatase (PLAP) by using a specific uncompetitive inhibitor, L-Phe-Gly-Gly along with the substrate para-nitrophenyl phosphate (pNPP), to elute scFvs from a phage-displayed immunoglobulin library. These antibodies were directed to the active site and inhibited enzyme activity. An uncompetitive inhibitor acts by stabilizing the enzyme-substrate (ES) complex. In the present work, we report the characteristics of a clone VE5, selected by the same method. This clone has a higher binding affinity for ES complex than for enzyme alone. This is true for all the three isozymes (placental, bone and intestinal) tested. However, the other synthetic small molecular substrate, disodium phenyl phosphate inhibits phage binding. The clone possibly binds to the conserved structures of the active site of the AP isozymes and the higher affinity binding to AP-pNPP complex reflects the method of selection. Such anti-enzyme antibodies have a possible potential role in dissecting structure-function relationship of enzymatic antigens.

The N-terminal membrane anchor domain of the membrane-bound prostacyclin synthase involved in the substrate presentation of the coupling reaction with cyclooxygenase

To mimic the native conditions, the cyclooxygenase (COX)/prostaglandin I(2) synthase (PGIS) coupling reaction system was used to determine the coordination of PGIS with COX for the biosynthesis of prostacyclin (PGI(2)) using arachidonic acid (AA) as a substrate in a membrane-bound environment. The membrane-bound PGIS exhibited a faster isomerization of PGH(2) produced by COX to PGI(2) than the detergent-solubilized PGIS. To determine whether the N-terminal domain of PGIS responds to the facilitation of PGH(2) movement (presentation) from COX to the active site of PGIS, the first 20 residues of PGIS (Delta20-PGIS) were deleted and expressed in COS-7 cells. Delta20-PGIS retained membrane-bound properties and exhibited a slower substrate presentation property. Furthermore, a chimeric molecule (PGIS/TXAS(8-27)) with the replacement of the first 20 residues of PGIS by the corresponding membrane anchor region (residues 8-27) of thromboxane A(2) synthase was created to evaluate the mechanism influencing the biosynthesis of PGI(2) in coordination with COX. The chimera revealed a multiple fold delay in the PGH(2) presentation in low range concentrations of AA (0.3-3muM) at 30s reactions. However, the delay could be recovered by a longer incubation time in high range concentrations of AA (>10muM), but not in low range concentrations of AA. These results demonstrated that the N-terminal domain of PGIS plays a role in the facilitation of the substrate presentation to the PGIS active site in low concentrations of AA, which may be a physiological condition. The TXAS N-terminal domain could not replace the function of the corresponding domain of PGIS, indicating that the facilitation of the substrate presentation is specific.

Determination of the membrane contact residues and solution structure of the helix F/G loop of prostaglandin I2 synthase

From our topological arrangement model of prostaglandin I(2) synthase (PGIS) created by homology modeling and topology studies, we hypothesized that the helix F/G loop of PGIS contains a membrane contact region distinct from the N-terminal membrane anchor domain. To provide direct experimental data we have explored the relationship between the endoplasmic reticulum (ER) membrane and the PGIS F/G loop using a constrained synthetic peptide to mimic PGIS residues 208-230 cyclized on both ends through a disulfide bond with added Cys residues. The solution structure and the residues important for membrane contact of the constrained PGIS F/G loop peptide were investigated by high-resolution 1H two-dimensional nuclear magnetic resonance (2D NMR) experiments and a spin label incorporation technique. Through the combination of 2D NMR experiments in the presence of dodecylphosphocholine (DPC) micelles used to mimic the membrane environment, complete 1H NMR assignments of the F/G loop segment have been obtained and the solution structure of the peptide has been determined. The PGIS F/G loop segment shows a defined helix turn helix conformation, which is similar to the three-dimensional crystallography structure of P450BM3 in the corresponding region. The orientation and the residues contacted with the membrane of the PGIS F/G loop were evaluated from the effect of incorporation of a spin-labeled 12-doxylstearate into the DPC micelles with the peptide. Three residues in the peptide corresponding to the PGIS residues L217 (L11), L222 (L16), and V224 (V18) have been demonstrated to contact the DPC micelles, which implies that the residues are involved in contact with the ER membrane in the native membrane-bound PGIS. These results provided the first experimental evidence to localize the membrane contact residues in the F/G loop region of microsomal P450 and are valuable to further define and understand the membrane topology of PGIS and those of other microsomal P450s in the native membrane environment.

Substrate access channel topology in membrane-bound prostacyclin synthase

Results from our molecular-modelling and site-directed-mutagenesis studies of prostaglandin I(2) synthase (PGIS) have suggested that the large PGIS cytoplasmic domain is anchored to the endoplasmic reticulum (ER) membrane by the N-terminal segment in a way that orients the substrate access channel opening to face the membrane. To test this hypothesis we have explored the accessibility of the PGIS substrate channel opening to site-specific antibodies. The working three-dimensional PGIS model constructed by protein homology modelling was used to predict surface portions near the substrate access channel opening. Two peptides corresponding to the surface immediately near the opening [residues 66-75 (P66-75) and 95-116 (P95-116)], and two other peptides corresponding to the surface about 10-20 A (1 A=0.1 nm) away from the opening [residues 366-382 (P366-382) and 472-482 (P472-482)] were used to prepare site-specific antibodies. All four antipeptide antibodies specifically recognized the synthetic segments of human PGIS and recombinant PGIS, as shown by binding assays and Western-blot analysis. The site-specific antibodies were used to probe the accessibility of the substrate access channel opening in transiently transfected COS-1 cells expressing recombinant human PGIS, and in spontaneously transformed human endothelial cell line ECV cells expressing endogenous human PGIS. Immunofluorescence staining was performed for cells selectively permeabilized with streptolysin O and for cells whose membranes were permeabilized with detergent. Antibodies to peptides in the immediate vicinity of the substrate channel (P66-75 and P95-116) bound to their targets only after general permeabilization with Triton X-100. In contrast, the two antibodies to peptides further from the channel opening (P366-382 and P472-482) bound to their targets even in cells with intact ER membranes. These observations support our topology model in which the PGIS substrate access channel opening is positioned close to the ER membrane.