Protein Disulfide Isomerase Active Sites Research Articles

AGR2: a secreted protein worthy of attention in diagnosis and treatment of breast cancer.

AGR2 is a secreted protein widely existing in breast. In precancerous lesions, primary tumors and metastatic tumors, the expression of AGR2 is increased, which has aroused our interest. This review introduces the gene and protein structure of AGR2. Its endoplasmic reticulum retention sequence, protein disulfide isomerase active site and multiple protein binding sequences endow AGR2 with diverse functions inside and outside breast cancer cells. This review also enumerates the role of AGR2 in the progress and prognosis of breast cancer, and emphasizes that AGR2 can be a promising biomarker and a target for immunotherapy of breast cancer, providing new ideas for early diagnosis and treatment of breast cancer.

PDI Cleavage of Disulfide Bonds within the TP Receptor Inhibits Signaling through Gα 13

G-protein coupled receptors (GPCRs) are the most abundant superfamily of cell surface receptors. Approximately 35% of approved drugs target GPCRs and class A GPCRs account for ~85% of this superfamily. Class A GPCRs are characterized in part by two highly conserved disulfides in their extracellular domains that are thought to stabilize protein structure. Whether these disulfides can be enzymatically modified to influence G-protein signaling is not known. We and others have previously shown that disulfide bond modification by thiol isomerases such as protein disulfide isomerase (PDI) represent a previously unrecognized level of control of thrombus formation. We therefore evaluated the ability of recombinant PDI (rPDI) to modulate signaling through modification of the canonical disulfides within platelet GPCRs. Exposure of platelets to rPDI had no effect on stimulation through PAR1, PAR4, or the α 2A-adrenergic receptor. In contrast, rPDI exposure dramatically decreased platelet activation induced by the TP receptor agonists U46619 or arachidonic acid, implicating rPDI-mediated modulation of TP receptor signaling. Consistent with this finding, rPDI blocked U46619-mediated activation of α IIbβ 3, α-granule release, and dense granule release. Conversely, inhibition of endogenous PDI using either inhibitory antibodies or PDI-targeted small molecules enhanced TP receptor-mediated platelet aggregation and granule release, indicating that endogenous platelet PDI influences TP receptor signaling. Inhibition of TP receptor-mediated signaling required PDI active site cysteines since rPDI mutants lacking these cysteines lost inhibitory activity. Evaluation of the inhibitory activity of different PDI fragments showed that the PDI substrate binding domain is also critical for inhibitory activity. The ability to inhibit signaling through the TP receptor was specific for rPDI since incubation with other recombinant thiol isomerases including ERp57, ERp5, and ERp72 had no effect. To determine the specific modifications to TP receptor canonical disulfides induced by PDI, HEK cells were transfected with TP receptor and exposed to rPDI. TP receptor was subsequently immunopreciptated and disulfide-linked peptide analysis was performed using mass spectrometry. Compared to untreated controls, TP receptor exposed to rPDI demonstrated cleavage of Cys11-Cys102 and Cys105-Cys183 bonds and the generation of a new Cys102-Cys183 bond. To determine how modification of the disulfide bonding pattern affected signaling through the TP receptor, we evaluated signaling through specific Gα subunits in platelets. The platelet TP receptor signals through Gα q (which couples to phospholipase and increases calcium flux) and Gα 13 (which couples to RhoA and myosin light chain kinase). PDI-mediated cleavage of the platelet TP receptor resulted in biased signaling, with substantial inhibition of G α13-mediated RhoA-GTP activation and myosin light chain phosphorylation and little effect on Gα q-mediated calcium flux. These results show how PDI can modify platelet signaling and represent the first demonstration that a thiol isomerase can modulate the function of a GPCR via rearrangement of canonical disulfide bonds. DisclosuresNo relevant conflicts of interest to declare.

Iron Binding Properties of Recombinant Class A Protein Disulfide Isomerase from Arabidopsis thaliana.

The protein disulfide isomerase (PDI) family comprises a wide set of enzymes mainly involved in thiol-disulfide exchange reactions in the endoplasmic reticulum. Class A PDIs (PDI-A) constitute the smallest members of the family, consisting of a single thioredoxin (TRX) module without any additional domains. To date, their catalytic activity and cellular function are still poorly understood. To gain insight into the role of higher-plant class A PDIs, the biochemical properties of rAtPDI-A, the recombinant form of Arabidopsis thaliana PDI-A, have been investigated. As expressed, rAtPDI-A has only little oxidoreductase activity, but it appears to be capable of binding an iron-sulfur (Fe-S) cluster, most likely a [2Fe-2S] center, at the interface between two protein monomers. A mutational survey of all cysteine residues of rAtPDI-A indicates that only the second and third cysteines of the CXXXCKHC stretch, containing the putative catalytic site CKHC, are primarily involved in cluster coordination. A key role is also played by the lysine residue. Its substitution with glycine, which restores the canonical PDI active site CGHC, does not influence the oxidoreductase activity of the protein, which remains marginal, but strongly affects the binding of the cluster. It is therefore proposed that the unexpected ability of rAtPDI-A to accommodate an Fe-S cluster is due to its very unique CKHC motif, which is conserved in all higher-plant class A PDIs, differentiating them from all other members of the PDI family.

Protein Succination‐A Biomarker of Mitochondrial Stress in Adipose Tissue Inhibits Protein Function and is Regulated by Nutrient Balance

S‐(2‐succino)cysteine (2SC) is an irreversible post‐translational modification formed by the reaction of fumarate with protein cysteines, also known as protein succination. 2SC is a biomarker of glucotoxicity driven mitochondrial stress in the adipocyte. Here, we investigated if 2SC is a mechanistic mediator of adipocyte stress via modification of the critical chaperone protein disulfide isomerase (PDI) and examined if 2SC levels could be regulated by caloric restriction.Fumarase knockdown or control 3T3‐L1 adipocytes were matured in normal (5 mM) or high (30 mM) glucose for 8 days, or in 30 mM glucose for 4 days and then switched to 5 mM glucose for 4 days. Adipocyte lysates and adipose tissue protein from control and diabetic mice were analyzed by immunoblotting with an anti‐2SC antibody. The fluorescent PDI specific substrate DiEosinGSSG was utilized to assess PDI enzymatic activity in adipocytes. LC‐MS/MS mass spectrometry was utilized to identify the specific site of recombinant PDI succination.2SC significantly increases in adipocytes matured in high glucose and in the adipose tissue of diabetic mice. Succination significantly lowers PDI reductase activity in fumarase knockdown adipocytes and in the adipose tissue of diabetic mice versus control. MS/MS analysis identified an important PDI active site cysteine as a target of succination. 2SC is turned over in adipocytes when maturation medium is returned to normal glucose.2SC inhibits PDI reductase activity facilitating adipocyte stress as PDI is no longer able to re‐fold proteins within the adipocyte. The turnover of succinated proteins when nutrient balance is restored illustrates the importance of a balanced diet in preventing obesity and diabetes‐related dysfunction in adipose tissue. Our data demonstrates a novel biochemical mechanism mediating nutrient induced stress in the adipocyte, and that this stress is ameliorated when reasonable caloric restriction is employed, ensuring healthy functional adipose tissue.Support or Funding InformationNational Institute of Health 1F31DK108559‐01(AMM), 1R56DK105087‐01(NF) and 1‐11‐JF‐13 from the American Diabetes Association (NF).

Nitroarachidonic acid (NO2AA) inhibits protein disulfide isomerase (PDI) through reversible covalent adduct formation with critical cysteines

BackgroundNitroarachidonic acid (NO2AA) exhibits pleiotropic anti-inflammatory actions in a variety of cell types. We have recently shown that NO2AA inhibits phagocytic NADPH oxidase 2 (NOX2) by preventing the formation of the active complex. Recent work indicates the participation of protein disulfide isomerase (PDI) activity in NOX2 activation. Cysteine (Cys) residues at PDI active sites could be targets for NO2AA- nitroalkylation regulating PDI activity which could explain our previous observation. MethodsPDI reductase and chaperone activities were assessed using the insulin and GFP renaturation methods in the presence or absence of NO2AA. To determine the covalent reaction with PDI as well as the site of reaction, the PEG-switch assay and LC–MS/MS studies were performed. Results and conclusionsWe determined that both activities of PDI were inhibited by NO2AA in a dose- and time- dependent manner and independent from release of nitric oxide. Since nitroalkenes are potent electrophiles and PDI has critical Cys residues for its activity, then formation of a covalent adduct between NO2AA and PDI is feasible. To this end we demonstrated the reversible covalent modification of PDI by NO2AA. Trypsinization of modified PDI confirmed that the Cys residues present in the active site a′ of PDI were key targets accounting for nitroalkene modification. General significancePDI may contribute to NOX2 activation. As such, inhibition of PDI by NO2AA might be involved in preventing NOX2 activation. Future work will be directed to determine if the covalent modifications observed play a role in the reported NO2AA inhibition of NOX2 activity.

Abstract 5513: Discovery of an orally active small-molecule irreversible inhibitor of protein disulfide isomerase (PDI) for ovarian cancer treatment.

Abstract Protein disulfide isomerase (PDI), an endoplasmic reticulum (ER) chaperone protein, catalyzes disulfide bond breakage, formation and rearrangement. The effect of PDI inhibition on ovarian cancer progression is not yet clear, and there is a need for potent, selective and safe small-molecule inhibitors of PDI. Here, we report a new class of novel propynoic acid carbamoyl methyl amides (PACMAs) that are active against a panel of human ovarian cancer cell lines. Using fluorescent derivatives, 2D gel electrophoresis, and mass spectrometry, we established that PACMA 31, one of the most active analogues, acts as an irreversible small-molecule inhibitor of PDI, forming a covalent bond with PDI's active site cysteines. We also demonstrated that PDI activity is essential for the survival and proliferation of human ovarian cancer cells. In vivo, PACMA 31 showed tumor targeting ability and significantly suppressed ovarian tumor growth without causing toxicity to normal tissues. These irreversible small-molecule PDI inhibitors represent a new approach for the development of targeted anticancer agents for ovarian cancer therapy, and can also serve as useful probes for investigating the biology of PDI-implicated pathways. Citation Format: Shili Xu, Alexey N. Butkevich, Roppei Yamada, Yu Zhou, Bikash Debnath, Roger Duncan, Ebrahim Zandi, Nicos A. Petasis, Nouri Neamati. Discovery of an orally active small-molecule irreversible inhibitor of protein disulfide isomerase (PDI) for ovarian cancer treatment. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5513. doi:10.1158/1538-7445.AM2013-5513

The Reduction Potential of the Active Site Disulfides of Human Protein Disulfide Isomerase Limits Oxidation of the Enzyme by Ero1α

Disulfide formation in newly synthesized proteins entering the mammalian endoplasmic reticulum is catalyzed by protein disulfide isomerase (PDI), which is itself thought to be directly oxidized by Ero1α. The activity of Ero1α is tightly regulated by the formation of noncatalytic disulfides, which need to be broken to activate the enzyme. Here, we have developed a novel PDI oxidation assay, which is able to simultaneously determine the redox status of the individual active sites of PDI. We have used this assay to confirm that when PDI is incubated with Ero1α, only one of the active sites of PDI becomes directly oxidized with a slow turnover rate. In contrast, a deregulated mutant of Ero1α was able to oxidize both PDI active sites at an equivalent rate to the wild type enzyme. When the active sites of PDI were mutated to decrease their reduction potential, both were now oxidized by wild type Ero1α with a 12-fold increase in activity. These results demonstrate that the specificity of Ero1α toward the active sites of PDI requires the presence of the regulatory disulfides. In addition, the rate of PDI oxidation is limited by the reduction potential of the PDI active site disulfide. The inability of Ero1α to oxidize PDI efficiently likely reflects the requirement for PDI to act as both an oxidase and an isomerase during the formation of native disulfides in proteins entering the secretory pathway.

Ero1α Is Expressed on Blood Platelets in Association with Protein-disulfide Isomerase and Contributes to Redox-controlled Remodeling of αIIbβ3

Recent evidence supports a role of protein-disulfide isomerase (PDI) in redox-controlled remodeling of the exofacial domains of α(IIb)β(3) in blood platelets. The aim of this study was to explain whether Ero1α can be responsible for extracellular reoxidation of the PDI active site. We showed that Ero1α can be found on platelets and is rapidly recruited to the cell surface in response to platelet agonists. It is physically associated with PDI and α(IIb)β(3), as suggested by colocalization analysis in confocal microscopy and confirmed by immunoprecipitation experiments. Apart from monomeric oxidized Ero1α, anti-α(IIb)β(3) immunoprecipitates showed the presence of several Ero1α-positive bands that corresponded to the complexes α(IIb)β(3)-PDI-Ero1α, PDI-Ero1α, and Ero1α-Ero1α dimers. It binds more efficiently to the activated α(IIb)β(3) conformer, and its interaction is inhibited by RGD peptides. Ero1α appears to be involved in the regulation of α(IIb)β(3) receptor activity because of the following: (a) blocking the cell surface Ero1α by antibodies leads to a decrease in platelet aggregation in response to agonists and a decrease in fibrinogen and PAC-1 binding, and (b) transfection of MEG01 with Ero1α increases α(IIb)β(3) receptor activity, as indicated by increased binding of fibrinogen.

A Structural Disulfide of Yeast Protein-disulfide Isomerase Destabilizes the Active Site Disulfide of the N-terminal Thioredoxin Domain

Protein-disulfide isomerase (PDI) is an essential catalyst of disulfide formation and isomerization in the eukaryotic endoplasmic reticulum. PDI has two active sites at either end of the molecule, each containing two cysteines that facilitate thiol-disulfide exchange. In addition to its four catalytic cysteines, PDI possesses two non-active site cysteines whose location and separation distance varies by organism. In higher eukaryotes, the non-active site cysteines are located in the C-terminal half of the protein sequence and are separated by 30 amino acids. In contrast, the internal cysteines of PDI from lower eukaryotes are located near the N-terminal active site and are much closer together in sequence. The function of these cysteines and the significance of their unique location in yeast PDI have been unclear. Previous data (Xiao, R., Wilkinson, B., Solovyov, A., Winther, J. R., Holmgren, A., Lundstrom-Ljung, J., and Gilbert, H. F. (2004) J. Biol. Chem. 279, 49780-49786) suggest that the internal cysteines exist as a disulfide in the endoplasmic reticulum of Saccharomyces cerevisiae. By coupling mass spectrometry with a gel-shift technique that allows us to measure the redox potentials of the PDI active sites in the presence and absence of the non-active site cysteines, we find that the non-active site cysteines form a disulfide that is stable even in a very reducing environment and demonstrate that this disulfide exists to destabilize the N-terminal active site disulfide, making it a better oxidant by 18-fold. Consistent with this finding, we show that mutating the non-active site cysteines to alanines disrupts both the oxidase and isomerase activities of PDI in vitro.

Reduction-Reoxidation Cycles Contribute to Catalysis of disulfide Isomerization by Protein-disulfide Isomerase

Protein-disulfide isomerase (PDI) catalyzes the formation and isomerization of disulfides during oxidative protein folding. This process can be error-prone in its early stages, and any incorrect disulfides that form must be rearranged to their native configuration. When the second cysteine (CGHC) in the PDI active site is mutated to Ser, the isomerase activity drops by 7-8-fold, and a covalent intermediate with the substrate accumulates. This led to the proposal that the second active site cysteine provides an escape mechanism, preventing PDI from becoming trapped with substrates that isomerize slowly (Walker, K. W., and Gilbert, H. F. (1997) J. Biol. Chem. 272, 8845-8848). Escape also reduces the substrate, and if it is invoked frequently, disulfide isomerization will involve cycles of reduction and reoxidation in preference to intramolecular isomerization of the PDI-bound substrate. Using a gel-shift assay that adds a polyethylene glycol-conjugated maleimide of 5 kDa for each sulfhydryl group, we find that PDI reduction and oxidation are kinetically competent and essential for isomerization. Oxidants inhibit isomerization and oxidize PDI when a redox buffer is not present to maintain the PDI redox state. Reductants also inhibit isomerization as they deplete oxidized PDI. These rapid cycles of PDI oxidation and reduction suggest that PDI catalyzes isomerization by trial and error, reducing disulfides and oxidizing them in a different configuration. Disulfide reduction-reoxidation may set up critical folding intermediates for intramolecular isomerization, or it may serve as the only isomerization mechanism. In the absence of a redox buffer, these steady-state reduction-oxidation cycles can balance the redox state of PDI and support effective catalysis of disulfide isomerization.

A small-molecule catalyst of protein folding in vitro and in vivo.

The formation of native disulfide bonds between cysteine residues often limits the rate and yield of protein folding. The enzyme protein disulfide isomerase (PDI) catalyzes the interchange of disulfide bonds in substrate proteins. The two -Cys-Gly-His-Cys- active sites of PDI provide a thiol that has a low pKa value and a disulfide bond of high reduction potential (Eo'). A synthetic small-molecule dithiol, (+/-)-trans-1,2-bis(2-mercaptoacetamido)cyclohexane (BMC), has a pKa value of 8.3 and an Eo' value of -0.24 V. These values are similar to those of the PDI active sites. BMC catalyzes the activation of scrambled ribonuclease A, an inactive enzyme with non-native disulfide bonds, and doubles the yield of active enzyme. A monothiol analog of BMC, N-methylmercaptoacetamide, is a less efficient catalyst than BMC. BMC in the growth medium of Saccharomyces cerevisiae cells increases by > threefold the heterologous secretion of Schizosaccharomyces pombe acid phosphatase, which has eight disulfide bonds. This effect is similar to that from the overproduction of PDI in the S. cerevisiae cells, indicating that BMC, like PDI, can catalyze protein folding in vivo. A small-molecule dithiol with a low thiol pKa value and high disulfide Eo' value can mimic PDI by catalyzing the formation of native disulfide bonds in proteins, both in vitro and in vivo.

The Relative Protein Disulphide Isomerase (PDI) Activities of Gonadotropins, Thioredoxin and PDI

The three-dimensional configuration assumed by a polypeptide chain occurs spontaneously, with or without the participation of molecular chaperones, and is due to the combined interactions of the amino acids in the chain. In the case of disulphide-bond containing proteins, one or more enzymes catalyse the random cleavage and correct reformation of nascent protein disulphide bonds, e.g. protein disulfide-isomerase (PDI). PDI was first discovered in the ninteen sixties and was purified from bovine liver by Anfinsen and coworkers. The PDI active site contains reducible disulphide groups that participate in the reactivation of denatured protein. Generally, its activity is measured with respect to the catalysis of correct disulphide formation, and therefore reactivation, of reduced RNAase. Alternatively oxidized denatured (‘scrambled’) RNAase can be used as substrate and disulphide isomerization leading to reactivation measured. That thioredoxin (TRX), a ubiquitous, monomeric, multifunctional protein may have a role analogous to that of PDI was first suggested because its active site includes an amino acid sequence (Trp-Cys-Gly-Pro-Cys) similar to that in the active site of PDI (Trp-Cys-Gly-His-Cys). These vicinyl Cys residues of TRX and PDI can undergo reversible oxidation to form disulphide bonds. Recently Boniface and Reichert [ l ] pointed out that the Bsubunits of gonadotropic hormones also contain similar vicinyl Cys residues. Bovine, ovine or porcine LH-8 have the sequence His-Cys-Gly-Pro-Cys at positions 89-93 and bovine, ovine, and porcine FSH-8 have a similar sequence (His-Cys-Gly-Lys-Cys) in the same position. They found experimentally that purified ovine FSH and bovine LH preparations were about 60 and 300 times as active as thioredoxin, respectively. We used the assay of Pigiet et al. [2] with reduced, denatured RNAase as substrate to compare the activities of gonadotropins @LH and pFSH), PDI and thioredoxin.

Peptide binding to protein disulfide isomerase occurs at a site distinct from the active sites.

Protein disulfide isomerase (PDI) is a multifunctional protein resident in the lumen of the rough endoplasmic reticulum that facilitates protein folding via disulfide bond isomerization. Previously we determined that PDI binds a variety of peptides that can be covalently attached to this protein via a photoreactive cross-linker. We have now investigated the relationship between the peptide binding site and the ability of PDI to catalyze disulfide bond isomerization. PDI has two identical sequences, -WCGHCK-, that have been demonstrated to be important in PDI-catalyzed disulfide isomerization. We have found that other proteins containing these thioredoxin-like active site sequences do not bind the photoreactive peptide probes. Moreover, although chemical modification of the 2 cysteines within the thioredoxin-like active site regions completely inhibits PDI-catalyzed disulfide isomerization, these modifications do not affect peptide binding by PDI. Both of these observations suggest that peptide binding occurs at a site other than the putative PDI active sites. To localize the site in PDI at which binding occurs, we used a radiolabeled peptide photoaffinity probe. Peptide fragments generated by cleavage of 125I-peptide-labeled PDI with cyanogen bromide yielded a single 8-kDa polypeptide fragment containing the 125I-labeled peptide site, but neither of the putative catalytic sites of PDI. An 125I-labeled tryptic peptide was generated from this cyanogen bromide fragment and determined by microsequencing to contain residues 451-476 of PDI; this 26-residue peptide is noteworthy because of its extremely high content of acidic amino acids. Based on these findings we conclude that the peptide binding site is located in the COOH-terminal domain of the protein, and it is distinct from the two active sites for PDI-catalyzed disulfide isomerization and from the region of PDI that has estrogen receptor sequence similarity.