Chemical Implications for Antitumor and Antiviral Prostaglandins: Reaction of Δ7-Prostaglandin A1 and Prostaglandin A1 Methyl Esters with Thiols

Kinetic assessment of Michael addition reactions of alpha, beta-unsaturated carbonyl compounds to amino acid and protein thiols

Humans have extensive adverse exposure to alpha,beta-unsaturated carbonyl compounds (ABuCs) as these are major toxins in smoke and exhaust fumes, as well as products of lipid peroxidation. In contrast, another ABuC, dimethylfumarate, is used to treat psoriasis and multiple sclerosis. ABuCs undergo Michael adduction with amine, imidazole and thiol groups, with reaction at Cys residues predominating. Here we report rate constants, k₂, for ABuCs (acrolein, crotonaldehyde, dimethylfumarate, cyclohex-1-en-2-one, cyclopent-1-en-2-one) with Cys residues present on N-Ac-Cys, GSH, bovine serum albumin, creatine kinase, papain, glyceraldehyde-3-phosphate dehydrogenase, and both wild-type and the C151S mutant of Keap-1. k₂ values for N-Ac-Cys and GSH vary by > 250-fold, indicating a marked ABuC structure dependence, with acrolein the most reactive. There is also considerable variation in k₂ between protein Cys groups, with these significantly greater than for GSH. A linear inverse correlation for acrolein with the thiol pK_a indicates that the thiolate anion is the reactive species. The modest k₂ for GSH rationalizes the detection of protein adducts of ABuCs in cells. The k₂ values for dimethylfumarate also vary markedly, with the Cys151 residue on Keap-1 being particularly reactive, with the C151S mutant giving a much lower k₂ value. The data for crotonaldehyde, dimethylfumarate, and cyclohex-1-en-2-one show little correlation with the Cys pK_a values, indicating that steric/electronic interactions, rather than Cys ionization are important. These data indicate that protein Cys residues, and particularly Cys151 on Keap-1, react readily with dimethylfumarate, and this may help rationalize the use of this compound as a therapeutic agent.

Protein Lipoxidation: Basic Concepts and Emerging Roles

Protein lipoxidation is a non-enzymatic post-translational modification that consists of the covalent addition of reactive lipid species to proteins. This occurs under basal conditions but increases in situations associated with oxidative stress. Protein targets for lipoxidation include metabolic and signalling enzymes, cytoskeletal proteins, and transcription factors, among others. There is strong evidence for the involvement of protein lipoxidation in disease, including atherosclerosis, neurodegeneration, and cancer. Nevertheless, the involvement of lipoxidation in cellular regulatory mechanisms is less understood. Here we review basic aspects of protein lipoxidation and discuss several features that could support its role in cell signalling, including its selectivity, reversibility, and possibilities for regulation at the levels of the generation and/or detoxification of reactive lipids. Moreover, given the great structural variety of electrophilic lipid species, protein lipoxidation can contribute to the generation of multiple structurally and functionally diverse protein species. Finally, the nature of the lipoxidised proteins and residues provides a frameshift for a complex interplay with other post-translational modifications, including redox and redox-regulated modifications, such as oxidative modifications and phosphorylation, thus strengthening the importance of detailed knowledge of this process.

How does cross-conjugation influence thiol additions to enones? A computational study of thiol trapping by the naturally occurring divinyl ketones zerumbone and α-santonin

DFT calculations show how the kinetics and thermodynamics of thiol additions to enones are affected by incorporation of the enone into a cross-conjugated divinyl ketone moiety.

Mass Spectrometry Characterization of Thiol Conjugates Linked to Polyoxygenated Polyunsaturated Fatty Acid Species

Radical mediated oxidation of polyunsaturated fatty acids (PUFA) is known to generate a series of polyoxygenated cyclic products (PUFA-O_n, n ≥ 3). Here, we describe the characterization of glutathione (GSH) conjugates bound to polyoxygenated docosahexaenoic (DHA-O_n, n = 3-9), arachidonic (ARA-O_n, n = 3-7), α-linolenic (ALA-O₃), and linoleic (LA-O₃) acid species. Similar conjugates were also characterized for N-acetylcysteine (NAC) and Cu,Zn-superoxide dismutase (SOD1). Extensive LC-MS/MS characterization using a synthetic α-linolenic hydroxy-endoperoxide (ALA-O₃) derivative revealed at least two types of mechanisms leading to thiol adduction: a mechanism involving the nucleophilic attack by thiolate anion on 1,2-dioxolane to form a sulfenate ester-bonded conjugate and a mechanism involving cleavage of the dioxolane to form a α,β-unsaturated carbonyl followed by the Michael addition reaction. Finally, we detected a GSH conjugate with hydroxy-endoperoxide derived from linoleic acid (LA-O₃) in mice liver. In summary, our study reveals the formation of a series of thiol conjugates that are bound to highly oxygenated PUFA species. GSH conjugates described in our study may potentially play relevant roles in redox and inflammatory processes, especially under high oxygen tension conditions.

Identification and Molecular Characterization of Peroxisome Proliferator-Activated Receptor δ as a Novel Target for Covalent Modification by 15-Deoxy-Δ12,14-prostaglandin J2

PPARδ belongs to the peroxisome proliferator-activated receptor (PPAR) family of nuclear receptors. Upon activation by an agonist, PPARδ controls a variety of physiological processes via regulation of its target genes. 15-Deoxy-Δ12,14-prostaglandin J2 (15d-PGJ2) is a cyclopentenone prostaglandin that features an electrophilic, α,β-unsaturated ketone (an enone) in the cyclopentenone ring. Many of 15d-PGJ2's biological effects result from covalent interaction between C9 and the thiol group of a catalytic cysteine (Cys) in target proteins. In this study, we investigated whether 15d-PGJ2 activates PPARδ by forming a covalent adduct. Our data show that 15d-PGJ2 activates PPARδ's transcriptional activity through formation of a covalent adduct between its endocyclic enone at C9 and Cys249 in the receptor's ligand-binding domain. As expected, no adduct formation was seen following a Cys-to-Ser mutation at residue 249 (C249S) of PPARδ or with a PGD2/PGJ2 analogue that lacks the electrophilic C9. Furthermore, the PPARδ C249S mutation weakened induction of the receptor's DNA binding activity by 15d-PGJ2, which highlights the biological significance of our findings. Calculated chemical properties as well as data from molecular orbital calculations, reactive molecular dynamics simulations, and intrinsic reaction coordinate modeling also supported the selectivity of 15d-PGJ2's C9 toward PPARδ's Cys thiol. In summary, our results provide the molecular, chemical, and structural basis of 15d-PGJ2-mediated PPARδ activation, designating 15d-PGJ2 as the first covalent PPARδ ligand to be identified.

Organocatalytic umpolung annulative dimerization of ynones for the synthesis of 5-alkylidene-2-cyclopentenones

A novel phosphine-catalyzed umpolung [3 + 2]-annulative dimerization of ynones was developed to furnish functionally rich 5-alkylidene-2-cyclopentenones. In this protocol, ynone acts as both C2 and C3 synthons, which undergo [3 + 2]-annulative dimerization.

Prevention of oxytosis-induced c-Raf down-regulation by (arylthio)cyclopentenone prostaglandins is neuroprotective

Prolonged exposure to high concentrations of glutamate leads to cell type specific glutathione depletion and resulting oxidative stress, known as oxytosis. As a result of glutathione depletion, accumulation of reactive oxygen species and Ca²⁺ influx are increased; however, the specific target of oxytosis has yet to be identified. In the present study, we focused on the effect of glutamate-induced oxidative stress on the extracellular-regulated protein kinase (ERK) pathway using the murine hippocampal HT22 cell line. Although the contribution of the ERK pathway to glutamate-induced oxytosis in HT22 cells is controversial, Western blot analysis revealed that glutamate caused down-regulation of mitogen-activated protein kinase kinase kinase (c-Raf) and a resulting decrease in the phosphorylation of c-Raf, as well as of mitogen-activated protein kinase kinase1/2 (MEK1/2) and ERK1/2, downstream components of the c-Raf/MEK/ERK pathway. Furthermore, neuroprotective (arylthio)cyclopentenone prostaglandins prevented glutamate-induced c-Raf down-regulation and consequently maintained the basal activity of c-Raf and its downstream signaling components. A pull-down assay using biotin-labeled cyclopentenone prostaglandins revealed that they preferentially bound to c-Raf relative to other signaling molecules of the ERK pathway, including Ras, MEK1/2, and ERK. These results suggest that neuroprotective (arylthio)cyclopentenone prostaglandins directly bind to c-Raf protein and protect cells from down-regulation of the c-Raf protein itself, resulting in neuroprotection against oxidative stress.

Covalent Modifiers: A Chemical Perspective on the Reactivity of α,β-Unsaturated Carbonyls with Thiols via Hetero-Michael Addition Reactions

Although Michael acceptors display a potent and broad spectrum of bioactivity, they have largely been ignored in drug discovery because of their presumed indiscriminate reactivity. As such, a dearth of information exists relevant to the thiol reactivity of natural products and their analogues possessing this moiety. In the midst of recently approved acrylamide-containing drugs, it is clear that a good understanding of the hetero-Michael addition reaction and the relative reactivities of biological thiols with Michael acceptors under physiological conditions is needed for the design and use of these compounds as biological tools and potential therapeutics. This Perspective provides information that will contribute to this understanding, such as kinetics of thiol addition reactions, bioactivities, as well as steric and electronic factors that influence the electrophilicity and reversibility of Michael acceptors. This Perspective is focused on α,β-unsaturated carbonyls given their preponderance in bioactive natural products.

Reversibility of the thia-Michael reaction of cytotoxic C5-curcuminoid and structure-activity relationship of bis-thiol-adducts thereof

C₅-curcuminoids [a.k.a. bis(arylmethylidene)acetones] are curcumin analogues bearing a reactive cross-conjugated dienone structure essential for eliciting cytotoxicity. To gain insight into the mode of action of C₅-curcuminoids, we investigated the reversibility of the thia-Michael reaction of 1,5-bis(3,5-bis(methoxymethoxy)phenyl)-1,4-pentadiene-3-one, named GO-Y030 which is the most potent cytotoxic C₅-curcuminoid, using spectroscopic methods. A panel of GO-Y030-bis-thiol-adducts were synthesized and the structure-reactivity relationship regarding the retro thia-Michael reaction as well as the cell growth inhibitory activity against human colon cancer HCT116 were evaluated. Some C₅-curcuminoid thiol-adducts exhibited comparable cytotoxicity with GO-Y030, demonstrating their potential use as prodrugs. These results imply that C₅-curcuminoids elicit cytotoxicity by covalently interacting with various biothiols via a reversible thia-Michael reaction.

Synthesis and Biological Investigation of Δ(12)-Prostaglandin J3 (Δ(12)-PGJ3) Analogues and Related Compounds

A series of Δ(12)-prostaglandin J3 (Δ(12)-PGJ3) analogues and derivatives were synthesized employing an array of synthetic strategies developed specifically to render them readily available for biological investigations. The synthesized compounds were evaluated for their cytotoxicity against a number of cancer cell lines, revealing nanomolar potencies for a number of them against certain cancer cell lines. Four analogues (2, 11, 21, and 27) demonstrated inhibition of nuclear export through a covalent addition at Cys528 of the export receptor Crm1. One of these compounds (i.e., 11) is currently under evaluation as a potential drug candidate for the treatment of certain types of cancer. These studies culminated in useful and path-pointing structure-activity relationships (SARs) that provide guidance for further improvements in the biological/pharmacological profiles of compounds within this class.

ROSics: chemistry and proteomics of cysteine modifications in redox biology

Post-translational modifications (PTMs) occurring in proteins determine their functions and regulations. Proteomic tools are available to identify PTMs and have proved invaluable to expanding the inventory of these tools of nature that hold the keys to biological processes. Cysteine (Cys), the least abundant (1-2%) of amino acid residues, are unique in that they play key roles in maintaining stability of protein structure, participating in active sites of enzymes, regulating protein function and binding to metals, among others. Cys residues are major targets of reactive oxygen species (ROS), which are important mediators and modulators of various biological processes. It is therefore necessary to identify the Cys-containing ROS target proteins, as well as the sites and species of their PTMs. Cutting edge proteomic tools which have helped identify the PTMs at reactive Cys residues, have also revealed that Cys residues are modified in numerous ways. These modifications include formation of disulfide, thiosulfinate and thiosulfonate, oxidation to sulfenic, sulfinic, sulfonic acids and thiosulfonic acid, transformation to dehydroalanine (DHA) and serine, palmitoylation and farnesylation, formation of chemical adducts with glutathione, 4-hydroxynonenal and 15-deoxy PGJ2, and various other chemicals. We present here, a review of relevant ROS biology, possible chemical reactions of Cys residues and details of the proteomic strategies employed for rapid, efficient and sensitive identification of diverse and novel PTMs involving reactive Cys residues of redox-sensitive proteins. We propose a new name, "ROSics," for the science which describes the principles of mode of action of ROS at molecular levels.

Total synthesis of Δ¹²-prostaglandin J₃, a highly potent and selective antileukemic agent

A catalytic asymmetric total synthesis of the potent and selective antileukemic Δ(12)-prostaglandin J3 (Δ(12)-PGJ3) is described. The convergent synthesis proceeded through intermediates 2 and 3, formed enantioselectively from readily available starting materials and coupled through an aldol reaction followed by dehydration to afford stereoselectively the cyclopentenone alkylidene structural motif of the molecule.

The 19S Deubiquitinase inhibitor b-AP15 is enriched in cells and elicits rapid commitment to cell death

b-AP15 [(3E,5E)-3,5-bis[(4-nitrophenyl)methylidene]-1-(prop-2-enoyl)piperidin-4-one] is a small molecule inhibitor of the ubiquitin specific peptidase (USP) 14/ubiquitin carboxyl-terminal hydrolase (UCH) L5 deubiquitinases of the 19S proteasome that shows antitumor activity in a number of tumor models, including multiple myeloma. b-AP15 contains an α,β-unsaturated carbonyl unit that is likely to react with intracellular nucleophiles such as cysteine thiolates by Michael addition. We found that binding of b-AP15 to USP14 is partially reversible, and that inhibition of proteasome function is reversible in cells. Despite reversible binding, tumor cells are rapidly committed to apoptosis/cell death after exposure to b-AP15. We show that b-AP15 is rapidly taken up from the medium and enriched in cells. Enrichment provides an explanation of the stronger potency of the compound in cellular assays compared with in vitro biochemical assays. Cellular uptake was impaired by 30-minute pretreatment of cells with low concentrations of N-ethylmaleimide (10 µM), suggesting that enrichment was thiol dependent. We report that in addition to inhibition of deubiquitinases, b-AP15 inhibits the selenoprotein thioredoxin reductase (TrxR). Whereas proteasome inhibition was closely associated with cell death induction, inhibition of TrxR was not. TrxR inhibition is, however, likely to contribute to triggering of oxidative stress observed with b-AP15. Furthermore, we present structure-activity, in vivo pharmacokinetic, and hepatocyte metabolism data for b-AP15. We conclude that the strong enrichment of b-AP15 in cells and a rapid commitment to apoptosis/cell death are factors that likely contribute to the strong antitumor activity of this compound.

Asymmetric Synthesis of Substituted Thiolanes through Domino Thia-Michael-Henry Dynamic Covalent Systemic Resolution using Lipase Catalysis

Dynamic systems based on consecutive thia-Michael and Henry reactions were generated and transformed using lipase-catalyzed asymmetric transformation. Substituted thiolane structures with three contiguous stereocenters were resolved in the process in high yields and high enantiomeric excesses.

Modification of cysteine residues by cyclopentenone prostaglandins: interplay with redox regulation of protein function

Cyclopentenone prostaglandins (cyPG) are endogenous lipid mediators involved in the resolution of inflammation and the regulation of cell proliferation and cellular redox status. Upon exogenous administration they have shown beneficial effects in models of inflammation and tissue injury, as well as potential antitumoral actions, which have raised a considerable interest in their study for the development of therapeutic tools. Due to their electrophilic nature, the best-known mechanism of action of these mediators is the covalent modification of proteins at cysteine residues through Michael addition. Identification of cyPG targets through proteomic approaches, including MS/MS analysis to pinpoint the modified residues, is proving critical to characterize their mechanisms of action. Among the targets of cyPG are proinflammatory transcription factors, proteins involved in cell defense, such as the regulator of the antioxidant response Keap1 and detoxifying enzymes like GST, and key signaling proteins like Ras proteins. Moreover, cyPG may interact with redox-active small molecules, such as glutathione and hydrogen sulfide. Much has been learned about cyPG in the past few years and this knowledge has also contributed to clarify both pharmacological actions and signaling mechanisms of these and other electrophilic lipids. Given the fact that many cyPG targets are involved in or are targets for redox regulation, there is a complex interplay with redox-induced modifications. Here we address the modification of protein cysteine residues by cyPG elucidated by proteomic studies, paying special attention to the interplay with redox signaling.

Redox-dependent anti-inflammatory signaling actions of unsaturated fatty acids

Unsaturated fatty acids are metabolized to reactive products that can act as pro- or anti-inflammatory signaling mediators. Electrophilic fatty acid species, including nitro- and oxo-containing fatty acids, display salutary anti-inflammatory and metabolic actions. Electrophilicity can be conferred by both enzymatic and oxidative reactions, via the homolytic addition of nitrogen dioxide to a double bond or via the formation of α,β-unsaturated carbonyl and epoxide substituents. The endogenous formation of electrophilic fatty acids is significant and influenced by diet, metabolic, and inflammatory reactions. Transcriptional regulatory proteins and enzymes can sense the redox status of the surrounding environment upon electrophilic fatty acid adduction of functionally significant, nucleophilic cysteines. Through this covalent and often reversible posttranslational modification, gene expression and metabolic responses are induced. At low concentrations, the pleiotropic signaling actions that are regulated by these protein targets suggest that some classes of electrophilic lipids may be useful for treating metabolic and inflammatory diseases.

Lipoxidation adducts with peptides and proteins: deleterious modifications or signaling mechanisms?

Protein lipoxidation refers to the modification by electrophilic lipid oxidation products to form covalent adducts, which for many years has been considered as a deleterious consequence of oxidative stress. Oxidized lipids or phospholipids containing carbonyl moieties react readily with lysine to form Schiff bases; alternatively, oxidation products containing α,β-unsaturated moieties are susceptible to nucleophilic attack by cysteine, histidine or lysine residues to yield Michael adducts, overall corresponding to a large number of possible protein adducts. The most common detection methods for lipoxidized proteins take advantage of the presence of reactive carbonyl groups to add labels, or use antibodies. These methods have limitations in terms of specificity and identification of the modification site. The latter question is satisfactorily addressed by mass spectrometry, which enables the characterization of the adduct structure. This has allowed the identification of lipoxidized proteins in physiological and pathological situations. While in many cases lipoxidation interferes with protein function, causing inhibition of enzymatic activity and increased immunogenicity, there are a small number of cases where lipoxidation results in gain of function or activity. For certain proteins lipoxidation may represent a form of redox signaling, although more work is required to confirm the physiological relevance and mechanisms of such processes. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

Monocarbonyl curcumin analogues: heterocyclic pleiotropic kinase inhibitors that mediate anticancer properties

Curcumin is a biologically active component of curry powder. A structurally related class of mimetics possesses similar anti-inflammatory and anticancer properties. Mechanism has been examined by exploring kinase inhibition trends. In a screen of 50 kinases relevant to many forms of cancer, one member of the series (4, EF31) showed ≥85% inhibition for 10 of the enzymes at 5 μM, while 22 of the proteins were blocked at ≥40%. IC50 values for an expanded set of curcumin analogues established a rank order of potencies, and analyses of IKKβ and AKT2 enzyme kinetics for 4 revealed a mixed inhibition model, ATP competition dominating. Our curcumin mimetics are generally selective for Ser/Thr kinases. Both selectivity and potency trends are compatible with protein sequence comparisons, while modeled kinase binding site geometries deliver a reasonable correlation with mixed inhibition. Overall, these analogues are shown to be pleiotropic inhibitors that operate at multiple points along cell signaling pathways.

Influence of base and structure in the reversible covalent conjugate addition of thiol to polycyclic enone scaffolds

The energetics of thiol addition and elimination reactions to bicyclic enones derived from an indole core structure were explored using (1)H NMR and density functional theory (DFT) calculations. The agreement between experiment and theory is excellent, and the combined results reveal that even minor changes in the conformation of the enone, substituents on the scaffold, and the use of different bases have a signficant influence on product distribution. A potential application of these principles is in the rational design of new reversible covalent enzyme inhibitors.

Targeting of histone acetyltransferase p300 by cyclopentenone prostaglandin Δ(12)-PGJ(2) through covalent binding to Cys(1438)

Inhibitors of histone acetyltransferases (HATs) are perceived to treat diseases like cancer, neurodegeneration, and AIDS. On the basis of previous studies, we hypothesized that Cys(1438) in the substrate binding site could be targeted by Δ(12)-prostaglandin J(2) (Δ(12)-PGJ(2)), a cyclopentenone prostaglandin (CyPG) derived from PGD(2). We demonstrate here the ability of CyPGs to inhibit p300 HAT-dependent acetylation of histone H3. A cell-based assay system clearly showed that the α,β-unsaturation in the cyclopentenone ring of Δ(12)-PGJ(2) was crucial for the inhibitory activity, while the 9,10-dihydro-15-deoxy-Δ(12,14)-PGJ(2), which lacks the electrophilic carbon (at carbon 9), was ineffective. Molecular docking studies suggested that Δ(12)-PGJ(2) places the electrophilic carbon in the cyclopentenone ring well within the vicinity of Cys(1438) of p300 to form a covalent Michael adduct. Site-directed mutagenesis of the p300 HAT domain, peptide competition assay involving p300 wild type and mutant peptides, followed by mass spectrometric analysis confirmed the covalent interaction of Δ(12)-PGJ(2) with Cys(1438). Using biotinylated derivatives of Δ(12)-PGJ(2) and 9,10-dihydro-15-deoxy-Δ(12,14)-PGJ(2), we demonstrate the covalent interaction of Δ(12)-PGJ(2) with the p300 HAT domain, but not the latter. In agreement with the in vitro filter binding assay, CyPGs were also found to inhibit H3 histone acetylation in cell-based assays. In addition, Δ(12)-PGJ(2) also inhibited the acetylation of the HIV-1 Tat by recombinant p300 in in vitro assays. This study demonstrates, for the first time, that Δ(12)-PGJ(2) inhibits p300 through Michael addition, where α,β-unsaturated carbonyl function is absolutely required for the inhibitory activity.