Suppression of inflammatory responses to 12-O-tetradecanoyl-phorbol-13-acetate and carrageenin by YM-26734, a selective inhibitor of extracellular group II phospholipase A2

Clinical development and informatics analysis of natural and semi-synthetic flavonoid drugs: A critical review

BACKGROUND

Flavonoids are one of the most important metabolites with vast structural diversity and a plethora of potential pharmacological applications, which have drawn considerable attention in the laboratory. Nevertheless, it remains uncertain how many candidates were progressed to clinical application.

AIM OF REVIEW

We carried out a critical review of natural and semi-synthetic flavonoid drugs and candidates undergoing different clinical phases worldwide by applying an adequate search method and conducted a brief cheminformatic and bioinformatic analysis. It was expected that the obtained results might narrow the screening scope and reduce the cost of drug research and development.

KEY SCIENTIFIC CONCEPTS OF REVIEW

To our knowledge, this is the most systematic summarization of flavonoid-based drugs and clinical candidates to date. It was found that a total of 19 flavonoid-based drugs have been approved for the market, and of these, natural flavonoids accounted for 52.6%. Besides, a total of 36 flavonoid-based clinical candidates are undergoing or suspended in different phases, and of these, natural flavonoids account for 44.4%. Thus, natural flavonoids remain the best option for finding novel agents/active templates, and when investigated in conjunction with synthetic chemicals and biologicals, they offer the potential to discover novel structures that can lead to effective agents against a variety of human diseases. Additionally, flavonoid-based marketed drugs have been successful in cardiovascular treatment, and the related drugs account for more than 30% of marketed drugs. However, the use of flavonoids as antineoplastic and immunomodulating agents is not likely for approximately 50% of the candidates suspended in the clinical stage. Interestingly, the marketed drugs covered a broader range of chemical spaces based on size, polarity, and three-dimensional structure compared to the clinical candidates. In addition, flavonoid glycosides with poor oral bioavailability account for 36.8% of the marketed drugs, and thus, they could be thoroughly investigated.

Selective inhibition of neurogenic, but not agonist-induced contractions by phospholipase A2 inhibitors points to presynaptic phospholipase A2 functions in contractile neurotransmission to human prostate smooth muscle

BACKGROUND

Phospholipases A2 (PLA2 ) may be involved in α1 -adrenergic contraction by formation of thromboxane A2 in different smooth muscle types. However, whether this mechanism occurs with α1 -adrenergic contractions of the prostate, is still unknown. While α1 -adrenoceptor antagonists are the first line option for medical treatment of voiding symptoms in benign prostatic hyperplasia (BPH), improvements are limited, probably by nonadrenergic contractions including thromboxane A2 . Here, we examined effects of PLA2 inhibitors on contractions of human prostate tissues.

METHODS

Prostate tissues were obtained from radical prostatectomy. Contractions were induced by electric field stimulation (EFS) and by α1 -adrenergic agonists in an organ bath, after application of the cytosolic PLA2 inhibitors ASB14780 and AACOCF3, the secretory PLA2 inhibitor YM26734, the leukotriene receptor antagonist montelukast, or of solvent to controls.

RESULTS

Frequency-dependent contractions of human prostate tissues induced by EFS were inhibited by 25% at 8 Hz, 38% at 16 Hz and 37% at 32 Hz by ASB14780 (1 µM), and by 32% at 16 Hz and 22% at 32 Hz by AACOCF3 (10 µM). None of both inhibitors affected contractions induced by noradrenaline, phenylephrine or methoxamine. YM26734 (3 µM) and montelukast (0.3 and 1 µM) neither affected EFS-induced contractions, nor contractions by α1 -adrenergic agonists, while all contractions were substantially inhibited by silodosin (100 nM).

CONCLUSIONS

Our findings suggest presynaptic PLA2 functions in prostate smooth muscle contraction, while contractions induced by α1 -adrenergic agonists occur PLA2 -independent. Lacking sensitivity to montelukast excludes an involvement of PLA2 -derived leukotrienes in promotion of contractile neurotransmission.

Modulation of immunity by the secreted phospholipase A2 family

Among the phospholipase A2 (PLA2 ) superfamily, which typically catalyzes the sn-2 hydrolysis of phospholipids to yield fatty acids and lysophospholipids, the secreted PLA2 (sPLA2 ) family contains 11 isoforms in mammals. Individual sPLA2 s have unique enzymatic specificity toward fatty acids and polar heads of phospholipid substrates and display distinct tissue/cellular distributions, suggesting their distinct physiological functions. Recent studies using knockout and/or transgenic mice for a full set of sPLA2 s have revealed their roles in modulation of immunity and related disorders. Application of mass spectrometric lipidomics to these mice has enabled to identify target substrates and products of individual sPLA2 s in given tissue microenvironments. sPLA2 s hydrolyze not only phospholipids in the plasma membrane of activated, damaged or dying mammalian cells, but also extracellular phospholipids such as those in extracellular vesicles, microbe membranes, lipoproteins, surfactants, and dietary phospholipids, thereby exacerbating or ameliorating various diseases. The actions of sPLA2 s are dependent on, or independent of, the generation of fatty acid- or lysophospholipid-derived lipid mediators according to the pathophysiological contexts. In this review, we make an overview of our current understanding of the roles of individual sPLA2 s in various immune responses and associated diseases.

Discovery of uncompetitive inhibitors of SapM that compromise intracellular survival of Mycobacterium tuberculosis

SapM is a secreted virulence factor from Mycobacterium tuberculosis critical for pathogen survival and persistence inside the host. Its full potential as a target for tuberculosis treatment has not yet been exploited because of the lack of potent inhibitors available. By screening over 1500 small molecules, we have identified new potent and selective inhibitors of SapM with an uncompetitive mechanism of inhibition. The best inhibitors share a trihydroxy-benzene moiety essential for activity. Importantly, the inhibitors significantly reduce mycobacterial burden in infected human macrophages at 1 µM, and they are selective with respect to other mycobacterial and human phosphatases. The best inhibitor also reduces intracellular burden of Francisella tularensis, which secretes the virulence factor AcpA, a homologue of SapM, with the same mechanism of catalysis and inhibition. Our findings demonstrate that inhibition of SapM with small molecule inhibitors is efficient in reducing intracellular mycobacterial survival in host macrophages and confirm SapM as a potential therapeutic target. These initial compounds have favourable physico-chemical properties and provide a basis for exploration towards the development of new tuberculosis treatments. The efficacy of a SapM inhibitor in reducing Francisella tularensis intracellular burden suggests the potential for developing broad-spectrum antivirulence agents to treat microbial infections.

Recent progress in phospholipase A₂ research: from cells to animals to humans

Mammalian genomes encode genes for more than 30 phospholipase A₂s (PLA₂s) or related enzymes, which are subdivided into several classes including low-molecular-weight secreted PLA₂s (sPLA₂s), Ca²+-dependent cytosolic PLA₂s (cPLA₂s), Ca²+-independent PLA₂s (iPLA₂s), platelet-activating factor acetylhydrolases (PAF-AHs), lysosomal PLA₂s, and a recently identified adipose-specific PLA. Of these, the intracellular cPLA₂ and iPLA₂ families and the extracellular sPLA₂ family are recognized as the "big three". From a general viewpoint, cPLA₂α (the prototypic cPLA₂ plays a major role in the initiation of arachidonic acid metabolism, the iPLA₂ family contributes to membrane homeostasis and energy metabolism, and the sPLA₂ family affects various biological events by modulating the extracellular phospholipid milieus. The cPLA₂ family evolved along with eicosanoid receptors when vertebrates first appeared, whereas the diverse branching of the iPLA₂ and sPLA₂ families during earlier eukaryote development suggests that they play fundamental roles in life-related processes. During the past decade, data concerning the unexplored roles of various PLA₂ enzymes in pathophysiology have emerged on the basis of studies using knockout and transgenic mice, the use of specific inhibitors, and information obtained from analysis of human diseases caused by mutations in PLA₂ genes. This review focuses on current understanding of the emerging biological functions of PLA₂s and related enzymes.

Simplified YM-26734 inhibitors of secreted phospholipase A2 group IIA

Simplified analogs of YM-26734, a known inhibitor of secreted phospholipase A(2) (sPLA(2)) group IIA, were synthesized and found to also display potent inhibition at low nanomolar concentrations. Analogs were based on the didodecanoylphloroglucinol portion of YM-26734 which contains the predicted active site calcium binding group.

Spinal phospholipase A2 in inflammatory hyperalgesia: role of the small, secretory phospholipase A2

Current work emphasizes that peripheral tissue injury and inflammation results in a heightened sensitivity to subsequent noxious input (hyperalgesia) that is mediated in large part by the spinal synthesis and release of eicosanoids, in particular prostaglandins. Secreted phospholipase A(2)s (sPLA(2)s) form a class of structurally related enzymes that release arachidonic acid from cell membranes that is further processed to produce eicosanoids. We hypothesized that spinal sPLA(2)s may contribute to inflammation-induced hyperalgesia. Spinal cord tissue and cerebrospinal fluid were collected from rats for assessment of sPLA(2) protein expression and sPLA(2) activity. A basal sPLA(2) protein expression and activity was detected in spinal cord homogenate (87+/-17 pmol/min/mg), though no activity could be detected in cisternal cerebrospinal fluid, of naive rats. The sPLA(2) activity did not change in spinal cord tissue or cerebrospinal fluid assessed over 8 h after injection of carrageenan into the hind paw. However, the sPLA(2) activity observed in spinal cord homogenates was suppressed by addition of LY311727, a selective sPLA(2) inhibitor. To determine the role of this spinal sPLA(2) in hyperalgesia, we assessed the effects of lumbar intrathecal (IT) administration of LY311727 in rats with chronic IT catheters in three experimental models of hyperalgesia. IT LY311727 (3-30 microg) dose-dependently prevented intraplantar carrageenan-induced thermal hyperalgesia and formalin-induced flinching, at doses that had no effect on motor function. IT LY311727 also suppressed thermal hyperalgesia induced by IT injection of substance P (30 nmol). Using in vivo spinal microdialysis, we found that IT injection of LY311727 attenuated prostaglandin E(2) release into spinal dialysate otherwise evoked by the IT injection of substance P. Taken together, this work points to a role for constitutive sPLA(2)s in spinal nociceptive processing.

Phospholipase A2 inhibitors in development

To date, three isoforms of phospholipase A2 (PLA2) have been identified. Of these, the two Ca2+-dependent isoforms, secretory (sPLA2) and cytosolic phospholipase A2 (cPLA2), are targets for new anti-inflammatory drugs. The catalytic mechanisms and functions of the third isoform, Ca2+-independent cytosolic phospholipase A2 (iPLA2), are unknown at present. sPLA2 and cPLA2 are both implicated in the release of arachidonic acid and prophlogistic lipid mediators. However, recent findings provide evidence that cPLA2 is the dominant isoform in various kinds of inflammation, such as T-cell-mediated experimental arthritis. A triple function of PLA2-derived lipid mediators has been suggested: causing immediate inflammatory signs, involvement in secondary processes, e.g., superoxide free radical (O2) generation, apoptosis, or tumour necrosis factor-alpha (TNF-alpha)-cytotoxicity, and controlling the expression and activation of pivotal proteins implicated in inflammation and cell development, e.g., cytokines, adhesion proteins, proteinases, NF-kappaB, fos/jun/AP-1, c-Myc, or p21ras. In the past, research predominantly focused on the development of sPLA2 inhibitors; however, present techniques enable discrimination of cPLA2, sPLA2, and iPLA2, and specific inhibitors of each of the three isoforms are likely to appear soon. Over the last decade, between 40 and 50 sPLA2 inhibitors have been described; and the list is growing. However, of these, few have the potential for clinical success, and those that do are predominantly active site-directed inhibitors, e.g., BMS-181162, LY311727, ARL-67974, FPL67047, SB-203347, Ro-23-9358, YM-26734, and IS-741. At present, there are no likely clinical candidates emerging from the ranks of cPLA2 and iPLA2 inhibitors in development. Indications for which PLA2 inhibitors are being pursued include, sepsis, acute pancreatitis, inflammatory skin and bowel diseases, asthma, and rheumatoid arthritis. The three main obstacles to the successful development of PLA2 inhibitors include, insufficient oral bioavailability, low affinity for the enzyme corresponding to low in vivo efficacy and insufficient selectivity.

Induction of distinct sets of secretory phospholipase A(2) in rodents during inflammation

Although the expression of the prototypic secretory phospholipase A(2) (sPLA(2)), group IIA (sPLA(2)-IIA), is known to be up-regulated during inflammation, it remains uncertain if other sPLA(2) enzymes display similar or distinct profiles of induction under pathological conditions. In this study, we investigated the expression of several sPLA(2)s in rodent inflammation models. In lipopolysaccharide (LPS)-treated mice, the expression of sPLA(2)-V, and to a lesser extent that of sPLA(2)-IID, -IIE, and -IIF, were increased, whereas that of sPLA(2)-X was rather constant, in distinct tissues. 12-O-Tetradecanoylphorbol-13-acetate (TPA)-induced mouse ear edema, in which the expression of sPLA(2)-IID, -IIF and -V was increased, was significantly reduced by YM-26734, a competitive sPLA(2)-IIA inhibitor that turned out to inhibit sPLA(2)-IID, -IIE, -V and -X as well. In contrast, sPLA(2)-IIA was dominant in carageenin-induced pleurisy in rats, where the accumulation of exudate fluids and leukocytes was significantly ameliorated by YM-26734. These results indicate that distinct sPLA(2)s can participate in inflammatory diseases according to tissues, animal species, and types of inflammation.

Inducible cyclooxygenase-derived 15-deoxy(Delta)12-14PGJ2 brings about acute inflammatory resolution in rat pleurisy by inducing neutrophil and macrophage apoptosis

Failure of acute inflammation to resolve leads to persistence of the inflammatory response and may contribute to the development of chronic inflammation. Thus, an understanding of inflammatory resolution will provide insight into the etiology of chronic inflammation. In an acute pleurisy, polymorphonuclear leukocytes (PMNs) were found to predominate at the onset of the lesion but decreased in number by undergoing apoptosis, the principal mechanism by which PMNs died in this model. PMNs were progressively replaced by monocytes, which differentiated into macrophages. As with PMNs, macrophages also underwent programmed cell death leading to an abatement of the inflammatory response and eventual resolution. It was found that apoptosis of both these inflammatory cell types was mediated by pro-resolving cyclooxygenase 2-derived 15deoxyDelta12-14PGJ2, which is uniquely expressed during active resolution. Although PMN programmed cell death is well understood, the observation that macrophages apoptose during resolution of acute inflammation is less well described. These results provide insight into the mechanisms that switch off acute inflammation and prevent complications of wound healing and potentially the development of immune-mediated chronic inflammation.

Secreted phospholipases A2 induce the expression of chemokines in microvascular endothelium

Acute respiratory distress syndrome (ARDS) is characterized by alterations in microvascular permeability. In ARDS secreted phospholipase A(2) (sPLA(2)) IB and IIA are found to be highly upregulated. In this study, we therefore investigated the influence of exogenously added sPLA(2)-IB and sPLA(2)-IIA on the production of chemokines and adhesion molecules in lung microvascular endothelial cells (LMVEC). Treatment of LMVEC with sPLA(2)s resulted in a significant increase in the production of chemokines and adhesion molecules due to an increased expression of their mRNA and in an enhanced release of oleic acid. The upregulation of chemokines and adhesion molecules by LPS was stronger in the presence of sPLA(2). Activation of NF-kappaB occurred upon stimulation with sPLA(2). Moreover the MAPkinase pERK seems to be involved since a specific pERK inhibitor, e.g., U0126, but not a p38Kinase inhibitor, e.g., SB203580 prevented sPLA(2)-induced chemokine upregulation. Our data therefore suggest that LMVEC are a highly sensitive target for the direct action of extracellular sPLA(2)s.

Regulation of the cellular expression of secretory and cytosolic phospholipases A2, and cyclooxygenase-2 by peptide growth factors

Secretory group II (sPLA2) and cytosolic (cPLA2) phospholipases A2 and cyclooxygenase-2 (Cox-2) play a pivotal role in release of proinflammatory eicosanoids. Excessive activity of sPLA2 per se can also propagate inflammation. Endogenous control of the above enzymes has not been completely elucidated. We investigated the combined impact of promoting cytokines and inhibitory peptide growth factors on the expression of mRNA of the above enzymes, on protein content and extracellular release of sPLA2 and on PGE2 production in osteoblasts (FRCO). The synthesis and release of sPLA2 were enhanced by about 20-fold by 0.5 ng/ml IL-1beta or by 50 ng/ml of TNFalpha. Coaddition of both cytokines resulted in synergistic 150-fold increase in the release of sPLA2 implying the existence of two paths of induction. IL-1beta and TNFalpha markedly enhanced the transcription of sPLA2 mRNA. Kinetic study showed that IL-1/TNF initiated sPLA2 release after 12 h, reaching maximum at 48 h. IL-1alpha was a weak stimulator of sPLA2 release, whereas IL-6, IL-8, IGF, IFN-gamma, growth hormone, insulin and GM-CSF were not stimulatory. Peptide growth hormones TGFbeta, PDGF-BB, EGF and bFGF markedly inhibited the extracellular release of sPLA2. TGFbeta and PDGF-BB significantly reduced the level of sPLA2 mRNA, thus acting upon transcription whereas EGF and bFGF were not inhibitory, acting rather upon the translational or posttranslational steps. IL-1/TNF and growth factors had no significant effect on cPLA2 mRNA expression. Cox-2 mRNA expression was markedly enhanced by IL-1/TNF and suppressed by all growth factors tested. Cytokines enhanced the extracellular release of PGE2 and further enhancement was induced by growth factors with the exception of TGFbeta. Cycloheximide abolished completely the release of sPLA2 and markedly reduced the release of PGE2 from cytokine-stimulated FRCO, regardless of whether growth factors were present or not. NS-398, a specific inhibitor of Cox-2 abolished almost completely the release of PGE2 from cytokine-stimulated cells, regardless of the presence of growth factors. Thus, different signalling mechanisms are involved in the impact of growth factors on mRNA expression of sPLA2, cPLA2 and Cox-2. The differences between the impact on FRCO sPLA2 and that reported in other cells, imply that endogenous control of arachidonic acid cascade is cell-specific.

Suppression of inflammatory responses by surfactin, a selective inhibitor of platelet cytosolic phospholipase A2

Surfactin inhibits platelet and spleen cytosolic 100 kDa phospholipase A2 (PLA2). In contrast, this same compound enhances rat platelet group II PLA2 activity by approximately 2-fold and slightly increases group I PLA2 activity from porcine pancreas and Naja naja venom in vitro. Surfactin does not affect a Ca2+ -independent PLA2 partially purified from bovine brain. Thus, this compound inhibits selectively the cytosolic form of PLA2. Based on in vitro studies utilizing preincubation of surfactin with the enzyme, dialysis, and increased concentrations of substrates, the inhibitory effect of surfactin appears to be due to a direct interaction with the enzyme. Linear regression analysis of the linear portion of a concentration-response curve reveals an IC50 of 8.5 microM. To further determine the inhibitory pattern, a Dixon plot was constructed to show that the inhibition by surfactin is competitive, but not uncompetitive, with an inhibition constant of Ki = 4.7 microM in 50 mM Tris-HCl buffer, pH 8.0, at 37 degrees. Surfactin blocked non-stimulated and calcium ionophore A23187-stimulated release of arachidonic acid from monkey kidney CV-1 cells, which contain a cytosolic 100 kDa PLA2 as the major activity, as shown in an anionic exchange DEAE-5PW high performance liquid chromatography profile and western blotting analysis. Surfactin ameliorated inflammation induced by several chemicals. That is, it exhibited in vivo anti-inflammatory activity in several tested inflammatory reactions including 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced mouse ear edema, carrageenan-induced rat paw edema, and acetic acid-induced mouse writhing. These results demonstrate that surfactin is a selective inhibitor for cytosolic PLA2 and a putative anti-inflammatory agent through the inhibitory effect produced by direct interaction with cytosolic PLA2, and that inhibition of cytosolic PLA2 activity may suppress inflammatory responses.

Beta-lactams SB 212047 and SB 216754 are irreversible, time-dependent inhibitors of coenzyme A-independent transacylase

The enzyme coenzyme A-independent transacylase (CoA-IT) has been demonstrated to be the key mediator of arachidonate remodeling, a process that moves arachidonate into 1-ether-containing phospholipids. Blockade of CoA-IT by reversible inhibitors has been shown to block the release of arachidonate in stimulated neutrophils and inhibit the production of eicosanoids and platelet-activating factor. We describe novel inhibitors of CoA-IT activity that contain a beta-lactam nucleus. beta-Lactams were investigated as potential mechanism-based inhibitors of CoA-IT on the basis of the expected formation of an acyl-enzyme intermediate complex. Two beta-lactams, SB 212047 and SB 216754, were shown to be specific, time-dependent inhibitors of CoA-IT activity (IC50 = 6 and 20 microM, respectively, with a 10-min pretreatment time). Extensive washing and dilution could not remove the inhibition, suggesting it was irreversible. In stimulated human monocytes, SB 216754 decreased the production of eicosanoids in a time-dependent manner. In an in vivo model of phorbol ester-induced ear inflammation, SB 216754 was able to inhibit indices of both edema and cell infiltration. Taken together, the results support two hypotheses: 1) CoA-IT activity is important for the production of inflammatory lipid mediators in stimulated cells and in vivo and 2) the mechanism by which CoA-IT acts to transfer arachidonate is through an acyl-enzyme intermediate.

Morelloflavone, a novel biflavonoid inhibitor of human secretory phospholipase A2 with anti-inflammatory activity

The flavanonylflavone morelloflavone inhibited secretory phospholipase A2 (PLA2) in vitro, with a high potency on the human recombinant synovial and bee venom enzymes (IC50 = 0.9 and 0.6 microM, respectively). The inhibition was apparently irreversible. In contrast, the compound was inactive on cytosolic PLA2 activity from human monocytes. Morelloflavone scavenged reactive oxygen species generated by human neutrophils (IC50 = 2.7 and 1.8 microM for luminol and lucigenin, respectively) but did not modify cellular responses such as degranulation or eicosanoid release. This biflavonoid exerted anti-inflammatory effects in animal models, with a potent inhibition of 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced ear inflammation in mice after topical administration. In this test, morelloflavone was found to decrease oedema and myeloperoxidase levels in ear homogenates ID50 = 58.5 and 74.3 micrograms/ear, respectively). In contrast, this biflavonoid failed to modify arachidonic acid-induced ear inflammation or eicosanoid levels in ear homogenates. A significant anti-inflammatory effect was also observed in the mouse paw carrageenan edema after oral administration, with the highest inhibition at 3 hr after induction of inflammation. Morelloflavone is an inhibitor of secretory PLA2 with selectivity for groups II and III enzymes and may be a pharmacological tool. In addition, it shows anti-inflammatory activity apparently not related to the synthesis of eicosanoids, but likely dependent on other mechanisms such as scavenging of reactive oxygen species.

Involvement of secretory phospholipase A2 activity in the zymosan rat air pouch model of inflammation

1. In the zymosan rat air pouch model of inflammation we have assessed the time dependence of phospholipase A2 (PLA2) accumulation in the inflammatory exudates as well as cell migration, myeloperoxidase activity, prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) levels. 2. A significant increase in PLA2 activity was detected in 1,200 g supernatants of exudates 8 h after injection of zymosan into rat air pouch. This event coincided with peaks in cell accumulation (mainly neutrophils) and myeloperoxidase activity in exudates and was preceded by a rise in eicosanoid levels. 3. This enzyme (without further purification) behaved as a secretory type II PLA2 with an optimum pH at 7-8 units, lack of selectivity for arachidonate release and dependence on mM calcium concentrations for maximal activity. 4. The PLA2 inhibitors manoalide and scalaradial inhibited this enzyme activity in vitro in a concentration-dependent manner. Scalaradial also inhibited zymosan stimulated myeloperoxidase release in vitro. 5. Injection of the marine PLA2 inhibitor scalaradial together with zymosan into the pouch at doses of 0.5, 1 and 5 mumol per pouch resulted in a dose-dependent inhibition of PLA2 activity in exudates collected 8 h later. Myeloperoxidase levels and cell migration were also decreased, while eicosanoid levels were not modified. 6. Colchicine administration to rats prevented infiltration and decreased PLA2 levels in the 8 h zymosan-injected air pouch. 7. These results indicate that during inflammatory response to zymosan in the rat air pouch a secretory PLA2 activity is released into the exudates. The source of this activity is mainly the neutrophil which migrates into the pouch. 8. Scalaradial exerts anti-inflammatory effects in the zymosan air pouch.

Effects of marine 2-polyprenyl-1,4-hydroquinones on phospholipase A2 activity and some inflammatory responses

Three 2-polyprenyl-1,4-hydroquinone derivatives (2-heptaprenyl-1,4-hydroquinone: IS1, 2-octaprenyl-1,4-hydroquinone: IS2 and 2-[24-hydroxy]-octaprenyl-1,4-hydroquinone: IS3) isolated from the Mediterranean sponge Ircinia spinosula, were evaluated for effects on phospholipase A2 activity of different origin (Naja naja venom, human recombinant synovial fluid and bee venom), as well as on human neutrophil function and mouse ear oedema induced by 12-O-tetradecanoylphorbol 13-acetate (TPA). IS1 interacted minimally with these responses. In contrast, IS2 and IS3 inhibited human recombinant synovial phospholipase A2 in a concentration-dependent manner, with minor effects on the rest of the enzymes. Both compounds slightly affected superoxide generation and degranulation in human neutrophils, whereas they decreased thromboxane B2 and leukotriene B4 synthesis and release in a mixed suspension of human platelets and neutrophils stimulated by ionophore A23187, with IC50 values in the microM range. IS3 was the most effective inhibitor of the synthesis of thromboxane B2 by human platelet microsomes and of leukotriene B4 by high speed supernatants from human neutrophils. IS2 and IS3 showed topical anti-inflammatory activity against the TPA-induced ear inflammation in mice, with similar effects on oedema and a higher inhibition of IS3 on leukocyte migration, estimated as myeloperoxidase activity in supernatants of ear homogenates. Some structure-activity relationships were established since differences in the prenylated chain attached to the hydroquinone moiety result in important modifications of these inflammatory responses.

A natural disruption of the secretory group II phospholipase A2 gene in inbred mouse strains

The synovial fluid or group II secretory phospholipase A2 (sPLA2) has been implicated as an important agent involved in a number of inflammatory processes. In an attempt to determine the role of sPLA2 in inflammation, we set out to generate sPLA2-deficient mice. During this investigation, we observed that in a number of inbred mouse strains, the sPLA2 gene was already disrupted by a frameshift mutation in exon 3. This mutation, a T insertion at position 166 from the ATG of the cDNA, terminates out of frame in exon 4, resulting in the disruption of the calcium binding domain in exon 3 and loss of both activity domains coded by exons 4 and 5. The mouse strains C57BL/6, 129/Sv, and B10.RIII were found to be homozygous for the defective sPLA2 gene, whereas outbred CD-1:SW mice had variable genotype at this locus. BALB/c, C3H/HE, DBA/1, DBA/2, NZB/BIN, and MRL lpr/lpr mice had a normal sPLA2 genotype. The sPLA2 mRNA was expressed at very high levels in the BALB/c mouse small intestine, whereas in the small intestine of the sPLA2 mutant mouse strains, sPLA2 mRNA was undetectable. In addition, PLA2 activity in acid extracts of the small intestine were approximately 40 times higher in BALB/c than in the mutant mice. Transcription of the mutant sPLA2 gene resulted in multiple transcripts due to exon skipping. None of the resulting mutant mRNAs encoded an active product. The identification of this mutation should not only help define the physiological role of sPLA2 but also has important implications in mouse inflammatory models developed by targeted mutagenesis.

Regulation of phospholipase A2 enzymes: selective inhibitors and their pharmacological potential

The area of PLA2 research has grown immensely over the past 20 years. There is a better understanding of the kinetics, or factors that affect the kinetics, of the different forms of PLA2. New forms of PLA2 are being discovered, such as the cPLA2, which fit the role of an intracellularly regulated enzyme. Multiple forms of PLA2 tend to complicate the elucidation of the cellular mechanisms that regulate AA release and the subsequent eicosanoid production. Because of the factors that affect PLA2 kinetics and the unknown nature of the PLA2 that regulates AA release (there may be more than one), it has been difficult to design or isolate specific inhibitors. This review discussed selected classes of inhibitors because these have generated the most intense research in the field. There is a multitude of structurally diverse compounds reported in the literature that have been reported to be inhibitors of PLA2 in vitro and some have been reported to have anti-inflammatory activity (Wilkerson, 1990; Connolly and Robinson, 1993a). It is clear from a brief survey of the literature that the bulk of PLA2 inhibitors have topical anti-inflammatory activity. This may be due to the nature of these inhibitors: because they are hydrophobic they may be more readily absorbed in the skin whereas when given orally they may not be absorbed. To data, manoalide has been clinically evaluated in man and a new Bristol-Myers Squibb retenoid derivative may enter clinical trials for psoriasis (BMS-181162 (XVI)); however, there are no PLA2 inhibitors on the market or significantly advanced in clinical development (Table III). This indicates the lack of understanding of this enzyme for the development of relevant inhibitors, which is related to the lack of understanding of the relevant PLA2 that regulates AA release and eicosanoid biosynthesis. The concept of regulation of eicosanoid biosynthesis by PLA2 inhibition and decreased AA availability still remains a viable therapeutic approach for the treatment of inflammatory diseases. The proof of this concept has not been obtained because of the complex nature of PLA2 and the multiple forms of PLA2 in the cell. Clinical results with cyclooxygenase inhibitors and recent clinical results with inhibitor of 5-lipoxygenase demonstrate that if inhibition of PLA2 results in reduction in both lipid mediators, a good anti-inflammatory compound should result. The added advantage of PLA2 inhibitors would be the reduction of PAF levels; however, the clinical results with potent and specific PAF antagonists has been less encouraging about the potential benefits of reduction in PAF levels.(ABSTRACT TRUNCATED AT 400 WORDS)

Exogenous group II phospholipase A2 induces prostaglandin E2 production in mouse peritoneal macrophages

Cultures of mouse peritoneal resident macrophages produced prostaglandin E2 when exposed to extracellular group II phospholipase A2. The response to group II phospholipase A2 was concentration dependent, and prostaglandin E2 production in response to 1 microgram/ml purified group II enzyme was comparable to the maximal response elicited by lipopolysaccharide. Group II phospholipase A2 required millimolar concentrations of extracellular Ca2+ for the induction of prostaglandin E2 production, as well as for phospholipase A2 activity. YM-26734 (4-(3,5-didodecanoyl-2,4,6-trihydroxyphenyl)-7-hydroxy-2-(4-hydroxyph eny l) chroman), a selective inhibitor of group II phospholipase A2, inhibited not only the enzyme activity but also the prostaglandin E2 production-inducing activity of group II phospholipase A2 in a concentration-dependent manner. These findings suggest that group II phospholipase A2 released into the extracellular space may induce prostaglandin E2 production through hydrolysis of plasma membrane phospholipids. Taken together with the previous finding that YM-26734 suppressed inflammatory responses in vivo, these results suggest that group II phospholipase A2 may play a role in the excitation and/or progression of inflammatory processes through the production of eicosanoids.

Effects of flavonoids on Naja naja and human recombinant synovial phospholipases A2 and inflammatory responses in mice

Six flavonoid derivatives were tested for their influence on Naja naja and human recombinant synovial phospholipase A2. They showed a selectivity for the last enzyme with IC50 = 14.3, 17.6, 12.2 and 28.2 microM for quercetagetin, kaempferol-3-O-galactoside, scutellarein and scutellarein-7-O-glucuronide, respectively, while reduced effects were observed for hispidulin and hibifolin. After topical application all the flavonoids inhibited 12-O-tetradecanoylphorbol-13-acetate-induced ear oedema in mice with a potency comparable to that of indomethacin and they were also able to inhibit carrageenan-induced mouse paw oedema at a dose of 150 mg/kg p.o. The blockade of the free hydroxyl at C-7 or C-6 reduced the anti-inflammatory activity and also the inhibitory effect on human recombinant synovial phospholipase A2. These results are in accordance with the notion that group II phospholipases A2 may play a role in experimental inflammation, although several mechanisms seems to be involved in the anti-inflammatory effect of this group of flavonoids.