The natural products chemistry of West Indian gorgonian octocorals

Biochemical warfare on the reef: the role of glutathione transferases in consumer tolerance of dietary prostaglandins

BACKGROUND

Despite the profound variation among marine consumers in tolerance for allelochemically-rich foods, few studies have examined the biochemical adaptations underlying diet choice. Here we examine the role of glutathione S-transferases (GSTs) in the detoxification of dietary allelochemicals in the digestive gland of the predatory gastropod Cyphoma gibbosum, a generalist consumer of gorgonian corals. Controlled laboratory feeding experiments were used to investigate the influence of gorgonian diet on Cyphoma GST activity and isoform expression. Gorgonian extracts and semi-purified fractions were also screened to identify inhibitors and possible substrates of Cyphoma GSTs. In addition, we investigated the inhibitory properties of prostaglandins (PGs) structurally similar to antipredatory PGs found in high concentrations in the Caribbean gorgonian Plexaura homomalla.

PRINCIPAL FINDINGS

Cyphoma GST subunit composition was invariant and activity was constitutively high regardless of gorgonian diet. Bioassay-guided fractionation of gorgonian extracts revealed that moderately hydrophobic fractions from all eight gorgonian species examined contained putative GST substrates/inhibitors. LC-MS and NMR spectral analysis of the most inhibitory fraction from P. homomalla subsequently identified prostaglandin A(2) (PGA(2)) as the dominant component. A similar screening of commercially available prostaglandins in series A, E, and F revealed that those prostaglandins most abundant in gorgonian tissues (e.g., PGA(2)) were also the most potent inhibitors. In vivo estimates of PGA(2) concentration in digestive gland tissues calculated from snail grazing rates revealed that Cyphoma GSTs would be saturated with respect to PGA(2) and operating at or near physiological capacity.

SIGNIFICANCE

The high, constitutive activity of Cyphoma GSTs is likely necessitated by the ubiquitous presence of GST substrates and/or inhibitors in this consumer's gorgonian diet. This generalist's GSTs may operate as 'all-purpose' detoxification enzymes, capable of conjugating or sequestering a broad range of lipophilic gorgonian compounds, thereby allowing this predator to exploit a range of chemically-defended prey, resulting in a competitive dietary advantage for this species.

Actions of octocoral and tobacco cembranoids on nicotinic receptors

Nicotinic acetylcholine receptors (AChRs) are pentameric proteins that form agonist-gated cation channels through the plasma membrane. AChR agonists and antagonists are potential candidates for the treatment of neurodegenerative diseases. Cembranoids are naturally occurring diterpenoids that contain a 14-carbon ring. These diterpenoids interact with AChRs in complex ways: as irreversible inhibitors at the agonist sites, as noncompetitive inhibitors, or as positive modulators, but no cembranoid was ever shown to have agonistic activity on AChRs. The cembranoid eupalmerin acetate displays positive modulation of agonist-induced currents in the muscle-type AChR and in the related gamma-aminobutyric acid (GABA) type A receptor. Moreover, cembranoids display important biological effects, many of them mediated by nicotinic receptors. Cembranoids from tobacco are neuroprotective through a nicotinic anti-apoptotic mechanism preventing excitotoxic neuronal death which in part could result from anti-inflammatory properties of cembranoids. Moreover, tobacco cembranoids also have anti-inflammatory properties which could enhance their neuroprotective properties. Cembranoids from tobacco affect nicotine-related behavior: they increase the transient initial ataxia caused by first nicotine injection into naive rats and inhibit the expression of locomotor sensitization to repeated injections of nicotine. In addition, cembranoids are known to act as anti-tumor compounds. In conclusion, cembranoids provide a promising source of lead drugs for many clinical areas, including neuroprotection, smoking-cessation, and anti-cancer therapies.

Molecular Rearrangements in the Construction of Complex Molecules

A personal account is given of the development in my laboratory of designed molecular rearrangements and their application in target-directed synthesis.

Cembranoid diterpenes from the Caribbean sea whip Eunicea knighti

Three new cembranoid diterpenes, knightol (1), knightol acetate (2), and knightal (3), along with the known asperdiol (4) and asperdiol acetate (5), were isolated as major compounds from the sea whip Eunicea knighti collected from the Colombian Caribbean. The structures and absolute configurations of 1-5 were determined on the basis of spectroscopic analyses and by a combination of chemical and NMR methods, multiple correlations observed in a ROESY experiment, and using the modified Mosher method. Additionally, five semisynthetic compounds, 6-10, obtained during the chemical transformations of the natural compounds are here reported for the first time. All compounds were tested for antimicrobial activity against marine bacteria associated with heavily fouled surfaces and were also screened for antiquorum sensing (QS) activity. Compounds 1, 3, and 8 showed significant antimicrobial activity against bacterial isolates, and 1, 3, 7, and 8 showed excellent anti-QS inhibition activity measured by means of bioluminescence inhibition with biosensor model systems.

A unified strategy for enantioselective total synthesis of cladiellin and briarellin diterpenes: total synthesis of briarellins E and F and the putative structure of alcyonin and revision of its structure assignment

Enantioselective total syntheses of briarellin E (12) and briarellin F (13), as well as the structure originally proposed for the cladiellin diterpene alcyonin (10), have been realized. Comparison of the spectral data for synthetic 10, natural alcyonin, cladiellisin (33), and cladiellaperoxide (34), as well as chemical transformations of 10 and natural alcyonin, suggest that the structure of this coral metabolite is allylic peroxide 11. The unified approach detailed herein can be used to access both C4-deoxygenated and C4-oxygenated cladiellins and briarellins. The central step in these syntheses is acid-promoted condensation of (Z)-alpha,beta-unsaturated aldehydes 17 with cyclohexadienyl diols 18 to form intermediates 16 incorporating the hexahydroisobenzofuran core and five stereocenters of these marine diterpenes (Scheme 1 ).

Sesquiterpene variability in the gorgonian genus Plexaurella

Gorgonian corals have proven to be a prolific source of a variety of biologically active terpenes. Corals of the genus Plexaurella are known to be the source of a diverse array of sesquiterpenes, and past reports have indicated variability in terpene content. This study involved an examination of the terpene chemistry of 109 individuals of five of the six known species of this genus in an attempt to identify links between sesquiterpene chemistry and species, location and depth. The data indicated a high level of variability of sesquiterpene content and essentially no correlation between species and sesquiterpene chemistry. There was also no correlation between location or depth and sesquiterpene content. We therefore conclude that Plexaurella spp. are chemically indistinguishable species with respect to terpene chemistry.

Diterpenes from gorgonian corals

Covering: 1995 to April 2008. Gorgonian corals continue to provide a wealth of novel structures, many of which exhibit potentially useful biological activity. Notably, the families Briareidae, Gorgoniidae and Plexauridae have been demonstrated to contain a wide variety of natural products including steroids, acetogenins, sesquiterpenes and diterpenes. The most common of the gorgonian natural products are the diterpenes, and the intent of this review is to describe such compounds isolated from gorgonian corals, with a focus on the structures of new compounds as well as their biological activity. There have been developments improving our understanding of the biosynthetic origin of selected diterpenes, and these will also be discussed. This review describes 602 new compounds from 177 articles.

Frajunolides E-K, briarane diterpenes from Junceella fragilis

Chemical investigation of the gorgonian octocoral Junceella fragilis, collected in Taiwan, resulted in the isolation of seven new briarane-type diterpenes, frajunolides E-K (1-7), in addition to 14 known briaranes, praelolide, junceellin, junceellolides A-E, and K, 11a,20a-epoxy-4-deacetoxyjunceelolide D, umbraculolide A, junceellonoid A, and juncins Y, Z, and ZI, as well as ergosterol peroxide. The structures of 1-7 were determined by analysis of HRESIMS and 2D NMR spectroscopic data. Cytotoxicity and in vitro anti-inflammatory activities of compounds 1-7 were also evaluated.

The effect of bleaching on the terpene chemistry of Plexaurella fusifera: evidence that zooxanthellae are not responsible for sesquiterpene production

The close association between marine invertebrates, zooxanthellae, and numerous bacteria gives rise to the question of the identity of the actual producer of secondary metabolites. In fall of 2005, a widespread bleaching event occurred throughout the Caribbean Sea in which some colonies of the gorgonian coral Plexaurella fusifera bleached. This study investigated whether zooxanthellae play a key role in the biosynthesis of secondary metabolite terpenes from P. fusifera. The extent of bleaching was examined by chlorophyll A analysis and also by zooxanthellae isolation and cell counting. The bleached and unbleached colonies were found to contain similar concentrations of eremophilene as the major terpene, and both exhibited similar biosynthetic capability as evaluated by the transformation of [C(1)-(3)H]-farnesyl diphosphate to the sesquiterpenes. Differences in bacterial communities between the bleached and unbleached colonies were analyzed using molecular techniques, and preliminary indications are that unbleached and bleached corals are dominated by low G + C firmicutes and gammaproteobacteria, respectively. It therefore appears that terpene biosynthesis can proceed independently of the zooxanthellae in P. fusifera, suggesting that the coral or a bacterium is the biosynthetic source.

Proteomic identification, cDNA cloning and enzymatic activity of glutathione S-transferases from the generalist marine gastropod, Cyphoma gibbosum

Glutathione S-transferases (GST) were characterized from the digestive gland of Cyphoma gibbosum (Mollusca; Gastropoda), to investigate the possible role of these detoxification enzymes in conferring resistance to allelochemicals present in its gorgonian coral diet. We identified the collection of expressed cytosolic Cyphoma GST classes using a proteomic approach involving affinity chromatography, HPLC and nano-spray liquid chromatography-tandem mass spectrometry (LC-MS/MS). Two major GST subunits were identified as putative mu-class GSTs; while one minor GST subunit was identified as a putative theta-class GST, apparently the first theta-class GST identified from a mollusc. Two Cyphoma GST cDNAs (CgGSTM1 and CgGSTM2) were isolated by RT-PCR using primers derived from peptide sequences. Phylogenetic analyses established both cDNAs as mu-class GSTs and revealed a mollusc-specific subclass of the GST-mu clade. These results provide new insights into metazoan GST diversity and the biochemical mechanisms used by marine organisms to cope with their chemically defended prey.

Mode of cembranoid action on embryonic muscle acetylcholine receptor

The mechanism of eupalmerin acetate (EUAC) actions on the embryonic muscle nicotinic acetylcholine receptor (nAChR) in BC3H-1 cells was studied by using whole-cell and single-channel patch-clamp current measurements. With whole-cell currents, EUAC did not act as an agonist on this receptor. Coapplication of 30 microM EUAC with 50 microM, 100 microM, or 500 microM carbamoylcholine (CCh) reversibly inhibited the current amplitude, whereas, with 20 microM CCh, current was increased above control values in the presence of EUAC. EUAC concentration curves (0.01-40 microM) obtained with 100 microM and 500 microM CCh displayed slope coefficients, n(H), significantly smaller than one, suggesting that EUAC bound to several sites with widely differing affinities on the receptor molecule. The apparent rate of receptor desensitization in the presence of EUAC and CCh was either slower than or equal to that obtained with CCh alone. The major finding from single-channel studies was that EUAC did not affect single-channel conductance or the ability of CCh to interact with the receptor. Instead, EUAC acted by increasing the channel closing rate constant. The results are not consistent with the competitive model for EUAC inhibition, with the sequential open-channel block model, or with inhibition by increased desensitization. The data are best accounted for by a model in which EUAC acts by closed-channel block at low concentrations, by positive modulation at intermediate concentrations, and by negative allosteric modulation of the open channel at high concentrations.

Bipinnatins K-Q, minor cembrane-type diterpenes from the West Indian Gorgonian Pseudopterogorgia kallos: isolation, structure assignment, and evaluation of biological activities

An extensive chemical study of the secondary metabolites found in the crude organic extract of the gorgonian octocoral Pseudopterogorgia kallos has led to the isolation of seven new cembranolides, bipinnatins K-Q ( 2- 8), and one known compound, bipinnatin E ( 9). The molecular structures of compounds 2- 8, many of which contain unusual structural features, were assigned mainly by 2D NMR spectroscopic methods and X-ray crystallographic analysis. The discovery of compounds 2- 8 may lend support to previously proposed mechanisms for the biosynthesis of recently isolated bioactive natural products from the same gorgonian specimen. The in vitro cytotoxicity of bipinnatins 4, 6, and 7 against the NCI tumor cells MCF breast cancer, NCI-H460 non-small cell lung cancer, and SF-268 CNS cancer was evaluated. However, only bipinnatin Q ( 6) displayed significant cytotoxic activity. Some of the compounds isolated proved to be inhibitors of acetylcholine receptors.

Oxygenated cembranoids from a Formosan soft coral Sinularia gibberosa

Chemical investigation of a Formosan soft coral, Sinularia gibberosa, has led to the isolation of eight oxygenated cembranoids, 1- 8, including seven new compounds, gibberosenes A-G ( 2- 8). None of these compounds were found to be cytotoxic toward a limited panel of cancer cell lines. Compound 1 significantly inhibited the accumulation of the pro-inflammatory COX-2 protein of the LPS-stimulated RAW264.7 macrophage cells.

Eupalmerin acetate, a novel anticancer agent from Caribbean gorgonian octocorals, induces apoptosis in malignant glioma cells via the c-Jun NH2-terminal kinase pathway

The marine ecosystem is a vast but largely untapped resource for potential naturally based medicines. We tested 15 compounds derived from organisms found in the Caribbean Sea (14 gorgonian octocoral-derived compounds and one sponge-derived compound) for their anticancer effects on human malignant glioma U87-MG and U373-MG cells. Eupalmerin acetate (EPA) was chosen as the lead compound based on its longer-term stability and greater cytotoxicity than those of the other compounds we tested in these cell types. EPA induced G(2)-M cell cycle arrest and apoptosis via the mitochondrial pathway; it translocated Bax from the cytoplasm to the mitochondria and dissipated the mitochondrial transmembrane potential in both cell types. EPA was found to increase phosphorylated c-Jun NH(2)-terminal kinase (JNK) by >50% in both U87-MG and U373-MG cells. A specific JNK inhibitor, SP600125, inhibited EPA-induced apoptosis, confirming the involvement of the JNK pathway in EPA-induced apoptotic cell death. Furthermore, 7 days of daily intratumoral injections of EPA significantly suppressed the growth of s.c. malignant glioma xenografts (P < 0.01, on day 19). These results indicate that EPA is therapeutically effective against malignant glioma cells in vitro and in vivo and that it, or a similar marine-based compound, may hold promise as a clinical anticancer agent.

A General Strategy for Synthesis of Both (6Z)- and (6E)-Cladiellin Diterpenes: Total Syntheses of (−)-Cladiella-6,11-dien-3-ol, (+)-Polyanthellin A, (−)-Cladiell-11-ene-3,6,7-triol, and (−)-Deacetoxyalcyonin Acetate

The first total synthesis of an (E)-cladiellin diterpene, (-)-cladiella-6,11-diene-3-ol (1), was accomplished featuring an intramolecular amide enolate alkylation-intramolecular Diels-Alder strategy. In addition, a highly stereo-, regio-, and chemoselective synthetic strategy for other members of the cladiellin diterpenes such as (+)-polyanthellin A (2), (-)-cladiell-11-ene-3,6,7-triol (3), and (-)-deacetoxyalcyonin acetate (4) was developed utilizing the synthetic (E)-cladiellin 1 as a common intermediate by taking advantage of the unique chemical properties of its C(6)-(E)-oxatricyclic skeleton.

Elucidation of the biosynthetic origin of the anti-inflammatory pseudopterosins

The pseudopterosins are a family of diterpene glycosides isolated from the gorgonian coral Pseudopterogorgia elisabethae. These metabolites exhibit potent anti-inflammatory activity, and this review describes our efforts to elucidate their biosynthetic origin. A radioactivity-guided isolation was used to identify the terpene cyclase product. In addition, a detailed NMR-guided search for potential biosynthetic intermediates identified metabolites which were tested by incubating 3H-labeled analogues with a cell-free extract of the coral. All labeled metabolites were generated biosynthetically, and radiochemical purity was established by a combination of HPLC purification and derivatization. In summary, pseudopterosins are produced by a cyclization of geranylgeranyl diphosphate to elisabethatriene, aromatization to erogorgiaene, two successive oxidations to 7,8-dihydroxyerogorgiaene and a glycosylation to afford a seco-pseudopterosin as a key intermediate. A dehydrogenation leads to amphilectosins which undergo ring closures to yield the pseudopterosins.

Establishing the absolute configuration of the asbestinins: enantioselective total synthesis of 11-acetoxy-4-deoxyasbestinin D

A highly stereoselective synthesis of 11-acetoxy-4-deoxyasbestinin D (1) has been completed in 26 linear steps. The synthesis hinges on a selective glycolate aldol addition to establish the C-2 stereocenter, a ring-closing metathesis reaction to complete the oxonene, and an intramolecular Diels-Alder cycloaddition to establish the relative configuration at C-1, C-10, and C-14. This initial total synthesis of an asbestinin also serves to confirm the absolute configuration of this subclass of the C-2-C-11-cyclized cembranoid natural products.

Tobacco cembranoids protect the function of acute hippocampal slices against NMDA by a mechanism mediated by alpha4beta2 nicotinic receptors

Nicotine has been reported to be neuroprotective in experimental and epidemiological studies. In addition to nicotine, tobacco and cigarette smoke contain cembranoids, which are antagonists of neuronal nicotinic receptors (nAChR). Exposure of hippocampal slices to N-methyl-D-aspartate (NMDA) decreases the population spikes (PS). This parameter has been used as a measure of excitotoxicity. Surprisingly, both nicotine and tobacco cembranoids protected against NMDA and this neuroprotection was not blocked by methyllycaconitine (MLA), an antagonist of alpha7 nAChR. On the contrary, MLA had a neuroprotective effect of its own. We examined the effect of the tobacco cembranoid (1S,2E,4R,6R,7E,11E)-cembra-2,7,11-triene-4,6-diol (4R) on the neuroprotection against NMDA. DHbetaE, a selective antagonist of alpha4beta2 nAChR, inhibited the neuroprotection by nicotine, 4R, and MLA, suggesting the involvement of alpha4beta2 nAChRs in the neuroprotection. The cell-signaling pathways underlying the neuroprotection by 4R and by nicotine are different. The activity of phosphatidylinositol-3 kinase (PI3K) was required in both cases; however, 4R required the activity of L-type calcium channels and CAM kinase, whereas nicotine required the extracellular signal regulated kinase-1,2 (ERK) and protein kinase C (PKC). In addition, 4R did not enhance total phospho-ERK-1/2 but increased the amount of total Akt/PKB phosphorylated on the activation site and of glycogen synthase kinase 3-beta phosphorylated on the inhibitory site. Total levels of phosphoenzymes are presented instead of the ratio of phospho- over total enzyme because in preliminary experiments total ERK-1/2 levels were slightly increased by 4R. In conclusion, these findings demonstrate that there are two different nicotinic neuroprotective mechanisms mediated by alpha4beta2.

DABCO-Catalyzed Reaction of α-Halo Carbonyl Compounds with Dimethyl Acetylenedicarboxylate: A Novel Method for the Preparation of Polysubstituted Furans and Highly Functionalized 2H-Pyrans

[Chemical reaction: see text] Polysubstituted furans and highly functionalized 2H-pyrans were prepared in good yields by DABCO-catalyzed reactions of alpha-halo carbonyl compounds with dimethyl acetylenedicarboxylate (DMAD) at room temperature.