Paul H. M. Harrison

Synthesis and inhibitory action on HMG-CoA synthase of racemic and optically active oxetan-2-ones (β-Lactones)

A homologous series of both C3-unsubstituted and C3-methyl substituted oxetan-2-ones (beta-lactones) was investigated as potential inhibitors of yeast 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) synthase. Several reported methods for racemic beta-lactone synthesis were studied for preparation of the target series. In addition, a novel aluminum-based Lewis acid obtained by combination of Et2AlCl with (1R,2R)-2-[(diphenyl)hydroxymethyl] cyclohexan-1-ol was studied for the asymmetric [2 + 2] cycloaddition of aldehydes and trimethylsilylketene. This Lewis acid exhibited good reactivity but variable enantioselectivity (22-85% ee). In in vitro assays using both native and recombinant HMG-CoA synthase from Saccharomyces cerevisiae, oxetan-2-ones mono-substituted at C4 with linear alkyl chains gave IC50s that decreased monotonically with chain length up to 10 carbons and then rose rapidly for longer chains. The trans isomers of 3-methyl-4-alkyl-oxetan-2-ones showed a similar trend but had 1.3- to 5.6-fold lower IC50s. The results imply a substantial hydrophobic pocket in this enzyme that interacts with both C-3 and C-4 substituents of oxetan-2-one inhibitors.

Pramanicin induces apoptosis in Jurkat leukemia cells: A role for JNK, p38 and caspase activation

Pramanicin is a novel anti-fungal drug with a wide range of potential application against human diseases. It has been previously shown that pramanicin induces cell death and increases calcium levels in vascular endothelial cells. In the present study, we showed that pramanicin induced apoptosis in Jurkat T leukemia cells in a dose- and time-dependent manner. Our data reveal that pramanicin induced the release of cytochrome c and caspase-9 and caspase-3 activation, as evidenced by detection of active caspase fragments and fluorometric caspase assays. Pramanicin also activated c-jun N-terminal kinase (JNK), p38 and extracellular signal-regulated kinases (ERK 1/2) with different time and dose kinetics. Treatment of cells with specific MAP kinase and caspase inhibitors further confirmed the mechanistic involvement of these signalling cascades in pramanicin-induced apoptosis. JNK and p38 pathways acted as pro-apoptotic signalling pathways in pramanicin-induced apoptosis, in which they regulated release of cytochrome c and caspase activation. In contrast the ERK 1/2 pathway exerted a protective effect through inhibition of cytochrome c leakage from mitochondria and caspase activation, which were only observed when lower concentrations of pramanicin were used as apoptosis-inducing agent and which were masked by the intense apoptosis induction by higher concentrations of pramanicin. These results suggest pramanicin as a potential apoptosis-inducing small molecule, which acts through a well-defined JNK- and p38-dependent apoptosis signalling pathway in Jurkat T leukemia cells.

Twisted amides: crystal and optimized structures, and molecular geometry analysis of 1-acetyl-3,4,7,8-tetramethylglycoluril and 1,6-diacetyl-3,4,7,8-tetramethylglycoluril

Abstract The crystal structures of racemic 1-acetyl-3,4,7,8-tetramethylglycoluril ( 3 ) and 1,6-diacetyl-3,4,7,8-tetramethylglycoluril ( 4 ) were determined by X-ray diffraction. Compound 3 forms dimers in which two complementary N–H⋯O C hydrogen bonds link a pair of enantiomeric molecules, while 4 crystallizes in two very similar but symmetry unrelated geometries. Unlike the parent 3,4,7,8-tetramethylglycoluril ( 1 ), both 3 and 4 are asymmetric: they exhibit a large dihedral angle between the two bridgehead-to-bridgehead substituent bonds (∠Me–C–C–Me∼−23 and −24°, respectively). Further, the plane through one acetyl group in 4 is twisted with respect to the plane through the tetrahydroimidazolone ring to which it is attached, while the other acetyl group is close to coplanar with its corresponding ring, as is the case for the acetyl group of 3 . The experimental structures were used as a basis for evaluating optimized geometries at four levels of theory: restricted Hartree–Fock (ab initio) using 3-21G and 6-31G ∗ basis sets and semiempirical AM1 and PM3 methods. General and specific geometric criteria of comparison are defined and used to evaluate the optimized structures. Using this scheme for comparison, the 6-31G ∗ geometries are closer to the experimental geometries than the 3-21G geometries, and both are found to reproduce the X-ray geometries far better than the two semiempirical methods. The agreement between the conformations of the ab initio and the experimental geometries supports the hypothesis that the asymmetry in these molecules is dominated by intramolecular effects, as opposed to those of crystal packing. The results are consistent with an inherent asymmetry of the electron density between N1 and N6 in 3 , and a corresponding induced effect in 4 as a result of the twisting in the amide moiety.

β-Lactone natural products and derivatives inactivate homoserine transacetylase, a target for antimicrobial agents.

Homoserine transacetylase (HTA) catalyzes the transfer of an acetyl group from acetyl-CoA to the hydroxyl group of homoserine. This is the first committed step in the biosynthesis of methionine (Met) from aspartic acid in many fungi, Gram-positive and some Gram-negative bacteria. The enzyme is absent in higher eukaryotes and is important for microorganism growth in Met-poor environments, such as blood serum, making HTA an attractive target for new antimicrobial agents. HTA catalyzes acetyl transfer via a double displacement mechanism facilitated by a classic Ser–His–Asp catalytic triad located at the bottom of a narrow actives site tunnel. We explored the inhibitory activity of several β-lactones to block the activity of HTA. In particular, the natural product ebelactone A, a β-lactone with a hydrophobic tail was found to be a potent inactivator of HTA from Haemophilus influenzae. Synthetic analogs of ebelactone A demonstrated improved inactivation characteristics. Covalent modification of HTA was confirmed by mass spectrometry, and peptide mapping identified Ser143 as the modified residue, consistent with the known structure and mechanism of the enzyme. These results demonstrate that β-lactone inhibitors are excellent biochemical probes of HTA and potential leads for new antimicrobial agents.

Efficient claisen-type condensation between acyl units bound to a molecular template

Acylation of 3,4,7,8-tetramethylglycoluril1 (1) provides the monoacyl derivative 3 which can be acylated further with LDA and acyl chlorides. The resulting symmetric or asymmetric diacyl derivatives 4 undergo efficient base-catalyzed acyl transfer to provide 2-(acylacyl)glycolurils 5,6. Thus, 1 acts as a template to allow facile condensations between acyl units.

Pramanicin Analog Induces Apoptosis in Human Colon Cancer Cells: Critical Roles for Bcl-2, Bim, and p38 MAPK Signaling

Pramanicin (PMC) is an antifungal agent that was previously demonstrated to exhibit antiangiogenic and anticancer properties in a few in vitro studies. We initially screened a number of PMC analogs for their cytotoxic effects on HCT116 human colon cancer cells. PMC-A, the analog with the most potent antiproliferative effect was chosen to further interrogate the underlying mechanism of action. PMC-A led to apoptosis through activation of caspase-9 and -3. The apoptotic nature of cell death was confirmed by abrogation of cell death with pretreatment with specific caspase inhibitors. Stress-related MAPKs JNK and p38 were both activated concomittantly with the intrinsic apoptotic pathway. Moreover, pharmacological inhibition of p38 proved to attenuate the cell death induction while pretreatment with JNK inhibitor did not exhibit a protective effect. Resistance of Bax −/− cells and the protective nature of caspase-9 inhibition indicate that mitochondria play a central role in PMC-A induced apoptosis. Early post-exposure elevation of cellular Bim and Bax was followed by a marginal Bcl-2 depletion and Bid cleavage. Further analysis revealed that Bcl-2 downregulation occurs at the mRNA level and is critical to mediate PMC-A induced apoptosis, as ectopic Bcl-2 expression substantially spared the cells from death. Conversely, forced expression of Bim proved to significantly increase cell death. In addition, analyses of p53−/− cells demonstrated that Bcl-2/Bim/Bax modulation and MAPK activations take place independently of p53 expression. Taken together, p53-independent transcriptional Bcl-2 downregulation and p38 signaling appear to be the key modulatory events in PMC-A induced apoptosis.

Pramanicin, an antifungal agent, raises cytosolic Ca2+ and causes cell death in vascular endothelial cells

The effects of a newly discovered antifungal agent, pramanicin, on cytosolic Ca(2+) and cell viability of cultured bovine pulmonary artery endothelial cells and on endothelium-dependent relaxation of dog carotid arterial rings were investigated by digital dynamic fluorescence ratio imaging and morphological and contractility studies, respectively. Pramanicin 100 microM, previously shown to cause maximal endothelium-dependent and NO-mediated vascular relaxation, induced a small transient elevation of cytosolic Ca(2+) concentration in Ca(2+)-free medium; subsequent introduction of 1 mM Ca(2+) caused a steady, nonsaturating increase of Ca(2+), which could be brought down to the basal level by the addition of EGTA. At the single cell level, the elevation of cytosolic Ca(2+) initiates from the cell periphery and progresses toward the central region. When added to the plateau phase of phenylephrine-induced contraction, pramanicin induced a slow endothelium-dependent relaxation, which could be reversed with the NO synthase inhibitor, L-NOARG. When preincubated with vascular tissue, pramanicin resulted in an irreversible loss of endothelial function characterized by the lack of carbachol-induced relaxation. Pramanicin caused cell injury characterized by plasmalemmal bleb formation, leading to cell death characterized by Trypan blue staining of the nuclei in cultured vascular endothelial cells in a concentration- and time-dependent manner. Such pramanicin-induced cell death was not associated with Ca(2+)-mediated or NO-mediated mechanisms. The time course of Ca(2+) elevation corresponds with that of pramanicin-induced relaxation of precontracted arterial rings, whereas the time course of endothelial cell death corresponds to that of pramanicin-induced loss of endothelial function as assessed by carbachol-induced relaxation. The pramanicin analogue, PMC-A, a by-product of the biosynthesis of pramanicin, in which the epoxy group is replaced by a CC bond, caused little endothelial-dependent relaxation, but it was able to cause endothelial cell dysfunction, albeit to a lesser extent compared to pramanicin, suggesting a role of the epoxy group in pramanicin for its vasorelaxant effect.

Biosynthesis of ebelactone A: isotopic tracer, advanced precursor and genetic studies reveal a thioesterase-independent cyclization to give a polyketide β-lactone

Macrocyclization of polyketides generates arrays of molecular architectures that are directly linked to biological activities. The four-membered ring in oxetanones (β-lactones) is found in a variety of bioactive polyketides (for example, lipstatin, hymeglusin and ebelactone), yet details of its molecular assembly have not been extensively elucidated. Using ebelactone as a model system, and its producer Streptomyces aburaviensis ATCC 31860, labeling with sodium [1-13C,18O2]propionate afforded ebelactone A that contains 18O at all oxygen sites. The pattern of 13C–18O bond retention defines the steps for ebelactone biosynthesis, and demonstrates that β-lactone ring formation occurs by attack of a β-hydroxy group onto the carbonyl moiety of an acyclic precursor. Reaction of ebelactone A with N-acetylcysteamine (NAC) gives the β-hydroxyacyl thioester, which cyclizes quantitatively to give ebelactone A in aqueous ethanol. The putative gene cluster encoding the polyketide synthase (PKS) for biosynthesis of 1 was also identified; notably the ebelactone PKS lacks a terminal thioesterase (TE) domain and no stand alone TE was found. Thus the formation of ebelactone is not TE dependent, supporting the hypothesis that cyclization occurs on the PKS surface in a process that is modeled by the chemical cyclization of the NAC thioester.

The biosynthesis of pramanicin in Stagonospora sp. ATCC 74235: a modified acyltetramic acid

Biosynthetic incorporations of acetate, malonate and serine precursors which had been isotopically labelled with 2H, 13C, 15N and 18O into pramanicin 1 in Stagonospora sp. ATCC 74235 were demonstrated. Intact incorporation of a starter acetate and six extender malonates generates the acyclic, hydrophobic tail. A further intact acetate, in preference to malonate, and a serine entity which is incorporated only as the L-enantiomer and with the OC–CH(N)–CH2 entity intact, provide the pyrrolidone ring and hydroxymethyl group of 1. The results are fully consistent with a biosynthetic pathway involving an acyltetramic acid (2). The olefinic precursor 3 of the epoxide in 1 is described, and is also shown to co-occur in the cultures. The ratio of 1∶3 can be controlled by addition of precursors.

The biosynthesis of pramanicin: origin of the carbon skeleton

Incorporation of labelled acetates and serine into pramanicin 1 in Stagonospora sp. ATCC 74235 shows that the carbon skeleton of 1 is derived from eight acetate units and a serine residue, implying that the biosynthesis of 1 proceeds via an acyl–tetramic acid intermediate 3.