Journal of lipid mediators最新文献

The activity of purified 5-lipoxygenases (5-LO) shows a requirement for the presence of phosphatidylcholine or other lipids, in addition to Ca2+ and ATP. The enzymatic activity of purified human 5-lipoxygenase was dependent on the ratio of arachidonic acid to phospholipids rather than on the bulk arachidonic acid concentration, suggesting that the concentration of substrate at the lipid-water interface is important for the rate of the 5-LO reaction. Enzyme activity was also examined using vesicles of dimyristoylphosphatidylmethanol/1-palmitoyl-2-arachidonoylphosp hat idylcholine. Using PLA2 to release arachidonic acid from phospholipid, the ratio of leukotriene A4 (detected as trans-LTB4 isomers) to 5-hydroperoxyeicosatetraenoic acid (5-HPETE) accumulated depended on the 5-LO concentration and was relatively independent of the amount of PLA2. The ratio of leukotriene A4 to 5-HPETE production increased with the amount of 5-LO (1-15 micrograms/ml) to reach values (> 10) similar to those observed with ionophore-challenged human leukocytes. The data are consistent with the catalysis of 5-LO occurring at the surface of phospholipid vesicles with the 5-HPETE product being re-utilized by the LTA4 synthase reaction of 5-lipoxygenase under conditions of limiting arachidonic acid availability.

A high affinity binding site for 14(R),15(S)-EET, one of the major cytochrome P-450 metabolites of arachidonic acid (AA) in blood vessels, liver, kidney and urine of patients with pregnancy-induced hypertension, has been identified in a membrane preparation from guinea pig mononuclear (GPM) cells. Using a radioligand assay, binding of 14(R),15(S)-[3H]EET to its receptor site was saturable, specific and reversible. Scatchard analysis of saturation binding studies yielded a dissociation constant (Kd) of 5.7 x 10(-9) M, and maximum number of binding sites (Bmax) of 2.4 pmol/mg membrane protein. The specificity of the binding site was determined by competition studies. 14(S),15(R)-EET and 8,9-EET had a Ki of 6.3 and 8.8 nM, respectively, followed by 12(R)-HETE and LTD4. 12(S)-HETE and 5,6-EET were even less effective as a competitive inhibitor of radioligand and binding with Ki values from 2 to 20 microM. Receptor antagonists for TxA2, LTB4, LTD4 and PAF failed to displace 14(R),15(S)-[3H]EET from its binding site on GPM cell membranes. The results correlate well with the reported biological functions of 14,15-EET. In view of its potent biological activities, 14,15-EET may exert its cellular function through the binding and activation of its stereo-specific cell surface binding sites or receptor.

Our understanding of the anti-inflammatory actions of glucocorticoid hormones has significantly advanced in the past year with the discovery of a second cyclooxygenase gene which we call 'glucocorticoid-regulated inflammatory prostaglandin G/H synthase' (griPGHS). In mouse fibroblasts and human monocytes levels of griPGHS mRNA and protein rise dramatically in response to growth factors, cytokines, and oncogene activation. These inductions are markedly suppressed by the glucocorticoid hormone, dexamethasone. This stands in contrast to the behavior of the previously cloned cyclooxygenase (PGHS), which appears to be constitutively expressed. Thus far, data show that griPGHS is increased in many other systems where prostaglandin biosynthesis is regulated, including models of inflammation, tissue injury, and hormonal control of reproductive processes. Thus, griPGHS is likely to be a key mediator of many clinically relevant processes and as such represents an important target for both steroidal and nonsteroidal anti-inflammatory agents.

We characterized a specific binding site for pancreatic-type phospholipase A2 (PLA2-I) in several tissues and cells. The PLA2-I binding protein was purified from bovine corpus luteum membranes, which had a mass of 190 kDa. The purified protein, which possessed a binding capacity with high affinity and specificity for a mammalian mature type of PLA2-I, was a glycoprotein having a core protein of approx. 150 kDa and its carbohydrate moieties might be required for ligand recognition. PLA2-I elicited several biological responses in tissues and cells; i.e., cell proliferation and eicosanoid production, possibly through its specific binding site.

5-Lipoxygenase-activating protein (FLAP) plays an essential role in cellular leukotriene (LT) synthesis and represents the target of three classes of LT biosynthesis inhibitors. We have taken three approaches to localize regions of FLAP involved in the binding of these inhibitors. A comparison of the amino acid sequences of FLAP from eight mammalian species identifies regions of the protein which are highly conserved and consequently may be involved in functional and inhibitor binding properties of the protein. Conversely, amino acids not conserved amongst these species are unlikely to play an essential role in inhibitor binding. Immunoprecipitation of peptide fragments of FLAP cross-linked to photoaffinity analogues of LT biosynthesis inhibitors following site-specific peptide cleavage indicates that the inhibitor attachment site is amino-terminal to 72Trp. Taken together, the cross-species analysis and photoaffinity labelling studies suggest a region within the first hydrophilic loop of FLAP which may be important for inhibitor binding. Site-directed mutagenesis of human FLAP followed by the analysis of FLAP mutants in a radioligand binding assay was used to more accurately define critical amino acid residues within this region. Mutagenesis studies reveal that mutants containing deletions of amino acids in regions of FLAP not conserved between species retain the ability to specifically bind inhibitors. Furthermore, mutants containing deletions in a highly conserved region of the protein (residues 42-61) do not bind inhibitors. These studies have therefore localized specific amino acids of FLAP which are essential for inhibitor binding. The roles that these amino acids play in inhibitor binding and may play in 5-LO activation is under investigation.

The present results demonstrate leukotriene and lipoxin synthesis in human bone marrow and link these findings to biological effects in the same tissue. However, the mechanisms behind the described effects on myeloid progenitor cell growth are presently unknown. It is conceivable that both leukotrienes and lipoxins may act through modulation of endogenous cytokine production. However, it should be noted, that these lipoxygenase products totally failed to induce colony growth in the absence of GM-CSF. Moreover, the role of lipoxins in the bone marrow needs to be further clarified, since LXA4 induced both synergistic (with GM-CSF) and antagonistic (with LTC4) effects on progenitor cell growth. A possible pathophysiological role for leukotrienes and lipoxins may be suggested in chronic myelogenous leukemia. Thus, the capacity of hematological cells from CML patients to synthesize LTC4 was significantly increased. In addition, we have recently reported that CML platelets possessed a markedly decreased ability to participate in transcellular synthesis of the potential inhibitors of myelopoiesis, LXA4 and 5(S),12(S)-diHETE (Stenke et al., 1991b). Moreover, the production of these compounds was totally abolished in platelets obtained from CML patients in blastic crisis. Further studies should aim at defining the mechanisms behind the regulatory actions of leukotrienes and lipoxins in normal and leukemic human myelopoiesis.

A structure-activity study has been performed with respect to EP 157, a PGH2 analogue with a diphenylmethoxime omega-chain which behaves as an IP-receptor agonist. Bicyclo[2.2.1]heptene/heptane and bicyclo[2.2.2]octene/octane analogues were the most potent, with a cyclohexene analogue being less potent. Oxabicyclo[3.2.2]nonane, oxabicyclo[3.2.1]octane and oxabicyclo[2.2.1]heptane analogues were of low potency. Introduction of ether oxygen(s) into the omega-chain generally maintained potency, whereas 6-oxa-1,4-m- and 6-oxa-1,4-p-interphenylene analogues were inactive. Diphenylmethylazine analogues were also potent platelet inhibitors, but saturation of one of the phenyl rings abolished activity. Replacement of the oxime function by an ether group abolished activity. The results are discussed in relation to octimibate, a triphenylimidazoloxyalkanoic acid which is also an IP-receptor agonist, and to the possibility of IP-receptor subtypes.