A novel approach to structure proof of glyceryl ether-containing glycerophospholipids. Base-catalyzed methanolysis of platelet-activating factor (AGEPC) at 60 degrees C

Identification of an intracellular receptor for lysophosphatidic acid (LPA): LPA is a transcellular PPARgamma agonist

Lysophosphatidic acid (LPA) is a pluripotent lipid mediator acting through plasma membrane-associated LPA(x) receptors that transduce many, but not all, of its effects. We identify peroxisome proliferator-activated receptor gamma (PPARgamma) as an intracellular receptor for LPA. The transcription factor PPARgamma is activated by several lipid ligands, but agonists derived from physiologic signaling pathways are unknown. We show that LPA, but not its precursor phosphatidic acid, displaces the drug rosiglitazone from the ligand-binding pocket of PPARgamma. LPA and novel LPA analogs we made stimulated expression of a PPAR-responsive element reporter and the endogenous PPARgamma-controlled gene CD36, and induced monocyte lipid accumulation from oxidized low-density lipoprotein via the CD36 scavenger receptor. The synthetic LPA analogs were effective PPARgamma agonists, but were poor ones for LPA(1), LPA(2), or LPA(3) receptor transfected cells. Transfection studies in yeast, which lack nuclear hormone and LPA(x) receptors, show that LPA directly activates PPARgamma. A major growth factor of serum is LPA generated by thrombin-activated platelets, and media from activated platelets stimulated PPARgamma function in transfected RAW264.7 macrophages. This function was suppressed by ectopic LPA-acyltransferase expression. LPA is a physiologic PPARgamma ligand, placing PPARgamma in a signaling pathway, and PPARgamma is the first intracellular receptor identified for LPA. Moreover, LPA produced by stimulated plasma platelets activates PPARgamma in nucleated cells.

Identification of a novel growth factor-like lipid, 1-O-cis-alk-1'-enyl-2-lyso-sn-glycero-3-phosphate (alkenyl-GP) that is present in commercial sphingolipid preparations

Lysophosphatidic acid, a member of the acidic phospholipid autacoid (APA) family of lipid mediators, elicits diverse cellular effects that range from mitogenesis to the prevention of programmed cell death. Sphingosine 1-phosphate and sphingosylphosphorylcholine have also been proposed to be ligands of the APA receptors. However, key observations that provide the foundation of this hypothesis have not been universally reproducible, leading to a controversy in the field. We provide evidence that 1-O-cis-alk-1'-enyl-2-lyso-sn-glycero-3-phosphate (alkenyl-GP) is present in some commercial sphingolipid preparations and is responsible for many of their APA-like effects, which were previously attributed to sphingosylphosphorylcholine. Alkenyl-GP was generated by acidic and basic methanolysis from ethanolamine lysoplasmalogen, which was present in the sphingomyelin fraction that is used to manufacture sphingosylphosphorylcholine. We present the structural identification of alkenyl-GP, using 1H and 13C NMR, Fourier transform infrared spectrometry, and mass spectrometry. Alkenyl-GP was a potent activator of the mitogen-activated protein kinases ERK1/2 and elicited a mitogenic response in Swiss 3T3 fibroblasts. In contrast, sphingosylphosphorylcholine at a concentration of 10 microM was only a weak mitogen and only weakly activated the extracellular signal-regulated protein kinases. Alkenyl-GP has recently been detected as an injury-induced component in the anterior chamber of the eye (Liliom, K., Guan, Z., Tseng, H., Desiderio, D. M., Tigyi, G., and Watsky, M. (1998) Am. J. Physiol. 274, C1065-C1074), indicating that this lipid is a naturally occurring member of the APA mediator family.

Solid-phase extraction on silica cartridges as an aid to platelet-activating factor enrichment and analysis

Solid-phase extraction methods using pre-packed silica cartridges and various elution solvents have been developed and evaluated as chromatographic means to enrich biological lipid extracts for platelet-activating factor (PAF). The optimized procedure advanced selectively removed the major tissue/blood neutral lipids and non-choline-containing phospholipids from complex lipid mixtures and yielded thereby a choline phospholipid fraction markedly enriched in bioactive PAF. Some tested solid-phase extraction procedures, while capable of resolving choline phospholipids from other polar and non-polar species, were detrimental to PAF's bioactivity and evidenced considerable loss or degradation of this analyte. It is concluded that, with solvents of appropriate composition, strength and polarity, solid-phase extraction on silica cartridges has several unique advantages over conventional thin-layer and column chromatographic methods presently in use for PAF enrichment from biological sources.

Radioligand competitive binding methodology for the evaluation of platelet-activating factor (PAF) and PAF-receptor antagonism using intact canine platelets

High-affinity, stereoselective, and ligand-selective specific binding of 3H-labeled platelet-activating factor (PAF) to its receptor on the dog platelet is reproducible over wide mass and concentration ranges of [3H]PAF. The [3H]PAF specific binding can be competitively inhibited by low picogram amounts of nonlabeled PAF. These observations have led to the formulation of radioligand competitive binding methodology for the detection and estimation of PAF in a biological lipid sample and the quantitative evaluation of PAF-receptor antagonism. The methodology is predicated upon correlation between the ability of a PAF analog/biological lipid sample/(synthetic) substance to inhibit [3H]PAF specific binding to the washed canine platelet and the known inhibition of [3H]PAF specific binding by standard, nonradioactive PAF. Application of this methodology to lipid extracts of human saliva has uncovered the finding that subjects with upper respiratory infection and chronic allergies have high saliva PAF contents. Pharmacologically active antiallergy agents known to inhibit PAF-induced pathology in animal models of disease were demonstrated, with the methology advanced, to act as PAF-receptor antagonists, and their potencies were quantified. These investigations indicate that the system proposed, in its ease, economy, sensitivity, specificity and capacity, has practical value for detecting and estimating PAF in biological lipid extracts and for evaluating PAF-receptor antagonism.