Structural characteristics of lysophosphatidic acid biological targets

Tandem pore TWIK-related potassium channels and neuroprotection

TWIK-related potassium channels (TREK) belong to a subfamily of the two-pore domain potassium channels family with three members, TREK1, TREK2 and TWIK-related arachidonic acid-activated potassium channels. The two-pore domain potassium channels is the last big family of channels being discovered, therefore it is not surprising that most of the information we know about TREK channels predominantly comes from the study of heterologously expressed channels. Notwithstanding, in this review we pay special attention to the limited amount of information available on native TREK-like channels and real neurons in relation to neuroprotection. Mainly we focus on the role of free fatty acids, lysophospholipids and other neuroprotective agents like riluzole in the modulation of TREK channels, emphasizing on how important this modulation may be for the development of new therapies against neuropathic pain, depression, schizophrenia, epilepsy, ischemia and cardiac complications.

Autotaxin

Autotaxin is a protein of approximately 900 amino acids discovered in the early 1990s. Over the past 15 years, a strong association between cancer cells and autotaxin production has been observed. Recent publications indicate that autotaxin and the capacity of cancer to metastasise are intimately linked. The discovery of new molecular targets in pharmacology is a mixture of pure luck, hard work and industrial strategy. Despite a crucial and desperate need for new therapeutic tools, many targets are approached in oncology, but only a few are validated and end up at the patient bed. Outside the busy domain of kinases, few targets have been discovered that can be useful in treating cancer, particularly metastatic processes. The fortuitous relationship between autotaxin and lysophosphatidic acid renders the results of observations made in the diabetes/obesity context considerably important. The literature provides observations that may aid in redesigning experiments to validate autotaxin as a potential oncology target.

Unique ligand selectivity of the GPR92/LPA5 lysophosphatidate receptor indicates role in human platelet activation

Lysophosphatidic acid (LPA) is a ligand for LPA(1-3) of the endothelial differentiation gene family G-protein-coupled receptors, and LPA(4-8) is related to the purinergic family G-protein-coupled receptor. Because the structure-activity relationship (SAR) of GPR92/LPA(5) is limited and whether LPA is its preferred endogenous ligand has been questioned in the literature, in this study we applied a combination of computational and experimental site-directed mutagenesis of LPA(5) residues predicted to interact with the headgroup of LPA. Four residues involved in ligand recognition in LPA(5) were identified as follows: R2.60N mutant abolished receptor activation, whereas H4.64E, R6.62A, and R7.32A greatly reduced receptor activation. We also investigated the SAR of LPA(5) using LPA analogs and other non-lysophospholipid ligands. SAR revealed that the rank order of agonists is alkyl glycerol phosphate > LPA > farnesyl phosphates >> N-arachidonoylglycine. These results confirm LPA(5) to be a bona fide lysophospholipid receptor. We also evaluated several compounds with previously established selectivity for the endothelial differentiation gene receptors and found several that are LPA(5) agonists. A pharmacophore model of LPA(5) binding requirements was developed for in silico screening, which identified two non-lipid LPA(5) antagonists. Because LPA(5) transcripts are abundant in human platelets, we tested its antagonists on platelet activation and found that these non-lipid LPA(5) antagonists inhibit platelet activation. The present results suggest that selective inhibition of LPA(5) may provide a basis for future anti-thrombotic therapies.

Lysophosphatidyl choline modulates mechanosensitive L-type Ca2+ current in circular smooth muscle cells from human jejunum

The L-type Ca2+ channel expressed in gastrointestinal smooth muscle is mechanosensitive. Direct membrane stretch and shear stress result in increased Ca2+ entry into the cell. The mechanism for mechanosensitivity is not known, and mechanosensitivity is not dependent on an intact cytoskeleton. The aim of this study was to determine whether L-type Ca2+ channel mechanosensitivity is dependent on tension in the lipid bilayer in human jejunal circular layer myocytes. Whole cell currents were recorded in the amphotericin-perforated-patch configuration, and lysophosphatidyl choline (LPC), lysophosphatidic acid (LPA), and choline were used to alter differentially the tension in the lipid bilayer. Shear stress (perfusion at 10 ml/min) was used to mechanostimulate L-type Ca2+ channels. The increase in L-type Ca2+ current induced by shear stress was greater in the presence of LPC (large head-to-tail proportions), but not LPA or choline, than in the control perfusion. The increased peak Ca2+ current also did not return to baseline levels as in control conditions. Furthermore, steady-state inactivation kinetics were altered in the presence of LPC, leading to a change in window current. These findings suggest that changes in tension in the plasmalemmal membrane can be transmitted to the mechanosensitive L-type Ca2+ channel, leading to altered activity and Ca2+ entry in the human jejunal circular layer myocyte.

Farnesyl phosphates are endogenous ligands of lysophosphatidic acid receptors: inhibition of LPA GPCR and activation of PPARs

Oligoprenyl phosphates are key metabolic intermediates for the biosynthesis of steroids, the side chain of ubiquinones, and dolichols and the posttranslational isoprenylation of proteins. Farnesyl phosphates are isoprenoid phosphates that resemble polyunsaturated fatty alcohol phosphates, which we have recently shown to be the minimal pharmacophores of lysophosphatidic acid (LPA) receptors. Here we examine whether farnesyl phosphates can interact with the cell surface and nuclear receptors for LPA. Both farnesyl phosphate and farnesyl diphosphate potently and specifically antagonized LPA-elicited intracellular Ca(2+)-mobilization mediated through the LPA(3) receptor, while causing only modest inhibition at the LPA(2) receptor and no measurable effect at the LPA(1) receptor. Farnesol also inhibited LPA(3) but was much less effective. The estimated dissociation constant of LPA(3) for farnesyl phosphate is 48+/-12 nM and 155+/-30 nM for farnesyl diphosphate. The transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma) binds to and is activated by LPA and its analogs including fatty alcohol phosphates. We found that both farnesyl phosphate and diphosphate, but not farnesol, compete with the binding of the synthetic PPARgamma agonist [(3)H]rosiglitazone and activate the PPARgamma-mediated gene transcription. Farnesyl monophosphate at 1 microM, but not diphosphate, activated PPARalpha and PPARbeta/delta reporter gene expression. These results indicate new potential roles for the oligoprenyl phosphates as potential endogenous modulators of LPA targets and show that the polyisoprenoid chain is recognized by some LPA receptors.

Phosphonothioate and Fluoromethylene Phosphonate Analogues of Cyclic Phosphatidic Acid: Novel Antagonists of Lysophosphatidic Acid Receptors§

Isoform-selective antagonists of the lysophosphatidic acid (LPA) G-protein coupled receptors (GPCRs) have important potential uses in cell biology and clinical applications. Novel phosphonothioate and fluoromethylene phosphonate analogues of carbacyclic phosphatidic acid (ccPA) were prepared by chemical synthesis. The pKa values of these amphilic phosphonolipids and the parent cyclic phosphonate were measured titrimetrically using the Yasuda-Shedlovsky extrapolation. The pharmacological properties of these and other ccPA analogues were characterized for LPA receptor (LPAR) subtype-specific agonist and antagonist activity using Ca2+-mobilization assays in RH7777 cells expressing the individual EDG-family GPCRs. In particular, the phosphonothioate ccPA analogue inhibited Ca2+ release through LPA1/LPA3 activation and was an LPA1/LPA3 antagonist. The monofluoromethylene phosphonate ccPA analogue was also a potent LPA1/LPA3 antagonist. In contrast, the difluoromethylene phosphonate ccPA analogue was a weak LPAR agonist, while ccPA itself had neither agonist nor antagonist activity.

Synthesis of Cyclic Phosphonate Analogues of (Lyso)phosphatidic Acid Using a Ring-Closing Metathesis Reaction

We describe a versatile and efficient method for the preparation of acyloxy-substituted six-membered cyclic phosphonates using the ring-closing metathesis. After closure, the key cyclic phosphonate intermediate was dihydroxylated and converted to a new class of conformationally constrained PA and LPA analogues. The oleoyloxy-substituted cyclic phosphonate 4 had unique receptor-selective properties as a ligand, showing partial activation of the LPA2 GPCR and weak antagonism of the LPA1 GPCR.