2D binary QSAR modeling of LPA3 receptor antagonism

A structurally diverse dataset of 119 compounds was used to develop and validate a 2D binary QSAR model for the LPA(3) receptor. The binary QSAR model was generated using an activity threshold of greater than 15% inhibition at 10 microM. The overall accuracy of the model on the training set was 82%. It had accuracies of 55% for active and 91% for inactive compounds, respectively. The model was validated using an external test set of 10 compounds. The accuracy on the external test set was 60% overall, identifying three out of seven actives and all three inactive compounds. This model was combined with similarity searching to rapidly screen libraries and select 14 candidate LPA(3) antagonists. Experimental assays confirmed 13 of these (93%) met the 15% inhibition threshold defining actives. The successful application of the model to select candidates for screening demonstrates the power of this binary QSAR model to prioritize compound selection for experimental consideration.

Structure-based drug design identifies novel LPA3 antagonists

Compound 5 ([5-(3-nitrophenoxy)-1,3-dioxo-1,3-dihydro-2-isoindol-2-yl]acetic acid) was identified as a weak selective LPA(3) antagonist (IC(50)=4504 nM) in a virtual screening effort to optimize a dual LPA(2 and 3) antagonist. Structure-based drug design techniques were used to prioritize similarity search matches of compound 5. This strategy rapidly identified 10 novel antagonists. The two most efficacious compounds identified inhibit activation of the LPA(3) receptor by 200 nM LPA with IC(50) values of 752 nM and 2992 nM. These compounds additionally define changes to our previously reported pharmacophore that will improve its ability to identify more potent and selective LPA(3) receptor antagonists. The results of the combined computational and experimental screening are reported.

Lysophosphatidic acid-induced platelet shape change revealed through LPA(1-5) receptor-selective probes and albumin

Lysophosphatidic acid (LPA), a component of mildly-oxidized LDL and the lipid rich core of atherosclerotic plaques, elicits platelet activation. LPA is the ligand of G protein-coupled receptors (GPCR) of the EDG family (LPA(1-3)) and the newly identified LPA(4-7) subcluster. LPA(4), LPA(5) and LPA(7) increase cellular cAMP levels that would induce platelet inhibition rather than activation. In the present study we quantified the mRNA levels of the LPA(1-7) GPCR in human platelets and found a rank order LPA(4) = LPA(5) > LPA(7) > LPA(6) = LPA(2) >> LPA(1) > LPA(3). We examined platelet shape change using a panel of LPA receptor subtype-selective agonists and antagonists and compared them with their pharmacological profiles obtained in heterologous LPA(1-5) receptor expression systems. Responses to different natural acyl and alkyl species of LPA, and octyl phosphatidic acid analogs, alpha-substituted phosphonate analogs, N-palmitoyl-tyrosine phosphoric acid, N-palmitoyl-serine phosphoric acid were tested. All of these compounds elicited platelet activation and also inhibited LPA-induced platelet shape change after pre-incubation, suggesting that receptor desensitization is likely responsible for the inhibition of this response. Fatty acid free albumin (10 microM) lacking platelet activity completely inhibited platelet shape change induced by LPA with an IC(50) of 1.1 microM but had no effect on the activation of LPA(1,2,3,&5) expressed in endogenously non-LPA-responsive RH7777 cells. However, albumin reduced LPA(4) activation and shifted the dose-response curve to the right. LPA(5) transiently expressed in RH7777 cells showed preference to alkyl-LPA over acyl-LPA that is similar to that in platelets. LPA did not increase cAMP levels in platelets. In conclusion, our results with the pharmacological compounds and albumin demonstrate that LPA does not induce platelet shape change simply through activation of LPA(1-5), and the receptor(s) mediating LPA-induced platelet activation remains elusive.

Chiral vinylphosphonate and phosphonate analogues of the immunosuppressive agent FTY720

The first enantioselective synthesis of chiral isosteric phosphonate analogues of FTY720 is described. One of these analogues, FTY720-(E)-vinylphosphonate (S)-5, but not its R enantiomer, elicited a potent antiapoptotic effect in intestinal epithelial cells, suggesting that it exerts its action via the enantioselective activation of a receptor. (S)-5 failed to activate the sphingosine 1-phosphate type 1 (S1P(1)) receptor.

Lysophosphatidic acid (LPA)-induced vasodilator-stimulated phosphoprotein mediates lamellipodia formation to initiate motility in PC-3 prostate cancer cells

Prostate cancer remains the most frequently diagnosed malignancy and the second leading cause of cancer mortality among men in the United States. Hormone refractory, metastatic disease has no molecular therapeutics to date and survival is poor. Lysophosphatidic acid (LPA) is a bioactive lipid exhibiting motility, invasive, growth, proliferative and survival effects in multiple cancer cell lineages. Cells express different combinations of LPA-specific G protein-coupled receptors, LPA(1), LPA(2) LPA(3), and LPA(4) as well as other LPA receptors, which bind LPA and thereby regulate lipid signaling. The role of specific LPA receptors in functional outcomes of lysolipid signaling remains to be fully elucidated in prostate cancer. We hypothesized that LPA can initiate cell migration through specific LPA receptors by activating actin-associating proteins involved in motility, including the vasodilator-stimulated phosphoprotein (VASP). In the present study, we demonstrate that LPA-induced lamellipodia formation in cells is dependent on LPA receptor-mediated phosphorylation of VASP, demonstrating a previously unknown regulation by LPA. LPA induces phosphorylation of VASP at Ser(157), through protein kinase A (PKA) since the stimulation was abrogated by PKA inhibition. In addition, we found the effects of LPA-induced lamellipodia formation and migration were reduced by knockdown of either VASP or LPA receptor expression, suggesting that LPA receptor-induced VASP phosphorylation is a critical mediator of migration initiation. Thus the LPA(2) and LPA(3) receptors, in addition to the previously implicated LPA(1) receptor, play a role in cellular motility potentially contributing to invasion and metastases. Emerging drugs targeting the LPA pathway may be beneficial for the treatment of metastatic progression in prostate cancer.

Dual activity lysophosphatidic acid receptor pan-antagonist/autotaxin inhibitor reduces breast cancer cell migration in vitro and causes tumor regression in vivo

Signal transduction modifiers that modulate the lysophosphatidic acid (LPA) pathway have potential as anticancer agents. Herein, we describe metabolically stabilized LPA analogues that reduce cell migration and invasion and cause regression of orthotopic breast tumors in vivo. Two diastereoisomeric alpha-bromophosphonates (BrP-LPA) were synthesized, and the pharmacology was determined for five LPA G protein-coupled receptors (GPCRs). The syn and anti diastereomers of BrP-LPA are pan-LPA GPCR antagonists and are also nanomolar inhibitors of the lysophospholipase D activity of autotaxin, the dominant biosynthetic source of LPA. Computational models correctly predicted the diastereoselectivity of antagonism for three GPCR isoforms. The anti isomer of BrP-LPA was more effective than syn isomer in reducing migration of MDA-MB-231 cells, and the anti isomer was superior in reducing invasion of these cells. Finally, orthotopic breast cancer xenografts were established in nude mice by injection of MB-231 cells in an in situ cross-linkable extracellular matrix. After 2 weeks, mice were treated with the BrP-LPA alone (10 mg/kg), Taxol alone (10 mg/kg), or Taxol followed by BrP-LPA. All treatments significantly reduced tumor burden, and BrP-LPA was superior to Taxol in reducing blood vessel density in tumors. Moreover, both the anti- and syn-BrP-LPA significantly reduced tumors at 3 mg/kg.

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.

Alkoxymethylenephosphonate analogues of (Lyso) phosphatidic acid stimulate signaling networks coupled to the LPA2 receptor

An efficient stereocontrolled synthesis afforded alkoxymethylenephosphonate (MP) analogues of lysophosphatidic acid (LPA) and phosphatidic acid (PA). The pharmacological properties of MP-LPA and MP-PA analogues were characterized for LPA receptor subtype-specific agonist and antagonist activity using Ca(2+)-mobilization assays in RH7777 cells expressing the individual LPA(1)-LPA(3) receptors and CHO cells expressing LPA(4). In addition, activation of a PPARgamma reporter gene construct expressed in CV-1 cells was assessed. These metabolically stabilized LPA analogues exhibited an unexpected pattern of partial agonist/antagonist activity for the LPA G-protein-coupled receptor family and the intracellular LPA receptor PPARgamma. Analogues were compared with 18:1 LPA for activation of downstream signaling in HT-29 colon cancer cells, which exclusively express LPA(2), and both SKOV3 and OVCAR3 ovarian cancer cells, which express LPA(1), LPA(2), and LPA(3). Unexpectedly, reverse phase protein arrays showed that four MP-LPA and MP-PA analogues selectively activated downstream signaling in HT-29 cells with greater potency than LPA. In particular, the oleoyl MP-LPA analogue strongly promoted phosphorylation and activation of AKT, MEK, and pS6 in HT-29 cells in a concentration-dependent manner. In contrast, the four MP-LPA and MP-PA analogues were equipotent with LPA for pathway activation in the SKOV3 and OVCAR3 cells. Taken together, these results suggest that the MP analogues may selectively activate signaling via the LPA(2) receptor subtype, while simultaneously suppressing signaling through the LPA(1) and LPA(3) subtypes.

The early- and late stages in phenotypic modulation of vascular smooth muscle cells: differential roles for lysophosphatidic acid

Lysophosphatidic acid (LPA) has been implicated as causative in phenotypic modulation (PM) of cultured vascular smooth muscle cells (VSMC) in their transition to the dedifferentiated phenotype. We evaluated the contribution of the three major LPA receptors, LPA1 and LPA2 GPCR and PPARgamma, on PM of VSMC. Expression of differentiated VSMC-specific marker genes, including smooth muscle alpha-actin, smooth muscle myosin heavy chain, calponin, SM-22alpha, and h-caldesmon, was measured by quantitative real-time PCR in VSMC cultures and aortic rings kept in serum-free chemically defined medium or serum- or LPA-containing medium using wild-type C57BL/6, LPA1, LPA2, and LPA1&2 receptor knockout mice. Within hours after cells were deprived of physiological cues, the expression of VSMC marker genes, regardless of genotype, rapidly decreased. This early PM was neither prevented by IGF-I, inhibitors of p38, ERK1/2, or PPARgamma nor significantly accelerated by LPA or serum. To elucidate the mechanism of PM in vivo, carotid artery ligation with/without replacement of blood with Krebs solution was used to evaluate contributions of blood flow and pressure. Early PM in the common carotid was induced by depressurization regardless of the presence/absence of blood, but eliminating blood flow while maintaining blood pressure or after sham surgery elicited no early PM. The present results indicate that LPA, serum, dissociation of VSMC, IGF-I, p38, ERK1/2, LPA1, and LPA2 are not causative factors of early PM of VSMC. Tensile stress generated by blood pressure may be the fundamental signal maintaining the fully differentiated phenotype of VSMC.

Identification of non-lipid LPA3 antagonists by virtual screening

In the present study, we utilized virtual screening to identify LPA(3) antagonists. We have developed a three-point structure-based pharmacophore model based on known LPA(3) antagonists. This model was used to mine the NCI database. Docking, pharmacophore development, and database mining produced new, non-lipid leads. Experimental testing of seven computationally selected pharmacophore hits produced one potentiator and three antagonists, one of which displays both LPA(3) selectivity and nanomolar potency. Similarity searching in the ChemBridge database using the most promising lead as the search target produced four additional LPA(3) antagonists and a potent dual LPA(1&2) antagonist.

Subtype-specific residues involved in ligand activation of the endothelial differentiation gene family lysophosphatidic acid receptors

Lysophosphatidic acid (LPA) is a ligand for three endothelial differentiation gene family G protein-coupled receptors, LPA(1-3). We performed computational modeling-guided mutagenesis of conserved residues in transmembrane domains 3, 4, 5, and 7 of LPA(1-3) predicted to interact with the glycerophosphate motif of LPA C18:1. The mutants were expressed in RH7777 cells, and the efficacy (E(max)) and potency (EC(50)) of LPA-elicited Ca(2+) transients were measured. Mutation to alanine of R3.28 universally decreased both the efficacy and potency in LPA(1-3) and eliminated strong ionic interactions in the modeled LPA complexes. The alanine mutation at Q3.29 decreased modeled interactions and activation in LPA(1) and LPA(2) more than in LPA(3). The mutation W4.64A had no effect on activation and modeled LPA interaction of LPA(1) and LPA(2) but reduced the activation and modeled interactions of LPA(3). The R5.38A mutant of LPA(2) and R5.38N mutant of LPA(3) showed diminished activation by LPA; however, in LPA(1) the D5.38A mutation did not, and mutation to arginine enhanced receptor activation. In LPA(2), K7.36A decreased the potency of LPA; in LPA(1) this same mutation increased the E(max). In LPA(3), R7.36A had almost no effect on receptor activation; however, the mutation K7.35A increased the EC(50) in response to LPA 10-fold. In LPA(1-3), the mutation Q3.29E caused a modest increase in EC(50) in response to LPA but caused the LPA receptors to become more responsive to sphingosine 1-phosphate (S1P). Surprisingly micromolar concentrations of S1P activated the wild type LPA(2) and LPA(3) receptors, indicating that S1P may function as a weak agonist of endothelial differentiation gene family LPA receptors.

Synthesis, pharmacology, and cell biology of sn-2-aminooxy analogues of lysophosphatidic acid

An efficient enantioselective synthesis of sn-2-aminooxy (AO) analogues of lysophosphatidic acid (LPA) that possess palmitoyl and oleoyl acyl chains is presented. Both sn-2-AO LPA analogues are agonists for the LPA1, LPA2, and LPA4 G-protein-coupled receptors, but antagonists for the LPA3 receptor and inhibitors of autotaxin (ATX). Moreover, both analogues stimulate migration of intestinal epithelial cells in a scratch wound assay.

The lysophosphatidic acid 2 receptor mediates down-regulation of Siva-1 to promote cell survival

Lysophosphatidic acid (LPA) promotes cell survival through the activation of G protein-coupled LPA receptors. However, whether different LPA receptors activate distinct anti-apoptotic signaling pathways is not yet clear. Here we report a novel mechanism by which the LPA(2) receptor targets the proapoptotic Siva-1 protein for LPA-dependent degradation, thereby attenuating Siva-1 function in DNA damage response. The carboxyl-terminal tail of the LPA(2) receptor, but not LPA(1) or LPA(3) receptor, specifically associates with the carboxyl cysteine-rich domain of Siva-1. Prolonged LPA stimulation promotes the association of Siva-1 with the LPA(2) receptor and targets both proteins for ubiquitination and degradation. As a result, adriamycin-induced Siva-1 protein stabilization is attenuated by LPA in an LPA(2)-dependent manner, and the function of Siva-1 in promoting DNA damage-induced apoptosis is inhibited by LPA pretreatment. Consistent with this result, inhibition of the LPA(2) receptor expression increases Siva-1 protein levels and augments adriamycin-induced caspase-3 cleavage and apoptosis. Together, these findings reveal a critical and specific role for the LPA(2) receptor through which LPA directly inactivates a critical component of the death machinery to promote cell survival.

The lysophosphatidic acid type 2 receptor is required for protection against radiation-induced intestinal injury

BACKGROUND & AIMS

We recently identified lysophosphatidic acid (LPA) as a potent antiapoptotic agent for the intestinal epithelium. The objective of the present study was to evaluate the effect of octadecenyl thiophosphate (OTP), a novel rationally designed, metabolically stabilized LPA mimic, on radiation-induced apoptosis of intestinal epithelial cells in vitro and in vivo.

METHODS

The receptors and signaling pathways activated by OTP were examined in IEC-6 and RH7777 cell lines and wild-type and LPA(1) and LPA(2) knockout mice exposed to different apoptotic stimuli.

RESULTS

OTP was more efficacious than LPA in reducing gamma irradiation-, camptothecin-, or tumor necrosis factor alpha/cycloheximide-induced apoptosis and caspase-3-8, and caspase-9 activity in the IEC-6 cell line. In RH7777 cells lacking LPA receptors, OTP selectively protected LPA(2) but not LPA(1) and LPA(3) transfectants. In C57BL/6 and LPA(1) knockout mice exposed to 15 Gy gamma irradiation, orally applied OTP reduced the number of apoptotic bodies and activated caspase-3-positive cells but was ineffective in LPA(2) knockout mice. OTP, with higher efficacy than LPA, enhanced intestinal crypt survival in C57BL/6 mice but was without any effect in LPA(2) knockout mice. Intraperitoneally administered OTP reduced death caused by lethal dose (LD)(100/30) radiation by 50%.

CONCLUSIONS

Our data indicate that OTP is a highly effective antiapoptotic agent that engages similar prosurvival pathways to LPA through the LPA(2) receptor subtype.

Alpha-substituted phosphonate analogues of lysophosphatidic acid (LPA) selectively inhibit production and action of LPA

Isoform-selective agonists and antagonists of the lysophosphatidic acid (LPA) G-protein-coupled receptors (GPCRs) have important potential applications in cell biology and therapy. LPA GPCRs regulate cancer cell proliferation, invasion, angiogenesis, and biochemical resistance to chemotherapy- and radiotherapy-induced apoptosis. LPA and its analogues are also feedback inhibitors of the enzyme lysophospholipase D (lysoPLD, also known as autotaxin), a central regulator of invasion and metastasis. For cancer therapy, the ideal therapeutic profile would be a metabolically stabilized pan-LPA receptor antagonist that also inhibits lysoPLD. Herein we describe the synthesis of a series of novel alpha-substituted methylene phosphonate analogues of LPA. Each of these analogues contains a hydrolysis-resistant phosphonate mimic of the labile monophosphate of natural LPA. The pharmacological properties of these phosphono-LPA analogues were characterized in terms of LPA receptor subtype-specific agonist and antagonist activity using Ca(2+) mobilization assays in RH7777 and CHO cells expressing the individual LPA GPCRs. In particular, the methylene phosphonate LPA analogue is a selective LPA(2) agonist, whereas the corresponding alpha-hydroxymethylene phosphonate is a selective LPA(3) agonist. Most importantly, the alpha-bromomethylene and alpha-chloromethylene phosphonates show pan-LPA receptor subtype antagonist activity. The alpha-bromomethylene phosphonates are the first reported antagonists for the LPA(4) GPCR. Each of the alpha-substituted methylene phosphonates inhibits lysoPLD, with the unsubstituted methylene phosphonate showing the most potent inhibition. Finally, unlike many LPA analogues, none of these compounds activate the intracellular LPA receptor PPARgamma.

Lysophosphatidic acid induces prostate cancer PC3 cell migration via activation of LPA(1), p42 and p38alpha

Prostate cancer cell migration is an essential event both in the progression of prostate cancer and in the steps leading to metastasis. We report here that lysophosphatidic acid (LPA), a potent bioactive phospholipid, induces prostate cancer PC3 cell migration via the activation of the LPA(1) receptor, which is linked to a PTX-sensitive activation mechanism of the mitogen-activated protein kinases (MAPK). Our results demonstrate that parallel activation of ERK1/2 and p38, but not JNK, is responsible for LPA-stimulated PC3 cell migration. Furthermore, using small interfering RNA (siRNA) technology, and overexpressing dominant-negative mutants of p38 MAPK isotypes of alpha, beta, gamma and delta, we have identified that the activation of ERK2 (p42) and p38alpha, but not of ERK1 and the other isoforms of p38 MAPK, is required for LPA-induced migration. Our study provides the first evidence for a functional role of p42 and p38alpha in LPA-induced mammalian cell migration, and also demonstrates, for the first time, that the receptor LPA(1) mediates prostate cancer cell migration. The results of the present study suggest that LPA, the receptor LPA(1), ERK2 and p38alpha are important regulators for prostate cancer cell invasion and thus could play a significant role in the development of metastasis.

Sphingosine 1-phosphate pKa and binding constants: intramolecular and intermolecular influences

The dissociation constant for an ionizable ligand binding to a receptor is dependent on its charge and therefore on its environmentally-influenced pKa value. The pKa values of sphingosine 1-phosphate (S1P) were studied computationally in the context of the wild type S1P1 receptor and the following mutants: E3.29Q, E3.29A, and K5.38A. Calculated pKa values indicate that S1P binds to S1P1 and its site mutants with a total charge of -1, including a +1 charge on the ammonium group and a -2 charge on the phosphate group. The dissociation constant of S1P binding to these receptors was studied as well. The models of wild type and mutant proteins originated from an active receptor model that was developed previously. We used ab initio RHF/6-31+G(d) to optimize our models in aqueous solution, where the solvation energy derivatives are represented by conductor-like polarizable continuum model (C-PCM) and integral equation formalism polarizable continuum model (IEF-PCM). Calculation of the dissociation constant for each mutant was determined by reference to the experimental dissociation constant of the wild type receptor. The computed dissociation constants of the E3.29Q and E3.29A mutants are three to five orders of magnitude higher than those for the wild type receptor and K5.38A mutant, indicating vital contacts between the S1P phosphate group and the carboxylate group of E3.29. Computational dissociation constants for K5.38A, E3.29A, and E3.29Q mutants were compared with experimentally determined binding and activation data. No measurable binding of S1P to the E3.29A and E3.29Q mutants was observed, supporting the critical contacts observed computationally. These results validate the quantitative accuracy of the model.

Identification of the hydrophobic ligand binding pocket of the S1P1 receptor

Sphingosine 1-phosphate (S1P), a naturally occurring sphingolipid mediator and also a second messenger with growth factor-like actions in almost every cell type, is an endogenous ligand of five G protein-coupled receptors (GPCRs) in the endothelial differentiation gene family. The lack of GPCR crystal structures sets serious limitations to rational drug design and in silico searches for subtype-selective ligands. Here we report on the experimental validation of a computational model of the ligand binding pocket of the S1P1 GPCR surrounding the aliphatic portion of S1P. The extensive mutagenesis-based validation confirmed 18 residues lining the hydrophobic ligand binding pocket, which, combined with the previously validated three head group-interacting residues, now complete the mapping of the S1P ligand recognition site. We identified six mutants (L3.43G/L3.44G, L3.43E/L3.44E, L5.52A, F5.48G, V6.40L, and F6.44G) that maintained wild type [32P]S1P binding with abolished ligand-dependent activation by S1P. These data suggest a role for these amino acids in the conformational transition of S1P1 to its activated state. Three aromatic mutations (F5.48Y, F6.44G, and W6.48A) result in differential activation, by S1P or SEW2871, indicating that structural differences between the two agonists can partially compensate for differences in the amino acid side chain. The now validated ligand binding pocket provided us with a pharmacophore model, which was used for in silico screening of the NCI, National Institutes of Health, Developmental Therapeutics chemical library, leading to the identification of two novel nonlipid agonists of S1P1.

Phosphorothioate analogues of alkyl lysophosphatidic acid as LPA3 receptor-selective agonists

The metabolically stabilized LPA analogue 1-oleoyl-2-O-methyl-rac-glycerophosphorothioate (OMPT) was recently shown to be a potent subtype-selective agonist for LPA3, a G-protein-coupled receptor (GPCR) in the endothelial differentiation gene (EDG) family. Further stabilization was achieved by replacing the sn-1 O-acyl group with an O-alkyl ether. A new synthetic route for the enantiospecific synthesis of the resulting alkyl LPA phosphorothioate analogues is described. The pharmacological properties of the alkyl OMPT analogues were characterized for subtype-specific agonist activity using Ca2+-mobilization assays in RH7777 cells expressing the individual EDG family LPA receptors. Alkyl OMPT analogues induced cell migration in cancer cells mediated through LPA1. Alkyl OMPT analogues also activated Ca2+ release through LPA2 activation but with less potency than sn-1-oleoyl LPA. In contrast, alkyl OMPT analogues were potent LPA3 agonists. The alkyl OMPTs 1 and 3 induced cell proliferation at submicromolar concentrations in 10T 1/2 fibroblasts. Interestingly, the absolute configuration of the sn-2 methoxy group of the alkyl OMPT analogues was not recognized by any of the LPA receptors in the EDG family. By using a reporter gene assay for the LPA-activated nuclear transcription factor PPARgamma, we demonstrated that phosphorothioate diesters have agonist activity that is independent of their ligand properties at the LPA-activated GPCRs. The availability of new alkyl LPA analogues expands the scope of structure-activity studies and will further refine the molecular nature of ligand-receptor interactions for this class of GPCRs.

Protean agonism of the lysophosphatidic acid receptor-1 with Ki16425 reduces nerve growth factor-induced neurite outgrowth in pheochromocytoma 12 cells

We report here a novel role for the constitutively active lysophosphatidic acid receptor-1 (LPA(1)) receptor in providing Gbetagamma subunits for use by the Trk A receptor. This enhances the ability of nerve growth factor (NGF) to promote signalling and cell response. These conclusions were based on three lines of evidence. Firstly, the LPA(1) receptor was co-immunoprecipitated with the Trk A receptor from lysates, suggesting that these proteins form a complex. Secondly, Ki16425, a selective protean agonist of the LPA(1) receptor, decreased constitutive basal and LPA-induced LPA(1) receptor-stimulated GTPgammaS binding. Ki16425 reduced the LPA-induced activation of p42/p44 mitogen activated protein kinase (MAPK), while acting as a weak stimulator of p42/p44 MAPK on its own, properties typical of a protean agonist. Significantly, Ki16425 also reduced the NGF-induced stimulation of p42/p44 MAPK and inhibited NGF-stimulated neurite outgrowth. Thirdly, the over-expression of the C-terminal GRK-2 peptide, which sequesters Gbetagamma subunits, reduced the NGF-induced activation of p42/p44 MAPK. In contrast, the stimulation of PC12 cells with LPA leads to a predominant G(i)alpha2-mediated Trk A-independent activation of p42/p44 MAPK, where Gbetagamma subunits play a diminished role. These findings suggest a novel role for the constitutively active LPA(1) receptor in regulating NGF-induced neuronal differentiation.

Lipid phosphate phosphatase-1 regulates lysophosphatidate-induced fibroblast migration by controlling phospholipase D2-dependent phosphatidate generation

Lysophosphatidate (LPA) stimulates cell migration and division through a family of G-protein-coupled receptors. Lipid phosphate phosphatase-1 (LPP1) regulates the degradation of extracellular LPA as well as the intracellular accumulation of lipid phosphates. Here we show that increasing the catalytic activity of LPP1 decreased the pertussis toxin-sensitive stimulation of fibroblast migration by LPA and an LPA-receptor agonist that could not be dephosphorylated. Conversely, knockdown of endogenous LPP1 activity increased LPA-induced migration. However, LPP1 did not affect PDGF- or endothelin-induced migration of fibroblasts in Transwell chamber and "wound healing" assays. Thus, in addition to degrading exogenous LPA, LPP1 controls signaling downstream of LPA receptors. Consistent with this conclusion, LPP1 expression decreased phospholipase D (PLD) stimulation by LPA and PDGF, and phosphatidate accumulation. This LPP1 effect was upstream of PLD activation in addition to the possible metabolism of phosphatidate to diacylglycerol. PLD(2) activation was necessary for LPA-, but not PDGF-induced migration. Increased LPP1 expression also decreased the LPA-, but not the PDGF-induced activation of important proteins involved in fibroblast migration. These included decreased LPA-induced activation of ERK and Rho, and the basal activities of Rac and Cdc42. However, ERK and Rho activation were not downstream targets of LPA-induced PLD(2) activity. We conclude that the intracellular actions of LPP1 play important functions in regulating LPA-induced fibroblast migration through PLD2. LPP1 also controls PDGF-induced phosphatidate formation. These results shed new light on the roles of LPP1 in controlling wound healing and the growth and metastasis of tumors.

Inhibition of transcellular tumor cell migration and metastasis by novel carba-derivatives of cyclic phosphatidic acid

Cyclic phosphatidic acid (1-acyl-sn-glycerol-2,3-cyclic phosphate; cPA) is a naturally occurring analog of lysophosphatidic acid (LPA) with a variety of distinctly different biological activities from those of LPA. In contrast to LPA, a potent inducer of tumor cell invasion, palmitoyl-cPA inhibits FBS- and LPA-induced transcellular migration and metastasis. To prevent the conversion of cPA to LPA we synthesized cPA derivatives by stabilizing the cyclic phosphate ring; to prevent the cleavage of the fatty acid we generated alkyl ether analogs of cPA. Both sets of compounds were tested for inhibitory activity on transcellular tumor cell migration. Carba derivatives, in which the phosphate oxygen was replaced with a methylene group at either the sn-2 or the sn-3 position, showed much more potent inhibitory effects on MM1 tumor cell transcellular migration and the pulmonary metastasis of B16-F0 melanoma than the natural pal-cPA. The antimetastatic effect of carba-cPA was accompanied by the inhibition of RhoA activation and was not due to inhibition of the activation of LPA receptors.

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.

Integrin signalling regulates the nuclear localization and function of the lysophosphatidic acid receptor-1 (LPA1) in mammalian cells

We show that LPA1 (lysophosphatidic acid receptor-1) is constitutively localized in the nucleus of mammalian cells. LPA1 also traffics from cell membranes to the nucleus in response to LPA (lysophosphatidic acid). Several lines of evidence suggest an important role for cell-matrix interaction in regulating the constitutive nuclear localization of LPA1. First, the RGDS peptide, which blocks cell matrix-induced integrin clustering and cytoskeletal rearrangement, reduced the number of cells containing LPA1 in the nucleus. Secondly, a higher proportion of cells contained nuclear LPA1 when adhesion on fibronectin-coated glass was compared with adherence to polylysine-coated glass. Thirdly, pre-treatment of cells with the Rho kinase inhibitor (Y27632) or the myosin light chain kinase inhibitor (ML9) reduced the number of cells containing nuclear LPA1. The addition of LPA and/or Ki16425 (which binds to LPA1) to isolated nuclei containing LPA1 induced the phosphorylation of several proteins with molecular masses of 34, 32, 14 and 11 kDa. These findings demonstrate that trafficking of LPA1 to the nucleus is influenced by cell-matrix interactions and that nuclear LPA1 may be involved in regulating intranuclear protein phosphorylation and signalling.

Carba analogs of cyclic phosphatidic acid are selective inhibitors of autotaxin and cancer cell invasion and metastasis

Autotaxin (ATX, nucleotide pyrophosphate/phosphodiesterase-2) is an autocrine motility factor initially characterized from A2058 melanoma cell-conditioned medium. ATX is known to contribute to cancer cell survival, growth, and invasion. Recently ATX was shown to be responsible for the lysophospholipase D activity that generates lysophosphatidic acid (LPA). Production of LPA is sufficient to explain the effects of ATX on tumor cells. Cyclic phosphatidic acid (cPA) is a naturally occurring analog of LPA in which the sn-2 hydroxy group forms a 5-membered ring with the sn-3 phosphate. Cellular responses to cPA generally oppose those of LPA despite activation of apparently overlapping receptor populations, suggesting that cPA also activates cellular targets distinct from LPA receptors. cPA has previously been shown to inhibit tumor cell invasion in vitro and cancer cell metastasis in vivo. However, the mechanism governing this effect remains unresolved. Here we show that 3-carba analogs of cPA lack significant agonist activity at LPA receptors yet are potent inhibitors of ATX activity, LPA production, and A2058 melanoma cell invasion in vitro and B16F10 melanoma cell metastasis in vivo.

Synthesis of photoactivatable analogues of lysophosphatidic acid and covalent labeling of plasma proteins

[reaction: see text] Lysophosphatidic acids bearing a benzophenone group in either the sn-1 or sn-2 chain of an oleoyl-type ester or oleyl-type ether chain and (32)P in the phosphate group were synthesized. The benzophenone moiety was introduced by selective hydroboration of the double bond of enyne 11 at low temperature, followed by a Suzuki reaction with 4-bromobenzophenone. The key intermediates for the preparation of ester-linked lysophosphatidic acid (LPA) 1 and 3 were obtained in one pot by a modified DIBAL-H reduction of orthoformate intermediate 22. These probes were shown to covalently modify a single protein target in rat plasma containing albumin and several protein targets in rat plasma containing a low level of albumin.

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.

The functional PDGFbeta receptor-S1P1 receptor signaling complex is involved in regulating migration of mouse embryonic fibroblasts in response to platelet derived growth factor

We report here that mouse embryonic fibroblasts (MEF) express a functional PDGFbeta receptor-S1P(1) receptor complex. The S1P(1) receptor is constitutively active and functions to enhance PDGF-stimulated migration of MEF. This was based on three pieces of evidence. Firstly, the S1P(1) receptor and PDGFbeta receptor are co-immunoprecipitated from cell lysates using anti-PDGFbeta receptor antibody. These findings suggest that the receptors form a complex in MEF. Secondly, inverse agonism of the S1P(1) receptor with SB649146 to eliminate the constitutive activity of the S1P(1) receptor reduced the PDGF-induced activation of p42/p44 MAPK in MEF. Thirdly, SB649146 inhibited the migration of MEF in response to the selective S1P(1) receptor agonist, SEW2871 or PDGF. In contrast, S1P inhibited PDGF-stimulated MEF migration, possibly mediated by the inhibitory S1P(2) receptor. These findings resolve an important issue regarding the functional role of the S1P(1) receptor in regulating MEF migration and suggest an important role within the context of PDGFbeta receptor-S1P(1) receptor complex signaling.

MicroRNA trafficking and human cancer

MicroRNAs (miRNAs) are short endogenous noncoding small RNA molecules with 21 to 25 nucleotides (nt) in length, which negatively regulate gene expression either by degrading specific mRNA or impeding translation at the transcriptional and post-transcriptional level. Recent advances suggest that miRNAs play prominent roles in development, genome organization, viral and transposon defense, and human disease, including neurodegenerative and metabolic diseases. Surprisingly, miRNAs are now linked with a variety of cancers, in which a reduced expression or overexpression may lead to oncogene or tumor suppressor gene-like actions. Here we will overview the miRNA pathway and its regulation with emphasis on the links with human cancer. Finally potential applications of miRNA in tumor diagnosis and therapy will be discussed.

Lipid phosphate phosphatase-1 regulates lysophosphatidic acid- and platelet-derived-growth-factor-induced cell migration

LPPs (lipid phosphate phosphatases) are members of a family of enzymes that catalyse the dephosphorylation of lipid phosphates. The only known form of regulation of this family of enzymes is via de novo expression of LPP isoforms in response to growth factors. In this respect, we evaluated the effect of moderate increases in the expression of recombinant LPP1 on signal transduction by both G-protein-coupled receptors and receptor tyrosine kinases. We present evidence for a novel role of LPP1 in reducing PDGF (platelet-derived growth factor)- and lysophosphatidic acid-induced migration of embryonic fibroblasts. We demonstrate that the overexpression of LPP1 inhibits cell migration by reducing the PDGF-induced activation of p42/p44 MAPK (mitogen-activated protein kinase). This appears to occur via a mechanism that involves the LPP1-induced down-regulation of typical PKC (protein kinase C) isoform(s), which are normally required for PDGF-induced activation of p42/p44 MAPK and migration. In this regard, DAG (diacylglycerol) levels are high and sustained in cells overexpressing LPP1, suggesting a dynamic interconversion of phosphatidic acid into DAG by LPP1. This may account for the effects of LPP1 on cell migration, as sustained DAG is known to down-regulate PKC isoforms in cells. Therefore the physiological changes in the expression levels of LPP1 might represent a heterologous desensitization mechanism for attenuating PKC-mediated signalling and regulation of cell migration.

Identification of Darmstoff analogs as selective agonists and antagonists of lysophosphatidic acid receptors

Darmstoff describes a family of gut smooth muscle-stimulating acetal phosphatidic acids initially isolated and characterized from the bath fluid of stimulated gut over 50 years ago. Despite similar structural and biological profiles, Darmstoff analogs have not previously been examined as potential LPA mimetics. Here, we report a facile method for the synthesis of potassium salts of Darmstoff analogs. To understand the effect of stereochemistry on lysophosphatidic acid mimetic activity, synthesis of optically pure stereoisomers of selected Darmstoff analogs was achieved starting with chiral methyl glycerates. Each Darmstoff analog was evaluated for subtype-specific LPA receptor agonist/antagonist activity, PPARgamma activation, and autotaxin inhibition. From this study we identified compound 12 as a pan-antagonist and several pan-agonists for the LPA(1-3) receptors. Introduction of an aromatic ring in the lipid chain such as analog 22 produced a subtype-specific LPA(3) agonist with an EC(50) of 692 nM. Interestingly, regardless of their LPA(1/2/3) ligand properties all of the Darmstoff analogs tested activated PPARgamma. However, these compounds are weak inhibitors of autotaxin. The results indicate that Darmstoff analogs constitute a novel class of lysophosphatidic acid mimetics.

Sphingosine 1-phosphate analogue recognition and selectivity at S1P4 within the endothelial differentiation gene family of receptors

Synergistic computational and experimental studies provided previously unforeseen details concerning the structural basis of S1P (sphingosine 1-phosphate) recognition by the S1P4 G-protein-coupled receptor. Similarly to reports on the S1P1 receptor, cationic and anionic residues in the third transmembrane domain (R3.28 and E3.29 at positions 124 and 125) form ion pairs with the phosphate and ammonium of S1P, and alanine mutations at these positions abolished specific S1P binding, S1P-induced receptor activation and cell migration. Unlike findings on the S1P1 receptor, no cationic residue in the seventh transmembrane domain interacts with the phosphate. Additionally, two previously undiscovered interactions with the S1P polar headgroup have been identified. Trp186 at position 4.64 in the fourth transmembrane domain interacts by a cation-pi interaction with the ammonium group of S1P. Lys204 at position 5.38 forms an ion pair with the S1P. The S1P4 and S1P1 receptors show differences in binding-pocket shape and electrostatic distributions that correlate with the published structure-activity relationships. In particular, the binding pocket of mS1P4 (mouse S1P4) has recognition sites for the anionic phosphate and cationic ammonium groups that are equidistant from the end of the non-polar tail. In contrast, the binding pocket of hS1P1 (human S1P4) places the ammonium recognition site 2 A (1 A=0.1 nm) closer to the end of the non-polar tail than the phosphate recognition site.

Different residues mediate recognition of 1-O-oleyllysophosphatidic acid and rosiglitazone in the ligand binding domain of peroxisome proliferator-activated receptor gamma

Here we showed that a naturally occurring ether analog of lysophosphatidic acid, 1-O-octadecenyl-2-hydroxy-sn-glycero-3-phosphate (AGP), is a high affinity partial agonist of the peroxisome proliferator-activated receptor gamma (PPARgamma). Binding studies using the PPARgamma ligand binding domain showed that [32P]AGP and [3H]rosiglitazone (Rosi) both specifically bind to PPARgamma and compete with each other. [32P]AGP bound PPARgamma with an affinity (Kdapp 60 nm) similar to that of Rosi. However, AGP displaced approximately 40% of bound [3H]Rosi even when applied at a 2000-fold excess. Activation of PPARgamma reporter gene expression by AGP and Rosi showed similar potency, yet AGP-mediated activation was approximately 40% that of Rosi. A complex between AGP and PPARgamma was generated using molecular modeling based on a PPARgamma crystal structure. AGP-interacting residues were compared with Rosi-interacting residues identified within the Rosi-PPARgamma co-crystal complex. These comparisons showed that the two ligands occupy partially overlapping positions but make different hydrogen bonding and ion pairing interactions. Site-specific mutants of PPARgamma were prepared to examine individual ligand binding. H323A and H449A mutants showed reduced binding of Rosi but maintained binding of AGP. In contrast, the R288A showed reduced AGP binding but maintained Rosi binding. Finally, alanine replacement of Tyr-473 abolished binding and activation by Rosi and AGP. These observations indicate that the endogenous lipid mediator AGP is a high affinity ligand of PPARgamma but that it binds via interactions distinct from those involved in Rosi binding. These distinct interactions are likely responsible for the partial PPARgamma agonism of AGP.

Cell migration activated by platelet-derived growth factor receptor is blocked by an inverse agonist of the sphingosine 1-phosphate receptor-1

We have previously identified a novel complex between the platelet-derived growth factor (PDGF)beta receptor and the sphingosine 1-phosphate receptor-1 (S1P1). The complex permits the utilization of active G-protein subunits (made available by constitutively active S1P1 receptor) by the PDGFbeta receptor kinase to transmit signals to p42/p44 MAPK in response to PDGF. Therefore, an inverse agonist of the S1P1 receptor is predicted to reduce signal transduction from PDGFbeta receptor tyrosine kinase by blocking the constitutive activity of the G-protein coupled receptor. SB649146 is a novel inverse agonist of the S1P1 receptor. First, SB649146 displaced the S1P1 receptor agonist dihydrosphingosine 1-phosphate from membranes expressing the recombinant S1P1 receptor. Second, SB649146 reduced basal recombinant S1P1 receptor-induced GTPgammaS binding and S1P-induced GTPgammaS binding in membranes. Third, SB649146 blocked the S1P-induced activation of p42/p44 MAPK in airway smooth muscle cells, a response that is mediated by the S1P1 receptor. We now report that inverse agonism of the S1P1 receptor with SB649146 reduced the endocytosis of the PDGFbeta receptor-S1P1 receptor complex and the stimulation of p42/p44 MAPK and cell migration in response to PDGF. These findings are the first to report that a GPCR inverse-agonist reduces growth factor-induced receptor tyrosine kinase signaling, fundamentally broadening their mechanism of action. The data obtained with SB649146 also suggest that the constitutively active endogenous S1P1 receptor enhances PDGF-induced cell migration.

Synthesis and pharmacological evaluation of second-generation phosphatidic acid derivatives as lysophosphatidic acid receptor ligands

Short-chain phosphatidic acid derivatives, dioctanoyl glycerol pyrophosphate (DGPP 8:0, 1) and phosphatidic acid 8:0 (PA 8:0, 2), were previously identified as subtype-selective LPA(1) and LPA(3) receptor antagonists. Recently, we reported that the replacement of the phosphate headgroup by thiophosphate in a series of fatty alcohol phosphates (FAP) improves agonist as well as antagonist activities at LPA GPCR. Here, we report the synthesis of stereoisomers of PA 8:0 analogs and their biological evaluation at LPA GPCR, PPARgamma, and ATX. The results indicate that LPA receptors stereoselectively interact with glycerol backbone modified ligands. We observed entirely stereospecific responses by dioctyl PA 8:0 compounds, in which (R)-isomers were found to be agonists and (S)-isomers were antagonists of LPA GPCR. From this series, we identified compound 13b as the most potent LPA(3) receptor subtype-selective agonist (EC(50)=3 nM), and 8b as a potent and selective LPA(3) receptor antagonist (K(i)=5 nM) and inhibitor of ATX (IC(50)=600 nM). Serinediamide phosphate 19b was identified as an LPA(3) receptor specific antagonist with no effect on LPA(1), LPA(2), and PPARgamma.

Identification of Residues Responsible for Ligand Recognition and Regioisomeric Selectivity of Lysophosphatidic Acid Receptors Expressed in Mammalian Cells

The endothelial differentiation gene family encodes three highly homologous G protein-coupled receptors for lysophosphatidic acid (LPA). Based on baculoviral overexpression studies, differences have been proposed in the structure-activity relationship (SAR) of these receptors. We have compared the SAR of the individual receptors either overexpressed transiently at high or at lower levels following stable transfection in LPA-nonresponsive RH7777 cells. The SAR in transfected RH7777 cells was markedly different from that described in insect cells. The LPA(3) receptor has been proposed to be selectively activated by unsaturated LPA species and shows a strong preference for sn-2 versus the sn-1 acyl-LPA regioisomer. Because of the short half-life of sn-2 LPA due to acyl migration under some conditions, we have synthesized acyl migration-resistant analogs using an acetyl group in place of the free hydroxyl group in order to evaluate LPA receptor SAR. Only LPA(1) and LPA(2) showed regioisomeric preference and only for the 18:2 fatty acyl-stabilized LPA sn-1 regioisomer. To identify residues involved in ligand recognition of LPA(3), we developed and validated computational models of LPA(3) complexes with the analogs studied. The models revealed that Arg-3.28 and Gln-3.29 conserved within the LPA-selective endothelial differentiation gene receptors and the more variable Lys-7.35 and Arg-5.38 of LPA(3) form critical interactions with the polar headgroup of LPA. The models identified Leu-2.60 and Val-7.39 of LPA(3) underlying the regioisomer-selective interaction with the acetyl group of the stabilized regioisomers. Mutation of Leu-2.60 to alanine selectively increased the EC(50) of the sn-2 acetyl-LPA regioisomers, whereas alanine replacement of Val-7.39 profoundly affected both regioisomers.

S1P1-selective in vivo-active agonists from high-throughput screening: off-the-shelf chemical probes of receptor interactions, signaling, and fate

The essential role of the sphingosine 1-phosphate (S1P) receptor S1P(1) in regulating lymphocyte trafficking was demonstrated with the S1P(1)-selective nanomolar agonist, SEW2871. Despite its lack of charged headgroup, the tetraaromatic compound SEW2871 binds and activates S1P(1) through a combination of hydrophobic and ion-dipole interactions. Both S1P and SEW2871 activated ERK, Akt, and Rac signaling pathways and induced S1P(1) internalization and recycling, unlike FTY720-phosphate, which induces receptor degradation. Agonism with receptor recycling is sufficient for alteration of lymphocyte trafficking by S1P and SEW2871. S1P(1) modeling and mutagenesis studies revealed that residues binding the S1P headgroup are required for kinase activation by both S1P and SEW2871. Therefore, SEW2871 recapitulates the action of S1P in all the signaling pathways examined and overlaps in interactions with key headgroup binding receptor residues, presumably replacing salt-bridge interactions with ion-dipole interactions.

Synthesis, Structure−Activity Relationships, and Biological Evaluation of Fatty Alcohol Phosphates as Lysophosphatidic Acid Receptor Ligands, Activators of PPARγ, and Inhibitors of Autotaxin†

We previously reported that fatty alcohol phosphates (FAP) represent a minimal pharmacophore required to interact with lysophosphatidic acid (LPA) receptors. To improve the activity of the first-generation saturated FAP series, a structure-activity relationship (SAR) study was carried out that includes modifications to the headgroup and alkyl side chain of the FAP pharmacophore. A series of unsaturated (C(10)-C(18)) FAP, headgroup-modified hydrolytically stable saturated (C(10)-C(18)) alkyl phosphonates, and saturated and unsaturated (C(10)-C(18)) thiophosphate analogues were synthesized and evaluated for activity in RH7777 cells transfected with individual LPA(1)(-3) receptors, in PC-3 cells and in human platelets that endogenously express all three isoforms. In this series we identified several LPA(1)- and LPA(3)-selective antagonists with IC(50) values in the nanomolar range. Oleoyl-thiophosphate (15g) was shown to be a pan-agonist, whereas tetradecyl-phosphonate (16c) was identified as a pan-antagonist. These compounds were also tested for the ability to activate the transcription factor PPARgamma, an intracellular receptor for LPA, in CV1 cells transfected with the PPRE-Acox-Rluc reporter gene. All the FAP tested, along with the previously reported LPA GPCR antagonists dioctanoyl glycerol pyrophosphate (2), Ki16425 (6), and the agonist OMPT (3), were activators of PPARgamma. The pan-agonist oleoyl-thiophosphate (15g) and pan-antagonist tetradecyl-phosphonate (16c) mimicked LPA in inhibiting autotaxin, a secreted lysophospholipase D that produces LPA in biological fluids.

Thrombogenic and atherogenic activities of lysophosphatidic acid

Lysophosphatidic acid (LPA) has been identified as a biologically active lipid in mildly-oxidized LDL, human atherosclerotic lesions, and the supernatant of activated platelets. The evidence that LPA has thrombogenic and atherogenic activities has increased substantially in recent years. Supporting the thrombogenic activity of LPA, analysis of the core region of human carotid plaques revealed recently the presence of alkyl- and acyl-molecular species from LPA with high platelet-activating potency (16:0 alkyl-LPA, 20:4 acyl-LPA). LPA, lipid extracts of atherosclerotic plaques, and the lipid-rich core elicited shape change and, in synergy with other platelet stimuli, aggregation of isolated platelets. This effect was completely abrogated by prior incubation of platelets with LPA receptor antagonists. Furthermore, LPA at concentrations approaching those found in vivo, induced platelet shape change, aggregation, and platelet-monocyte aggregate formation in blood. LPA-stimulated platelet aggregation was mediated by the ADP-stimulated activation of the P2Y(1) and P2Y(12) receptors. Supporting its atherogenic activity, LPA is a mitogen and motogen to vascular smooth muscle cells (VSMCs) and an activator of endothelial cells and macrophages. Recently, LPA has been identified as an agonist of the peroxisome proliferator activating receptor gamma (PPARgamma), which is a key regulator of atherogenesis. LPA elicits progressive neointima formation, which is fully abolished by GW9662, an antagonist of PPARgamma. We propose that LPA plays a central role in eliciting vascular remodeling and atherogenesis. Furthermore, upon rupture of lipid-rich atherosclerotic plaques, LPA may trigger platelet aggregation and intra-arterial thrombus formation. Antagonists of LPA receptors might be useful in preventing LPA-elicited thrombus formation and neointima formation in patients with cardiovascular diseases.