E2F7 drives autotaxin/Enpp2 transcription via chromosome looping: Repression by p53 in murine but not in human carcinomas.

Dysregulation of the autotaxin (ATX, Enpp2)-lysophosphatidic acid (LPA) signaling in cancerous cells contributes to tumorigenesis and therapy resistance. We previously found that ATX activity was elevated in p53-KO mice compared to wild-type (WT) mice. Here, we report that ATX expression was upregulated in mouse embryonic fibroblasts from p53-KO and p53R172H mutant mice. ATX promoter analysis combined with yeast one-hybrid testing revealed that WT p53 directly inhibits ATX expression via E2F7. Knockdown of E2F7 reduced ATX expression and chromosome immunoprecipitation showed that E2F7 promotes Enpp2 transcription through cooperative binding to two E2F7 sites (promoter region -1393 bp and second intron 996 bp). Using chromosome conformation capture, we found that chromosome looping brings together the two E2F7 binding sites. We discovered a p53 binding site in the first intron of murine Enpp2, but not in human ENPP2. Binding of p53 disrupted the E2F7-mediated chromosomal looping and repressed Enpp2 transcription in murine cells. In contrast, we found no disruption of E2F7-mediated ENPP2 transcription via direct p53 binding in human carcinoma cells. In summary, E2F7 is a common transcription factor that upregulates ATX in human and mouse cells but is subject to steric interference by direct intronic p53 binding only in mice.

LPA suppresses HLA-DR expression in human melanoma cells: a potential immune escape mechanism involving LPAR1 and DR6-mediated release of IL-10.

While immune checkpoint inhibitors (ICIs) are promising in the treatment of metastatic melanoma, about half of patients do not respond well to them. Low levels of human leukocyte antigen-DR (HLA-DR) in tumors have been shown to negatively influence prognosis and response to ICIs. Lysophosphatidic acid (LPA) is produced in large amounts by melanoma and is abundantly present in the tumor microenvironment. LPA induces the release of various cytokines and chemokines from tumor cells, which affect cancer development, metastasis, and tumor immunity. In the present study, we investigated the role of LPA-induced IL-10 release in regulating HLA-DR expression and the underlying mechanisms in human melanoma cells. We showed that LPA (0.001-10 µM) dose-dependently increased DR6 transcript levels through activating LPAR1 in HEK293T cells. Knockdown of NF-κB1 abrogated the LPA-increased DR6 expression without affecting basal DR6 expression in both A2058 and A375 melanoma cell lines. LPA (10 µM) significantly increased IL-10 transcripts in A2058 and A375 melanoma cells, the effect was abolished by pharmacological inhibition of LPAR1 or knockdown of DR6. We found a statistically significant correlation between the expression of LPAR1, DR6 and IL-10 in human melanoma tissue and an association between increased expression of LPAR1 and reduced effectiveness of ICI therapy. We demonstrated that LPA (10 µM) markedly suppressed HLA-DR expression in both A375 and A2058 melanoma cells via activating the LPAR1-DR6-IL-10 pathway. These data suggest that the LPAR1-DR6-IL-10 autocrine loop could constitute a novel mechanism used by tumor cells to evade immunosurveillance by decreasing HLA-DR expression.

Molecular modelling guided design, synthesis and QSAR analysis of new small molecule non-lipid autotaxin inhibitors.

The lysophospholipase D autotaxin (ATX) generates lysophosphatidic acid (LPA) that activates six cognate G-protein coupled receptors (GPCR) in cancerous cells, promoting their motility and invasion. Four novel compounds were generated aided by molecular docking guided design and synthesis techniques to obtain new dual inhibitors of ATX and the lysophosphatidic acid receptor subtype 1 (LPAR1). Biological evaluation of these compounds revealed two compounds, 10 and 11, as new ATX enzyme inhibitors with potencies in the range of 218-220 nM and water solubility (>100 µg/mL), but with no LPAR1 inhibitory activity. A QSAR model was generated that included four newly designed compounds and twenty-one additional compounds that we have reported previously. The QSAR model provided excellent predictability of the pharmacological activity and potency among structurally related drug candidates. This model will be highly useful in guiding the synthesis of new ATX inhibitors in the future.

Lysophosphatidic acid type 2 receptor agonists in targeted drug development offer broad therapeutic potential.

The growth factor-like lipid mediator, lysophosphatidic acid (LPA), is a potent signaling molecule that influences numerous physiologic and pathologic processes. Manipulation of LPA signaling is of growing pharmacotherapeutic interest, especially because LPA resembles compounds with drug-like features. The action of LPA is mediated through activation of multiple types of molecular targets, including six G protein-coupled receptors that are clear targets for drug development. However, the LPA signaling has been linked to pathological responses that include promotion of fibrosis, atherogenesis, tumorigenesis, and metastasis. Thus, a question arises: Can we harness, in an LPA-like drug, the many beneficial activities of this lipid without eliciting its dreadful actions? We developed octadecyl thiophosphate (OTP; subsequently licensed as Rx100), an LPA mimic with higher stability in vivo than LPA. This article highlights progress made toward developing analogs like OTP and exploring prosurvival and regenerative LPA signaling. We determined that LPA prevents cell death triggered by various cellular stresses, including genotoxic stressors, and rescues cells condemned to apoptosis. LPA2 agonists provide a new treatment option for secretory diarrhea and reduce gastric erosion caused by nonsteroidal anti-inflammatory drugs. The potential uses of LPA2 agonists like OTP and sulfamoyl benzoic acid-based radioprotectins must be further explored for therapeutic uses.

Role of autotaxin in cancer stem cells.

Stem cells are a rare subpopulation defined by the potential to self-renew and differentiate into specific cell types. A population of stem-like cells has been reported to possess the ability of self-renewal, invasion, metastasis, and engraftment of distant tissues. This unique cell subpopulation has been designated as cancer stem cells (CSC). CSC were first identified in leukemia, and the contributions of CSC to cancer progression have been reported in many different types of cancers. The cancer stem cell hypothesis attempts to explain tumor cell heterogeneity based on the existence of stem cell-like cells within solid tumors. The elimination of CSC is challenging for most human cancer types due to their heightened genetic instability and increased drug resistance. To combat these inherent abilities of CSC, multi-pronged strategies aimed at multiple aspects of CSC biology are increasingly being recognized as essential for a cure. One of the most challenging aspects of cancer biology is overcoming the chemotherapeutic resistance in CSC. Here, we provide an overview of autotaxin (ATX), lysophosphatidic acid (LPA), and their signaling pathways in CSC. Increasing evidence supports the role of ATX and LPA in cancer progression, metastasis, and therapeutic resistance. Several studies have demonstrated the ATX-LPA axis signaling in different cancers. This lipid mediator regulatory system is a novel potential therapeutic target in CSC. In this review, we summarize the evidence linking ATX-LPA signaling to CSC and its impact on cancer progression and metastasis. We also provide evidence for the efficacy of cancer therapy involving the pharmacological inhibition of this signaling pathway.

Rapid disruption of intestinal epithelial tight junction and barrier dysfunction by ionizing radiation in mouse colon in vivo: protection by N-acetyl-l-cysteine.

The goals of this study were to evaluate the effects of ionizing radiation on apical junctions in colonic epithelium and mucosal barrier function in mice in vivo. Adult mice were subjected to total body irradiation (4 Gy) with or without N-acetyl-l-cysteine (NAC) feeding for 5 days before irradiation. At 2-24 h postirradiation, the integrity of colonic epithelial tight junctions (TJ), adherens junctions (AJ), and the actin cytoskeleton was assessed by immunofluorescence microscopy and immunoblot analysis of detergent-insoluble fractions for TJ and AJ proteins. The barrier function was evaluated by measuring vascular-to-luminal flux of fluorescein isothiocyanate (FITC)-inulin in vivo and luminal-to-mucosal flux in vitro. Oxidative stress was evaluated by measuring protein thiol oxidation. Confocal microscopy showed that radiation caused redistribution of occludin, zona occludens-1, claudin-3, E-cadherin, and ß-catenin, as well as the actin cytoskeleton as early as 2 h postirradiation, and this effect was sustained for at least 24 h. Feeding NAC before irradiation blocked radiation-induced disruption of TJ, AJ, and the actin cytoskeleton. Radiation increased mucosal permeability to inulin in colon, which was blocked by NAC feeding. The level of reduced-protein thiols in colon was depleted by radiation with a concomitant increase in the level of oxidized-protein thiol. NAC feeding blocked the radiation-induced protein thiol oxidation. These data demonstrate that radiation rapidly disrupts TJ, AJ, and the actin cytoskeleton by an oxidative stress-dependent mechanism that can be prevented by NAC feeding.

Interaction of platelet-derived autotaxin with tumor integrin αVß3 controls metastasis of breast cancer cells to bone.

Autotaxin (ATX), through its lysophospholipase D activity controls physiological levels of lysophosphatidic acid (LPA) in blood. ATX is overexpressed in multiple types of cancers, and together with LPA generated during platelet activation promotes skeletal metastasis of breast cancer. However, the pathophysiological sequelae of regulated interactions between circulating LPA, ATX, and platelets remain undefined in cancer. In this study, we show that ATX is stored in α-granules of resting human platelets and released upon tumor cell-induced platelet aggregation, leading to the production of LPA. Our in vitro and in vivo experiments using human breast cancer cells that do not express ATX (MDA-MB-231 and MDA-B02) demonstrate that nontumoral ATX controls the early stage of bone colonization by tumor cells. Moreover, expression of a dominant negative integrin αvß3-Δ744 or treatment with the anti-human αvß3 monoclonal antibody LM609, completely abolished binding of ATX to tumor cells, demonstrating the requirement of a fully active integrin αvß3 in this process. The present results establish a new mechanism for platelet contribution to LPA-dependent metastasis of breast cancer cells, and demonstrate the therapeutic potential of disrupting the binding of nontumor-derived ATX with the tumor cells for the prevention of metastasis.

Mitigation of radiation injury by selective stimulation of the LPA(2) receptor.

Due to its antiapoptotic action, derivatives of the lipid mediator lysophosphatidic acid (LPA) provide potential therapeutic utility in diseases associated with programmed cell death. Apoptosis is one of the major pathophysiological processes elicited by radiation injury to the organism. Consequently, therapeutic explorations applying compounds that mimic the antiapoptotic action of LPA have begun. Here we present a brief account of our decade-long drug discovery effort aimed at developing LPA mimics with a special focus on specific agonists of the LPA(2) receptor subtype, which was found to be highly effective in protecting cells from apoptosis. We describe new evidence that 2-((3-(1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)propyl)thio)benzoic acid (GRI977143), a prototypic nonlipid agonist specific to the LPA(2) receptor subtype, rescues apoptotically condemned cells in vitro and in vivo from injury caused by high-dose γ-irradiation. GRI977143 shows the features of a radiomitigator because it is effective in rescuing the lives of mice from deadly levels of radiation when administered 24h after radiation exposure. Our findings suggest that by specifically activating LPA(2) receptors GRI977143 activates the ERK1/2 prosurvival pathway, effectively reduces Bax translocation to the mitochondrion, attenuates the activation of initiator and effector caspases, reduces DNA fragmentation, and inhibits PARP-1 cleavage associated with γ-irradiation-induced apoptosis. GRI977143 also inhibits bystander apoptosis elicited by soluble proapoptotic mediators produced by irradiated cells. Thus, GRI977143 can serve as a prototype scaffold for lead optimization paving the way to more potent analogs amenable for therapeutic exploration. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Hits of a high-throughput screen identify the hydrophobic pocket of autotaxin/lysophospholipase D as an inhibitory surface.

Autotaxin (ATX), a lysophospholipase D, plays an important role in cancer invasion, metastasis, tumor progression, tumorigenesis, neuropathic pain, fibrotic diseases, cholestatic pruritus, lymphocyte homing, and thrombotic diseases by producing the lipid mediator lysophosphatidic acid (LPA). A high-throughput screen of ATX inhibition using the lysophosphatidylcholine-like substrate fluorogenic substrate 3 (FS-3) and ∼10,000 compounds from the University of Cincinnati Drug Discovery Center identified several small-molecule inhibitors with IC50 vales ranging from nanomolar to low micromolar. The pharmacology of the three most potent compounds: 918013 (1; 2,4-dichloro-N-(3-fluorophenyl)-5-(4-morpholinylsulfonyl) benzamide), 931126 (2; 4-oxo-4-{2-[(5-phenoxy-1H-indol-2-yl)carbonyl]hydrazino}-N-(4-phenylbutan-2-yl)butanamide), and 966791 (3; N-(2,6-dimethylphenyl)-2-[N-(2-furylmethyl)(4-(1,2,3,4-tetraazolyl)phenyl)carbonylamino]-2-(4-hydroxy-3-methoxyphenyl) acetamide), were further characterized in enzyme, cellular, and whole animal models. Compounds 1 and 2 were competitive inhibitors of ATX-mediated hydrolysis of the lysophospholipase substrate FS-3. In contrast, compound 3 was a competitive inhibitor of both FS-3 and the phosphodiesterase substrate p-nitrophenyl thymidine 5'-monophosphate. Computational docking and mutagenesis suggested that compounds 1 and 2 target the hydrophobic pocket, thereby blocking access to the active site of ATX. The potencies of compounds 1-3 were comparable to each other in each of the assays. All of these compounds significantly reduced invasion of A2058 human melanoma cells in vitro and the colonization of lung metastases by B16-F10 murine melanoma cells in C57BL/6 mice. The compounds had no agonist or antagonist effects on select LPA or sphingosine 1-phosphate receptors, nor did they inhibit nucleotide pyrophosphatase/phosphodiesterase (NPP) enzymes NPP6 and NPP7. These results identify the molecular surface of the hydrophobic pocket of ATX as a target-binding site for inhibitors of enzymatic activity.

Emerging roles of lysophosphatidic acid receptor subtype 5 (LPAR5) in inflammatory diseases and cancer.

Lysophosphatidic acid (LPA) is a bioactive lipid mediator that regulates a variety of cellular functions such as cell proliferation, migration, survival, calcium mobilization, cytoskeletal rearrangements, and neurite retraction. The biological actions of LPA are mediated by at least six G protein-coupled receptors known as LPAR1-6. Given that LPAR1-3 were among the first LPARs identified, the majority of research efforts have focused on understanding their biology. This review provides an in-depth discussion of LPAR5, which has recently emerged as a key player in regulating normal intestinal homeostasis and modulating pathological conditions such as pain, itch, inflammatory diseases, and cancer. We also present a chronological overview of the efforts made to develop compounds that target LPAR5 for use as tool compounds to probe or validate LPAR5 biology and therapeutic agents for the treatment of inflammatory diseases and cancer.

Paneth cell dysfunction in radiation injury and radio-mitigation by human α-defensin 5.

Introduction: The mechanism underlying radiation-induced gut microbiota dysbiosis is undefined. This study examined the effect of radiation on the intestinal Paneth cell α-defensin expression and its impact on microbiota composition and mucosal tissue injury and evaluated the radio-mitigative effect of human α-defensin 5 (HD5). Methods: Adult mice were subjected to total body irradiation, and Paneth cell α-defensin expression was evaluated by measuring α-defensin mRNA by RT-PCR and α-defensin peptide levels by mass spectrometry. Vascular-to-luminal flux of FITC-inulin was measured to evaluate intestinal mucosal permeability and endotoxemia by measuring plasma lipopolysaccharide. HD5 was administered in a liquid diet 24 hours before or after irradiation. Gut microbiota was analyzed by 16S rRNA sequencing. Intestinal epithelial junctions were analyzed by immunofluorescence confocal microscopy and mucosal inflammatory response by cytokine expression. Systemic inflammation was evaluated by measuring plasma cytokine levels. Results: Ionizing radiation reduced the Paneth cell α-defensin expression and depleted α-defensin peptides in the intestinal lumen. α-Defensin down-regulation was associated with the time-dependent alteration of gut microbiota composition, increased gut permeability, and endotoxemia. Administration of human α-defensin 5 (HD5) in the diet 24 hours before irradiation (prophylactic) significantly blocked radiation-induced gut microbiota dysbiosis, disruption of intestinal epithelial tight junction and adherens junction, mucosal barrier dysfunction, and mucosal inflammatory response. HD5, administered 24 hours after irradiation (treatment), reversed radiation-induced microbiota dysbiosis, tight junction and adherens junction disruption, and barrier dysfunction. Furthermore, HD5 treatment also prevents and reverses radiation-induced endotoxemia and systemic inflammation. Conclusion: These data demonstrate that radiation induces Paneth cell dysfunction in the intestine, and HD5 feeding prevents and mitigates radiation-induced intestinal mucosal injury, endotoxemia, and systemic inflammation.

Virtual screening for LPA2-specific agonists identifies a nonlipid compound with antiapoptotic actions.

Lysophosphatidic acid (LPA) is a highly potent endogenous lipid mediator that protects and rescues cells from programmed cell death. Earlier work identified the LPA2 G protein-coupled receptor subtype as an important molecular target of LPA mediating antiapoptotic signaling. Here we describe the results of a virtual screen using single-reference similarity searching that yielded compounds 2-((9-oxo-9H-fluoren-2-yl)carbamoyl)benzoic acid (NSC12404), 2-((3-(1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)propyl)thio)benzoic acid (GRI977143), 4,5-dichloro-2-((9-oxo-9H-fluoren-2-yl)carbamoyl)benzoic acid (H2L5547924), and 2-((9,10-dioxo-9,10-dihydroanthracen-2-yl)carbamoyl) benzoic acid (H2L5828102), novel nonlipid and drug-like compounds that are specific for the LPA2 receptor subtype. We characterized the antiapoptotic action of one of these compounds, GRI977143, which was effective in reducing activation of caspases 3, 7, 8, and 9 and inhibited poly(ADP-ribose)polymerase 1 cleavage and DNA fragmentation in different extrinsic and intrinsic models of apoptosis in vitro. Furthermore, GRI977143 promoted carcinoma cell invasion of human umbilical vein endothelial cell monolayers and fibroblast proliferation. The antiapoptotic cellular signaling responses were present selectively in mouse embryonic fibroblast cells derived from LPA(1&2) double-knockout mice reconstituted with the LPA2 receptor and were absent in vector-transduced control cells. GRI977143 was an effective stimulator of extracellular signal-regulated kinase 1/2 activation and promoted the assembly of a macromolecular signaling complex consisting of LPA2, Na⁺ - H⁺ exchange regulatory factor 2, and thyroid receptor interacting protein 6, which has been shown previously to be a required step in LPA-induced antiapoptotic signaling. The present findings indicate that nonlipid LPA2-specific agonists represent an excellent starting point for development of lead compounds with potential therapeutic utility for preventing the programmed cell death involved in many types of degenerative and inflammatory diseases.

Prometastatic Effect of ATX Derived from Alveolar Type II Pneumocytes and B16-F10 Melanoma Cells.

Although metastases are the principal cause of cancer-related deaths, the molecular aspects of the role of stromal cells in the establishment of the metastatic niche remain poorly understood. One of the most prevalent sites for cancer metastasis is the lungs. According to recent research, lung stromal cells such as bronchial epithelial cells and resident macrophages secrete autotaxin (ATX), an enzyme with lysophospholipase D activity that promotes cancer progression. In fact, several studies have shown that many cell types in the lung stroma could provide a rich source of ATX in diseases. In the present study, we sought to determine whether ATX derived from alveolar type II epithelial (ATII) pneumocytes could modulate the progression of lung metastasis, which has not been evaluated previously. To accomplish this, we used the B16-F10 syngeneic melanoma model, which readily metastasizes to the lungs when injected intravenously. Because B16-F10 cells express high levels of ATX, we used the CRISPR-Cas9 technology to knock out the ATX gene in B16-F10 cells, eliminating the contribution of tumor-derived ATX in lung metastasis. Next, we used the inducible Cre/loxP system (Sftpc-CreERT2/Enpp2fl/fl) to generate conditional knockout (KO) mice in which ATX is specifically deleted in ATII cells (i.e., Sftpc-KO). Injection of ATX-KO B16-F10 cells into Sftpc-KO or Sftpc-WT control littermates allowed us to investigate the specific contribution of ATII-derived ATX in lung metastasis. We found that targeted KO of ATX in ATII cells significantly reduced the metastatic burden of ATX-KO B16-F10 cells by 30% (unpaired t-test, p = 0.028) compared to Sftpc-WT control mice, suggesting that ATX derived from ATII cells could affect the metastatic progression. We detected upregulated levels of cytokines such as IFNγ (unpaired t-test, p < 0.0001) and TNFα (unpaired t-test, p = 0.0003), which could favor the increase in infiltrating CD8+ T cells observed in the tumor regions of Sftpc-KO mice. Taken together, our results highlight the contribution of host ATII cells as a stromal source of ATX in the progression of melanoma lung metastasis.

Revisiting the role of lysophosphatidic acid in stem cell biology.

Stem cells possess unique biological characteristics such as the ability to self-renew and to undergo multilineage differentiation into specialized cells. Whereas embryonic stem cells (ESC) can differentiate into all cell types of the body, somatic stem cells (SSC) are a population of stem cells located in distinct niches throughout the body that differentiate into the specific cell types of the tissue in which they reside in. SSC function mainly to restore cells as part of normal tissue homeostasis or to replenish cells that are damaged due to injury. Cancer stem-like cells (CSC) are said to be analogous to SSC in this manner where tumor growth and progression as well as metastasis are fueled by a small population of CSC that reside within the corresponding tumor. Moreover, emerging evidence indicates that CSC are inherently resistant to chemo- and radiotherapy that are often the cause of cancer relapse. Hence, major research efforts have been directed at identifying CSC populations in different cancer types and understanding their biology. Many factors are thought to regulate and maintain cell stemness, including bioactive lysophospholipids such as lysophosphatidic acid (LPA). In this review, we discuss some of the newly discovered functions of LPA not only in the regulation of CSC but also normal SSC, the similarities in these regulatory functions, and how these discoveries can pave way to the development of novel therapies in cancer and regenerative medicine.

Dysregulation of lysophospholipid signaling by p53 in malignant cells and the tumor microenvironment.

The TP53 gene has been widely studied for its roles in cell cycle control, maintaining genome stability, activating repair mechanisms upon DNA damage, and initiating apoptosis should repair mechanisms fail. Thus, it is not surprising that mutations of p53 are the most common genetic alterations found in human cancer. Emerging evidence indicates that dysregulation of lipid metabolism by p53 can have a profound impact not only on cancer cells but also cells of the tumor microenvironment (TME). In particular, intermediates of the sphingolipid and lysophospholipid pathways regulate many cellular responses common to p53 such as cell survival, migration, DNA damage repair and apoptosis. The majority of these cellular events become dysregulated in cancer as well as cell senescence. In this review, we will provide an account on the seminal contributions of Prof. Lina Obeid, who deciphered the crosstalk between p53 and the sphingolipid pathway particularly in modulating DNA damage repair and apoptosis in non-transformed as well as transformed cells. We will also provide insights on the integrative role of p53 with the lysophosphatidic acid (LPA) signaling pathway in cancer progression and TME regulation.

Regulation of Tumor Immunity by Lysophosphatidic Acid.

The tumor microenvironment (TME) may be best conceptualized as an ecosystem comprised of cancer cells interacting with a multitude of stromal components such as the extracellular matrix (ECM), blood and lymphatic networks, fibroblasts, adipocytes, and cells of the immune system. At the center of this crosstalk between cancer cells and their TME is the bioactive lipid lysophosphatidic acid (LPA). High levels of LPA and the enzyme generating it, termed autotaxin (ATX), are present in many cancers. It is also well documented that LPA drives tumor progression by promoting angiogenesis, proliferation, survival, invasion and metastasis. One of the hallmarks of cancer is the ability to modulate and escape immune detection and eradication. Despite the profound role of LPA in regulating immune functions and inflammation, its role in the context of tumor immunity has not received much attention until recently where emerging studies highlight that this signaling axis may be a means that cancer cells adopt to evade immune detection and eradication. The present review aims to look at the immunomodulatory actions of LPA in baseline immunity to provide a broad understanding of the subject with a special emphasis on LPA and cancer immunity, highlighting the latest progress in this area of research.

Regulation of tumor cell - Microenvironment interaction by the autotaxin-lysophosphatidic acid receptor axis.

The lipid mediator lysophosphatidic acid (LPA) in biological fluids is primarily produced by cleavage of lysophospholipids by the lysophospholipase D enzyme Autotaxin (ATX). LPA has been identified and abundantly detected in the culture medium of various cancer cell types, tumor effusates, and ascites fluid of cancer patients. Our current understanding of the physiological role of LPA established its role in fundamental biological responses that include cell proliferation, metabolism, neuronal differentiation, angiogenesis, cell migration, hematopoiesis, inflammation, immunity, wound healing, regulation of cell excitability, and the promotion of cell survival by protecting against apoptotic death. These essential biological responses elicited by LPA are seemingly hijacked by cancer cells in many ways; transcriptional upregulation of ATX leading to increased LPA levels, enhanced expression of multiple LPA GPCR subtypes, and the downregulation of its metabolic breakdown. Recent studies have shown that overexpression of ATX and LPA GPCR can lead to malignant transformation, enhanced proliferation of cancer stem cells, increased invasion and metastasis, reprogramming of the tumor microenvironment and the metastatic niche, and development of resistance to chemo-, immuno-, and radiation-therapy of cancer. The fundamental role of LPA in cancer progression and the therapeutic inhibition of the ATX-LPA axis, although highly appealing, remains unexploited as drug development to these targets has not reached into the clinic yet. The purpose of this brief review is to highlight some unique signaling mechanisms engaged by the ATX-LPA axis and emphasize the therapeutic potential that lies in blocking the molecular targets of the LPA system.

The LPA2 receptor agonist Radioprotectin-1 spares Lgr5-positive intestinal stem cells from radiation injury in murine enteroids.

Rapidly proliferating cells are highly sensitive to ionizing radiation and can undergo apoptosis if the oxidative and genotoxic injury exceed the defensive and regenerative capacity of the cell. Our earlier work has established the antiapoptotic action of the growth factor-like lipid mediator lysophosphatidic acid (LPA). Activation of the LPA2 GPCR has been hypothesized to elicit antiapoptotic and regenerative actions of LPA. Based on this hypothesis we developed a novel nonlipid agonist of LPA2, which we designated Radioprotectin-1 (RP-1). We tested RP-1 at the six murine LPA GPCR subtypes using the transforming growth factor alpha shedding assay and found that it had a 25 nM EC50 that is similar to that of LPA18:1 at 32 nM. RP-1 effectively reduced apoptosis induced by γ-irradiation and the radiomimetic drug Adriamycin only in cells that expressed LPA2 either endogenously or after transfection. RP-1 reduced γ-H2AX levels in irradiated mouse embryonic fibroblasts transduced with the human LPA2 GPCR but was ineffective in vector transduced MEF control cells and significantly increased clonogenic survival after γ-irradiation. γ-Irradiation induced the expression of lpar2 transcripts that was further enhanced by RP-1 exposure within 30 min after irradiation. RP-1 decreased the mortality of C57BL/6 mice in models of the hematopoietic and gastrointestinal acute radiation syndromes. Using Lgr5-EGFP-CreER;Tdtomatoflox transgenic mice, we found that RP-1 increased the survival and growth of intestinal enteroids via the enhanced survival of Lgr5+ intestinal stem cells. Taken together, our results suggest that the LPA2-specific agonist RP-1 exerts its radioprotective and radiomitigative action through specific activation of the upregulated LPA2 GPCR in Lgr5+ stem cells.