Lysophosphatidic Acid Receptors in CancerABIR MUKHERJEE, JINHUA WU, YONGLING GONG, and XIANJUN FANG
31.1. INTRODUCTION
In the past decade, there have been considerable advances in our understand-ing of the sources and biological functions of lysophospholipids. The best characterized of these are lysophosphatidic acid (LPA) with a glycerol back-bone and the related sphingosine 1-phosphate (S1P) containing sphingosine (1, 2). These lysophospholipids are not only metabolites in biosynthesis of more complex lipids in eukaryotic and prokaryotic cells but also have emerged as pluripotent intercellular mediators to induce hormone- and growth factor-like responses in their target cells.
LPA is a key intermediate in de novo lipid synthesis in all cells formed by acylation of glycerol 3-phosphate catalyzed by glycerophosphate acyltransfer-ase (3, 4). LPA is subsequently converted to phosphatidic acid (PA) by LPA acyltransferase and further to other phospholipids and triacylglycerols. These conversions occur intracellularly in the endoplasmic reticulum or mitochon-dria. The pathway is conserved in lower organisms in which extracellular LPA actions are not observed. There is no evidence for LPA release into the extra-cellular fluid or leaflets of plasma membranes. Thus, LPA produced through this route is not considered to be involved in extracellular LPA signaling. Another potential intracellular pathway for the synthesis of LPA is the phos-phorylation of monoacylglycerol by acylglycerol kinase (AGK) located at the mitochondria (5). Similarly, the contribution of AGK to the physiological levels of LPA is unknown due to the lack of information on potential LPA transporters responsible for secreting LPA from intracellular locations.
The molecular pathways for extracellular LPA production are better under-stood. The mechanism has been best characterized in activated platelets that are responsible for increased LPA levels in serum compared to the whole blood or plasma (6–8). LPA can be produced by activated platelets from newly generated, membrane-associated PA by the action of phospholipase D (PLD) followed by phospholipase A1 (PLA1) or phospholipase A2 (PLA2)-mediated deacylation (6, 7). However, the bulk of LPA arising from platelet activation results from the sequential cleavage of serum and membrane phospholipids to lysophospholipids by PLA1 and PLA2 secreted from platelets, followed by conversion to LPA by lysophospholipase D (lysoPLD) present in the plasma (6, 9, 10). The plasma lysoPLD is autotaxin (ATX) (11, 12), a member of the nucleotide pyrophosphatase and phosphodiesterase family of exo- and ecto-enzymes (13). Homozygous deletion of ATX leads to embryonic lethality at E9.5 due to impaired vessel formation in the yolk sac and embryo proper (14, 15), indicating that ATX is essential for embryonic vasculature. Blood LPA levels in ATX heterozygous mice are about half of those present in wild-type littermates, indicating that ATX is a major LPA-generating enzyme in the circulation (14, 15).
The LPA homeostasis is regulated mainly by the LPA synthesizing enzyme ATX and lipid phosphate phosphatases (LPPs) including LPP1/PAP2A, LPP3/PAP2B, and LPP2/PAP2C (16). LPPs are located at the cell membrane with the catalytic domain facing the extracellular media. The LPP enzymes dephos-phorylate LPA and other phospholipids at the external leaflet of the mem-brane (17, 18). Overexpression of LPPs has been shown to antagonize overall cellular responses to LPA and to reduce LPA concentrations in the medium (19, 20). In contrast, inhibition of LPP enzyme activity sensitizes platelets to LPA-induced responses as well as increases thrombin-induced LPA produc-tion (20). Together, these observations suggest that LPPs play a critical role in controlling levels of extracellular LPA or membrane accessible LPA. Tomsig et al. recently reported that LPP1 functions as a LPP that specifically catalyzes dephosphorylation of LPA (21).
31.2. G PROTEIN-COUPLED RECEPTORS (GPCRS) FOR LPA
Most biological actions of LPA are mediated through LPA activation of spe-cific GPCRs (22). At least six GPCRs have been identified to mediate a wide range of biological actions of LPA (23–29). The LPA1/Edg2, LPA2/Edg4, and LPA3/Edg7 receptors are members of the endothelial differentiation gene (Edg) family, sharing 50–57% homology in their amino acid sequences (23–26). These LPA receptors couple to Gi, Gq, and G12/13 that, in turn, activate diverse pathways including stimulation of phospholipase C (PLC) (30), inhibi-tion of adenylyl cyclase (30), and activation of mitogen-activated protein kinases (MAPKs) (31) and PI3K (32). Activation of these signaling events
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downstream of LPA receptors culminates in changes in cell morphology, growth, survival, migration, and tumor cell invasion (33).
In addition to the Edg LPA1–3 receptors, GPR23/P2Y9/LPA4 of the puriner-gic receptor family and the related GPR92/LPA5, and P2Y5/LPA6 have been identified as new LPA receptors (27–29). GPR87 was proposed to be the seventh LPA receptor (34), but further evidence is needed to validate its role as an additional LPA receptor. This new subgroup of non-Edg LPA receptors is structurally distant from the Edg LPA1–3 receptors (27–29). These novel LPA receptors couple to Gq, G12/13, and Gs, leading to activation of PLC, the small GTPase Rho, and adenylyl cyclase, respectively.
Only minor abnormalities such as craniofacial dysmorphism and defective sucking behavior were found in LPA1-deficient mice (35). However, further analysis of these Lpar1 knockout mice subjected to pathophysiological condi-tions has revealed that LPA1 is required for the initiation of neuropathic pain (36) and promotion of pulmonary and renal fibrosis (37, 38). Disruption of the Lpar2 gene results in no obvious abnormalities in mice (39). Targeted deletion of Lpar3 leads to identification of a specific function of this LPA receptor in female reproduction. LPA3-deficient female mice show a delayed implantation and defective embryo spacing, associated with reduced uterine expression of Cox-2 mRNA (40). Among the non-Edg LPA receptors, two independent groups have reported phenotypes of Lpar4 knockout mice (41, 42). In mice of C57BL/6 background, loss of LPA4 seems to cause partial embryonic lethality (41). However, in the C57BL/6 and 129/Sv mixed background, Lee et al. did not observe any apparent abnormalities in embryonic development (42). Instead, LPA4 null embryonic fibroblasts are more responsive to LPA-induced migration, pointing to the possibility that LPA4 plays a negative role in the regulation of cell motility (42). The observations in genetic models of LPA receptors suggest the diverse LPA receptors may act redundantly or antago-nistically to regulate cellular responses to LPA.
31.3. LPA1 IN CANCER
The potential role of LPA in the pathogenesis of cancers has received more and more attention in recent years. In addition to growth and survival-promoting effects, LPA is a potent motogen, driving random migration, che-motaxis, and tumor cell invasion (12, 33, 42, 43). Interestingly, the major LPA synthesizing enzyme ATX was originally identified from culture supernatants as a tumor cell motility-stimulating factor (11, 12). Stimulation of cell motility by LPA and ATX could contribute to the metastatic phenotype of cancer cells.
Although each of the Edg LPA receptors has been linked to promoting cell motility in certain cellular contexts, the LPA1 receptor is apparently the primary receptor subtype that plays a fundamental role in mediation of cell motility. Knockdown of the LPA1 receptor expression or pharmacological
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inhibition of LPA1 with Ki16425 blocks the chemotactic response to LPA consistently in different lineages of mammalian cells (44). In nude mice, down-regulation of LPA1 expression or activity in breast cancer cell lines inhibited their metastasis to the bone (45). Conversely, topical expression of LPA1 alone in the nonresponsive cell lines such as Rh7777 (12), B103 (46), and SKBr-3 (45, 47) is sufficient to restore migratory response to LPA or to induce bone metastasis in vivo. The most convincing evidence to support a critical role for LPA1 in cell motility stemmed from the study by Hama et al., who showed mouse skin fibroblasts devoid of LPA1 failed to migrate toward LPA, while the cells lacking LPA2 showed a normal response similar to wild-type cells (44). Loss of LPA1 was associated with the deficiency in activation of Rac by LPA in these skin fibroblasts. The observation is consistent with the fact that LPA1 couples to Gi that links to activation of the downstream target Rac. Rac, along with Rho downstream of G12/13, mediate cell migration in a coordinate manner. Rac promotes lamellipodia protrusion and forward movement, whereas RhoA regulates actomyosin-driven cytoskeleton contraction and detachment of the rear of migrating cells (48).
Compared to other Edg LPA receptors, LPA1 is most widely expressed in tissues (49). Gene expression studies failed to show any consensus increase in LPA1 expression between normal and malignant cells (50–54). Instead, some expression or array analyses indicate modest decreases in LPA1 mRNA expres-sion in primary tumors (50, 52–55). However, most of these results are statisti-cally insignificant or lack appropriate normal controls for comparison.
One of the most exciting findings about regulation of LPA1 in cancer is that the metastatic tumor suppressor Nm23 inhibits LPA1 expression (56). Nm23 is the first identified metastasis suppressor gene (MSG) that, by definition, inhibits the process of metastasis but not growth of primary tumors (56). At least three biological functions of Nm23 may contribute to its metastasis-suppressive activity. These include the histidine kinase activity that phosphory-lates ATP-citrate lyase, aldolase C, and the kinase suppressor of ras, Nm23 interaction/inactivation of cellular and viral proteins that stimulate metastasis, and regulation of expression of metastatic proteins (56, 57). Manipulation of Nm23 expression in lung and breast cancer cells has revealed a number of genes that repressed by Nm23 including Wnt5B, uPA, MMPs, CTGF, c-Met, and recently, LPA1 (58–60). Among these targets repressed by Nm23, only LPA1 is capable of functionally overcoming the antimetastatic effects of Nm23 (57, 61). Topical expression of LPA2 only shows a minor effect (56), suggesting that suppression of LPA1 expression is an important and major downstream effector of the Nm23 metastasis suppressor.
LPA1 mRNA expression levels were analyzed in two published microarray cohorts of human breast carcinomas (62, 63). When the cohorts were separated into high and low Nm23-H1 expressing sets, LPA1 expression inversely cor-related with Nm23-H1 (61). In addition, an inverse correlation was also observed by immunohistochemical staining of LPA1 and Nm23-M1 in hepato-cellular carcinomas from wild-type and Nm23-M1 null mice (61). The next
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question is whether LPA1 expression is indeed upregulated in metastatic cancer compared to primary tumors. Such analysis is only available in ovarian cancer where LPA1 expression seems to be higher in metastatic tissues than in primary ovarian tumors (64).
The importance of LPA1 in the regulation of metastatic potential of cancer cells is further highlighted by the observation that LPA1 may lie downstream of fibroblast growth factor receptor 4 (FGFR4) (65). The germline polymor-phism of FGFR4 results in substitution of glycine by arginine at codon 388 (G388 to R388). The R388 FGFR4 does not increase tumor incidence. However, clinical studies demonstrate that cancer patients with the R388 polymorphism are more likely to develop metastatic diseases (66). This phenotype seems to correlate with higher levels of LPA1 receptor expression and migratory and invasive responses to LPA in tumors carrying R388 FGFR (65). In contrast, the G388 FGFR4 variant serves as a repressor of LPA1 receptor expression and LPA-dependent cellular effects (65).
Another interesting study suggests that LPA1 also downregulates the func-tion of the p53 tumor suppressor (67). Stimulation of lung carcinoma cell lines with LPA induces proteasomal degradation of p53 associated with inhibition of p53 nuclear accumulation and p53-dependent transcription. The effect of LPA on p53 could be also reproduced in hepatocellular carcinoma cells by overexpression of LPA receptors including LPA1 but not its inactive mutant form LPA1-R124A (67).
31.4. LPA2 IN CANCER
Different from LPA1, the LPA2 receptor does not seem to be a major mediator of cellular migratory response to LPA (68, 69). However, strong evidence indicates that expression of this receptor is most commonly increased in human malignancies. We previously examined mRNA expression of LPA1, LPA2, and LPA3 in normal ovarian epithelial cells, SV40 T antigen-immortalized ovarian epithelial cells, and ovarian cancer cell lines (70, 71). LPA2 and/or LPA3 mRNAs were highly expressed in ovarian cancer cell lines but were not present in normal ovarian epithelial cells (70, 71). The observation was later extended to the primary ovarian cancer samples directly obtained from patients. LPA2 is overexpressed in about 30% of ovarian cancers, whereas LPA3 is overexpressed in about 45% of ovarian cancers (70). Thus, a majority of ovarian cancers overexpress LPA2, LPA3, or both of them. These include early and late stages of ovarian cancer.
Many other groups have reported abnormal expression of the LPA2 recep-tor in various malignancies. For instance, overexpression of LPA2 mRNA was observed in differentiated thyroid cancer (72). In breast cancer, LPA2 overex-pression was observed in more than half (57%) of the most common invasive ductal carcinoma (73). Interestingly, LPA2 overexpression is more common in postmenopausal breast cancer patients (67%) than in premenopausal patients
666 LPA RECEPTORS IN CANCER
(45%) (73). LPA2 was also commonly overexpressed in gastric cancer (74). The increase was more prevalent in the intestinal type (67%) than in the diffuse type of gastric cancer (32%). Of particular interest, LPA2 overexpres-sion in the diffuse-type gastric cancer correlated with a higher rate of lym-phatic and venous invasion, lymphatic metastasis, and advanced tumor stages (74). In colorectal cancer, LPA2 mRNA was found to be consistently increased, while that of LPA1 was lower compared to the matched normal intestinal mucosa (55). So, the LPA2/LPA1 ratio was dramatically augmented in all 26 cancer specimens examined. These studies of breast, gastric, and colorectal cancers also showed increases in immunohistochemical staining of LPA2 protein in carcinoma cells compared to the surrounding normal epithelial tissues (55, 73, 74).
These observations on increased LPA2 expression in malignancies did not establish a cause–effect relationship for this receptor in oncogenesis. Further, these observations did not answer how LPA2 is upregulated mechanistically in cancer cells. A number of recent studies, however, have provided compelling evidence that LPA2 is causally linked to tumorigenesis in animals. Ectopic expression of LPA2 in ovarian cancer cell lines enhances their tumorigenicity and aggressiveness in subcutaneous and intraperitoneal xenograft models in nude mice (75). Although LPA1 and LPA3 also showed partial effects, LPA2 was consistently the most effective in these assays. More recently, Liu et al reported that transgenic expression of LPA receptors driven by the MMTV promoter leads to development of mammary tumors including estrogen receptor-positive and -negative breast carcinomas (76). After repeated breed-ing cycles, the female mice developed late-onset invasive mammary carcino-mas, which was preceded by chronic mastitis, hyperplasia, and intraepithelial neoplasia, suggesting that LPA signaling underlies these preneoplastic pro-cesses culminating in mammary tumorigenesis (76). Once again, LPA2 trans-genic mice showed a more aggressive tumorigenic phenotype compared to the LPA1 or LPA3 transgenic animals. The LPA2 transgenic model in the ovary has been also reported (77). The directed expression of LPA2 did not seem to be sufficient to induce ovarian tumorigenesis but resulted in overexpression of the oncogenic factors vascular endothelial growth factor (VEGF) and urokinase plasminogen activator (uPA) in the transgenic mice (77).
The most direct evidence to implicate LPA2 in cancer stems from the recent studies by Yun’s group using the Lpar2 null mice (78, 79). Mice lacking LPA2 do not show any physiological abnormalities (39). However, compared to wild-type mice, the LPA2-deficient mice were more resistant to intestinal tumor formation induced by colitis or by ApcMin mutation (78, 79). The mechanism for the oncogenic action of LPA2 remains elusive. Most studies have been focused on the ability of LPA to stimulate expression of oncogenic protein factors including IL-6, VEGF, HIF1a, c-Myc, cyclin D1, Krüppel-like factor 5,and Cox-2 (80–84). LPA2 seems to be more effective than other LPA recep-tors in driving the transcriptional effects of LPA on these LPA target genes. Furthermore, LPA2 contains a postsynaptic density 95, disks large, and zonula
DEREGULATION OF OTHER LPA RECEPTORS IN CANCER 667
occludens-1 (PDZ) binding motif at the carboxyl terminal end that enables interaction with multiple PDZ scaffold proteins (85, 86). Of particular interest is the Na+/H+ exchanger regulatory factor 2 (NHERF-2), which likely potenti-ates LPA2-dependent gene expression and other LPA-driven cellular pro-cesses (85–87). This is in sharp contrast to the LPA1 receptor where deletion of the PDZ domains enhances the receptor activity or makes it constitutively active (88). Several recent studies provide further evidence to support interac-tions of diverse PDZ scaffold proteins with the LPA2 PDZ binding domain, which regulate many aspects of LPA2 biological functions (87, 89, 90).
31.5. DEREGULATION OF OTHER LPA RECEPTORS IN CANCER
Different from LPA1 and LPA2, LPA3 is quite unique in that its expression is limited to a few normal tissues such as the uterus (40, 70). There is inconsistent information on its expression in cancer. For instance, it is overexpressed in ovarian cancer (70), prostate cancer (91), and superficial bladder cancer (53). However, the overexpression of LPA3 was not seen in many other types of cancer (Fig. 31.1). Recent studies in murine and human cancer cell lines suggest that LPA3 could be epigenetically silenced in tumor cells via hypermethylation of its promoter (92). There is also evidence that LPA3 inhibits LPA1-mediated tumor cell migration (93). Thus, a generalized role for LPA3 in cancer remains to be fully determined, although this receptor is capable of eliciting many cel-lular responses to LPA when exogenously overexpressed in mammalian cells.
Compared to the Edg LPA receptors, little is known about the biological roles in cancer of the novel subtype of LPA receptors LPA4–6. LPA4 mRNA is expressed in cells of both mesenchymal and epithelial origins (42). Expression profiling analyses have not led to any statistically meaningful conclusions about its expression in cancer (Fig. 31.1). Studies in culture, however, indicate that a majority of cancer cell lines lack significant expression of LPA4 mRNA when compared to the Edg LPA receptors (44, 94). This highlights the possibil-ity that LPA4 expression is generally repressed in malignant cells in contrast to the Edg LPA receptors. To determine whether LPA4 expression is sup-pressed in association with cell transformation, we transformed NIH 3T3 cells with activated Ras, Raf, or Mos oncogene. The oncogene-expressing clones showed transformed morphology and grew colonies in soft agar (data not shown). These Ras-, Raf-, or Mos-transformed clones expressed LPA4 mRNA at much reduced levels as revealed by Northern blotting (Fig. 31.2, left panel). The dramatic decrease in LPA4 in oncogene-transformed cells was readily reproduced by regular RT-PCR (Fig. 31.2, right panel). The transformation-linked downregulation of LPA4 was specific as expression of LPA1 or LPA2 was not compromised in Ras-, Raf-, or Mos-transformed cells.
GPR87, also known as GPR95 and recently proposed to be an additional LPA receptor (LPA7) (22, 34), is expressed at low levels in most tissues with the exception of the prostate, placenta, head, and neck (95). An interesting
668 LPA RECEPTORS IN CANCER
Figure 31.1. Expression of LPA1–6 receptors in human malignancies and their corre-sponding normal tissues. Oncomine™ (Compendia Bioscience, Ann Arbor, MI) was used for analysis and visualization of microarray data from various published research articles. Expression levels of the LPA1–6 receptors in the indicated normal and cancer tissues were depicted as heat map of their log2 median-centered intensities. The numbers of normal and tumor samples examined in each category were indicated in brackets. (A) breast versus invasive ductal breast carcinoma (51), (B) ovary versus ovarian serous adenocarcinoma (52), (C) bladder versus superficial bladder cancer (53), and (D) colon versus colon carcinoma (54). (See color insert.)
A
B
C
D
Ovary (4) Ovarian Serous Adenocarcinoma (41)
Bladder (48) Superficial Bladder Cancer (28)
Colon (10) Colon Carcinoma (5)
Invasive Ductal Breast Carcinoma (33)Breast (9)
observation with GPR87 is its overexpression specifically in squamous cell carcinomas of the lung, cervix, skin, urinary bladder, testis, head, and neck (95). Earlier studies demonstrated that GPR87 possesses a prosurvival activity in cancer cells (96). It was later found that DNA damage and genotoxic stress induce GPR87 expression in a p53-dependent fashion (95, 97). This induction is likely mediated through a p53-responsive element present in the GPR87 promoter. Other p53 family members, p63 and p73, also promote GPR87 expression (95). Strikingly, knockdown of GPR87 expression sensitizes cancer cells to cell killing by genotoxic drugs, indicating that GPR87 mediates cell viability effect downstream of p53 (95, 97). Thus, targeting of GPR87 offers an opportunity for enhancing drug efficacy in cancer, especially in squamous cell carcinoma. In spite of all these interesting observations, no information is available on whether these biological functions of GPR87 rely on its putative ligand LPA.
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31.6. POTENTIAL MUTATIONS OF LPA RECEPTORS IN CANCER
Although not common, mutations of GPCRs are associated with cancer development. For example, activating mutations in the thyroid-stimulating hormone receptor are found in 30% of human thyroid adenomas and some carcinomas (98). It has been long speculated that certain LPA receptors may be activated or inactivated through genetic mutation in cancer. Recent finding that P2Y5/LPAR6 is mutated and responsible for hypotrichosis simplex (29) has further stimulated the interest in the possible mutation and dysfunction of certain LPA receptors in neoplastic tissues. Recently, Obo et al. reported missense mutations of Lpar1 in more than 40% rat hepatocellular carcinomas induced by N-nitrosodiethylamine (DEN) or by a choline-deficient l-amino acid-defined (CDAA) diet (99). A similar frequency of Lpar1 missense muta-tions was detected in lung adenocarcinoma in rats induced by N-nitrosobis(2-hydroxypropyl)amine (BHP) (100). The biological consequences of these mutations are not understood. Interestingly, various human cancer cell lines also harbor mutant forms of LPAR1 or LPAR2 (92, 101). It will be of interest to determine whether such mutations are indeed present in primary human cancers.
31.7. ATX IN CANCER
In addition to the altered expression of LPA receptors, there is also evidence for amplifying LPA signaling in cancer through overexpression of the major
Figure 31.2. Downregulation of LPA4 expression in NIH 3T3 cells transformed by oncogenes. NIH 3T3 cells were transformed with activated Ras, Raf, or Mos oncogene. LPA4 mRNA expression in oncogene-transformed cell lines (Raf transformed clone Raf-1; Ras transformed clones Ras-1 and Ras-2; and Mos transformed clone Mos-1), vector control cells (vector), and parental NIH 3T3 cells (parental) was analyzed by Northern blotting (left panel) and confirmed by RT-PCR (right panel).
NIH 3T3 NIH 3T3
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670 LPA RECEPTORS IN CANCER
LPA-producing enzyme ATX. Serum ATX protein levels were elevated in patients with hematological malignancies including B-cell-derived follicle lym-phoma and Hodgkin’s lymphoma (102). The increases in ATX protein in these patients correlated with higher LPA levels in the plasma. It was not clear whether the increased ATX in these patients was made and secreted by tumor cells. Prostate cancer is a prototype model of stepwise progression of tumori-genesis. ATX was found to be absent in non-neoplastic prostate cells but elevated in a majority of high-grade intraepithelial neoplasia (103). There are also observations to support elevated expression of ATX in renal carcinoma (104), glioblastoma multiforme (94), poorly differentiated thyroid cancer (105), and non-small-cell lung carcinomas (106). Thus, ATX overexpression appears to be a common route to amplify LPA production and action in tumor microenvironments.
31.8. FUTURE PROSPECTS
Significant progresses have been made toward better understanding of the oncogenic actions of LPA. The field has been benefitted from knockout and transgenic animal models targeting LPA receptors or LPA production. Earlier studies of these genetic models lacking Lpar1, Lpar2, or even both of them did not provide direct evidence for LPA signaling in cancer (35, 39). More substantial studies of the susceptibility of these mice to carcinogenesis have clearly demonstrated that LPA signaling through its receptors, particularly the LPA2 receptor, contributes to oncogenic processes (78, 79). Overexpression of LPA receptors or ATX in transgenic models also supports the tumorigenic role of LPA (76). Therefore, LPA receptors and LPA metabolism are attractive novel anticancer targets. Several groups have developed a receptor subtype-specific antagonist of known LPA receptors with the most successful one, Ki16425, as a selective inhibitor for LPA1 and LPA3 (107). Due to the promi-nent role of LPA1 in the regulation of cell motility and tumor cell invasion, Ki16425 and other LPA1-specific blockers could be of potential application in the prevention of metastasis. Relatively speaking, limited progresses have been made in the identification and validation of LPA2-specific blockers. Among all LPA receptors, LPA2 has been found to be most consistently ele-vated in various human malignancies (55, 70–74). Therefore, effective and specific antagonists of LPA2 are highly desired. A number of potential inhibi-tors have been identified through high-throughput screenings (108). However, these compounds have not been thoroughly characterized and validated by independent groups as specific and potent antagonists for LPA2.
Further, alteration of LPA production or catabolism in the tumor microen-vironment may provide additional approaches to control LPA levels and LPA-associated tumor-promoting activities. A number of natural and synthetic compounds have been described to inhibit ATX activity and invasive pheno-
REFERENCES 671
types of tumor cells (109, 110). More recently, calcium-independent phospho-lipase A2 (iPLA2), an enzyme that could be potentially involved in LPA production from membrane phospholipids (9), is required for tumorigenicity of ovarian and prostate cancer cell lines in xenograft mouse models (111–113). Intriguingly, iPLA2 activity in both host animals and injected tumor cells is needed for tumorigenicity of ovarian cancer cell lines (112). If this is mediated through regulation of LPA production, iPLA2 could offer an alternative way for the intervention of LPA action and tumor growth.
ACKNOWLEDGMENTS
The work was supported by the NIH/NCI grant 2R01CA102196 (X.F.), the Massey Cancer Center pilot project grant (X.F.), and the NIH grant P30 CA16059 to the Massey Cancer Center of Virginia Commonwealth University.
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