Lipid phosphate phosphatase-1 in the regulation of lysophosphatidate signaling

Targeting Lysophosphatidic Acid in Cancer: The Issues in Moving from Bench to Bedside

Since the clear demonstration of lysophosphatidic acid (LPA)'s pathological roles in cancer in the mid-1990s, more than 1000 papers relating LPA to various types of cancer were published. Through these studies, LPA was established as a target for cancer. Although LPA-related inhibitors entered clinical trials for fibrosis, the concept of targeting LPA is yet to be moved to clinical cancer treatment. The major challenges that we are facing in moving LPA application from bench to bedside include the intrinsic and complicated metabolic, functional, and signaling properties of LPA, as well as technical issues, which are discussed in this review. Potential strategies and perspectives to improve the translational progress are suggested. Despite these challenges, we are optimistic that LPA blockage, particularly in combination with other agents, is on the horizon to be incorporated into clinical applications.

Lysophosphatidic acid counteracts glucagon-induced hepatocyte glucose production via STAT3

Hepatic glucose production (HGP) is required to maintain normoglycemia during fasting. Glucagon is the primary hormone responsible for increasing HGP; however, there are many additional hormone and metabolic factors that influence glucagon sensitivity. In this study we report that the bioactive lipid lysophosphatidic acid (LPA) regulates hepatocyte glucose production by antagonizing glucagon-induced expression of the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK). Treatment of primary hepatocytes with exogenous LPA blunted glucagon-induced PEPCK expression and glucose production. Similarly, knockout mice lacking the LPA-degrading enzyme phospholipid phosphate phosphatase type 1 (PLPP1) had a 2-fold increase in endogenous LPA levels, reduced PEPCK levels during fasting, and decreased hepatic gluconeogenesis in response to a pyruvate challenge. Mechanistically, LPA antagonized glucagon-mediated inhibition of STAT3, a transcriptional repressor of PEPCK. Importantly, LPA did not blunt glucagon-stimulated glucose production or PEPCK expression in hepatocytes lacking STAT3. These data identify a novel role for PLPP1 activity and hepatocyte LPA levels in glucagon sensitivity via a mechanism involving STAT3.

Enhanced apoptosis through farnesol inhibition of phospholipase D signal transduction

Farnesol is a catabolite of the cholesterol biosynthetic pathway that preferentially causes apoptosis in tumorigenic cells. Phosphatidylcholine (PC), phosphatidic acid (PA), and diacylglycerol (DAG) were able to prevent induction of apoptosis by farnesol. Primary alcohol inhibition of PC catabolism by phospholipase D augmented farnesol-induced apoptosis. Exogenous PC was unable to prevent the increase in farnesol-induced apoptosis by primary alcohols, whereas DAG was protective. Farnesol-mediated apoptosis was prevented by transformation with a plasmid coding for the PA phosphatase LPP3, but not by an inactive LPP3 point mutant. Farnesol did not directly inhibit LPP3 PA phosphatase enzyme activity in an in vitro mixed micelle assay. We propose that farnesol inhibits the action of a DAG pool generated by phospholipase D signal transduction that normally activates an antiapoptotic/pro-proliferative target.

Delta-catenin/NPRAP (neural plakophilin-related armadillo repeat protein) interacts with and activates sphingosine kinase 1

Sphingosine kinase (SPHK) is a key enzyme catalysing the formation of sphingosine 1-phosphate (SPP), a lipid messenger that is implicated in the regulation of a wide variety of important cellular events acting through intracellular, as well as extracellular, mechanisms. However, the molecular mechanism of intracellular actions of SPP remains unclear. Here, we have identified delta-catenin/NPRAP (neural plakophilin-related armadillo repeat protein) as a potential binding partner for SPHK1 by yeast two-hybrid screening. From co-immunoprecipitation analyses, the C-terminal portion of delta-catenin/NPRAP containing the seventh to tenth armadillo repeats was found to be required for interaction with SPHK1. Endogenous delta-catenin/NPRAP was co-localized with endogenous SPHK1 and transfected delta-catenin/NPRAP was co-localized with transfected SPHK1 in dissociated rat hippocampal neurons. MDCK (Madin-Darby canine kidney) cells stably expressing delta-catenin/NPRAP contained elevated levels of intracellular SPP. In a purified system delta-catenin/NPRAP stimulated SPHK1 in a dose-dependent manner. Furthermore, delta-catenin/NPRAP-induced increased cell motility in MDCK cells was completely inhibited by dimethylsphingosine, a specific inhibitor of SPHK1. These results strongly suggest that at least some of delta-catenin/NPRAP functions, including increased cell motility, are mediated by an SPHK-SPP signalling pathway.

A lysophosphatidic acid analogue is revealed as a potent inhibitor of phosphatidylcholine synthesis, inducing apoptosis

A previous study demonstrated that cross-desensitization experiments performed with the lysophosphatidic acid (LPA) analogues (R)- and (S)-N-palmitoyl-norleucinol 1-phosphate (PNPAs) inhibited LPA-induced platelet aggregation without any stereospecificity. Here we report opposite biological effects of the two enantiomers on mitogenesis of IMR-90 fibroblasts in relation to their respective metabolism. (R)PNPA was proliferative, while (S)PNPA induced apoptosis by specifically inhibiting phosphatidylcholine biosynthesis at the last step of the CDP-choline pathway controlled by cholinephosphotransferase. This effect was not direct but required dephosphorylation of PNPAs by ecto-lipid phosphate phosphatase before cellular uptake of the generated N-palmitoyl-norleucinols (PNOHs). Inhibition of cholinephosphotransferase by the derivative (S)PNOH was confirmed by an in vitro assay. (S)PNPA proapoptotic effects led us to clarify the mechanism linking cholinephosphotransferase inhibition to apoptosis. Three proapoptotic responses were observed: the activation of caspase-3, the production of ceramides from newly synthesized pools (as demonstrated by the inhibitor Fumonisin B1) and finally the activation of stress-activated protein kinase, p38 and c-Jun N-terminal kinases 1/2, as a result of ceramide increase. Thus our data demonstrate that synthetic analogues of LPA might display stereospecific effects leading to apoptosis independently of classical LPA-activated pathways.

Role for 18:1 Lysophosphatidic Acid as an Autocrine Mediator in Prostate Cancer Cells

Lysophosphatidic acid (LPA) is a lipid mediator that may play an important role in growth and survival of carcinomas. In this study, LPA production and response were characterized in two human prostate cancer (CaP) cell lines: PC-3 and Du145. Bombesin, a neuroendocrine peptide that is mitogenic for CaP cells, stimulated focal adhesion kinase phosphorylation and activated the extracellular signal-regulated kinase/mitogen-activated protein kinase pathway. Similar responses were elicited by 18:1 LPA (oleoyl-LPA). Studies using radioisotopic labeling revealed that both PC-3 and Du145 generate LPA and that LPA production is increased by bombesin. The kinetics of bombesin-induced phospholipase D activation and LPA production were similar. Using electrospray ionization mass spectrometry, 18:1 LPA was found to be an abundant LPA species in CaP cell medium. Structure activity studies of acyl-LPAs revealed that 18:1 LPA is most efficacious for activation of extracellular signal-regulated kinase and phospholipase D in CaP cells. Incubation with 18:1 LPA caused homologous desensitization of LPA response, whereas bombesin caused heterologous desensitization. LPA was present at nanomolar levels in medium from bombesin-treated cells. LPA extracted from the medium induced calcium mobilization in CaP cells. These results demonstrate that bioactive LPA is generated by CaP cells in response to a mitogen and suggest that 18:1 LPA can act as an autocrine mediator.

Expression of ectolipid phosphate phosphohydrolases in 3T3F442A preadipocytes and adipocytes. Involvement in the control of lysophosphatidic acid production

Because of its production by adipocytes and its ability to increase preadipocyte proliferation, lysophosphatidic acid (LPA) could participate in the paracrine control of adipose tissue development. The aim of the present study was to determine which enzyme activities are involved in exogenous LPA hydrolysis by preadipocytes and adipocytes. Using a quantitative method, we observed that extracellular LPA rapidly disappeared from the culture medium of 3T3F442A preadipocytes. This disappearance was strongly slowed down in the presence of the phosphatase inhibitors, sodium vanadate and sodium pervanadate. By using [(33)P]LPA on intact 3T3F442A preadipocytes, we found that 90% of LPA hydrolysis resulted from LPA phosphatase activity biochemically related to previously described ectolipid phosphate phosphohydrolases (LPPs). Quantitative real time reverse transcriptase-PCR revealed that 3T3F442A preadipocytes expressed mRNAs of three known Lpp gene subtypes (1, 2, and 3), with a predominant expression of Lpp1 and Lpp3. Differentiation of 3T3F442A preadipocytes into adipocytes led to an 80% reduction in ecto-LPA phosphatase activity, with a concomitant down-regulation in Lpp1, Lpp2, and Lpp3 mRNA expression. Despite this regulation, treatment of 3T3F442A adipocytes with sodium vanadate increased LPA production in the culture medium, suggesting the involvement of ecto-LPA phosphatase activity in the control of extracellular production of LPA by adipocytes. In conclusion, these data demonstrate that hydrolysis of extracellular LPA by preadipocytes and adipocytes mainly results from a dephosphorylation activity. This activity (i) occurs at the extracellular face of cell membrane, (ii) exhibits biochemical characteristics similar to those of the LPP, (iii) is negatively regulated during adipocyte differentiation, and (iv) plays an important role in the control of extracellular LPA production by adipocytes. Ecto-LPA phosphatase activity represents a potential target to control adipose tissue development.

Molecular cloning and expression of pulmonary lipid phosphate phosphohydrolases

Pulmonary lipid phosphate phosphohydrolase (LPP) was shown previously to hydrolyze phosphatidic acid and lysophosphatidic acid in purified rat lung plasma membranes. To better investigate the nature of pulmonary LPP isoforms and their role in the lung, LPPs were cloned by RT-PCR from both adult rat lung and type II cell RNA. The RT-PCR generated LPP1 (849 bp), up to three LPP1 variants, and LPP3 (936 bp) cDNAs. The three LPP1 variants include LPP1a (852 bp) and two novel isoforms, LPP1b (697 bp) and LPP1c (1004 bp). The pulmonary LPP1 and LPP3 isoforms are essentially identical to the previously cloned rat liver and intestinal LPPs, respectively, and the LPP1a isoform has 80% sequence identity to the human homolog. The LPP2 isoform was not detected in lung by RT-PCR. Northern analyses revealed that the mRNAs for LPP1 and LPP3 increase in fetal rat lung in late gestation to day 1 after birth. These mRNAs decrease somewhat during the neonatal period but increase slightly during postnatal development. Expression of LPP1, LPP1a, and LPP3 cDNAs in HEK 293 cells established that they encode functional LPP. In contrast, the novel isoforms LPP1b and LPP1c contain frameshifts that would result in premature termination, producing putative catalytically inactive polypeptides of 30 and 76 amino acids, respectively. Further investigation of the LPP1b isoform revealed that it was present across a variety of tissues, although at lower levels than LPP1/1a. Transient mammalian expression of LPP1b failed to increase phosphatidate phosphohydrolase activity in HEK 293 cells.

Pulmonary lipid phosphate phosphohydrolase in plasma membrane signalling platforms

Lipid phosphate phosphohydrolase (LPP) has recently been proposed to have roles in signal transduction, acting sequentially to phospholipase D (PLD) and in attenuating the effects of phospholipid growth factors on cellular proliferation. In this study, LPP activity is reported to be enriched in lipid-rich signalling platforms isolated from rat lung tissue, isolated rat type II cells and type II cell-mouse lung epithelial cell lines (MLE12 and MLE15). Lung and cell line caveolin-enriched domains (CEDs), prepared on the basis of their detergent-insolubility in Triton X-100, contain caveolin-1 and protein kinase C isoforms. The LPP3 isoform was predominantly localized to rat lung CEDs. These lipid-rich domains, including those from isolated rat type II cells, were enriched both in phosphatidylcholine plus sphingomyelin (PC+SM) and cholesterol. Saponin treatment of MLE15 cells shifted the LPP activity, cholesterol, PC+SM and caveolin-1 from lipid microdomains to detergent-soluble fractions. Elevated LPP activity and LPP1/1a protein are present in caveolae from MLE15 cells prepared using the cationic-colloidal-silica method. In contrast, total plasma membranes had a higher abundance of LPP1/1a protein with low LPP activity. Phorbol ester treatment caused a 3.8-fold increase in LPP specific activity in MLE12 CEDs. Thus the activated form of LPP1/1a may be recruited into caveolae/rafts. Transdifferentiation of type II cells into a type I-like cell demonstrated enrichment in caveolin-1 levels and LPP activity. These results indicate that LPP is localized in caveolae and/or rafts in lung tissue, isolated type II cells and type II cell lines and is consistent with a role for LPP in both caveolae/raft signalling and caveolar dynamics.

Expression of a catalytically inactive sphingosine kinase mutant blocks agonist-induced sphingosine kinase activation. A dominant-negative sphingosine kinase

Sphingosine kinase (SK) catalyzes the formation of sphingosine 1-phosphate (S1P), a lipid messenger that plays an important role in a variety of mammalian cell processes, including inhibition of apoptosis and stimulation of cell proliferation. Basal levels of S1P in cells are generally low but can increase rapidly when cells are exposed to various agonists through rapid and transient activation of SK activity. To date, elucidation of the exact signaling pathways affected by these elevated S1P levels has relied on the use of SK inhibitors that are known to have direct effects on other enzymes in the cell. Furthermore, these inhibitors block basal SK activity, which is thought to have a housekeeping function in the cell. To produce a specific inhibitor of SK activation we sought to generate a catalytically inactive, dominant-negative SK. This was accomplished by site-directed mutagenesis of Gly(82) to Asp of the human SK, a residue identified through sequence similarity to the putative catalytic domain of diacylglycerol kinase. This mutant had no detectable SK activity when expressed at high levels in HEK293T cells. Activation of endogenous SK activity by tumor necrosis factor-alpha (TNFalpha), interleukin-1beta, and phorbol esters in HEK293T cells was blocked by expression of this inactive sphingosine kinase (hSK(G82D)). Basal SK activity was unaffected by expression of hSK(G82D). Expression of hSK(G82D) had no effect on TNFalpha-induced activation of protein kinase C and sphingomyelinase activities. Thus, hSK(G82D) acts as a specific dominant-negative SK to block SK activation. This discovery provides a powerful tool for the elucidation of the exact signaling pathways affected by elevated S1P levels following SK activation. To this end we have employed the dominant-negative SK to demonstrate that TNFalpha activation of extracellular signal-regulated kinases 1 and 2 (ERK1,2) is dependent on SK activation.