Inhibitors of intracellular phosphatidic acid production: novel therapeutics with broad clinical applications

Simultaneous estimation of lisofylline and pentoxifylline in rat plasma by high performance liquid chromatography-photodiode array detector and its application to pharmacokinetics in rat

Lisofylline (LSF) is an anti-inflammatory and immunomodulatory agent with proven activity in serious infections associated with cancer chemotherapy, hyperoxia-induced acute lung injury, autoimmune disorders including type-1 diabetes (T1DM) and islet rejection after islet transplantation. It is also an active metabolite of another anti-inflammatory agent, Pentoxifylline (PTX). LSF bears immense therapeutic potential in multiple pharmacological activities and hence appropriate and accurate quantification of LSF is very important. Although a number of analytical methods for quantification of LSF and PTX have been reported for pharmacokinetics and metabolic studies, each of these have certain limitations in terms of large sample volume required, complex extraction procedure and/or use of highly sophisticated instruments like LC-MS/MS. The aim of current study is to develop a simple reversed-phase HPLC method in rat plasma for simultaneous determination of LSF and PTX with the major objective of ensuring minimum sample volume, ease of extraction, economy of analysis, selectivity and avoiding use of instruments like LC-MS/MS to ensure a widespread application of the method. A simple liquid-liquid extraction method using methylene chloride as extracting solvent was used for extracting LSF and PTX from rat plasma (200μL). Samples were then evaporated, reconstituted with mobile phase and injected into HPLC coupled with photo-diode detector (PDA). LSF, PTX and 3-isobutyl 1-methyl xanthine (IBMX, internal standard) were separated on Inertsil® ODS (C18) column (250×4.6mm, 5μm) with mobile phase consisting of A-methanol B-water (50:50v/v) run in isocratic mode at flow rate of 1mL/min for 15min and detection at 273nm. The method showed linearity in the concentration range of 50-5000ng/mL with LOD of 10ng/mL and LLOQ of 50ng/mL for both LSF and PTX. Weighted linear regression analysis was also performed on the calibration data. The mean absolute recoveries were found to be 80.47±3.44 and 80.89±3.73% for LSF and PTX respectively. The method was successfully applied for studying the pharmacokinetics of LSF and PTX after IV bolus administration at dose of 25mg/kg in Wistar rat. In conclusion, a simple, sensitive, accurate and precise reversed-phase HPLC-UV method was established for simultaneous determination of LSF and PTX in rat plasma.

Lisofylline, a novel anti-inflammatory agent, enhances glucose-stimulated insulin secretion in vivo and in vitro: studies in prediabetic and normal rats

Previous studies from this laboratory have shown that the novel anti-inflammatory agent, lisofylline (LSF), improves oral glucose tolerance in streptozotocin (STZ) diabetic rats. Subsequent studies suggested that the improved glucose tolerance could be the result of enhanced beta-cell functioning. The possibility that LSF enhancement of insulin release in these animals is the result of direct effects of this agent on a residual population of functionally normal beta cells was further evaluated in these studies. In vivo studies: 6- to 8-week-old male rats were administered STZ (35 mg/kg body weight) intravenously. After 10 days, LSF administration (25 mg/kg body weight, twice daily) was initiated in the treated group (n = 11) for comparison with the vehicle-injected controls (n = 10). Body weight, food intake, and serum glucose and insulin levels were monitored weekly. Glucose and insulin responses to an oral glucose bolus were measured at 4 to 5 weeks as an index of LSF effects on impaired glucose tolerance. Glucose areas under curve (AUC) during the 2-hour tolerance tests in the LSF-treated rats (n = 11) were 23,390 +/- 253 versus 29,390 +/- 1,006 mg/dL x min (P <.0001) in the vehicle-injected rats (n = 10). Improved glucose tolerance was associated with increases in blood insulin levels in the LSF-treated rats, AUC (+LSF) = 6,564 +/- 66 versus 5,127 +/- 633 microU/mL x min in the vehicle-injected STZ-rats (not significant [NS]). These observations suggested that the improved glucose tolerance is the result of direct effects of LSF on glucose-induced release of insulin.

IN VITRO STUDIES

the validity of this hypothesis was subsequently tested using isolated perfused pancreas preparations from normal rats. In this series of experiments, 12-week-old animals were used, and pancreases were perfused in situ using single-pass technique. Three levels of LSF were directly infused into individual pancreas preparations and included 20 (n = 5), 40 (n = 4), and 60 (n = 4) micromol/L. First (minutes 3 to 10) and second (minutes 13 to 35) phase glucose-stimulated (300 mg/dL) insulin response areas (AUC) for the 2 phases measured in the LSF-infused pancreases were compared with AUC in vehicle-infused pancreases (n = 4). At LSF concentrations of 20 and 40 micromol/L, total insulin released during the first phase of glucose stimulation was more than twice that of the controls (3,919 +/- 739 and 3,643 +/- 630 microU, respectively v 1,481 +/- 269 microU, P <.03). A total of 60 micromol/L LSF did not significantly enhance first phase glucose-induced insulin secretion. Second phase comparisons of total insulin released in the LSF-infused versus the controls showed differences of comparable magnitude (about 2-fold) with statistical significance (P <.03) observed at all 3 levels of LSF. These findings demonstrate that LSF enhances glucose-stimulated insulin release in vitro. Enhanced beta-cell functioning by LSF likely represents an important factor underlying improved glucose tolerance in vivo. In addition, the in vitro observations in normal rat pancreas indicate that the LSF effect is not limited to beta-cell dysfunction per se. These results support the conclusion that agents, such as LSF, may have therapeutic benefits in type 2 diabetes.

ATP-independent fatty acyl-coenzyme A synthesis from phospholipid: coenzyme A-dependent transacylation activity toward lysophosphatidic acid catalyzed by acyl-coenzyme A:lysophosphatidic acid acyltransferase

CoA-dependent transacylation activity in microsomes is known to catalyze the transfer of fatty acids between phospholipids and lysophospholipids in the presence of CoA without the generation of free fatty acids. We previously found a novel acyl-CoA synthetic pathway, ATP-independent acyl-CoA synthesis from phospholipids. We proposed that: 1) the ATP-independent acyl-CoA synthesis is due to the reverse reaction of acyl-CoA:lysophospholipid acyltransferases and 2) the reverse and forward reactions of acyltransferases can combine to form a CoA-dependent transacylation system. To test these proposals, we examined whether or not recombinant mouse acyl-CoA:1-acyl-sn-glycero-3-phosphate (lysophosphatidic acid, LPA) acyltransferase (LPAAT) could catalyze ATP-independent acyl-CoA synthetic activity and CoA-dependent transacylation activity. ATP-independent acyl-CoA synthesis was indeed found in the membrane fraction from Escherichia coli cells expressing mouse LPAAT, whereas negligible activity was observed in mock-transfected cells. Phosphatidic acid (PA), but not free fatty acids, served as an acyl donor for the reaction, and LPA was formed from PA in a CoA-dependent manner during acyl-CoA synthesis. These results indicate that the ATP-independent acyl-CoA synthesis was due to the reverse reaction of LPAAT. In addition, bacterial membranes containing LPAAT catalyzed CoA-dependent acylation of LPA; PA but not free fatty acid served as an acyl donor. These results indicate that the CoA-dependent transacylation of LPA consists of 1) acyl-CoA synthesis from PA through the reverse action of LPAAT and 2) the transfer of the fatty acyl moiety of the newly formed acyl-CoA to LPA through the forward reaction of LPAAT.

Lymphoid/nonlymphoid compartmentalization of donor leukocyte chimerism in rat recipients of heart allografts, with or without adjunct bone marrow

BACKGROUND

The role of leukocyte migration and chimerism in organ allograft acceptance has been obscured by the lack of information about the late localization of the donor cells.

METHODS

Male Lewis rat-->female Brown Norway abdominal heart transplantation was performed under tacrolimus immunosuppression (days 0-13, 20, and 27) with or without donor bone marrow and (in bone marrow subgroups) a 1-week postoperative course of a possibly chimerism-enhancing drug. Using rat sex-determining region-Y-specific oligonucleotide primers, we determined the donor DNA concentration by polymerase chain reaction in serial venous blood samples for 100 days and in tissue specimens when animals were killed.

RESULTS

Chimerism was detected out to 56 days in 89% of the blood samples but in none of the samples at 100 days. However, donor DNA was detected when animals were killed in 95% of the native hearts, 80% of the skin biopsy specimens, and 23% of the spleens. The presence and quantity of early and late chimerism were strongly correlated the administration of adjunct bone marrow and with a reduction in the vasculopathy and inflammation index in the cardiac allografts. Marginally significant further increases in chimerism and/or reductions in chronic heart rejection beyond those achieved with adjunct bone marrow alone were associated with additional treatment with the growth factors Flt-3 ligand, granulocyte colony-stimulating factor, and a recombinant molecular variant of interleukin-6 (interleukin-6 mutein) but not with hepatocyte growth factor or lisofylline.

CONCLUSIONS

The previously suspected shift of early chimerism in the blood and lymphoid organs to dominance in host nonlymphoid tissues is consistent with the dual mechanisms of clonal exhaustion and immune indifference, governed by antigen migration and localization, that have been postulated elsewhere to account for organ allograft acceptance.

Characterization of a human lysophosphatidic acid acyltransferase that is encoded by a gene located in the class III region of the human major histocompatibility complex

Sequence analysis of cDNA clones corresponding to a number of genes located in the class III region of the human major histocompatibility complex (MHC), in the chromosome band 6p21.3, has shown that the G15 gene encodes a 283-amino acid polypeptide with significant homology over the entire polypeptide with the enzyme lysophosphatidic acid acyltransferase (LPAAT) from different yeast, plant, and bacterial species. The amino acid sequence of the MHC-encoded human LPAAT (hLPAATalpha) is 48% identical to the recently described hLPAAT (Eberhardt, C., Gray, P. W., and Tjoelker, L. W. (1997) J. Biol. Chem. 272, 20299-20305), which is encoded by a gene located on chromosome 9p34.3. LPAAT is the enzyme that in lipid metabolism converts lysophosphatidic acid (LPA) into phosphatidic acid (PA). The expression of the hLPAATalpha polypeptide in the baculovirus system and in mammalian cells has shown that it is an intracellular protein that contains LPAAT activity. Cell extracts from insect cells overexpressing hLPAATalpha were analyzed in different LPAAT enzymatic assays using, as substrates, different acyl acceptors and acyl donors. These cell extracts were found to contain up to 5-fold more LPAAT activity compared with control cell extracts, indicating that the hLPAATalpha specifically converts LPA into PA, incorporating different acyl-CoAs with different affinities. The hLPAATalpha polypeptide expressed in the mammalian Chinese hamster ovary cell line was found, by confocal immunofluorescence, to be localized in the endoplasmic reticulum. Due to the known role of LPA and PA in intracellular signaling and inflammation, the hLPAATalpha gene represents a candidate gene for some MHC-associated diseases.

Suppression of proliferation and phosphorylation of Jak3 and STAT5 in malignant T-cell lymphoma cells by derivatives of octylamino-undecyl-dimethylxanthine

IL-2R signal transduction involves tyrosine phosphorylation of several proteins including Jak3 and STAT5. In the present study we examined the effect of two octylamino-undecyl-dimethylxanthine (OUDMX) derivatives, designated CT2576 and CT5589, on proliferation and protein tyrosine phosphorylation in human malignant T-cell lymphoma lines. These T-cell lines (PB-1, 2A, and 2B), obtained from a progressive T-cell lymphoma involving skin, are IL-2 independent but have constitutively activated IL-2R-associated signal transduction pathway common to IL-2 and several other cytokines: IL-4, IL-7, IL-9, and IL-15. CT2576, characterized previously on the functional level as an inhibitor of IL-2 signaling and, on the biochemical level, as an inhibitor of phosphatidic acid biosynthesis, suppressed completely growth of the malignant T cell lymphoma lines. CT5589 which is a novel analog of the CT2576, displayed a similar, although weaker, effect. Furthermore, both CT compounds inhibited constitutive tyrosine phosphorylation of two proteins: Jak3 and STAT5 which are key downstream elements in the signal transduction pathway activated by IL-2 and the other cytokines. The CT compounds inhibited also Jak3 phosphorylation induced by IL-2 in the IL-2 dependent SZ-4 cells. Inhibition of phosphorylation by CT2576 and CT5589 was only partially selective since phosphorylation of several other proteins was also affected. Phosphorylation of many others was, however, unaffected. These findings demonstrate that the OUDMX derivatives suppress proliferation of malignant T lymphocytes. Furthermore, they suggest that this suppression may be mediated by inhibition of the IL-2R-associated Jak/STAT signaling pathway. A potential role for OUDMX derivatives in therapy of human T-cell lymphoma should be further explored.

Modulating phosphatidic acid metabolism decreases oxidative injury in rat lungs

We determined that lisofylline, a potent inhibitor of oleate- and linoleate-containing phosphatidic acid formation (half-maximal inhibitory concentration = 40 nM), prevented oxidant-mediated capillary leak in isolated rat lungs given interleukin-8 (IL-8) intratracheally and perfused with human neutrophils. Lung leak was prevented by lung, but not neutrophil, lisofylline pretreatment. Furthermore, although lisofylline inhibited IL-8-stimulated neutrophil production of phosphatidic acid in vitro, it did not prevent IL-8-stimulated neutrophil adherence, chemotaxis, or intracellular calcium mobilization or N-formyl-Met-Leu-Phe (fMLP)-stimulated oxidant production in vitro. Lisofylline also prevented acute capillary leak in isolated rat lungs perfused only with the oxidant generator purine-xanthine oxidase but did not scavenge O2-(+) or H2O2 in vitro. Finally, lisofylline-mediated protection against lung leak in both models was associated with alterations in lung membrane free fatty acid acyl composition (as reflected by the decreased ratio [linoleate + oleate]/[palmitate]). We conclude that lisofylline prevented both neutrophil-dependent and neutrophil-independent oxidant-induced capillary leak in isolated rat lungs and that protection appears to be mediated by blocking intrinsic lung linoleoyl phosphatidic acid metabolism. We speculate that lisofylline, in addition to our previously reported effects on cytokine signaling by intrapulmonary mononuclear cells, alters intrinsic pulmonary capillary membrane composition and renders this barrier less vulnerable to oxidative damage.

Effect in supralethally irradiated rats of granulocyte colony-stimulating factor and lisofylline on hematopoietic reconstitution by syngeneic bone marrow or whole organ passenger leukocytes

We have previously shown the existence of migratory hematopoietic stem cells in adult solid organs. This study demonstrates that granulocyte colony-stimulating factor (G-CSF) and lisofylline, a phosphatidic acid inhibitor that suppresses hematopoiesis-inhibiting cytokines, can enhance the engraftment of organ-based hematopoietic stem cells. When syngeneic heart grafts or liver nonparenchymal cells were transplanted into lethally irradiated (9.5 Gy) Lewis rats, complete hematopoietic reconstitution and animal survival were significantly improved by treating the recipient with G-CSF or, to a lesser extent, with lisofylline. Pretreatment of hepatic nonparenchymal cell donors with G-CSF, but not lisofylline, also resulted in striking improvement of recipient survival which was associated with an augmented subpopulation of donor stem cells. The results suggest that these drugs can be used to enhance the chimerism that we postulate to be the basis of organ allograft acceptance.

Endogenous natural killer enhancing factor-B increases cellular resistance to oxidative stresses

Natural killer-enhancing factor (NKEF) was identified and cloned on the basis of its ability to increase NK cytotoxicity. Two genes, NKEF-A and -B, encode NKEF proteins and sequence analysis presented suggests that each belongs to a highly conserved family of antioxidants. To examine the antioxidant potential of NKEF, we transfected the coding region of NKEF-B cDNA into the human endothelial cell line ECV304. The stable transfectant, B/1, was found to overexpress NKEF-B gene transcript and protein. We subjected B/1 to oxidative stress by either culturing them with glucose oxidase (GO), which continuously generates hydrogen peroxide, or by direct addition of hydrogen peroxide. We found that B/1 cells were more resistant than control cell lines. Resistance to hydrogen peroxide was originally thought to be mediated mainly by catalase and the glutathione cycle. Therefore, we used inhibitors to block the two pathways and found that B/1 cells were more resistant to oxidative stress than control cells when we used inhibitors to preblock either pathway. We also examined the cellular inflammatory responses to oxidized low-density lipoprotein (LDL) and bacterial lipopolysaccharide (LPS) by measuring monocyte adhesion to endothelial cells in vitro and found that B/1 cells were resistant to such responses. Lastly, we found that B/1 cells were more resistant to a novel chemotherapeutic agent CT-2584, which appears to kill tumor cells by stimulating production of reactive oxygen intermediates in mitochondria. These results demonstrate that the NKEF-B is an antioxidant that protects cells from oxidative stress, chemotherapy agents, and inflammation-induced monocyte adhesion. Furthermore, its expression may mediate cellular responses to proinflammatory molecules.

The lost chord: microchimerism and allograft survival

Recent evidence suggests that passenger leukocytes migrate after organ transplantation and produce persistent chimerism, which is essential for sustained survival of the allografts. Here, Thomas Starzl and colleagues argure that this hematolymphopoietic chimerism provides an important framework for the interpretation of basic and therapeutically oriented transplantataion research.

Phospholipid regulation of a cyclic AMP-specific phosphodiesterase (PDE4) from U937 cells

The regulation of phosphodiesterase-4 (PDE4) by various phospholipids was explored using PDE4s partially purified from U937 cells. Preincubation (5 min, 4 degrees C) of the large molecular weight PDE4 denoted "Peak 2 PDE4" with mixed phosphatidic acids (PAs) produced a 2-fold increase in its Vmax without changing its Km (approximately 2 microM) for cyclic AMP. This "activation" was not limited to PAs with specific fatty acid substituents: Synthetic PAs containing saturated and/or unsaturated fatty acids 16-20 carbons long produced similar effects. Lysophosphatidic acids (LPAs) and phosphatidylserines (PSs) also induced PDE4 activation, whereas phosphatidylcholines (PCs), phosphatidylethanolamines (PEs) and diacylglycerol did not. Antibodies to a peptide region near the PDE4 catalytic site specifically inhibited PA-induced activation. The data demonstrate that anionic phospholipids can act as non-essential activators of a leukocyte PDE4, and suggest biochemical crosstalk between phospholipid-dependent and cyclic AMP-dependent signalling pathways in human leukocytes.