Expression of Gab1 lacking the pleckstrin homology domain is associated with neoplastic progression

An in vitro transformation system of carcinogen-treated Syrian hamster embryo (SHE) cell cultures represents multistep genetic and nongenetic changes that develop during the neoplastic progression of normal cells to tumor cells in vivo. During this neoplastic progression, SHE cells demonstrate an altered response to epidermal growth factor (EGF). In the present report, we examined the role of the adapter protein Gab1 (Grb2-associated binder-1) in the neoplastic progression of SHE cells. We used two asbestos-transformed SHE cell clones in different neoplastic stages: a 10W+8 clone, which is immortal and retains the ability to suppress the tumorigenicity of tumor cells in cell-cell hybrid experiments, and a 10W-1 clone, which has lost this tumor suppressor ability. 10W+8 cells expressed full-length 100-kDa Gab1 and associated 5.2-kb mRNA. Upon repeated cell passaging, 10W-1 cells showed increasing expression of a novel 87-kDa form of Gab1 as well as 4.6-kb mRNA with diminishing expression of the original 100-kDa Gab1. cDNA encoding the 87-kDa Gab1 predicts a form of Gab1 lacking the amino-terminal 103 amino acids (Gab1(Delta1-103)), which corresponds to loss of most of the pleckstrin homology (PH) domain. Gab1(Delta1-103) retains the ability to be phosphorylated in an EGF-dependent manner and to associate with the EGF receptor and SHP-2 upon EGF stimulation. The endogenous expression of Gab1(Delta1-103) in 10W-1 cells appeared closely related to EGF-dependent colony formation in soft agar. Moreover, transfection and expression of Gab1(Delta1-103), but not Gab1, in 10W+8 cells enhanced their EGF-dependent colony formation in soft agar. These results demonstrate that Gab1 is a target of carcinogen-induced transformation of SHE cells and that the expression of a Gab1 variant lacking most of the PH domain plays a specific role in the neoplastic progression of SHE cells.

Identification of epidermal growth factor receptor- Grb2-associated binder-1-SHP-2 complex formation and its functional loss during neoplastic cell progression

The adaptor protein Grb2-associated binder-1 (Gab1) is known to bind to the SHP-2 tyrosine phosphatase on epidermal growth factor (EGF) receptor stimulation. To clarify the roles of these two proteins in EGF receptor (EGFR) signaling and determine their possible alteration during neoplastic cell progression, we studied these proteins in a Syrian hamster embryo (SHE) cell line model of neoplastic progression. Specifically, we used asbestos-transformed SHE fibroblasts: the 10W+8 clone, which is immortal but nontumorigenic; and the 10W2T clone, which is tumorigenic. Gab1 was detected, and the EGF-dependent formation of the EGFR-Gab1-SHP-2 complex was observed in 10W+8 cells. After cloning hamster Gab1 cDNA, exogenous expression of Gab1 significantly enhanced EGF-dependent mitogenic activity in 10W+8 cells. On the other hand, Gab1 was not detected in 10W2T cells, and the EGF-dependent association of SHP-2 with EGFR was also absent. Exogenous Gab1 expression in transfected 10W2T cells restored the EGF-dependent association of SHP-2 with EGFR, although it only showed a marginal effect on EGF-dependent mitogenic activity. Thus, Gab1 plays a pivotal role in the EGFR signaling pathway via the formation of the EGFR-Gab1-SHP-2 complex, and alteration in the expression and function of Gab1 is implicated in the neoplastic progression of SHE cells.

The linoleic acid metabolite, 13-HpODE augments the phosphorylation of EGF receptor and SHP-2 leading to their increased association

In previous studies with Syrian hamster embryo fibroblasts, we found that a specific lipoxygenase metabolite of linoleic acid, 13(S)-hydroperoxyoctadecadienoic acid (HpODE), enhanced epidermal growth factor (EGF) signal transduction in a tumor suppressor gene plus phenotype (supB+); with a diminished response to 13(S)-HpODE in a tumor suppressor gene minus phenotype (supB-). This differential response was attributed to differences in the rate of EGF receptor (EGFR) dephosphorylation. To further define the molecular basis for these observations, in this report we examine the interaction of phosphorylated EGFR with the SH2 domain-containing protein tyrosine phosphatase, SHP-2, a positive modulator of EGF dependent cell growth. SHP-2 associated with phosphorylated EGFR to a greater extent in supB+ cells when compared to supB-. This differential association could not be accounted for by differences between suppressor gene phenotypes in SHP-2 protein level or mutations in the molecular sequence. The addition of 13(S)-HpODE stimulated a concentration-dependent increase in EGF-dependent phosphorylation of SHP-2 and its association with EGFR. A more dramatic response was observed in the supB+ cells. Differences in SHP-2 interaction with EGFR may account, in part, for phenotypic differences in the growth rates and responsiveness to EGF between the supB+ and supB- cells. EGFR-SHP-2 association appears to play an important role in the regulation of EGFR signal transduction.

The linoleic acid metabolite, (13S)-hydroperoxyoctadecadienoic acid, augments the epidermal growth factor receptor signaling pathway by attenuation of receptor dephosphorylation. Differential response in Syrian hamster embryo tumor suppressor phenotypes

In Syrian hamster embryo (SHE) fibroblasts, epidermal growth factor receptor (EGFR) tyrosine kinase activity regulates the metabolism of endogenous linoleic acid to (13S)-hydroperoxyoctadecadienoic acid (13S)-HPODE). (13S)-HPODE stimulates EGF-dependent mitogenesis in a SHE cell phenotype, which expresses tumor suppressor genes (supB+), but was not effective in a variant that does not express these suppressor genes (supB-). In the present study, we have investigated the potential effects of this lipid metabolite on the EGFR signaling pathways in these two SHE cell lines. Treatment of quiescent SHE cells with EGF produced a rapid, transient increase in the tyrosine phosphorylation of EGFR. Dependence on EGF concentration for EGFR tyrosine phosphorylation was similar in both SHE cell lines, but a more prolonged phosphorylation was detected in the supB- variant. Incubation of supB+ cells with (13S)-HPODE and EGF increased EGFR autophosphorylation and tyrosine phosphorylation on several signaling proteins with Src homology-2 domains including GTPase-activating protein. The lipid metabolite did not significantly alter EGF-dependent tyrosine phosphorylation in the supB- variant. Tyrosine phosphorylation of mitogen-activated protein (MAP) kinase was also measured. The addition of (13S)-HPODE increased the extent and duration of MAP kinase tyrosine phosphorylation in supB+ cells but not in the supB- variant. MAP kinase activity in supB+ cells, as measured in immunoprecipitates from cells after the addition of EGF, was increased by the presence of (13S)-HPODE. The addition of (13S)-HPODE did not directly alter EGFR kinase activity or the internalization of the EGFR. However, the addition of (13S)-HPODE to supB+ cells extended the tyrosine phosphorylation of the EGFR in response to EGF. The dephosphorylation of the EGFR was measured directly, and a slower rate was observed in the supB- compared with the supB+ cells. Incubation of the supB+ cells with (13S)-HPODE attenuated the dephosphorylation of the EGFR. Thus, (13S)-HPODE stimulates EGF-dependent mitogenesis and up-regulation of EGF-dependent tyrosine phosphorylation by inhibiting the dephosphorylation of the EGFR. This study shows that a metabolite of an essential dietary fatty acid, linoleic acid, can modulate tyrosine phosphorylation and activity of key signal transduction proteins in a growth factor mitogenic pathway.

Epidermal growth factor-stimulated production of esterified 13(S)-hydroxyoctadecadienoic acid is associated with tumor suppressor phenotype in Syrian hamster embryo fibroblasts

Epidermal growth factor (EGF) stimulates the lipoxygenase metabolism of linoleic acid to 13(S)-hydroxyoctadecadienoic acid (HODE) in Syrian hamster embryo (SHE) fibroblasts. 13(S)-HODE is a potent and specific enhancer of EGF-dependent DNA synthesis in normal phenotypic SHE cells (supB+), but is inactive in variant SHE cells that have lost tumor suppressor gene function (supB-). EGF activation of quiescent SHE cells results in increased levels of 13-HODE esterified in cellular phospholipid and triglyceride. Steric analyses suggest that this metabolite is generated in part by direct oxygenation of membrane lipids by an n-6 lipoxygenase. In studies on the uptake and mobilization of 13-HODE in SHE cells, we observed EGF to stimulate a time- and dose-dependent incorporation and reacylation of the mono-hydroxy linoleate metabolite. The level of 13-HODE uptake in supB+ cells is twice that of supB-. Among classes of phospholipids, radiolabeled 13-HODE is esterified predominantly into phosphatidylcholine and this distribution pattern is similar for both SHE cell lines. Pretreatment of cells with the tyrosine kinase inhibitor methyl-2,5-dihydroxycinnamate blocks EGF-stimulated HODE incorporation. Inhibition of tyrosine phosphatase activity with vanadate potentiates HODE uptake in supB+ but not supB- cells. Moreover, activation of protein kinase C with phorbol ester stimulates HODE incorporation in the supB+ line only. The differential effects of EGF on 13-HODE uptake and mobilization in supB+ and supB- cells appear to be related to loss of the tumor suppressor phenotype. EGF-stimulated generation of esterified 13-HODE may be an important biological process in determining the mechanism and site of HODE interaction with the mitogenic signaling pathway.

Role of hydroperoxyeicosatetraenoic acids in oxidative stress-induced activating protein 1 (AP-1) activity

We have previously reported that hydrogen peroxide, an active oxygen species and a cellular oxidant, induces c-Fos and c-Jun mRNA expression and DNA synthesis in vascular smooth muscle cells and that these events require arachidonic acid release and metabolism through the lipoxygenase pathway. Here we have identified the eicosanoids that mediate the hydrogen peroxide-induced growth-related events in these cells. Hydrogen peroxide stimulated the production of 12- and 15-hydroperoxyeicosatetraenoic acids in vascular smooth muscle cells. Both 12- and 15-hydroperoxyeicosatetraenoic acids induced the expression of c-Fos and c-Jun protein and increased activating protein 1 (AP-1) activity, as measured by AP-1-DNA binding and AP-1-dependent human collagenase promoter-driven chloramphenicol acetyltransferase reporter gene transcription. Hydrogen peroxide and arachidonic acid also induced the expression of c-Fos and c-Jun protein and AP-1 activity. Nordihydroguaiaretic acid, an inhibitor of the lipoxygenase pathway, significantly inhibited both hydrogen peroxide and arachidonic acid-stimulated c-Fos and c-Jun protein expression and AP-1 activity. Together, these findings suggest that hydrogen peroxide induces the production of eicosanoids and that the eicosanoids are potential mediators of the oxidative stress-stimulated growth-related events in vascular smooth muscle cells.

Regulation of 13(S)-hydroxyoctadecadienoic acid biosynthesis in Syrian hamster embryo fibroblasts by the epidermal growth factor receptor tyrosine kinase

Metabolism of arachidonic and linoleic acid can be regulated by polypeptide growth factors in a variety of cell types. In Syrian hamster embryo (SHE) fibroblasts, epidermal growth factor (EGF) stimulates the conversion of exogenous linoleic acid to 13(S)-hydroxyoctadecadienoic acid (HODE). Inhibition of 13-HODE biosynthesis blocks the EGF-mitogenic response in SHE cells, and 13-HODE and its hydroperoxy precursor are potent and highly specific enhancers of EGF-dependent DNA synthesis. We demonstrated that EGF stimulates a biphasic production and release of endogenous 13-HODE. Through development of a stable isotope-dilution GC/MS assay for 13-HODE, we observed 13-HODE production as early as 5 min after EGF stimulation, and this initial phase peaked at 1 hr. A second rise in 13-HODE formation was seen at 2-4 hr, and this phase plateaued at 4-6 hr at a level of 30-40 ng/10(6) cells. EGF stimulation of 13-HODE biosynthesis is not mediated by transcriptional or translational regulation of the inducible form of prostaglandin H synthase. Based on enzyme inhibitor studies and structural characterization of products, the linoleate metabolite is apparently formed by an n-6 lipoxygenase that remains to be characterized. EGF stimulation of 13-HODE formation is linked with activation of the EGF receptor tyrosine kinase. Inhibition of EGF receptor tyrosine kinase activity with methyl-2,5-dihydroxycinnamate blocked EGF-dependent linoleic acid metabolism and EGF-regulated DNA synthesis. Potentiation of the EGF receptor tyrosine phosphorylation cascade through treatment of SHE cells with the tyrosine phosphatase inhibitor vanadate resulted in a 3-fold increase in EGF-stimulated 13-HODE production and a corresponding enhancement of the EGF mitogenic response. The coupling of EGF-regulated linoleic acid metabolism with the EGF receptor tyrosine kinase activity suggests the importance of specific linoleate compounds in mediating mitogenic signal transduction.

Regulation of the adhesion of a human breast carcinoma cell line to type IV collagen and vitronectin: roles for lipoxygenase and protein kinase C

We have investigated the regulation of adhesion of metastatic human breast carcinoma cells to various protein substrates in the presence or absence of the protein kinase C (PKC) activator, 12-tetradecanoyl phorbol 13-acetate (TPA) or calcium ionophore A23187 (A23187). Both TPA and A23187 dramatically enhanced MDA-MB-435 cell adhesion to type IV collagen (collagen IV), vitronectin, and, to some extent, fibronectin and laminin. Adhesion to BSA and polylysine were not affected. TPA and A23187 induced substantial dose-dependent effects that were apparent after 30- and 60-min incubations, respectively, whereas a phorbol ester, which does not activate PKC, had no effect. A23187, but not TPA, induced a release of arachidonic acid (AA) from MDA-MB-435 cells. Nordihydroguaiaretic acid, a lipoxygenase inhibitor, prevented A23187 and exogenous AA, but not TPA, from stimulating cell adhesion to collagen IV. In contrast, the increase in adhesion to vitronectin induced by A23187 and AA was, at best, only partially inhibited by nordihydroguaiaretic acid treatment. Calphostin C, a PKC inhibitor, blocked the stimulation of adhesion by A23187, exogenous AA, and TPA to both collagen IV and vitronectin. Together, these results suggest that calcium mobilization activates the release of AA and its metabolism through a lipoxygenase pathway leading to a rapid increase of MDA-MB-435 cell adhesion to collagen IV, whereas other mechanisms regulate adhesion to vitronectin. Finally, PKC activation, occurring downstream from calcium mobilization or the AA effects, is a key event involved in the regulation of adhesion to both proteins.

Human immunodeficiency virus type 1 envelope protein does not stimulate either prostaglandin formation or the expression of prostaglandin H synthase in THP-1 human monocytes/macrophages

Prostaglandin E2 is observed at elevated levels during human immunodeficiency virus (HIV) infection and thus may contribute to the HIV-dependent immunosuppression. The mechanisms responsible for this increase are not understood. Evidence indicates that the viral envelope proteins perturb membrane signaling mediated by the CD4 receptor, suggesting that the free envelope protein and/or the intact virus may be responsible for the increase in prostaglandin E2 levels. In this study, we have used THP-1 human monocytes and THP-1 cells differentiated by 12-O-tetradecanoylphorbol-13-acetate treatment into macrophages to determine if the HIV envelope protein, gp120, or an anti-CD4 receptor antibody stimulates prostaglandin formation by interacting with the CD4 receptor. Incubation of THP-1 cells with OKT4A antibody greatly stimulated the CD4-p56lck receptor complex as estimated by enhanced p56lck autophosphorylation, while the gp120 gave small but significant responses. Monocytic THP-1 cells poorly metabolized arachidonic acid to prostaglandin E2 and thromboxane B2 as measured by high-pressure liquid chromatography analysis. Western blot (immunoblot) and Northern (RNA) blot analyses revealed that unstimulated monocytes expressed little prostaglandin H synthase 1 and 2 (PGHS-1 and -2). Incubation of the monocytes with lipopolysaccharide, OKT4A, or gp120 did not increase the formation of prostaglandins. The expression of PGHS-1 or PGHS-2 was also not increased. Differentiation of the monocytes to macrophages by 12-O-tetradecanoylphorbol-13-acetate treatment resulted in increased expression of PGHS-1 and increased formation of prostaglandins compared with that for the monocytes. Lipopolysaccharide stimulation of the macrophages increased the formation of prostaglandins and increased the expression of PGHS-2 in the macrophages. However, OKT4A or gp120 preparation, at concentrations that stimulated p56lck autophosphorylation, did not enhance the formation of prostaglandins or the expression of PGHS-1 or PGHS-2. OKT4A and gp120 also did not stimulate the release of arachidonic acid, indicating that phospholipase A2 was not activated by the CD4 receptor in either the THP-1 monocytes or macrophages. These results indicate that activation of the CD4-p56lck receptor signal transduction pathway by the HIV envelope protein does not increase prostaglandin formation.

Prostaglandin H synthase-2 is induced in Syrian hamster embryo cells in response to basic fibroblast growth factor

Metabolites of arachidonic acid and linoleic acid can serve as regulators of the epidermal growth factor signal transduction system in Syrian hamster embryo (SHE) fibroblasts. We have now investigated the possible role of these lipids in modulating the signal transduction of basic fibroblast growth factor (bFGF), a potent mitogen to SHE fibroblasts. The addition of bFGF (0.1 to 1.0 ng/ml) to serum-deprived SHE cells stimulated a six- to sevenfold increase in the incorporation of thymidine into DNA. Structural analysis indicated that bFGF stimulated the metabolism of exogenous and endogenous arachidonic acid to primarily PGE2, PGF2 alpha, and PGD2, with lesser amounts of uncharacterized prostaglandins observed. The metabolism of linoleic acid in SHE cells was not affected by bFGF. bFGF stimulated the expression of the inducible form of prostaglandin H synthase (PGHS-2) as determined by Northern analysis using murine PGHS-2 cDNA as the probe. PGHS-2 protein in the SHE cells was also increased by bFGF as determined by Western analysis using antibodies specific for PGHS-2. Levels of the constitutive (PGHS-1) enzyme and mRNA were not altered by bFGF. Preincubation of the cells with 1-2 microM dexamethasone significantly inhibited bFGF-stimulated expression of PGHS-2 protein and mRNA. Dexamethasone potently inhibited bFGF induced mitogenesis in these cells. Pretreatment of SHE cells with indomethacin inhibited bFGF-dependent mitogenesis, as well as endogenously produced PGE2. The data suggests that regulation of PGHS-2 expression may be an element of the bFGF mitogenic signal transduction pathway.

Activation of mitogen-activated protein kinases by arachidonic acid and its metabolites in vascular smooth muscle cells

Previous studies from this laboratory and others suggest that arachidonic acid and its metabolites play important roles in a variety of biological processes such as signal transduction, contraction, chemotaxis, and cell growth and differentiation. Here we studied the effect of arachidonic acid on mitogen-activated protein (MAP) kinases in vascular smooth muscle cells (VSMC). Arachidonic acid activated MAP kinases in VSMC in a time- and dose-dependent manner. Nordihydroguaiaretic acid (NDGA), a potent inhibitor of the lipoxygenase system, significantly blocked the arachidonic acid-induced activation of MAP kinases, whereas indomethacin, an inhibitor of cyclooxygenase, had no effect. In VSMC, arachidonic acid was converted to 15-hydroxyeicosatetraenoic acid (15-HETE); NDGA inhibited the formation of this HETE. Exogenous addition of 15-HETE to VSMC caused stimulation of MAP kinases. Depletion of protein kinase C attenuated both the arachidonic acid- and 15-HETE-induced activation of MAP kinases in VSMC. Together these results suggest that 1) arachidonic acid activates MAP kinases in VSMC; 2) 15-HETE, a 15-lipoxygenase product of arachidonic acid, at least in part, mediates the arachidonic acid effect on MAP kinases; and 3) protein kinase C appears to be important in arachidonic acid activation of MAP kinases. Therefore, MAP kinases may play an important role in arachidonic acid signaling of VSMC growth and function.

Cellular proliferation and lipid metabolism: importance of lipoxygenases in modulating epidermal growth factor-dependent mitogenesis

In this article we have reviewed and discussed the results of our investigation of lipid metabolites as modulators of epidermal growth factor (EGF) signaling pathways. We have studied epidermal growth factor-dependent mitogenesis in BALB/c 3T3 and Syrian hamster embryo (SHE) cells in culture. We observed that EGF stimulates the formation of prostaglandins in BALB/c 3T3 cells and their formation appears to be necessary for EGF dependent mitogenesis. EGF did not stimulate PGE2 formation in SHE cells and in fact, exogenously added PGE2 inhibited mitogenesis. In both cell lines, EGF stimulated the formation of lipoxygenase-derived 13(S)-hydroxyoctadecadienoic acid (13-HODE) and inhibition of 13-HODE formation attenuated mitogenesis. The addition of 13-(S)-HODE enhanced EGF-dependent mitogenesis but when added alone, the compound was not mitogenic. Other metabolites, including lipoxygenase metabolites of arachidonic acid, were either weak simulators of EGF-dependent mitogenesis or essentially inactive. The 13(S)-HODE appears to be formed by an apparently unique lipoxygenase that is regulated by the tyrosine kinase activity of the EGF receptor. The mechanisms by which lipids, particularly the lipoxygenase-derived linoleic acid metabolites, modulate the EGF signaling pathways leading to cell proliferation is discussed. The possible significance of lipoxygenase and prostaglandin H synthase-dependent metabolism of unsaturated fatty acids in breast and colon cancer is also discussed.

Structure-activity relationship for potentiation of EGF-dependent mitogenesis by oxygenated metabolites of linoleic acid

Epidermal growth factor induces the oxygenation of linoleic acid in Syrian hamster embryo fibroblasts, and the lipoxygenase-derived products potentiate the mitogenic signal. We have further characterized the linoleate metabolites of growth factor-activated cells by chiral phase HPLC analysis. The primary product was identified as the pure (S) enantiomer of 13-hydroxyoctadecadienoic acid (HODE). In comparison to 13(R)-HODE isomer, only the biologically derived 13(S)-HODE was active in augmenting DNA synthesis as assessed by [3H]thymidine incorporation. To extend these investigations, we defined the structural requirements of analogous lipid compounds necessary for stimulation of mitogenesis in these cells. Carbon-chain length, degree of unsaturation, type of oxidized functionality, position of oxygenated moiety, double-bond geometry, and chirality were all identified as factors that modulate the mitogenic activity of related compounds. The results demonstrate a high degree of specificity for (S)-isomer hydro(pero)xylinoleic acid metabolites in stimulating DNA synthesis and further define the relationship between linoleic acid metabolism and growth-factor-dependent cell growth.

Multigenic evasion of inflammation by poxviruses

Analyses of different cowpox virus (Brighton Red strain [CPV-BR]) mutants indicate that there is a minimum of three genes encoded by CPV-BR that are nonessential for virus replication in tissue culture but are involved in inhibiting the generation of an inflammatory response in the chicken embryo chorioallantoic membrane (CAM) model. The CPV-BR-encoded anti-inflammatory genes include the gene encoding the 38-kDa protein (also called 38K, crmA, SPI-2, or VV-WR-ORF-B13R), a tumor necrosis factor receptor homolog, and an unidentified gene that maps to the right end of the CPV genome. The kinetics of triggering of an inflammatory response at the site of virus infection as well as the magnitude of the response is dependent on the virus-encoded inhibitor that is deleted. Virus yields recovered from pocks decreased in proportion to the magnitude of the inflammatory response. The deletion of these identified inhibitors of inflammation was associated with attenuation of the mutant viruses in mice. These data confirm the existence of multiple poxvirus-encoded host defense modifiers whose function is to block the generation of an inflammatory response at the site of virus infection, which allows enhanced virus replication and potentially facilitates virus transmission.

Investigation of the oxygenation of phospholipids by the porcine leukocyte and human platelet arachidonate 12-lipoxygenases

When arachidonate 12-lipoxygenase purified from porcine leukocytes was incubated aerobically with 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine, the phospholipid reacted at up to 30% of the rate of a free fatty acid substrate; the esterified arachidonic acid was oxygenated predominantly to the (12S)-12-hydroperoxy product. The porcine leukocyte enzyme was also capable of metabolizing phosphatidylcholine containing esterified (15S)-15-hydroperoxy-5,8,11,13-eicosatetraenoic acid; oxygenation occurred predominantly at the 14R position. Reaction with mitochondrial and endoplasmic membranes of rat liver produced esterified (12S)-12-hydroperoxy-5,8,10,14-eicosatetraenoic acid and (13S)-13-hydroperoxy-9,11-octadecadienoic acid as major oxygenation products. Thus, porcine leukocyte 12-lipoxygenase is capable of oxygenating not only free polyenoic fatty acids but also more complex substrates such as phospholipids and biomembranes. In contrast, the human platelet 12-lipoxygenase is almost inactive with these esterified polyenoic fatty acids. In regard to the function of these enzymes, the leukocyte-type of 12-lipoxygenase has similar catalytic activities to the mammalian 15-lipoxygenase and its physiological function may include the structural modification of membrane lipids.

Poxvirus-induced alteration of arachidonate metabolism

Recent evidence suggests that orthopoxviruses have an obligate requirement for arachidonic acid metabolites during replication in vivo and in vitro. Our report indicates that a virus family (Poxviridae) possesses multiple genes that function to regulate arachidonate metabolism. Analyses of BS-C-1 cells infected with cowpox virus or vaccinia virus detected enhanced arachidonate product formation from both the cyclooxygenase (specifically prostaglandins E2 and F2 alpha) and lipoxygenase (specifically 15-hydroxyeicosatetraenoic acid and 12-hydroxyeicosatetraenoic acid) pathways. In contrast, human parainfluenza type 3 or herpes simplex virus type 1 infections did not increase arachidonate metabolism. Results were consistent with a virus early-gene product either directly mediating or inducing a host factor that mediated the up-regulation of arachidonate metabolism, although vaccinia growth factor was not responsible. In addition, the cowpox virus 38-kDa protein-encoding gene, which is associated with inhibition of an inflammatory response, correlated with inhibition of formation of a product biochemically characteristic of (14R,15S)-dihydroxyeicosatetraenoic acid. We propose that orthopoxvirus-induced up-regulation of arachidonic acid metabolism during infection renders the infected cells susceptible to generation of inflammatory mediators from both the cyclooxygenase and the lipoxygenase pathways, and poxviruses, therefore, possess at least one gene (38K) that can alter the lipoxygenase-metabolite spectrum.

Modulation of the epidermal growth factor mitogenic response by metabolites of linoleic and arachidonic acid in Syrian hamster embryo fibroblasts. Differential effects in tumor suppressor gene (+) and (-) phenotypes

Specific metabolites of arachidonic and linoleic acid have been proposed as serving a regulatory function in growth factor signal transduction in fibroblasts. In studies with Syrian hamster embryo (SHE) fibroblasts, we found lipoxygenase inhibitors to be potent blockers of epidermal growth factor (EGF)-dependent mitogenesis. Analytical chemical characterization of arachidonic and linoleic acid metabolism in SHE cells demonstrated that the major lipoxygenase product was 13-hydroxyoctadecadienoic acid (HODE). EGF stimulation of quiescent SHE cells resulted in an enhancement of HODE biosynthesis. The primary arachidonate products were prostaglandin E2 and F2 alpha formed via the cyclooxygenase pathway. Inhibition of cyclooxygenase activity did not alter the EGF-mitogenic response in SHE cells. Addition of lipoxygenase-derived linoleate metabolites (10(-10)-10(-6) M) produced a 2-4-fold potentiation of EGF-stimulated [3H]thymidine incorporation in SHE cells. Interestingly, the linoleate products did not enhance the EGF mitogenic effect in variant SHE cells that had lost tumor suppressor gene function. These results were confirmed by autoradiographic studies of DNA synthesis and suggest that loss of tumor suppressor phenotype correlates with a lack of responsiveness to linoleate products in signal transduction. In studies on the mechanism of EGF regulation of linoleic acid metabolism, inhibitors of EGF receptor tyrosine kinase activity were observed to block EGF-stimulated HODE biosynthesis. In addition, both cyclohexamide and actinomycin D attenuated the ability of EGF to increase linoleic acid metabolism in SHE cells. EGF induction of the linoleate pathway appears to be linked to activation of the EGF receptor and may be modulated at transcriptional or translational levels.

Platelet-activating factor provokes release of mucin-like glycoproteins from guinea pig respiratory epithelial cells via a lipoxygenase-dependent mechanism

Primary cultures of guinea pig tracheal epithelial cells maintained in an air/liquid interface system that maintains differentiated characteristics were grown to near confluence and exposed for 1 h to platelet-activating factor (PAF) on both apical and basal sides. PAF provoked release of high-molecular-weight mucin-like glycoproteins (MLG) from the cells, with maximal stimulation occurring at 10(-8) and 10(-9) M. The inactive form of PAF, lyso-PAF, was without effect. Indomethacin, the cyclooxygenase inhibitor, did not affect secretion stimulated by PAF, but nordihydroguiaretic acid (NDGA), a mixed cyclooxygenase and lipoxygenase inhibitor, attenuated secretion stimulated by PAF in a concentration-dependent manner. High performance liquid chromatography assay of the culture medium after addition of PAF revealed increased production of 15-, 12-, and 5-hydroxyeicosatetraenoic acids (15-, 12-, and 5-HETEs). The stimulatory effect of PAF on both mucin secretion and formation of HETEs was inhibited by the PAF receptor antagonists, CV-3988 and Ro 19 3704, with Ro 19 3704 acting at a concentration 10-fold lower than CV-3988 in inhibiting both effects. When added exogenously to the cell cultures, the combination of 5-, 12-, and 15-HETEs stimulated MLG release in a concentration-dependent manner. The results suggest that PAF stimulates release of MLG by guinea pig airway epithelium in vitro by a mechanism involving binding of PAF to receptors on epithelial cell surfaces, stimulation of lipoxygenase metabolism of arachidonic acid to HETEs within the epithelium, and stimulation of secretion by these epithelial-derived HETEs via an autocrine or paracrine mechanism.

Studies on the reduction of endogenously generated prostaglandin G2 by prostaglandin H synthase

Prostaglandin H synthase oxidizes arachidonic acid to prostaglandin G2 (PGG2) via its cyclooxygenase activity and reduces PGG2 to prostaglandin H2 by its peroxidase activity. The purpose of this study was to determine if endogenously generated PGG2 is the preferred substrate for the peroxidase compared with exogenous PGG2. Arachidonic acid and varying concentrations of exogenous PGG2 were incubated with ram seminal vesicle microsomes or purified prostaglandin H synthase in the presence of the reducing cosubstrate, aminopyrine. The formation of the aminopyrine cation free radical (AP.+) served as an index of peroxide reduction. The simultaneous addition of PGG2 with arachidonic acid did not alter cyclooxygenase activity of ram seminal vesicle microsomes or the formation of the AP.+. This suggests that the formation of AP.+, catalyzed by the peroxidase, was supported by endogenous endoperoxide formed from arachidonic acid oxidation rather than by the reduction of exogenous PGG2. In addition to the AP.+ assay, the reduction of exogenous versus endogenous PGG2 was studied by using [5,6,8,9,11,12,14,15-2H]arachidonic acid and unlabeled PGG2 as substrates, with gas chromatography-mass spectrometry techniques to measure the amount of reduction of endogenous versus exogenous PGG2. Two distinct results were observed. With ram seminal vesicle microsomes, little reduction of exogenous PGG2 was observed even under conditions in which all of the endogenous PGG2 was reduced. In contrast, studies with purified prostaglandin H synthase showed complete reduction of both exogenous and endogenous PGG2 using similar experimental conditions. Our findings indicate that PGG2 formed by the oxidation of arachidonic acid by prostaglandin H synthase in microsomal membranes is reduced preferentially by prostaglandin H synthase.

Epidermal growth factor stimulates linoleic acid metabolism in BALB/c 3T3 fibroblasts

One response of BALB/c 3T3 cells to epidermal growth factor (EGF) is the release and subsequent metabolism of arachidonic acid. Prostaglandins generated from EGF treatment appear to play a role in the mitogenic signal. Lipoxygenase inhibitors (nordihydroguaiaretic acid and 5,8,11,14-eicosatetraynoic acid) were previously shown to be very effective in blocking EGF-stimulated DNA synthesis; however, only low levels of lipoxygenase-derived arachidonate metabolites were detected. In an extension of these investigations, we have now found that EGF stimulates lipoxygenase metabolites of linoleic acid in BALB/c 3T3 fibroblasts. In the presence of EGF (10 ng/ml), the cells converted 10-15% of exogenous linoleic acid (10 microM) to hydroxy fatty acids that were isolated on reverse phase high performance liquid chromatography. No linoleate metabolites were detected in the absence of EGF. The isolated compounds were characterized further by straight phase high performance liquid chromatography, UV spectroscopy, and gas chromatography-mass spectrometry analyses, and they were identified as 13-hydroxyoctadecadienoic acid and 9-hydroxyoctadecadienoic acid. The hydroxy metabolites and their hydroperoxy precursors produced a 2- to 4-fold potentiation of EGF-stimulated [3H]thymidine incorporation in BALB/c 3T3 cells. These linoleate derivatives stimulated DNA synthesis at concentration ranges of 10(-8) to 10(-6) M. Thus, linoleic acid metabolism might be an important element in the EGF-regulated cascade of biochemical events leading to fibroblast mitogenesis.

A short-chain aldehyde is a major lipoxygenase product in arachidonic acid-stimulated porcine leukocytes

Porcine leukocytes convert exogenous arachidonic acid to a complex array of products derived via the 5-, 12-, and 15-lipoxygenase pathways of metabolism. The major monohydroxylated metabolite following addition of 100 microM arachidonic acid is 12-hydroxyeicosatetraenoic acid. Of the more polar compounds on reverse-phase high pressure liquid chromatography, the most prominent is a previously uncharacterized arachidonate product which chromatographs near to the omega-oxidized metabolites of leukotriene B4. The structure of this new product was examined by high pressure liquid chromatography, UV, NMR, and also by gas chromatography-mass spectrometry of several derivatives; it was identified as 12-oxododeca-5,8,10-(Z,Z,E)-trienoic acid. It is proposed that this C-12 trienal acid is formed from 12-hydroperoxyeicosatetraenoic acid by a cleavage reaction catalyzed by the leukocyte 12-lipoxygenase in the presence of excess arachidonic acid and under anaerobic conditions. These conditions are satisfied by addition of 100 microM arachidonic acid to the leukocyte suspension (3 X 10(7) cells/ml); 12-hydroperoxyeicosatetraenoic acid is formed as the major product, excess arachidonic acid is available, and the concomitant leukocyte respiratory burst quickly depletes the solution of oxygen. Preliminary experiments indicate that this aldehyde product has significant biological activity in the activation of leukocytes. In the course of an intense inflammatory reaction it is conceivable that the conditions for synthesis of this C-12 trienal acid and related aldehydes could prevail; such aldehydes would constitute an additional class of lipoxygenase product which exacerbates the process of inflammation.