Identification of a bipartite nuclear localization sequence necessary for nuclear import of 5-lipoxygenase

CF Patients’ Airway Epithelium and Sex Contribute to Biosynthesis Defects of Pro-Resolving Lipids

Specialized pro-resolving lipid mediators (SPMs) as lipoxins (LX), resolvins (Rv), protectins (PD) and maresins (MaR) promote the resolution of inflammation. We and others previously reported reduced levels of LXA4 in bronchoalveolar lavages from cystic fibrosis (CF) patients. Here, we investigated the role of CF airway epithelium in SPMs biosynthesis, and we evaluated its sex specificity. Human nasal epithelial cells (hNEC) were obtained from women and men with or without CF. Lipids were quantified by mass spectrometry in the culture medium of hNEC grown at air-liquid interface and the expression level and localization of the main enzymes of SPMs biosynthesis were assessed. The 5-HETE, LXA4, LXB4, RvD2, RvD5, PD1 and RvE3 levels were significantly lower in samples derived from CF patients compared with non-CF subjects. Within CF samples, the 12-HETE, 15-HETE, RvD3, RvD4, 17-HODHE and PD1 were significantly lower in samples derived from females. While the mean expression levels of 15-LO, 5-LO and 12-LO do not significantly differ either between CF and non-CF or between female and male samples, the SPMs content correlates with the level of expression of several enzymes involved in SPMs metabolism. In addition, the 5-LO localization significantly differed from cytoplasmic in non-CF to nucleic (or nuclear envelope) in CF hNEC. Our studies provided evidence for lower abilities of airway epithelial cells derived from CF patients and more markedly, females to produce SPMs. These data are consistent with a contribution of CF airway epithelium in the abnormal resolution of inflammation and with worse pulmonary outcomes in women.

Lack of 5-lipoxygenase in intramembranous and endochondral 129Sv mice skeleton and intramembranous healing

OBJECTIVE

To analyze the lack of 5-lipoxygenase (5LO) on dental socket healing and post-natal phenotype of intramembranous and endochondral bones.

DESIGN

Wild type (WT) 129/SvEv (n = 20) and 5LO knockout (5LOKO) (n = 20) male mice underwent tooth extraction of the upper right incisor and were euthanized after 7, 14, and 30 day time points for the evaluation of dental socket healing and histological phenotyping of intramembranous (IM) and endochondral (EC) bones. Microscopic analysis of alveolar sockets included histopathological description, histomorphometry, and immunohistochemistry for 5LO, cyclooxygenase 2 (COX2), and tartrate resistant acid phosphatase (TRAP).

RESULTS

Histological phenotyping revealed thicker cortical bone in EC bones (femur and vertebra) of 5LOKO mice compared to WTs, with no differences in collagenous content. Although dental socket healing was similarly observed in both groups, WT mice revealed increased numbers of COX-2+ and 5LO+ cells during bone maturing stage, with a decrease of TRAP+ cells at day 30. On the other hand, an increased quantity of fibroblasts was observed at day 7 in 5LOKO group, as well as increased inflammatory infiltrate and significantly decreased TRAP+ cells at final stages of alveolar socket healing in comparison to WTs.

CONCLUSIONS

The lack of 5LO in 5LOKO mice resulted in thicker cortical of EC, but not of IM post natal bones. Furthermore, genetic deletion of 5LO in the 5LOKO mice directly affected the inflammatory response during socket healing, influencing initial and late phases of bone repair in a model of post-tooth extraction in 129Sv WT and 5LOKO mice.

A new treatment for severe pulmonary arterial hypertension based on an old idea: inhibition of 5-lipoxygenase

It has been generally accepted that severe forms of pulmonary arterial hypertension are associated with inflammation. Plasma levels in patients with severe pulmonary arterial hypertension show elevated levels of interleukins and mediators of inflammation and histologically the diseased small pulmonary arterioles show infiltrates of inflammatory and immune cells. Here, we review the literature that connects pulmonary hypertension with the arachidonic acid/5-lipoxygenase-derived leukotriens. This mostly preclinical background data together with the availability of 5-lipoxygenase inhibitors and leukotriene receptor blockers provide the rationale for testing the hypothesis that 5-lipoxygenase products contribute to the pathobiology of severe pulmonary arterial hypertension in a subgroup of patients.

The role of the LTB4-BLT1 axis in chemotactic gradient sensing and directed leukocyte migration

Directed leukocyte migration is a hallmark of inflammatory immune responses. Leukotrienes are derived from arachidonic acid and represent a class of potent lipid mediators of leukocyte migration. In this review, we summarize the essential steps leading to the production of LTB₄ in leukocytes. We discuss the recent findings on the exosomal packaging and transport of LTB₄ in the context of chemotactic gradients formation and regulation of leukocyte recruitment. We also discuss the dynamic roles of the LTB₄ receptors, BLT1 and BLT2, in mediating chemotactic signaling in leukocytes and contrast them to other structurally related leukotrienes that bind to distinct GPCRs. Finally, we highlight the specific roles of the LTB₄-BLT1 axis in mediating signal-relay between chemotaxing neutrophils and its potential contribution to a wide variety of inflammatory conditions including tumor progression and metastasis, where LTB₄ is emerging as a key signaling component.

Phosphorylation of serine 271 on 5-lipoxygenase and its role in nuclear export

The enzyme 5-lipoxygenase (5-LO) initiates the biosynthesis of leukotrienes, inflammatory mediators involved in immune diseases and defense. The subcellular localization of 5-LO is regulated, with nuclear import commonly leading to increased leukotriene production. We report here that 5-LO is constitutively phosphorylated on Ser-271 in transfected NIH 3T3 cells. This residue is nested in a classical nuclear export sequence, and phosphorylated Ser-271 5-LO was exclusively found in the nucleus by immunofluorescence and by fractionation techniques. Mutation of Ser-271 to Ala allowed nuclear export of 5-LO that was blocked by the specific nuclear export inhibitor leptomycin b, suggesting that phosphorylation of Ser-271 serves to interfere with exportin-1-mediated nuclear export. Consistent with previous reports that purified 5-LO can be phosphorylated on Ser-271 in vitro by MAPK-activated protein kinase 2, the nuclear export of 5-LO was increased by either treatment with the p38 inhibitor SB 203,580 or co-expression of a kinase-deficient p38 MAPK. Nuclear export of 5-LO can also be induced by KN-93, an inhibitor of Ca2+/calmodulin-dependent kinase II, and the effects of SB 203,580 plus KN-93 are additive. Finally, HeLa cells, which lack nuclear 5-LO, also lack constitutive phosphorylation of Ser-271. Taken together, these results indicate that the phosphorylation of Ser-271 serves to inhibit the nuclear export of 5-LO. This action works in concert with nuclear import, which is regulated by phosphorylation on Ser-523, to determine the subcellular distribution of 5-LO, which in turn regulates leukotriene biosynthesis.

Regulation of pro-inflammatory responses by lipoxygenases via intracellular reactive oxygen species in vitro and in vivo

Reactive oxygen species (ROS) performs a pivotal function as a signaling mediator in receptor-mediated signaling. However, the sources of ROS in this signaling have yet to be determined, but may include lipoxygenases (LOXs) and NADPH oxidase. The stimulation of lymphoid cells with TNF-alpha, IL-1beta, and LPS resulted in significant ROS production and NF-kappaB activation. Intriguingly, these responses were markedly abolished via treatment with the LOXs inhibitor nordihydroguaiaretic acid (NDGA). We further examined in vivo anti-inflammatory effects of NDGA in allergic airway inflammation. Both intraperitoneal and intravenous NDGA administration attenuated ovalbumin (OVA)-induced influx into the lungs of total leukocytes, as well as IL-4, IL-5, IL-13, and TNF-alpha levels. NDGA also significantly reduced serum levels of OVA-specific IgE and suppressed OVA-induced airway hyperresponsiveness to inhaled methacholine. The results of our histological studies and flow cytometric analyses showed that NDGA inhibits OVA-induced lung inflammation and the infiltration of CD11b+ macrophages into the lung. Collectively, our findings indicate that LOXs performs an essential function in pro-inflammatory signaling via the regulation of ROS regulation, and also that the inhibition of LOXs activity may have therapeutic potential with regard to the treatment of allergic airway inflammation.

5-Lipoxygenase: Regulation and possible involvement in atherosclerosis

This review article focuses on two aspects regarding 5-lipoxygenase. First, mechanisms for activation of the enzyme. Second, the involvement of 5-lipoxygenase and leukotrienes in atherosclerosis.

Hydroxyeicosatetraenoic acids released through the cytochrome P-450 pathway regulate 3T6 fibroblast growth

Eicosanoids participate in the regulation of cellular proliferation. Thus, we observed that prostaglandin E(2) interaction with membrane receptors is involved in the control of 3T6 fibroblast growth induced by serum. However, our results suggested that another arachidonic acid pathway might be implicated in these events. Our results show that 3T6 fibroblasts synthesized hydroxyeicosatetraenoic acids (HETEs) such as 12-HETE through the cytochrome P-450 (CYP450) pathway. However, 3T6 fibroblasts did not produce leukotriene B(4) (LTB(4)), and lipoxygenase inhibitors and LT antagonists failed to inhibit 3T6 fibroblast growth induced by FBS. In contrast, we observed that CYP450 inhibitors such as SKF-525A, 17-octadecynoic acid, 1-aminobenzotriazole, and 6-(2-propargyloxyphenyl)hexanoic acid reduced 12(S)-HETE levels, 3T6 fibroblast growth, and DNA synthesis induced by FBS. The impairment of DNA synthesis and 3T6 fibroblast growth induced by SKF-525A were reversed by exogenous addition of HETEs. Moreover, we report that 5-HETE, 12(S)-HETE, and 15(S)-HETE are mitogenic on 3T6 fibroblast in the absence of another growth factor, and this effect was dependent on the activation of the phosphatidylinositol-3-kinase pathway. In conclusion, our results show that HETEs, probably produced by CYP450, are involved in the control of 3T6 fibroblast growth.

Regulating leukotriene synthesis: the role of nuclear 5-lipoxygenase

Leukotrienes are lipid messengers involved in autocrine and paracrine cellular signaling. They are synthesized from arachidonic acid by the 5-lipoxygenase pathway. Current models of this enzymatic pathway recognize that a key step in initiating leukotriene synthesis is the calcium-mediated movement of enzymes, including 5-lipoxygenase, to intracellular membranes. However, 5-lipoxygenase can be imported into or exported from the nucleus before calcium activation. As a result, its subcellular localization will affect its ability to be activated by calcium, as well as the membrane to which it binds and its interaction with other enzymes. This commentary focuses on the role of 5-lipoxygenase compartmentation in determining its regulation and, ultimately, leukotriene synthesis.

Regulation of 5-lipoxygenase enzyme activity

In this article, regulation of human 5-lipoxygenase enzyme activity is reviewed. First, structural properties and enzyme activities are described. This is followed by the activating factors: Ca2+, membranes, ATP, and lipid hydroperoxide. Also, studies on phosphorylation of 5-lipoxygenase and nuclear localization sequences are reviewed.

Phosphorylation by protein kinase a inhibits nuclear import of 5-lipoxygenase

The enzyme 5-lipoxygenase initiates the synthesis of leukotrienes from arachidonic acid. Protein kinase A phosphorylates 5-lipoxygenase on Ser(523), and this reduces its activity. We report here that phosphorylation of Ser(523) also shifts the subcellular distribution of 5-lipoxygenase from the nucleus to the cytoplasm. Phosphorylation and redistribution of 5-lipoxygenase could be produced by overexpression of the protein kinase A catalytic subunit alpha, by pharmacological activators of protein kinase A, and by prostaglandin E(2). Mimicking phosphorylation by replacing Ser(523) with glutamic acid caused cytoplasmic localization; replacement of Ser(523) with alanine prevented phosphorylation and redistribution in response to protein kinase A activation. Because Ser(523) is positioned within the nuclear localization sequence-518 of 5-lipoxygenase, the ability of protein kinase A to phosphorylate and alter the localization of green fluorescent protein fused to the nuclear localization sequence-518 peptide was also tested. Site-directed replacement of Ser(523) with glutamic acid within the peptide impaired nuclear accumulation; overexpression of the protein kinase A catalytic subunit alpha and pharmacological activation of protein kinase caused phosphorylation of the fusion protein at Ser(523), and the phosphorylated protein was found chiefly in the cytoplasm. Taken together, these results indicate that phosphorylation of Ser(523) inhibits the nuclear import function of a nuclear localization sequence, resulting in the accumulation of 5-lipoxygenase enzyme in the cytoplasm. As cytoplasmic localization can be associated with reduced leukotriene synthetic capacity, phosphorylation of Ser(523) serves to inhibit leukotriene production by both impairing catalytic activity and by placing the enzyme in a site that is unfavorable for action.

Stress-induced nuclear export of 5-lipoxygenase

A key enzyme for leukotriene biosynthesis is 5-lipoxygenase (5-LO), which we found is exported from the nucleus when p38 MAPK is activated. CHO-K1 cells stably express green fluorescent protein-5-lipoxygenase fusion protein (GFP-5LO), which is located predominantly in the nucleus, and is exported by anisomycin, hydrogen peroxide, and sorbitol, with activation of p38 MAPK. SB203580, an inhibitor of p38 MAPK, and Leptomycin B, an inhibitor of the nuclear export, blocked the anisomycin-induced export of GFP-5LO. When HEK293 cells were transformed with plasmids for wild-type GFP-5LO, GFP-5LO-S271A or GFP-5LO-S271E mutants, most wild-type GFP-5LO and GFP-5LO-S271A localized in the nucleus, but GFP-5LO-S271E localized in the cytosol. Thus, phosphorylation at Ser-271 of 5-LO is important for its export. Endogenous 5-LO in RBL cells stimulated with anisomycin was also exported from the nucleus. These results suggest that the nuclear export of 5-LO depends on the stress-induced activation of the p38 MAPK pathway.

Protein Kinase A Inhibits Leukotriene Synthesis by Phosphorylation of 5-Lipoxygenase on Serine 523

Leukotrienes (LTs) are lipid messengers generated by leukocytes that drive inflammation and modulate neighboring cell function. The synthesis of LTs from arachidonic acid is initiated by the enzyme 5-lipoxygenase (5-LO). We report for the first time that LT synthesis is inhibited by the direct action of protein kinase A (PKA) on 5-LO. The catalytic subunit of PKA directly phosphorylated 5-LO in vivo and in vitro and inhibited activity in intact cells and in vitro. Mutation of Ser-523 on human 5-LO prevented phosphorylation by PKA and restored LT synthesis. Treatment with PKA activators inhibited LTB(4) synthesis in 3T3 cells expressing wild type 5-LO but not in cells expressing the S523A mutant of 5-LO. The mechanism of inhibition of LTB(4) synthesis did not involve either reduced membrane association of activated 5-LO or redistribution of 5-LO from the nucleus to the cytoplasm. Instead, PKA phosphorylation of recombinant 5-LO inhibited in vitro activity, as did co-transfection of cells with 5-LO plus the catalytic subunit of PKA. Also, substitution of Ser-523 with glutamic acid, mimicking phosphorylation, resulted in the total loss of 5-LO activity. These results indicate that PKA phosphorylates 5-LO on Ser-523, which inhibits the catalytic activity of 5-LO and reduces cellular LT generation. Thus, PKA activation, as can occur in response to adenosine, prostaglandin E(2), beta-adrenergic agonists, and other mediators, is a means of directly reducing 5-LO activity and LT synthesis that may be important in limiting inflammation and maintaining homeostasis.

Nuclear prostaglandin receptors: role in pregnancy and parturition?

The key regulatory role of prostanoids [prostaglandins (PGs) and thromboxanes (TXs)] in the maintenance of pregnancy and initiation of parturition has been established. However, our understanding of how these events are fine-tuned by the recruitment of specific signaling pathways remains unclear. Whereas, initial thoughts were that PGs were lipophilic and would easily cross cell membranes without specific receptors or transport processes, it has since been realized that PG signaling occurs via specific cell surface G-protein coupled receptors (GPCRs) coupled to classical adenylate cyclase or inositol phosphate signaling pathways. Furthermore, specific PG transporters have been identified and cloned adding a further level of complexity to the regulation of paracrine action of these potent bioactive molecules. It is now apparent that PGs also activate nuclear receptors, opening the possibility of novel intracrine signaling mechanisms. The existence of intracrine signaling pathways is further supported by accumulating evidence linking the perinuclear localization of PG synthesizing enzymes with intracellular PG synthesis. This review will focus on the evidence for a role of nuclear actions of PGs in the regulation of pregnancy and parturition.

Nuclear localization of 5-lipoxygenase as a determinant of leukotriene B4 synthetic capacity

The enzyme 5-lipoxygenase (5-LO) initiates the synthesis of leukotrienes from arachidonic acid. In resting cells, 5-LO can accumulate in either the cytoplasm or the nucleoplasm and, upon cell stimulation, translocates to membranes to initiate leukotriene synthesis. Here, we used mutants of 5-LO with altered subcellular localization to assess the role that nuclear positioning plays in determining leukotriene B4 (LTB4) synthesis. Mutation of either a nuclear localization sequence or a phosphorylation site reduced LTB4 synthesis by 60%, in parallel with reduced nuclear localization of 5-LO. Mutation of both sites together or mutation of all three nuclear localization sequences on 5-LO inhibited LTB4 synthesis by 90% and abolished nuclear localization. Reduced LTB4 generation in mutants could not be attributed to differences in 5-LO amount, enzymatic activity, or membrane association. Instead, 5-LO within the nucleus acts at a different site, the nuclear envelope, than does cytosolic 5-LO, which acts at cytoplasmic and perinuclear membranes. The significance of this difference was suggested by evidence that exogenously derived arachidonic acid colocalized with activated nuclear 5-LO. These results unequivocally demonstrate that the positioning of 5-LO within the nucleus of resting cells is a powerful determinant of the capacity to generate LTB4 upon subsequent activation.

5-lipoxygenase and FLAP

The initial steps in the biosynthesis of leukotrienes from arachidonic acid are carried out by the enzyme 5-lipoxygenase (5-LO). In intact cells, the helper protein 5-LO activating protein (FLAP) is necessary for efficient enzyme utilization of endogenous substrate. The last decade has witnessed remarkable progress in our understanding of these two proteins. Here we review the molecular and cellular aspects of the expression, function, and regulation of 5-LO and FLAP.

Subcellular localization and tumor-suppressive functions of 15-lipoxygenase 2 (15-LOX2) and its splice variants

15-Lipoxygenase 2 (15-LOX2), the most abundant arachidonate (AA)-metabolizing enzyme expressed in adult human prostate, is a negative cell-cycle regulator in normal human prostate epithelial cells. Here we study the subcellular distribution of 15-LOX2 and report its tumor-suppressive functions. Immunocytochemistry and biochemical fractionation reveal that 15-LOX2 is expressed at multiple subcellular locations, including cytoplasm, cytoskeleton, cell-cell border, and nucleus. Surprisingly, the three splice variants of 15-LOX2 we previously cloned, i.e. 15-LOX2sv-a/b/c, are mostly excluded from the nucleus. A potential bi-partite nuclear localization signal (NLS),203RKGLWRSLNEMKRIFNFRR221, is identified in the N terminus of 15-LOX2, which is retained in all splice variants. Site-directed mutagenesis reveals that this putative NLS is only partially involved in the nuclear import of 15-LOX2. To elucidate the relationship between nuclear localization, enzymatic activity, and tumor suppressive functions, we established PCa cell clones stably expressing 15-LOX2 or 15-LOX2sv-b. The 15-LOX2 clones express 15-LOX2 in the nuclei and possess robust enzymatic activity, whereas 15-LOX2sv-b clones show neither nuclear protein localization nor AA-metabolizing activity. To our surprise, both 15-LOX2- and 15-LOX2sv-b-stable clones proliferate much slower in vitro when compared with control clones. More importantly, when orthotopically implanted in nude mouse prostate, both 15-LOX2 and 15-LOX2sv-b suppress PC3 tumor growth in vivo. Together, these results suggest that both 15-LOX2 and 15-LOX2sv-b suppress prostate tumor development, and the tumor-suppressive functions apparently do not necessarily depend on AA-metabolizing activity and nuclear localization.

Identification of two novel nuclear import sequences on the 5-lipoxygenase protein

The nuclear import of 5-lipoxygenase modulates its capacity to produce leukotrienes from arachidonic acid. However, the molecular determinants of its nuclear import are unknown. Recently, we used structural and functional criteria to identify a novel import sequence at Arg(518) on human 5-lipoxygenase (Jones, S. M., Luo, M., Healy, A. M., Peters-Golden, M., and Brock, T. G. (2002) J. Biol. Chem. 277, 38550-38556). However, this analysis also indicated that other import sequences must exist. Here, we identify two additional sites, at Arg(112) and Lys(158), as nuclear import sequences. Both sites were found to be common to 5-lipoxygenases from different species but not found on other lipoxygenases. Both sites also appeared to be a part of structures that were predominantly random loops. Peptide sequences at these sites were sufficient to direct nuclear import of green fluorescent protein. Mutation of basic residues in these sites impaired nuclear import and combinations of mutations at different sites were additive in effect. Mutations in all three sites were required to disable nuclear accumulation of 5-lipoxygenase in all cells. Significantly, mutation in these sites did not inhibit catalytic function. Taken together, these results indicate that nuclear import of 5-lipoxygenase may reflect the combined functional effects of three discrete import sequences. Mutation of individual sites can, by itself, impair nuclear import, which in turn could impact arachidonic acid metabolism.

Differentially expressed nucleolar TGF-beta1 target (DENTT) in mouse development

Differentially expressed nucleolar TGF-beta1 target (DENTT) is a recently identified gene whose mRNA is differentially affected by TGF-beta1 in TGF-beta1-responsive human lung cancer cells and who is a new member of the TSPY/TSPY-like/SET/NAP-1 (TTSN) protein superfamily. Here, we report that mouse DENTT mRNA contains a 2031-bp open reading frame that encodes a predicted polypeptide of 677-amino acids with a relative molecular mass of 77,671 Da. The mouse and human DENTT sequences show 77% and 78% homology at the nucleotide and amino acid level, respectively. Mouse DENTT is predicted to be a nuclear protein with two nuclear localization signals (NLS), two coiled-coil regions, and a domain that shows significant identity to a region that defines the TTSN superfamily. Green fluorescent protein (GFP)-tagged full-length mouse DENTT transfected into COS-7 cells showed localization predominantly in the nucleolus. Reverse transcription-polymerase chain reaction amplification, Northern hybridization, and Western blot analyses showed expression of mouse DENTT mRNA and protein throughout mouse embryogenesis. Immunohistochemical staining analysis showed that DENTT is expressed in multiple tissues in a defined spatiotemporal pattern during mouse embryogenesis. The heart and primitive brain were the first organs of the embryo that showed immunoreactivity for the DENTT antibody by day 8 of development (E8). In the developing mouse brain, the choroid plexus was intensely stained for DENTT in all stages of development. The spinal cord and dorsal root ganglia were also positive for DENTT staining beginning in the 11-day-old embryo (E11), where homogeneous immunostaining was observed throughout the developing neurons. By day 16 of development (E16), only a small subset of the neuronal population in the spinal cord and dorsal root ganglia was positively stained for DENTT. DENTT immunoreactivity increased steadily with maturation as the differentiation of cartilage and osteoblasts proceeded and reached a maximum in the growth plate during endochondral ossification. DENTT expression was also detected in multiple rodent cell types in vitro, including mouse F9 embryonal carcinoma (EC) cells. Addition of retinoic acid or sodium butyrate to F9 EC cells showed a rapid decrease in expression of DENTT protein occurring by 1 hr that continued to decrease to almost undetectable levels after 24 hr. Cotransfection of full-length mouse DENTT expression plasmid with 3TPLux or COL7A1Luc Luciferase reporter plasmids into F9 EC cells significantly increased the level of 3TPLux reporter transcription while decreasing the level of COL7A1Luc reporter transcription, suggesting that DENTT may play multiple roles in modulating transcriptional responses. These findings suggest new roles for the TTSN superfamily during embryogenesis and differentiation.

Defects in virion production caused by mutations affecting the C-terminal portion of the Moloney murine leukemia virus capsid protein

The capsid (CA) domain of the Moloney murine leukemia virus (Mo-MuLV) Gag protein has a unique carboxy terminus with a highly charged arginine-rich sequence. Mutant viruses harboring arginine-to-alanine mutations affecting this region of CA displayed significant defects in virion release, and the few viral particles produced were noninfectious. The interaction between the mutant Gag precursors was affected, as judged by the yeast two-hybrid assay. The results suggest that the unique carboxy terminus of CA in the Mo-MuLV plays an important role in Gag-Gag association during virion production.

Structural and functional criteria reveal a new nuclear import sequence on the 5-lipoxygenase protein

Leukotrienes are lipid mediators with important roles in immunity. The enzyme 5-lipoxygenase initiates leukotriene synthesis; nuclear import of 5-lipoxygenase modulates leukotriene synthetic capacity. In this study, we used structural and functional criteria to identify potential nuclear import sequences. Specifically, we sought basic residues that 1) were common to different 5-lipoxygenases but not shared with other lipoxygenases, 2) were found on random coil/loop structures, and 3) could be replaced without eliminating catalytic activity. Application of these criteria to the putative bipartite nuclear import sequence of 5-lipoxygenase revealed that this region formed an alpha-helix rather than a random coil, that the critical residue arginine 651 serves a structural role, and that mutation of this residue eliminated catalytic activity. A previously unrecognized region corresponding to residues 518-530 on human 5-lipoxygenase was found to be unique to 5-lipoxygenase and on a random coil. This region alone was sufficient to drive import of green fluorescent protein to the same degree as complete 5-lipoxygenase. Replacement of basic residues in this region of the complete protein was capable of eliminating nuclear import without abolishing catalytic activity. Surprisingly, two subpopulations of cells expressing 5-lipoxygenase with this mutated region could be discerned: those with strongly impaired import and those with normal import. Taken together, these results show that the previously identified region with a bipartite motif is not a functional import sequence, whereas the newly identified basic region constitutes a true nuclear import sequence. Moreover, we suggest that another sequence that can mediate nuclear import of 5-lipoxygenase remains to be identified.

Arachidonate 5-lipoxygenase

In this article, it has been attempted to review data primarily on the activation of human 5-lipoxygenase, in vitro and in the cell. First, structural properties and enzyme activities are described. This is followed by the activating factors: Ca2+, membranes, ATP, and lipid hydroperoxide. Also, studies on phosphorylation of 5-lipoxygenase, interaction with other proteins, and the intracellullar mobility of 5-lipoxygenase, are reviewed.

The C-terminal C1 cassette of the N -methyl-D-aspartate receptor 1 subunit contains a bi-partite nuclear localization sequence

The N -methyl-D-aspartate receptor (NMDAR) is a multimeric transmembrane protein composed of at least two subunits. One subunit, NR1, is derived from a single gene and can be subdivided into three regions: the N-terminal extracellular domain, the transmembrane regions, and the C-terminal intracellular domain. The N-terminal domain is responsible for Mg2+ metal ion binding and channel activity, while the transmembrane domains are important for ion channel formation. The intracellular C-terminal domain is involved in regulating receptor activity and subcellular localization. Our recent experiments indicated that the intracellular C-terminal domain, when expressed independently, localizes almost exclusively in the nucleus. An examination of the amino acid sequence reveals the presence of a putative nuclear localization sequence (NLS) in the C1 cassette of the NR1 intracellular C-terminus. Using an expression vector designed to test whether a putative NLS sequence is a valid, functional NLS, we have demonstrated that a bi-partite NLS does in fact exist within the NR1-1 C-terminus. Computer algorithms identified a putative helix-loop-helix motif that spanned the C0C1 cassettes of the C-terminus. These data suggest that the NR1 subunit may represent another member of a family of transmembrane proteins that undergo intramembrane proteolysis, releasing a cytosolic peptide that is actively translocated to the nucleus leading to alterations in gene regulation.

Nuclear-localization-signal-dependent and nuclear-export-signal-dependent mechanisms determine the localization of 5-lipoxygenase

5-Lipoxygenase (5-LO) metabolizes arachidonic acid to leukotriene A4, a key intermediate in leukotriene biosynthesis. To explore the molecular mechanisms of its cell-specific localization, a fusion protein between green fluorescent protein (GFP) and human 5-LO (GFP-5LO) was expressed in various cells. GFP-5LO was localized in the cytosol in HL-60 cells and in both the nucleus and the cytosol in RBL (rat basophilic leukaemia) cells, similarly to the native enzyme in these cells. The localization of GFP fusion proteins for mutant 5-LOs in a putative bipartite nuclear localization signal (NLS), amino acids 638-655, in Chinese hamster ovary (CHO)-K1 and Swiss3T3 cells revealed that this motif is important for the nuclear localization of 5-LO. A GFP fusion protein of this short peptide localized consistently in the nucleus. Leptomycin B, a specific inhibitor of nuclear export signal (NES)-dependent transport, diminished the cytosolic localization of 5-LO in HL-60 cells and that of GFP-5LO in CHO-K1 cells, suggesting that an NES-system might also function in determining 5-LO localization. Analysis of the localization of 5-LO during the cell cycle points to a controlled movement of this enzyme. Thus we conclude that a balance of NLS- and NES-dependent mechanisms determines the cell-type-specific localization of 5-LO, suggesting a nuclear function for this enzyme.

Sulphatides trigger polymorphonuclear granulocyte spreading on collagen-coated surfaces and inhibit subsequent activation of 5-lipoxygenase

Sulphatides are sulphate esters of galactocerebrosides that are present on the surfaces of many cell types and act as specific ligands to selectins. The present study was undertaken to investigate the effect of sulphatides on polymorphonuclear granulocyte (PMN) attachment, spreading and 5-lipoxygenase (5-LO) metabolism. Sulphatides, but not non-sulphated galactocerebrosides, dose-dependently enhanced attachment to collagen, as measured by the myeloperoxidase assay. Studies with blocking antibodies indicated that the increased attachment was mediated by CD11b/CD18 (Mac-1) beta 2 integrin. Scanning electron microscopy indicated that sulphatides also greatly enhanced the degree of cell spreading. In PMNs treated in suspension, sulphatides had no effect on the ionophore A23187-stimulated release of arachidonic acid and the synthesis of 5-LO metabolites. In contrast, in PMNs attached to collagen, the enzymic conversion of arachidonic acid by 5-LO was inhibited by sulphatides. Inhibition of 5-LO metabolism by sulphatides was observed even in the presence of exogenous substrate, suggesting that sulphatides directly inhibited 5-LO action. Consistent with this, sulphatides interfered with ionophore-induced translocation of the 5-LO to the nuclear envelope. Substances competing with sulphatide binding to cells, like dextran sulphate, or a strong inhibitor of cell spreading, like the actin-polymerizing agent jasplakinolide, prevented the effects of sulphatides on PMN attachment and spreading and leukotriene synthesis. We conclude that shape changes occurring in response to sulphatides specifically impair PMN leukotriene synthesis by inhibiting translocation of 5-LO.

Coupling between cyclooxygenase, terminal prostanoid synthase, and phospholipase A2

We have recently shown that two distinct prostaglandin (PG) E(2) synthases show preferential functional coupling with upstream cyclooxygenase (COX)-1 and COX-2 in PGE(2) biosynthesis. To investigate whether other lineage-specific PG synthases also show preferential coupling with either COX isozyme, we introduced these enzymes alone or in combination into 293 cells to reconstitute their functional interrelationship. As did the membrane-bound PGE(2) synthase, the perinuclear enzymes thromboxane synthase and PGI(2) synthase generated their respective products via COX-2 in preference to COX-1 in both the -induced immediate and interleukin-1-induced delayed responses. Hematopoietic PGD(2) synthase preferentially used COX-1 and COX-2 in the -induced immediate and interleukin-1-induced delayed PGD(2)-biosynthetic responses, respectively. This enzyme underwent stimulus-dependent translocation from the cytosol to perinuclear compartments, where COX-1 or COX-2 exists. COX selectivity of these lineage-specific PG synthases was also significantly affected by the concentrations of arachidonate, which was added exogenously to the cells or supplied endogenously by the action of cytosolic or secretory phospholipase A(2). Collectively, the efficiency of coupling between COXs and specific PG synthases may be crucially influenced by their spatial and temporal compartmentalization and by the amount of arachidonate supplied by PLA(2)s at a moment when PG production takes place.

Leukotriene B4: metabolism and signal transduction

Leukotriene B4 (LTB4) is known as one of the most potent chemoattractants and activators of leukocytes and is involved in inflammatory diseases. Enzymes involved in the biosynthesis and metabolism of LTB4 have been cloned, and their properties are well understood. Two G-protein-coupled receptors (BLT1 and BLT2) have been cloned and characterized. BLT1 and BLT2 are high- and low-affinity LTB4 receptors, respectively, and form a gene cluster in human and mouse. In this article recent findings on the metabolism of and the receptors for LTB4 are reviewed. We also discuss briefly a coreceptor role of BLT in HIV infection, and ion channel modification by LTB4.

Anti-inflammatory drugs: new multitarget compounds to face an old problem. The dual inhibition concept

In this short review we have tried to focus on some new relevant aspects of the pharmacological control of inflammation. The clinical availability of new drugs able to produce a selective inhibition of type 2 cyclooxygenase (COX-2), the enzyme thought to be mainly responsible for generating arachidonic-acid-derived inflammatory mediators, has been the origin of much hope. However, expectations of having an effective and completely safe non-steroidal anti-inflammatory drug (NSAID) have been only partially fulfilled. Emerging information has challenged some aspects of the original hypothesis indicating COX-2 as devoid of 'housekeeping' physiological functions. Moreover, the recently available clinical studies have indicated only a relatively small improvement in the tolerability of the newer 'selective' COX-2 inhibitors over the classical COX-1/COX-2 mixed type NSAIDs. The new appreciation of the role of other arachidonic acid derivatives, the leukotrienes (LTS), in producing and maintaining inflammation has generated considerable interest in drugs able to block LTS receptors or to produce a selective inhibition of 5-lipoxygenase (5-LO), the initial key enzyme of the leukotriene pathway. These drugs are now included among the effective therapies of asthma but appear, in the few clinical studies performed, to be an insufficient single therapeutic approach in other inflammatory diseases. Drugs able to block equally well both COX and 5-LO metabolic pathways (dual inhibitors) have been developed and experimentally evaluated in the last few years, but none are available on the market yet. The pharmacological rationale at the basis of their development is strong, and animal studies are indicative of a wide range of anti-inflammatory activity. What appears most impressive from the available studies on dual inhibitors is their almost complete lack of gastric toxicity, the most troublesome side effect of NSAIDs. The mechanism of the gastric-sparing properties of these drugs is not yet completely understood; however, it appears that leukotrienes significantly contribute to gastric epithelial injury particularly when these compounds represent the major arachidonic acid derivatives present in the gastric mucosa after inhibiton of prostanoid production.

Phorbol ester up-regulates capacities for nuclear translocation and phosphorylation of 5-lipoxygenase in Mono Mac 6 cells and human polymorphonuclear leukocytes

The leukotrienes are inflammatory mediators derived from arachidonic acid. It was demonstrated that the priming of leukocytes with phorbol-12-myristate-13-acetate (PMA) leads to the increased formation of 5-lipoxygenase (5-LO) products in parallel with the increased association of 5-LO with the nucleus and the activation of kinases that can phosphorylate 5-LO in vitro. Stimulation of the monocytic cell line Mono Mac 6 with calcium ionophore gave low 5-LO product formation and no detectable redistribution of 5-LO. However, after priming of Mono Mac 6 cells with phorbol esters, ionophore led to the association of 45% to 75% of cellular 5-LO with the nuclear membrane, to 5-LO kinase activation, to enhanced release of arachidonate, and to substantial leukotriene synthesis. Similar results were obtained for human polymorphonuclear leukocytes stimulated with low-dose ionophore. In addition, for each cell type, PMA priming up-regulated leukotriene biosynthesis in the presence of exogenous arachidonic acid. A protein kinase inhibitor, calphostin C, reduced the association of 5-LO with the nucleus and 5-LO kinase activity, and the formation of 5-LO products was inhibited. These results suggest that PMA up-regulates leukotriene biosynthesis not only by increasing the release of endogenous arachidonate, but also by increasing the capacity for 5-LO phosphorylation and for the translocation of 5-LO to the nucleus in leukocytes.

Identification of differentially expressed nucleolar TGF-beta1 target (DENTT) in human lung cancer cells that is a new member of the TSPY/SET/NAP-1 superfamily

The transforming growth factor-beta1 (TGF-beta1) responsive epithelial non-small-cell lung cancer (NSCLC) cell line NCI-H727 was used to identify potential target genes involved in TGF-beta1-mediated responses. Comparative cDNA expression patterns between cells treated with TGF-beta1 and those treated with vehicle were generated by differential mRNA display. One 496-bp fragment, differentially increased threefold by TGF-beta1 and hybridizing to a 2.7-kb mRNA species in NCI-H727 cells by Northern analysis, revealed no significant match to any known gene sequence. The mRNA transcript of this novel gene that we named differentially expressed nucleolar TGF-beta1 target (DENTT) is expressed in several normal human tissues, with the highest level of expression in brain. Human brain cDNA library screening and 5' rapid amplification of cDNA ends yielded full-length DENTT cDNA containing an 1899-bp open reading frame encoding a predicted 633-amino-acid protein with four potential nuclear localization signals (NLSs) and two coiled-coil regions. DENTT contains a conserved 191-residue domain that shows significant identity to, and defines, the TSPY/TSPY-like/SET/NAP-1 superfamily. Enhanced green fluorescent protein (EGFP)-tagged full-length DENTT transfected into COS-7 cells showed nucleolar and cytoplasmic localization. Transfection of EGFP-tagged DENTT NLS deletion constructs lacking the bipartite NLS-1 were excluded from the nucleolus. While NLS-1 is necessary for nucleolar localization of DENTT, it is not sufficient for sole nucleolar localization. Our data show that DENTT mRNA induction by TGF-beta1 correlates with induction of TGF-beta1 mRNA, induction of extracellular matrix gene expression, and inhibition of colony formation in soft agarose in TGF-beta1 responsive NSCLC cells when exposed to TGF-beta1. TGF-beta1 does not induce DENTT mRNA expression in TGF-beta1 nonresponsive NSCLC cells. Our data suggest that this novel TGF-beta1 target gene has distinct domains for direction to different subnuclear locations.

Intracellular compartmentalization of leukotriene synthesis: unexpected nuclear secrets

Leukotrienes are important lipid mediators implicated in the regulation of various cellular processes and in disease states as well as homeostasis. Regulation of leukotriene biosynthesis is therefore of considerable interest. Although the levels of expression and catalytic activity of leukotriene-forming proteins have long been recognized as important determinants of leukotriene biosynthesis, it has recently become apparent that their intracellular compartmentalization also affects the integrated output of this biosynthetic pathway. In this minireview, we focus on the unexpected discovery that the nucleus is the key intracellular site for leukotriene biosynthesis and discuss the mechanisms that regulate protein localization and the potential implications of these findings.

The N-terminal "beta-barrel" domain of 5-lipoxygenase is essential for nuclear membrane translocation

5-Lipoxygenase is the key enzyme in the formation of leukotrienes, which are potent lipid mediators of asthma pathophysiology. This enzyme translocates to the nuclear envelope in a calcium-dependent manner for leukotriene biosynthesis. Eight green fluorescent protein (GFP)-lipoxygenase constructs, representing the major human and mouse enzymes within this family, were constructed and their cDNAs transfected into human embryonic kidney 293 cells. Of these eight lipoxygenases, only the 5-lipoxygenase was clearly nuclear localized and translocated to the nuclear envelope upon stimulation with the calcium ionophore. The N-terminal "beta -barrel" domain of 5-lipoxygenase, but not the catalytic domain, was necessary and sufficient for nuclear envelope translocation. The GFP-N-terminal 5-lipoxygenase domain translocated faster than GFP-5-lipoxygenase. beta-Barrel/catalytic domain chimeras with 12- and 15-lipoxygenase indicated that only the N-terminal domain of 5-lipoxygenase could carry out this translocation function. Mutations of iron atom binding ligands (His550 or deletion of C-terminal isoleucine) that disrupt nuclear localization do not alter translocation capacity indicating distinct determinants of nuclear localization and translocation. Moreover, data show that GFP-5-lipoxygenase beta-barrel containing constructs can translocate to the nuclear membrane whether cytoplasmic or nuclear localized. Thus, the predicted beta-barrel domain of 5-lipoxygenase may function like the C2 domain within protein kinase C and cytosolic phospholipase A(2) with unique determinants that direct its localization to the nuclear envelope.

Cellular components that functionally interact with signaling phospholipase A(2)s

Accumulating evidence has suggested that cytosolic phospholipase A(2) (cPLA(2)) and several secretory PLA(2) (sPLA(2)) isozymes are signaling PLA(2)s that are functionally coupled with downstream cyclooxygenase (COX) isozymes for prostaglandin (PG) biosynthesis. Arachidonic acid (AA) released by cPLA(2) and sPLA(2)s is supplied to both COX-1 and COX-2 in the immediate, and predominantly to COX-2 in the delayed, PG-biosynthetic responses. Vimentin, an intermediate filament component, acts as a functional perinuclear adapter for cPLA(2), in which the C2 domain of cPLA(2) associates with the head domain of vimentin in a Ca(2+)-sensitive manner. The heparin-binding signaling sPLA(2)-IIA, IID and V bind the glycosylphosphatidylinositol-anchored heparan sulfate proteoglycan glypican, which plays a role in sorting of these isozymes into caveolae and perinuclear compartments. Phospholipid scramblase, which facilitates transbilayer movement of anionic phospholipids, renders the cellular membranes more susceptible to signaling sPLA(2)s. There is functional cooperation between cPLA(2) and signaling sPLA(2)s in that prior activation of cPLA(2) is required for the signaling sPLA(2)s to act properly. cPLA(2)-derived AA is oxidized by 12/15-lipoxygenase, the products of which not only augment the induction of sPLA(2) expression, but also cause membrane perturbation, leading to increased cellular susceptibility to the signaling sPLA(2)s. sPLA(2)-X, a heparin-non-binding sPLA(2) isozyme, is capable of releasing AA from intact cells in the absence of cofactors. This property is attributed to its ability to avidly hydrolyze zwitterionic phosphatidylcholine, a major phospholipid in the outer plasma membrane. sPLA(2)-V can also utilize this route in several cell types. Taken together, the AA-releasing function of sPLA(2)s depends on the presence of regulatory cofactors and interfacial binding to membrane phospholipids, which differ according to cell type, stimuli, secretory processes, and subcellular distributions.