New aspects of the role of hydroxyeicosatetraenoic acids in cell growth and cancer development

Old and new generation lipid mediators in acute inflammation and resolution

Originally regarded as just membrane constituents and energy storing molecules, lipids are now recognised as potent signalling molecules that regulate a multitude of cellular responses via receptor-mediated pathways, including cell growth and death, and inflammation/infection. Derived from polyunsaturated fatty acids (PUFAs), such as arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), each lipid displays unique properties, thus making their role in inflammation distinct from that of other lipids derived from the same PUFA. The diversity of their actions arises because such metabolites are synthesised via discrete enzymatic pathways and because they elicit their response via different receptors. This review will collate the bioactive lipid research to date and summarise the findings in terms of the major pathways involved in their biosynthesis and their role in inflammation and its resolution. It will include lipids derived from AA (prostanoids, leukotrienes, 5-oxo-6,8,11,14-eicosatetraenoic acid, lipoxins and epoxyeicosatrienoic acids), EPA (E-series resolvins), and DHA (D-series resolvins, protectins and maresins).

Role of eicosanoids on intestinal epithelial homeostasis

The intestinal epithelium is a highly dynamic system that is continuously renewed by a process involving cell proliferation and differentiation. Moreover, it is the main interface with the external environment, and maintenance and regulation of the epithelial structure and epithelial barrier function are key determinants of digestive health and host well being. The tight junction, a multiprotein complex composed of transmembrane proteins associated with the cytoskeletal peri-junctional ring of actin and myosin, is an essential component of this barrier that is strictly regulated in a spatio-temporal manner by a complex signaling network. Defects in the intestinal epithelial barrier function have been observed in inflammatory bowel disease, and a classic example of the connection between inflammation and cancer is the increased risk of colorectal cancer in patients with inflammatory bowel disease. In recent years, several molecules have emerged as critical players contributing to inflammation-associated colorectal cancer. For example, eicosanoids derived from arachidonic acid are proposed as mediators involved in the regulation of epithelial structure/function. Interestingly, the tissue concentration of eicosanoids increases during mucosal inflammation and colorectal cancer development. This overview focuses on the physiological and physiopathological roles of eicosanoids in cell growth/cell differentiation/apoptosis and in the paracellular permeability of the intestinal epithelium. A better understanding of these processes will foster new ideas for the development of therapies for these chronic disorders.

Oxpholipin 11D: an anti-inflammatory peptide that binds cholesterol and oxidized phospholipids

Background

Many Gram-positive bacteria produce pore-forming exotoxins that contain a highly conserved, 12-residue domain (ECTGLAWEWWRT) that binds cholesterol. This domain is usually flanked N-terminally by arginine and C-terminally by valine. We used this 14-residue sequence as a template to create a small library of peptides that bind cholesterol and other lipids.

Methodology/Results

Several of these peptides manifested anti-inflammatory properties in a predictive in vitro monocyte chemotactic assay, and some also diminished the pro-inflammatory effects of low-density lipoprotein in apoE-deficient mice. The most potent analog, Oxpholipin-11D (OxP-11D), contained D-amino acids exclusively and was identical to the 14-residue design template except that diphenylalanine replaced cysteine-3. In surface plasmon resonance binding studies, OxP-11D bound oxidized (phospho)lipids and sterols in much the same manner as D-4F, a widely studied cardioprotective apoA-I-mimetic peptide with anti-inflammatory properties. In contrast to D-4F, which adopts a stable α-helical structure in solution, the OxP-11D structure was flexible and contained multiple turn-like features.

Conclusion

Given the substantial evidence that oxidized phospholipids are pro-inflammatory in vivo, OxP-11D and other Oxpholipins may have therapeutic potential.

mRNA levels of enzymes and receptors implicated in arachidonic acid metabolism in gliomas

BACKGROUND

Gliomas are tumors of the central nervous system derived from glial cells. They show cellular heterogeneity and lack specific diagnostic markers. Although a possible role for the eicosanoid cascade has been suggested in glioma tumorigenesis, the relationship between enzymes and receptors implicated in arachidonic acid metabolism, with histological tumor type has not yet been determined.

DESIGN AND METHODS

Quantitative real-time reverse transcription-polymerase chain reaction was performed to measure and compare transcript levels of enzymes and receptors implicated in both lipoxygenase and cyclooxygenase pathways between oligodendrogliomas, astrocytomas, glioblastomas and mixed oligoastrocytomas.

RESULTS

Arachidonic acid metabolism-related enzymes and receptor transcripts (i) were underexpressed in classical oligodendrogliomas compared to astrocytomas and/or glioblastomas, (ii) differed between astrocytomas and glioblastomas and (iii) had an intermediate expression in mixed oligoastrocytomas.

CONCLUSIONS

mRNA levels of enzymes and receptors implicated both in lipoxygenase and cyclooxygenase pathways differed significantly in gliomas according to the histological type.

12-Lipoxygenase Products Reduce Insulin Secretion and {beta}-Cell Viability in Human Islets

CONTEXT

Inflammation is increasingly recognized as an important contributing factor in diabetes mellitus. Lipoxygenases (LOs) produce active lipids that promote inflammatory damage by catalyzing the oxidation of linoleic and arachidonic acid, and LO is expressed in rodent and human islets. Little is known about the differential effect of the various hydroxyeicosatetraenoic acids (HETEs) that result from LO activity in human islets.

OBJECTIVE

We compared the effects of 12-LO products on human islet viability and function.

DESIGN

Human islets were treated with stable compounds derived from LOs: 12(S)-HETE, 15HETE, 12HPETE, and 12RHETE and then examined for insulin secretion and islet viability. The p38-MAPK (p38) and JNK stress-activated pathways were investigated as mechanisms of 12-LO-mediated islet inhibition in rodent and human islets.

RESULTS

Insulin secretion was consistently reduced by 12(S)-HETE and 12HPETE. 12(S)-HETE at 1 nm reduced viability activity by 32% measured by MTT assay and increased cell death by 50% at 100 nm in human islets. These effects were partially reversed with lisofylline, a small-molecule antiinflammatory compound that protects mitochondrial function. 12(S)-HETE increased phosphorylated p38-MAPK (pp38) protein activity in human islets. Injecting 12-LO siRNA into C57BL/6 mice reduced 12-LO and pp38-MAPK protein levels in mouse islets. The addition of proinflammatory cytokines increased pp38 levels in normal mouse islets but not in siRNA-treated islets.

CONCLUSIONS

These data suggest that 12(S)-HETE reduces insulin secretion and increases cell death in human islets. The 12-LO pathway is present in human islets, and expression is up-regulated by inflammatory cytokines. Reduction of 12-LO activity could thus provide a new therapeutic approach to protect human beta-cells from inflammatory injury.

15-hydroxyeicosatetraenoic acid is a preferential peroxisome proliferator-activated receptor beta/delta agonist

Peroxisome proliferator-activated receptor (PPARs) modulate target gene expression in response to unsaturated fatty acid ligands, such as arachidonic acid (AA). Here, we report that the AA metabolite 15-hydroxyeicosatetraenoic acid (15-HETE) activates the ligand-dependent activation domain (AF2) of PPARbeta/delta in vivo, competes with synthetic agonists in a PPARbeta/delta ligand binding assay in vitro, and triggers the interaction of PPARbeta/delta with coactivator peptides. These agonistic effects were also seen with PPARalpha and PPARgamma, but to a significantly weaker extent. We further show that 15-HETE strongly induces the expression of the bona fide PPAR target gene Angptl4 in a PPARbeta/delta-dependent manner and, conversely, that inhibition of 15-HETE synthesis reduces PPARbeta/delta transcriptional activity. Consistent with its function as an agonistic ligand, 15-HETE triggers profound changes in chromatin-associated PPARbeta/delta complexes in vivo, including the recruitment of the coactivator cAMP response element-binding protein binding protein. Both 15R-HETE and 15S-HETE are similarly potent at inducing PPARbeta/delta coactivator binding and transcriptional activation, indicating that 15-HETE enantiomers generated by different pathways function as PPARbeta/delta agonists.

Primers on molecular pathways - lipoxygenases: their role as an oncogenic pathway in pancreatic cancer

Different evidence supports a functional role of enzymes involved in lipid metabolic pathways, such as lipoxygenases (LOXs) and their metabolite derivatives, in carcinogenesis. LOX enzymes catalyze the dioxygenation of arachidonic acid into hydroxyperoxyeicosatetraenoic acids, which is followed by their conversion to their corresponding eicosanoids as hydroxyeicosatetraenoic acids, leukotrienes, lipoxins and hepoxilins, which in turn act as cellular messengers. Subcellular LOX enzyme localization varies according to the LOX and cellular type regulating different cell functions. LOX enzymes or their products may exert their biological effects in different modes, either intracellular or in other cells. Numerous clinical studies on expression of LOXs in human tumors as well as in animal models indicate different roles of distinct LOX isoforms in carcinogenesis. In fact, different LOXs exhibit either protumorigenic or antitumorigenic activities and modulate the tumor response in a tissue-specific manner. Moreover, the LOX pathways are involved in the spread and metastasis of several cancers, including pancreas, through the activation of several cellular signaling pathways which modify gene expression affecting cellular proliferation, survival, migration and extracellular matrix production. In this review we focus on the important role and different mechanisms of action of LOX pathways in the regulation of pancreatic cancer initiation and progression. A novel approach for pancreatic cancer chemoprevention would involve targeting LOX activities, alone or in combination with other pathways as a major anticancer strategy.

Targeted quantitative analysis of eicosanoid lipids in biological samples using liquid chromatography-tandem mass spectrometry

The eicosanoids are a large family of arachidonic acid oxidation products that contain 20 carbon atoms. Cyclooxygenase (COX)-derived eicosanoids have important roles as autacoids involved in the regulation of cardiovascular function and tumor progression. Lipoxygenase (LO)-derived eicosanoids have been implicated as important mediators of inflammation, asthma, cardiovascular disease and cancer. Cytochrome P-450 (P450)-derived eicosanoids are both vasodilators and vasoconstrictors. There is intense interest in the analysis of reactive oxygen species (ROS)-derived isoprostanes (isoPs) because of their utility as biomarkers of oxidative stress. Enzymatic pathways of eicosanoid formation are regioselective and enantioselective, whereas ROS-mediated eicosanoid formation proceeds with no stereoselectivity. Many of the eicosanoids are also present in only pM concentrations in biological fluids. This presents a formidable analytical challenge because methodology is required that can separate enantiomers and diastereomers with high sensitivity and specificity. However, the discovery of atmospheric pressure ionization (API)/MS methodology of electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and electron capture (EC) APCI has revolutionized our ability to analyze endogenous eicosanoids. LC separations of eicosanoids can now be readily coupled with API ionization, collision induced dissociation (CID) and tandem MS (MS/MS). This makes it possible to efficiently conduct targeted eicosanoid analyses using LC-multiple reaction motoring (MRM)/MS. Several examples of targeted eicosanoid lipid analysis using conventional LC-ESI/MS have been discussed and some new data on the analysis of eicosanoids using chiral LC-ECAPCI/MS has been presented.

Lipids as targets for novel anti-inflammatory therapies

Lipids serve important functions as membrane constituents and also as energy storing molecules. Besides these functions certain lipid species have now been recognized as signalling molecules that regulate a multitude of cellular responses including cell growth and death, and also inflammatory reactions. Bioactive lipids are generated by hydrolysis from membrane lipids mainly by phospholipases giving rise to fatty acids and lysophospholipids that either directly exert their function or are further converted to active mediators. This review will summarize the present knowledge about bioactive lipids that either promote or attenuate inflammatory reactions. These lipids include polyunsaturated fatty acids (PUFA), eicosanoids including the epoxyeicosatrienoic acids (EET), peroxisome proliferation activating receptor (PPAR) activators, cannabinoids and the sphingolipids ceramide, sphingosine 1-phosphate and sphingosylphosphorylcholine.