Biosynthesis of 15-deoxy-delta12,14-PGJ2 and the ligation of PPARgamma

Activated human T lymphocytes express cyclooxygenase-2 and produce proadipogenic prostaglandins that drive human orbital fibroblast differentiation to adipocytes

The differentiation of preadipocyte fibroblasts to adipocytes is a crucial process to many disease states including obesity, cardiovascular, and autoimmune diseases. In Graves' disease, the orbit of the eye can become severely inflamed and infiltrated with T lymphocytes as part of the autoimmune process. The orbital fibroblasts convert to fat-like cells causing the eye to protrude, which is disfiguring and can lead to blindness. Recently, the transcription factor peroxisome proliferator activated receptor (PPAR)-gamma and its natural (15d-PGJ2) and synthetic (thiazolidinedione-type) PPAR-gamma agonists have been shown to be crucial to the in vitro differentiation of preadipocyte fibroblasts to adipocytes. We show herein several novel findings. First, that activated T lymphocytes from Graves' patients drive the differentiation of PPAR-gamma-expressing orbital fibroblasts to adipocytes. Second, this adipogenic differentiation is blocked by nonselective small molecule cyclooxygenase (Cox)-1/Cox-2 inhibitors and by Cox-2 selective inhibitors. Third, activated, but not naïve, human T cells highly express Cox-2 and synthesize prostaglandin D2 and related prostaglandins that are PPAR-gamma ligands. These provocative new findings provide evidence for how activated T lymphocytes, through production of PPAR-gamma ligands, profoundly influence human fibroblast differentiation to adipocytes. They also suggest the possibility that, in addition to the orbit, T lymphocytes influence the deposition of fat in other tissues.

Prostaglandin D2 and J2-series (PGJ2, Delta12-PGJ2) prostaglandins stimulate IL-6 and MCP-1, but inhibit leptin, expression and secretion by 3T3-L1 adipocytes

Prostaglandin D(2) and its derivatives PGJ(2) and Delta(12)-PGJ(2) strongly stimulate the synthesis and secretion by white adipocytes of the neurotrophin NGF. Here we have explored whether PGD(2) and the J(2)-series prostaglandins have pervasive effects on adipokine production. The influence of these prostaglandins on the production of the adipocyte hormones leptin and adiponectin, and the inflammatory factors IL-6 and monocyte chemoattractant protein 1 (MCP-1), were examined in 3T3-L1 adipocytes. PGD(2) induced a reduction in adiponectin and leptin mRNA, and the secretion of these adipokines was also inhibited, the effect being greater with leptin (up to 10-fold) than with adiponectin (twofold). In contrast, PGD(2) induced a marked stimulation of IL-6 and MCP-1 expression; with IL-6, this was rapid, the mRNA level increasing by >50-fold by 1 h. The rise in mRNA was accompanied by an increase in IL-6 and MCP-1 release (up to 100- and 6.5-fold, respectively). The effects of PGD(2) were generally mirrored by PGJ(2) and Delta(12)-PGJ(2); Delta(12)-PGJ(2) was a particularly strong stimulator of IL-6 production. These results indicate that PGD(2) and the J(2)-series prostaglandins PGJ(2) and Delta(12)-PGJ(2) can have major effects on the synthesis and release of key adipokines. Such effects could be important in the inflammatory response in adipose tissue.

PPAR gamma and human metabolic disease

The nuclear receptor family of PPARs was named for the ability of the original member to induce hepatic peroxisome proliferation in mice in response to xenobiotic stimuli. However, studies on the action and structure of the 3 human PPAR isotypes (PPARalpha, PPARdelta, and PPARgamma) suggest that these moieties are intimately involved in nutrient sensing and the regulation of carbohydrate and lipid metabolism. PPARalpha and PPARdelta appear primarily to stimulate oxidative lipid metabolism, while PPARgamma is principally involved in the cellular assimilation of lipids via anabolic pathways. Our understanding of the functions of PPARgamma in humans has been increased by the clinical use of potent agonists and by the discovery of both rare and severely deleterious dominant-negative mutations leading to a stereotyped syndrome of partial lipodystrophy and severe insulin resistance, as well as more common sequence variants with a much smaller impact on receptor function. These may nevertheless have much greater significance for the public health burden of metabolic disease. This Review will focus on the role of PPARgamma in human physiology, with specific reference to clinical pharmacological studies, and analysis of PPARG gene variants in the abnormal lipid and carbohydrate metabolism of the metabolic syndrome.

Essential role for hematopoietic prostaglandin D2 synthase in the control of delayed type hypersensitivity

Hematopoietic prostaglandin D(2) synthase (hPGD(2)S) metabolizes cyclooxygenase-derived prostaglandin (PG) H(2) to PGD(2), which is dehydrated to cyclopentenone PGs, including 15-deoxy-Delta(12,14)-PGJ(2) (15d-PGJ(2)). PGD(2) acts through two receptors (DP1 and DP2/CRTH2), whereas 15d-PGJ(2) can activate peroxisome proliferator-activated receptors or inhibit a range of proinflammatory signaling pathways, including NF-kappaB. Despite eliciting asthmatic and allergic reactions through the generation of PGD(2), it is not known what role hPGD(2)S plays in T helper (Th)1-driven adaptive immunity. To investigate this question, the severity and duration of a delayed type hypersensitivity reaction was examined in hPGD(2)S knockout and transgenic mice. Compared with their respective controls, knockouts displayed a more severe inflammatory response that failed to resolve, characterized histologically as persistent acute inflammation, whereas transgenic mice had little detectable inflammation. Lymphocytes isolated from inguinal lymph nodes of hPGD(2)S(-/-) animals showed hyperproliferation and increased IL-2 synthesis effects that were rescued by 15d-PGJ(2), but not PGD(2), working through either of its receptors. Crucially, 15d-PGJ(2) exerted its suppressive effects through the inhibition of NF-kappaB activation and not through peroxisome proliferator-activated receptor signaling. In contrast, lymph node cultures from transgenics proliferated more slowly and synthesized significantly less IL-2 than controls. Therefore, contrary to its role in driving Th2-like responses, this report shows that hPGD(2)S may act as an internal braking signal essential for bringing about the resolution of Th1-driven delayed type hypersensitivity reactions. Consequently, hPGD(2)S-derived cyclopentenone PGs may protect against inflammatory diseases, where T lymphocytes play a pathogenic role, as in rheumatoid arthritis, atopic eczema, and chronic rejection.

The in vitro real-time oscillation monitoring system identifies potential entrainment factors for circadian clocks

BACKGROUND

Circadian rhythms are endogenous, self-sustained oscillations with approximately 24-hr rhythmicity that are manifested in various physiological and metabolic processes. The circadian organization of these processes in mammals is governed by the master oscillator within the suprachiasmatic nuclei (SCN) of the hypothalamus. Recent findings revealed that circadian oscillators exist in most organs, tissues, and even in immortalized cells, and that the oscillators in peripheral tissues are likely to be coordinated by SCN, the master oscillator. Some candidates for endogenous entrainment factors have sporadically been reported, however, their details remain mainly obscure.

RESULTS

We developed the in vitro real-time oscillation monitoring system (IV-ROMS) by measuring the activity of luciferase coupled to the oscillatory gene promoter using photomultiplier tubes and applied this system to screen and identify factors able to influence circadian rhythmicity. Using this IV-ROMS as the primary screening of entrainment factors for circadian clocks, we identified 12 candidates as the potential entrainment factor in a total of 299 peptides and bioactive lipids. Among them, four candidates (endothelin-1, all-trans retinoic acid, 9-cis retinoic acid, and 13-cis retinoic acid) have already been reported as the entrainment factors in vivo and in vitro. We demonstrated that one of the novel candidates, 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2), a natural ligand of the peroxisome proliferator-activated receptor-gamma (PPAR-gamma), triggers the rhythmic expression of endogenous clock genes in NIH3T3 cells. Furthermore, we showed that 15d-PGJ2 transiently induces Cry1, Cry2, and Roralpha mRNA expressions and that 15d-PGJ2-induced entrainment signaling pathway is PPAR-gamma--and MAPKs (ERK, JNK, p38MAPK)-independent.

CONCLUSION

Here, we identified 15d-PGJ2 as an entrainment factor in vitro. Using our developed IV-ROMS to screen 299 compounds, we found eight novel and four known molecules to be potential entrainment factors for circadian clocks, indicating that this assay system is a powerful and useful tool in initial screenings.

Collecting duct-specific deletion of peroxisome proliferator-activated receptor gamma blocks thiazolidinedione-induced fluid retention

The peroxisome proliferator-activated receptor subtype gamma (PPARgamma) ligands, namely the synthetic insulin-sensitizing thiazolidinedione (TZD) compounds, have demonstrated great potential in the treatment of type II diabetes. However, their clinical applicability is limited by a common and serious side effect of edema. To address the mechanism of TZD-induced edema, we generated mice with collecting duct (CD)-specific disruption of the PPARgamma gene. We found that mice with CD knockout of this receptor were resistant to the rosiglitazone- (RGZ) induced increases in body weight and plasma volume expansion found in control mice expressing PPARgamma in the CD. RGZ reduced urinary sodium excretion in control and not in conditional knockout mice. Furthermore, RGZ stimulated sodium transport in primary cultures of CD cells expressing PPARgamma and not in cells lacking this receptor. These findings demonstrate a PPARgamma-dependent pathway in regulation of sodium transport in the CD that underlies TZD-induced fluid retention.

Cyclopentenone eicosanoids as mediators of neurodegeneration: a pathogenic mechanism of oxidative stress-mediated and cyclooxygenase-mediated neurotoxicity

The activation of cyclooxygenase enzymes in the brain has been implicated in the pathogenesis of numerous neurodegenerative conditions. Similarly, oxidative stress is believed to be a major contributor to many forms of neurodegeneration. These 2 distinct processes are united by a common characteristic: the generation of electrophilic cyclopentenone eicosanoids. These cyclopentenone compounds are defined structurally by the presence of an unsaturated carbonyl moiety in their prostane ring, and readily form Michael adducts with cellular thiols, including those found in glutathione and proteins. The cyclopentenone prostaglandins (PGs) PGA2, PGJ2, and 15-deoxy-delta(12,14) PGJ2, enzymatic products of cyclooxygenase-mediated arachidonic acid metabolism, exert a complex array of potent neurodegenerative, neuroprotective, and anti-inflammatory effects. Cyclopentenone isoprostanes (A2/J2-IsoPs), products of non-enzymatic, free radical-mediated arachidonate oxidation, are also highly bioactive, and can exert direct neurodegenerative effects. In addition, cyclopentenone products of docosahexaenoic acid oxidation (cyclopentenone neuroprostanes) are also formed abundantly in the brain. For the first time, the formation and biological actions of these various classes of reactive cyclopentenone eicosanoids are reviewed, with emphasis on their potential roles in neurodegeneration. The accumulating evidence suggests that the formation of cyclopentenone eicosanoids in the brain may represent a novel pathogenic mechanism, which contributes to many neurodegenerative conditions.

Differential impact of prostaglandin H synthase 1 knockdown on platelets and parturition

Platelet activation is a hallmark of severe preeclampsia, and platelet PGH synthase 1-derived (PGHS1-derived) thromboxane A(2) (TxA(2)) has been implicated in its pathogenesis. However, genetic disruption of PGHS1 delays parturition. We created hypomorphic PGHS1 (PGHS1(Neo/Neo)) mice, in which the substantial but tissue-dependent variability in the inhibition of PGHS1-derived eicosanoids achieved by low-dose aspirin treatment is mimicked, to assess the relative impact of this strategy on hemostatic and reproductive function. Depression of platelet TxA(2) by 98% in PGHS1(Neo/Neo) mice decreased platelet aggregation and prevented thrombosis. Similarly, depression of macrophage PGE(2) by 75% was associated with selectively impaired inflammatory responses. PGF(2alpha) at 8% WT levels was sufficient to induce coordinated temporal oxytocin receptor (OTR) expression in uterus and normal ovarian luteolysis in PGHS1(Neo/Neo) mice at late gestation, while absence of PGHS1 expression in null mice delayed OTR induction and the programmed decrease of serum progesterone during parturition. Thus, extensive but tissue-dependent variability in PG suppression, as occurs with low-dose aspirin treatment, prevents thrombosis and impairs the inflammatory response but sustains parturition. PGHS1(Neo/Neo) mice provide a model of low-dose aspirin therapy that elucidates how prevention or delay of preeclampsia might be achieved without compromising reproductive function.

Nitrolinoleic acid: an endogenous peroxisome proliferator-activated receptor gamma ligand

Nitroalkene derivatives of linoleic acid (nitrolinoleic acid, LNO2) are formed via nitric oxide-dependent oxidative inflammatory reactions and are found at concentrations of approximately 500 nM in the blood of healthy individuals. We report that LNO2 is a potent endogenous ligand for peroxisome proliferator-activated receptor gamma (PPARgamma; Ki approximately 133 nM) that acts within physiological concentration ranges. This nuclear hormone receptor (PPARgamma) regulates glucose homeostasis, lipid metabolism, and inflammation. PPARgamma ligand activity is specific for LNO2)and not mediated by LNO2 decay products, NO donors, linoleic acid (LA), or oxidized LA. LNO2 is a significantly more robust PPARgamma ligand than other reported endogenous PPARgamma ligands, including lysophosphatidic acid (16:0 and 18:1), 15-deoxy-Delta12,14-PGJ2, conjugated LA and azelaoyl-phosphocholine. LNO2 activation of PPARgamma via CV-1 cell luciferase reporter gene expression analysis revealed a ligand activity that rivals or exceeds synthetic PPARgamma agonists such as rosiglitazone and ciglitazone, is coactivated by 9 cis-retinoic acid and is inhibited by the PPARgamma antagonist GW9662. LNO2 induces PPARgamma-dependent macrophage CD-36 expression, adipocyte differentiation, and glucose uptake also at a potency rivaling thiazolidinediones. These observations reveal that NO-mediated cell signaling reactions can be transduced by fatty acid nitration products and PPAR-dependent gene expression.

Cellular mechanisms of redox cell signalling: role of cysteine modification in controlling antioxidant defences in response to electrophilic lipid oxidation products

The molecular mechanisms through which oxidized lipids and their electrophilic decomposition products mediate redox cell signalling is not well understood and may involve direct modification of signal-transduction proteins or the secondary production of reactive oxygen or nitrogen species in the cell. Critical in the adaptation of cells to oxidative stress, including exposure to subtoxic concentrations of oxidized lipids, is the transcriptional regulation of antioxidant enzymes, many of which are controlled by antioxidant-responsive elements (AREs), also known as electrophile-responsive elements. The central regulator of the ARE response is the transcription factor Nrf2 (NF-E2-related factor 2), which on stimulation dissociates from its cytoplasmic inhibitor Keap1, translocates to the nucleus and transactivates ARE-dependent genes. We hypothesized that electrophilic lipids are capable of activating ARE through thiol modification of Keap1 and we have tested this concept in an intact cell system using induction of glutathione synthesis by the cyclopentenone prostaglandin, 15-deoxy-Delta12,14-prostaglandin J2. On exposure to 15-deoxy-Delta12,14-prostaglandin J2, the dissociation of Nrf2 from Keap1 occurred and this was dependent on the modification of thiols in Keap1. This mechanism appears to encompass other electrophilic lipids, since 15-A(2t)-isoprostane and the lipid aldehyde 4-hydroxynonenal were also shown to modify Keap1 and activate ARE. We propose that activation of ARE through this mechanism will have a major impact on inflammatory situations such as atherosclerosis, in which both enzymic as well as non-enzymic formation of electrophilic lipid oxidation products are increased.

15-Deoxy-delta12,14-PGJ2: endogenous PPARgamma ligand or minor eicosanoid degradation product?

15-Deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) affects gene transcription by activating PPARgamma and by covalent addition to transcription factors and signaling molecules, However, it is not known whether the high concentrations of 15d-PGJ2 required for these responses are consistent with physiological levels. A new study suggests that in vivo 15d-PGJ2 levels are actually several orders of magnitude below the levels required to induce many of the biological effects attributed to this molecule.