Jacques Maclouf

Activation of human aortic smooth-muscle cells is inhibited by PPARα but not by PPARγ activators

Peroxisome proliferator-activated receptors (PPARs) are key players in lipid and glucose metabolism and are implicated in metabolic disorders predisposing to atherosclerosis, such as dyslipidaemia and diabetes. Whereas PPARγ promotes lipid storage by regulating adipocyte differentiation, PPARα stimulates the β-oxidative degradation of fatty acids. PPARα-deficient mice show a prolonged response to inflammatory stimuli, suggesting that PPARα is also a modulator of inflammation. Hypolipidaemic fibrate drugs are PPARα ligands that inhibit the progressive formation of atherosclerotic lesions, which involves chronic inflammatory processes, even in the absence of their atherogenic lipoprotein-lowering effect,. Here we show that PPARα is expressed in human aortic smooth-muscle cells, which participate in plaque formation and post-angioplasty re-stenosis. In these smooth-muscle cells, we find that PPARα ligands, and not PPARγ ligands, inhibit interleukin-1-induced production of interleukin-6 and prostaglandin and expression of cyclooxygenase-2. This inhibition of cyclooxygenase-2 induction occurs transcriptionally as a result of PPARα repression of NF-κB signalling. In hyperlipidaemic patients, fenofibrate treatment decreases the plasma concentrations of interleukin-6, fibrinogen and C-reactive protein. We conclude that activators of PPARα inhibit the inflammatory response of aortic smooth-muscle cells and decrease the concentration of plasma acute-phase proteins, indicating that PPARα in the vascular wall may influence the process of atherosclerosis and re-stenosis.

Localization of distinct F2-isoprostanes in human atherosclerotic lesions.

F2-Isoprostanes are prostaglandin (PG) isomers formed in situ in cell membranes by peroxidation of arachidonic acid. 8-epi PGF2alpha and IPF2alpha-I are F2-isoprostanes produced in humans which circulate in plasma and are excreted in urine. Measurement of F2-isoprostanes may offer a sensitive, specific, and noninvasive method for measuring oxidant stress in clinical settings where reactive oxygen species are putatively involved. We determined whether isoprostanes were present in human atherosclerotic lesions, where lipid peroxidation is thought to occur in vivo. 8-epi PGF2alpha ranged from 1.310-3.450 pmol/micromol phospholipid in atherectomy specimens compared with 0.045-0.115 pmol/micromol phospholipid (P < 0.001) in vascular tissue devoid of atherosclerosis. Corresponding values of IPF2alpha-I were 5.6-13.8 vs. 0.16-0.44 pmol/micromol phospholipid (P < 0.001). Levels of the two isoprostanes in vascular tissue were highly correlated (r = 0.80, P < 0.0001). Immunohistochemical studies confirmed that foam cells adjacent to the lipid necrotic core of the plaque were markedly positive for 8-epi PGF2alpha. These cells were also reactive with anti-CD68, an epitope specific for human monocyte/macrophages. 8-epi PGF2alpha immunoreactivity was also detected in cells positive for anti-alpha-smooth muscle actin antibody, which specifically recognizes vascular smooth muscle cells. Our results indicate that 8-epi PGF2alpha and IPF2alpha-I, two distinct F2-isoprostanes and markers of oxidative stress in vivo, are present in human atherosclerotic plaque. Quantitation of these chemically stable products of lipid peroxidation in target tissues, as well as in biological fluids, may aid in the rational development of antioxidant drugs in humans.

The relationship of hydroxyeicosatetraenoic acids and F2-isoprostanes to plaque instability in human carotid atherosclerosis

Evidence for increased oxidant stress has been reported in human atherosclerosis. However, no information is available about the importance of in situ oxidant stress in relation to plaque stability. This information is relevant because the morbidity and mortality of atherosclerosis are essentially the consequences of acute ischemic syndromes due to unstable plaques. We studied 30 carotid atherosclerotic plaques retrieved by endarterectomy from 18 asymptomatic (stable plaques) and 12 symptomatic patients (unstable plaques). Four normal arteries served as controls. After lipid extraction and ester hydrolysis, quantitation of different indices of oxidant stress were analyzed, including hydroxyeicosatetraenoic acids (HETEs), epoxyeicosatetraenoic acids (EETs), ketoeicosatetraenoic acids (oxo-ETEs), and F2-isoprostanes using online reverse-phase high-performance liquid chromatography tandem mass spectrometry (LC/MS/MS). All measurements were carried out in a strictly double-blind procedure. We found elevated levels of the different compounds in atherosclerotic plaques. Levels of HETEs were 24 times higher than EETs, oxo-ETEs, or F2-isoprostanes. Levels of HETEs, but not those of EETs, oxo-ETEs or F2-isoprostanes, were significantly elevated in plaques retrieved from symptomatic patients compared with those retrieved from asymptomatic patients (1, 738 +/- 274 vs. 1,002 +/- 107 pmol/ micromol lipid phosphorous, respectively; P < 0.01). One monooxygenated arachidonate species, 9-HETE, which cannot be derived from known enzymatic reactions, was the most abundant and significant compound observed in plaques, suggesting that nonenzymatic lipid peroxidation predominates in advanced atherosclerosis and may promote plaque instability.

Role of NF-κB in the Antiproliferative Effect of Endothelin-1 and Tumor Necrosis Factor-α in Human Hepatic Stellate Cells INVOLVEMENT OF CYCLOOXYGENASE-2

During chronic liver diseases, hepatic stellate cells (HSC) acquire an activated myofibroblast-like phenotype and proliferate and synthesize fibrosis components. Endothelin-1 (ET-1), which inhibited the growth of human myofibroblastic HSC, increased the formation of two NF-κB DNA binding complexes; this effect was also observed with tumor necrosis factor-α (TNF-α). The complexes were identified as the p50/p50 and p50/p65 NF-κB dimers. Activation of NF-κB was associated with the degradation of the inhibitory protein IκB-α; no IκB-β was detected. Activation of NF-κB and degradation of IκB-α were prevented by the NF-κB inhibitors sodium salicylate and MG-132. In addition to cyclooxygenase-1 (COX-1), COX-2 is also constitutively expressed in human HSC, and the use of dexamethasone and of SC-58125, a selective COX-2 inhibitor, revealed that COX-2 accounts for basal COX activity. Moreover, COX-2 mRNA and protein were up-regulated by ET-1 and TNF-α, whereas COX-1 was unaffected. Induction of COX-2 and stimulation of COX activity by ET-1 and TNF-α were prevented by sodium salicylate and MG-132, suggesting that activation of NF-κB by either factor is needed for stimulation of COX-2. Finally, SC-58125 and dexamethasone reduced the growth inhibitory effect of ET-1 and TNF-α, indicating that activation of COX-2 is required for inhibition of HSC proliferation. Taken together, our results suggest that NF-κB, by inducing COX-2 expression, may play an important role in the negative regulation of human myofibroblastic HSC proliferation.

Expression of cyclooxygenase-1 and -2 in ovine endometrium during the estrous cycle and early pregnancy.

In this study we investigated the expression of the two cyclooxygenases, cox-1 and -2, in sheep uterine tissues during the estrous cycle and early pregnancy. We identified the cox-2 isoform in the ovine uterus by Western blot and demonstrated that the two cyclooxygenases exhibited different patterns of expression. Cox-1 was expressed at steady state levels in the endometrium during the estrous cycle and comparable stages of pregnancy. In contrast, cox-2 was highly and transiently expressed from days 12–15 of the estrous cycle and declined thereafter to undetectable levels. Endometrium from early pregnant ewes showed a similar pattern of cox-2 expression, although there was a slower decrease beyond day 15. Immunohistochemical studies demonstrated that cox-1 was localized in both epithelial and stromal cells, whereas cox-2 was localized solely in the luminal epithelium and to a lesser extent in the superficial glands. Treatment of ovariectomized ewes with steroids indicated that expression of cox-1 remained...

Inducible nitric oxide synthase expression in activated rat microglial cultures is downregulated by exogenous prostaglandin E2 and by cyclooxygenase inhibitors

Prostaglandins and nitric oxide (NO) are among the numerous substances released by activated microglial cells, the brain resident macrophages, and they mediate several important microglial functions. We have previously shown that cyclooxygenase‐2 (COX‐2) and inducible NO synthase (iNOS), the two key enzymes in prostaglandin and NO synthesis, respectively, are rapidly co‐induced in rat neonatal microglial cultures activated by bacterial endotoxin (lipopolysaccharide [LPS]) and that COX‐2 expression appears to be under the negative control of endogenous as well as exogenous NO. In this study we show that exogenous prostaglandin E2 (PGE2), which is known to increase cyclic adenosine monophosphate (cAMP) levels in microglial cells, downregulates LPS‐induced iNOS expression in a dose‐dependent manner. The involvement of cAMP in the PGE2‐dependent inhibition of iNOS is supported by several pieces of evidence. First, iNOS expression was also inhibited by agents such as isoproterenol and forskolin, which cause an elevation of cAMP levels, and by dibutyryl cAMP (dbcAMP), a cAMP stable analogue. Second, the inhibitory effect of PGE2 was mimicked by 11‐deoxy‐16,16‐dm PGE2, a selective agonist at the PGE2 receptor subtype EP2, coupled to the activation of adenylyl cyclase, but not by sulprostone, a potent agonist at receptor subtypes EP3 and EP1, associated with an inhibition of adenylyl cyclase activity and intracellular Ca2+ elevation, respectively. Third, the inhibitory effect of PGE2 on NO synthesis was blocked by SQ 22,536, a specific inhibitor of adenylyl cyclase. Interestingly, the abrogation of endogenous prostanoid production by several COX inhibitors caused a reduction of iNOS expression, suggesting a positive modulatory effect of endogenous prostanoids of iNOS expression, as opposed to the inhibitory effect of exogenous PGE2.

Early expression of cyclo‐oxygenase‐2 during sporadic colorectal carcinogenesis

Regular administration of non‐steroidal anti‐inflammatory drugs (NSAIDs) may reduce the incidence of colorectal cancer by targeting cyclo‐oxygenase‐2 (Cox‐2), a key enzyme in arachidonic acid metabolism. To evaluate the role of Cox‐2 in sporadic colorectal cancer development, Cox‐2 expression was investigated by immunohistochemistry in 85 adenomas, 53 carcinomas, 34 hyperplastic lesions and 104 samples of histologically normal mucosa adjacent to adenoma or carcinoma. In addition, Cox‐2 mRNA expression was assessed by reverse transcription‐polymerase chain reaction (RT‐PCR) in six adenomas and 14 carcinomas with paired grossly normal mucosa. Immunohistochemistry for the proliferation‐associated antigen Ki‐67 and in situ end labelling for demonstrating apoptotic bodies were also used to analyse the associations between Cox‐2 expression and proliferation and apoptosis. Cox‐2 protein expression was increased in 76/85 (89·4 per cent) adenomas and 44/53 (83·0 per cent) carcinomas compared with normal mucosa. Cox‐2 protein expression was unrelated either to the degree of dysplasia or to the size of the adenomas (p > 0·50, p > 0·10, respectively) or to differentiation, Dukes stage or lymph node metastasis of carcinomas (all p > 0·50). Interestingly, 20/34 (58·8 per cent) hyperplastic lesions adjacent to adenomas or carcinomas displayed expression higher than in normal mucosa (18·3 per cent) (p < 0·0001) but lower than in adenomas or carcinomas (p < 10−5, p < 0·001, respectively). There were no correlations between Cox‐2 protein expression and proliferative or apoptotic index in either adenomas or carcinomas (all p > 0·25). Cox‐2 mRNA expression was significantly increased in adenomas and carcinomas compared with normal mucosa (p < 0·005, p < 0·001, respectively). There were no differences between adenomas and carcinomas in either protein or mRNA levels (p > 0·25, p > 0·90, respectively). These data indicate that enhanced expression of Cox‐2 occurs early during colorectal carcinogenesis and may contribute to tumour formation. Copyright

Radioimmunoassay of 11-dehydrothromboxane B2 in human plasma and urine

Because of the discrepancy between the capacity of platelets to synthesize thromboxane B2 ex vivo and the actual synthetic rate in vivo, measurement of thromboxane B2 in plasma is highly influenced by sampling-related artifacts. We have developed and validated a radioimmunoassay for a major enzymatic derivative of thromboxane B2 with an extended plasma half-life, i.e., 11-dehydrothromboxane B2. The binding of the tracer is displaced by as low as 1 pg/ml of the homologous ligand, with a high degree of specificity for the open ring structure as well as for the omega side-chain. This method can detect changes in the plasma concentration and urinary excretion of 11-dehydrothromboxane B2 associated with stimulated short-term increases of thromboxane B2 secretion in the human circulation.

Cyclooxygenases-1 and −2 of Endothelial Cells Utilize Exogenous or Endogenous Arachidonic Acid for Transcellular Production of Thromboxane

The presence of prostaglandin (PG) H in the supernatant of human umbilical vein endothelial cells (HUVEC) stimulated by thrombin restores the capacity of aspirin-treated platelets to generate thromboxane (TX) B. Induction of cyclooxygenase-2 (Cox-2) by interleukin (IL)-1α or a phorbol ester increases this formation. HUVEC treated with aspirin lost their capacity to generate PGs but recovery occurred after 3- or 6-h induction of Cox-2 with phorbol ester or IL-1α. Enzyme activity of the newly synthesized Cox-2 in aspirin-treated cells, evaluated after immunoprecipitation, was similar to untreated cells but after 18 h of cell stimulation only 50-60% recovery of Cox-1 was observed. The use of SC58125, a selective Cox-2 inhibitor, confirmed these findings in intact cells. Cyclooxygenase activity was related to the amount of Cox proteins present in the cells, but after induction of Cox-2, contribution of the latter to PG production was 6-8-fold that of Cox-1. Aspirin-treated or untreated cells were incubated in the absence or presence of SC58125 and stimulated by thrombin, the ionophore A23187, or exogenous arachidonic acid. The production of endogenous (6-keto-PGF, PGE, PGF) versus transcellular (TXB) metabolites was independent of the inducer, the source of arachidonic acid and the Cox isozyme. However, in acetylsalicylic acid-treated cells, after 6-h stimulation with IL-1α, newly synthesized Cox-2 produced less TXB than 6-keto-PGF compared to untreated cells. At later times (>18 h), there was no metabolic difference between the cells. These studies suggest that in HUVEC, Cox compartmentalization occurring after short-term activation may selectively affect transcellular metabolism, but not constitutive production, of PGs.

PHOSPHORYLATION OF THE THROMBOXANE RECEPTOR ALPHA , THE PREDOMINANT ISOFORM EXPRESSED IN HUMAN PLATELETS

A single gene encodes the human thromboxane receptor (TP), of which there are two identified splice variants, α and β. Both isoforms are rapidly phosphorylated in response to thromboxane agonists when overexpressed in human embryonic kidney 293 cells; this phenomenon is only slightly altered by inhibitors of protein kinase C. Pharmacological studies have defined two classes of TP in human platelets; sites that bind the agonist I-BOP with high affinity support platelet shape change. Low affinity sites, which irreversibly bind the antagonist GR 32191, transduce platelet activation and aggregation. Isoform-specific antibodies permitted detection of TPα, but not TPβ, from human platelets, although mRNA for both isoforms is present. A broad protein band of 50–60 kDa, reflecting the glycosylated receptor, was phosphorylated upon activation of platelets for 2 min with I-BOP. This was a rapid (∼30 s) and transient (maximum, 2–4 min) event and was inhibited by TP antagonists. Both arachidonic acid and low concentrations of collagen stimulated TPα phosphorylation, which was blocked by cyclooxygenase inhibition or TP antagonism. Blockade of the low affinity TP sites with GR 32191 prevented I-BOP-induced TPα phosphorylation. This coincided with agonist-induced platelet aggregation and activation but not shape change. Also, activation of these sites with the isoprostane iPF2α-III induced platelet shape change but not TPα phosphorylation. Heterologous TP phosphorylation was observed in aspirin-treated platelets exposed to thrombin, high concentrations of collagen, and the calcium ionophore A 23187. Both homologous and heterologous agonist-induced phosphorylation of endogenous TPα was blocked by protein kinase C inhibitors. TPα was the only isoform detectably translated in human platelets. This appeared to correspond to the activation of the low affinity site defined by the antagonist GR 32191 and not activated by the high affinity agonist, iPF2α-III. Protein kinase C played a more important role in agonist-induced phosphorylation of native TPα in human platelets than in human embryonic kidney 293 cells overexpressing recombinant TPα.