Garret A. FitzGerald

Prostaglandins and Inflammation

Prostaglandins are lipid autacoids derived from arachidonic acid. They both sustain homeostatic functions and mediate pathogenic mechanisms, including the inflammatory response. They are generated from arachidonate by the action of cyclooxygenase isoenzymes, and their biosynthesis is blocked by nonsteroidal antiinflammatory drugs, including those selective for inhibition of cyclooxygenase-2. Despite the clinical efficacy of nonsteroidal antiinflammatory drugs, prostaglandins may function in both the promotion and resolution of inflammation. This review summarizes insights into the mechanisms of prostaglandin generation and the roles of individual mediators and their receptors in modulating the inflammatory response. Prostaglandin biology has potential clinical relevance for atherosclerosis, the response to vascular injury and aortic aneurysm.

Platelet Activation in Unstable Coronary Disease

Pathological and clinical studies have suggested that platelets have a role in the pathogenesis of unstable angina and myocardial infarction. However, the relation of platelet activation to episodic ischemia in patients with unstable angina is unknown. We assessed the biosynthesis of thromboxane and prostacyclin as indexes of platelet activation in patients with stable and unstable coronary disease by physicochemical analysis of metabolites in plasma and urine. Prostacyclin biosynthesis was markedly elevated in patients with acute myocardial infarction and correlated with plasma creatine kinase (r = 0.795; P less than 0.001). The largest rise in thromboxane synthesis was observed in patients with unstable angina, in whom 84 percent of the episodes of chest pain were associated with phasic increases in the excretion of thromboxane and prostacyclin metabolites. However, 50 percent of such increases were not associated with chest pain, possibly reflecting silent myocardial ischemia. These data indicate that platelet activation occurs during spontaneous ischemia in patients with unstable angina. The increment in prostacyclin biosynthesis during such episodes may be a compensatory response of vascular endothelium that limits the degree or effects of platelet activation. If so, biochemically selective inhibition of the synthesis or action of thromboxane A2 would be desirable in the treatment of unstable angina. In contrast, thromboxane inhibitors or antagonists would not be expected to be effective in patients with chronic stable angina, in whom there was no increase in the formation of thromboxane A2.

Prostaglandin E2 regulates vertebrate haematopoietic stem cell homeostasis

Haematopoietic stem cell (HSC) homeostasis is tightly controlled by growth factors, signalling molecules and transcription factors. Definitive HSCs derived during embryogenesis in the aorta–gonad–mesonephros region subsequently colonize fetal and adult haematopoietic organs. To identify new modulators of HSC formation and homeostasis, a panel of biologically active compounds was screened for effects on stem cell induction in the zebrafish aorta–gonad–mesonephros region. Here, we show that chemicals that enhance prostaglandin (PG) E2 synthesis increased HSC numbers, and those that block prostaglandin synthesis decreased stem cell numbers. The cyclooxygenases responsible for PGE2 synthesis were required for HSC formation. A stable derivative of PGE2 improved kidney marrow recovery following irradiation injury in the adult zebrafish. In murine embryonic stem cell differentiation assays, PGE2 caused amplification of multipotent progenitors. Furthermore, ex vivo exposure to stabilized PGE2 enhanced spleen colony forming units at day 12 post transplant and increased the frequency of long-term repopulating HSCs present in murine bone marrow after limiting dilution competitive transplantation. The conserved role for PGE2 in the regulation of vertebrate HSC homeostasis indicates that modulation of the prostaglandin pathway may facilitate expansion of HSC number for therapeutic purposes.

Oxidative Stress and Cardiovascular Injury Part I: Basic Mechanisms and In Vivo Monitoring of ROS

Oxidative stress has been associated with diverse pathophysiological events, including cancer, renal disease, and neurodegeneration. More recently, it has become apparent that reactive oxygen species (ROS) also play a role in the development of vasculopathies, including those that define atherosclerosis, hypertension, and restenosis after angioplasty. The “response to injury” hypothesis developed by Russell Ross in the late 1970s suggested that atherosclerosis, at least, resulted from an initial injury to endothelial cells, leading to impaired endothelial function and subsequent macrophage infiltration and smooth muscle dysfunction. Many investigators then focused on oxidation of LDL and its interaction with the endothelium as the initial injury leading to the formation of fatty streaks and ultimately atherogenesis. It is now clear not only that diverse ROS are produced in the vessel wall, but that they individually and in combination contribute to many of the abnormalities associated with vascular disease. There are many ROS that play central roles in vascular physiology (Figure 1) and pathophysiology, the most important of which are nitric oxide (NO·), superoxide (O2·−), hydrogen peroxide (H2O2) and peroxynitrite (ONOO·−). NO· is normally produced by endothelial nitric oxide synthase (eNOS) in the vasculature, but in inflammatory states, inducible NOS can be expressed in macrophages and smooth muscle cells and contributes to NO·production. NO·is a crucial mediator of endothelium-dependent vasodilation and may also play a role in platelet aggregation and in maintaining the balance between smooth muscle cell growth and differentiation. Superoxide results from one electron reduction of oxygen by a variety of oxidases (Figure 2). When O2·− is produced in concert with NO·, they rapidly react to form the highly reactive molecule ONOO·−. ONOO·− is an important mediator of lipid peroxidation and protein nitration, including oxidation of LDL, which has dramatic proatherogenic effects. …

BMAL1 and CLOCK, Two Essential Components of the Circadian Clock, Are Involved in Glucose Homeostasis

Circadian timing is generated through a unique series of autoregulatory interactions termed the molecular clock. Behavioral rhythms subject to the molecular clock are well characterized. We demonstrate a role for Bmal1 and Clock in the regulation of glucose homeostasis. Inactivation of the known clock components Bmal1 (Mop3) and Clock suppress the diurnal variation in glucose and triglycerides. Gluconeogenesis is abolished by deletion of Bmal1 and is depressed in Clock mutants, but the counterregulatory response of corticosterone and glucagon to insulin-induced hypoglycaemia is retained. Furthermore, a high-fat diet modulates carbohydrate metabolism by amplifying circadian variation in glucose tolerance and insulin sensitivity, and mutation of Clock restores the chow-fed phenotype. Bmal1 and Clock, genes that function in the core molecular clock, exert profound control over recovery from insulin-induced hypoglycaemia. Furthermore, asynchronous dietary cues may modify glucose homeostasis via their interactions with peripheral molecular clocks.

Vascular and upper gastrointestinal effects of non-steroidal anti-inflammatory drugs: Meta-analyses of individual participant data from randomised trials

Summary Background The vascular and gastrointestinal effects of non-steroidal anti-inflammatory drugs (NSAIDs), including selective COX-2 inhibitors (coxibs) and traditional non-steroidal anti-inflammatory drugs (tNSAIDs), are not well characterised, particularly in patients at increased risk of vascular disease. We aimed to provide such information through meta-analyses of randomised trials. Methods We undertook meta-analyses of 280 trials of NSAIDs versus placebo (124 513 participants, 68 342 person-years) and 474 trials of one NSAID versus another NSAID (229 296 participants, 165 456 person-years). The main outcomes were major vascular events (non-fatal myocardial infarction, non-fatal stroke, or vascular death); major coronary events (non-fatal myocardial infarction or coronary death); stroke; mortality; heart failure; and upper gastrointestinal complications (perforation, obstruction, or bleed). Findings Major vascular events were increased by about a third by a coxib (rate ratio [RR] 1·37, 95% CI 1·14–1·66; p=0·0009) or diclofenac (1·41, 1·12–1·78; p=0·0036), chiefly due to an increase in major coronary events (coxibs 1·76, 1·31–2·37; p=0·0001; diclofenac 1·70, 1·19–2·41; p=0·0032). Ibuprofen also significantly increased major coronary events (2·22, 1·10–4·48; p=0·0253), but not major vascular events (1·44, 0·89–2·33). Compared with placebo, of 1000 patients allocated to a coxib or diclofenac for a year, three more had major vascular events, one of which was fatal. Naproxen did not significantly increase major vascular events (0·93, 0·69–1·27). Vascular death was increased significantly by coxibs (1·58, 99% CI 1·00–2·49; p=0·0103) and diclofenac (1·65, 0·95–2·85, p=0·0187), non-significantly by ibuprofen (1·90, 0·56–6·41; p=0·17), but not by naproxen (1·08, 0·48–2·47, p=0·80). The proportional effects on major vascular events were independent of baseline characteristics, including vascular risk. Heart failure risk was roughly doubled by all NSAIDs. All NSAID regimens increased upper gastrointestinal complications (coxibs 1·81, 1·17–2·81, p=0·0070; diclofenac 1·89, 1·16–3·09, p=0·0106; ibuprofen 3·97, 2·22–7·10, p<0·0001; and naproxen 4·22, 2·71–6·56, p<0·0001). Interpretation The vascular risks of high-dose diclofenac, and possibly ibuprofen, are comparable to coxibs, whereas high-dose naproxen is associated with less vascular risk than other NSAIDs. Although NSAIDs increase vascular and gastrointestinal risks, the size of these risks can be predicted, which could help guide clinical decision making. Funding UK Medical Research Council and British Heart Foundation.

A Functional Genomics Strategy Reveals Rora as a Component of the Mammalian Circadian Clock

The mammalian circadian clock plays an integral role in timing rhythmic physiology and behavior, such as locomotor activity, with anticipated daily environmental changes. The master oscillator resides within the suprachiasmatic nucleus (SCN), which can maintain circadian rhythms in the absence of synchronizing light input. Here, we describe a genomics-based approach to identify circadian activators of Bmal1, itself a key transcriptional activator that is necessary for core oscillator function. Using cell-based functional assays, as well as behavioral and molecular analyses, we identified Rora as an activator of Bmal1 transcription within the SCN. Rora is required for normal Bmal1 expression and consolidation of daily locomotor activity and is regulated by the core clock in the SCN. These results suggest that opposing activities of the orphan nuclear receptors Rora and Rev-erb alpha, which represses Bmal1 expression, are important in the maintenance of circadian clock function.

Oxidative Stress and Cardiovascular Injury: Part II: Animal and Human Studies

This review so far has considered the role of vascular cells in the generation of reactive oxygen species (ROS) and novel approaches to their detection in integrated systems such as animal models of vascular disease and humans. We now turn attention to evidence consistent with a role for ROS in models of human disease and in discrete patient populations. We also briefly comment on the outcomes of clinical trials of antioxidants. Mainly, these have been disappointing. However, it is premature to conclude that the oxidation hypothesis of disease causality has been adequately tested. Animal studies, for the most part, support a fundamental role for ROS in cardiovascular disease. Any controversy could in part reflect the use of ineffective antioxidants and the selection of models in which ROS generation is of marginal relevance to the measured outcome. Both of these issues require a quantitatively accurate measurement of drug effect (ie, antioxidant capacity) before hypotheses relating to the role of oxidant stress can be addressed rationally. These issues pertain to clinical trials also, as we will discuss. However, animal studies permit administration of much more powerful antioxidants (eg, rate constant for interaction of superoxide dismutase (SOD) with O2·− ≈1.6×109 · mol/L−1 · s−1) than is possible in humans (eg, rate constant for vitamin E ≈0.59 mol/L−1 · s−1) or, like in the case of vitamin E, doses of the antioxidant in excess of those usually applied in clinical research. ### Atherosclerosis Animal models of atherosclerosis have documented that all the constituents of the plaque produce and use ROS. Lesion formation is associated with the accumulation of lipid peroxidation products1,2 and induction of inflammatory genes,3 inactivation of NO·resulting in endothelial dysfunction, 4,5 activation of matrix metalloproteinases,6 and increased smooth muscle cell growth.7 …

Endogenous biosynthesis of prostacyclin and thromboxane and platelet function during chronic administration of aspirin in man.

To assess the pharmacologic effects of aspirin on endogenous prostacyclin and thromboxane biosynthesis, 2,3-dinor-6-keto PGF1 alpha (PGI-M) and 2,3-dinor-thromboxane B2 (Tx-M) were measured in urine by mass spectrometry during continuing administration of aspirin. To define the relationship of aspirin intake to endogenous prostacyclin biosynthesis, sequential urines were initially collected in individuals prior to, during, and subsequent to administration of aspirin. Despite inter- and intra-individual variations, PGI-M excretion was significantly reduced by aspirin. However, full mass spectral identification confirmed continuing prostacyclin biosynthesis during aspirin therapy. Recovery of prostacyclin biosynthesis was incomplete 5 d after drug administration was discontinued. To relate aspirin intake to indices of thromboxane biosynthesis and platelet function, volunteers received 20 mg aspirin daily followed by 2,600 mg aspirin daily, each dose for 7 d in sequential weeks. Increasing aspirin dosage inhibited Tx-M excretion from 70 to 98% of pretreatment control values; platelet TxB2 formation from 4.9 to 0.5% and further inhibited platelet function. An extended study was performed to relate aspirin intake to both thromboxane and prostacyclin generation over a wide range of doses. Aspirin, in the range of 20 to 325 mg/d, resulted in a dose-dependent decline in both Tx-M and PGI-M excretion. At doses of 325-2,600 mg/d Tx-M excretion ranged from 5 to 3% of control values while PGI-M remained at 37-23% of control. 3 d after the last dose of aspirin (2,600 mg/d) mean Tx-M excretion had returned to 85% of control, whereas mean PGI-M remained at 40% of predosing values. Although the platelet aggregation response (Tmax) to ADP ex vivo was inhibited during administration of the lower doses of aspirin the aggregation response returned to control values during the final two weeks of aspirin administration (1,300 and 2,600 mg aspirin/d) despite continued inhibition of thromboxane biosynthesis. These results suggest that although chronic administration of aspirin results in inhibition of endogenous thromboxane and prostacyclin biosynthesis over a wide dose range, inhibition of thromboxane biosynthesis is more selective at 20 than at 2,600 mg aspirin/d. However, despite this, inhibition of platelet function is not maximal at the lower aspirin dosage. Doses of aspirin in excess of 80 mg/d resulted in substantial inhibition of endogenous prostacyclin biosynthesis. Thus, it is unlikely that any dose of aspirin can maximally inhibit thromboxane generation without also reducing endogenous prostacyclin biosynthesis. These results also indicate that recovery of endogenous prostacyclin biosynthesis is delayed following aspirin administration and that the usual effects of aspirin on platelet function ex vivo may be obscured during chronic aspirin administration in man.

Human platelet/erythroleukemia cell prostaglandin G/H synthase: cDNA cloning, expression, and gene chromosomal assignment.

Platelets metabolize arachidonic acid to thromboxane A2, a potent platelet aggregator and vasoconstrictor compound. The first step of this transformation is catalyzed by prostaglandin (PG) G/H synthase, a target site for nonsteroidal antiinflammatory drugs. We have isolated the cDNA for both human platelet and human erythroleukemia cell PGG/H synthase using the polymerase chain reaction and conventional screening procedures. The cDNA encoding the full‐length protein was expressed in COS‐M6 cells. Microsomal fractions from transfected cells produced prostaglandin endoperoxide‐derived products which were inhibited by indomethacin and aspirin. Mutagenesis of the serine residue at position 529, the putative aspirin acetylation site, to an asparagine reduced cyclooxygenase activity to barely detectable levels, an effect observed previously with the expressed sheep vesicular gland enzyme. Platelet‐derived growth factor and phorbol ester differentially regulated the expression of PGG/H synthase mRNA levels in the megakaryocytic/platelet‐like HEL cell line. The PGG/H synthase gene was assigned to chromosome 9 by analysis of a human‐hamster somatic hybrid DNA panel. The availability of platelet PGG/H synthase cDNA should enhance our understanding of the important structure/function domains of this protein and its gene regulation.—Funk, C. D.; Funk, L. B.; Kennedy, M. E.; Pong, A. S.; FitzGerald, G. A. Human platelet/erythroleukemia cell prostaglandin G/H synthase: cDNA cloning, expression, and gene chromosomal assignment. FASEB J. 5: 2304–2312; 1991.