The William Harvey Research Institute St. Bartholomew's and the Royal London School of Medicine and Dentistry London, UK
c.thiemermann@qmul.ac.uk
New findings in mice suggest that corticosteroids mediate nitric oxide production in the endothelium, which in turn protects the heart against damage when deprived of oxygen. The mechanism explains, at least in part, the cardioprotective effects of these anti-inflammatory agents. (pages 473−479)
Heart disease is the leading cause of death worldwide, and its prevalence is rapidly increasing in non-industrialized countries. Myocardial infarction is the irreversible necrosis of heart muscle secondary to prolonged oxygen deprivation; it results in impaired contractile function, increased predisposition to arrhythmias and other long-term complications. In the United States alone, each year approximately 1.5 million people suffer from acute myocardial infarction and 500,000−700,000 die from a coronary artery−related event. Corticosteroids—potent anti-inflammatory and immunosuppressive agents commonly used in the treatment of several disorders—protect the heart from ischemic injury1, but before these compounds can be considered for therapy in ischemic heart disease, a better understanding of their mechanism of action must emerge. In this issue, Hafezi-Moghadam and colleagues at Harvard Medical School suggest that regulation of endothelial nitric oxide (NO) synthase (eNOS) activity plays a key role in the process2.
Myocardial infarction and other acute coronary syndromes occur in response to an imbalance of oxygen supply and demand; reperfusion of the previously ischemic myocardium and the associated inflammatory response contribute to the development of cardiac injury. Although anti-inflammatory corticosteroids protect the heart against ischemic injury1, they also reduce wound healing and scar formation, leading to the development of cardiac aneurysms and potentially fatal cardiac ruptures3. The adverse effects of corticosteroids have been attributed to their genomic effects. Corticosteroids bind to nuclear glucocorticoid receptors, which in turn modulate the expression of target genes by binding to DNA sequences containing the glucocorticoid response elements (GRE). Which target genes are involved in the known cardioprotective effects of corticosteroids remains an important question.
The study by Hafezi-Moghadam and colleagues reports that high-dose corticosteroids protect the heart and cremaster muscle of the mouse against ischemia-reperfusion injury by causing the rapid (within 10−60 min), non-transcriptional activation of eNOS2. Specifically, the authors show that binding of dexamethasone to the glucocorticoid receptor results in the stimulation of phosphatidylinositol 3-kinase and protein kinase Akt, which cause activation of eNOS and enhanced NO formation. Once formed by vascular endothelial cells, NO diffuses to adjacent cells and activates soluble guanylate cyclase (sGC) by binding to the iron on its heme component. Activation of sGC results in a reduction in intracellular calcium concentration, which in turn mediates many (but not all) of the effects of NO, including vasodilatation and inhibition of platelet and neutrophil adhesion (Fig. 1).
Figure 1. Proposed mechanisms by which corticosteroids mediate cardioprotection.
The binding of dexamethasone to the glucocorticoid (GC) receptor results in the stimulation of phosphatidylinositol 3-kinase (PI3-kinase) and the protein kinase Akt. Activated Akt phosphorylates (P) and thereby activates eNOS, leading to enhanced NO production. Once formed by vascular endothelial cells, NO diffuses to adjacent cells and activates soluble guanylate cyclase, resulting in vasodilation and inhibition of platelet and neutrophil adhesion. In addition to eNOS activation, inhibition of NF-B activation and/or enhanced expression of annexin 1 may also contribute to the cardioprotective effects of glucocorticoids.
In wild-type mice, the cardioprotective effects of corticosteroids were abolished by antagonists of the nuclear glucocorticoid receptor and inhibitors of eNOS activitiy (NG-nitroarginine methyl ester)3. Most notably, no cardioprotective effects of corticosteroids were found in mice in which the eNOS gene had been deleted (eNOS-/- mice) 3. The authors, therefore, propose that the rapid activation of eNOS in response to the non-nuclear actions of glucocorticoid receptors will result in enhanced formation of NO, which may mediate beneficial effects of corticosteroids in cardiovascular disese.
The authors' hypothesis that the increase in eNOS activity is the key mechanism by which corticosteroids reduce tissue injury caused by ischemia-reperfusion is difficult to prove—and may well be disputed by other investigators. Clearly, the administration of high doses (40 mg/kg) of glucocorticosteroids will cause a multitude of effects in vivo. These include the reported activation of eNOS, but also the enhanced expression of annexin I (or lipocortin-1) and the inhibition of the transcription factor, nuclear factor-B (NF-B). In my opinion, the latter two effects may well contribute to the observed anti-ischemic effects of high-dose corticosteroid therapy.
Indeed, regional myocardial ischemia and reperfusion result both in the expression (in leukocytes) of annexin I4 and in the activation of NF-B5,
6. There is evidence that inhibiting the activation of NF-B exerts cardioprotective effects in rodents. For instance, reperfusion (for 15 min) of the previously ischemic myocardium in rodents results in the degradation of inhibitor B (IB)5,
6, activation of IB kinase complex (IKK) and subsequent translocation of NF-B into the nucleus5. Inhibition of IKK activity with parthenolide, a sesquiterpene lactone, results in a significant reduction of ischemia-reperfusion injury as well as the associated inflammatory response7. Similarly, inhibition of the activation of IKK activity with the cyclopentanone prostaglandin 15-deoxy8,
9 prostaglandin J2 (15d-PGJ2)10 reduces myocardial infarct size in rats subjected to regional myocardial ischemia and reperfusion6.
Preventing NF-B activation in the heart results in the reduced expression of several genes. These include the inducible isoform of NO synthase (iNOS), which results in a reduced formation of peroxynitrite5,
6. Also affected are the chemokine MCP-1 and adhesion molecule vascular adhesion molecule-1 (VCAM-1)6, and thereby the adhesion, activation and extravasation of polymorphonuclear leukocytes5,
6. There is also evidence that the administration of either annexin I or the annexin I peptide Ac2-26 reduces the infarct size as well as the inflammatory response caused by regional myocardial ischemia and reperfusion in the rat4. Thus, it is possible that the cardioprotective effects of glucocorticoids are due to one or more of the following pharmacological effects: activation of eNOS, inhibition of the activation of NF-B, and/or enhanced expression of annexin 1.
The current study supports growing evidence that links protein kinase Akt to the upregulation of eNOS activity and cardioprotection. For instance, the finding that dexamethasone activates protein kinase Akt has recently been reported11 and activation of Akt enhances eNOS expression12,
13. Similarly, it is known that estrogen upregulates eNOS activity via activation of protein kinase Akt8. Enhanced activation of protein kinase Akt in the heart of female rats (as compared to their male litter mates) may help to explain gender differences in the incidence of coronary heart disease14. It is tempting to speculate that the cardioprotective effects of the acute administration of estrogen in animals with acute myocardial infarction14 or in patients undergoing balloon angioplasty15 are—at least in part—due to the activation of eNOS activity.
Although the study by Hafezi-Moghadam and colleagues supports the view that high-dose corticosteroids protect the heart against ischemia-reperfusion injury, it is likely that any suggestion that corticosteroids should be used in the therapy of patients with acute myocardial infarction, or even to prevent acute coronary syndromes, will be met with some scepticism. Even if the use of corticosteroids were limited to acute—rather than chronic—administration, it is unclear whether such therapy would lead to scar thinning and the development of cardiac aneurysms.
The hypothesis of the authors that 'glucocorticoid receptor modulators', which selectively activate a specific signal transduction pathway, may one day become novel weapons in our therapeutic arsenal to combat ischemic heart disease is intriguing. Indeed, there is recent evidence that statins (HMG-CoA-reductase inhibitors) activate protein kinase Akt and enhance eNOS activity16, an effect that may well contribute to the cardioprotective effects of these blockbuster drugs17. Thus, the ability to enhance eNOS activity to protect the vasculature, and ultimately the heart and brain, against ischemic episodes appears not to be limited to corticosteroids, but is shared by other stimuli. These stimuli include shear stress, exercise, hormones and xenobiotics that activate protein kinase Akt.
The study by Hafezi-Moghadam and colleagues should provide an impetus for both scientists and clinicians to gain a better insight into the relationship between regulation of eNOS activity and ischemic heart disease.
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B activation and/or enhanced expression of annexin 1 may also contribute to the cardioprotective effects of glucocorticoids.
and PPAR-
) reduce myocardial infarct size. FASEB J. (in the press).
-Estradiol, but not 
