doi: 10.1161/01.RES.0000016843.82450.8C
© 2002 American Heart Association, Inc.
Editorials |
Novel AT1 Receptor–Independent Functions of Losartan
From the Department of Cell Biology and Molecular Medicine, Cardiovascular Research Institute, The University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark, NJ.
Correspondence to Junichi Sadoshima, MD, PhD, Cardiovascular Research Institute UMDNJ, New Jersey Medical School, 185 South Orange Ave, MSB G-609, Newark, NJ 07103-2714. E-mail sadoshju{at}umdnj.edu
Key Words: losartan • cyclooxygenase-2 • indomethacin • platelet aggregation • EXP3179
A lthough currently available angiotensin II type 1 (AT1) receptor blockers (ARBs) share a common mechanism of action, blocking AT1 receptors, their receptor binding kinetics and biological actions are not identical.1 Two ARBs, namely losartan and candesartan cilexetil, are prodrugs that are converted to active drugs in vivo. After oral administration, losartan undergoes the first-pass metabolism in the liver and is converted into EXP3174, whose affinity to the AT1 receptor is 10 times higher than that of losartan. It should be noted that losartan is not a typical prodrug because it has significant ARB activity and only 14% of the administered dose is converted to EXP3174.2 This indicates that pharmacological effects of losartan are mediated not only by losartan itself but also by EXP3174 and possibly other metabolites as well.
There is growing evidence to suggest that losartan has AT1 receptor–independent actions primarily related to antiinflammatory and antiaggregatory mechanisms (Table). These actions have been speculated to be independent of the AT1 receptor blockade primarily because these properties are not shared by other ARBs, such as candesartan and valsartan, or by angiotensin-converting enzyme inhibitors. Furthermore, losartan blocks vasoconstriction and platelet aggregation induced by the thromboxane A2 (TXA2) analog, U46619, and displaces ligand binding to the TXA2 receptor, suggesting that losartan can act as a dual-receptor antagonist.3 Interestingly, EXP3174 and irbesartan, another ARB, both of which have the imidazole moiety of biphenyl tetrazole in their structure similar to losartan, interact with the TXA2 receptor. Thus, the chemical structure appears to determine the AT1 receptor–independent actions of the ARBs.4 However, because stimulation of the AT1 receptor alone is also able to stimulate inflammation and thrombosis through upregulation of inflammatory cytokines and prostaglandins (PGs),5 the significance of such AT1 receptor–independent actions of losartan over its AT1 receptor antagonism has been difficult to prove.
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In this issue of Circulation Research, Krämer et al6 have shown that EXP3179, another metabolite of losartan, possesses antiinflammatory and antiaggregatory properties, despite the fact that it does not interfere with ligand binding to the AT1 receptor. After a single oral dose of 100 mg losartan in patients, inhibition of PGF2
production was observed over 6 to 8 hours in vivo. By that time, a significant amount of EXP3179 has been produced through the hepatic circulation.
Although EXP3179 is structurally similar to indomethacin, a conventional nonsteroidal antiinflammatory drug and an inhibitor of both cyclooxygenase (COX)-1 and COX-2, the precise mechanism of its antiinflammatory and antiaggregatory actions remains to be elucidated. Inhibition of PGF2 synthesis reflects that EXP3179 works as a COX inhibitor. Like other antiinflammatory agents, however, EXP3179 may also exert its action through COX inhibition-independent effects (reviewed in Tegeder et al7). Many nonsteroidal antiinflammatory drugs inhibit transcription factors, such as NF-
B, AP-1, or CCAAT enhancer-binding proteins (C/EBPs), thereby negatively regulating transcription of proinflammatory genes, including cytokines and cell adhesion molecules.7 Indomethacin may exert its antiinflammatory activities through activation of peroxisome proliferator-activated receptor 
.8 These antiinflammatory agents also affect activities of various protein kinases, including IB kinase ß,9 mitogen-activated protein kinases, Cdk, and Src.7,10 EXP3179 may share these mechanisms, thereby negatively regulating mRNA expression of COX-2 and ICAM-1, important mediators of inflammation (Figure). EXP3179 may also act as a receptor antagonist for TXA2 receptors to prevent U46619-induced platelet aggregation.
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Stimulation of the AT1 receptor causes both activation of phospholipase A2 and upregulation of COX-211,12 in vascular smooth muscle cells, which in turn mediate production of eicosanoids, including PGE2 and TXA2, and cell proliferation. Thus, losartan should block cellular responses mediated by PGs through multiple mechanisms in vivo: AT1 receptor blockade by losartan and EXP3174, suppression of COX-2 expression by EXP3179, and blockade of TXA2 receptor by all 3 compounds. In this regard, losartan may be effective in treating atherosclerosis, where both the local renin-angiotensin system and the inflammatory process are stimulated.13,14
Myocardial COX-2 expression is upregulated in failing hearts15 as well as after myocardial infarction.16 Increased production of PGs, including PGE2 and PGF2, stimulates cardiac hypertrophy,17,18 whereas selective inhibition of COX-2 prevents myocardial infiltration of inflammatory cells after myocardial infarction.16 By contrast, COX-2 prevents cardiac myocyte apoptosis19 and mediates the late phase of ischemic preconditioning.20 Thus, whether or not downregulation of COX-2 by EXP3179 is salutary in treatment of patients with coronary artery diseases or heart failure remains to be elucidated.21,22 Although treatment of heart failure patients with losartan did not show survival advantage over captopril in the ELITE (Evaluation of Losartan In The Elderly) II study,23 losartan treatment may be beneficial for treatment of a subset of heart failure patients if inflammation is considered to be a component in the pathogenesis of heart failure.
The observation by Krämer et al6 provides us with an important precaution regarding the interpretation of experimental data obtained by using losartan in vivo. Inhibition of in vivo phenomenon by losartan alone may not necessarily indicate the involvement of the AT1 receptor, because losartan has AT1 receptor–independent actions. Ideally, involvement of the AT1 receptor should be confirmed by using other ARBs with distinct structural properties. It would be also important to confirm that stimulating the AT1 receptor by angiotensin II elicits an opposite response from losartan treatment.
Several important questions regarding the properties of EXP3179 remain unanswered. First, it is unknown if EXP3179 is solely responsible for the antiinflammatory action of losartan. This issue can be addressed if one can inhibit the metabolism of losartan in the liver P450 pathway at multiple stages. Second, losartan and EXP3174 also work as TXA2 receptor antagonists.3,4 The potency of antiaggregatory and antiinflammatory actions of EXP3179 should be compared with that of losartan and EXP3174. Third, recent evidence suggests that COX-2 is not entirely inducible, and its constitutive expression in vascular endothelium may critically regulate the generation of prostacyclin, which is a vasodilator and inhibitor of platelet aggregation.21 In this regard, how EXP3179 affects production of prostacyclin needs to be determined. Finally, it will be interesting to determine the crucial chemical structure that confers antiinflammatory actions to EXP3179.
Acknowledgments
This work was supported by grants from NIH and AHA National. We thank Drs S.F. Vatner and S. Soler for critical reading of this article.
Footnotes
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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