doi: 10.1161/01.RES.0000142314.55287.a1
© 2004 American Heart Association, Inc.
Editorials |
Beating Ischemia
A New Feat of EETs?
From the Institute of Molecular Cardiology, University of Louisville, Louisville, Ky.
Correspondence to Aruni Bhatnagar, PhD, Division of Cardiology, Department of Medicine, Delia Baxter Bldg, 580 S Preston St, Room 421F, University of Louisville, Louisville, KY 40202. E-mail aruni{at}louisville.edu
See related article, pages 506–514
Key Words: cytochrome P450 • myocardial ischemia • epoxyeicosatrienoic acid (EET) • mitochondria • ATP-dependent K+ channel
It is well-known that myocardial ischemia results in the hydrolysis of phospholipids and the generation of arachidonic acid. Free arachidonic acid also could accumulate as a result of hormonal activation or the failure of fatty acid oxidation in the ischemic heart. What happens next to arachidonic acid and how its many metabolites (see the Figure) affect myocardial function is not entirely clear, but questions related to these issues are turning out some of the most fascinating and provocative answers in cardiovascular physiology. Much of the current evidence, however, is conflicting. Increased arachidonic acid metabolism via cyclooxygenases mediates the cardioprotective effects of the late phase of ischemic preconditioning1 and provides long-term protection to the heart against ischemic injury.2 However, treatment with cytochrome P450 (CYP) inhibitors3 or inhibition of mitochondrial calcium-independent phospholipase A2 (which liberates arachidonic acid from phospholipid stores) reduces infarct size,4 suggesting that increased generation of arachidonic acid and its metabolites could be detrimental to the ischemic heart. Adding new fuel to this debate is the elegant study by Seubert et al,5 published in the current issue of Circulation Research, which shows that increased CYP-dependent epoxidation of arachidonic acid to cis-epoxyeicosatrienoic acids (EETs) could decrease myocardial ischemic injury. Using transgenic mice overexpressing human CYP2J2 in the heart, these investigators demonstrate improved postischemic recovery of the heart and suggest that this protection is attributable to the activation of the mitochondrial KATP channel (mitoKATP) and the stimulation of the p42/p44 MAP kinase pathway. Their convincing demonstration of a novel anti-ischemic role of CYP2J2 opens up a new area of research into the role of EETs in cardioprotection and points toward new avenues for understanding and assessing the involvement of arachidonic acid and its metabolites in regulating myocardial function and ischemic injury. The importance of this work is further underscored by the recent report6 that polymorphism leading to reduced gene activity of CYP2J2 is associated with an increased risk of coronary artery disease, indicating a cardioprotective role of CYP2J2 in humans.
|
terminus. These reactions generate HETEs. Epoxidation of the acid forms 14,15-, 11,12-, 8,9-, and 5,6-EETs either as a R,S or a S,R enantiomer. The EETs could be further metabolized via multiple pathways, all of which may not be present in the same tissue.The biology of CYPs is complex. It reveals its secrets only in a committed relationship, and the multiplicity of CYP isoforms and their myriad products elude a casual grasp. However, this complexity belies the widespread physiological, pathological, and toxicological significance of these enzymes. The CYPs catalyze the controlled addition of molecular oxygen to carbon-carbon bonds, which is one of the most elegant reactions devised by nature and is used by all organisms to remove and detoxify foreign chemicals. It is, however, a Faustian bargain. The reaction proceeds via the formation of an alkoxyl radical-like intermediate and leaks superoxide and hydrogen peroxide,7 both of which could be highly toxic. Consequently, CYP-based detoxification of alcohols, drugs, and chemicals often results in increased radical generation and causes oxidative stress.7 The protective effects of CYP inhibitors (chloramphenicol, cimetidine, and sulfaphenazole) against myocardial ischemic injury have been linked to their ability to prevent CYP-derived free radical formation during reperfusion.3 This is in sharp contrast to the protective effects of CYP2J2 overexpression described by Seubert et al.5 In their thorough characterization of CYP2J2 transgenic mice, they did not find any gross changes in cardiac function that would be expected if free radical generation in the heart was increased. Apparently, not all CYPs are created equal. The offending CYP that generates free radicals in the rat heart has been linked to the 2C family,3 but because both 2J and 2C families generate similar metabolites, it is not clear why one isoform is protective while the other causes injury. Maybe the 2C isoforms are leakier and generate more free radicals than the more tightly coupled 2J isoforms, which, when expressed, prevent free radical generation during hypoxia reoxygenation.8 Alternatively, increased free radical generation may be one of the mechanisms of cardioprotection associated with chronic overexpression of CYP2J2. Clearly, much work remains to be done. Potentially, what is needed most is dogged characterization of each of the CYP isoforms in the heart, identification of their specific metabolites, and elucidation of their specific roles in regulating myocardial physiology, particularly their involvement in cardioprotective signaling and free radical generation.
Seubert et al5 propose that CYP2J2-derived EETs induce cardioprotection by stimulating the mitoKATP channel and ERK. They present evidence showing that perfusion with 11,12-EET improves postischemic recovery in wild-type hearts, and that inhibitors of mitoKATP channels or the MEK inhibitor PD98059 abolish improved functional recovery in CYP2J2 transgenic hearts. Collectively, these data provide compelling evidence supporting the view that CYP2J2-derived EETs prevent ischemic injury by stimulating the previously well-characterized pathways of cardioprotection.9,10 However, the protection seen in transgenic hearts was less than that in wild-type hearts treated with a mitoKATP opener, suggesting that EETs only partially activate the channel and presumably a synergistic effect of ERK and mitoKATP activation is needed to confer cardioprotection. Thus, it will be interesting to further characterize the nature of this protection and to see whether it involves other pathways that are stimulated by ischemic preconditioning, such as the activation of protein kinase C and NF-
B leading to the upregulation of nitric oxide synthases, cyclooxygenase-2, and manganese-superoxide dismutase.9 Because protection attributable to CYP2J2 overexpression was completely abolished by inhibiting its epoxygenase activity, it appears that the catalytic activity of the enzyme is needed during ischemia, and that its effects are not simply attributable to upregulation of other cardioprotective genes. Nevertheless, cardioprotective signaling involves concerted activation of several interdependent pathways.9 Hence, delineation of the role of CYPs in these signaling pathways could add a new facet to the polygenic myocardial responses that mediate preconditioning and attenuate ischemic injury.
Additional proof of the involvement of CYP-derived eicosanoids in cardioprotection is provided by the observation that treatment with an epoxygenase inhibitor reduced postischemic recovery in wild-type hearts. These observations suggest that CYPs play an important role in preventing ischemic injury, not only when their expression is enhanced artificially in transgenic experiments but also in a normal wild-type heart. Based on these observations, it appears likely that CYP-derived EETs are generated endogenously in wild-type hearts and that these eicosanoids protect the heart from ischemic injury. Nonetheless, the extent of increase in ischemic injury on inhibiting CYPs was small. One reason for this may be that EETs get esterified to membrane phospholipids, particularly phosphatidylcholine and phosphatidylinositol, which represent a major fraction (>85%) of the endogenous EET pool in mammalian cells.11 Therefore, inhibition of EET synthesis may not immediately abolish their effects and the role of EETs in modulating ischemic injury may be more significant than gleaned from inhibitor data alone. Thus it will be interesting to determine whether phospholipids esterified to EETs are present in the heart, whether the biological activity of the EETs requires hydrolysis of these esters, and whether changes in EET formation or storage are affected by ischemia. Furthermore, in addition to forming phospholipid esters, EETs could be metabolized via other biochemical pathways (see the Figure), which could uniquely alter, extinguish, or modify their reactivity.11 Although Seubert et al5 found that myocytes converted EETs to inactive DHETs (indicating the presence of epoxide hydrolase in myocytes), other pathways of EET metabolism in the heart are largely unknown but are likely to attract attention because of the anti-ischemic effects of CYP2J2 demonstrated in this study. Similarly, other more direct aspects of EET biochemistry remain unclear. The most critical of these appears to be a better definition of the molecular nature of EET targets. Although previous studies show that EETs regulate vascular homeostasis and can modulate ion channel activity, many questions related to the mechanisms by which these eicosanoids affect cardiovascular processes remain unanswered. For instance, do EETs elicit their effects by binding to as-yet-unidentified receptors? Do they bind to or influence previously described receptors, or do they simply exert their effects by changing the biophysical properties of the membrane microdomains near transporters and ion channels?12 Seubert et al5 report that 11,12- and 14,15-EETs activate mitoKATP. However, 11,12-EET also activates the cardiac sarcolemmal KATP channels by decreasing the sensitivity of the channel to cytoplasmic ATP.13 Because sarcolemmal and mitochondrial KATP channels have overlapping ligand-binding affinities, these observations suggest that the KATP channel or proteins associated with this channel could represent one set of EET receptors. Other potential candidates are protein kinases, ADP ribotransferases, and GTP-binding proteins, as well as calcium channels, which seem to be directly activated by EETs.11,12
Finally, the work of Seubert et al5 suggests an important mechanism by which the environment influences the heart and its sensitivity to ischemia because of changes in CYP expression and activity. In the past, CYPs have been studied largely for their participation in hepatic drug detoxification and disposition. Their role in cardiovascular toxicity, however, is less clear. Because the heart and blood vessels are not sites of primary exposure, and they do not participate in systemic detoxification, the cardiovascular effects of environmental toxicants and pollutants have been largely ignored. Nonetheless, as overwhelming epidemiological and experimental data on the health effects of smoking and ambient air particles so starkly demonstrate, adverse cardiovascular events are often the most common outcome of pollutant exposure.14 Given that multiple CYP isoforms are expressed in blood vessels and heart, and that the activities of these enzymes profoundly influence myocardial ischemic injury and vascular tone, inhibition or induction of these enzymes by pollutants, drugs, and chemicals could represent a critical interface between the environment and cardiovascular health.
Footnotes
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
References
1. Shinmura K, Tang XL, Wang Y, Xuan YT, Liu SQ, Takano H, Bhatnagar A, Bolli R. Cyclooxygenase-2 mediates the cardioprotective effects of the late phase of ischemic preconditioning in conscious rabbits. Proc Natl Acad Sci U S A. 2000; 97: 10197–10202.
2. Camitta MG, Gabel SA, Chulada P, Bradbury JA, Langenbach R, Zeldin DC, Murphy E. Cyclooxygenase-1 and -2 knockout mice demonstrate increased cardiac ischemia/reperfusion injury but are protected by acute preconditioning. Circulation. 2001; 104: 2453–2458.
3. Granville DJ, Tashakkor B, Takeuchi C, Gustafsson AB, Huang C, Sayen MR, Wentworth P, Jr., Yeager M, Gottlieb RA. Reduction of ischemia and reperfusion-induced myocardial damage by cytochrome P450 inhibitors. Proc Natl Acad Sci U S A. 2004; 101: 1321–1326.
4. Williams SD, Gottlieb RA. Inhibition of mitochondrial calcium-independent phospholipase A2 (iPLA2) attenuates mitochondrial phospholipid loss and is cardioprotective. Biochem J. 2002; 362: 23–32.[CrossRef][Medline] [Order article via Infotrieve]
5. Seubert J, Yang B, Bradbury JA, Graves J, Degraff LM, Gabel S, Gooch R, Foley J, Newman J, Mao L, Rockman HA, Hammock BD, Murphy E, Zeldin DC. Enhanced postischemic functional recovery in CYP2J2 transgenic hearts involves mitochondrial ATP-sensitive K+ channels and p42/p44 MAPK pathway. Circ Res. 2004; 95: 506–514.
6. Spiecker M, Darius H, Hankeln T, Soufi M, Sattler AM, Schaefer JR, Node K, Borgel J, Mugge A, Lindpainter K, Huesing A, Maisch B, Zeldin DC, Liao JK. Risk of coronary artery disease associated with polymorphism of a cytochrome P450 epoxygenase, CYP2J2. Circulation. 2004. In press.
7. Ortiz de Montellano RR. Cytochrome P450 Structure, Mechanism, and Biochemistry. New York: Plenum; 1986.
8. Yang B, Graham L, Dikalov S, Mason RP, Falck JR, Liao JK, Zeldin DC. Overexpression of cytochrome P450 CYP2J2 protects against hypoxia-reoxygenation injury in cultured bovine aortic endothelial cells. Mol Pharmacol. 2001; 60: 310–320.
9. Bolli R. The late phase of preconditioning. Circ Res. 2000; 87: 972–983.
10. O’Rourke B. Myocardial KATP channels in preconditioning. Circ Res. 2000; 87: 845–855.
11. Zeldin DC. Epoxygenase pathways of arachidonic acid metabolism. J Biol Chem. 2001; 276: 36059–36062.
12. Fleming I. Cytochrome p450 and vascular homeostasis. Circ Res. 2001; 89: 753–762.
13. Lu T, Hoshi T, Weintraub NL, Spector AA, Lee HC. Activation of ATP-sensitive K+ channels by epoxyeicosatrienoic acids in rat cardiac ventricular myocytes. J Physiol. 2001; 537: 811–827.
14. Bhatnagar A. Cardiovascular pathophysiology of environmental pollutants. Am J Physiol Heart Circ Physiol. 2004; 286: H479–H485.
This article has been cited by other articles:
 |
|
 |
|
  K. Nithipatikom, J. M. Moore, M. A. Isbell, J. R. Falck, and G. J. Gross Epoxyeicosatrienoic acids in cardioprotection: ischemic versus reperfusion injury Am J Physiol Heart Circ Physiol, August 1, 2006; 291(2): H537 - H542. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. Nithipatikom, M. P. Endsley, J. M. Moore, M. A. Isbell, J. R. Falck, W. B. Campbell, and G. J. Gross Effects of selective inhibition of cytochrome P-450 {omega}-hydroxylases and ischemic preconditioning in myocardial protection Am J Physiol Heart Circ Physiol, February 1, 2006; 290(2): H500 - H505. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||