Circulation Research. 2006;98:298-300
doi: 10.1161/01.RES.0000208091.94643.6f
doi: 10.1161/01.RES.0000208091.94643.6f
© 2006 American Heart Association, Inc.
Editorials |
A Female Way to Protect the Heart
Say NO to Calcium
From the Dipartimento di Chimica Biologica, Università di Padova, Italy.
Correspondence to Prof Fabio Di Lisa, Dipartimento di Chimica Biologica, Università di Padova, Viale G. Colombo 3, I-35121 Padova, Italy. E-mail dilisa{at}civ.bio.unipd.it
See related article, pages 403–411
Key Words: nitric oxide • calcium • ischemia • gender • S-nitrosylation • calcium
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Introduction |
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 Top Introduction Increased NO Formation Underlies...NOS Activation Antagonizes...Additional Protective Mechanisms...References |
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Both clinical and experimental observations support the concept that female hearts are less susceptible to myocardial injury caused by ischemia and reperfusion.1,2 A large body of evidence indicates that estrogen is involved in gender-related mechanisms of protection. The protective effect of 17-ß-estradiol (E2) administration was described as a reduced extent of necrosis in rabbit hearts undergoing coronary occlusion followed by reperfusion.3 This initial observation, as well as the reduced myocardial injury in female hearts, has been confirmed in different animal species using various experimental protocols.4–11 Myocardial protection was shown to be associated with activation of estrogen receptors (ER),5 and specifically of ER
.6 Besides the well-established cytosolic/nuclear localization,12,13 ER, or at least proteins recognized by anti-ER antibodies, have also been detected at the level of plasma membrane and mitochondria.14–17 However, the lack of a typical transmembrane domain in cytosolic ER casts doubts about these additional membrane receptors.15 The activated estrogen–receptor complex triggers the synthesis of specific mRNAs and the production of a number of proteins that are responsible for the various effects elicited in the different cell types. Along with these "genomic" effects, additional processes termed "nongenomic" or alternative occur rapidly and independently of protein synthesis.18 Among the many pathways that can modify the susceptibility to ischemic injury in female hearts, the relevance of nitric oxide (NO) signaling was addressed by Sun et al in this issue of Circulation Research.19
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Increased NO Formation Underlies Myocardial Protection in Females |
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TopIntroductionIncreased NO Formation Underlies...NOS Activation Antagonizes...Additional Protective Mechanisms...References |
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Previous studies from Murphy’s group investigated gender differences in hearts subjected to ischemia reperfusion (I/R) protocols under conditions having elevated levels of intracellular Ca2+ in common. Invariably, on reperfusion a reduced degree of injury was detected in hearts from female rodents as compared with male littermates.7–9 Highlighting a pivotal role of NO generation in the protective mechanism, it was also noted that eNOS content was higher in female hearts9 consistent with estrogen induction of NOS expression.20 The NOS inhibitor L-NAME abolished the gender differences.8 NOS-mediated protection was linked to intracellular Ca2+ homeostasis by showing that, on isoproterenol addition, SR calcium loading was lower in female hearts.11 On the other hand, increased NO availability has been shown to decrease the activity of L-type Ca2+ channels.21 Therefore, it was hypothesized that the higher content of eNOS in female hearts might reduce the intracellular rise in Ca2+ by decreasing its entry through L-type Ca2+ channels. This hypothesis appears to be convincingly demonstrated by the study of Sun et al.19
Confirming and extending previous reports, female mouse hearts pretreated with isoproterenol before ischemia displayed decreased injury as compared with hearts from male mice. In addition, besides a higher content of eNOS associated with caveolin-3 at the plasma membrane, in isoproterenol-pretreated female hearts ischemia caused n-NOS translocation from SR to sarcolemma. This finding confirms a previous report documenting intracellular redistribution of nNOS in infarcted hearts of senescent rats.22 The role of NOS in myocardial protection was supported not only by pharmacological approaches, but also by the absence of male–female differences in mice lacking eNOS or nNOS, suggesting that both isoforms have to be present to limit I/R-induced injury. This concept was supported by the assessment of S-nitrosothiol content that, reflecting an increased NOS activity, was higher in female wild-type mice, yet gender differences were absent in eNOS- and nNOS-null mice. Notably, in isoproterenol-treated hearts protection was observed only when S-nitrosothiol content was above 15 pmol/mg protein, suggesting that the combined activities of both eNOS and nNOS is required to provide high rates of S-nitrosothiol formation.
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NOS Activation Antagonizes Intracellular [Ca2+] Rise |
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TopIntroductionIncreased NO Formation Underlies...NOS Activation Antagonizes...Additional Protective Mechanisms...References |
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To validate the link between increased NO formation and decreased intracellular Ca2+ accumulation, it was imperative to demonstrate that one or more proteins involved in Ca2+ homeostasis were S-nitrosylated, and that such a covalent modification was causally related to functional changes. Indeed, this was the case with the L-type Ca2+ channel
1 subunit that was identified as the predominant S-nitrosylated protein in membrane fractions. The degree of 1 subunit nitrosylation was increased in isoproterenol-treated hearts and even more when this treatment was followed by ischemia and reperfusion. This increase was larger in female than in male hearts from wild-type mice, whereas gender differences were abrogated by NOS inhibitors and were not detected in mice lacking eNOS or nNOS. Finally, this novel biochemical evidence of S-nitrosylation was causally correlated with a decreased function of L-type Ca2+ channels by showing that after isoproterenol addition females (1) have less of an increase in systolic Ca2+ under conditions where SR function was blocked, and (2) a reduced increase in SR Ca2+. In addition, the isoproterenol-induced increase in Ca2+ current was smaller in female than in male hearts, and again this difference was abrogated by NOS blockade. Of note, the direct assessment of Ca2+ current allows ruling out that the differences in Ca2+ levels between female and male hearts might be caused by other factors, such as action potential duration or Na–Ca exchanger activity. Future studies should identify the residue(s) that are S-nitrosylated in 1 subunit to add relevant information to the current understanding of structure-function relationships in L-type Ca2+ channels. These findings provide a direct demonstration that the activity of L-type Ca2+ channel is decreased by its S-nitrosylation, a concept that was previously supported only by indirect evidence. This lends convincing support to a protective mechanism characterizing female hearts whereby an initial increased entry of Ca2+ stimulates the activity of NOS isoforms localized in the plasma membrane, so that the increased NO production can be easily targeted to the L-type Ca2+ channels. The consequent decrease in Ca2+ influx is likely to result in a reduced intracellular Ca2+ overload during ischemia, eventually favoring the maintenance of tissue viability on reperfusion. Such a sequence of events might represent a crucial mechanism underlying the well-established protection afforded by NO that has not yet been conclusively elucidated in molecular terms.23
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Additional Protective Mechanisms in Female Hearts |
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TopIntroductionIncreased NO Formation Underlies...NOS Activation Antagonizes...Additional Protective Mechanisms...References |
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The inhibition of Ca2+ influx attributable to S-nitrosylation of L-type Ca2+ channels, though relevant, represents one of the many ways through which estrogen protects ischemic hearts. It must be pointed out that in the absence of isoproterenol treatment I/R injury was not different in female and male mice. It is tempting to speculate that stimulation by circulating estrogen, that is lacking in the perfused heart model used by Sun et al, might trigger additional protective mechanisms. These include long-term effects, such as synthesis of antioxidant and prosurvival proteins,17,24–26 and short-term responses, such as activation of PI3K/AKT.27,28 In particular, this latter process appears to reduce the formation of reactive oxygen species decreasing the activity of the proapoptotic p38a MAPK concomitant with upregulation of the prosurvival isoform p38ß.29 Therefore, the NO-dependent mechanism of reduced Ca2+ accumulation elucidated by Sun et al19 is likely to contribute to a protective network activated by estrogen stimulation.
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Acknowledgments |
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This work was supported by Grants from CNR, FIRB, and MIUR.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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Related Article:
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Hypercontractile Female Hearts Exhibit Increased S-Nitrosylation of the L-Type Ca2+ Channel 1 Subunit and Reduced Ischemia/Reperfusion Injury
- Junhui Sun, Eckard Picht, Kenneth S. Ginsburg, Donald M. Bers, Charles Steenbergen, and Elizabeth Murphy
Circ. Res. 2006 98: 403-411.[Abstract] [Full Text] [PDF]
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