Abstract
The effects of estrogen are mediated through two functionally distinct receptors, estrogen receptor α (ER- α ), and estrogen receptor β (ER- β ), both of which are expressed in the cardiovascular system. The etiology of cardiovascular disease is believed to result in part from the loss of endogenous estrogen, indicating that estrogen and its receptors may play important roles in the prevention of cardiovascular disease in women.
Introduction
The incidence of cardiovascular disease among women is low before menopause but increases after the onset of menopause [ Mendelsohn and Karas, 1999 ]. This etiology is believed to result in part from the loss of endogenous estrogen, and indicates that estrogen and its receptors may play important roles in the prevention of cardiovascular disease in women. The effects of estrogen are mediated through two functionally distinct receptors, estrogen receptor α (ER- α ), and estrogen receptor β (ER- β ), both of which are expressed in the cardiovascular system [ Mendelsohn and Karas, 1999 ]. ER knock-out (ERKO) mice have provided useful information about the roles of ERs in the cardiovascular system, and in this perspective we summarize the cardiovascular roles of ERs as learnt from ERKO mice.
Pathophysiological roles of ER- α and ER- β : the carotid artery injury model.
ER- α KO mice
In an initial study, the effects of estrogen on wild-type and ER- α KO mice were compared [ Iafrati et al., 1997 ]. Increases in both vascular medial cross-sectional area and in smooth muscle cell proliferation were observed after vascular injury in both groups of mice ( Figure 1 ). Furthermore, estrogen inhibited all measures of vascular injury in WT and ER- α KO mice to a similar extent. These results indicated that ER- α is not necessary for the inhibitory effects of estrogen in response to vascular injury, and suggested that ER- β might mediate the protective effects of estrogen in this context.
Figure 1 . Different roles of ERα and ERβ in the cardiovascular system.
A. After treatment with vehicle, increases in vascular medial area and smooth muscle cell proliferation are observed in WT and ER α KOst mice after vascular injury. However, estrogen inhibits these changes in WT mice, but not in ER α KOst mice. Carotid artery injury model from ER α KOst mice indicates that ER α mediates the protective effects of estrogen on response to vascular injury. B. ER β KO mice have hypertension and multiple abnormalities in ion channel function. In vessels from ER β KO mice, estrogen increases vasoconstriction. These cardiovascular phenotypes in ER β KO mice demonstrate that ER β has an important role in the physiology of the cardiovascular system.
ER- β KO mice
In further studies [ Karas et al., 1999 ], ER- β KO mice were compared to WT mice to directly examine the role of ER- β in mediating the vascular effects of estrogen. Surprisingly, there was no difference between WT and ER- β KO mice when comparing medial areas of the uninjured, injured, placebo-treated and estrogen-treated injured vessels from mice. These studies suggested that either ER- α or ER- β is sufficient to protect against vascular injury, or that another, unidentified ER could mediate the vascular protective effects of estrogen.
ER- α / β (double) KO mice
In order to directly demonstrate a role for ERs in cardiovascular protection, a study of vascular injury response in mice with disrupted versions of both ER- α and ER- β was performed [ Karas et al., 2001 ]. The vascular medial areas were increased in both WT and double KO mice by vascular injury. Estrogen inhibited increases in vascular medial cross-sectional area following injury in WT mice, but not in double KO mice, and this was the first direct evidence that ERs mediate the vascular protective effects of estrogen. However, estrogen inhibited vascular smooth muscle cell proliferation following injury in double KO mice, as well as inducing an increase in uterine weight. The latter findings could be explained by (i) a receptor-independent mechanism, (ii) an unidentified receptor responsive to estrogen, or (iii) residual activity of the ER- α splice variant previously reported in the parental ER- α KO mice [ Couse et al., 1995 ].
ER- α KO Strasbourg (ER- α KOst) mice
The effects of estrogen on the response to vascular injury were studied in null mouse line generated in Strasbourg in which ER- α was completely ablated (ER- α KOst) [Brouchet et al., 2001; Dupont et al., 2000; Pare et al., 2002]. After vascular injury in these mice, estrogen has no effect on any measure of vascular injury, including medial cross-sectional area, smooth muscle cell proliferation and reendothelialization ( Figure 1 ). This study conclusively shows that ER- α mediates the inhibition of the vascular injury response by estrogen.
Physiological role of ER- α and ER- β
ER- α KO mice have abnormal reproductive function, but are otherwise normal, without gross or histological cardiovascular abnormalities. ER- β KO mice, however, develop sustained systolic and diastolic hypertension as they age [ Zhu et al., 2002 ]. Furthermore, in WT mouse blood vessels, estrogen attenuates vasoconstriction. In contrast, estrogen augments vasoconstriction in blood vessels from ER- β KO mice. Moreover, vascular smooth muscle cells isolated from ER- β KO mice demonstrate multiple abnormalities of ion channel function. Taken together, these data support an essential role for ER- β in regulation of vascular function and blood pressure.
Nitric oxide production: Nitric oxide (NO) plays an important role as a regulator in the nervous, immune, and cardiovascular system [ Moncada et al., 1991 ]. In WT and ER- β KO mice, estrogen increases the production of endothelial NO, an effect not observed in ER- α KOst mice [ Pendaries et al., 2002 ]. In WT mice, ER- β , and not ER- α , mediates estrogen-dependent attenuation of endothelial independent vasoconstriction by induction of NO synthase [ Zhu et al., 2002 ]. These results indicate that ER- α mediates estrogen-induced alteration of endothelial NO production, whereas ER- β controls estrogen-induced, endothelial-independent NO production.
In conclusion, studies of ERKO mice have elucidated somewhat the different roles of ERs in the cardiovascular system. ER- α seems to play an important role in the pathophysiology of the vessel wall, whereas ER- β , in contrast, has an important role in the physiology of the cardiovascular system, i.e. the regulation of vascular function and blood pressure.
Abbreviations
- ER- α Kost
Null mouse line generated in Strasbourg in which ER- α is completely ablated
- NO
nitric oxide
References
- Brouchet L. , Krust A. , Dupont S. , Chambon P. , Bayard F. , Arnal J. F. Estradiol accelerates reendothelialization in mouse carotid artery through estrogen receptor- α but not estrogen receptor- β . Circulation . 2001 ; 103 : 423 – 8 . doi: 10.1161/01.cir.103.3.423. [DOI] [PubMed] [Google Scholar]
- Couse J. F. , Curtis S. W. , Washburn T. F. , Lindzey J. , Golding T. S. , Lubahn D. B. , Smithies O. , Korach K. S. Analysis of transcription and estrogen insensitivity in the female mouse after targeted disruption of the estrogen receptor gene . Mol Endocrinol . 1995 ; 9 : 1441 – 54 . doi: 10.1210/mend.9.11.8584021. [DOI] [PubMed] [Google Scholar]
- Dupont S. , Krust A. , Gansmuller A. , Dierich A. , Chambon P. , Mark M. Effect of single and compound knockouts of estrogen receptors α (ERalpha) and β (ERbeta) on mouse reproductive phenotypes . Development . 2000 ; 127 : 4277 – 91 . doi: 10.1242/dev.127.19.4277. [DOI] [PubMed] [Google Scholar]
- Iafrati M. D. , Karas R. H. , Aronovitz M. , Kim S. , Sullivan T. R., Jr. , Lubahn D. B. , O'Donnell T. F., Jr. , Korach K. S. , Mendelsohn M. E. Estrogen inhibits the vascular injury response in estrogen receptor α -deficient mice . Nat Med . 1997 ; 3 : 545 – 8 . doi: 10.1038/nm0597-545. [DOI] [PubMed] [Google Scholar]
- Karas R. H. , Hodgin J. B. , Kwoun M. , Krege J. H. , Aronovitz M. , Mackey W. , Gustafsson J. A. , Korach K. S. , Smithies O. , Mendelsohn M. E. Estrogen inhibits the vascular injury response in estrogen receptor β -deficient female mice . Proc Natl Acad Sci U S A . 1999 ; 96 : 15133 – 6 . doi: 10.1073/pnas.96.26.15133. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Karas R. H. , Schulten H. , Pare G. , Aronovitz M. J. , Ohlsson C. , Gustafsson J. A. , Mendelsohn M. E. Effects of estrogen on the vascular injury response in estrogen receptor α , β (double) knockout mice . Circ Res . 2001 ; 89 : 534 – 9 . doi: 10.1161/hh1801.097239. [DOI] [PubMed] [Google Scholar]
- Mendelsohn M. E. , Karas R. H. The protective effects of estrogen on the cardiovascular system . N Engl J Med . 1999 ; 340 : 1801 – 11 . doi: 10.1056/NEJM199906103402306. [DOI] [PubMed] [Google Scholar]
- Moncada S. , Palmer R. M. , Higgs E. A. Nitric oxide: physiology, pathophysiology, and pharmacology . Pharmacol Rev . 1991 ; 43 : 109 – 42 . [PubMed] [Google Scholar]
- Pare G. , Krust A. , Karas R. H. , Dupont S. , Aronovitz M. , Chambon P. , Mendelsohn M. E. Estrogen receptor- α mediates the protective effects of estrogen against vascular injury . Circ Res . 2002 ; 90 : 1087 – 92 . doi: 10.1161/01.res.0000021114.92282.fa. [DOI] [PubMed] [Google Scholar]
- Pendaries C. , Darblade B. , Rochaix P. , Krust A. , Chambon P. , Korach K. S. , Bayard F. , Arnal J. F. The AF-1 activation-function of ERalpha may be dispensable to mediate the effect of estradiol on endothelial NO production in mice . Proc Natl Acad Sci U S A . 2002 ; 99 : 2205 – 10 . doi: 10.1073/pnas.042688499. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Zhu Y. , Bian Z. , Lu P. , Karas R. H. , Bao L. , Cox D. , Hodgin J. , Shaul P. W. , Thoren P. , Smithies O. Abnormal vascular function and hypertension in mice deficient in estrogen receptor β . Science . 2002 ; 295 : 505 – 8 . doi: 10.1126/science.1065250. [DOI] [PubMed] [Google Scholar]