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(Circulation Research. 2001;89:534.)
© 2001 American Heart Association, Inc.

Integrative Physiology

Effects of Estrogen on the Vascular Injury Response in Estrogen Receptor ,ß (Double) Knockout Mice

Richard H. Karas, Henny Schulten, Gary Pare, Mark J. Aronovitz, Claes Ohlsson, Jan-Ake Gustafsson, Michael E. Mendelsohn

From the Molecular Cardiology Research Institute, Department of Medicine (R.H.K., H.S., G.P., M.J.A., M.E.M.), and Division of Cardiology, New England Medical Center Hospitals and Tufts University School of Medicine, Boston, Mass; Department of Internal Medicine (C.O.), Sahlgrenska University Hospital, Gothenburg, Sweden; and Department of Medical Nutrition (J.-A.G.), Karolinska Institute, NOVUM, Huddinge, Sweden.

Correspondence to Michael E. Mendelsohn, MD, Tufts University School of Medicine, New England Medical Center, Molecular Cardiology Research Institute, 750 Washington St, Box 80, Boston, MA 02111. E-mail mmendelsohn{at}lifespan.org

	Abstract

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Abstract — The two known estrogen receptors, ER and ERß, mediate the effects of estrogen in all target tissues, including blood vessels. We have shown previously that estrogen inhibits vascular injury response to the same extent in female wild-type (WT), ER knockout (ERKO_CH), and ERß knockout (ERßKO_CH) mice. We generated mice harboring disruptions of both ER and ERß genes (ER,ßKO_CH) by breeding and studied the effect of 17ß-estradiol (E2) on vascular injury responses in ovariectomized female ER,ßKO_CH mice and WT littermates. E2 inhibited increases in vascular medial area following injury in the WT mice but not in the ER,ßKO_CH mice, demonstrating for the first time that the two known estrogen receptors are necessary and sufficient to mediate estrogen inhibition of a component of the vascular injury response. Surprisingly, as in WT littermates, E2 still significantly increased uterine weight and inhibited vascular smooth muscle cell (VSMC) proliferation following injury in the ER,ßKO_CH mice. These data support that the role of estrogen receptors differs for specific components of the vascular injury response in the ER,ßKO_CH mice. The results leave unresolved whether E2 inhibition of VSMC proliferation in ER,ßKO_CH mice is caused by a receptor-independent mechanism, an unidentified receptor responsive to estrogen, or residual activity of the ER splice variant reported previously in the parental ERKO_CH mice. These possibilities may be resolved by studies of mice in which ER has been fully disrupted (ERKO_St), which are in progress.

Key Words: animal models • vascular injury • 17ß-estradiol • hormone action

	Introduction

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The cardiovascular effects of estrogen are diverse. Estrogen has both systemic effects on circulating factors (eg, cholesterol, cytokines, coagulation/fibrinolytic factors) and direct effects on the blood vessel wall (eg, regulation of vasomotor tone, vascular cell proliferation; reviewed in Mendelsohn and Karas,¹ Farhat et al,² and Mendelsohn³) Some of the effects of estrogen occur rapidly, whereas others require prolonged estrogen exposure. At physiologically relevant concentrations of estrogen, both the rapid and the longer-term cardiovascular effects of estrogens are mediated by estrogen receptors.^3–6 To date, two estrogen receptors (ERs) have been described, ER and ERß (reviewed in Gustafsson [1997],⁷ Katzenellenbogen and Korach,⁸ and Gustafsson [1999]⁹). Although their physiological relevance in the vasculature is incompletely understood, ER and ERß are expressed in endothelial cells and vascular smooth muscle cells (VSMCs),^10,11–16 the predominant cells present in vascular tissues.

Using wild-type (WT) and estrogen receptor knockout (KO) mice, we have previously studied the role of ER and ERß in mediating the vascular protective effects of estrogen in a mouse carotid artery injury model.^10,17,18 Previous studies used mice developed at the University of North Carolina, Chapel Hill, which harbor gene deletions of either ER (ERKO_CH)¹⁹ or ERß (ERßKO_CH).²⁰ These studies show that treatment of ovariectomized female mice with nanomolar concentrations of 17ß-estradiol (E2) inhibits the response to vascular injury to equivalent levels in wild-type,¹⁷ ERKO_CH,¹⁰ and ERßKO_CH mice.^18,21 These findings suggest that ER and ERß are able to complement one another such that each receptor alone is sufficient to mediate the vascular protective effects of estrogen, or that the vascular protective effects of estrogen are mediated by an ER/ERß-independent pathway. To distinguish between these two hypotheses, we undertook the present study examining the effect of estrogen on the response to vascular injury in ER,ßKO_CH (double) estrogen receptor knockout mice.

	Materials and Methods

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Animals
A total of over 820 animals were required to ultimately generate the ER,ßKO_CH mice used in this study. These mice, which have been extensively studied, do not express ER proteins in any tissue.^{19,20,22–24} However, ERKO_CH mice have been shown previously to express mRNA for 2 partial ER transcripts, one of which retains the hormone- and DNA-binding domains of full-length ER, and can both bind estradiol and mediate hormone-induced gene expression.²⁴ This partial ER transcript, detectable only by reverse transcriptase- polymerase chain reaction approaches, may account for the low level of residual specific binding of estradiol in uterine tissue of the ERKO_CH mice.²⁴ ER,ßKO_CH mice were generated by extensive crossbreeding in one of our laboratories (C.O.) as follows: male and female double heterozygous (ER^+/-ß^+/-) mice were created from the parental ERKO_CH and ERßKO_CH mice. Female offspring with the wild-type ER^+/+ß^+/+ genotype and the double receptor knockout ER^-/-ß^-/- genotype were included in the present study,^19,20,25 and the latter are referred to as ER,ßKO_CH. More than 830 mice were required to generate a sufficient number of ER,ßKO_CH female mice for the study. All mice were of mixed C57BL/6J/129 backgrounds. Genotyping of tail DNA was performed at 4 weeks of age and again at the end of the vascular injury experiment on tail snips. The ER gene was analyzed with the following primer pairs. Primers AACTCGCCGGCTGCCACTTACCAT and CATCAGCGGGCTAGGCGACACG for the WT gene correspond to flanking regions in the targeted exon 2. They produce a fragment of 320 bp. Primers TGTGGCCGGCTGGGTGTG and GGCGCTGGGCTCGTTCTC for the knockout gene correspond to part of the Neo cassette and the flanking exon 2. They produce a 700-bp fragment. Genotyping of the ERß gene has been described previously.²⁶ (The primers used for the ERß gene were one primer in intron 2 [ßNHD4-25; 5'-AGAATGTTGCACTG- CCCCTGCTGC-3'], one in intron 3 [C1wt-27; 5'-GGAGTAGAA- ACAAGCAATCCAGACATC-3'], and one in the Neo cassette [Neo-25; 5'-GCAGCCTCTGTTCCACATACACTTC-3']. A 650-bp product [ßNHD4-25 and C1wt-27] was amplified for the homozygous wild-type [WT; +/+] mice, and a 450-bp [ßNHD4-25 and Neo-25] product was amplified for the homozygous mutant [-/-] mice; both bands were amplified for the heterozygous [+/-] mice). Animals had free access to fresh water and food pellets.

Carotid Artery Injury
All procedures and protocols were approved by the New England Medical Center Animal Research Committee. A detailed description of the vascular injury protocol has been published previously.^10,17,18 Briefly, 7 days following ovariectomy, 26 wild-type and 23 ER,ßKO_CH female mice, average age 12 to 13 weeks, were randomized to receive either placebo pellets (-E2) or 17ß-estradiol- containing pellets (+E2; 0.1 mg/21-day pellets, Innovative Research) at a dose previously shown to produce physiologically relevant (ie, low nanomolar) concentrations of circulating 17ß-estradiol (E2). A week after pellets were implanted, the mice were anesthetized with inhaled isoflurane and subjected to unilateral vascular injury by intraluminal passage of a wire into the left common carotid artery resulting in endothelial denudation. At this time, osmotic minipumps calibrated to release bromodeoxyuridine (BrdU) over the 14 days of the experiment were implanted subcutaneously, as described.¹⁷ The mice were allowed to recover and return to normal activity.

Tissue Harvest
Two weeks following vascular injury, the mice were anesthetized with 2.5% isoflurane and blood was taken for cholesterol and estradiol determinations. Uteri where then extracted and wet weights obtained, followed by harvesting of a small segment of the tail for confirmation of prestudy genotype analysis by reverse transcriptase- polymerase chain reaction. Both carotid arteries were harvested next after perfusion fixation with 10% formalin at 150 mm Hg for 4 minutes. A small segment of small intestine was also harvested for use as a positive control for the BrdU stains (see next section).

Immunohistochemistry and Morphometry
Following embedding in paraffin, parallel sections from all 98 carotid arteries were stained as previously described for hematoxylin/eosin and for elastin.^10,17,18,27 Immunodetection of BrdU-labeled cells was also performed as previously described.^10,17,18 A section of small intestine was included on each slide to serve as a positive control for the BrdU stain. One section of each vessel was not exposed to a primary antibody, serving as a negative control. VSMCs were identified by immunostaining for smooth muscle specific actin, performed on a Vantana automated slide staining machine using clone 1A4 from Sigma as the primary antibody. BrdU-stained VSMCs and unstained VSMC nuclei were counted by two independent observers. A VSMC proliferation index also was calculated by dividing the number of BrdU-labeled VSMCs by the total number of unstained VSMCs for each section. Medial area was determined on serial elastin-stained sections using the ImagePro Plus software. All observations were made by observers blinded to both the genotype and the treatment group of the specimen. Only one vessel developed an occlusive thrombosis, and this mouse from the WT-E2 group was excluded from analysis.

Additional Assays
Circulating concentrations of E2 were measured by radioimmunoassay using a commercially available kit (Ultra-Sensitive Estradiol RIA, Diagnostic System Laboratories Inc), according to the manufacturer’s instructions. Pooled samples of serum (200 µL) from each of the treatment groups were each assayed in triplicate, and the mean value is reported as representative of the group. Control experiments confirmed the linearity of the assay between the range of 0.018 and 2.78 nmol/L. Circulating concentrations of total and HDL cholesterol, and triglycerides were determined as previously described.¹⁷ At the time of euthanasia, the correct genotype of each mouse was confirmed by a second independent set of PCR studies using tail-snip DNA as described above.

Statistical Analysis
For all analyses, differences between the 4 treatment groups were first examined with a one-way ANOVA after the normality of the data were tested. Post hoc comparisons were made with the Student-Newman-Keuls test. Two group comparisons were made with the Student’s t test. All statistics were performed using SigmaStat software. A value of P0.05 was considered significant.

	Results

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As in our previous studies, implantation of E2-containing pellets in the ovariectomized female mice restored circulating levels of E2 to physiological levels in both the WT and ER,ßKO_CH mice (0.43±0.03 nmol/L [115±9 pg/mL] and 0.55±0.03 nmol/L [148±7 pg/mL], respectively; Table). There were no significant differences in body weight, blood pressure, or heart rate at baseline or at the time of injury between any of the groups (Table). Cholesterol levels also were similar between the WT and ER,ßKO_CH animals, and not significantly changed by estrogen treatment (Table). In the WT animals, E2 treatment resulted in the expected approximately 10-fold increase in uterine weight compared with the -E2 group (Figure 1; P<0.001 WT+E2 versus WT-E2). The uterine weights were lower in the placebo-treated ER,ßKO_CH mice than in the placebo-treated WT mice (P<0.01 ER,ßKO_CH-E2 versus WT-E2). Surprisingly, E2 replacement to physiological levels also significantly increased uterine weight in the ER,ßKO_CH mice (5.8-fold; Figure 1; P<0.01 ER,ßKO_CH+E2 versus ER,ßKO_CH-E2), although to a lesser extent than in the WT mice. Uterine weight increased in every animal assigned to receive E2-containing pellets, confirming proper placement and function of the pellets.

View this table: [in this window] [in a new window]   Table 1. Effects of E2 on Wild-Type and ER,ßKOCH Mice

View larger version (14K): [in this window] [in a new window]   Figure 1. Uterine weights of wild-type and ER,ßKOCH mice treated with placebo or 17B-estradiol. Wet weights of uteri were determined at the time of euthanasia in wild-type and ER,ßKOCH mice. Mice received either placebo pellets (-E2) or 17ß-estradiol-containing pellets (+E2) for a total of 3 weeks before harvest. Bars represent the mean value for each group, ±SEM. *P<0.001 vs -E2.

The 2 primary endpoints for the mouse carotid artery injury model are well established^10,17,18 and include (1) change in medial area and (2) extent of VSMC proliferation, both assessed 14 days after unilateral endothelial denudation injury. The medial-area endpoint reflects the combined effects of alterations in extracellular matrix constituents and cellular volume, whereas the VSMC proliferation endpoint specifically indicates the degree of mitogenesis that occurs in these cells in response to the vascular injury.

Response to Vascular Injury: Medial Area
Medial areas were determined by computerized morphometric analysis of elastin-stained sections of each carotid artery. A summary of the medial area results is presented in Figure 2. The medial areas of the uninjured carotid arteries were similar in the WT (n=26) and ER,ßKO_CH (n=23) mice (15.5±0.3x 10^-3 mm² versus 15.3±0.5x10^-3 mm², respectively; P=NS). In the placebo-treated WT mice, injury induced an increase in the medial area to 21.3±1.6x10^-3 mm² (P<0.01 WT-E2 versus uninjured). Estrogen replacement in the WT mice significantly inhibited the injury-induced increase in medial area, resulting in a medial area of 16.0±1.4x10^-3 mm² (P=0.03 WT+E2 versus WT-E2). The medial area of the injured carotid in the WT+E2 animals was indistinguishable from that of the uninjured contralateral vessel (P=0.7 WT+E2 versus uninjured).

View larger version (41K): [in this window] [in a new window]   Figure 2. Medial areas of uninjured and injured carotid arteries from wild-type and ER,ßKOCH mice treated with vehicle or 17ß-estradiol. a through f, Representative elastin-stained sections from uninjured (Uninj) and injured (Inj) carotid arteries from wild-type (WT) and ER,ßKOCH mice treated with placebo pellets (-E2), or 17ß-estradiol-containing pellets (+E2) are shown (x400). The medial area of each specific section shown is listed in parentheses. The medial area was determined on the entire section. a, Wild-type, uninjured (15.5x10-3 mm2). b, Wild-type, injured, placebo-treated (22.2x10-3 mm2). c, Wild-type, injured, E2-treated (15.4x10-3 mm2). d, ER,ßKOCH, uninjured (14.7x10-3 mm2). e, ER,ßKOCH, injured, placebo-treated (21.6x10-3 mm2). f, ER,ßKOCH, injured, E2-treated (18.9x10-3 mm2). g, Summary of medial area results for all mice. In panel b, the external elastic lamina is identified by the red arrow, and the internal elastic lamina is identified by the black arrow. In panel g, bars represent the mean value for each group, ±SEM. *P<0.05 vs the inj, -E2 group within the same genotype.

In the placebo-treated ER,ßKO_CH mice, injury induced a significant increase in medial area (20.3±1.1x10^-3 mm²; P<0.01 ER,ßKO_CH-E2 versus uninjured), similar to the degree of injury response in the placebo-treated WT mice (P=0.6 ER,ßKO_CH-E2 versus WT-E2). In contrast to the findings in the WT mice, in the ER,ßKO_CH mice E2 treatment did not significantly inhibit the injury-induced increase in medial area (19.0±0.7x10^-3 mm² in the ER,ßKO_CH+E2, P=0.5 versus ER,ßKO_CH-E2). Thus, the injury-induced increase in medial area was similar in the placebo-treated WT and ER,ßKO_CH mice, but E2 replacement no longer inhibited the increase in medial area following injury in the ER,ßKO_CH mice. In contrast to any of our previous studies using this model, in over 300 animals (References 10, 17, 18, and unpublished results, 2000–2001), these results in the ER,ßKO_CH mice represent the first instance that E2 has failed to protect against an endpoint analyzed to quantify the vascular response to carotid injury.

Response to Vascular Injury: VSMC Proliferation
VSMC proliferation was assessed by immunohistochemical detection of BrdU-labeled VSMCs in sections from each carotid artery. A summary of these results is presented in Figure 3. There were no differences in the morphological appearance or total number of nuclei in the uninjured vessels of the WT and ER,ßKO_CH mice (95±15 versus 93±14 cells/section, respectively; P=NS). This supports that there are no obvious developmental abnormalities in the formation of the vasculature in the ER,ßKO_CH mice. As expected, BrdU-labeled VSMCs were only rarely detected in the uninjured vessels of either the WT or the ER,ßKO_CH mice (mean <1 labeled cell per vessel for WT and ER,ßKO_CH groups; P=NS between groups). As in our previous studies using this injury technique, a small amount of neointima was observed in a minority of injured vessels and there were no significant differences in neointimal formation in the WT compared with the ER,ßKO_CH mice (data not shown). In the placebo-treated WT mice, injury increased the VSMC proliferation index from <0.1 to 1.1±0.3 (P<0.001 WT-E2 versus uninjured). This increase in VSMC proliferation was significantly attenuated by E2 treatment in the WT mice, resulting in a VSMC proliferation index of 0.5±0.1 (P<0.05 WT+E2 versus WT-E2).

View larger version (41K): [in this window] [in a new window]   Figure 3. Vascular smooth muscle cell proliferation of uninjured and injured carotid arteries from wild-type and ER,ßKOCH mice treated with vehicle or 17ß-estradiol. a through f, Representative BrdU-stained sections from uninjured (Uninj) and injured (Inj) carotid arteries from wild-type (WT) and ER,ßKOCH mice treated with placebo pellets (-E2) or 17ß-estradiol-containing pellets (+E2) are shown (x400). The VSMC proliferation index (BrdU-labeled VSMCs/unstained VSMCs) of each specific section shown is listed in parentheses. The VSMC proliferation index was determined on the entire section. a, Wild-type, uninjured. b, Wild-type, injured, placebo-treated (0.90). c, Wild-type, injured, E2-treated (0.54). d, ER,ßKOCH, uninjured. e, ER,ßKOCH, injured, placebo-treated (0.77). f, ER,ßKOCH, injured, E2-treated (0.46). g, Summary of VSMC BrdU-labeling results for all mice. In panel g, bars represent the mean value for each group, ±SEM. *P<0.05 vs the Inj, -E2 group within the same genotype.

In the placebo-treated ER,ßKO_CH mice, injury also induced an increase in the VSMC proliferation index from <0.1 to 0.8±0.1 (P<0.001 ER,ßKO_CH-E2 versus uninjured). The injury-induced increase in VSMC proliferation in the placebo-treated ER,ßKO_CH mice was similar to that observed in the placebo-treated WT mice (P=0.4 ER,ßKO_CH-E2 versus WT-E2). As in the WT mice, E2 treatment of the ER,ßKO_CH mice significantly attenuated the injury-induced increase in the VSMC proliferation index (0.4±0.1; P<0.05 ER,ßKO_CH+E2 versus ER,ßKO_CH-E2; P=0.6 ER,ßKO_CH+E2 versus WT+E2). Thus, injury induced an increase in VSMC proliferation to similar degrees in the placebo-treated WT and ER,ßKO_CH mice, and the inhibitory effect of E2 on the injury-induced increase in VSMCs observed in the WT animals was retained in the ER,ßKO_CH mice. These findings demonstrate maintenance of the protective effect of E2 on VSMC proliferation in the ERKO_CH mice, in surprising contrast to the loss of the protective effect of E2 on the medial-area endpoint presented above.

	Discussion

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Recent data demonstrate clearly that estrogen receptors mediate both genomic and nongenomic effects in vascular cells at physiological levels of estrogen.^3–6 ER was the first and only known estrogen receptor from 1985, when it was initially cloned,²⁸ until 1996, when ERß was discovered.^29–31 Although ER and ERß are the products of distinct genes, they share relatively high degrees of homology in the DNA binding domain (95%) and in the ligand binding domain (55%). Recent studies have begun to define receptor-specific differences in ligand binding affinity,^29,32–34 receptor interacting proteins,^35,36 and pharmacologic agonists and antagonists.^37–39 These findings highlight the potential for differences in the physiological effects of these two receptors.

In 1995, we first reported that estrogen inhibits the vascular injury response in normolipemic wild-type mice,¹⁷ as it does in other animal models (reviewed in Mendelsohn and Karas¹). A subsequent study showed equivalent inhibition of the vascular injury response in the ERKO_CH mice created by Lubahn et al¹⁹ compared with their littermate wild-type controls.¹⁰ A more recent study in which ERKO_CH mice were bred with the hyperlipidemic apolipoprotein E knockout mice demonstrated that in this model estrogen no longer decreased plaque size, but continued to significantly alter plaque complexity, decreasing the number of lesions with fibrous caps, calcifications, and cholesterol clefts.⁴⁰ These studies support the existence of ER-independent vascular protective effects of E2 or of residual ER activity in the ERKO_CH mice.²⁴ The discovery of ERß,²⁹ identification of its expression in vascular cells and tissues,¹⁰ and demonstration that vascular injury dramatically enhances vascular cell expression of ERß but not ER^13,16 all raised expectations that ERß might mediate the vascular protective effects of estrogen. However, a subsequent carotid artery injury study in the ERßKO mice demonstrated that E2 continued to inhibit the response to injury equally well in ERßKO and WT mice.¹⁸ These studies, which used the only available ER knockout mice at that time,^19,20 suggested that neither ER nor ERß by themselves are necessary for the protective effect of estrogen in the vasculature.

In the present study, the effect of E2 on the usual two vascular injury endpoints, medial area and VSMC proliferation, was compared in ER,ßKO_CH and WT mice. In the wild-type mice, E2 inhibited the injury-induced increase in medial area as it has in each of our previous studies.^10,17,18 In contrast, in the ER,ßKO_CH mice, E2 did not prevent the injury-induced increase in medial area. Loss of the protective effect of estrogen on the medial-area endpoint in the ER,ßKO_CH mice supports that ER and ERß mediate this effect of estrogen. Thus, for the first time, ER and ERß are implicated as physiologically relevant mediators of one component of the vascular injury response. Taken together with the results of previous single estrogen receptor knockout experiments,^10,18 these data support that the ER and ERß can functionally complement one another in vivo, such that either receptor alone can mediate the inhibitory effect of E2 on the medial area thickening endpoint (see subsequent caveat).

The E2-mediated inhibition of VSMC proliferation observed in the WT mice in the present study also is consistent with our previous observations in the WT mice.^10,17,18 Quite surprisingly, however, estrogen also inhibited VSMC proliferation in the ER,ßKO_CH mice to a similar extent as in the WT mice. One interpretation of the preserved estrogenic effect on VSMC proliferation in ER,ßKO_CH mice is the exciting possibility that there are estrogen receptors other than ER and ERß that have not yet been identified. However, to date, extensive searching has failed to identify any likely candidates (authors’ unpublished results, 1997–2001).

It is important to note that E2-mediated inhibition of VSMC proliferation was not the only estrogenic effect observed in the ER,ßKO_CH mice. Estrogen exposure also resulted in a significant increase in ER,ßKO_CH uterine weights (Figure 1). This demonstrates that preservation of an effect of estrogen in the ER,ßKO_CH mice is not confined to, or specific for, the cardiovascular system. The estrogen responsiveness of the uterus in these mice raises an additional consideration. The ER,ßKO_CH mice used in the present study were produced by breeding ERKO_CH and ERßKO_CH mice.^19,20,22,23 Uteri of ERKO_CH mice express mRNA for 2 partial ER transcripts.²⁴ Although one of these transcripts is nonfunctional, the other transcript, E1, is a splice variant of the wild-type ER in which only a portion of exon 2 is omitted.²⁴ The protein encoded by this mutant sequence appears to bind estrogen with an affinity similar to that of the wild-type receptor, and to be capable of mediating hormone-induced gene expression.²⁴ In this prior study, immunoblot analyses of ERKO_CH uterus did not detect expression of a protein from the E1 splice variant.²⁴ However, it remains quite possible that the estrogenic effects observed on VSMC proliferation and uterine weight in the present study are mediated by expression of low levels of this mutant ER. If the E1 transcript mediates the effects of estrogen on uterine weight and/or VSMC proliferation in the ER,ßKO_CH mice (and the parental ERKO_CH mice¹⁰), this would have widespread implications for the published reports that have used ERKO_CH mice to study various physiological systems (reviewed in Couse and Korach⁴¹).

The mechanism by which estrogen continues to inhibit VSMC proliferation and increase uterine weight in the ER,ßKO_CH mice thus remains unclear. Receptor-independent effects of estradiol could be involved, although there is little physiological evidence in support of this explanation.¹ The presence of an unidentified, novel estrogen receptor could also account for the persistent ability of E2 to inhibit VSMC proliferation, although no ER has been identified despite extensive searching. However, another member of the steroid hormone receptor superfamily⁴² (eg, ERR, ERRß, ERR, or an orphan receptor) with the ability to respond to estrogen could be involved. Alternatively, a low level of expression of the E1 estrogen receptor splice variant identified by Couse et al²⁴ in the ERKO_CH parental strain could mediate residual estrogen responsiveness in ER,ßKO_CH mice. The explanation for persistent effects of estrogen in this study may be resolved by our ongoing studies of the effect of E2 on the vascular injury response in mice recently created in the Chambon laboratory in Strasbourg, France (ERKO_St),⁴³ in which ER has been fully disrupted. Understanding the mechanism by which estrogen responsiveness is retained in the ER,ßKO_CH mice has important implications for our understanding of estrogen receptor-mediated signaling in all target tissues, including the cardiovascular system.

	Acknowledgments

 
This work was supported in part by NIH P50 HL63494, NIH R01 HL55309, and NIH R01 HL56069 (M.E.M.); NIH R01 HL61298 (R.H.K.); the Swedish Medical Research Council and the Swedish Foundation for Strategic Research (C.O.); and by KaraBio AB and the Swedish Cancer Fund (J.-A.G.). The authors would like to thank Kristen Whitehead and Sharon Lynch for expert preparation of this manuscript. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of NIH.

Received June 13, 2001; accepted August 7, 2001.

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