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Renal Unit (M.W., M.S., R.T.) and the Department of Obstetrics and Gynecology (J.L.E., R.T.), Massachusetts General Hospital, Boston, Massachusetts 02114; Endocrinology, Diabetes, and Hypertension Division (E.W.S.), Brigham and Womens Hospital, Boston, Massachusetts 02115; Renal Division (C.L., V.P.S., S.A.K.), Beth Israel-Deaconess Medical Center, Boston, Massachusetts 02215; Magee-Womens Research Institute and Department of Obstetrics and Gynecology and Reproductive Sciences (C.A.H., R.B.N., A.R., A.D., A.S.M.S., J.M.R.), University of Pittsburgh, Pittsburgh, Pennsylvania 15213; and Department of Epidemiology (R.B.N., J.M.R.), University of Pittsburgh Graduate School of Public Health, Pittsburgh, Pennsylvania 15261
Address all correspondence and requests for reprints to: Ravi Thadhani, M.D., M.P.H., Bulfinch 127, 55 Fruit Street, Massachusetts General Hospital, Boston, Massachusetts 02114. E-mail: rthadhani{at}partners.org.
| Abstract |
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| Introduction |
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Alterations in angiogenesis during early pregnancy contribute to the incomplete remodeling of uterine spiral arterioles and abnormal placental vascular development that are central to the pathogenesis of preeclampsia (4). Altered placental vascular development is thought to lead to the elaboration of soluble factors of placental origin that induce the maternal preeclampsia syndrome (5). Soluble fms-like tyrosine kinase (sFlt-1) is a secreted splice variant of Flt-1 that antagonizes vascular endothelial growth factor (VEGF) and placental growth factor (PlGF) by preventing their binding to their target tissue receptors Flt-1 and kinase insert domain-containing receptor (6). It has been recently shown that increased sFlt1 occurs as early as 56 wk before the onset of clinical symptoms of preeclampsia (7), whereas significant reductions in PlGF are detectable as early as the first trimester in women who subsequently develop preeclampsia (8). Furthermore, administration of exogenous sFlt1 to pregnant rats leads to reduced PlGF, hypertension, proteinuria, and glomerular endothelial injury, suggesting an important pathogenic role for angiogenic factors in preeclampsia (6). Because alterations in angiogenesis and insulin resistance are also associated with CVD (9), we hypothesized that these alterations would be evident in postpartum women with a history of preeclampsia before the development of hypertension or CVD.
| Subjects and Methods |
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50%) and the universitys prenatal outpatient clinic (
50%), which serves a mostly low-income, racially diverse population. Consecutive subjects who developed preeclampsia during their first completed (nulliparous) pregnancy and participants with normotensive pregnancies within the same timeframe were mailed a letter cosigned by their primary caregivers and the investigators in which they were offered an opportunity to opt out of the current study. Women who chose not to opt out were contacted by the investigators and offered inclusion. In total, we examined 29 normotensive women with a history of preeclampsia during their first completed (nulliparous) pregnancy (18 from MGH and 11 from MWH) and 32 normotensive women (18 from MGH and 14 from MWH) at 18.0 ± 9.7 months postpartum in the General Clinical Research Centers at the Massachusetts Institute of Technology and the MWH. The study was approved by the MGH and MWH human research committees, and all subjects provided written informed consent. At both sites, preeclampsia was defined according to research criteria as hypertension (>140/90 mm Hg) and proteinuria (either 2+ or greater by dipstick or 300 mg or greater per 24 h) that first appeared after 20 wk of gestation and resolved within 12 wk postpartum (1). Because preeclampsia often presents near term and other disorders can lead to preterm delivery, to prevent misclassification of pregnancy outcome, all women in the normal pregnancy group had delivered at term (>38 wk). At both sites, women with current pregnancy; diabetes; or a history of gestational diabetes, chronic hypertension, proteinuria, or serum creatinine greater than 1.0 mg/dl were excluded.
Subjects underwent a history and physical examination and a urine pregnancy test. Blood was collected on the morning after an overnight fast for measurement of insulin, glucose, free VEGF, and sFlt-1. Samples were processed immediately, stored at 80 C for no longer than 18 months and were thawed only for the current study. Glucose was measured using standard glucose oxidase assays with intra- and interassay coefficients of variation (CVs) less than 2%. Insulin levels were measured using the Lincos RIA (Linco Research, St. Charles, MO), which does not cross-react with proinsulin. The intra- and interassay CVs were less than 8%. Commercial ELISA kits were used for sFlt-1 and free VEGF (R&D Systems, Minneapolis, MN). The intra- and interassay CVs for sFlt-1 and VEGF were 3.5 and 5.6 and 8.1 and 10.9, respectively. All samples were run in duplicate by technicians blinded to pregnancy outcome.
Univariate comparisons between the pregnancy outcome groups were performed using two-sample t tests, Wilcoxon rank sum test, or Fisher exact test as appropriate. Due to rightward skewing of the VEGF and homeostasis model of insulin resistance (HOMAIR) distributions, these variables were analyzed after natural log transformations. Pearson correlation coefficients between markers of angiogenesis, HOMAIR, and covariates were calculated. Analysis of covariance was used to examine the association between sFlt-1 and preeclampsia adjusted for potential confounding. Logistic regression was used to calculate odds ratios for having had prior preeclampsia given levels of postpartum markers and adjust for potential confounding.
| Results |
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There was no association between menstrual cycle phase and sFlt-1, VEGF, or HOMAIR. Nonetheless, because Flt-1 expression might be increased in the menstrual phase of the cycle (11), we reanalyzed the difference in mean sFlt-1 levels between the pregnancy outcome groups after excluding women who were evaluated at the clinical research centers on d 15 of their cycle. After excluding these 10 women, there remained a statistically significant increase in sFlt-1 levels among the preeclampsia group, compared with the normal pregnancy group (41.6 ± 6.7 vs. 30.8 ± 10.3 pg/ml, P < 0.01). As noted, adjusting for menstrual cycle phase did not mitigate the association between increased sFLt-1 and prior preeclampsia.
Next, we examined the likelihood of having had prior preeclampsia according to postpartum levels of insulin resistance and angiogenesis. The odds ratio of having had prior preeclampsia among women in the upper quartile (>75th percentile) of the HOMAIR distribution vs. the remaining women (<75th percentile) was 4.9 (95% confidence interval 1.2, 23.8; P = 0.01). The odds ratio for preeclampsia comparing the upper quartile (>75th percentile) of sFlt-1 vs. the remaining women (<75th percentile) was also 4.9 (95% confidence interval 1.2, 23.8; P = 0.01), but different groups of women constituted the upper quartiles of the HOMAIR and sFlt-1 distributions. Only five women were in the upper quartiles of both and all five had a history of preeclampsia. When women who were in either the upper HOMAIR or the upper sFlt-1 quartiles (n = 27) were compared with women in the lower quartiles of each (n = 34), the odds ratio of having had preeclampsia increased to 5.7 (1.7, 20.0; P < 0.01). Adjusting for differences in age, race, postpartum duration, BP, BMI, smoking, lactation status, or contraceptive use did not appreciably alter the observed association among increased postpartum sFlt-1, HOMAIR, and prior preeclampsia.
| Discussion |
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Studies during pregnancy (6) suggest that the increased concentrations of sFlt-1 we observed should be associated with decreased levels of free VEGF. Instead, there was a trend toward increased free VEGF levels in the preeclampsia group. Furthermore, a negative correlation between sFlt-1 and VEGF was not observed, and the relative molar concentrations of sFlt-1 were at least 20-fold lower than VEGF, suggesting that sFlt-1 does not meaningfully influence circulating levels of free VEGF in the basal postpartum state. Prior studies of patients with established CVD also found increased levels of circulating VEGF, but its role in atherogenesis remains controversial (13, 14, 15, 16, 17, 18). Some studies suggest a beneficial effect of enhanced angiogenesis in coronary and peripheral vascular disease models (19), but recent data suggest that increased VEGF could actually worsen vascular disease by promoting neoangiogenesis in atherosclerotic plaques (20). Nonetheless, our results are mostly in agreement with prior cross-sectional clinical studies (13, 14, 15, 16, 17, 18) and extend the results from populations with prevalent CVD back to a population at high risk of CVD years before its development.
Women with a history of preeclampsia also demonstrated increased HOMAIR. Insulin resistance is associated with CVD; prospective studies during pregnancy suggest that it is associated with increased risk of preeclampsia, and previous postpartum studies similarly identified increased insulin resistance in women with a history of preeclampsia (21, 22). Therefore, insulin resistance appears to be a potential mechanism linking preeclampsia and future CVD. Although the molecular pathways by which insulin resistance might link preeclampsia with future CVD are unclear, oxidative stress and inflammation, which are features of both diseases, may be involved. For example, the vascular superoxide-producing nicotinamide adenine dinucleotide phosphate reduced oxidase, an enzyme that can be activated by hyperinsulinemia or associated excess free fatty acids, may be particularly important in oxidative stress and inflammation (23, 24, 25, 26). Interestingly, we observed significant linear correlation between HOMAIR and VEGF. Insulin induces the expression of VEGF mRNA (27) and reduced expression of VEGF mRNA in insulin-resistant states can be reversed with insulin (9, 28). Therefore, it is tempting to speculate that increased insulin resistance at baseline along with alterations in angiogenesis might act synergistically to predispose certain women to preeclampsia and perhaps future CVD. Indeed, a recent prospective study (29) during pregnancy identified such an interaction: whereas insulin-resistant women and women with disordered angiogenesis were at increased risk of preeclampsia (odds ratio 4.1 and 8.7, respectively), women who demonstrated both alterations were at markedly increased risk (odds ratio 15.1). In this study, there was a suggestion that women with the highest levels of both sFlt-1 and HOMAIR had the greatest odds of prior preeclampsia; however, our small sample size did not permit formal testing of interaction between the two factors.
We acknowledge limitations of this pilot study. First, a small number of subjects were examined once after pregnancy and the difference between the groups sFlt-1 levels were statistically significant, although relatively small. This is the first study, however, to examine markers of angiogenesis in postpartum women with a history of preeclampsia. Second, the basal levels of sFlt-1 appear to be too low to influence circulating VEGF concentrations, raising the possibility that although sFlt-1 levels are elevated in women with a history of preeclampsia, they may not play a clinically significant role in the postpartum state. Alternatively, it is possible that in disease states such as atherosclerosis, hypoxia could induce sFlt-1 to higher, potentially pathogenic levels in especially those patients with higher baseline levels, such as women with a history of preeclampsia, and thereby contribute to acceleration of atherosclerosis. Finally, it is possible that the circulating levels of the angiogenesis factors we measured may not fully reflect their levels at the endothelial surface. Third, the small number of subjects precluded stratification according to the severity of preeclampsia. We hypothesize that women with severe preeclampsia who are at the highest risk for future CVD (2, 30, 31, 32) and who demonstrate the highest levels of sFlt-1 during pregnancy (6, 7) might also demonstrate the highest levels postpartum. Certainly larger studies in the future should investigate whether such a dose-response relationship exists among the severity of preeclampsia, its risk of recurrence, and the magnitude of increase in postpartum sFlt-1 levels. Fourth, we compared women with previous preeclampsia with women with uncomplicated pregnancies. Whether other adverse pregnancy outcomes such as preterm labor or gestational diabetes might also be associated with postpartum alterations in angiogenesis factors is still unknown.
Finally, it is important to acknowledge that we cannot exclude the possibility that preeclampsia was the cause of our postpartum observations rather than a consequence of alterations that antedated pregnancy. This distinction may be less important if further studies confirm that altered angiogenesis either contributes directly to future CVD or at least helps identify high-risk women who might benefit most from early and aggressive cardiovascular risk factor modification. Indeed, a point of this study is to convey to the general medical community the possibility that perhaps closer attention to pregnancy outcome could help identify women at higher risk for future CVD, even when certain evidence of that risk, such as obesity, might have been present before pregnancy, as it was in this study. Whereas obesity is indeed a powerful risk factor for preeclampsia (33), most obese women still do not develop preeclampsia. Conversely, lean women who are insulin resistant are at increased risk of preeclampsia, suggesting an independent association between insulin resistance and preeclampsia (10). Furthermore, separating the independent effects of obesity from its downstream consequences, such as insulin resistance, is difficult and perhaps statistically inappropriate given that they likely share a common causal pathway (34, 35). Importantly, increased sFlt-1 levels were independent of BMI in this study. Therefore, our results suggest that the phenotype of postpartum women with prior preeclampsia appears to include not only the obesity-insulin resistance syndrome but also alterations in angiogenesis factors.
| Footnotes |
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1 M.W. and C.A.H. contributed equally to this work. ![]()
Abbreviations: BMI, Body mass index; BP, blood pressure; CV, coefficient of variation; CVD, cardiovascular disease; HOMAIR, homeostasis model of insulin resistance; MOMS, MGH Obstetric Maternal Study; PlGF, placental growth factor; sFlt-1, soluble fms-like tyrosine kinase; VEGF, vascular endothelial growth factor.
Received March 24, 2004.
Accepted September 15, 2004.
| References |
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