Preeclampsia and Pregnancy-Related Hypertensive Disorders

Abstract

Our understanding of hypertension during pregnancy and in particular, preeclampsia has changed dramatically over the last decade. During the last year (2014–15), several articles published in Hypertension have provided important insights into the pathogenesis of preeclampsia and its related complications^1–38. In addition, Hypertension also published some key research communications that translated important basic science observations into the clinic. Some of these articles are briefly discussed, highlighting their significance to our understanding of the mechanism of the disease, to predict the disease and treat or prevent hypertension during pregnancy and other preeclampsia related complications.

Biomarkers for Early Pregnancy Prediction of Preeclampsia

Biomarkers for the early detection of preeclampsia are critical for risk stratification and testing therapies to prevent preeclampsia. In a recent study by Kenny et al, samples from the Screening for Pregnancy Endpoints (SCOPE) consortium (N=5623 women) were used to evaluate the role of 47 different plasma biomarkers for predicting preeclampsia between 14–16 weeks of gestation¹¹. While they found a number of biomarkers that were statistically significant in women who subsequently developed preeclampsia, only placental growth factor, a pro-angiogenic factor that has been extensively studied showed a modest predictive ability. However, when the biomarkers (placental growth factor and cystatin C) were combined with clinical information such as mean arterial pressure, and body mass index at 14 to 16 weeks' gestation, and maternal doppler scans, the predictive ability for early onset preeclampsia improved to area under the curve of 0.90 in a training cohort and 0.83 in a validation cohort. In contrast to the early onset data, the predictive ability for term preeclampsia was quite poor. These data suggest a concept of 2-stage screening with screening in first or early second trimester for early onset disease and with screening during 3^rd trimester for late onset disease may be a more fruitful strategy^39,40,. As the authors correctly point out, this study represents one of the most comprehensive evaluations of putative biomarkers in early pregnancy to predict preeclampsia, using a rigorous prospective cohort design and with robust immunoassays on a Luminex platform. These findings provide a rationale for further evaluation of the utility of a multivariate model that includes biomarkers and clinical factors as predictive and monitoring tools in prospective clinical trials of preeclampsia.

Santillan and colleagues recently hypothesized the use for plasma arginine vasopressin (AVP) measurements in the prediction of preeclampsia²⁸. In order to assess AVP levels, copeptin was used as a clinically biomarker of AVP secretion. Copeptin is a glycopeptide that makes up the C-terminal portion of prepro–arginine vassopressin (preproAVP), which is the precursor protein of AVP, a vasoactive neuropituitary hormone that plays a role in water homeostasis and in the regulation of vascular tone. Because AVP is short-lived and susceptible to degradation, vasopressin assays are not reliable. In contrast, copeptin is more stable and high throughput robust assays are available for use in a number of diseases⁴¹. Copeptin was measured throughout pregnancy in maternal plasma from preeclamptic and control women. The authors reported that maternal plasma copeptin was significantly higher throughout preeclamptic pregnancies versus control pregnancies. Despite controlling for clinically significant confounders (age, body mass index, chronic essential hypertension, twin gestation, diabetes mellitus, and history of preeclampsia) using multivariate regression, they found that the association of higher copeptin concentration and the development of preeclampsia remained significant. ROC analyses also revealed that as early as the sixth week of gestation, elevated maternal plasma copeptin concentration is a highly significant predictor of preeclampsia throughout pregnancy. While these data implicate AVP release as a novel predictive biomarker for preeclampsia very early in pregnancy, further larger-scale clinical studies must be performed to confirm the prediction of preeclampsia by copeptin. Moreover, studies are necessary to identify causes of enhanced AVP production in these patients.

Yeung et al also recently published a follow-up study describing copeptin as a new, emerging predictive biomarker specific for preeclampsia³⁷. Using a nested case-controlled study from the Calcium for Preeclampsia Prevention (CPEP) cohort, they found that serum copeptin levels were elevated in preeclampsia during and prior to the onset of clinical symptoms. Importantly, copeptin was not altered in other disorders such as gestational hypertension, gestational diabetes or preterm birth without preeclampsia. These data suggest that serum copeptin alterations are specific to preeclampsia. Authors also noted that copeptin levels did not correlate with circulating angiogenic factors. These data suggest that copeptin measurements may add additional information and could be used in an algorithm with angiogenic factors for the prediction of preeclampsia.

In addition proposing the use for plasma AVP measurements in the prediction of preeclampsia and Santillan and colleagues also hypothesized a possible involvement of AVP in the pathogenesis of preeclampsia^28,42. They reported that chronic infusion of AVP during pregnancy in C57BL/6J mice, caused pregnancy-specific hypertension, renal glomerular endotheliosis, proteinuria, and intrauterine growth restriction. While they suggested that chronic AVP infusion as a novel and clinically relevant model of preeclampsia in mice, addition studies in other species must be performed to confirm that AVP plays a causative role in preeclampsia. It will also be important to demonstrate that the chronic infusion rate of AVP produces a physiologically relevant concentration of plasma AVP that is consistent with the levels of AVP observed in preeclamptic women.

Role of Statins in Women with Severe Preeclampsia

The search for the ability to prevent preeclampsia as well as the mechanisms of its pathogenesis in order to develop a therapy that safely prolongs gestation has been extensive. Brownfoot el al present mechanistic data that pravastatin, a HMG-CoA inhibitor that has been FDA approved for the treatment of hypercholesterolemia may have a therapeutic benefit in preeclampsia by downregulating antiangiogenic factors such as soluble fms-like tyrosine kinase 1 (sFlt1) that contribute to the pathogenesis of the disorder². Authors show in elegant cell culture studies that pravastatin reduced sFlt1 secretion from primary endothelial cells, purified cytotrophoblasts and placental organ cultures obtained from women with preeclampsia. Interestingly the regulation of sFlt1 by pravastatin was mediated via the HMG-CoA reductase pathway. Finally, authors performed a pilot study in 4 women with severe preterm preeclampsia and report that the drug is well tolerated without any adverse effects. Interestingly pravastatin therapy was associated with an improvement in the clinical features such as blood pressure and proteinuria and reduction in circulating sFlt1 levels. Taken together with prior studies demonstrating benefit of pravastatin in other animal models of preeclampsia.^43,44, these data suggest that pravastatin may be a promising therapeutic for prevention and treatment of preeclampsia. Clinical trials to test the safety and efficacy of statins in women with established preeclampsia have been initiated^45,46,.

Preeclampsia and Increased Risk of Cardiovascular Disease Later in Life

Over the past decade, information has accumulated indicating that women who have had a preeclamptic pregnancy are at increased long-term risk of premature cardiovascular disease and mortality. This increased in risk ranges from a doubling of risk in all cases to an eight to nine fold increase in women with preeclampsia who gave birth before 34 weeks of gestation^30,47–50. The severity of this increased risk has been recognized by the American Heart Association, which now recommends that a pregnancy history be part of the evaluation of cardiovascular risk in women. While pre-existing cardiovascular disease risk factors are associated with increased risk of developing preeclampsia it is not known whether preeclampsia merely unmasks risk or contributes directly to future cardiovascular disease. To attempt to address this issue important unanswered question, Pruthi et al utilized a mouse model of soluble fms-like tyrosine kinase-1 overexpression to test the hypothesis that elevated levels of sFlt-1, as seen in preeclamptic women, causes an enhanced vascular response to future vessel injury²⁴. The authors report that overexpression of sFlt-1 in pregnant CD1 mice induced hypertension and glomerular disease characteristics resembling human preeclampsia. Two months postpartum, sFlt-1 levels and blood pressure normalized and cardiac size and function by echocardiography and renal histology were similar in preeclampsia-exposed compared with control mice. The authors then challenged the mice with unilateral carotid injury. Interestingly, sFlt-1-exposed mice had significantly enhanced vascular remodeling with increased vascular smooth muscle cell proliferation vessel fibrosis compared with control pregnancy. In the contralateral uninjured vessel, there was no difference in remodeling after exposure to sFlt-1. These data support the concept that vessels exposed to excess levels of sFlt-1 retain a persistently enhanced vascular response to injury despite resolution of sFlt-1 levels after delivery. While cardiovascular disease in later life in women with preeclampsia may be due to common risk factors for both conditions, the findings from this study suggest that a component of residual injury from sFlt-1 cannot be completely excluded. The underlying mechanisms responsible for the residual injury from sFlt-1 remain unknown. Moreover, whether these findings can be translated to the human condition of preeclampsia is unclear. It is also unclear whether other soluble placental factors that have been implicated in pathogenesis also have a residual injury on the maternal vasculature after pregnancy. This is certainly an area of research that requires further investigation.

Endothelin-1 and Endothelin type A (ET_A) and B (ET_B) Receptors in the Pathogenesis of Preeclampsia

There is growing evidence to suggest an important role for endothelin-1 (ET-1) in the pathophysiology of preeclampsia^51,52. ET-1 was identified as a potent endothelium-derived vasoconstrictor, the most potent vasoconstrictor known. Derived from a longer 203-amino acid precursor known as preproendothelin, the active peptide is proteolytically cleaved into its final 21-amino acid form. Much of the research on endothelin-1 has focused on the role of the endothelin type A (ET_A) receptor found in the vascular smooth muscle and how they serve as important regulators of ET-1 dependent vasoconstriction and cellular proliferation. However, there is another ET-1 receptor, the endothelin type B (ET_B) receptor which is found, among other locations, on vascular endothelial and renal epithelial cells. In contrast to ET_A receptor activation, activation of ET_B receptors conveys a vasodilatory response through the production of nitric oxide and cyclooxygenase metabolites.³⁴ The exact role of endothelin and the relative contributions of the ET_A and ET_B receptors to preeclampsia are not fully elucidated though recent studies suggest that the system has great potential as a target for the treatment of preeclamspsia.^51,52

Multiple studies have examined circulating levels of ET-1 in normal pregnant and preeclamptic cohorts, and found elevated levels of plasma ET-1 in the preeclamptic group, with some studies indicating that the level of circulating ET-1 correlates with the severity of the disease symptoms, though this is not a universal finding^51,52. Verdonk et al recently investigated the relationship between disturbed angiogenic balance, arterial pressure, and ET-1 in pregnant women with a high (≥85; n=38) or low (<85) soluble Fms-like tyrosine kinase-1/placental growth factor ratio³¹. Plasma ET-1 levels were increased in women with a high ratio. In addition, plasma ET-1 correlated positively with soluble Fms-like tyrosine kinase-1 and negatively with plasma renin concentration Moreover, urinary protein correlated with plasma ET-1 and mean arterial pressure. The authors concluded that a high antiangiogenic state associates with ET-1 activation, which together with the increased mean arterial pressure may underlie the parallel reductions in renin in preeclampsia.

A number of experimental models of preeclampsia (placental ischemia, sFlt-1 infusion, TNF-α infusion, and AT1-AA infusion) are also associated with elevated tissue levels of ET-1^51,52. The fact that hypertension in pregnant rats, induced by placental ischemia or chronic infusion of sFlt-1, TNF-α, or AT1-AA can be completely attenuated by ET_A receptor antagonism, strongly suggests that ET-1 appears to be a final common pathway linking factors produced during placental ischemia to elevations in blood pressure^51,52.

While numerous studies have focused on endothelin receptor type-A in preeclampsia, the role of vasodilator endothelial ET_B receptor in preeclampsia is unclear. To test whether downregulation of endothelial ET_B receptor expression/activity plays a role in preeclampsia, Mazzuca et al recently examined mesenteric microvascular function, ET_A receptor and ET_B receptor levels and blood pressure in a rat model of preeclampsia produced by reduction of uteroplacental perfusion pressure (RUPP)¹⁵. The authors reported that ET_B receptor expression/activity is reduced in pregnant rats with RUPP and may explain the increased BP and ET-1 vasoconstriction and reduced ET_B receptor-mediated relaxation in placental ischemia-induced hypertension. Based on their data the authors suggested that ET_B receptor could be an important target in preeclampsia and using pharmacological approaches with ET_B receptor agonists may represent a novel approach in managing preeclampsia.

Immune Mechanisms and Regulatory T cells in Preeclampsia

Preeclampsia is a multisystemic syndrome during pregnancy that is often associated with intrauterine growth retardation and immunologic dysregulation involving decreases in T regulatory cells⁵³. Przybyl et al recently evaluated the effect of upregulating regulatory T cells in transgenic rat model for preeclampsia achieved by mating female rats transgenic for human angiotensinogen with rats transgenic for human renin²⁵. The authors utilized a superagonistic monoclonal antibody for CD28 to upregulate regulatory T cells. They reported that Tregs are reduced in the uteroplacental unit in the transgenic rat model and the superagonistic monoclonal antibody for CD28 was effective in restoring Tregs in preeclamptic placentas, as well as in maternal circulation and spleen. Interestingly, they found that restoration of Tregs did not ameliorate the maternal phenotype. However, they demonstrated improved IUGR, reversed the brain sparing effect, showed improved brain size with an increase in mature neurons, and reduced reactive astrogliosis in the cortex, indicating an improved fetal development. Moreover, restoration of Tregs reduced the number of severely retarded pups. The improvement of fetal outcome was independent of altering the antiangiogenic balance and of altering trophoblast cell invasion in the uteroplacental unit. The improvement of fetal outcome, independent of reducing hypertension in the transgenic model of preeclampsia are very exciting results and deserves further investigation in other animal models of preeclampsia associated with immunologic dysregulation.

Numerous studies have demonstrated that women with preeclampsia have exaggerated immune responses characterized by elevated pro-inflammatory cytokines, autoantibodies, activated immune cells, and oxidative stress. Various cellular sources of oxidative stress, such as neutrophils, monocytes, and CD4+ T cells have been suggested⁵³. Wallace et al recently examined the role of circulating and placental CD4+ T cells in mediating oxidative stress in response to placental ischemia³³. They evaluated CD4+ T cells and oxidative stress in preeclamptic and normal pregnant women, placental ischemic and normal pregnant rats, and normal pregnant recipient rats of placental ischemic CD4+ T cells. The authors demonstrated that the placental T-cell profile mirrors that in the circulation (increased Th17/ decreased Tregs) and that these cells are sources of oxidative stress molecules and stimulate reactive oxygen species from vascular cells. Moreover, they reported that transfer of CD4+ T cells into normal pregnant rats results in oxidative stress. They also reported that NADPH oxidase inhibition attenuated oxidative stress, hypertension, and T cells from increasing in response to placental ischemia or in response to adoptive transfer of T cells during pregnancy. The results from this comprehensive study highlight importance of circulating and placental CD4+ T cells in mediating the increase in oxidative stress in response to placental ischemia.