Decreased plasma hemopexin activity is associated with increased placental and monocyte AT-1 receptor expression and lower plasma AT-1 receptor in preeclampsia

Introduction: Hemopexin (Hx) is a free heme scavenger, which also has protease activity. During healthy pregnancy, Hx activity increases, while in women with preeclampsia, Hx activity was decreased. This is likely due to high levels of its natural inhibitor, ATP. As active Hx induced AT-1R shedding in vitro, it was suggested that decreased Hx activity in preeclampsia leads to higher AT-1R expression and subsequently increased angiotensin II sensitivity. In the current study, we aimed to evaluate whether changes in Hx activity during pregnancy and in preeclampsia correlated to in vivo shedding of the AT-1R. Methods: Hx activity, as well as plasma ATP levels and alkaline phosphatase (AP) activity, were measured in nonpregnant, healthy pregnant and earlyand late-onset preeclamptic women. In addition, angiotensin II type 1 receptor (AT-1R) expression on peripheral blood monocytes and in the placenta was determined using flow cytometry and immunohistochemistry. Plasma AT-1R levels were measured with western blotting. Results: Plasma Hx and AP activity increased in healthy pregnant women compared with nonpregnant women. This associated with lower monocyte AT-1R expression and higher plasma AT-1R levels. Plasma Hx activity decreased and ATP levels increased in women with preeclampsia, while placental and monocyte AT-1R expression was increased and plasma AT-1R levels were decreased in preeclamptic women. Conclusions: Our data suggest that also in vivo active Hx activity sheds the AT-1R from tissue, such as monocytes and placenta. Therefore active Hx may play a role in the development of the decreased responsiveness to angiotensin II in healthy pregnancy, while the decreased Hx activity in preeclampsia may result in increased angiotensin II sensitivity. CHAPTER 7 | HEMOPEXIN ACTIVITY AND EXTRACELLULAR ATP IN THE PATHOGENESIS OF PREECLAMPSIA | FLOOR SPAANS 123 CHAPTER 7 | HEMOPEXIN ACTIVITY AND EXTRACELLULAR ATP IN THE PATHOGENESIS OF PREECLAMPSIA | FLOOR SPAANS 7 Introduction Hypertension and proteinuria in the second half of pregnancy are the main symptoms of preeclampsia, the most common pregnancy complication. Preeclampsia occurs in around 3-5% percent of all pregnancies [1] and, due to lack of knowledge about the exact pathophysiology, an adequate therapy for the disease is still unavailable. Symptoms of preeclampsia may develop early (before 34 weeks) and late (after 34 weeks) in pregnancy. Although similar maternal symptoms are observed in these two subtypes of preeclampsia, the pathophysiologies of earlyand late-onset disease have been assumed to be different [2,3]. In early-onset preeclampsia, which has been mostly studied, a central role for the placenta in the pathogenesis has been recognized. Poor placentation in the first trimester is thought to lead to placental oxidative stress and hypoxia, which may result in the production of various stress-related factors such as anti-angiogenic factors, cytokines, microparticles and DAMPs (like extracellular ATP) [4,5] from the placenta. These are thought to contribute to systemic inflammation and endothelial dysfunction, which then leads to hypertension and proteinuria [4]. In contrast, less is known about the pathophysiology of late-onset disease, but it was shown to be associated with various maternal factors, such as diabetes mellitus, a BMI >25 and a family history of hypertension [3,6,7]. Signs of hypoxia have been shown in the placenta of late onset preeclamptic women [8]. It has recently been suggested that this may be due to poor placentation, but in a milder form [9]. Therefore problems with placental perfusion and symptoms of preeclampsia may not arise until the end of pregnancy in these women [9]. Hemopexin (Hx) is a plasma protein that is mainly known as a free heme scavenger [10]. However, Hx was recently found to have serine protease activity [11]. This activity could be inhibited by extracellular nucleotides, such as ATP [11], and increased by ATP hydrolysing enzymes such as alkaline phosphatase (AP) [11]. Interestingly, during healthy pregnancy, the protease activity of Hx was shown to be increased [12]. Hx activity started to rise early in pregnancy (week 10-15) and remained high until the end of pregnancy [13]. Although the physiological role of increased Hx activity during pregnancy is not exactly known, we have recently shown that active Hx can shed the Angiotensin II type 1 receptor (AT-1R) from endothelial cells and monocytes [13], suggesting that the increased Hx activity may contribute to decreased sensitivity to angiotensin II [14]. This suggestion is in line with the observation that the timing of the increase in Hx activity coincided with the timing of the decrease in angiotensin II sensitivity, i.e. both processes start around week 10-15 of pregnancy [15]. In women with early-onset preeclampsia, decreased Hx activity was found in plasma. This decreased Hx activity was suggested to be due to increased plasma levels of ATP, its natural inhibitor [13]. We hypothesized that the decreased Hx activity in preeclamptic women may lead to decreased shedding of the AT-1R from the vascular wall [13] and therefore result in