Perfusion of human placenta with hemoglobin introduces preeclampsia-like injuries that are prevented by α₁-microglobulin

Abstract

Background

Preeclamptic women have increased plasma levels of free fetal hemoglobin (HbF), increased gene expression of placental HbF and accumulation of free HbF in the placental vascular lumen. Free hemoglobin (Hb) is pro-inflammatory, and causes oxidative stress and tissue damage.

Methodology

To show the impact of free Hb in PE, we used the dual ex vivo placental perfusion model. Placentas were perfused with Hb and investigated for physical parameters, Hb leakage, gene expression and morphology. The protective effects of α₁-microglobulin (A1M), a heme- and radical-scavenger and antioxidant, was investigated.

Results

Hb-addition into the fetal circulation led to a significant increase of the perfusion pressure and the feto-maternal leakage of free Hb. Morphological damages similar to the PE placentas were observed. Gene array showed up-regulation of genes related to immune response, apoptosis, and oxidative stress. Simultaneous addition of A1M to the maternal circulation inhibited the Hb leakage, morphological damage and gene up-regulation. Furthermore, perfusion with Hb and A1M induced a significant up-regulation of extracellular matrix genes.

Significance

The ex vivo Hb-perfusion of human placenta resulted in physiological and morphological changes and a gene expression profile similar to what is observed in PE placentas. These results underline the potentially important role of free Hb in PE etiology. The damaging effects were counteracted by A1M, suggesting a role of this protein as a new potential PE therapeutic agent.

Introduction

Preeclampsia (PE) is a leading cause of maternal and fetal morbidity and mortality. Despite extensive research, PE still remains enigmatic and is called the disease of theories by many obstetricians [1]. Clinical manifestations, i.e. hypertension and proteinuria, appear from 20 weeks of gestation and onwards, but the underlying mechanisms may begin already at the time of implantation [2]. Up to date, there are no established prognostic and/or diagnostic markers for the disease. The only cure still is termination of pregnancy with delivery of the fetus and removal of the placenta.

PE evolves in two stages where the first stage is initiated by a defective placentation. A growing body of studies shows that uneven blood perfusion, hypoxia and oxidative stress follow as a consequence of the defect in placentation, further aggravating the impairment of placental functions [3], [4]. Stage two is characterized by the appearance of clinical symptoms such as hypertension, proteinuria and edema, which are caused by a general vascular endothelial dysfunction leading to a general organ failure and damage. The link between stage one and two is still unclear but several different factors and explanations have been suggested [5].

By using gene and protein profiling techniques, we have previously been able to show increased mRNA levels of fetal hemoglobin (HbF) in the placental tissue and evidence of free HbF in the placental vascular lumen in PE [6]. Furthermore, we have shown increased plasma and serum concentrations of HbF in the mother, suggesting that free HbF leaks over the blood-placenta barrier, into the maternal circulation where the plasma concentration is increasing from early pregnancy and later correlates to the severity of the the disease [7], [8].

Free Hb is a highly reactive molecule that is capable of damaging and disrupting cell membranes [9]. Also, it binds and inactivates nitric oxide (NO)[10], with vasoconstriction as a consequence. The metabolites of Hb, free heme and iron, damage lipids, protein and DNA through direct oxidation and/or generation of reactive oxygen species (ROS) [11]. In fact, free heme, bilirubin, and biliverdin have been identified among 14 metabolites in a metabolomic signature of preeclampsia using first trimester plasma [12]. Due to the lipophilic nature of the heme-group, it intercalates membranes and has destabilizing effects on the cytoskeleton [13]. Heme is also a pro-inflammatory molecule that activates neutrophils [11]. Several important Hb-detoxification systems work in parallel to prevent Hb-induced oxidative stress and tissue damage. Haptoglobin is a glycoprotein that forms a complex with free Hb, and is one of the primary Hb scavengers in plasma. In fact, a haptoglobin polymorphism has been associated with essential hypertension, which is predisposing for developing PE [14]. Free heme is primarily scavenged by hemopexin, but this activity is reduced in PE [15]. The haptoglobin-Hb and hemopexin–heme complexes are cleared from the circulation by the two receptor-mediated pathways CD163 and CD91, and subsequently degraded in lysosomes [16].

α₁-microglobulin (A1M), a 26 kDa plasma and tissue protein, has recently been described as a heme- and radical-scavenger with anti-oxidative, cell-protective and repair properties [17], [18], [19], [20]. A1M is mainly synthesized in the liver and distributed via the blood-stream to the extra-vascular compartment in all tissues [21]. Due to its small size, A1M is filtered in the renal glomeruli and partially re-absorbed in the tubuli [21], [22]. Recent reports have shown that A1M is a heme- and radical-scavenger, involved in the defense against oxidative stress induced by free Hb and participating in the degradation of heme [18], [19], [23]. Its synthesis is up-regulated, both in liver and peripheral cells, as a consequence of elevated concentrations of free Hb, heme and ROS [24].

We have hypothesized, that early events, including hypoxia, during development of PE cause over-production and release of free HbF, which induces oxidative stress with damage to the blood-placenta barrier and leakage of free HbF into the maternal circulation. Thus, circulating free HbF may be one of the important factors, linking stage 1 to stage 2, leading to endothelial dysfunction and subsequently the clinical manifestations characterizing PE. The levels of A1M are elevated in maternal plasma, serum, urine and placental tissue from women with PE suggesting that the protein is involved in a defence reaction against the Hb-insult [8]. Hypothesizing that A1M, and other defence systems, are overwhelmed in PE, we propose that the disease may be treated by addition of exogenous A1M.

In this study we used the dual placental perfusion system, which is a well-established model to study the placental function ex vivo [25], in order to systematically decipher the effects of free Hb in an isolated healthy placenta. We have previously shown that ex vivo perfusion of human placenta under control conditions leads to mild oxidative stress with changes resembling those described in vivo in PE, such as increased secretion of pro-inflammatory cytokines and release of syncytiotrophoblast membranes [26], [27], [28]. Physical and morphological parameters were recorded and related to the global gene expression and electron microscopy (EM) data. Furthermore, the protective and potentially therapeutic effects of A1M were evaluated.