Preeclampsia (PE) is characterised by high levels and activity of the transcription factor Nuclear Factor kappa B (NFĸB) in the maternal blood and placental cells. This factor is responsible for the regulation of over 400 genes known to influence processes related to inflammation, apoptosis and angiogenesis, and cellular responses to oxidative stress and hypoxia. Although high NFĸB activity induces hypoxia and inflammation, which are beneficial for the process of implantation, NFĸB level should be reduced in the later stages of physiological pregnancy to favour maternal immunosuppression and maintain gestation. It is believed that the downregulation of NFĸB activity by pharmacotherapy might be a promising way to treat preeclampsia.
1. Introduction
Preeclampsia (PE) is responsible for 5–10% of pregnancy complications and is recognized as one of the most common reasons for maternal and foetal death. While it generally appears after week 20 of pregnancy, cases with very early (i.e., before week 20) and very late symptoms (i.e., in the first 6 weeks after birth) have been reported
[1][2]. The guidelines presented by international obstetrics and gynaecology societies characterise preeclampsia as the sudden occurrence of hypertension (i.e., >140 mmHg systolic or >90 mmHg diastolic) in previously normotensive women, accompanied by proteinuria (i.e., >300 mg for 24 h or at least 2+ on a dipstick). Preeclampsia can also be recognised by the presence of hypertension complicated by at least one of the following symptoms: serum creatinine level >1 mg/dL, elevated transaminase levels, thrombocytopenia, haemolysis, neurological disorders, or uteroplacental dysfunction (i.e., foetal growth restriction)
[3][4].
A considerable body of evidence indicates that blood from patients with preeclampsia demonstrates a high level of inflammatory factors (such as: TNF-α, IFN-γ, IL6 or IL1) and reactive oxygen species (ROS), and that preeclamptic placentas display features indicative of chronic hypoxia throughout the gestation period
[5][6][7]. Inflammation, oxidative stress, ineffective angiogenesis, and hypoxia are regulated at the cellular level by numerous pathways, most of which are under the control of nuclear factor kappa B (NFĸB)
[8]. NFĸB level and activity are both upregulated in maternal and placental cells, and as this upregulation is strongly linked with the pathomechanism of preeclampsia, it is possible that preeclampsia could be treated by drugs targeting the mechanism of NFĸB activation. Interestingly a number of agents already adopted for the prophylaxis and treatment of preeclampsia are believed to modulate NFĸB activity.
2. NFĸB and Its Relationship with Preeclampsia Development
To prepare for pregnancy, the endometrium demonstrates an extensive increase in NFĸB expression to prepare maternal tissues for the opening of the implantation window in the case of fertilization
[9][10][11], and this NFĸB activation continues in the uterus during the implantation period
[8][9][10]. NFĸB activation affects the regulation of inflammatory factors, such as TNFα, IL6, IL8, and IFN-γ, secreted by the endometrial cells, as well as by the natural killer cells (NK), macrophages, dendritic cells, or lymphocytes of the maternal immune system that are recruited to the site of implantation
[7].
It is possible that the intensity of inflammatory reaction, related to the strength of NFĸB activation, might play a significant role in the success of implantation. The rise in the levels of proinflammatory cytokines may disturb the delicate inflammatory reactions at the feto–maternal interface during implantation, leading to its failure or pregnancy loss
[12]. Moreover, such abnormal maternal inflammation has been also found to impair the remodeling of uterine spiral arteries and alter uteroplacental perfusion, leading to the development of features of preeclampsia in a rat model
[13]. Abnormal, shallow placentation forces trophoblastic cells to live under hypoxic conditions, which generates oxidative stress and intensifies the inflammatory reaction. These processes are strongly influenced by the presence of NFĸB, whose level and activity is downregulated over the course of a non-complicated gestation. In preeclampsia, NFĸB level and activity are significantly elevated in the maternal blood and have been found to be up to 10-fold higher in placentas than controls
[14][15].
Interestingly, although preeclampsia is characterised by elevated NFĸB activity, its mechanism of activation remains poorly explored. It is postulated that oxidative stress favours the degradation of NFĸB inhibitors in lymphocytes and aortic endothelial cells by proteosomes
[16][17]; however, no such findings have been noted in human umbilical vein endothelial cells or preeclamptic placental samples
[18][19].
Unstimulated cells demonstrate only basal levels of NFĸB in the cytoplasm. This level is maintained by various inhibitors, of which IĸBα (NF-Kappa B Inhibitor Alpha) and IĸBβ (NF-Kappa B Inhibitor Beta) are the most common. These bind to NFĸB and prevent its activation i.e., its phosphorylation and translocation into nucleus. However, in environments rich in reactive oxygen species or cytokines, NFĸB activation takes place, driven by various NFĸB activators. Among these, IKKα (Inhibitor of Nuclear Factor Kappa B Kinase Subunit Alpha), IKKβ (Inhibitor of Nuclear Factor Kappa B Kinase Subunit Beta), IKKγ (Inhibitor of Nuclear Factor Kappa B Kinase Subunit Gamma), and CK2 (Casein kinase 2) are implicated in the three most widely-studied pathways: canonical, non-canonical, and atypical
[20].
Interestingly, in preeclamptic placentas, the canonical, non-canonical, and atypical activation pathways do not seem to play a significant role in the process of NFĸB activation. In the canonical pathway, various NFĸB activators (i.e., IKKα, IKKβ and IKKγ) are downregulated, whereas the inhibitors (i.e., IĸBα, IĸBβ) are upregulated
[18][21]. Similarly, NFĸB activators participating in the non-canonical (i.e., IKKα) and atypical pathways (i.e., CK2) are also downregulated. This suggests that preeclamptic placentas employ specific NFĸB activation mechanics: their activity is independent of IKKα, IKKβ, IKKγ, and CK2, and avoids the cytoplasmic and proteosomal degradation by NFĸB inhibitors such as IĸBα or IĸBβ. Some studies suggest that this mechanism may be dependent on the activity of a p53/RSK1 (Tumour Protein p53/Ribosomal Protein S6 Kinase A1) complex
[18][20].
Independently of the molecular mechanism of NFĸB activation, this factor is strongly linked with oxidative stress and inflammation. Under such conditions, placental cells secrete a range of proteins controlling vascular function, such as arginase II, endothelin-1 or soluble fms-like tyrosine kinase 1 (sFlt-1), and undergo apoptosis, shedding apoptotic debris into the maternal circulation. All of these factors contribute to maternal endothelial dysfunction, which the main cause of clinical symptoms in patients with preeclampsia
[22][23][24][25][26][27].
3. Targeting NFĸB by Aspirin
Early supplementation with low doses of aspirin, i.e., acetylsalicylic acid (ASA), is effective in preventing preeclampsia. However, although aspirin treatment improves the outcome of pregnancy, reducing the risk of preterm preeclampsia by approximately 30–62%, conflicting data exists regarding the optimal dose and time of initiation of ASA intake
[28][29][30]. In numerous countries, the guidelines prepared by cardiology, gynaecology and obstetrics societies recommend the initiation of aspirin treatment before week 20 (optimum before week 16) of gestation among high- and moderate-risk women (
Table 1).
Table 1. Recommendations given by selected organisations regarding ASA supplementation as preeclampsia prophylaxis among moderate and high-risk groups *.
Selected Word Organisation (Year) |
ASA Dose |
Initiation ASA Supplement |
Bibliograph |
World Health Organisation (WHO) (2011) |
75 mg/day |
<week 20 |
[31] |
German Society of Gynaecology and Obstetrics (DGGG) (2015) |
100 mg/day |
no data |
[32] |
French Society of Cardiology (FESC)/French Society of Hypertension (2016) |
75–160 mg/day |
<week 20 |
[33] |
The American College of Obstetricians and Gynaecologists (ACOG) (2018) |
81 mg/day |
week 12–28 Optimum <16 |
[34] |
European Society of cardiology (ESC)/European Society of Hypertension (ESH) (2018) |
100–150 mg/day |
week 12 |
[35] |
New Zealand Committee of the Royal Australian & New Zealand College of Obstetricians & Gynaecologists (RANZCOG) New Zealand College of Midwives (NZCOM) (2018) |
≥75 mg/day optimum 100 mg/day |
week 12 |
[36] |
The International Society for the Study of Hypertension in Pregnancy (ISSHP) (2018) |
75–162 mg/day |
<week 20 Optimum <16 |
[37] |
International Federation of Gynaecology and Obstetrics (FIGO) (2019) |
150 mg/day (at night) |
week 11–14 |
[38] |
National Institute for Health and Care Excellence (NICE) (2019) |
75–150 mg/day ** |
week 12 |
[39] |
Polish Society of Hypertension (PTNT), Polish Cardiac Society (PTK) and Polish Society of Gynaecologists and Obstetricians (PTGiP) (2019) |
100–150 mg/day |
<week 16 |
[40] |
International Society of Hypertension (ISH) (2020) |
75–162 mg/day |
week 12 |
[41] |
Society for Maternal-Foetal Medicine (SMFM) (2020) |
81 mg/day |
week 12–28 Optimum <16 |
[42] |