4. Discussion
PE is a multisystem disorder that remains a major cause of maternal and fetal morbidity and mortality. Yet, no preventive measures are known to preclude the occurrence of the disease. Free radical damage and several metabolic derangement have been implicated in the pathophysiology of this condition (Rodrigo et al., 2005). This study aimed to evaluate the role of oxidative stress and markers of tissue damage as indicators risk and of pathogenesis of preeclampsia in pregnant Sudanese women.
In the present study the ages of the two study groups were not statistically different, while the body mass index was significantly higher in preeclamptic women than control group.
It has been established that hypertension during preeclampsia is associated with increased maternal vascular sensitivity to angiotensin II and agonistic autoantibodies to the angiotensin II type I receptor via activation of endothelin-1 and oxidative stress (Brewer et al., 2013). Several approaches to improve control of blood pressure were highlighted: inhibition of ANG II activity, by either blocking angiotensin II type 1 receptors or angiotensin-converting enzyme, or by preventing oxidative stress by administration of antioxidants (Palm and Nordquist, 2011). Without a proper intervention, microvascular endothelial cell injury plays a central role in the pathogenesis of preeclampsia. On that account, end organ damage is mostly directed towards organ systems highly dependent on the microvasculature for normal function including the kidney (Turner et al., 2015).
The excessive cellular activity associated with the process of placental ischemia leads to overproduction of uric acid which serves as a marker of the disease, with abnormal levels seen much earlier than the detection of proteinuria (Enaruna et al., 2014). This study revealed that the mean serum uric acid and creatinine concentrations were significantly higher in preeclamptic women compared to controls. Uric acid concentrations showed significantly positive association with the lipid parameters TC, LDL, TG, and the selected markers of complications LDH, AST, CK, and creatinine. Previous studies, indicated the association of uric acid level in preeclampsia, and its insinuative function as a marker of oxidative stress providing index for early diagnosis and clinical severity of the PE (Enaruna et al., 2014, Magna and Sitikantha, 2012). Other reports also showed that, serum uric acid and creatinine concentrations seem to be useful parameters to predict maternal complications and the management of women with PE (Razia et al., 2013, Suchanda et al., 2011, Sangeeta et al., 2013). In normal pregnancy, uric acid concentration initially falls 25-35% due to elevation in renal clearance secondary to increased glomerular filtration rate (GFR) or reduced proximal tubular re-absorption due to changes in its production rate. Later in pregnancy the serum uric acid level increases possibly due to raised fetal production, altered renal function and increased activity of xanthine oxidase (Corominas et al., 2014, Bainbridge and Roberts, 2008, Powers et al., 2006).
Ischemic placenta in preeclampstic women subsequently becomes hypoxic leading to its tissue breakdown which provides an additional source of purines for generation of uric acid by xanthine oxidase (Bargale et al., 2011, Anjum Sayyed, 2013).
Furthermore, uric acid can promote endothelial inflammation, damage and dysfunction, therefore PE, which is characterized by widespread endothelial dysfunction might be promulgated by uric acid (Martin and Brown, 2010). Uric acid decreases nitric oxide production by the endothelial cells due to its endothelial dysfunction action (Coutinho et al., 2007). In addition, it stimulates human monocytes to produce the pro-inflammatory cytokines IL-1? IL-6 and TNF-?, which are also elevated in the circulation of experimentally induced hyperuricemic animals as well as preeclamptic women. In preeclamptic women the increased concentration of circulating TNF-? was positively related with circulating uric acid concentrations (Bainbridge and Roberts, 2008). Acute rise in uric acid concentrations in animal models induces endothelial progenitor cell (EPC) mobilization. Uric acid was predicated to act as a signal for endothelial damage promoting repair of damaged vessels through EPC mobilization(Patschan et al., 2007).
The increased systemic oxidative stress can decrease nitric oxide bioavailability and cause pregnancy complications. It has been postulated that nitric oxide is important for facilitating the renal hemodynamic changes that occur during a normal pregnancy and blood pressure regulation (Hung et al., 2010).
Because during gestation there is a physiological increase in the glomerular filtration rate (GFR) and reduction of serum creatinine, even mild elevations of the latter can indicate renal function insufficiency. Severe PE can lead to deterioration of kidney function, chiefly in women with previous chronic kidney disease (CKD) and GFR below 40 mL/min/1.73m2 (2.5 mg/dl will experience preterm delivery, and >40% develop preeclampsia. Women who initiated pregnancy with a serum creatinine>2.0 mg/dl had a high (33%) likelihood of an accelerated decline in renal function during or immediately after pregnancy (Maynard and Thadhani, 2009). Hence, it is reasonable to consider the concentrations of uric acid and creatinine in women with PE.
PE can affect every maternal organ, predominantly the vascular, renal, hepatic, cerebral and coagulation systems. LDH levels were significantly elevated in women with PE and eclampsia and significantly correlated with high blood pressure as well as poor maternal and perinatal outcome. Extracellular activity of LDH increases under the condition of oxidative stress, since the cell integrity can be disrupted during the lipid peroxidation process (Jaiswar et al., 2011). Higher serum LDH levels were associated with increased incidence of maternal complications like placental abruption, renal failure, HELLP syndrome, cerebrovascular accidents; preeclamptic women with higher LDH concentrations >800 IU/l were more likely to have complications and neonatal death (Jaiswar et al., 2011). In this study 35 out of 111 patients (31.53%) had moderately to highly elevated LDH levels (600–800 IU/l).
Liver may undergo periportal hemorrhagic necrosis giving rise to elevated enzyme levels (Sibai, 2004). In contrast to normal pregnancy which is generally associated with normal AST, in PE AST concentrations was found to be elevated (Delic and Stefanovic, 2010). In accordance, the present study showed that the serum level of AST was significantly higher in preeclamptic women than control group. Similar findings were reported and revealed that the serum LDH and AST concentrations were significantly higher in preeclamptic patients compared to normal pregnant women and stated that LDH and AST may be increased due to liver damage (Rubina and Tabassum, 2008).
Creatine kinase (CK) is associated with high blood pressure, and it is also a better indicator of heart or muscle damage (Johnsen et al., 2014). CK is a main predictor of blood pressure, and this is thought to largely depend on high resistance artery contractility (Taherzadeh et al., 2015). LDH and CK were found to be elevated due to cellular damage in preeclamptic women, and have a significant association with various maternal complications specially cardiovascular and fetal outcomes (Purnima and Sonal, 2013, Jaiswar et al., 2011).
In this study, risk estimates considering complications parameters LDH, CK, uric acid and creatinine, preeclamptic women have had 2, 2.7, 5.8, and 1.6 times risk of developing tissue damage respectively. In the regression model, CK and LDH elucidated as important predictors for increase of the blood pressure.
PE is characterized by disturbed extravillous trophoblast migration toward uterine spiral arteries leading to increased uteroplacental vascular resistance and by vascular dysfunction resulting in reduced systemic vasodilatory properties. Its pathogenesis is mediated by an altered bioavailability of NO and tissue damage caused by increased levels of ROS (Matsubara et al., 2015). Vascular function modulated by the interference of ROS and NO. Increased ROS production seems to suppress the expression and function of endothelial NO synthase (Boger et al., 2010).
NO exhibits multiple and complex biological functions and many of its effects can be mostly attributed to its strong oxidant capacity. Thus, NO is an important mediator of immune homeostasis and host defense, and changes in its generation or action can contribute to pathologic states (Wander and Jean, 2010).
In this study, maternal serum NO levels were significantly low in the preeclamptic women compared to controls. NO was negatively and significantly correlated with SBP, DBP, TC, LDL, TG, LDH, AST, and CK, While, HDL and TAC showed positive and significant correlation. Several reports ascertain the decreased levels of NO in PE, and its negative correlation with body weight, systolic blood pressure and diastolic blood pressure (Ehsanipoor et al., 2013, Sharma et al., 2011, Saha et al., 2013, Tayal et al., 2014). In contrast, Norris et al. (1999) reported that the production of NO was significantly higher in the uteroplacental, feto-placental, and peripheral circulation in PE compared to normotensive pregnancies. They attributed the marked increase in NO levels to compensatory mechanism to the pathological effect of PE (Norris et al., 1999).
In normal pregnancy, NO derived from endothelial nitric oxide synthase (eNOS) contributes to the maintenance of vascular tone to increase uterine blood flow (Moncada and Higgs, 2006). An up-regulation of eNOS, resulting in increased NO production has been shown to contribute to increases in uteroplacental blood flow via changes in vascular tone (Sandrim et al., 2008). In addition, there is evidence that genetic eNOS polymorphisms may affect the susceptibility to hypertensive disorders of pregnancy (Sandrim et al., 2010). A study on 3 polymorphisms of the eNOS gene and the plasma nitric oxide concentrations was conducted in a population of Chinese origin from mainland China. Two variants 298Asp allele and eNOS 4a were strongly associated with higher plasma NO concentrations in pregnant women and suggested to modulate PE susceptibility (Chen et al., 2014).Vascular tone is altered by the interference of ROS and NO, increased ROS production seems to suppress the expression of eNOS and hence reduced production of NO (Farrow et al., 2008). In fact, several studies have shown that impaired vascular relaxation in PE has been attributed to reduced bioavailability of NO produced via endothelial NOS (Eleuterio et al., 2013). A reduction in the bioavailability of NO and the imbalance between ROS and antioxidant activity seem to play a critical role in endothelial dysfunction contributing to raised in blood pressure and hence pathogenesis of PE (Sankaralingam et al., 2010, Alpoim et al., 2013). NO is also a potent inhibitor of platelet aggregation and activation by both cGMP-dependent and -independent mechanisms (Crane et al., 2005). Drugs that target various components of the nitric oxide soluble guanylylcyclase pathway can help to increase NO bioavailability, and the delivery of exogenous NO is an attractive therapeutic option (Johal et al., 2014).
Total antioxidant capacity (TAC) parameter summarizes the overall activity of antioxidants and antioxidant enzymes (Prior et al., 2005). Evaluating oxidative stress by measuring TAC can lead to a better understanding of free radical damage in oxidative stress related diseases like PE which would be useful to identify the patients with increased risk of progression of the disease and also for monitoring and optimization of antioxidant therapy (Suresh and Annam, 2013). In our study, the serum level of TAC was statistically low in the preeclamptic patients compared to controls. TAC was negatively correlated with SBP, DBP, BMI, TC, LDL and LDH, while it was positively correlated with HDL and NO. Risk estimate considering antioxidants revealed that pregnant women that have low level of NO and TAC had 7.6 and 2.6 times higher risk to develop oxidative stress which in turn leads to development of PE. Study findings are in agreement with the study of Hasan and Dina (2014) who reported that the serum level of TAC was significantly low in preeclamptic cases signifying that the decrease in TAC leads to an imbalance between prooxidants and antioxidants in those women that go on to develop PE(Hasan and Dina, 2014). Oxidative stress reflects an imbalance between the formation of oxidative substances and the innate antioxidants that make up the endogenous defense system (Buonocore et al., 2010). During normal pregnancy there is a slight increase in oxidative stress, even in the presence of antioxidant systems (Raijmakers et al., 2001). In PE, the Abnormal vascular development of the blood vessels in the placenta leads to reduced placental perfusion and induce hypoxia which is by itself a potent stimulus for ROS formation (Rodrigo et al., 2005).
A genome-wide transcriptomic view identified genes involved in lipid metabolism, angiogenesis, hormone activity, and inflammation to be significantly altered in placenta from obese women. These studies provide evidence for increased lipids and decreased TAC in placenta from obese women, and pinpointed key signaling pathways (increased JNK/FoxO4 signaling) and downstream mediators (HIF-1? and VEGF-A) that provide a link between maternal-obesity, placental inflammation/oxidative stress, and altered angiogenic factors. Obesity provokes cellular stress, which may in turn adversely affect placental development and function(Saben et al., 2014). Antioxidants may be utilized to greater extent to counteract and scavenging free radicals, resulting in the reduction of their levels (Manisha et al., 2015).
The limitations of this study include the relatively small sample size. There were no follow up criteria set. Even with this limitation, the findings of this study serve as ground for further studies to better understanding the associations of total antioxidant capacity and nitric oxide levels with preeclampsia.