Uric acid as a pathogenic factor in preeclampsia

Risk factors and mediators of the vascular dysfunction associated with hypertension in pregnancy

Normal pregnancy is associated with significant hemodynamic changes and vasodilation in the uterine and systemic circulation in order to meet the metabolic demands of the mother and developing fetus. Hypertension in pregnancy (HTN-Preg) and preeclampsia (PE) are major complications and life-threatening conditions to both the mother and fetus. PE is precipitated by various genetic, dietary and environmental factors. Although the initiating events of PE are unclear, inadequate invasion of cytotrophoblasts into the uterine artery is thought to reduce uteroplacental perfusion pressure and lead to placental ischemia/hypoxia. Placental hypoxia induces the release of biologically active factors such as growth factor inhibitors, anti-angiogenic proteins, inflammatory cytokines, reactive oxygen species, hypoxia-inducible factors, and antibodies to vascular angiotensin II receptor. These bioactive factors affect the production/activity of various vascular mediators in the endothelium, smooth muscle and extracellular matrix, leading to severe vasoconstriction and HTN. As an endothelial cell disorder, PE is associated with decreased vasodilator mediators such as nitric oxide, prostacyclin and hyperpolarizing factor and increased vasoconstrictor mediators such as endothelin, angiotensin II and thromboxane A(2). PE also involves enhanced mechanisms of vascular smooth muscle contraction including intracellular free Ca(2+) concentration ([Ca(2+)](i)), and [Ca(2+)](i) sensitization pathways such as protein kinase C, Rho-kinase and mitogen-activated protein kinase. Changes in extracellular matrix composition and matrix metalloproteases activity also promote vascular remodeling and further vasoconstriction in the uterine and systemic circulation. Characterization of the predisposing risk factors, the biologically active factors, and the vascular mediators associated with PE holds the promise for early detection, and should help design specific genetic and pharmacological tools for the management of the vascular dysfunction associated with HTN-Preg.

Factors associated with persistent hypertension after puerperium among women with pre-eclampsia/eclampsia in Mulago hospital, Uganda

BACKGROUND

Women with severe pre-eclampsia/eclampsia are at risk of developing chronic hypertension in future. Chronic hypertension may manifest initially as persistent hypertension at the end of the puerperium. The objective was to determine the incidence and maternal biochemical, hematological and socio-demographic risk factors for persistent hypertension in patients with pre-eclampsia/eclampsia.

METHODS

This was a prospective cohort study conducted from November 2008 to May 2009 at Mulago hospital labor ward and postnatal clinic. Participants were 200 women managed for pre-eclampsia/eclampsia and followed up to the end of the puerperium. Data was collected through using pre-coded interviewer-administered questionnaires, checking medical records and laboratory investigations. STATA (release 9) software was used for data analysis. At bivariate analysis, the relative risk of persistent hypertension was estimated at the 95% confidence level. Using multivariate logistic regression analysis, factors that were independently associated with persistent hypertension were evaluated.

RESULTS

Fifty four (27.7%) out of the total 195 women had persistent hypertension after puerperium. Serum creatinine and the age of the patient were the only factors associated with persistence of hypertension after puerperium.

CONCLUSION

Nearly every one in four mothers with pre-eclampsia/eclampsia are at risk of persistent hypertension after the puerperium. Serum creatinine, serum uric acid and participants' age were the only factors independently associated with persistence of hypertension after the puerperium.

Uric acid concentrations are associated with insulin resistance and birthweight in normotensive pregnant women

OBJECTIVE

We sought to investigate whether uric acid concentrations are increased in pregnant women with insulin resistance and to correlate both with fetal growth.

STUDY DESIGN

Uric acid, glucose, and insulin were measured in plasma at 20.4 (+/-2.0) weeks' gestation in 263 women. The association between uric acid and insulin resistance, as estimated using the homeostasis model assessment (HOMA), was analyzed and related to birthweights.

RESULTS

In 212 (80.6%) women who remained normotensive throughout pregnancy, HOMA increased 1.23 U per 1-mg/dL increase in uric acid (95% confidence interval, 1.07-1.42; P=.003). Infants born to normotensive women in the upper quartile of uric acid and lowest HOMA quartile weighed 435.6 g less than infants of women with highest uric acid and HOMA quartiles (P<.005).

CONCLUSION

Increasing uric acid concentrations were associated with insulin resistance in midpregnancy. Hyperuricemia was associated with lower birthweight in normotensive women, and this effect was attenuated by insulin resistance.

Uric acid attenuates trophoblast invasion and integration into endothelial cell monolayers

Hyperuricemia develops as early as 10 wk of gestation in women who later develop preeclampsia. At this time the invasive trophoblast cells are actively remodeling the uterine spiral arterioles, integrating into and finally replacing the vascular endothelial lining. In the nonpregnant population uric acid has several pathogenic effects on vascular endothelium. We therefore sought to examine the effects of uric acid (0-7 mg/dl) on trophoblast cell invasion through an extracellular matrix using an in vitro Matrigel invasion assay. We also assessed trophoblast integration into a uterine microvascular endothelial cell monolayer in a trophoblast-endothelial cell coculture model. Additionally, we addressed the importance of redox signaling and trophoblast-induced endothelial cell apoptosis. Uric acid elicited a concentration-dependent attenuation of trophoblast invasion and integration into a uterine microvascular endothelial cell monolayer. The attenuated trophoblast integration appeared to be the result of reduced trophoblast-induced endothelial cell apoptosis, likely through the intracellular antioxidant actions of uric acid. In a test of relevance, pooled serum (5% vol/vol) from preeclamptic women attenuated the ability of trophoblast cells to integrate into the endothelial cell monolayers compared with pooled serum from healthy pregnant controls, and this response was partially rescued when endogenous uric acid was previously removed with uricase. Taken together these data support the hypothesis that elevations in circulating uric acid in preeclamptic women contribute to the pathogenesis of the disorder, in part, through attenuation of normal trophoblast invasion and spiral artery vascular remodeling.

Recent advances in arginine metabolism: roles and regulation of the arginases

As arginine can serve as precursor to a wide range of compounds, including nitric oxide, creatine, urea, polyamines, proline, glutamate and agmatine, there is considerable interest in elucidating mechanisms underlying regulation of its metabolism. It is now becoming apparent that the two isoforms of arginase in mammals play key roles in regulation of most aspects of arginine metabolism in health and disease. In particular, work over the past several years has focused on the roles and regulation of the arginases in vascular disease, pulmonary disease, infectious disease, immune cell function and cancer. As most of these topics have been considered in recent review articles, this review will focus more closely on results of recent studies on expression of the arginases in endothelial and vascular smooth muscle cells, post-translational modulation of arginase activity and applications of arginase inhibitors in vivo.

Increased xanthine oxidase in the skin of preeclamptic women

Xanthine oxioreductase is the holoenzyme responsible for terminal purine catabolism. Under conditions of metabolic stress or heightened proinflammatory cytokine production, this enzyme is preferentially in its oxidized form, xanthine oxidase, with catalytic action that generates uric acid and the free radical superoxide. As preeclampsia is characterized by heightened inflammation, oxidative stress, and hyperuricemia, it has been proposed that xanthine oxidase plays a pivotal role in this hypertensive disorder of pregnancy. We sought to determine whether xanthine oxidase protein content was higher in maternal tissue of preeclamptic mothers, compared to healthy pregnant controls, using immunohistochemical analysis of skin biopsies. We further compared xanthine oxidase immunoreactivity in skin biopsies from preeclamptic women and patients with several inflammatory conditions. In preeclamptic women, intense xanthine oxidase immunoreactivity was present within the epidermis. By contrast, only very faint xanthine oxidase staining was observed in skin biopsies from healthy pregnant controls. Further, a role for inflammation in the increase of xanthine oxidase was suggested by similar findings of heightened xanthine oxidase immunoreactivity in the skin biopsies from nonpregnant individuals diagnosed with conditions of systemic inflammation. The finding of increased xanthine oxidase in maternal tissue, most likely as the result of heightened maternal inflammation, suggests maternal xanthine oxidase as a source of free radical and uric acid generation in preeclampsia.

Hypothesis: could excessive fructose intake and uric acid cause type 2 diabetes?

We propose that excessive fructose intake (>50 g/d) may be one of the underlying etiologies of metabolic syndrome and type 2 diabetes. The primary sources of fructose are sugar (sucrose) and high fructose corn syrup. First, fructose intake correlates closely with the rate of diabetes worldwide. Second, unlike other sugars, the ingestion of excessive fructose induces features of metabolic syndrome in both laboratory animals and humans. Third, fructose appears to mediate the metabolic syndrome in part by raising uric acid, and there are now extensive experimental and clinical data supporting uric acid in the pathogenesis of metabolic syndrome. Fourth, environmental and genetic considerations provide a potential explanation of why certain groups might be more susceptible to developing diabetes. Finally, we discuss the counterarguments associated with the hypothesis and a potential explanation for these findings. If diabetes might result from excessive intake of fructose, then simple public health measures could have a major impact on improving the overall health of our populace.

Uric acid inhibits placental system A amino acid uptake

Hyperuricemia, a common clinical characteristic of preeclamptic pregnancies, has historically been considered a marker of reduced renal function in preeclamptic women. More recently it has been suggested that uric acid may directly contribute to pathological cell signaling events involved in disease progression as well as maternal and fetal pregnancy outcomes including fetal growth restriction. We hypothesize that the increased frequency of restricted fetal growth seen in relation to increasing uric acid concentrations in preeclamptic women is in part the result of uric acid-induced reductions in amino acid transport across the placenta. The objective of the current study was to examine the effects of uric acid on human placental System A amino acid transport using a primary placental villous explant model. Further, we examined the necessity of uric acid uptake and the role of redox signaling as a potential mechanism through which uric acid may attenuate System A activity. Placental uptake of a radiolabeled amino acid analogue, specific to the System A transporter, was reduced in a concentration-dependent fashion with increasing uric acid (0-7 mg/dL), corresponding to uric acid concentrations measured in healthy pregnant and preeclamptic women in the third trimester. Uric acid-induced reduction in System A activity was partially reversed by NADPH oxidase inhibition and completely eliminated by antioxidant treatment. This study demonstrates inhibition of placental System A amino acid transport with uric acid treatment, as a result of uric acid-induced stimulation of intracellular redox signaling cascades. These findings may be relevant to the increased frequency of fetal growth restriction observed in hyperuricemic preeclampsia. Additionally the results of this study, indicating a detrimental effect of hyperuricemia on amino acid transport in the placenta, at concentrations present in women with preeclampsia, also suggest a role for uric acid in the pathophysiology of preeclampsia.