Nitric Oxide and Vascular Endothelial Growth Factor during Pregnancy in Sickle Cell Disease

BACKGROUND: Vascular Endothelial Growth Factor (VEGF) is a multifunctional cytokine that may be clinically relevant in sickle cell disease (SCD). In additional to its role as a potent activator of angiogenesis, VEGF is also a potent inducer of Nitric Oxide (NO) release. Patients with SCD have impaired NO metabolism and decreased bioavailability of NO due to chronic hemolysis which may contribute to the vasculopathy that is present in this disorder. High levels of plasma arginase and the resultant decreased arginine/ornithine ratios have also been proposed to contribute to low NO levels in SCD patients. We have previously reported that pregnant SCD patients have higher circulating levels of the naturally occurring VEGF antagonist, sFLT-1, when compared to pregnant African American controls. High levels of sFLT-1 are associated with poor pregnancy outcomes including preeclampsia and delivery of small for gestational age (SGA) infants.

OBJECTIVE: In this study we wanted to assess the levels of NO, free VEGF, sFLT-1 and arginase activity during pregnancy in SCD. We also sought to explore the relationship between NO and sFLT-1 with pregnancy outcome.

METHODS: We performed an analysis of previously collected plasma/serum samples obtained from the Pregnancy, Infection and Nutrition (PIN) study cohort at UNC- Chapel Hill. Subjects were matched on race, gestational age, BMI and smoking status. Levels of free VEGF were quantified by luminex analysis and confirmed by ELISA. NO was quantified via its metabolite NOx using a modified Griess method in de-proteinated serum/plasma. Arginase activity was quantified by ELISA. Pregnancy outcomes were previously recorded by PIN study personnel.

RESULTS: We found that levels of free VEGF and NO were significantly lower in pregnant women with SCD than in matched pregnant controls (VEGF: 5.8 pg/ml vs. 13.56 pg/ml, p = 0.03, NOx: 14.31 μM vs. 26.99 μM, p=0.0009). We also found a significant inverse correlation between NOx levels and sFLT-1 levels (r²=0.6421, p = 0.0094), but no relationship between NOx and arginase activity in plasma. There was also no difference in arginase activity between pregnant SCD subjects and controls. Finally, pregnant SCD subjects that delivered an SGA infant had significantly lower levels of NOx than those women that did not.

CONCLUSIONS: These results suggest that VEGF may play a role in NO release during pregnancy in SCD. In addition, low NO levels may contribute to poor pregnancy outcomes in pregnant SCD women.