Background: Sickle cell anemia (SCA) is characterized by chronic hemolysis and endothelial dysfunction (ED). Plasma hemoglobin (pHb) and its heme component released from intravascular hemolysis (IH) are among the most important factors contributing to ED. Unfortunately, the importance of IH to the development of an ED and the effects of hydroxyurea therapy on IH and ED in SCA remains unclear.
Aims: We evaluated plasma levels of IH and ED markers among Brazilian SCA patients not receiving hydroxyurea therapy (HbSS), and compared with those of hydroxyurea-treated SCA patients (HbSS_HU) and healthy controls (HbAA).
Methods: A cross-sectional study of 60 SCA consenting patients (32 HbSS and 28 HbSS_HU; 19-42 years) in steady state, who are being followed up at the Blood Center, Pernambuco (HEMOPE) and 32 HbAA controls. The IH markers were serum Lactate Dehydrogenase [LDH] and total heme measured by enzymatic colorimetric tests, and pHb measured by enzyme-linked immunosorbent assay (ELISA). The ED markers were plasma von Willebrand factor (vWF:Ag) and vWF ristocetin cofactor activity (vWF:Rco) levels measured by the latex enhanced immunoassay. The other ED markers were the antigen of vWF-cleaving protease (ADAMTS13:Ag), thrombospondin-1 and endothelin-1 levels measured by ELISA, and ADAMTS13 Activity (ADAMTS13:Act) measured by FRETS-VWF73 method. The study was approved by the Ethics Committee of the State University of Campinas and HEMOPE under protocol No: 1.863.428. Data were analyzed using GraphPad Prism 6.
Results: The pHb and LDH were significantly increased in HbSS than HbSS_HU patients (Figures 1A and 1B). Plasma levels of vWF:Ag, vWF:Rco, serum levels of total heme, thrombospondin-1 and endothelin-1 were significantly increased in HbSS and HbSS_HU patients compared to HbAA controls (Figures 1C and 2A, 2B, 2E, 2F), while serum level of thrombospondin-1 level was elevated in HbSS than HbSS_HU patients (Figure 2E). Similarly, ADAMTS13:Ag levels and ADAMTS13 activity were significantly lower in HbSS and HbSS_HU patients than HbAA controls, while ADAMTS13 activity levels were significantly elevated in HbSS_HU patients compared to HbSS patients (Figure 2D). In HbSS_HU patients, the ADAMTS13:Act was negatively correlated with heme and LDH [r = -0.47, p = 0.013; and r = -0.44, p = 0.023 respectively]. In additional, heme was positively correlated with vWF:Ag and LDH [(r = 0.47, p = 0.017) and (r = 0.56, p = 0.003) respectively]. In HbSS patients, LDH and pHb were positively correlated (r = 0.44, p = 0.014).
Conclusions: Hydroxyurea therapy was associated with a reduced levels of LDH, pHb and thrombospondin-1 levels, and increased levels of ADAMTS13 activity in SCA patients. Increased ADAMTS13 activity levels may be attributed to reduction of pHb and thrombospondin-1 levels because previous invitro studies have shown that thrombospondin-1 or pHb are bound to vWF. Thus, vWF is restrained from ADAMTS13 activity and cleavage, and hyperreactive vWF might accumulate as a consequence of inhibition of ADAMTS13 activity. Increased thrombospondin-1, pHb and hyperreactive vWF levels are proposed to participate in sickle cell adhesion and promote thrombotic complications. Therefore, our results demonstrate an additional clinical benefit for the use of hydroxyurea in these patients.
No relevant conflicts of interest to declare.
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