Heat-Shock Protein-70, -27 and Peroxiredoxin-2 Show Chaperone Function in Sickle Red Cells: Evidences from Transgenic Sickle Cell Mouse Model.

Sickle cell disease (SCD) is a worldwide-distributed hereditary red cell (RBCs) disorder characterized by the pathologic hemoglobin-S (HbS). The acute clinical manifestations of SCD are strictly related to HbS polymerization, to generation of dense RBCs, to their interaction with the abnormal activated vascular endothelial cells and to amplified inflammatory response. To identify new biomarkers associated with acute SCD events, we studied the RBCs membrane proteome from human SCD erythrocytes (n=20) fractioned according to density compared to normal RBCs (n=20), we divided RBCs into 2 fractions: fraction 1 (F1) corresponding to the least dense RBCs (density < 1.074) and fraction 2 (F2) corresponding to the densest RBCs (density > 1.095). None of the patients were under hydroxyurea treatment and they did not receive transfusion in the sixmonths before the study. Bi-dimensional electrophoresis (2-DE) followed by MS-analysis of RBCs F1 and F2 from both control and SCD subjects was carried out and 65 proteins differently expressed were identified. We focused on molecular chaperones. In SCD RBCs, we found that the amount of HSP27, HSP70 and Prx-2 recruited to the membrane was higher than in controls, Prx-2 was present as monomers and dimmers that were more abundant in SCD RBCs than in normal controls. Then, we exposed F1 and F2 fractions from normal and SCD patients to in vitro cellular stress (deoxygenation). In sickle RBCs, deoxygenation induced

1. increase membrane recruitment of HSP27 in F1,

2. modulation of HSP70 membrane binding in F2;

3. reduction of Prx-2 monomers with increase Prx-2 dimerization in F1, but slightly changes in Prx-2 monomers in F2.

These data suggest that in SCD RBCs HSP-27, -70 and Prx-2 are recruited to the membrane in response to cellular stress, acting as molecular chaperones to assist proteins in regaining their functional conformation. In order to evaluate their role in vivo and because increase serum levels of HSP70 has been recently reported in few SCD patients, we used transgenic sickle cell SAD mice as a model for SCD and exposed to hypoxia (8%-O2) respectively for 2–12–48 hours, mimicking acute human SCD vaso-occlusive-crisis (VOCs). We studies SAD mice and wild-type mice (C57/B6, WT) divided into groups of 6 animals each. We evaluated HSP70, HSP27 and Prx-2 expression on RBCs membrane, hematological parameters, RBCs density, and cation content. We found that HSP70 and HSP27 bound to RBCs membrane respectively after 12 hours and 48 hours (hrs) of hypoxia, while Prx-2 membrane binding was modulated during hypoxia. At the different time schedule, we observed a marked reduction in red cell K+ at 12 and 48 hrs hypoxia, associated with significant increase of the dense RBCs at 48 hrs hypoxia. Our data indicate that in SCD, HSP70, HSP27 and Prx-2 membrane recruitment is modulated in vivo during acute VOCs, supporting their novel role as RBCs membrane protein protectors and as new markers of severity of RBCs membrane damage during acute sickle VOCs.