The Mechanism of Proinflammatory HDL Generation in Sickle Cell Disease Is Linked to Cell-Free Hemoglobin via Haptoglobin

In sickle cell disease (SCD), the inflammatory properties of high-density lipoprotein (HDL) can be changed by cell-free hemoglobin (Hb), which is released into the blood during hemolysis. Hb in the plasma of SCD patients or mice can bind with HDL specifically inducing an inflammatory reaction. In our study, we found increased amounts of inflammatory factor proteins in the chronic oxidative state of SCD with higher levels of Hb, haptoglobin (Hp) and hemopexin (Hx) in the apolipoprotein A-I (ApoA-1) particles of HDL and the role of HDL is changed from being anti-inflammatory to proinflammatory. Our results also suggest Hp and Hx, the scavengers of Hb in HDL, are positively associated with inflammatory levels in SCD patients. HDL retained its inflammatory inhibition role in Hp-/- mice, with less Hb accumulation. Hx may further prevent inflammatory reaction because its level will be even higher when lack of Hx. We therefore demonstrated that Hp is indispensable during the process whereby Hb associates with HDL and plays a clear proinflammatory role. Therefore, it is essential to break the binding between Hb and Hp for treatment. The dissociation of Hb/Hp/Hx complexes may also play an important role in the study of other inflammatory angiogenesis-related diseases.