Macrophage Activation in Sickle Cell Disease: The Role of Sphingolipid Metabolism in the Disease State
Lane, Alicia Renee
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Sickle cell disease (SCD) is a disorder in which defective hemoglobin causes sickling of red blood cells, inducing painful vaso-occlusive crises when blood flow is blocked at sites of red blood cell (RBC) clotting that can ultimately result in organ failure or death. This work demonstrates that sphingolipid metabolism is dysregulated in SCD and that this pathway can be targeted pharmacologically to prevent vaso-occlusion. We suggest a pathway in which the sickling of RBCs in SCD activates acid sphingomyelinase, altering the distribution and concentration of sphingolipids in the RBC membrane and resulting in the production of sphingolipid-rich microparticles that are secreted and can interact with cells in circulation. Sphingosine-1-phosphate (S1P) is believed to be a key modulator of SCD because it is stored at high concentrations in RBCs. Sphingolipid metabolism was confirmed to be dysregulated in SCD; most notably, S1P was significantly elevated in RBCs, and plasma, and microparticles, and the activity of acid sphingomyelinase and concentration of its byproduct, microparticles, were significantly elevated in SCD RBCs. Treatment of monocytes with S1P and SCD RBCs increased their adhesion over four-fold to endothelial cells, indicating that altered sphingolipid distribution in RBCs may contribute to vaso-occlusion through increasing myeloid cell adhesion. A cytokine profile of macrophages treated with SCD microparticles suggest that microparticles play a role in this process by increasing the secretion of inflammatory cytokines associated with SCD crises, including MIP-1α, IL-6, and TNF-α. Pilot in vitro studies in RBCs and in vivo studies in mice implicate that drugs targeting the sphingolipid metabolic pathway may be more effective treatment options than blood transfusions in managing SCD and preventing vaso-occlusive crises.