Sickle cell disease is a devastating hemolytic disease characterized by endothelial cell dysfunction and chronic vasculopathy. Hemolysis releases hemoglobin in the vasculature, which when oxidized to methemoglobin can release toxic heme that promotes a pro-oxidative, pro-inflammatory and pro-thrombotic environment. Despite advances in limiting hemolysis, sickle cell patients continue to have increased morbidity and mortality. The clinician must still deal with the consequences of hemolysis. Currently, there are no treatments directed at hemoglobin and heme removal and their detoxification. We hypothesize that excess heme is central to the endothelial cell dysfunction and vasculopathy in sickle cell disease. The plasma of sickle cell patients is deficient in haptoglobin and hemopexin, the two primary hemoglobin and heme-scavenging proteins. Once these cytoprotective heme-scavenging proteins are overwhelmed, heme oxygenase-1 and ferritin (especially ferritin heavy chain with its ferroxidase activity) detoxify heme and protect endothelial cells against free heme and redox-active iron. In this proposal, we will test the hypothesis that the endothelial cell dysfunction and vasculopathy in sickle cell disease is largely attributable to excess heme in three specific aims:
Aim # 1. Investigate heme's ability to induce an inflammatory response through toll-like receptor 4 (TLR4) signaling and Weibel-Palade body exocytosis leading to vascular stasis and endothelial cell dysfunction in transgenic sickle mice;
Aim #2. Examine whether removing and detoxifying hemoglobin and heme will inhibit endothelial cell activation and vascular stasis, stimulate cytoprotective responses and lessen organ pathology in transgenic sickle mice;
Aim #3. Determine whether upregulation of Nrf2, a transcriptional regulator of anti-oxidant defenses, will induce proteins that remove and detoxify heme and inhibit heme-mediated vasculopathy in hemolytic transgenic sickle mice. Successful accomplishment of these aims will be the basis of novel therapies that remove and detoxify hemoglobin and heme in sickle cell patients, with the goal of improving endothelial cell function and lessening oxidative stress, inflammation and vaso-occlusion.

Public Health Relevance

Despite remarkable improvements in the lifespan and quality of life of patients with sickle cell disease, devastating organ dysfunction, painful crises and eary death remain all too prevalent. The studies proposed here will examine how free heme derived from the hemolysis of sickle red blood cells affects the blood vessels of sickle cell patients. New therapies are proposed that target and limit the adverse vascular effects of heme.

National Institute of Health (NIH)
Research Project (R01)
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Molecular and Cellular Hematology (MCH)
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Mitchell, Phyllis
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University of Minnesota Twin Cities
Internal Medicine/Medicine
Schools of Medicine
United States
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Vercellotti, Gregory M; Belcher, John D (2014) Not simply misshapen red cells: multimolecular and cellular events in sickle vaso-occlusion. J Clin Invest 124:1462-5
Belcher, John D; Chen, Chunsheng; Nguyen, Julia et al. (2014) Heme triggers TLR4 signaling leading to endothelial cell activation and vaso-occlusion in murine sickle cell disease. Blood 123:377-90