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Unraveling Novel Roles of Xanthine Oxidase in Sickle Cell Disease

Schmidt, Heidi M (2022) Unraveling Novel Roles of Xanthine Oxidase in Sickle Cell Disease. Doctoral Dissertation, University of Pittsburgh. (Unpublished)

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Hemolysis, the release of hemoglobin and heme from red blood cells, is a defining characteristic of hemolytic diseases such as sickle cell disease (SCD); however, the mechanisms that enable the organism to cope with excessive hemoglobin and heme when canonical mediators are depleted remain poorly understood. Xanthine oxidase (XO) is one enzyme that has been reported to have elevated activity in SCD. XO oxidizes hypoxanthine to xanthine and xanthine to uric acid as part of purine degradation. A byproduct of these reactions is the production of O2•- or H2O2. Increased production of oxidants by elevated XO activity has been thought to cause endothelial damage. We hypothesized that XO could have condition dependent dichotomous functions in SCD where under basal XO function endothelial damage results from increased oxidant production and under heme overload conditions XO regulates a secondary mechanism of heme scavenging and degradation. To test this hypothesis, we treated bone marrow transplanted sickle mice with the XO inhibitor febuxostat for 10 weeks. Treated mice had less hemolysis and improved pulmonary vascular function, identifying XO as a key driver of hemolysis during basal SCD conditions. Next, we studied the role of XO during heme crisis by injecting bone marrow transplanted sickle mice with hemin. Mice lacking hepatic XO had reduced 24-hour survival indicating, for the first time, a protective mechanism for XO. We found that XO is released in response to free heme in a toll like receptor 4-dependent manner to increase circulating levels of XO. Subsequent studies revealed that XO can bind free heme and facilitate degradation to prevent platelet activation and aggregation. Based on these findings, we hypothesize that XO activity is elevated in SCD in anticipation of a heme crisis event. XO activity could create a microenvironment on the endothelial surface designed to specifically protect the endothelium by binding free heme, inducing a heme splitting reaction, and chelating the free iron released with uric acid. However, the protective functions during a crisis could come at a cost over time, as elevated XO activity at baseline could elicit chronic endothelial cell damage and exacerbate hemolysis in hemolytic diseases.


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Item Type: University of Pittsburgh ETD
Status: Unpublished
CreatorsEmailPitt UsernameORCID
Schmidt, Heidi Mhes81@pitt.eduhes81
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Thesis AdvisorStraub, Adam
Committee ChairPagano,
Committee MemberFreeman,
Committee MemberSchopfer,
Committee MemberJackson,
Committee MemberStolz,
Committee MemberKelley,
Date: 28 November 2022
Date Type: Publication
Defense Date: 7 December 2021
Approval Date: 28 November 2022
Submission Date: 4 January 2022
Access Restriction: No restriction; Release the ETD for access worldwide immediately.
Number of Pages: 148
Institution: University of Pittsburgh
Schools and Programs: School of Medicine > Pharmacology and Chemical Biology
Degree: PhD - Doctor of Philosophy
Thesis Type: Doctoral Dissertation
Refereed: Yes
Uncontrolled Keywords: Xanthine Oxidase, Sickle Cells Disease, Hemolytic Conditions
Date Deposited: 29 Nov 2022 04:05
Last Modified: 29 Nov 2022 23:23


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