HYDROXYUREA REVERSES DYSFUNCTIONAL UBIQUITIN-PROTEASOMAL SYSTEM IN SICKLE CELL DISEASE AND SUPPRESSES POSTTRANSLATIONAL ALTERATIONS IN HEMOGLOBIN AND CELL MEMBRANES
Sirsendu Jana, PhD1, Michael Brad Strader, PhD1, Fantao Meng, PhD1,Michael Heaven, PhD2, Arun Shet, MD3, Swee Lay Thein, MD3, and Abdu I. Alayash, PhD1.
1Laboratory of Biochemistry and Vascular Biology, Center for Biologics Evaluation and Research, Food and Drug Administration (FDA), 2Vulcan Analytical, Birmingham Al, 3Sickle Cell Branch, National Heart, Lung and Blood Institute (NHLBI), National Institutes of Health (NIH).
Introduction: Intracellular oxidative stress and oxidative modifications of sickle cell hemoglobin (HbS) play an important role in the pathogenesis of sickle cell disease (SCD). We recently reported transgenic mice studies revealing microparticles (MP) proteome differences between SCD and control mice expressing human HbS and HbA, respectively. Hb-dependent oxidative reactions and consequent posttranslational modifications of Hb βCys93 were central to red cell membrane changes that included modification of band3, and ubiquitination of proteins (Jana S et al., JCI Insights, 2018 3:120451). Ubiquitination is an important post-translational modification required for several biological functions including degradation by the ubiquitin-proteasomal system (UPS) proteolytic pathway. Proteins susceptible to oxidative damage are therefore likely degraded by UPS machinery. When these animals were treated with hydroxyurea (HU) they were able to reduce oxidative stress by controlling Hb oxidation side reactions. As a follow-up study, we have characterized human RBC derived MP proteomes of control, untreated and HU treated SCD patient samples to identify the mechanistic basis of how HU treatment reduces oxidative stress.
Methods: We used a variety of biochemical and hematological methods to investigate a group of sickle cell disease (SCD) patients (n= 22) who are either on HU treatment (n=10) or without HU treatment (n=12) and a group of ethnically matched controls (n=4). We also performed an additional proteomic analysis on a subset of these patients, including a separate longitudinal study in which SCD patients (n=2) were followed before and after treatment with HU.
Results: Immunoprecipitation experiments on RBCs obtained from untreated SCD patients revealed the presence of extensive ubiquitination contrary to those samples obtained from HU treated patients and controls. High proteasomal activity was found in SCD RBCs suggesting accumulated polyubiquitinated proteins found in these samples were not a byproduct of proteasomal inhibition but rather due to imbalance in the redox state of SS RBCs. In addition to Hb oxidation and oxidative modifications (including βCys93), our results revealed differences in the SCD proteome (from both control and untreated groups) including upregulation of phosphorylation and ubiquitination of proteins that are known to interact directly with band3 and are functionally involved in MP formation. Ubiquitination of Hb βLys145 and βLys96 were more abundant in SS patient's samples as well as phosphorylation of band3 (a prerequisite process for band3 clustering and MPs release). As revealed by the separate longitudinal study, HU treatment uniformly reversed ubiquination and phosphorylation of proteins involved in SCD induced MP formation.
Conclusion: These mechanistic analyses of SCD RBC derived MPs suggest a potential involvement of ubiquitination and phosphorylation in SCD pathogenesis and provide additional insight into the therapeutic mechanisms of HU treatment.
Disclosures
No relevant conflicts of interest to declare.
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