Loss of NRF2 Function Worsens the Pathophysiology of Sickle Cell Disease in a Transgenic Mouse Model

The basic leucine zipper transcription factor, nuclear factor (erythroid-derived 2)-like 2 (NRF2) plays a critical role in the cellular antioxidant response to control oxidative stress. We and others previously demonstrated that NRF2 activation enhances γ-globin gene transcription and fetal hemoglobin expression in human primary erythroid progenitors through changes in chromatin structure (Zhu et al., Haematologica 2017). In this study, we investigated the protective role of NRF2 in reversing the pathophysiology of sickle cell disease (SCD) in a SCD/NRF2 knockout (SCD/NRF2^KO) transgenic model created in our lab by crossbreeding Townes SCD mice (Ryan et al., Science 1997) and NRF2 knockout mice (Kuroha et al., J Biochem 1998). The NRF2 gene is transmitted through autosomal recessive Mendelian genetics for wild-type, heterozygote and NRF2^KO pups. By contrast, the SCD/NRF2^KO genotype was present in <2% of pups. In addition, the fertility and litter size of SCD/NRF2^KO females were lower than SCD^WT mice, suggesting a critical role of NRF2 in the survival of pups during gestation.

To determine the hematopoietic effect of NRF2^KO in SCD, we monitored the complete blood count with differential and reticulocyte count. There was no significant change in any parameters except higher total white blood cell counts in the SS/NRF2^KO mouse suggesting increased inflammation. Examination of globin gene expression patterns in SS/NRF2^KO mice showed reduced γ-globin gene expression during erythroid differentiation in the E13.5 and E18.5 fetal liver, adult spleen and bone marrow. In addition, peripheral blood red cells had a 33% increase (p<0.05) in reactive oxygen species and a significant 38% increase in sickling under in vitro hypoxic conditions. We next characterized the effects of NRF2 loss on organ pathology. The SCD/NRF2^KO mice displayed greater splenomegaly indicating exaggerated hemolysis most likely due to high levels of reactive oxygen species. By 8-10 weeks of age, the SCD/NRF2^KO mouse showed significant inflammation by hematoxylin-eosin staining of the spleen, lung and liver when compared to SCD/NRF2^WT mice. Protein expression profiling by western blotting using adult spleen demonstrated downregulation of the antioxidant proteins heme oxygenase 1 (HMOX1), NADPH: quinone oxidoreductase 1 (NQO1) and catalase by 31%, 60%, and 48% respectively (p<0.05). To further support a severe disease phenotype, the expression of intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1) were increased by 1.7-fold and 2.3-fold (p<0.05) while vascular endothelial growth factor (VEGF) levels were not changed. Lastly, the expression of interleukin 1β (IL-1β), IL-6, tumor necrosis factor α (TNF-α), monocyte chemoattractant protein (MCP-1) and macrophage migration inhibitory factor (MIF-1) was elevated in SCD/NRF2^KO mice compared to SCD^WT mice. These data validate a critical role of NRF2 in ameliorating the phenotypic severity of SCD by protecting against red blood cell sickling, oxidative stress and tissue inflammation. Furthermore, the ability of NRF2 to mediate fetal hemoglobin induction provides a rationale for the development of therapeutic agents that activate NRF2 expression.