Exploiting the NRF2/Antioxidant Response Element Signaling Pathway to Induce γ-Globin Gene Expression and HbF Production

Abstract 2074

Induction of fetal hemoglobin (HbF) has proven therapeutic potential to treat sickle cell disease and β-thalassemia. However, agents known to be effective in humans, including hydroxyurea, DNMT inhibitors and butyrate derivatives are not ideal due to suppression of hematopoiesis and the possibility of long-term side effects. Two natural compounds, angelicin (Lampronti, et al, Eur J Hematol 2003) and resveratrol (Rodrigue, et al, Hematology 2001) have been found to induce γ-globin gene expression in K562 cells. These agents may be important lead compounds as they are generally non-cytotoxic and are being evaluated in ongoing human trials as cancer chemopreventative agents where these and several other agents are thought to work by activating antioxidant response pathway genes. The products of these genes are enzymes involved in antioxidant and detoxification activities and include NADPH-quinone oxireductase 1 (NQO1), glutamate-cysteine ligase (GCL) and glutathione S-transferase (GST). The activation of these genes is mediated by the transcription factor, NF-E2 related factor 2 (NRF2), which binds to a specific antioxidant response element (ARE) sequence (TGACnnnGCA) in target gene promoters. The proximal γ-globin promoter contains an ARE sequence between the two CAAT boxes, suggesting that it too may be activated by NRF2. This led us to hypothesize that drugs that activate the ARE/NRF2 pathway may provide a less toxic approach to HbF induction. To test this hypothesis, we treated K562 cells with various NRF2 pathway activators. We initially tested six compounds that are known to induce antioxidant response genes at doses that did not inhibit proliferation and found the most pronounced γ-globin induction with tert-butylhydroquinone (tBHQ) (2.8 fold). We next tested tBHQ in two different primary cell culture models: erythroid precursors isolated from normal human bone marrow and in vitro erythroid differentiation of primary human CD34+ cells. In both of these models, tBHQ treatment increased γ-globin steady state mRNA levels and induced expression of NRF2 target genes. Treatment of differentiating erythroid cells caused a dose dependent increase in γ/(γ+β) mRNA and % HbF. HPLC analysis revealed the highest non-toxic concentration of tBHQ, 5μM, produced 10% HbF while the positive control, 0.5μM of 5-Azacytidine, resulted in 12% HbF compared to the untreated control at 3%. Since similar mRNA induction was seen in K562 cells, we used these cells to characterize the mechanism of tBHQ induced γ-globin expression. First, we used siRNA to decrease NRF2 mRNA levels. This resulted in a greater than 75% knockdown of NRF2 mRNA and protein and reduced tBHQ induction of γ-globin and NQO1 gene expression by 90% and 75%, respectively, compared to samples transfected with scrambled siRNA (p < 0.01). Subsequent experiments showed that tBHQ treatment resulted in NRF2 translocation to the nucleus and binding to the NQO1 and γ-globin promoters but not at negative control sites. In addition, inducing NRF2 translocation by transiently suppressing levels of its inhibitor, KEAP1, did not result in full induction of γ-globin mRNA expression, suggesting that NRF2 translocation alone is not sufficient for γ-globin induction. However, when combined with tBHQ, suppression of KEAP1 did enhance γ-globin induction. Taken together, these results suggest that NRF2/ARE pathway activation induces γ-globin mRNA expression and HbF production in primary human erythroid cells and that this is a promising strategy for further pre-clinical development.