Heme Affects Sensitivity to Imatinib through Regulation of Nrf2 Activity in BCR/ABL-Positive Cell Lines.

Heme has been recognized as an important modulator of the functions of various cellular molecules. Recent work showed that heme binds to Bach1 transcription factor, resulting in the loss of the repressor activity of Bach1. Because Nrf2 transcription factor and Bach1 may competitively recognize the common gene regulatory elements, heme may have an effect on the Nrf2-madiated gene expressions. We recently reported that Nrf2 is involved in the resistance to imatinib in BCR/ABL-positive cells. Here we investigated the role of heme in the sensitivity to imatinib in human BCR/ABL-positive cell lines. Addition of hemin to KCL22 and KU812 cells resulted in significant increases in the IC50 values of imatinib in accordance with the abrogation of imatinib-mediated induction of apoptosis-related molecules, cleaved caspase 3 and cleaved PARP. The levels of phosphorylated BCR/ABL and phosphorylated ERK1/2 MAP kinase (active form) were both decreased by imatinib even when hemin was added, indicating that hemin does not interfere with imatinib-mediated inhibition of BCR/ABL function and that mechanisms of hemin-induced acquisition of imatinib-resistance is not related to BCR/ABL kinase activity. Hemin increased Nrf2/DNA complex formation at the antioxidant response element of the human γ-glutamylcystein synthetase (γ-GCS) light subunit gene promoter. Because γ-GCS is a rate-limiting enzyme of the synthesis of glutathione (GSH), which has been shown to be involved in resistance to imatinib, we next examined the level of GSH and found that the intracellular amount of GSH was increased by hemin treatment of these cells. In addition, the protein level of heme oxygenase-1, which is important for survival of chronic myeloid leukemia cells and the gene expression of which can be also regulated by Nrf2, was increased after hemin treatment. Addition of ascorbic acid, which suppresses Nrf2-dependent gene expression, partially restored the sensitivity to imatinib in hemin-treated cells. We also showed that inhibition of intracellular heme synthesis by succinyl acetone increased the sensitivity to imatinib in imatinib-resistant cell lines, KCL22/SR and KU812/SR. The results suggest that heme is critically involved in the sensitivity to imatinib, at least in part, through regulation of Nrf2 function.