Abstract 960: Targeting Egr-1 is an effective strategy for overcoming kinase inhibitor resistance in CML

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA The primary oncogene associated with CML is BCR/ABL which controls proliferative and survival signaling and is a potent inducer of reactive oxygen species (ROS). ROS play both positive and negative roles in proliferation and survival; this dual nature has been exploited by leukemia cells to promote growth, genomic instability, and drug resistance. However, the distinct molecular alterations that occur as a result of BCR/ABL-induced ROS are not well described. BCR/ABL-targeted therapeutics have improved clinical response rates, therefore the number of CML patients living with detectable disease burden is rising. Complete hematologic responses to tyrosine kinase inhibitor (TKI) therapy are seen in ∼10-30% of CML patients, so acquired drug resistance and relapse remain major issues. Thus, novel targets for combined therapeutics are needed. We have shown that early growth response 1 (Egr-1) is a BCR/ABL-dependent, redox-responsive transcription factor that modulates expression of the non-receptor tyrosine kinase Fyn in CML leading to proliferation and survival. Tissue microarray analysis of 26 CML patients (10 chronic phase, 6 accelerated phase, and 10 blast crisis), corroborated by western blotting on independent patient samples, showed that Egr-1 protein expression increased as CML progresses from chronic phase to the more treatment resistant accelerated phase and blast crisis. Egr-1 protein expression was also elevated 2.5 fold in a model of acquired pan-TKI resistance (K562-STI) when compared with parental K562 cells. When Egr-1 was genetically inhibited, proliferation of K562-STI cells decreased by 56%. Egr-1 knockdown was also sufficient to sensitize K562-STI cells to growth inhibition caused by first and second generation BCR/ABL-directed TKI, further implicating Egr-1 in acquired TKI resistance. Egr-1 is well known as a redox-responsive transcription factor, and we found that ROS were elevated in K562-STI vs. K562 cells. While analysis of mitochondrial respiration using a Seahorse Bioanalyzer showed no increase in mitochondrial respiration, spare respiratory capacity, nor proton leak in K562-STI vs. K562, there remained a basal level of oxygen consumption from non-mitochondrial sources in both cell lines. Interestingly, fluorigenic and western blotting assays showed increases in NADPH oxidase (NOX) activity (1.35 fold) and p47phox (2 fold), an essential component of the NOX complex, respectively in K562-STI cells, suggesting NOX as a source of ROS in these cells. To this end, inhibition of the NOX complex with diphenyleneiodonium decreased ROS levels and Egr-1 expression by 50% in K562-STI but not K562 cells. These data suggest that K562-STI have altered regulation of Egr-1 controlled, in part, by NOX. Together, our findings suggest that targeting the transcription factor Egr-1 directly, or through the NOX complex, may be beneficial for improving outcomes for CML patients. Citation Format: Mary E. Irwin, Roxsan Manshouri, Blake Johnson, Hesham M. Amin, Joya Chandra. Targeting Egr-1 is an effective strategy for overcoming kinase inhibitor resistance in CML. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 960. doi:10.1158/1538-7445.AM2014-960