Abstract 4308: A model of oncogene addiction: BCR-ABL hijacks signaling through a MEK-dependent process and inhibits growth factor-mediated signal transduction.
Authors: Gajan Jennifer , Lasater Elisabeth , Tajon Cheryl , Craik Charles , Shah Neil 
Published: August 2014

Abstract
Background: Chronic myeloid leukemia (CML) is characterized by the presence of the Philadelphia (Ph) chromosome and the fusion gene BCR-ABL, which has been shown to activate key signaling pathways, including PI-3 kinase, RAS/ERK and STAT5. ABL tyrosine kinase inhibitors (TKIs) are highly effective clinical agents that inhibit downstream signaling, trigger CML cell apoptosis, and provide clinical evidence for the phenomenon of “oncogene addiction," whereby cell survival becomes critically dependent upon the activity of a particular oncogene product. TKI therapy has largely failed to achieve the same degree of clinical success in other activated kinase-driven malignancies, suggesting that BCR-ABL is relatively unique in its ability to establish a state of oncogene addiction.
Results: To dissect the mechanism by which BCR-ABL coordinates STAT5 and RAS/ERK signaling to establish a state of oncogene addiction, JAK2 kinase activity and RAS-GTP levels were assessed in the CML patient-derived cell line K562. Surprisingly, basal levels of RAS-GTP were low and JAK2 kinase activity was not detectable. Treatment of K562 cells with erythropoietin (EPO) only very modestly activated JAK2. Similarly, EPO treatment after one hour of BCR-ABL inhibition also failed to rescue STAT5 or RAS/ERK activation. However, with more prolonged BCR-ABL kinase inhibition, we observed a time-dependent increase in the ability of EPO to rescue STAT5 and ERK phosphorylation. After 24hrs of BCR-ABL inhibition, EPO treatment led to a significant increase in both JAK2 activation and the GTP loading of RAS relative to vehicle treated cells. These data suggest that BCR-ABL kinase activity hampers the ability of JAK2 to activate STAT5 and RAS/MAPK . Further, 24hr treatment with the MEK inhibitor PD0325901 allowed for a comparable degree of EPO-mediated JAK2 activation, suggesting that the negative regulation of JAK2 is mediated at least in part by a MEK/ERK-dependent mechanism. These observations were subsequently validated in an isogenic system utilizing TF-1 cells. Similar to the EPO/JAK2 axis in K562 cells, TF1-BCR-ABL cells exhibit attenuated GM-CSF signaling relative to control TF1-puro cells, which was restored only after prolonged BCR-ABL inhibition. Significantly, we have observed that BCR/ABL-expressing cells irreversibly commit to apoptosis before growth factor signaling is fully restored.
Conclusions: Our data suggest that BCR-ABL-mediated oncogene addiction is a consequence of a high level of physiologic negative feedback mechanisms that dampen growth factor receptor signaling. Our findings also help reconcile recent evidence that JAK2 inhibition restores the sensitivity of CML stem/progenitor cells to ABL TKI treatment in vitro with the observation that JAK2 is completely dispensable for BCR-ABL-induced myeloid disease in mice.
Citation Format: Jennifer Gajan, Elisabeth Lasater, Cheryl Tajon, Charles Craik, Neil Shah. A model of oncogene addiction: BCR-ABL hijacks signaling through a MEK-dependent process and inhibits growth factor-mediated signal transduction. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4308. doi:10.1158/1538-7445.AM2013-4308