Abstract 1577: Contributions of KIT and ABL to the therapeutic responses of GIST and CML cells to imatinib mesylate

Abstract Most GISTs are caused by activating mutations of the KIT receptor tyrosine kinase and can effectively be treated with the small molecule kinase inhibitor imatinib mesylate (IM). IM is a potent inhibitor of KIT as well as ABL, a kinase that is constitutively activated through a chromosomal translocation (BCR-ABL) in chronic myeloid leukemia (CML). Since both KIT and ABL are expressed in GIST as well as CML cells, the present study was designed to determine the individual contributions of KIT and ABL inhibition, respectively, to the therapeutic responses in both malignancies. As expected, siRNA-mediated knock-down of KIT in the human GIST cell line GIST882 led to a significant increase of apoptosis and reduced cellular proliferation as measured by TUNEL and luminescence-based assays. Depletion of ABL1 in human K562 CML cells led to similar results. Immunoblotting for PARP cleavage as well as p27(Kip1) and cyclin A corroborated these findings. Reduction of ABL1 protein levels in GIST cells, however, was only associated with a minimal increase of apoptosis. Nevertheless, it resulted in a significantly reduced cellular proliferation when compared to controls and had approximately 50% of the anti-proliferative effect detected after knock-down of KIT in GIST cells. Combined knock-down of ABL1 and KIT in GIST did not show an additive effect. In contrast, siRNA-mediated knock-down of KIT in K562 cells did not have any significant effects on either cell survival or proliferative activity. Combined depletion of ABL1 and KIT in CML cells had a similar effect as knock-down of ABL1 alone. Immunoblotting analysis confirmed respective siRNA knockdowns, inhibition of downstream pathways and cellular proliferation as well as induction of apoptosis. In conclusion, ABL inhibition is unlikely to contribute to the induction of apoptosis after IM-treatment of GIST cells, however, it may cooperate with KIT inhibition to inhibit GIST cell proliferation. In contrast, inhibition of KIT does not seem to play a role in the therapeutic effect of IM in CML cells. Our results have implications for future strategies to improve the therapeutic effectiveness of targeted therapies in GISTs. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1577.