1. ¹Department of Pathology, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, USA
2. ²Department of Pathology, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA
3. ³Ludwig Center at Dana Farber Cancer Institute, Harvard Medical School, Boston, MA, USA

Correspondence: Dr S Bauer, Department of Internal Medicine/Cancer Research (West German Cancer Center), Hufelandstr. 55, 45122 Essen, Germany. E-mail: sebastian.bauer@uni-due.de; Professor J Fletcher, Department of Pathology, Brigham & Women's Hospital, 75 Francis Street, Boston, MA 02155, USA. E-mail: jfletcher@partners.org

Received 27 September 2006; Revised 12 March 2007; Accepted 18 April 2007; Published online 4 June 2007.

Abstract

Most gastrointestinal stromal tumor (GIST) patients respond to KIT inhibition with imatinib, yet will eventually exhibit resistance. Imatinib-resistance mechanisms are heterogeneous, and little is known about KIT functional roles in imatinib-resistant GIST. Biological consequences of biochemical inhibition of KIT, phosphatidyl-inositol-3-kinase (PI3-K), PLC, MAPK/ERK kinase/mitogen-activated protein kinase (MEK/MAPK), mammalian target of rapamycin (mTOR) and JAK were determined by immunoblotting for protein activation, and by cell proliferation and apoptosis assays in GIST cell lines from imatinib-sensitive GIST (GIST882), imatinib-resistant GISTs (GIST430 and GIST48) and KIT-negative GIST (GIST62). KIT activation was 3- to 6-fold higher in GIST430 and GIST48 than in GIST882, whereas total KIT expression was comparable in these three GIST lines. In addition to the higher set point for KIT activation, GIST430 and GIST48 had intrinsic imatinib resistance. After treatment with 1 M imatinib, residual KIT activation was 6- and 2.8-fold higher in GIST430 and GIST48, respectively, compared to GIST882. In all GIST lines, cell growth arrest resulted from PI3-K inhibition, and – to a lesser extent – from MEK/MAPK and mTOR inhibition. Inhibition of JAK/STAT or PLC did not affect cell proliferation. Similarly, only PI3-K inhibition resulted in substantial apoptosis in the imatinib-resistant GISTs. We conclude that GIST secondary KIT mutations can be associated with KIT hyperactivation and imatinib resistance. Targeting critical downstream signaling proteins, such as PI3-K, is a promising therapeutic strategy in imatinib-resistant GISTs.