Gain-of-function mutations in Kit
receptor tyrosine kinase result in the development of a variety of
cancers, such as mast cell tumours, gastrointestinal stromal tumours (GISTs),
acute myeloid leukemia, and
melanomas. The drug
imatinib, a selective inhibitor of Kit, is used for treatment of mutant Kit-positive
cancers. However, mutations in the Kit
kinase domain, which are frequently found in neoplastic mast cells, confer an
imatinib resistance, and
cancers expressing the mutants can proliferate in the presence of
imatinib. Recently, we showed that in neoplastic mast cells that endogenously express an
imatinib-resistant Kit mutant, Kit causes oncogenic activation of the
phosphatidylinositol 3-kinase-Akt (PI3K-Akt) pathway and the
signal transducer and activator of transcription 5 (STAT5) but only on endolysosomes and on the endoplasmic reticulum (ER), respectively. Here, we show a strategy for inhibition of the Kit-PI3K-Akt pathway in neoplastic mast cells by M-COPA (2-methylcoprophilinamide), an inhibitor of this secretory pathway. In M-COPA-treated cells, Kit localization in the ER is significantly increased, whereas endolysosomal Kit disappears, indicating that M-COPA blocks the biosynthetic transport of Kit from the ER. The drug greatly inhibits oncogenic Akt activation without affecting the association of Kit with PI3K, indicating that ER-localized Kit-PI3K complex is unable to activate Akt. Importantly, M-COPA but not
imatinib suppresses neoplastic mast cell proliferation through inhibiting anti-apoptotic Akt activation. Results of our M-COPA treatment assay show that Kit can activate Erk not only on the ER but also on other compartments. Furthermore, Tyr568/570, Tyr703, Tyr721, and Tyr936 in Kit are phosphorylated on the ER, indicating that these five
tyrosine residues are all phosphorylated before mutant Kit reaches the plasma membrane (PM). Our study provides evidence that Kit is
tyrosine-phosphorylated soon after synthesis on the ER but is unable to activate Akt and also demonstrates that M-COPA is efficacious for growth suppression of neoplastic mast cells.