Engagement of the CrkL adaptor in interferon alpha signalling in BCR-ABL-expressing cells

Interferon alpha (IFN alpha) has significant clinical activity in the treat ment of patients with chronic myelogenous leukaemia (CML), but the mechanis ms of its selective efficacy in the treatment of the disease are unknown. T he CrkL adaptor protein interacts directly with the BCR-ABL fusion protein that causes the malignant transformation and is constitutively phosphorylat ed in BCR-ABL-expressing cells. In the present study, we provide evidence t hat CrkL was engaged in IFN alpha -signalling in the CML-derived KT-1 cell line, which expresses BCR-ABL and is sensitive to the growth inhibitory eff ects of IFN alpha. CrkL is constitutively associated with BCR-ABL in these cells and treatment with IFN alpha had no effect on the BCR-ABL/CrkL intera ction. After IFN alpha stimulation, CrkL associated with Stat5, which also underwent phosphorylation in an IFN alpha -dependent manner. The interactio n of CrkL with Stat5 was facilitated by the function of both the SH2 and th e N-terminus SH3 domains of CrkL. The resulting CrkL-Stat5 complex transloc ated to the nucleus and could be detected in gel shift assays using element s derived from either the beta -casein promoter or the promoter of the PML gene, an IFN alpha -inducible gene that mediates growth inhibitory response s. In addition to its interaction with Stat5, CrkL interacts with C3G in KT -1 cells and such an interaction regulates the downstream activation of the small GTPase Rap1, which also mediates inhibition of cell proliferation. T hus, despite its engagement by BCR-ABL in CML-derived cells, CrkL mediates activation of downstream signalling pathways in response to the activated t ype I IFN receptor and such signals may contribute to the generation of the anti-proliferative effects of IFN alpha in CML.