The tetramerization domain-independent Ras activation by BCR-ABL oncoprotein in hematopoietic cells

BCR-ABL is a chimeric oncoprotein that exhibits deregulated tyrosine kinase activity and is implicated in the pathogenesis of Philadelphia chromosome-positive human leukemias. We have previously shown that BCR-ABL activates Ras signaling pathways required for transformation. To elucidate the mechanisms whereby BCR-ABL induced transformation in hematopoietic cells, we examined the biological effects of expression of a series of BCR-ABL mutants. We found that the Grb2 binding site-deleted BCR-ABL and the SH2 domain-deleted BCR-ABL, as well as the tetramerization domain-deleted BCR-ABL do not diminish the transforming properties of BCR-ABL in hematopoietic cells, although these mutations were previously shown to drastically reduce the transforming activity of BCR-ABL in fibroblasts. The tetramerization domain-deleted BCR-ABL did not induce tyrosine phosphorylation of CrkL, SHP-2, Vav and the interactions of BCR-ABL and Shc. However, Ras is activated, Shc is tyrosine phosphorylated and binds to Grb2 in the tetramerization domain-deleted BCR-ABL expressing hematopoietic cells. These results suggest that the tetramerization domain-independent Ras activation is mediated by Shc proteins and induces the transformation of hematopoietic cells.