Oncogenic activation of the proto-oncogene c- abl in human leukemias occurs as a result of the addition of exons from the gene bcr and truncation of the first abl exon. Analysis of tyrosine kinase activity and quantitative measurement of transformation potency in a single-step assay indicate that variation in bcr exon contribution results in a functional difference between p210 ^bcr-abl and p185 ^bcr-abl proteins. Thus, foreign upstream sequences are important in the deregulation of the kinase activity of the abl product, and the extent of deregulation correlates with the pathological effects of the bcr-abl proteins.

The c-Abl tyrosine kinase localizes to the cytoplasm and plasma membrane in addition to the nucleus. However, there is little information regarding a role for c-Abl in the cytoplasm/plasma membrane compartments. Here we report that a membrane pool of c-Abl is activated by the growth factors PDGF and EGF in fibroblasts. The pattern and kinetics of activation are similar to growth factor activation of Src family kinases. To determine whether a link existed between activation of c-Abl and members of the Src family, we examined c-Abl kinase activity in cells that expressed oncogenic Src proteins. We found that c-Abl kinase activity was increased by 10- to 20-fold in these cells, and that Src and Fyn kinases directly phosphorylated c-Abl in vitro. Furthermore, overexpression of wild-type Src potentiated c-Abl activation by growth factors, and a kinase-inactive form of Src reduced this activation, showing that Abl activation by growth factors occurs at least in part via activation of Src kinases. Significantly, we show that c-Abl has a functional role in the morphological response to PDGF. Whereas PDGF treatment of serum-starved wild-type mouse embryo fibroblasts resulted in distinct linear or circular/dorsal membrane ruffling, c-Abl-null cells demonstrated dramatically reduced ruffling in response to PDGF, which was rescued by physiological re-expression of c-Abl. These data identify c-Abl as a downstream target of activated receptor tyrosine kinases and Src family kinases, and show for the first time that c-Abl functions in the cellular response to growth factors.

The c-abl proto-oncogene encodes a cytoplasmic tyrosine kinase which is homologous to the src gene product in its kinase domain and in the upstream kinase regulatory domains SH2 (src homology region 2) and SH3 (src homology region 3). The murine v-abl oncogene product has lost the SH3 domain as a consequence of N-terminal fusion of gag sequences. Deletion of the SH3 domain is sufficient to render the murine c-abl proto-oncogene product transforming when myristylated N-terminal membrane localization sequences are also present. In contrast, the human BCR/ABL oncogene of the Philadelphia chromosome translocation has an intact SH3 domain and its product is not myristylated at the N terminus. To analyze the contribution of BCR-encoded sequences to BCR/ABL-mediated transformation, the effects of a series of deletions and substitutions were assessed in fibroblast and hematopoietic-cell transformation assays. BCR first-exon sequences specifically potentiate transformation and tyrosine kinase activation when they are fused to the second exon of otherwise intact c-ABL. This suggests that BCR-encoded sequences specifically interfere with negative regulation of the ABL-encoded tyrosine kinase, which would represent a novel mechanism for the activation of nonreceptor tyrosine kinase-encoding proto-oncogenes.