Transformation of hematopoietic cells by the Bcr-abl oncoprotein leads to constitutive tyrosine phosphorylation of a number of cellular polypeptides that function in normal growth factor-dependent cell proliferation. Recent studies have shown that the CrkL adaptor protein and the Cbl protooncoprotein are constitutively tyrosine phosphorylated and form a preformed complex in cells expressing Bcr-abl. In the current study, we have examined cytokine- dependent tyrosine phosphorylation of Cbl and its association with Crk proteins. Erythropoietin (EPO) and interleukin-3 induced a dose and time- dependent tyrosine phosphorylation of Cbl in both EPO-dependent Ba/F3 and DA- 3 transfectants, and the erythroid cell line HCD-57. Furthermore, once phosphorylated, Cbl associated with Crk adaptor proteins. Of the three Crk isoforms expressed in hematopoietic cells (CrkL, who is Crkll, and Crkl), tyrosine phosphorylated Cbl binds preferentially to CrkL and Crkll. The amount of Cbl associated with CrkL and Crkll exceeded the fraction of Cbl associated with Grb2 indicating that unlike other receptor systems, the Cbl- Crk association represents the dominant complex of Cbl in growth factor- stimulated hematopoietic cells. In factor-dependent hematopoietic cell lines, CrkL constitutively associated with the guanine nucleotide release factor, C3G, which is known to interact via Crk src-homology 3 (SH3) domains. Our data suggest that the inducible Cbl-Crk association is a proximal component of a signaling pathway downstream of multiple cytokine receptors.
ASJC Scopus subject areas
- Cell Biology