The c-Fes protein-tyrosine kinase suppresses cytokine-independent outgrowth of myeloid leukemia cells induced by Bcr-Abl.

The c-Fes protein-tyrosine kinase exhibits strong expression in myeloid hematopoietic cells. Previous studies have shown that Fes induces differentiation in the chronic myelogenous leukemia-derived cell line K-562, suggesting that the Fes signal for differentiation is dominant to the Bcr-Abl signal for transformation in these cells. In addition, Fes has been shown to associate with and phosphorylate Bcr on NH2-terminal sequences retained within Bcr-Abl. To determine whether Fes interacts directly with Bcr-Abl, kinase-inactive Bcr-Abl was coexpressed with Fes in 293T cells, and phosphorylation was assessed by anti-phosphotyrosine immunoblotting. Bcr-Abl was strongly phosphorylated by Fes under these conditions, suggestive of direct interaction. Similarly, tyrosine phosphorylation of kinase-inactive Fes was observed after coexpression with active Bcr-Abl. To test for the interaction of Fes with Bcr-Abl under physiological conditions, wild-type and kinase-defective Fes were stably expressed in the cytokine-dependent myeloid leukemia cell line, DAGM. Expression of either form of Fes alone did not affect the proliferation or interleukin 3 dependence of these cells. The DAGM/Fes cells were then infected with Bcr-Abl retroviruses, and their rates of cytokine-independent outgrowth were compared. Fes dramatically suppressed Bcr-Abl-induced DAGM cell outgrowth relative to a cell line expressing beta-galactosidase as a negative control. This effect required Fes tyrosine kinase activity, because the kinase-inactive form of Fes did not affect Bcr-Abl-induced cell outgrowth. The phosphotyrosine content of both wild-type and kinase-inactive Fes was strongly enhanced after coexpression with Bcr-Abl in DAGM cells, similar to the 293T result. Phosphorylation of wild-type Fes correlated with stimulation of Fes tyrosine kinase activity in the presence of Bcr-Abl. These results show that Fes and Bcr-Abl interact in myeloid cells, leading to Fes activation and suppression of Bcr-Abl-induced conversion to cytokine independence.