Direct Binding of CRKL to BCR-ABL Is Not Required for BCR-ABL Transformation

BCR/ABL P190 transgenic mice develop leukemia in the absence of Crkl

The Bcr/Abl fusion protein directly causes chronic myelogenous leukemia and Philadelphia-chromosome positive acute lymphoblastic leukemia. Multiple independent studies have implicated Crkl, a small adapter protein, in transduction of oncogenic signals of Bcr/Abl and Crkl tyrosine-phosphorylation is used as a diagnostic tool for Philadelphia-positive leukemia. To evaluate the contribution of Crkl to this type of leukemia, we generated mutant mice that lack Crkl expression. We found that the overall survival of P190 BCR/ABL crkl-/- mice was comparable to that of genetically matched P190 BCR/ABL crkl +/+ mice. Both genotypes developed lymphoid lineage leukemia/lymphoma. Western blot analysis of -/- and +/+ lymphomas showed that the related Crk protein was tyrosine phosphorylated and could be found complexed with Bcr-Abl P190. These data indicate that possible therapeutic approaches that target Crkl may be complicated by the presence of pathways that compensate for lack of Crkl function.

Erythropoietin Induces Tyrosine Phosphorylation of Jak2, STAT5A, and STAT5B in Primary Cultured Human Erythroid Precursors

We examined signaling by erythropoietin in highly purified human colony forming unit-erythroid cells, generated in vitro from CD34+ cells. We found that erythropoietin induces tyrosine phosphorylation of Jak2, STAT5A, and STAT5B. Tyrosine phosphorylation of Jak2 reaches a peak around 10 minutes after stimulation and is maximum at 5 U/mL of erythropoietin. Tyrosine phosphorylation of STAT5 is accompanied by the translocation of activated STAT5 to the nucleus as shown by electrophoretic mobility shift assay (EMSA) using 32Pi-labeled STAT5 binding site in the β-casein promoter. Tyrosine phosphorylation STAT1 or STAT3 was not detected in human erythroid precursors after stimulation with erythropoietin. Crkl, an SH2/SH3 adapter protein, becomes coimmunoprecipitated specifically with STAT5 from erythropoietin-stimulated erythroid cells; although it was shown to become associated with c-Cbl in the studies using cell lines. Thus, human erythroid precursors can be expanded in vitro in sufficient numbers and purity to allow its usage in signal transduction studies. This report sets a basis for further studies on signaling in primary cultured human erythroid precursors, which in turn contribute to our better understanding in the differentiation processes of erythrocytes and their precursors.