Phosphatidylinositol-3 kinase activity is regulated by BCR/ABL and is required for the growth of Philadelphia chromosome-positive cells

The BCR/ABL oncogenic tyrosine kinase is responsible for initiating and maintaining the leukemic phenotype of Philadelphia chromosome (Ph¹)-positive cells. Phosphatidylinositol-3 (PI-3) kinase is known to interact with and be activated by receptor and nonreceptor tyrosine kinases. We investigated whether PI-3 kinase associates with and/or is regulated by BCR/ABL, whether this interaction is functionally significant for Ph¹ cell proliferation, and, if so, whether inhibition of PI-3 kinase activity can be exploited to eliminate Ph¹-positive cells from bone marrow. We show that the p85α subunit of PI-3 kinase associates with BCR/ABL and that transient expression of BCR/ABL in fibroblasts and downregulation of BCR/ABL expression using antisense oligodeoxynucleotides (ODNs) in Ph¹ cells activates and inhibits, respectively, PI-3 kinase enzymatic activity. The use of specific ODNs or antisense constructs to downregulate p85α expression showed a requirement for p85α subunit in the proliferation of BCR/ABL-dependent cell lines and chronic myelogenous leukemia (CML) primary cells. Similarly, wortmannin, a specific inhibitor of the enzymatic activity of the p110 subunit of PI-3 kinase, inhibited growth of these cells. The growth of normal bone marrow end erythromyeloid, but not megakaryocyte, progenitors was inhibited by p85α antisense [S]ODNs, but wortmannin, at the concentrations tested, did not affect normal hematopoiesis. The proliferation of two BCR/ABL- and growth factor-independent cell lines was not affected by downregulation of the expression of the p85α subunit or inhibition of p110 enzymatic activity, confirming the specificity of the observed effects on Ph¹ cells. Thus, PI-3 kinase is one of the downstream effectors of BCR/ABL tyrosine kinase in CML cells. Moreover, reverse transcriptase-polymerase chain reaction performed on single colonies to detect BCR-ABL transcripts showed that wortmannin was able to eliminate selectively CML-blast crisis cells from a mixture of normal bone marrow and Ph¹ cells.