TYROSINE PHOSPHORYLATION AND ACTIVATION OF FOCAL ADHESION KINASE (P125(FAK)) BY BCR-ABL ONCOPROTEIN

Focal adhesion kinase (p125(FAK); FAK) is a protein tyrosine kinase that is tyrosine-phosphorylated in response to v-src-mediated transformation, cell adhesion, and stimulation with neuropeptides. To elucidate a possible functional relationship between FAK and BCR-ABL oncoprotein detected in Philadelphia chromosome-positive (Ph(+)) leukemias, we investigated the tyrosine phosphorylation state of FAK in a murine growth factor-dependent cell line and in its stable human bcr-abl cDNA transfectant. In interleukin-3 (IL-3)-dependent NFS/N1.H7 cells, tyrosine phosphorylation of BAK was not detected after stimulation with either IL-3 or Steel factor (SLF), both of which involve Ras-mediated signaling pathways. However, stable gene transfection with p210(bcr-abl) cDNA into H7 cells made these cells growth factor-independent for proliferation and resulted in constitutive tyrosine phosphorylation and kinase activation of FAK. Constitutive phosphorylation and activation of FAK was also observed in all Ph(+) leukemia cell lines examined-that is, K562, TS9;22, and YS9;22, which express p210(BCR-ABL), and NALM-21 and OM9;22, which express p185(BCR-ADL). Ph-negative (Ph(-)) cell lines, such as MO7e and JM, did not show any detectable tyrosine phosphorylation of FAK. FAK phosphorylation in BCR-ABL-expressing cells was inhibited in a dose-dependent manner by cytochalasin D, a reagent that disrupts the intracellular network of actin filaments, However, no suppression of kinase activity or protein expression of BCR-ABL was observed after treatment with cytochalasin D. A physical association between BCR-ABL and FAK was not apparent. These data suggest that BCR-ABL may be involved in the activation of FAK. Moreover, FAK may be distinct from components in Ras-mediated signaling cascades that are activated by stimulation of myeloid cells with various cytokines.