Abstract
The tyrosine kinase Bcr-Abl is the fusion product of a reciprocal translocation between chromosomes 9 and 22, known as Philadelphia chromosome and it is present in the leukemic cells of more than 95% of patients with chronic myeloid leukemia (CML). Overexpression of Bcr-Abl in myeloid cells activates various signaling pathways. Previous studies have demonstrated that certain Src family kinases, such as Hck and Lyn, are also targets of Bcr-Abl activity. Hck and Lyn are expressed and activated in CML blast-crisis patients and their increased expression correlates with disease progression or STI571 resistance in some CML patients. Resistance to STI571 seems to be mediated by amplification of or mutations in the Bcr-Abl gene, reducing sensitivity to this inhibitor; newer Abl inhibitors may be susceptible to the same mechanism of resistance. Alternative strategies for control of CML, including the biological relevance of the Bcr-Abl - Src family kinase pathway, are necessary. One such strategy is the use of a specific small molecule Src kinase inhibitor. Recently, a new class of compounds, 4-anilino-3-quinolinecarbonitrile Src kinase inhibitors, has been synthesized. One member of this class, SKI606, is a dual-specificity inhibitor of both Src family and Abl kinases. To investigate the effect in vitro of SKI-606, we analyzed human cell lines from CML patients in blast crisis (K562, MK2, LAMA) and CD34+ from 9 patients in CML blast crisis using a wide range of concentrations (0.01μM-10μM) of this novel agent. Cell cycle analysis, in particular for the cell lines, showed that a major effect of SKI606 is to alter cell cycle progression, producing G1/S arrest. SKI606 induced dose-dependent inhibition of proliferation with IC50 of 1μM at 24hr. Flow cytometric analysis with Annexin-V showed that SKI-606 induced apoptosis of 50% of cells at 48hr. Western blotting and immuno-blotting analyses showed reduced phosphorylation of Bcr-Abl and also of Lyn and Hck. We also demonstrated activation of Caspase-9, an effector cysteine-protease, after exposure to SKI606. These drug effects also reduced the oncogenic effects of the Bcr-Abl gene product in CD34+ cells from patients with CML blast crisis. SKI606 induced a dose-dependent inhibition of proliferation with an IC50 of 1μM at 48hr and induction of apoptosis at 72hr. Cytofluorimetric analysis after 72hr of exposure revealed marked accumulation of cells in the G1 phase of cell cycle, accompanied by a significant increase in the number of apoptotic cells. In some of these patient samples, we observed hypophosphorylation of Bcr-Abl, Hck and Lyn at low concentration of SKI606 (1uM at 24h, 10uM at 48h). Interestingly, CD34+ cells taken from two of our imatinib-resistant patients with Bcr-Abl point mutations (E255K and Y253H) in the P-loop region of the protein exhibited a significant increase of apoptosis (50%) and a block in G1 phase of the cell cycle after treatment with 1 μM SKI606 for 48h. Our study thus showed a potential therapeutic usefulness of the drug in treatment of CML, particularly in blast crisis phase. Ongoing gene expression profiles will contribute to further understanding of the drug mechanism.
Modulation of fibroblast proliferation by oxygen free radicals