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INTRODUCTION: Molecular abnormalities that interfere with beta-catenin degradation have been identified in human cancer, particularly in colorectal carcinoma. However, beta-catenin has been hypothesized to play a role in chronic myelogenous leukemia (CML) proliferation, specifically during blast crisis. The role of beta-catenin in CML proliferation was evaluated using multiple strategies to down-regulate beta-catenin.

METHODS: Human leukemia cell lines K562 (CML) and KU812 (CML, blast crisis) were cultured under standard conditions. Cells were transfected with siRNAs to beta-catenin or Bcr-abl via electroporation. Cells were treated with STI571 to inhibit Bcr-abl kinase activity and indomethacin and sulindac sulfone to down-regulate beta-catenin. Cellular proliferation was determined by direct counting of viable cells. Beta-catenin, c-Abl, and PARP protein levels were determined by western blotting. Caspase inhibitor (ZVD-fmk) was used to inhibit caspase-mediated protein degradation. For in vivo experiments, KU812 xenografts were established in balb-c/nude mice. Mice were treated systemically with STI571 and/or indomethacin.

RESULTS: KU812 cells have detectable beta-catenin protein levels at baseline while K562 cells do not. In KU812 cells, beta-catenin protein levels were down-regulated by beta-catenin siRNA treatment. Targeted beta-catenin knock-down by siRNA significantly decreased cell proliferation in vitro, comparable to the effect of siRNA-mediated Bcr-abl knock-down; control siRNA had no effect on KU812 cell growth. While inhibited by Bcr-abl siRNA, there was no effect of beta-catenin siRNA on the proliferation of K562 cells. Treatment of KU812 cells with indomethacin or sulindac sulfone caused a dose-dependent down-regulation of beta-catenin and a parallel inhibition of cell growth. The inhibitory effects of beta-catenin siRNA or indomethacin on cell growth were additive with the anti-leukemic effects of Bcr-abl siRNA or STI571. KU812 leukemia xenograft growth in balb-c/nude mice was inhibited by systemic treatment with STI571 or indomethacin, with combination treatment demonstrating additive effects.

CONCLUSION: While Bcr-abl translocation is responsible for hematopoietic progenitor cellular transformation to CML, beta-catenin may be involved in leukemia cell transformation from chronic phase to blast crisis. The results presented here suggest a linkage between beta-catenin expression and proliferation in some CML cells. Agents that down-regulate beta-catenin appear to have anti-leukemia cell activity, enhance the activity of STI571 and may represent a useful approach to the targeted therapy of CML.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA