STI571 Suppresses Proliferation by Restoring Nuclear Cyclin Dependent Kinase Inhibitors (CDKIs) while STI571+TRAIL Promotes Cell Death by Decreasing Cytoplasmic CDKIs.

Cyclin dependent kinase inhibitors (CDKIs), p27Kip1 and p21Cip1, function as cell cycle inhibitors when located in the nucleus. When localized to the cytoplasm, these CDKIs can function as anti-apoptotic molecules by sequestering/preventing the activation of pro-apoptotic proteins such as ASK1 and procaspase-3. Our lab has reported elevated cytoplasmic CDKIs and decreased nuclear CDKIs in hematopoietic cells expressing BCR/ABL, the oncogene found in more than 90% of cases of chronic myeloid leukemia. Within the past decade, STI571 has been shown highly promising for CML treatment. However, there is increasing evidence suggesting that the drug might function more as a suppressor of proliferation and less as a promoter of cell death. In the current studies, we differentiate the effect of STI571 on proliferation vs. survival by tracking the subcellular increase/decrease in CDKIs using MO7e cells engineered to express BCR/ABL. To determine if a correlation exists between STI571 resistance and levels of cytoplasmic anti-apoptotic CDKIs, we also investigated changes in levels of nuclear vs. cytoplasmic CDKIs in LAMA84 -S (sensitive to STI571) vs. LAMA84-R (resistant to STI571). And lastly, we tested whether activation of TRAIL would enhance cell death in STI571-resistant cells. STI571 treatment increases nuclear CDKIs, correlating directly with a decrease in proliferation of MO7e/p210 cells. However, the high levels of cytoplasmic CDKIs in MO7e/p210bcr/abl was not modulated following STI571 treatment and cell death was not prominent, unless growth factors were removed. Moreover, cytoplasmic p21Cip co-immunoprecipitated with ASK1 and procaspase 3. When compared with LAMA-S cells, LAMA-R cells expressed even higher levels of cytoplasmic CDKIs. Treatment with STI571 decreased cytoplasmic CDKIs in LAMA84-S cells and resulted in cell death. As hypothesized, LAMA84-R cells did not show reduction in cytoplasmic CDKIs and did not enter apoptosis. However, when treated with STI571 and TRAIL, LAMA84-R cells showed a decrease in cytoplasmic CDKIs, and increase in apoptosis. Based on these observations, we conclude that: 1. BCR/ABL expression reduces nuclear CDKIs but increases cytoplasmic CDKIs. 2. STI571 treatment restores nuclear CDKIs and reduces cell proliferation of BCR/ABL expressing cells under physiological conditions. 3. Treatment of STI571+TRAIL reduces cytoplasmic CDKIs and increases cell death of BCR/ABL expressing cells, most notably, the STI571-resistant cells. In conclusion, we show that the imbalance between nuclear (cell cycle inhibitor) and cytoplasmic (cell survival enhancer) CDKIs exist in BCR/ABL-hematopoietic cells.