A Novel Role for Bcl-x_L in the Context of p210 BCR/ABL B Cell Acute Lymphoblastic Leukemia (B-ALL).

While treatment with tyrosine kinase inhibitors is highly successful for patients diagnosed in the chronic phase of chronic myeloid leukemia, these drugs are inefficient for BCR/ABL associated B-cell acute lymphocytic leukemia (B-ALL). Therefore, it is necessary to identify molecular targets downstream of BCR/ABL to develop additional therapeutic approaches. Cells transformed by BCR/ABL are resistant to a wide variety of apoptotic stimuli and therapeutic strategies aimed at reinstating the apoptotic pathway appear as an attractive concept. Bcl-x_L is an antiapoptotic member of the Bcl-2 family of proteins and studies employing cell lines, as well as primary cells have linked BCR/ABL expression with increased levels of Bcl-x_L, resulting in resistance to chemotherapeutic agents. To define the role of Bcl-x_L in BCR/ABL associated B-ALL, we generated two inducible transgenic mouse models. In the first model, BCR/ABL and loss of Bcl-x expression are co-induced, and in the second model, leukemia is induced with expression of Bcl-x_L protein well above the levels found in wildtype lymphoblasts. Surprisingly, we found that deletion of Bcl-x_L did not inhibit leukemogenesis or affect apoptosis. Bcl-x deficient B-ALL mice rapidly succumbed to a B-ALL like disease with Bcl-x deficient B-ALL animals being moribund as early as 17 days after induction. By day 28, all mice (n=10) had died or had to be euthanized. Necropsy of animals suffering from Bcl-x deficient leukemia revealed massive lymphadenopathy, pleural effusion, and splenomegaly. While loss of Bcl-x in our B-ALL model led to a more severe phenotype with considerable tumor burden, no statistically significant difference was found between the survival time in Bcl-x deficient and wild type B-ALL animals due to development of pleural effusion in both models. The most prominent difference was the presence of mitotic figures in the peripheral blood, lymph node, and spleen of Bcl-x deficient B-ALL animals, suggestive of increased proliferation of Bcl-x deficient lymphoblasts. Cell cycle analysis of leukemic cells isolated from pleural effusion and spleen of Bcl-x deficient B-ALL mice demonstrated a significant increase of cells in S/G2/M phase (p ≤ 0.05) compared to wildtype lymphoblasts. Thus, loss of Bcl-x_L results in increased passage through the cell cycle, while expression of the protein limits the proliferation rate. To test this hypothesis, we generated a second model in which Bcl-x_L is expressed at higher levels than in wild type lymphoblasts. Overexpression of Bcl-x_L in BCR/ABL positive mice led to reduced proliferation as significantly fewer leukemic cells were present in the S phase than in controls substantiating a role for in Bcl-x_L proliferation of lymphoblasts. Initial studies performed to determine the mechanisms by which loss of Bcl-x leads to increased proliferation suggest that the protein may indirectly regulate stability of p27^Kip1. Our data show that cells from Bcl-x deficient B-ALL mice in G1 and S phase contain less p27, as a consequence of proteosomal degradation. Clearly, our model systems demonstrate an unexpected and novel role for Bcl-x_L in the context of BCR/ABL associated B-ALL. Ongoing studies are aimed at the identification of the mechanism and molecules through which Bcl-x_L is linked to cell cycle and proliferation of BCR/ABL transformed lymphoblasts.