No abstract

The response of Philadelphia chromosome (Ph(+)) acute lymphoblastic leukemia (ALL) to treatment by BCR-ABL tyrosine kinase inhibitors (TKIs) has been disappointing, often resulting in short remissions typified by rapid outgrowth of drug-resistant clones. Therefore, new treatments are needed to improve outcomes for Ph(+) ALL patients. In a mouse model of Ph(+) B-lineage ALL, MCL-1 expression is dysregulated by the BCR-ABL oncofusion protein, and TKI treatment results in loss of MCL-1 expression prior to the induction of apoptosis, suggesting that MCL-1 may be an essential prosurvival molecule. To test this hypothesis, we developed a mouse model in which conditional allele(s) of Mcl-1 can be deleted either during leukemia transformation or later after the establishment of leukemia. We report that endogenous MCL-1's antiapoptotic activity promotes survival during BCR-ABL transformation and in established BCR-ABL(+) leukemia. This requirement for MCL-1 can be overcome by overexpression of other antiapoptotic molecules. We further demonstrate that strategies to inhibit MCL-1 expression potentiate the proapoptotic action of BCL-2 inhibitors in both mouse and human BCR-ABL(+) leukemia cell lines. Thus, strategies focused on antagonizing MCL-1 function and expression would be predicted to be effective therapeutic strategies.

The anti-apoptotic function and tumor-associated expression of HSP70 is consistent with HSP70 functioning as a survival factor to promote tumorigenesis. However, its immunomodulatory activities to induce anti tumor immunity predict the suppression of tumor growth. Using the Hsp70.1/3−/− (Hsp70−/−) mouse model, we observed that tumor-derived HSP70 was neither required for cellular transformation nor for in vivo tumor growth. Hsp70−/− murine embryonic fibroblasts (MEFs) were transformed by E1A/Ras and generated tumors in immune deficient hosts as efficiently as WT transformants. Comparison of Bcr-Abl-mediated transformation of WT and Hsp70−/− bone marrow and progression of B cell leukemogenesis in vivo revealed no differences in disease onset or survival rates and Eμ-Myc driven lymphoma in Hsp70−/− mice was phenotypically indistinguishable from WT Eμ-Myc mice. However, Hsp70−/− E1A/Ras MEFs generated significantly larger tumors than their WT counterparts in C57BL/6J immune competent hosts. Concurrent with this was a reduction in intra-tumoral infiltration of innate and adaptive immune cells, including macrophages and CD8+ T cells. Evaluation of several potential mechanisms revealed an HSP70-chemokine-like activity to promote cellular migration. These observations support a role for tumor-derived HSP70 in facilitating anti-tumor immunity to limit tumor growth and highlight the potential consequences of anti-HSP70 therapy as an efficacious anti-cancer strategy.