Taspase1 is a Non-oncogene Mediator of Tumorigenesis and Maintenance

摘要: The clinical success of oncogene-targeted therapies substantiates the continued reliance of certain cancers upon the continued function of apical oncogenes involved in its genesis--a phenomenon known as "oncogene addiction." Though this shift from non-targeted, cytotoxic therapies offers new hope to patients, resistance to oncogene inactivation often remains an eventuality, and it is clear that further investigation is required to more effectively battle cancer using alternative therapeutic targets. Our studies on Taspase1 (threonine aspartase 1) reveal its role in coordinating cellular proliferation and apoptosis in cancer. Loss of Taspase1 by shRNA-mediated knockdown decreased cellular proliferation in a diverse set of cancer cell lines, including glioblastoma, melanoma, lung, breast, colon, and prostate, as well as increased sensitivity to death stimuli in melanoma and glioblastoma. Taspase1 loss impedes proliferation via up-regulation of the Cyclin dependent kinase inhibitor p27 and destabilizes the anti-apoptotic BCL-2 family member MCL-1. Decreased levels of MCL-1 in glioblastoma and melanoma due to Taspase1 loss sensitizes these cells to a variety of apoptotic stimuli, including anoikis, DNA damaging agents etoposide and doxorubicin, and to a targeted inhibitor of BCL-2 family proteins, ABT-737. We found that Taspase1 is over-expressed in multiple primary human tumors, including glioblastoma and melanoma, suggesting that Taspase1 is relevant to tumorigenesis in humans and that its inhibition could be clinically useful. In vitro examination of tumorigenic clones selected by their ability to form colonies in soft agar revealed an increase in Taspase1 expression compared to the unselected pool of MEFs transduced by defined oncogenes, suggesting an increased reliance on Taspase1 in the process of tumorigenic transformation. Yet, we demonstrate that Taspase1 is not an oncogene, and therefore, Taspase1 better fits a new class of cancer targets--the subordinate cellular machinery of oncogene-driven processes. Dependence on this machinery was recently termed "non-oncogene addiction." Taspase1 loss can sensitize cancer cells to both cytotoxic agents as well as oncogene-targeted agents, suggesting that inhibition of the non-oncogene addiction protease Taspase1 can complement traditional therapies. Through high throughput screening, we have identified a noncompetitive, small molecule inhibitor of Taspase1, named TASPIN-1, which inhibits Taspase1 in vitro and in cell culture. Murine fibroblasts treated with TASPIN-1 exhibit decreased proliferative capacity, while also demonstrating molecular characteristics similar to those observed with the acute, genetic loss of Taspase1. TASPIN-1 treatment exerts specific cytotoxicity in human breast cancer and glioblastoma cells that have high expression of Taspase1. Additionally, TASPIN-1 treated mice bearing U251 human glioblastoma xenografts exhibit tumor regression while the mice tolerated short term treatment relatively well. This suggests that pharmacological inhibition of Taspase1 is of potential therapeutic benefit in the treatment of cancer patients