Abstract 2262: Evidence that the translational initiation factor DAP5 plays a critical role in breast cancer metastasis

The metastatic process is highly inefficient. Among the thousands cells that escape daily from the primary tumor only a few survive and posses the ability of developing overt metastases in a distant organ. Yet, metastasis is the cause of 90% of cancer-related deaths. Despite its clinical importance, little is known about genetic and biochemical determinants of metastasis and new therapeutic targets are desperately needed. During their journey to colonize a distant organ, cancer cells undergo a wide variety of stresses (hypoxia, anoikis or colonization) that down-regulate global cap-dependent protein synthesis. In order to produce the proteins needed to overcome these stresses cancer cells rely on alternative mechanisms for translation initiation, typically through non-cap-dependent means such as internal ribosome entry (IRES). DAP5 is a poorly studied translation initiation factor known to mediate IRES-driven translation of cellular mRNAs that include anti-apoptotic factors and others factors likely involved in stress responses. Therefore, we hypothesized that DAP5 is crucial for the expression of the disseminated cancer cell proteome and cell survival during metastasis. We engineered highly metastatic murine 4T1 cancer cells to express a doxycycline (dox) inducible system shRNA silencing of DAP5 (4T1 shDAP5). These cells or the non silencing (NS) shRNA control cells were grown s.c. in immunocompetent syngeneic BALB-c mice. Dox induction was started by drinking water addition 12 days post cell injection and tumors measured twice weekly. Animals were sacrificed 18 days later, lungs collected and fixed in 4% PFA. While no significant effect was observed on cell proliferation in vitro, silencing DAP5 significantly decreased primary tumor growth and lung metastatic colonization. Importantly, when the same tumor study was performed in immunodeficient animals (NOD SCID γ) the inhibition of tumor growth with DAP5 silencing was completely abolished. These data suggest that DAP5 likely enables cancer cells to elude the immune response. However in this model, DAP5 silencing impaired the metastatic process and significantly decreased the number of lung metastases observed, implying that at least one of the mechanisms by which DAP5 prevents metastasis is immune system-independent. These results have been preliminarily confirmed in human metastatic breast cancer cells that efficiently disseminate to lungs. We engineered them with the same dox inducible system. When tes