Abstract 1572: Overexpression of Axl delays metastatic outgrowth of prostate cancer bone marrow disseminated tumor cells

Abstract The presence of detectable metastases in the bone marrow of prostate cancer patients represents lethal, incurable disease. These metastases that may emerge years or decades after prostatectomy are derived from small colonies of dormant tumor cells that disseminated from the primary tumor. Our group has previously shown that Gas6/Axl signaling is associated with disseminated tumor cell (DTC) dormancy in mouse models of prostate cancer metastasis. In this study we explored the functional significance of this association and tested the hypothesis that overexpression of Axl in prostate cancer cells would prolong their dormant phase in the bone marrow. Axl overexpression in the AR-positive Axl-negative human prostate cancer cell line C42B slowed proliferation in vitro. Select cells were completely dormant as detected by the lack of EdU incorporation over 40 hours, and this fraction was increased when co-cultured with bone-forming osteoblasts. To assess the functional effects of Axl on prostate cancer progression in vivo, we utilized a mouse model of prostate cancer metastasis in which subcutaneous tumors are established and surgically removed upon reaching 200mm3. At 10 weeks post-tumor removal, mice bearing C42B-Axl tumors did not have overt osteoblastic or osteolytic lesions in their femurs or tibiae, whereas mice bearing C42B-WT tumors had visible lesions by x-ray. Immunohistochemistry staining of mouse femurs and tibiae corroborated these findings. To confirm that the absence of metastases in mice with C42B-Axl tumors was not due to the inability of the cells to disseminate, we used immunofluorescence to detect human cells in the bone marrow of flushed femurs and tibiae. At 12 weeks post-tumor removal, fewer prostate cancer cells were detected in the bone marrow of mice with C42B-Axl tumors compared to those with C42B-WT tumors. Notably however, both groups had detectable prostate cancer cells in the bone marrow, indicating that the absence of C42B-Axl metastasis was not due to failure of primary tumor cells to disseminate. The presence of disseminated C42B-Axl cells in the bone marrow coupled with their failure to develop into clinical metastases at the time of C42B-WT metastatic outgrowth establishes a functional role for the association of Axl expression with DTC dormancy. Axl may be sufficient to induce and maintain dormancy, and ongoing experiments will address the requirement of Axl in this setting. Citation Format: Haley D. Axelrod, Kenneth C. Valkenburg, Kenneth J. Pienta. Overexpression of Axl delays metastatic outgrowth of prostate cancer bone marrow disseminated tumor cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1572.