Axl may support dormancy of disseminated tumor cells by inducing cell clustering and decreasing proliferation of metastatic prostate cancer cells

Abstract While the majority of prostate cancer (PCa) patients are cured by radical prostatectomy and/or radiation, 10-15% of these men eventually develop recurrent disease in the form of lethal metastases. The most common site for PCa metastasis is the bone marrow. The time from primary tumor removal to recurrence can range from 1-2 years to decades, and constitutes a period of minimal residual disease in which cancer cells remain in the body but are clinically undetectable. It is thought that these disseminated tumor cells (DTCs) are able to maintain a state of cellular dormancy in the bone marrow in which they are not proliferating or dying, and may be the seeds for lethal metastases. By understanding the mechanism(s) that regulate dormancy, we may be able to prevent metastasis altogether. Our group has previously found that the secreted bone marrow factor Gas6 and the expression of its receptor Axl are associated with dormant DTCs in xenograft mouse models. Thus, we hypothesize that Gas6 signaling through Axl mediates dormancy. In this study, we sought to functionally support this hypothesis both in vitro and in human PCa tissue. Overexpression of Axl in the C42B PCa cell line decreased proliferation in vitro, and this was independent of Gas6 when Axl was highly overexpressed. Axl overexpression also led to aggregation of C42B cells, suggestive of a possible role in circulating tumor cell (CTC) clustering to survive the circulation and inhabit the bone marrow. Furthermore, immunohistochemical staining of a PCa tissue microarray showed almost no Axl positivity in cancer cells, regardless of Gleason score and other clinical outcomes. Negative staining was also observed in sections from liver, lymph node, and tibial metastases. Consistent with this, Axl expression correlates with decreased incidence of biochemical recurrence and increased overall survival in the GRID and TCGA databases, respectively. These results indicate an antiproliferative role for Axl in PCa, and suggest its temporal expression during the natural history of PCa. Ongoing work includes assessing the expression of Axl on patient CTCs and DTCs, and using xenograft mouse models of metastasis to analyze the effects of Axl overexpression and knockout in PCa cell lines. Citation Format: Haley D. Axelrod, Kenneth C. Valkenburg, Jessica L. Hicks, Angelo M. DeMarzo, Kenneth J. Pienta. Axl may support dormancy of disseminated tumor cells by inducing cell clustering and decreasing proliferation of metastatic prostate cancer cells [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr B026.