The receptor tyrosine kinase Axl is over-expressed in a variety of cancers and is known to play a role in proliferation and invasion. Previous data from our lab indicates that Axl and its ligand GAS6 may play a role in establishing metastatic dormancy in the bone marrow microenvironment. In the current study, we found that Axl is highly expressed in metastatic prostate cancer (PCa) cell lines PC3 and DU145 and has negligible levels of expression in a non-metastatic cancer cell line LNCaP. Knockdown of Axl in PC3 and DU145 cells resulted in decreased expression of several mesenchymal markers including Snail, Slug, and N-cadherin, and enhanced expression of the epithelial marker E-cadherin, suggesting that Axl is involved in the epithelial to mesenchymal transition in PCa cells. The Axl-knockdown PC3 and DU145 cells also displayed decreased in vitro migration and invasion. Interestingly, when PC3 and DU145 cells were treated with GAS6, Axl protein levels were down-regulated. Moreover, CoCl2, a hypoxia mimicking agent, prevented GAS6 mediated down-regulation of Axl in these cell lines. Immunochemical staining of human PCa tissue microarrays demonstrated that Axl, GAS6 and Hif1-α (indicator of hypoxia) were all co-expressed in PCa and in bone metastases, compared to normal tissues. Together, our studies indicate that Axl plays a crucial role in PCa metastasis, and that GAS6 regulates the expression of Axl. Importantly, in a hypoxic tumor microenvironment Axl expression maintained leading to enhanced signaling.