2621

GAS6

growth arrest-specific 6;GAS6;growth arrest-specific 6

AXL receptor tyrosine kinase ligand|AXL stimulatory factor|growth arrest-specific protein 6

13q34

protein-coding

Our recent studies have shown that annexin II, expressed on the cell surface of osteoblasts, plays an important role in the adhesion of hematopoietic stem cells (HSCs) to the endosteal niche. Similarly, prostate cancer (PCa) cells express the annexin II receptor and seem to use the stem cell niche for homing to the bone marrow. The role of the niche is thought to be the induction and sustenance of HSC dormancy. If metastatic PCa cells occupy a similar or the same ecological niche as HSCs, then it is likely that the initial role of the HSC niche will be to induce dormancy in metastatic cells. In this study, we demonstrate that the binding of PCa to annexin II induces the expression of the growth arrest-specific 6 (GAS6) receptors AXL, Sky, and Mer, which, in the hematopoietic system, induce dormancy. In addition, GAS6 produced by osteoblasts prevents PCa proliferation and protects PCa from chemotherapy-induced apoptosis. Our results suggest that the activation of GAS6 receptors on PCa in the bone marrow environment may play a critical role as a molecular switch, establishing metastatic tumor cell dormancy.

The receptor tyrosine kinase Axl is overexpressed in a variety of cancers and is known to play a role in proliferation and invasion. Previous data from our laboratory indicate that Axl and its ligand growth arrest-specific 6 (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 cell lines PC3 and DU145 and has negligible levels of expression in a nonmetastatic 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-mesenchymal transition in prostate cancer 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 downregulated. Moreover, CoCl(2), a hypoxia mimicking agent, prevented GAS6-mediated downregulation of Axl in these cell lines. Immunochemical staining of human prostate cancer tissue microarrays showed that Axl, GAS6, and hypoxia-inducible factor-1alpha (Hif-1alpha; indicator of hypoxia) were all coexpressed in prostate cancer and in bone metastases compared with normal tissues. Together, our studies indicate that Axl plays a crucial role in prostate cancer metastasis and that GAS6 regulates the expression of Axl. Importantly, in a hypoxic tumor microenvironment Axl expression is maintained leading to enhanced signaling.