Previous work had isolated a transforming gene, axl, that defines a new structural class of receptor tyrosine kinase (RTK) uniquely characterized by an extracellular domain resembling adhesion molecules such as N-CAM. Subsequent work identified GAS-6, a protein S homologue, as the AXL ligand, and determined that AXL is overexpressed in the epithelial component of 30% of primary breast cancers, with coordinate expression of GAS-6 from stromal elements. Since the GAS-6 ligand is a mitogen for some AXL expressing breast cancer cell lines, AXL and GAS-6 may therefore function in a paracrine manner to modulate breast cancer biology. In their characterization of AXL signaling, however, we have uncovered complex functions for AXL which are dependent on its surface concentration and the presence of cooperating molecules: At lower levels, AXL acts solely as a ligand-dependent adhesion molecule. In the presence of putative interacting proteins such as the EGF receptor, a yet unidentified p180 phosphoprotein, or the AXL homologue, SKY, GAS-6 activation of AXL can induce cell survival or mitogenesis. At high concentrations or under other conditions that favor homodimerization, AXL will cause proliferation and transformation through activation of the RAS/MAPK pathway. These molecular interactions may be modulated by enzymatic cleavage of AXL by a calpain protease which may represent a general mechanism for receptor regulation. Thus, AXL is a multifunctional receptor tyrosine kinase involved in cell adhesion, cell viability, and transformation that may play a role in cancer biology, especially in breast carcinogenesis. They wish to define the interactions and the processing of the AXL receptor, and to dissect the role of AXL/GAS-6 in breast carcinogenesis. Specifically, they will determine the biological and the downstream biochemical consequences of AXL interactions with three surface molecules, EGFR, p180, and SKY. Using the yeast two-hybrid system, they have identified seven proteins that putatively bind to the kinase domain. The goal is to link specific biochemical pathways to the different biological functions of this receptor tyrosine lunase: adhesion, enhancement of cell survival, proliferation, transformation, and potentially DNA repair. They also propose to investigate the importance of an AXL protease to AXL function which we believe may be generalized to other RTKs including HER-2/c-erbB2. Finally, they wish to assess the role of AXL and GAS-6 in malignant transformation with a focus on breast cancer development. They will first correlate the level of expression of AXL and GAS-6 in primary breast cancers with other markers of breast cancer behaviour (ER/PR, %S phase, cathepsinD/EGFR expression). Moreover, they will test whether AXL alone or with GAS-6 is capable of inducmg mammary cancers in transgenic mice.
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