Pathway of the Month
- Axl belongs to the TAM family of receptor tyrosine kinases, which consists of Tyro3, Axl, and Mer. All three family members have similar structures and share a number of ligands, including the vitamin K–dependent ligands growth arrest protein 6 (Gas6) and protein S. In normal tissues, TAM receptor tyrosine kinases contribute to immune response regulation, including clearance of apoptotic cells and inhibition of cytotoxic immune activation in response to apoptosis. When cells undergo apoptosis, the polarity of the plasma membrane lipid bilayer is altered, externalizing the anionic phospholipid phosphatidylserine (PS).
- Angiogenesis, the growth of new capillary blood vessels, is critical for the growth and metastatic spread of tumors.1 The search for tumor-derived factors that stimulate angiogenesis led to the identification of vascular endothelial growth factor (VEGF, also known as VEGF-A) as an endothelial mitogen.2 This protein had previously been identified as vascular permeability-inducing factor secreted by tumor cells (VPF, for vascular permeability factor).3 The VEGF gene undergoes alternative splicing to yield at least five different isoforms, ranging in size from 121 to 206 amino acids, of which VEGF165 is the predominant form.