Axl as a mediator of cellular growth and survival

Abstract

The control of cellular growth and proliferation is key to the maintenance of homeostasis. Survival, proliferation, and arrest are regulated, in part, by Growth Arrest Specific 6 (Gas6) through binding to members of the TAM receptor tyrosine kinase family. Activation of the TAM receptors leads to downstream signaling through common kinases, but the exact mechanism within each cellular context varies and remains to be completely elucidated. Deregulation of the TAM family, due to its central role in mediating cellular proliferation, has been implicated in multiple diseases. Axl was cloned as the first TAM receptor in a search for genes involved in the progression of chronic to acute-phase leukemia, and has since been established as playing a critical role in the progression of cancer. The oncogenic nature of Axl is demonstrated through its activation of signaling pathways involved in proliferation, migration, inhibition of apoptosis, and therapeutic resistance. Despite its recent discovery, significant progress has been made in the development of effective clinical therapeutics targeting Axl. In order to accurately define the role of Axl in normal and diseased processes, it must be analyzed in a cell type-specific context.

Figure 1. Structures of the TAM receptors and their shared ligand, Gas6

(A) The TAM family of receptors share common extracellular structures, composed of two Ig-like domains for ligand binding and two fibronectin III domains. Axl and Mer have both been shown to yield soluble extracellular fragments by protease cleavage just outside their transmembrane domains. To date, this has not been demonstrated for Tyro3. Potential glycosylation sites are represented on each receptor; Axl, amino acids 43, 157, 198, 339, 345, 401; Tyro3, amino acids 63, 191, 230, 240, 293, 366, 380; Mer, amino acids 114, 170, 207, 215, 234, 294, 316, 329, 336, 354, 389, 395, 442 (confirmed), 454. (B) Gas6 is a vitamin K-dependent protein that binds Axl with higher affinity compared to Tyro3 or Mer. The Gla domain allows for cell membrane contact and the LG domains bind the Ig-like domains of the receptors.

Figure 2. Gas6 activation of Axl leads to homodimerization and activation of downstream signal cascades with functional consequences

The above signaling diagram represents events downstream of Gas6 binding and Axl homodimerization across many cell types. Gas6 binding to Axl creates a major contact formed between the LG1 domain of Gas6 and the Ig-like 1 domain of Axl, and a minor contact between the LG1 domain of Gas6 and the Ig-like 2 domain of Axl. Other ligands not shown: protein S contains the same domains as Gas6, and binds Tyro3 and Mer through its SHGB region; tubby and Tulp1 contain “minimal phagocytic determinants” (MPDs) in their N-termini which are essential for receptor binding; tubby binds Mer and Tulp1 binds all three TAM receptors.

Figure 3. Activation of Axl by heterodimerization with plasma membrane proteins leads to cell-specific consequences

Figure 4. Transcription factor binding to the Axl promoter

Adapted compilation of figures from multiple sources (Mudduluru, 2010; Mudduluru, 2011; Xu, 2011; Mudduluru, 2008; Hong, 2008; Vaughan, 2012). Putative HIF1α binding to HRE sequences is not shown. p53 interaction with CREB complex induces histone acetylation around CRE sites. YAP is a transcriptional cofactor for TEAD. Methylation of CCWGG sites are marked. CpG methylation is not shown, but occurs in 19 CpG sites within nucleotides −669 to −97. CpG methylation also occurs in Sp a, b, and c sites which prevents Sp factors from binding.

Figure 5. Representation of Axl in the blood-forming lineages

Axl contributes to the maintenance of HSCs and helps regulate differentiation of various HSC lineages. Axl plays a major role in the immune response by regulating inflammation and helping to clear apoptotic cells.