AXL Receptor Tyrosine Kinase
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Tyrosine-protein kinase receptor UFO is a protein that in human is encoded by the ''AXL''
gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
. The gene was initially designated as UFO, in allusion to the unidentified function of this protein. However, in the years since its discovery, research into AXL's expression profile and mechanism has made it an increasingly attractive target, especially for cancer therapeutics. In recent years, AXL has emerged as a key facilitator of immune escape and drug-resistance by cancer cells, leading to aggressive and metastatic cancers. AXL is a cell surface
receptor Receptor may refer to: * Sensory receptor, in physiology, any neurite structure that, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and respond ...
tyrosine kinase, part of the TAM family of kinases including TYRO3 and MERTK.


Gene

The Axl gene is evolutionarily conserved between vertebrate species. Human AXL is in close vicinity to the BCL3 oncogene, which is at 19q13.1-q13.2. This gene has two different alternatively spliced transcript variants.


Structure

The protein encoded by this gene is a member of the
receptor tyrosine kinase Receptor tyrosine kinases (RTKs) are the high-affinity cell surface receptors for many polypeptide growth factors, cytokines, and hormones. Of the 90 unique tyrosine kinase genes identified in the human genome, 58 encode receptor tyrosine kinas ...
subfamily. Although it is similar to other receptor tyrosine kinases, the Axl protein represents a unique structure of the extracellular region that juxtaposes IgL and FNIII repeats. The AXL protein is characterized by an extracellular structure consisting of two fibronectin type 3-like repeats and two immunoglobulin-like repeats along with its intracellular tyrosine kinase domain.


Function

The AXL receptor transduces signals from the
extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix (ICM), is a network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide structural and bio ...
into the
cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...
by binding growth factors like vitamin K-dependant protein growth-arrest-specific gene 6 ( GAS6). It is involved in the stimulation of
cell proliferation Cell proliferation is the process by which ''a cell grows and divides to produce two daughter cells''. Cell proliferation leads to an exponential increase in cell number and is therefore a rapid mechanism of tissue growth. Cell proliferation ...
,
migration Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...
, differentiation and
survival Survival or survivorship, the act of surviving, is the propensity of something to continue existing, particularly when this is done despite conditions that might kill or destroy it. The concept can be applied to humans and other living things ...
. Activation of Axl leads to autophosphorylation of its intracellular domain. Proteolytic cleavage of the AXL extracellular domain by the metalloproteinases ADAM10 and
ADAM17 A disintegrin and metalloprotease 17 (ADAM17), also called TACE (''tumor necrosis factor-α-converting enzyme''), is a 70-kDa enzyme that belongs to the ADAM protein family of disintegrins and metalloproteases, activated by substrate presentatio ...
can downregulate this signalling activity. Signalling pathways activated downstream of AXL include PI3K-AKT-mTOR, MEKERK, NF-κB, and JAK/STAT. This receptor can also mediate cell aggregation by homophilic binding. AXL protein is expressed in normal tissues, particularly in bone marrow stroma and myeloid cells, and in tumour cells and tumour vasculature. In cancer, AXL is expressed on the tumor cells as well as adjacent immune cells including
dendritic cell A dendritic cell (DC) is an antigen-presenting cell (also known as an ''accessory cell'') of the mammalian immune system. A DC's main function is to process antigen material and present it on the cell surface to the T cells of the immune system ...
s,
macrophage Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...
s, and NK cells. Axl is an inhibitor of the innate immune response. The function of activated AXL in normal tissues includes the efficient clearance of apoptotic material and the dampening of TLR-dependent inflammatory responses and natural killer cell activity. Axl expression on dendritic cells has been shown to limit their maturation and functions by limiting cholesterol mobilization for the assembly of lipid nanodomains at the cell surface. AXL is a putative driver of diverse cellular processes that are critical for the development, growth, and spread of tumours, including proliferation, invasiveness and migration, epithelial-to-mesenchymal transition, stemness, angiogenesis, and immune modulation. AXL has been implicated as a cancer driver and correlated with poor survival in numerous aggressive tumors including triple-negative breast cancer (TNBC), acute myeloid leukemia (AML), non-small-cell lung cancer (NSCLC), pancreatic cancer and ovarian cancer, among others.


Clinical significance

Axl was first isolated in 1988 and identified as an oncogene in a screen for transforming genes in patients with a
chronic myelogenous leukemia Chronic myelogenous leukemia (CML), also known as chronic myeloid leukemia, is a cancer of the white blood cells. It is a form of leukemia characterized by the increased and unregulated growth of myeloid cells in the bone marrow and the accumula ...
- that progressed to 'blast crisis'. Since then, increased AXL expression has been associated with numerous cancers including
lung cancer Lung cancer, also known as lung carcinoma, is a malignant tumor that begins in the lung. Lung cancer is caused by genetic damage to the DNA of cells in the airways, often caused by cigarette smoking or inhaling damaging chemicals. Damaged ...
,
breast cancer Breast cancer is a cancer that develops from breast tissue. Signs of breast cancer may include a Breast lump, lump in the breast, a change in breast shape, dimpling of the skin, Milk-rejection sign, milk rejection, fluid coming from the nipp ...
,
pancreatic cancer Pancreatic cancer arises when cell (biology), cells in the pancreas, a glandular organ behind the stomach, begin to multiply out of control and form a Neoplasm, mass. These cancerous cells have the malignant, ability to invade other parts of ...
,
ovarian cancer Ovarian cancer is a cancerous tumor of an ovary. It may originate from the ovary itself or more commonly from communicating nearby structures such as fallopian tubes or the inner lining of the abdomen. The ovary is made up of three different ...
,
colon cancer Colorectal cancer (CRC), also known as bowel cancer, colon cancer, or rectal cancer, is the development of cancer from the colon or rectum (parts of the large intestine). Signs and symptoms may include blood in the stool, a change in bowel ...
and
melanoma Melanoma is the most dangerous type of skin cancer; it develops from the melanin-producing cells known as melanocytes. It typically occurs in the skin, but may rarely occur in the mouth, intestines, or eye (uveal melanoma). In very rare case ...
among others, and shown to have a strong correlation with poor survival outcomes. AXL has been shown to be a key driver of drug-resistance to targeted therapies, immuno therapies and chemotherapy in various animal models. Based on current knowledge of AXL's role in therapy resistance, future studies will help to determine whether AXL has a translational application as a biomarker for predicting therapeutic response to established drugs. Recently, AXL has been implicated in chronic fibrotic diseases in several organs, including the liver. AXL also play an important role in
Zika virus Zika virus (ZIKV; pronounced or ) is a member of the virus family ''Flaviviridae''. It is spread by daytime-active ''Aedes'' mosquitoes, such as '' A. aegypti'' and '' A. albopictus''. Its name comes from the Ziika Forest of Uganda, where ...
and
SARS-CoV-2 Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is a strain of coronavirus that causes COVID-19, the respiratory illness responsible for the COVID-19 pandemic. The virus previously had the Novel coronavirus, provisional nam ...
infection, allowing for entry of the virus into host cells. This phenomenon is known to rely on
phosphatidylserine Phosphatidylserine (abbreviated Ptd-L-Ser or PS) is a phospholipid and is a component of the cell membrane. It plays a key role in cell cycle signaling, specifically in relation to apoptosis. It is a key pathway for viruses to enter cells via a ...
incorporated in the viral envelope during egress, which then binds to AXL via the adapter GAS6. AXL mediates internalization into the
endosome Endosomes are a collection of intracellular sorting organelles in eukaryotic cells. They are parts of the endocytic membrane transport pathway originating from the trans Golgi network. Molecules or ligands internalized from the plasma membra ...
from which these viruses escape and initiate replication.


As a drug target

Studies have shown that AXL knockdown leads to downregulation of transcription factors required for EMT, including Slug, Twist, and Zeb1, and to increased expression of E-cadherin.


Cancer

Several drugs classified as "AXL inhibitors" have entered clinical trials; however, many target multiple kinase receptors in addition to AXL. The most advanced AXL ''selective'' inhibitor is bemcentinib (BGB324 or R428), an oral small molecule currently in multiple Phase II clinical trials for NSCLC, TNBC, AML and melanoma. Bemcentinib is being pursued as monotherapy and as combination therapy with existing and emerging targeted therapies, immunotherapies and chemotherapy. A monoclonal antibody targeting AXL (YW327.6S2) and an AXL decoy receptor (GL2I.T) are currently in preclinical development. Additionally, an oral AXL inhibitor (TP-0903) is expected to enter Phase 1 clinical trial in November 2016 (in advanced solid tumours: NCT02729298). Astellas Pharma is currently testing gilteritinib (ASP2215), a dual FLT3-AXL tyrosine kinase inhibitor in
acute myeloid leukemia Acute myeloid leukemia (AML) is a cancer of the myeloid line of blood cells, characterized by the rapid growth of abnormal cells that build up in the bone marrow and blood and interfere with haematopoiesis, normal blood cell production. Sympt ...
(AML). In 2017, gilteritinib gained FDA orphan drug status for AML. These approved drugs and ongoing and pending clinical trials highlight the potentially wide-ranging safety and efficacy of AXL inhibition.


Interactions

AXL receptor tyrosine kinase has been shown to interact with TENC1. Also, it interacts with CBL, GRB2, LCK, NCK2, PIK3R1, PIK3R2, PIK3R3, PLCG1, SOCS1, and TNS2.


References


Further reading

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External links

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