Expression of Receptor Tyrosine Kinase Axl and its Ligand Gas6 in Rheumatoid Arthritis

Abstract

Angiogenesis and synovial cell hyperplasia are characteristic features of rheumatoid arthritis (RA). Many growth and survival factors use receptors belonging to the tyrosine kinase family that share conserved motifs within the intracellular catalytic domains. To understand further the molecular basis of cellular hyperplasia in RA, we have used degenerate primers based on these motifs and RNA obtained from the synovium of a patient with RA to perform reverse transcriptase-polymerase chain reaction. We report detection of the receptor tyrosine kinase (RTK) Axl in RA synovium and we document the expression pattern of Axl in capillary endothelium, in vascular smooth muscle cells of arterioles and veins, and in a subset of synovial cells in RA synovial tissue. Gas6 (for growth arrest-specific gene 6), which is a ligand for Axl and is related to the coagulation factor protein S, was found in synovial fluid and tissue from patients with RA and osteoarthritis. Axl expression and function was studied in human umbilical vein endothelial cells (HUVECs). Gas6 bound to HUVECs; soluble Axl inhibited this binding. Exogenous Gas6 protected HUVECs from apoptosis in response to growth factor withdrawal and from TNFalpha-mediated cytotoxicity. These findings may reveal a new aspect of vascular physiology, which may also be relevant to formation and maintenance of the abnormal vasculature in the rheumatoid synovium.

Signal Pathways in Up-regulation of Chemokines by Tyrosine Kinase MER/NYK in Prostate Cancer Cells

“…The MER tyrosine kinase was first described by Graham et al (3), and our laboratory independently cloned it as NYK (4). The AXL family kinases are transcriptionally regulated by extracellular stimuli, showing significant variation in expression among different tissues and different stages of diseases (5)(6)(7). Several functions including cell adhesion, migration, phagocytosis, and apoptosis regulation have been ascribed to this family of tyrosine kinases (8 -14).…”

“…A cDNA fragment encoding GAS6 was subcloned into pcDNA3.1 vector and V5-(His) 6 tagged at the COOH terminus. 293 cells were transfected with the GAS6-V5-(His) 6 plasmid, and supernatants were collected at different time points. The expression of GAS6 in supernatants was detected by anti-V5-Tag antibody (Invitrogen).…”

TheGrowth Arrest-SpecificGene Product Gas6 Promotes the Survival of Human Oligodendrocytes via a Phosphatidylinositol 3-Kinase-Dependent Pathway

“…Gas6 signaling can induce varied responses in specific cell types. Gas6 is mitogenic for fibroblasts and Schwann cells (Goruppi et al, 1996;Li et al, 1996), whereas in rat hippocampal neurons, fibroblasts, and endothelial cells Gas6/Axl signaling protects from apoptosis induced by serum withdrawal or tumor necrosis factor-␣ (Goruppi et al, 1997;O'Donnell et al, 1999;Healy et al, 2001;Funakoshi et al, 2002).…”

Effect of AXL on the epithelial-to-mesenchymal transition in non-small cell lung cancer

“…During EMT, epithelial cells loosen and cell-cell adhesion is lost (12). Furthermore, EMT is characterized by the downregulation of E-cadherin and cytokeratins, and the upregulation of mesenchymal proteins, including vimentin, fibronectin and N-cadherin (13,14). A previous study demonstrated that activation of the epidermal growth factor (EGF) receptor (EGFR) by EGF promoted EMT in hepatocellular carcinoma cell lines by altering the expression levels and molecular structures of EMT-associated markers, including the E-cadherin/β-catenin complex (14).…”

“…Previous studies demonstrated that downregulation of AXL expression in solid tumors by RNA interference (RNAi) was able to decrease cell invasion and proliferation, and increase the chemosensitivity of cells via the activation of AKT and mitogen-activated protein kinase (MAPK) pathways (10,11,13). In addition, AXL was identified as an oncogene in human leukemia cells (14)(15)(16), and the expression levels of AXL were shown to be increased in >50% of NSCLC cell lines (17).…”