Expression of receptor tyrosine kinase Axl and its ligand Gas6 in rheumatoid arthritis: evidence for a novel endothelial cell survival pathway.

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.