Signaling through the ARK tyrosine kinase receptor protects from apoptosis in the absence of growth stimulation

Abstract

ARK (AXL) is the prototype of a distinctive family of receptor tyrosine kinases which contain in their extracellular domains features reminiscent of cell adhesion molecules. ARK is capable of homophilic binding, which results in a degree of receptor activation, but can also be activated by a heterophilic ligand, Gas6, a member of the family of vitamin K dependent proteins that is preferentially expressed in quiescent cells. Since a number of tissues and cell lines express both ARK and Gas6, we studied the e ect of endogenous and exogenous Gas6 on the phenotype of ARK expressing cells. Here we show that constitutive expression of Gas6 in an NIH3T3 cell line that does not spontaneously express this protein does not result in cell transformation or uncontrolled growth, but protects from apoptosis induced by serum deprivation. Recombinant exogenous Gas6 was also capable of protecting cells from apoptosis at concentrations that did not result in signi®cant induction of DNA synthesis. Activation of ARK phosphorylation and a weak but signi®cant induction of MAP kinase activity accompanied the increased survival of cells treated with Gas6. The antiapoptotic e ect of ARK signaling was con®rmed by studies using ®broblasts from ARK knock-out mice, that showed that the absence of ARK resulted in higher levels of serum deprivation-induced apoptosis, that could not be rescued by the addition of Gas6. Interestingly ARK signaling protects from apoptosis induced by serum deprivation, myc overexpression, or by TNFa but not from u.v. irradiation or Staurosporine. These results suggest that a major function of Gas6 ± ARK signaling is that of increasing cell survival under conditions which do not allow cell proliferation.

Gas6-mediated survival in NIH3T3 cells activates stress signalling cascade and is independent of Ras

“…In addition to a weak mitogenic signal, the most prominent Gas6 feature is its ability to protect the cells from apoptosis induced by growth factor withdrawal (Goruppi et al, 1996;Nakano et al, 1996), TNFa (Bellosta et al, 1997) and several other genotoxic stress (SG unpublished results). Since Gas6 expression is enhanced at growth arrest (Man®oletti et al, 1993), it is likely that its survival-enhancing activity should be most relevant for quiescent cells, possibly indicating that signals departing from Axl act by interfering with speci®c elements involved in the apoptotic process.…”

“…The biological system used for our analysis consisted of serum starved NIH3T3 ®broblasts induced to die by complete growth factor removal from medium. Under these conditions Gas6 lacks any mitogenic activity, thus allowing the speci®c analysis of its survival pathway uncoupled from its mitogenic stimulation (Bellosta et al, 1997;Goruppi et al, 1996).…”

“…Gas6 was reported to e ciently prevent apoptosis induced by complete serum withdrawal from serum starved NIH3T3 cells (Bellosta et al, 1997;Goruppi et al, 1996). Such survival activity was shown to be uncoupled from mitogenesis since Gas6 was not able to induce BrdU incorporation in the complete absence of serum (Goruppi et al, 1996).…”

“…Gas6 was shown to act as a growth factor for ®broblasts, Schwann cells, chondrocytes, thyroid and vascular smooth muscle cells (Goruppi et al, 1996;Li et al, 1996;Loeser et al, 1997;Nakano et al, 1995;Tanaka et al, 1998). In most of the cellular systems Gas6 has been reported to protect cells from apoptosis independently of its growth stimulating activity (Avanzi et al, 1997;Bellosta et al, 1997;Goruppi et al, 1996;, therefore suggesting that induction of proliferation and prevention of apoptosis could involve elements in distinct (parallel) pathways.…”

Mechanisms of apoptosis by c-Myc

“…Tissue speci®c di erences are again hinted at because TGF-a mediates its e ects through the EGF receptor, but EGF was not found to promote survival in Rat1 cells (Harrington et al, 1994a). Lastly, activation of the tyrosine kinase receptor Ark (Axl) by its ligand Gas6 (a member of the vitamin K-dependent family of proteins preferentially expressed in quiescent cells) has been reported to suppress apoptosis by c-Myc (Bellosta et al, 1997). No mechanistic information is available concerning the signaling pathways used by TGF-a or Gas6/ARK.…”