DIFFERENTIAL ACTIVATION OF THE RAS EXTRACELLULAR-SIGNAL-REGULATED PROTEIN-KINASE PATHWAY IS RESPONSIBLE FOR THE BIOLOGICAL CONSEQUENCES INDUCED BY THE AXL RECEPTOR TYROSINE KINASE/

To understand the mechanism of Axl signaling, we have initiated studie s to delineate downstream components in interleukin-3-dependent 32D ce lls by using a chimeric receptor containing the recombinant epidermal growth factor (EGF) receptor extracellular and transmembrane domains a nd the Axl kinase domain (EAK [for EGF receptor-Axl kinase]). We have previously shown that upon exogenous EGF stimulation, 32D-EAK cells ar e capable of proliferation in the absence of interleukin-3. With this system, we determined that EAK-induced cell survival and mitogenesis a re dependent upon the Ras/extracellular-signal-regulated protein kinas e (ERK) cascade. Although the phosphatidylinositol-3 kinase pathway is activated upon EAK signaling, it appears to be dispensable for the bi ological actions of the Axl kinase. Furthermore, we demonstrated that different threshold levels of Ras/ERK activation are needed to induce a block to apoptosis or proliferation in 32D cells. Recently, we have identified an Axl ligand, GAS6. Surprisingly, GAS6-stimulated 32D-Axl cells exhibited no blockage to apoptosis or mitogenic response which i s correlated with the absence of Ras/ERK activation. Taken together, t hese data suggest that different extracellular domains dramatically al ter the intracellular response of the Axl kinase. Furthermore, our dat a suggest that the GAS6-Axl interaction does not induce mitogenesis an d that its exact role remains to be determined.