P. Mccloskey et al., ACTIVATION OF THE AXL RECEPTOR TYROSINE KINASE INDUCES MITOGENESIS AND TRANSFORMATION IN 32D CELLS, Cell growth & differentiation, 5(10), 1994, pp. 1105-1117
axl is a transforming receptor tyrosine kinase isolated from DNA of pa
tients with chronic myelogenous leukemia. Association of axl expressio
n with myelogenous leukemias and its expression in primitive hematopoi
etic cells suggests a role for axl in myeloid biology. To study the ce
llular function of axl, we constructed a chimeric receptor tyrosine ki
nase composed of the extracellular and transmembrane domains of the EG
F receptor and the cytoplasmic domain of axl; this chimera was named E
AK for EGFR-Axl-Kinase. The EAK chimeric receptor was expressed in the
mouse myeloid progenitor cell line 32D, which is dependent on interle
ukin 3 (IL-3) for proliferation and survival. Treatment of the 32D-EAK
cells with EGF stimulated the tyrosine phosphorylation of the axl kin
ase domain and enabled proliferation through EGF rather than IL-3. Thu
s, axl can effectively couple with mitogenic signaling pathways intrin
sic to 32D myeloid cells. Assay of proteins phosphorylated in response
to different cytokine treatments showed that IL-3 and EGF exposure pr
oduced unique profiles in the 32D-EAK cells. Furthermore, jak-2 is pho
sphorylated only in response to IL-3 treatment in these cells. This su
ggests that IL-3 receptor and axl transduce mitogenic signals through
separate pathways. In addition, exposure of cells expressing the chime
ric receptor to EGF for 19 days converted the cells to factor-independ
ent growth, a phenomenon not seen with other receptor tyrosine kinases
. Generation of this transformed phenotype is absolutely dependent on
axl activation by foster ligand. The tyrosine phosphorylation level of
the axl kinase domain in the factor-independent subclones is 40-fold
greater than the factor-dependent cells. The association of a unique a
xl phosphorylation level with the factor-independent phenotype suggest
s that there is a threshold phosphorylation level of the axl kinase fo
r transformation. The fact that activation of the axl receptor leads t
o transformation of 32D cells suggests that axl can play a role in leu
kemic conversion of myeloid cells, either through inappropriate expres
sion or improper activation.