Interleukin-3 (IL-3) stimulates in vitro blast cell proliferation in a
consistent proportion of acute myeloid leukemia (AML) cases, however
the degree of response varies from case to case and it is not related
to the FAB subtype or to other clinical parameters, IL-3-induced proli
feration of myeloid cells is mediated by the interaction with an heter
odimeric receptor (IL-3R) comprised of a ligand binding subunit denote
d alpha and a common transducing subunit designated as beta (beta(c)).
Ligand binding to the receptor activates a number of signaling molecu
les including proteins of the STATs (signal transducing and activators
of transcription) family, To elucidate the mechanisms responsible for
the abnormal proliferative response of AML cells to IL-3, we evaluate
d, both in the IL-3-dependent M-07e cell line and in 20 AML cases, the
activation of STAT1 p91 and its association with the beta(c) subunit,
On the basis of the in vitro proliferation assay, 11 out of 20 cases
were found to be responsive to IL-3 and eight out of 16 to GM-CSF, Our
results demonstrated that in M-07e cells and in six AML cases (five I
L-3 responsive and one unresponsive) p91 tyrosine phosphorylation was
ligand dependent, Ligand independent p91 tyrosine phosphorylation was
detected in 10 AML cases (five responsive and five unresponsive), p91
association with the beta(c) subunit was consistent with its ligand de
pendent activation and with the ability to form a DNA-binding complex
containing p91, In the remaining four cases (three unresponsive and on
e responsive) no p91 tyrosine phosphorylation and/or association were
detected, These findings, together with the observation that in five I
L-3 responsive cases p91 was constitutively phosphorylated, suggest th
at IL-3-mediated AML proliferation is only partially sustained by p91
activation and that other post-receptor molecules are required to achi
eve maximal proliferative response, Moreover structural abnormalities
of the receptor or of post-receptor signaling proteins may account for
the constitutive p91 phosphorylation and growth factor independent pr
oliferation observed in the unresponsive AML cases.