Interleukin‐3 (IL‐3) is an important regulator of hemopoiesis and considerable effort has been directed towards the study of its mechanism of signal transduction. In this paper, we describe the first molecular identification of a STAT transcription factor that is activated by IL‐3. STATs exist in a cytoplasmic, transcriptionally inactive form which, in response to extracellular signals, become tyrosine phosphorylated and translocate to the nucleus where they bind to specific DNA elements. Several of these DNA elements were found which bind proteins in an IL‐3‐responsive manner. Analysis of these bandshift complexes with available antibodies to the known STATs suggests that IL‐3 activates the DNA‐binding ability of STAT5, a protein which was originally characterized as a prolactin‐responsive transcription factor in sheep. IL‐5 and granulocyte‐macrophage colony stimulating factor (GM‐CSF), which share a common signaling receptor subunit with IL‐3, also activate STAT5. Unexpectedly, two murine STAT5 homologs, 96% identical to each other at the amino acid level, were isolated and IL‐3‐dependent GAS binding could be reconstituted in COS cells transfected with IL‐3 receptor and either STAT5 cDNA. In IL‐3‐dependent hemopoietic cells, both forms of STAT5 are expressed and activated in response to IL‐3.

Interleukin-10 (IL-10) limits inflammatory responses by inhibiting macrophage activation. In macrophages, IL-10 activates Stat1 and Stat3. We characterized IL-10 responses of the J774 mouse macrophage cell line, and of J774 cells expressing wild-type hIL-10R, mutant hIL-10R lacking two membrane-distal tyrosines involved in recruitment of Stat3 (hIL-10R-TyrFF), a truncated Stat3 (DeltaStat3) which acts as a dominant negative, or an inducibly active Stat3-gyraseB chimera (Stat3-GyrB). A neutralizing anti-mIL-10R monoclonal antibody was generated to block the function of endogenous mIL-10R. IL-10 inhibited proliferation of J774 cells and of normal bone marrow-derived macrophages, but not J774 cells expressing hIL-10RTyrFF. Dimerization of Stat3-GyrB by coumermycin mimicked the effect of IL-10, and expression of DeltaStat3 blocked the anti-proliferative activity of IL-10. For macrophage de-activation responses, hIL10R-TyrFF could not mediate inhibition of lipopolysaccharide-induced TNFalpha, IL-1beta or CD86 expression, while DeltaStat3 did not interfere detectably with these IL-10 responses. Thus signals mediating both anti-proliferative and macrophage de-activation responses to IL-10 require the two membrane-distal tyrosines of IL-10R, but Stat3 appears to function only in the anti-proliferative response.

Fifteen patients with refractory AML were treated in a phase 1 study with SU11248, an oral kinase inhibitor of fms-like tyrosine kinase 3 (Flt3), Kit, vascular endothelial growth factor (VEGF), and plateletderived growth factor (PDGF) receptors. Separate cohorts of patients received SU11248 for 4-week cycles followed by either a 2-or a 1-week rest period. At the starting dose level of 50 mg (n ‫؍‬ 13), no dose-limiting toxicities were observed. The most frequent grade 2 toxicities were edema, fatigue, and oral ulcerations.