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Analysis of antiapoptosis activity of human GM-CSF receptor.

Human GM-CSF (hGM-CSF) induces proliferation and sustains the viability of a mouse IL-3-dependent lymphoid cell line BA/F3 that expresses the functional hGM-CSF receptor (hGMR). To reveal an antiapoptotic mechanism of hGM-CSF, we analyzed various apoptotic markers of BA/F3 cells in various conditions. Within 24 hours of factor depletion, caspase 3-like, but not caspase 1-like, enzyme activity and DNA fragmentation were augmented. Analysis with the tyrosine kinase inhibitor (genistein) and an MEK1 inhibitor (PD98059) on antiapoptosis activity indicates that the activation of either the genistein-sensitive signaling pathway or the PD98059-sensitive signaling pathway of the betac subunit may be sufficient to suppress apoptosis through hGMR. Because hGMR mutants (which activate JAK2 but neither STAT5 nor the MAPK cascade) have antiapoptotic activity in BA/F3 cells, the involvement of JAK2, excluding the molecules mentioned earlier, for antiapoptosis activity seems likely. Because the JAK2 inhibitor AG-490 suppressed the antiapoptotic activity of hGM-CSF, the essential role for JAK2 activation to maintain the viability is considered. Interestingly, hGMR mutants, which lack MAPK cascade activation, require a higher dose of hGM-CSF than that for wild-type hGMR. Because the expression level and affinity to hGM-CSF among wild-type hGMR and mutant hGMR are the same, we speculated that biologic response is determined by a combination of strength of various signaling events.