Tyrosine-dependent and -independent mechanisms of STAT3 activation by the human granulocyte colony-stimulating factor (G-CSF) receptor are differentially utilized depending on G-CSF concentration

The granulocyte colony-stimulating factor receptor (G-CSF-R) activates mult iple STAT proteins. Although the membrane-proximal cytoplasmic region of th e G-CSF-R is necessary and sufficient for activation of STAT1 and STATE, ac tivation of STATE requires the membrane distal region that contains four ty rosines. Although one of these (Y704) has previously been shown to be invol ved in STATE activation from a truncated G-CSF-R derived from a patient wit h severe chronic neutropenia (SCN), this tyrosine is not required for STATE activation by the full-length G-CSF-R. To investigate possible alternative mechanisms of STATE activation, we generated a series of Ba/F3 cell transf ectants expressing the wild-type G-CSF-R or mutant receptors that either co mpletely lack tyrosines or retain just one of the four cytoplasmic tyrosine s of the G-CSF-R. We show that, at saturating G-CSF concentrations, STATE a ctivation from the full-length G-CSF-R is efficiently mediated by the C-ter minal domain in a manner independent of receptor tyrosines. In contrast, at low G-CSF concentrations, Y704 and Y744 of the G-CSF-R play a major role i n STATE activation. Both tyrosine-dependent and -independent mechanisms of STATE activation are sensitive to the Jak2 inhibitor AG-490, follow similar kinetics, and lead to transactivation of a STATE reporter construct, indic ating functional equivalence. STATE activation is also impaired, particular ly at nonsaturating G-CSF concentrations, in bone marrow cells from mice ex pressing a truncated G-CSF-R (gcsfr-Delta 715). These findings suggest that G-CSF-induced STAT3 activation during basal granulopoiesis (low G-CSF) and "emergency" granulopoiesis (high G-CSF) are differentially controlled. In addition, the data establish the importance of the G-CSF-R C-terminus in ST ATE activation in primary cells, which has implications for understanding w hy truncated G-CSF-R derived from SCN patients are defective in maturation signaling. (C) 1999 by The American Society of Hematology.