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The SH2 domain-containing protein tyrosine phosphatase SHP-1 is expressed widely in the hematopoietic system. SHP-1 has been shown to negatively control signal transduction from many cytokine receptors by direct docking to either the receptor itself, or to members of the Jak family of tyrosine kinases which are themselves part of the receptor complex.

The membrane-distal region of the cytoplasmic domain of human granulocyte colony-stimulating factor receptor (G-CSF-R) contains four conserved tyrosine residues: Y704, Y729, Y744, and Y764. Three of these (Y729, Y744, and Y764) are located in the C-terminal part of G-CSF-R, previously shown to be essential for induction of neutrophilic differentiation. To determine the role of the tyrosines in G-CSF–mediated responses, we constructed tyrosine-to-phenylalanine (Y-to-F) substitution mutants and expressed these in a differentiation competent subclone of 32D cells that lacks endogenous G-CSF-R. We show that all tyrosines can be substituted essentially without affecting the differentiation signaling properties of G-CSF-R. However, substitution of one specific tyrosine, ie, Y764, markedly influenced proliferation signaling as well as the timing of differentiation. 32D cells expressing wild-type (WT) G-CSF-R (or mutants Y704F, Y729F, or Y744F) proliferated in G-CSF–containing cultures until day 8 and then developed into mature neutrophils. In contrast, 32D/Y764F cells arrested in the G1 phase of the cell cycle within 24 hours and showed complete neutrophilic differentiation after 3 days of culture. This resulted in an average 30-fold reduction of neutrophil production as compared with the 32D/WT controls. Importantly, G-CSF–mediated activation of Shc, p21Ras and the induction of c-myc were severely reduced by substitution of Y764. These findings indicate that Y764 of G-CSF-R is crucial for maintaining the proliferation/differentiation balance during G-CSF–driven neutrophil development and suggest a role for multiple signaling mechanisms in maintaining this balance.

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