Protein tyrosine kinases in granulocyte colony stimulating factor receptor signal transduction, myeloid cell proliferation, and neutrophil activation

Granulocyte colony-stimulating factor activates protein kinase A in granulocytic but not monocytic precursors or neutrophils

Granulocyte colony-stimulating factor (G-CSF) regulates survival, proliferation, differentiation, and activation of myeloid cells. G-CSF-R signaling mechanisms other than tyrosine kinase activation have not been documented. We explored the potential involvement of cAMP-dependent protein kinase A (PKA) in G-CSF-R signal transduction. In this report, we provide the first direct evidence of PKA modulation by G-CSF-R. G-CSF treatment of granulocytic precursor cell lines (HL-60, NFS-60, KG-1) resulted in PKA activation, measured by phosphorylation of Kemptide, a peptide substrate. In contrast, the myelomonocytic cell lines (WEHI-3B,U-937) and peripheral blood neutrophils (PMNC) showed a rapid decrease in PKA activity in response to G-CSF. H-89, a specific inhibitor of PKA, blocked G-CSF-induced PKA activation in HL-60 cells but did not affect ligand-induced downmodulation of G-CSF-R. Indomethacin, an inhibitor of the cyclooxygenase pathway and prostaglandin synthesis, did not inhibit PKA induction in G-CSF-treated HL-60 cells. Our results demonstrate the involvement of PKA in G-CSF-R signal transduction and suggest a lineage-restricted, developmental stage-specific regulation of this pathway in myeloid cells.

G-CSF receptor-mediated up-regulation of c-fos but not c-raf mRNA expression in myeloid cells

Granulocyte colony stimulating factor (G-CSF) regulates survival, proliferation, and differentiation of myeloid precursor cells. One of the signaling mechanisms for the G-CSF receptor (G-CSF-R) involves the activation of Ras/MAP kinase (MEK) pathway. Since Raf is an important, common link between Ras and MEK in the Ras-Raf-MEK cascade, we studied the expression of c-raf mRNA in G-CSF-treated myeloid precursor cell lines--NFS-60 and HL-60. G-CSF did not alter c-raf mRNA expression in these cells up to 24 h, but induced a transient up-regulation of c-fos mRNA expression between 15-60 min post-treatment. Our results show that G-CSF triggers a de novo induction of c-fos but not c-raf mRNA, and suggests that G-CSF-R-mediated activation of Ras/MEK pathway may involve post-transcriptional mechanisms of Raf regulation.

Granulocyte colony-stimulating factor-induced activation of protein kinase-C in myeloid cells

Granulocyte colony stimulating factor (G-CSF) regulates survival, proliferation, differentiation, and activation of myeloid cells. It binds to a high affinity receptor (G-CSF-R) expressed on myeloid cells, for which the signal transduction mechanisms other than protein tyrosine kinase (PTK) activation have not been completely identified. We explored the potential involvement of protein kinase-C (PKC) in G-CSF-R signal transduction. In this report, we provide direct evidence of PKC activation by G-CSF-R. G-CSF treatment of peripheral blood neutrophils, granulocytic cell lines (HL-60, NFS-60, KG-1), and monocytic cell lines (WEHI-3B,U-937) resulted in PKC activation. Chelerythrine chloride and HA-100, an isoquinolinesulfonamide derivative, the specific inhibitors of PKC, 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid (BAPTA), a chelator of intracellular calcium, and 3,4,5-trimethoxybenzoic acid 8-(diethylamino)-octyl ester (TMB-8), an inhibitor of intracellular calcium release, blocked G-CSF-induced PKC activation in HL-60 cells, and reduced CD11b upregulation in neutrophils, but did not affect ligand-binding or down-modulation of G-CSF-R. Methyl 2,5-dihydroxycinnamate (MDHC), a potent inhibitor of protein tyrosine kinases (PTK), also inhibited PKC activation in response to G-CSF treatment, suggesting that PKC activation may occur downstream of PTK activation. Our results demonstrate the involvement of PKC in G-CSF-R signal transduction, and suggest a common signaling pathway in myeloid cells of granulocytic and monocytic lineages.