Activation of Akt kinase by granulocyte colony-stimulating factor (G-CSF): evidence for the role of a tyrosine kinase activity distinct from the janus kinases

Abstract

Activation of the serine/threonine kinase Akt has been shown to be a critical component for growth factor and cytokine stimulation of cell survival. Although some of the immediate upstream activators of Akt have been defined, the roles of tyrosine kinases in the activation of Akt are not well delineated. Granulocyte colony-stimulating factor (G-CSF) regulates the proliferation, differentiation, and survival of neutrophilic granulocytes. G-CSF exerts its actions by stimulating several signaling cascades after binding its cell surface receptor. Both Jak (Janus) and Src families of tyrosine kinases are stimulated by incubation of cells with G-CSF. In this report, we show that G-CSF stimulation of cells leads to activation of Akt. The membrane-proximal 55 amino acids of the G-CSF receptor cytoplasmic domain are sufficient for mediating Akt activation. However, activation of Akt appears to be downregulated by the receptor's carboxy-terminal region of 98 amino acids, a region that has been shown to be truncated in some patients with acute myeloid leukemia associated with severe congenital neutropenia. Furthermore, we demonstrate that G-CSF–induced activation of Akt requires the activities of Src family kinases but can be clearly dissociated from G-CSF–stimulated activation of Stats (signal transducers and activators of transcripton) by the Jak kinases. Thus, cytokine activation of the Jak/Stat and other signaling cascades can be functionally separated.