The cytokine-activated tyrosine kinase JAK2 activates Raf-1 in a p21ras-dependent manner.

Abstract

JAK2, a member of the Janus kinase superfamily was found to interact functionally with Raf-1, a central component of the ras/mitogen-activated protein kinase signal transduction pathway. Interferon-y and several other cytokines that are known to activate JAK2 kinase were also found to stimulate Raf-I kinase activity toward MEK-1 in mammalian cells. In the baculovirus coexpression system, Raf-1 was activated by JAK2 in the presence of p2lras. Under these conditions, a ternary complex of p2lras, JAK2, and Raf-1 was observed. In contrast, in the absence of p2lras, coexpression of JAK2 and Raf-1 resulted in an overall decrease in the Raf-1 kinase activity. In addition, JAK2 phosphorylated Raf-1 at sites different from those phosphorylated by pp6Ov-src. In mammalian cells treated with either erythropoietin or interferon-'y, a small fraction of Raf-1 coimmunoprecipitated with JAK2 in lysates of cells in which JAK2 was activated as judged by its state of tyrosine phosphorylation. Taken together, these data suggest that JAK2 and p21ras cooperate to activate Raf-1.The serine/threonine kinase Raf-1 is activated by numerous growth factors (1, 2) and is believed to play a central role in cell growth and differentiation. Binding of a growth factor to its tyrosine kinase receptor at the cell surface leads to p21ras activation that in turn recruits Raf-1 to the plasma membrane (3, 4) through a direct interaction with the N-terminal region of Raf-1. At the cell membrane, Raf-1 is activated in a p21ras-independent manner. Activated Raf-1 phosphorylates and activates MEK-1, leading to activation of the mitogenactivated protein (MAP) kinase cascade (5, 6). Unlike members of the tyrosine kinase receptor superfamily, the cytokine receptors have no intrinsic tyrosine kinase activity, but are dependent mainly upon the JAK family of tyrosine kinases with which they form a stable complex (7-10). Binding of a cytokine to its receptor causes the receptor to dimerize, thereby activating the associated JAK kinases through tyrosine phosphorylation. Activated JAK kinases in turn phosphorylate a number of important cellular substrates, chief among which are members of the STAT family of transcription factors (11)(12)(13)(14). Previous studies have documented that tyrosine phosphorylation of STATs leads to their oligomerization and is essential for activation of their DNA binding activity (15, 16).However, recently it has become apparent that subsequent phosphorylation of STATs on serine/threonine residues is required for their full activation as transcription factors (17,18). It has also been reported that JAK2, p2lras, and Raf-1 are required for the activation of MAP kinases by growth hormone stimulation (19), and in the interferon (IFN) system, STAT activation requires functionally activated MAP kinases (20). This focuses particular interest on the molecular mechanisms by which cytokine receptor family members couple to Raf-1 and to the MAP kinase cascade. Before the discovery of either MEK-1 as a physiological substrate of Raf-1 or ...

Interferonγ Activation of Raf-1 Is Jak1-dependent and p21ras-independent

“…However, this does not exclude the possibility that Jak1 may function through an intermediary kinase that may be responsible for the activation of Raf-1. Although it has been recently suggested that Jak2 can be coimmunoprecipitated with Raf-1 and p21 ras , this was observed by overexpression in nonmammalian insect cells (38). However, the authors also comment on their inability to show co-immunoprecipitation of Jak2 and Raf-1 in mammalian cells treated with IFN␥ and fail to address the issue of p21 ras -dependence of Raf-1 activation.…”

Meaningful relationships: the regulation of the Ras/Raf/MEK/ERK pathway by protein interactions

“…The other site is tyrosine-341, which is targeted by Src family kinases [37,38]. In addition, abl [39,40] and JAK (Janus kinase) [41,42] family tyrosine kinases also induce Raf-1 activation and tyrosine phosphorylation, but since the phosphorylation sites have not been mapped, it is not known whether they work through tyrosine-341. These tyrosine kinases can be co-immunoprecipitated with Raf-1, and, being found at the cell membrane, they may form part of the activation complex (Table 1 and Figure 4).…”

Prolactin receptor signal transduction in cells of the immune system

“…While at face value the Jak/Stat and Shc/Grb2/Vav/Sos/Ras/Rac/Raf/MEK/MAPK pathways may appear as linear cascades, multiple interactions between the signaling complexes associated with the PRLr may occur. Indeed, the documented interactions between Fyn-Vav, Jak-Fyn, Grb2-Vav, and JAK2-Raf indicate the generation of proximal receptor-based signals occurs as the result of associated multimeric transduction complexes (Xia et al 1996, Wang et al 1997.…”