Development - EPO-induced Jak-STAT pathway

EPO-induced Jak-STAT pathway

Erythropoiesis is a major pathway by which a pluripotent hematopoietic stem cell gives rise to mature end stage cells. Erythropoietin (EPO) is a lineage-specific hematopoietic growth factor required for survival, proliferation and differentiation of committed erythroid progenitor cells [1], [2], [3].

EPO exerts its effect by binding to the EPO receptor [4], which is composed of two identical subunits. Upon ligand binding, the two subunits dimerize and Janus kinase JAK2 is recruited to the receptor complex resulting in the phosphorylation of several tyrosine residues on the receptor. These phosphorylated tyrosine residues form docking sites for several molecules, including the signal transducer and activator of transcription (STAT) transcription factors. The STAT molecules are phosphorylated on a single tyrosine residue by JAK kinases, leading to dimerization and subsequent translocation to the nucleus where they act as transcription factors [5]. STAT5 factors (STAT5A and STAT5B) are the most prominent STAT proteins activated by EPO receptor [6], [7]. STAT3 and STAT1 are also, albeit to a lesser extent, activated by EPO stimulation [8], [9], [10].

CrkL, an adapter protein that is activated by binding to the EPO receptor, can directly associate with STAT5A and inhibit its DNA binding activity [11], [12]. Kinases c-Src and JAK2 cooperatively tyrosine-phosphorylate STAT5 [13].

EPO-induced activation of STATs is mainly involved in cell survival via up-regulation of the expression of antiapopototic factors, such as Bcl-XL, XIAP, Pim-1, Bcl-2 and transcription factor c-Myc [10], [14], [15], [16], [17].

Protein kinase Lyn directly associates with EPO receptor, binds to JAK2, and phosphorylates EPO receptor and STAT5 at tyrosine residues, thus playing a role in activation of JAK2/STAT5 signaling [18].

Calcium-binding protein Calmodulin physically interacts with EPO receptor and enhances JAK2-mediated signaling [19].

EPO also activates the canonical kinase cascade H-Ras - c-Raf-1- MEK1/2 - ERK1/2 by recruiting Shc/GRB2/Sos complex to the EPO receptor [20], [21].

Besides tyrosine phosphorylation of STAT3 by JAK2, leading to nuclear translocation, STAT3 proteins can be serine-phosphorylated by MSK1 kinase, which is activated upon MEK1/2 - ERK1/2 signal transduction cascade. It was demonstrated that serine phosphorylation of STAT3 augments its transactivational potential in erythroid cells [22].

c-Fes tyrosine kinase is also involved in EPO-induced activation of STAT3 [9], [23].

Expression of suppressors of cytokine signaling proteins SOCS1 and SOCS3 (, or CISH (Cytokine-inducible SH2-containing protein) negatively regulates EPO-mediated Jak-STAT signaling [24], [25]. On the other hand, SOCS3 binds to the GTPase activating protein p120GAP that attenuates H-Ras inhibition and activates MEK1/2 - ERK1/2 cascade to ensure cell survival and proliferation [26].

The adaptor protein APS is tyrosine phosphorylated by JAK2, followed by recruitment of the adaptor protein c-Cbl into the EPO receptor/JAK2 complex, which inhibits the JAK-STAT pathway [27].

Protein tyrosine phosphatases SHP-1 and SHP-2 are recruited to the EPO receptor/JAK2 complex and dephosphorylate JAK2, inhibiting JAK-STAT signaling [28], [29], [30]. SHP-1 is critical in negative regulation of STAT5 activation. SHP-1 is recruited to EPO receptor/JAK2 complex that results in the recruitment of Grb2 and its associated protein SOCS1. SOCS1 is targeted to JAK2, leading to JAK2/STAT5 signaling inactivation [31].

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