Development - EPO-induced MAPK pathway

EPO-induced MAPK pathway

Erythropoiesis is the main pathway of pluripotent hematopoietic stem cell development into mature end stage cells. Erythropoietin (Epo) is a major 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], 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 Epo receptor [5], [6], [7]. Activated JAK2 initiates Epo-induced JAK/STAT signaling [8].

Once phosphorylated Epo receptor recruits and activates a number of downstream adaptors and effectors including MAPK (mitogen-activated protein kinase) cascade and JNK (Jun N-terminal Kinases) pathway [9].

Activated Epo receptor complex recruits adapter proteins Shc and GRB2, leading to activation of the classical Shc / GRB2/ SOS/ Ras/c-Raf-1/ MEK/ ERK cascade, which is involved in cell proliferation [10].

Epo receptor, adapter protein Shc and phosphatidyl-inositol polyphosphate 5-phosphatase SHIP form a complex in hematopoietic cells, and SHIP is involved in Epo-induced signaling as an adapter protein [11], [12].

Transmembrane receptor c-Kit is the receptor for mast cell growth factor (MGF). c-Kit can interact with and phosphorylate the Epo receptor, resulting in enhanced erythroid cell differentiation and proliferation [10].

Epo receptor transduces signals by activating physically associated tyrosine kinases, mainly JAK2 and Lyn, and, thereby, inducing tyrosine phosphorylation of various substrates including the Epo receptor itself, adapter proteins CrkL and c-Cbl and tyrosine kinases Syk and Btk [13], [14], [15], [16]. CrkL associates with Shc and c-Cbl in hematopoietic cells. CrkL is constitutively associated with guanine nucleotide exchange factor C3G, that also binds Shc [17]. C3G modulates activity of the Ras family GTPases, such as RAP-1A [18], which, in turn, inhibits c-Raf-1 kinase signaling [19].

Phospholipase C (PLC-gamma 1) is an adapter protein which is rapidly tyrosine phosphorylated (e.g. by Btk kinase) upon Epo stimulation. PLC-gamma 1 interacts with GRB2 and SOS2, leading to activation of the ERK pathway [20].

Phosphorylated by Btk, Syk and Lyn, phospholipase PLC-gamma also triggers activation of several isoforms of protein kinase C, including PKC-alpha and PKC-epsilon, via diacylglycerol (DAG) production and Ca(2+) influx [21], [22], [23], [24]. PKC isoforms, in turn, can phosphorylate c-Raf-1 kinase, leading to MAPK cascade activation [25], [26].

In primary human erythroid progenitor cells, phosphatidylinositol-3 kinase, PI3K-gamma, is activated by low concentration of Epo. MEK1 and ERK1/2 kinases, through a Raf-independent signaling pathway, are signal mediators of PI3K-gamma [25]. PI3K-gamma is activated, probably, by H-Ras [27] and/or G-proteins beta/gamma [28].

Epo stimulation activates guanine nucleotide exchange factor VAV-1 [29]. VAV-1 is involved in proliferative signals in hematopoietic cells via activation of small GTPase Rac1 that stimulates both JNK1-3 kinases and MEK (MEK1 and MEK2)/ERK1/2 cascade [26], [30], [31], [32], [33], [34]. JNK1-3 and ERK1/2 kinases, in turn, phosphorylate transcription factors Elk-1, c-Jun and c-Fos that are crucial for cell proliferation [35], [36], [37].

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