Erythropoietin (Epo) elicits cell-specific responses upon interacting with its receptor EpoR. Activation of EpoR by Epo results in receptor homodimerization, which induces a cascade of signaling events.Epo is a major activator of the Ras/MAPK pathway. This activation is achieved via signaling through GRB2, SHC, GAB2 and the PTB domain containing proteins. GRB2 constitutively associates with SOS and binds to EpoR either via an interaction with GAB2 or by binding to EpoR-associated tyrosine-phosphorylated SHC. SHC recruits the GRB2-SOS complex to the plasma membrane, thereby promoting Ras activation. PKC, a potent activator of Ras and c-Raf, plays an important role in the Epo-induced activation of MAPK. Raf1 inhibits the activity of VDAC and functions as an important Anti-apoptotic factor. Activated c-Raf phosphorylates the dual functional protein kinases MEKs, which then phosphorylate and activate ERK/MAPKs. Activated ERKs in turn enhance transcriptional activity of Elk1 and CREB, which regulate the early gene expression in erythroid cells.
Epo activates MAPK through a novel pathway dependent on Gi association to EpoR. Following Epo stimulation, Gi is released from the receptor, resulting in subsequent activation of MAPKs. The tyrosine kinase JAK2 contributes to this pathway by acting downstream of GNβ-γ and upstream of Ras and PI3K. Epo signaling also activates the Ras/MAPK/ERK pathway by the use of a third group of interacting partners other than the SHC/GRB2/SOS complex or the Gi proteins. The adaptor protein CrkL, interacts with and recruits C3G to the activated EpoR. C3G independently initiates Epo-mediated MAPK signaling via the activation of Ras. Activated Ras then stimulates the Raf/MEK/ERK signaling cascade leading to the activation of Elk1, which enhances transcriptional activity of immediate early factors c-Fos and c-Jun through the serum response element. CrkL and C3G are also implicated in c-Raf activation via a Ras/PI3K-dependent mechanism involving the activation of Rac.
The transcriptional activation and stability of c-Jun is accomplished by JNK1, CKII and PPtase, whereas in case of c-Fos, ERK phosphorylates c-Fos, resulting in its increased transcriptional activity. PIN1 regulates AP-1-mediated transcription. As a consequence, the mitogenic action of Epo is directly linked to transcriptional regulatory events utilizing Ras as a molecular switch relaying upstream tyrosine kinase signals to a serine/threonine kinase cascade. Active ERKs also activate Cyclin D1, the critical regulator of the cell-cycle clock apparatus directing the G1-S phase progression during the cell cycle.
Activated EpoR ensures its control over this trail via Src tyrosine kinase. JAK2-activated FAK1 activates PI3K in a Syk-dependent manner. Activated PI3K phosphorylates membrane-bound PIP2 to generate PIP3, and PTEN acts as a negative regulator of this phosphorylation event. As a result, PI3K triggers the phosphorylation of Akt via PDK-1. Akt phosphorylates various signaling molecules such as GSK3, BAD and FkhR to promote cell survival. Akt prevents the nuclear translocation of FoxO3A, which inhibits apoptosis. BAD phosphorylation also stimulates certain anti-apoptotic signals that facilitate inhibition of Cytochrome C release and inactivation of caspase 1, 3, 8 and 9. Optimal function of GATA1 is essential for the biological function of Epo, and GATA1 plays a role at a number of steps of the erythroid differentiation program. (Upgraded 07/2020)