Erythropoietin: elucidating new cellular targets that broaden therapeutic strategies

Abstract

Given that erythropoietin (EPO) is no longer believed to have exclusive biological activity in the hematopoietic system, EPO is now considered to have applicability in a variety of nervous system disorders that can overlap with vascular disease, metabolic impairments, and immune system function. As a result, EPO may offer efficacy for a broad number of disorders that involve Alzheimer's disease, cardiac insufficiency, stroke, trauma, and diabetic complications. During a number of clinical conditions, EPO is robust and can prevent metabolic compromise, neuronal and vascular degeneration, and inflammatory cell activation. Yet, use of EPO is not without its considerations especially in light of frequent concerns that may compromise clinical care. Recent work has elucidated a number of novel cellular pathways governed by EPO that can open new avenues to avert deleterious effects of this agent and offer previously unrecognized perspectives for therapeutic strategies. Obtaining greater insight into the role of EPO in the nervous system and elucidating its unique cellular pathways may provide greater cellular viability not only in the nervous system but also throughout the body.

Figure 1. Erythropoietin (EPO) prevents the activation and proliferation of microglia during oxygen-glucose deprivation (OGD)

EPO (10ng/ml) was applied to microglial cultures (EOC-2) 1 hour prior to a 6 hour period of OGD. Proliferating cell nuclear antigen (PCNA) that can assess activation of microglia and bromodeoxyuridine (BrdU) that can follow microglia proliferation were performed with anti-mouse antibody against PCNA (1:100) or BrdU (1:100) and visualized through fluorescence conjugated anti-mouse IgG (1:50) for PCNA and Texas Red conjugated anti-mouse IgG for BrdU. BrdU (10 μM) and fluorodexyuridine (1 μM) were applied 1 hour prior to the time of fixation. Untreated control microglia have minimal PCNA and BrdU expression. Expression of PCNA and BrdU in microglia significantly increases during OGD exposure. In contrast, PCNA expression and BrdU expression is significantly less in microglia treated with EPO (10 ng/ml), illustrating the ability of EPO to prevent the activation and proliferation of inflammatory microglia during oxidative stress.

Figure 2. Erythropoietin (EPO) prevents apoptotic injury through a series of interconnected cellular pathways

With HIF-1 activation, EPO and the EPO receptor (EPOR) can increase cell survival, promote progenitor cell development, and control inflammatory cell activation through pathways that involve the Janus-tyrosine kinase 2 (Jak2) protein, protein kinase B (Akt), and signal transducer and activator of transcription (STAT) proteins. Interconnected pathways involve Wnt1, IκB kinase (IKK), IκB, inhibitors of apoptotic protein (IAPs), extracellular signal-related kinases (ERKs), the forkhead family member FOXO3a, glycogen synthase kinase-3β(GSK-3β), nuclear factor-κB (NF-κB), mitochondrial membrane potential (Δψ_m), cytochrome c, (Cyto-c), and caspases. Ultimately these pathways converge upon early apoptotic injury with phosphatidylserine (PS) exposure and later apoptotic DNA degradation.