A "classical" homodimeric erythropoietin receptor is essential for the antiapoptotic effects of erythropoietin on differentiated neuroblastoma SH-SY5Y and pheochromocytoma PC-12 cells

AKT pathway in neuroblastoma and its therapeutic implication

Neuroblastoma is a frequent pediatric tumor with a poor outcome in spite of aggressive treatment, even with autologous hematopoietic stem cell transplantation. The overall cure rate of 40% is unsatisfactory and new therapeutic strategies are urgently needed. AKT is a major mediator of survival signals that protect cells from apoptosis and regulate cell proliferation. The AKT signaling network is considered a key determinant of the biological aggressiveness of these tumors. In this article, the authors discuss the relation between activators of AKT in neuroblastoma, in particular, growth factors such as IGF-1, TRK, GDNF, VEGF and EGF, and their effects on tumoral proliferation, differentiation and apoptosis. Numerous other proteins interact with AKT in neuroblastoma. Several are relatively well characterized, such as PTEN and retinoic acid; others are new and potentially interesting, such as PKC and anaplastic lymphoma kinase. Specific inhibition of AKT has been studied, such as with LY249002, with significant effects on cell progression and apoptosis in tumoral cells. Moreover, a series of new drugs, such as geldanamycin and rapamycin, directly modify the expression of AKT in tumoral cells. Few specific inhibitors of AKT are available; less specific inhibitors are probably unsuitable therapeutic options in neuroblastoma. Drugs with a direct or indirect inhibitory effect on the AKT pathway, used alone or in combination with other drugs, seem to hold great promise as a new therapeutic modality in neuroblastoma.

Erythropoietin: elucidating new cellular targets that broaden therapeutic strategies

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.

Erythropoietin and oxidative stress

Unmitigated oxidative stress can lead to diminished cellular longevity, accelerated aging, and accumulated toxic effects for an organism. Current investigations further suggest the significant disadvantages that can occur with cellular oxidative stress that can lead to clinical disability in a number of disorders, such as myocardial infarction, dementia, stroke, and diabetes. New therapeutic strategies are therefore sought that can be directed toward ameliorating the toxic effects of oxidative stress. Here we discuss the exciting potential of the growth factor and cytokine erythropoietin for the treatment of diseases such as cardiac ischemia, vascular injury, neurodegeneration, and diabetes through the modulation of cellular oxidative stress. Erythropoietin controls a variety of signal transduction pathways during oxidative stress that can involve Janus-tyrosine kinase 2, protein kinase B, signal transducer and activator of transcription pathways, Wnt proteins, mammalian forkhead transcription factors, caspases, and nuclear factor kappaB. Yet, the biological effects of erythropoietin may not always be beneficial and may be poor tolerated in a number of clinical scenarios, necessitating further basic and clinical investigations that emphasize the elucidation of the signal transduction pathways controlled by erythropoietin to direct both successful and safe clinical care.

Neuroprotective properties and mechanisms of erythropoietin in in vitro and in vivo experimental models for hypoxia/ischemia

Besides its established function in erythropoiesis, erythropoietin (EPO) is currently also appreciated for its neuroprotective effects. The detrimental sequelae of prolonged cerebral hypoxia and ischemia have been shown to attenuate by EPO treatment. After binding to the EPO receptor, EPO is capable of initiating a cascade of events which--via different pathways--may lead to neuroprotection. The circumstances that determine which specific signalling route(s) are activated by EPO are largely unknown. We aim to provide the reader with a timely overview on the use of EPO in models of stroke and hypoxia-ischemia and to discuss the molecular events that underlie its neuroprotection.

The erythropoietin receptor in normal and cancer tissues

The hormone erythropoietin (EPO) is essential for the survival, proliferation and differentiation of the erythrocytic progenitors. The EPO receptor (EPO-R) of erythrocytic cells belongs to the cytokine class I receptor family and signals through various protein kinases and STAT transcription factors. The EPO-R is also expressed in many organs outside the bone marrow, suggesting that EPO is a pleiotropic anti-apoptotic factor. The controversial issue as to whether the EPO-R is functional in tumor tissue is critically reviewed. Importantly, most studies of EPO-R detection in tumor tissue have provided falsely positive results because of the lack of EPO-R specific antibodies. However, endogenous EPO appears to be necessary to maintain the viability of endothelial cells and to promote tumor angiogenesis. Although there is no clinical proof that the administration of erythropoiesis stimulating agents (ESAs) promotes tumor growth and mortality, present recommendations are that (i) ESAs should be administered at the lowest dose sufficient to avoid the need for red blood cell transfusions, (ii) ESAs should not be used in patients with active malignant disease not receiving chemotherapy or radiotherapy, (iii) ESAs should be discontinued following the completion of a chemotherapy course, (iv) the target Hb should be 12 g/dL and not higher and (v) the risks of shortened survival and tumor progression have not been excluded when ESAs are dosed to target Hb <12 g/dL.

Triple play: promoting neurovascular longevity with nicotinamide, WNT, and erythropoietin in diabetes mellitus

Oxidative stress is a principal pathway for the dysfunction and ultimate destruction of cells in the neuronal and vascular systems for several disease entities, not promoting the ravages of oxidative stress to any less of a degree than diabetes mellitus. Diabetes mellitus is increasing in incidence as a result of changes in human behavior that relate to diet and daily exercise and is predicted to affect almost 400 million individuals worldwide in another two decades. Furthermore, both type 1 and type 2 diabetes mellitus can lead to significant disability in the nervous and cardiovascular systems, such as cognitive loss and cardiac insufficiency. As a result, innovative strategies that directly target oxidative stress to preserve neuronal and vascular longevity could offer viable therapeutic options to diabetic patients in addition to more conventional treatments that are designed to control serum glucose levels. Here we discuss the novel application of nicotinamide, Wnt signaling, and erythropoietin that modulate cellular oxidative stress and offer significant promise for the prevention of diabetic complications in the nervous and vascular systems. Essential to this process is the precise focus upon diverse as well as common cellular pathways governed by nicotinamide, Wnt signaling, and erythropoietin to outline not only the potential benefits, but also the challenges and possible detriments of these therapies. In this way, new avenues of investigation can hopefully bypass toxic complications, or at the very least, avoid contraindications that may limit care and offer both safe and robust clinical treatment for patients.

Erythropoietin receptor transcription is neither elevated nor predictive of surface expression in human tumour cells

Erythropoietin receptor (EpoR) has been reported to be overexpressed in tumours and has raised safety concerns regarding the use of erythropoiesis-stimulating agents (ESAs) to treat anaemia in cancer patients. To investigate the potential for EpoR to be overexpressed in tumours, we have evaluated human tumours for amplification of the EPOR locus, levels of EPOR transcripts, and expression of surface EpoR protein. Gene amplification analysis of 1083 solid tumours found that amplification of the EPOR locus was rare with frequencies similar to other non-oncogenes. EPOR transcript levels in tumours and tumour cell lines were low in comparison with bone marrow and were equivalent to, or lower than, levels in normal tissues of tumour origin. Although EpoR mRNA was detected in some tumour lines, no EpoR could be detected on the cell surface using (125)I-Epo binding studies. This may be due to the lack of EpoR protein expression or lack of cell-surface-trafficking factors, such as Jak2. Taken together, we have found no evidence that EpoR is overexpressed in tumours or gets to the surface of tumour cells. This suggests that there is no selective advantage for tumours to overexpress EpoR and questions the functional relevance of EpoR gene transcription in tumours.

Vascular injury during elevated glucose can be mitigated by erythropoietin and Wnt signaling

Impacting a significant portion of the world's population with increasing incidence in minorities, the young, and the physically active, diabetes mellitus (DM) and its complications affect approximately 20 million individuals in the United States and over 100 million individuals worldwide. In particular, vascular disease from DM may lead to some of the most serious complications that can extend into both the cardiac and nervous systems. Unique strategies that can prevent endothelial cell (EC) demise and elucidate novel cellular mechanisms for vascular cytoprotection become vital for the prevention and treatment of vascular DM complications. Here, we demonstrate that erythropoietin (EPO), an agent that has recently been shown to extend cell viability in a number of systems extending beyond hematopoietic cells, prevents EC injury and apoptotic nuclear DNA degradation during elevated glucose exposure. More importantly, EPO employs Wnt1, a cysteine-rich glycosylated protein involved in gene expression, cell differentiation, and cell apoptosis, to confer EC cytoprotection and maintains the integrity of Wnt1 expression during elevated glucose exposure. In addition, application of anti-Wnt1 neutralizing antibody abrogates the protective capacity of both EPO and Wnt1, illustrating that Wnt1 is an important component in the cytoprotection of ECs during elevated glucose exposure. Intimately linked to this cytoprotection is the downstream Wnt1 pathway of glycogen synthase kinase (GSK-3beta) that requires phosphorylation of GSK-3beta and inhibition of its activity by EPO. Interestingly, inhibition of GSK-3beta activity during elevated glucose leads to enhanced EC survival, but does not synergistically improve protection by EPO or Wnt1, suggesting that EPO and Wnt1 are closely tied to the blockade of GSK-3beta activity. Our work exemplifies an exciting potential application for EPO in regards to the treatment of DM vascular disease complications and highlights a previously unrecognized role for Wnt1 and the modulation of the downstream pathway of GSK-3beta to promote vascular cell viability during DM.

Darbepoetin alfa protects podocytes from apoptosis in vitro and in vivo

Detachment or apoptosis of podocytes leads to proteinuria and glomerulosclerosis. There are no current interventions for diabetic or non-diabetic glomerular diseases specifically preventing podocyte apoptosis. Binding of erythropoiesis stimulating proteins (ESPs) to receptors on non-hematopoietic cells has been shown to have anti-apoptotic effects in vitro, in vivo, and in preliminary human studies. Recently, erythropoietin receptors were identified on podocytes; therefore, we tested effects of darbepoetin alfa in preventing podocyte apoptosis. Cultured immortalized mouse podocytes were treated with low-dose ultraviolet-C (uv-C) irradiation to induce apoptosis in the absence or presence of darbepoetin alfa. Apoptosis was quantified by Hoechst staining and by caspase 3 cleavage assessed by Western blots. Pretreatment with darbepoetin alfa significantly reduced podocyte apoptosis with this effect involving intact Janus family protein kinase-2 (JAK2) and AKT signaling pathways. Additionally, darbepoetin alfa was found protective against transforming growth factor-beta1 but not puromycin aminonucleoside induced apoptosis. Mice with anti-glomerular antibody induced glomerulonephritis had significantly less proteinuria, glomerulosclerosis, and podocyte apoptosis when treated with darbepoetin alfa. Our studies show that treatment of progressive renal diseases characterized by podocyte apoptosis with ESPs may be beneficial in slowing progression of chronic kidney disease.

Does erythropoietin have a dark side? Epo signaling and cancer cells

Erythropoietin (Epo) stimulates red blood cell production by docking with its cognate receptor on the erythroid progenitor cell and triggering an array of signaling pathways that inhibit apoptosis and promote cell proliferation and differentiation. In its pharmaceutical forms, epoetin and darbepoetin, Epo is widely used to treat various anemias, including those associated with cancer. The Epo receptor is also expressed by nonhematopoietic cells, including cancer cells, and Epo exhibits a "tissue-protective" effect on nonhematopoietic tissues, possibly mediated through a novel heteroreceptor, blocking apoptosis induced by a variety of insults. The unexpected results of several clinical studies in which Epo was used to treat cancer patients have now raised the question of a potential direct growth-promoting action of Epo on cancer cells.

Expression and function of erythropoietin receptors in tumors

Safety concerns surrounding the use of recombinant human erythropoietin (Epo) to treat anemia in cancer patients were raised after 2 recent clinical studies reported a worse survival outcome in patients who received epoetin alpha or epoetin beta compared with patients who received placebo. Although those findings contrasted with previous clinical studies, which demonstrated no difference in survival for cancer patients who received erythropoiesis-stimulating agents (ESAs), some investigators have suggested a potential role for ESAs in promoting tumor growth through 1) stimulation of Epo receptors (EpoR) expressed in tumors, 2) stimulation and formation of tumor vessels, and/or 3) enhanced tumor oxygenation. The first and second hypotheses appeared to be supported by some EpoR expression and ESA in vitro studies. However, these conclusions have been challenged because of poor specificity of EpoR-detection methodologies, conflicting data from different groups, and the lack of correlation between in vitro data and in vivo findings in animal tumor models. For this report, the authors reviewed the biology of EpoR in erythropoiesis and compared and contrasted the reported findings on the role of ESAs and EpoR in tumors.