Biological functions and therapeutic use of erythropoiesis-stimulating agents: perplexities and perspectives

Erythropoietin and co.: intrinsic structure and functional disorder

Erythropoietin (Epo) is a heavily glycosylated protein, with its main function being related to erythropoiesis, where it controls red blood cell production via interaction with the Epo receptor (EpoR). It also plays a number of important roles in various hormonal, growth factor, and cytokine pathways. These roles are defined by Epo partners, such as the homodimeric (EpoR)2 receptor, the heterodimeric EpoR/βCR receptor and hypoxia inducing factor (HIF). Although the main structural features of both Epo and EpoR are conserved in vertebrates, the secretion sites of Epo in mammals are different from those in other vertebrates. Both biosynthetic and synthetic analogues of this protein are available on the market. Several side effects, such as pure red cells aplaisa, increase the rate of cancer-related death in patients treated with recombinant Epo. The multifunctionality of Epo and the ability of this protein to serve as a hormone, a cytokine, and a growth factor suggest the presence of functional disorder, which is a typical "structural" feature of moonlighting proteins. The goal of this article is to evaluate the roles of intrinsic disorder in the functions of Epo and its primary interactors, EpoR, βCR, and HIF-1α.

Systemic administration of erythropoietin inhibits retinopathy in RCS rats

Objective

Royal College of Surgeons (RCS) rats develop vasculopathy as photoreceptors degenerate. The aim of this study was to examine the effect of erythropoietin (EPO) on retinopathy in RCS rats.

Methods

Fluorescein angiography was used to monitor retinal vascular changes over time. Changes in retinal glia and vasculature were studied by immunostaining. To study the effects of EPO on retinal pathology, EPO (5000 IU/kg) was injected intraperitoneally in 14 week old normal and RCS rats twice a week for 4 weeks. Changes in the retinal vasculature, glia and microglia, photoreceptor apoptosis, differential expression of p75 neurotrophin receptor (p75^NTR), pro-neurotrophin 3 (pro-NT3), tumour necrosis factor-α (TNFα), pigment epithelium derived factor (PEDF) and vascular endothelial growth factor-A (VEGF-A), the production of CD34⁺ cells and mobilization of CD34⁺/VEGF-R2⁺ cells as well as recruitment of CD34⁺ cells into the retina were examined after EPO treatment.

Results

RCS rats developed progressive capillary dropout and subretinal neovascularization which were accompanied by retinal gliosis. Systemic administration of EPO stabilized the retinal vasculature and inhibited the development of focal vascular lesions. Further studies showed that EPO modulated retinal gliosis, attenuated photoreceptor apoptosis and p75^NTR and pro-NT3 upregulation, promoted the infiltration of ramified microglia and stimulated VEGF-A expression but had little effect on TNFα and PEDF expression. EPO stimulated the production of red and white blood cells and CD34⁺ cells along with effective mobilization of CD34⁺/VEGF-R2⁺ cells. Immunofluorescence study demonstrated that EPO enhanced the recruitment of CD34⁺ cells into the retina.

Conclusions

Our results suggest that EPO has therapeutic potentials in treatment of neuronal and vascular pathology in retinal disease. The protective effects of EPO on photoreceptors and the retinal vasculature may involve multiple mechanisms including regulation of retinal glia and microglia, inhibition of p75^NTR-pro-NT3 signaling together with stimulation of production and mobilization of bone marrow derived cells.

Erythropoietin or darbepoetin for patients with cancer

BACKGROUND

Anaemia associated with cancer and cancer therapy is an important clinical factor in the treatment of malignant diseases. Therapeutic alternatives are recombinant human erythropoiesis stimulating agents (ESAs) and red blood cell transfusions.

OBJECTIVES

To assess the effects of ESAs to either prevent or treat anaemia in cancer patients.

SEARCH METHODS

This is an update of a Cochrane review first published in 2004. We searched the Central Register of Controlled Trials (CENTRAL), MEDLINE and EMBASE and other databases. Searches were done for the periods 01/1985 to 12/2001 for the first review, 1/2002 to 04/2005 for the first update and to November 2011 for the current update. We also contacted experts in the field and pharmaceutical companies.

SELECTION CRITERIA

Randomised controlled trials on managing anaemia in cancer patients receiving or not receiving anti-cancer therapy that compared the use of ESAs (plus transfusion if needed).

DATA COLLECTION AND ANALYSIS

Several review authors assessed trial quality and extracted data. One review author assessed quality assessment and extracted data, a second review author checked for correctness.

MAIN RESULTS

This update of the systematic review includes a total of 91 trials with 20,102 participants. Use of ESAs significantly reduced the relative risk of red blood cell transfusions (risk ratio (RR) 0.65; 95% confidence interval (CI) 0.62 to 0.68, 70 trials, N = 16,093). On average, participants in the ESAs group received one unit of blood less than the control group (mean difference (MD) -0.98; 95% CI -1.17 to -0.78, 19 trials, N = 4,715). Haematological response was observed more often in participants receiving ESAs (RR 3.93; 95% CI 3.10 to 3.71, 31 trials, N = 6,413). There was suggestive evidence that ESAs may improve Quality of Life (QoL). There was strong evidence that ESAs increase mortality during active study period (hazard ratio (HR) 1.17; 95% CI 1.06 to 1.29, 70 trials, N = 15,935) and some evidence that ESAs decrease overall survival (HR 1.05; 95% CI 1.00 to 1.11, 78 trials, N = 19,003). The risk ratio for thromboembolic complications was increased in patients receiving ESAs compared to controls (RR 1.52, 95% CI 1.34 to 1.74; 57 trials, N = 15,498). ESAs may also increase the risk for hypertension (fixed-effect model: RR 1.30; 95% CI 1.08 to 1.56; random-effects model: RR 1.12; 95% CI 0.94 to 1.33, 31 trials, N = 7,228) and thrombocytopenia/haemorrhage (RR 1.21; 95% CI 1.04 to 1.42; 21 trials, N = 4,507). There was insufficient evidence to support an effect of ESA on tumour response (fixed-effect RR 1.02; 95% CI 0.98 to 1.06, 15 trials, N = 5,012).

AUTHORS' CONCLUSIONS

ESAs reduce the need for red blood cell transfusions but increase the risk for thromboembolic events and deaths. There is suggestive evidence that ESAs may improve QoL. Whether and how ESAs affects tumour control remains uncertain. The increased risk of death and thromboembolic events should be balanced against the potential benefits of ESA treatment taking into account each patient's clinical circumstances and preferences. More data are needed for the effect of these drugs on quality of life and tumour progression. Further research is needed to clarify cellular and molecular mechanisms and pathways of the effects of ESAs on thrombogenesis and their potential effects on tumour growth.

Low or undetectable TPO receptor expression in malignant tissue and cell lines derived from breast, lung, and ovarian tumors

BACKGROUND

Numerous efficacious chemotherapy regimens may cause thrombocytopenia. Thrombopoietin receptor (TPO-R) agonists, such as eltrombopag, represent a novel approach for the treatment of chemotherapy-induced thrombocytopenia. The TPO-R MPL is expressed on megakaryocytes and megakaryocyte precursors, although little is known about its expression on other tissues.

METHODS

Breast, lung, and ovarian tumor samples were analyzed for MPL expression by microarray and/or quantitative reverse transcription-polymerase chain reaction (qRT-PCR), and for TPO-R protein expression by immunohistochemistry (IHC). Cell line proliferation assays were used to analyze the in vitro effect of eltrombopag on breast, lung, and ovarian tumor cell proliferation. The lung carcinoma cell lines were also analyzed for TPO-R protein expression by Western blot.

RESULTS

MPL mRNA was not detectable in 118 breast tumors and was detectable at only very low levels in 48% of 29 lung tumors studied by microarray analysis. By qRT-PCR, low but detectable levels of MPL mRNA were detectable in some normal (14-43%) and malignant (3-17%) breast, lung, and ovarian tissues. A comparison of MPL to EPOR, ERBB2, and IGF1R mRNA demonstrates that MPL mRNA levels were far lower than those of EPOR and ERBB2 mRNA in the same tissues. IHC analysis showed negligible TPO-R protein expression in tumor tissues, confirming mRNA analysis. Culture of breast, lung, and ovarian carcinoma cell lines showed no increase, and in fact, showed a decrease in proliferation following incubation with eltrombopag. Western blot analyses revealed no detectable TPO-R protein expression in the lung carcinoma cell lines.

CONCLUSIONS

Multiple analyses of breast, lung, and ovarian tumor samples and/or cell lines show no evidence of MPL mRNA or TPO-R protein expression. Eltrombopag does not stimulate growth of breast, lung, or ovarian tumor cell lines at doses likely to exert their actions on megakaryocytes and megakaryocyte precursors.

Emerging technologies in the delivery of erythropoietin for therapeutics

Deciphering the function of proteins and their roles in signaling pathways is one of the main goals of biomedical research, especially from the perspective of uncovering pathways that may ultimately be exploited for therapeutic benefit. Over the last half century, a greatly expanded understanding of the biology of the glycoprotein hormone erythropoietin (Epo) has emerged from regulator of the circulating erythrocyte mass to a widely used therapeutic agent. Originally viewed as the renal hormone responsible for erythropoiesis, recent in vivo studies in animal models and clinical trials demonstrate that many other tissues locally produce Epo independent of its effects on red blood cell mass. Thus, not only its hematopoietic activity but also the recently discovered nonerythropoietic actions in addition to new drug delivery systems are being thoroughly investigated in order to fulfill the specific Epo release requirements for each therapeutic approach. The present review focuses on updating the information previously provided by similar reviews and recent experimental approaches are presented to describe the advances in Epo drug delivery achieved in the last few years and future perspectives.

Targeting the erythropoietin receptor on glioma cells reduces tumour growth

Hypoxia has been shown to be one of the major events involved in EPO expression. Accordingly, EPO might be expressed by cerebral neoplastic cells, especially in glioblastoma, known to be highly hypoxic tumours. The expression of EPOR has been described in glioma cells. However, data from the literature remain descriptive and controversial. On the basis of an endogenous source of EPO in the brain, we have focused on a potential role of EPOR in brain tumour growth. In the present study, with complementary approaches to target EPO/EPOR signalling, we demonstrate the presence of a functional EPO/EPOR system on glioma cells leading to the activation of the ERK pathway. This EPO/EPOR system is involved in glioma cell proliferation in vitro. In vivo, we show that the down-regulation of EPOR expression on glioma cells reduces tumour growth and enhances animal survival. Our results support the hypothesis that EPOR signalling in tumour cells is involved in the control of glioma growth.

Thrombopoietin receptor levels in tumor cell lines and primary tumors

Thrombopoietin (TPO) receptor agonists represent a new approach for the treatment of thrombocytopenia, which may develop as a consequence of immune thrombocytopenia, chemotherapy treatment, chronic hepatitis C infection, or myelodysplastic syndromes. There are concerns that use of certain growth factors can hasten disease progression in some types of hematologic malignancies and solid tumors. In this study, expression of MPL (TPO-R) mRNA was examined in tumor cell lines, patient tumor samples (renal cell carcinoma, prostatic carcinoma, soft tissue and bony/cartilage sarcoma, colon cancer, and lymphoma), and normal tissues using microarray analysis and qRT-PCR. MPL mRNA is expressed at very low or undetectable levels compared with erythropoietin receptor (EPOR), human epidermal growth factor (ERBB2; HER2), and insulin-like growth factor-1 receptor (IGF1R) in these patient samples. These data suggest TPO-R agonists will likely preferentially stimulate proliferation and differentiation of cells of megakaryocytic lineage, potentially demonstrating their utility for correcting thrombocytopenia in clinical settings.