Involvement of erythropoietin-induced cytosolic free calcium mobilization in activation of mitogen-activated protein kinase and DNA synthesis in vascular smooth muscle cells

Vasculopathy in the setting of cardiorenal syndrome: roles of protein-bound uremic toxins

Chronic kidney disease (CKD) often leads to and accelerates the progression of cardiovascular disease (CVD), while CVD also causes kidney dysfunction. This bidirectional interaction leads to the development of a complex syndrome known as cardiorenal syndrome (CRS). CRS not only involves both the heart and the kidney but also the vascular system through a vast array of contributing factors. In addition to hemodynamic, neurohormonal, mechanical, and biochemical factors, nondialyzable protein-bound uremic toxins (PBUTs) are also key contributing factors that have been demonstrated through in vitro, in vivo, and clinical observations. PBUTs are ineffectively removed by hemodialysis because their complexes with albumins are larger than the pores of the dialysis membranes. PBUTs such as indoxyl sulfate and p-cresyl sulfate are key determinate and predictive factors for the progression of CVD in CKD patients. In CRS, both vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) exhibit significant dysfunction that is associated with the progression of CVD. PBUTs influence proliferation, calcification, senescence, migration, inflammation, and oxidative stress in VSMCs and ECs through various mechanisms. These pathological changes lead to arterial remodeling, stiffness, and atherosclerosis and thus reduce heart perfusion and impair left ventricular function, aggravating CRS. There is limited literature about the effect of PBUT on the vascular system and their contribution to CRS. This review summarizes current knowledge on how PBUTs influence vasculature, clarifies the relationship between uremic toxin-related vascular disease and CRS, and highlights the potential therapeutic strategies of uremic vasculopathy in the setting of CRS.

Erythropoietin and Nonhematopoietic Effects

Erythropoietin (EPO) is the main regulator of red blood cell production. Since the 1990s, EPO has been used for the treatment of anemia associated with end-stage renal failure and chemotherapy. The erythropoietin receptors were found on other organs such as the brain, spinal cord, heart and skin. In addition, it has been shown that many tissues produce and locally release EPO in response to hypoxic, biochemical and physical stress. In cellular, animal and clinical studies, EPO protects tissues from ischemia and reperfusion injury, has antiapoptotic effects and improves regeneration after injury. In this article, we mainly review the nonhematopoietic effects and new possible clinical indications for EPO.

Erythropoietin stimulates the coronary collateral development in patients with coronary chronic total occlusion

Objective

Erythropoietin (EPO) improves cardiac function and induces neovascularisation in post-myocardial infarction heart failure. The aim of this study was to analyse the association between the serum erythropoietin level and coronary collateral development in patients with coronary artery disease and chronic total occlusion.

Methods

A total of 168 patients consisting of 117 with coronary artery disease (CAD, (62 with chronic total occlusion (CTO), 55 without CTO)) and 51 with healthy coronary arteries were included in the study. The patients were assigned as coronary artery disease without CTO (group 0), CAD with CTO (group 1: poor collateral development, group 2: good collateral development) and normal coronary arteries (group 3).

Results

There was a significant positive correlation between serum EPO levels and the Rentrop scores in angiography (r = 0.243, p = 0.001). Similarly, a positive correlation was found between serum EPO levels and the Syntax scores (r = 0.253, p = 0.001). Echocardiography revealed a negative correlation between serum EPO levels and the cardiac ejection fraction (r = −0.210, p = 0.006).

Conclusions

Serum EPO is a useful biomarker for coronary collateral development in patients with CTO.

Erythropoietin Induces Excessive Neointima Formation: A Study in a Rat Carotid Artery Model of Vascular Injury

A therapeutic strategy that would mitigate the events leading to hyperplasia and facilitate re-endothelialization of an injured artery after balloon angioplasty could be effective for a long-term patency of the artery. It is hypothesized that erythropoietin (EPO), which has both anti-inflammatory and antiapoptotic properties, will prevent hyperplasia, and its ability to proliferate and mobilize endothelial progenitor cells will re-endothelialize the injured artery. To test this hypothesis, EPO (5000 IU/kg) in solution was injected intraperitoneally 6 hours before vascular injury and then on every alternate day for a week or as a single dose (5000 IU/kg) in a sustained release gel formulation 1 week before the vascular injury. Morphometric analysis revealed nearly continuous re-endothelialization of the injured artery in EPO solution-treated animals (90% vs less than 20% in saline control); however, the treatment also caused excessive neointima formation (intima/media ratio, 2.10 ± 0.09 vs 1.60 ± 0.02 saline control, n = 5, P < .001). The EPO gel also induced similar excessive neointima formation. Immunohistochemical analysis of the injured arteries from the animals treated with EPO solution demonstrated a significant angiogenic response in adventitia and media, thus explaining the formation of excessive neointima. Although the results are in contrast to expectation, they explain a greater degree of stenosis seen in hemodialysis access fistulas in patients who are on EPO therapy for anemic condition. The results also caution the use of EPO, particularly in patients who are at a risk of vascular injury or are suffering from an atherosclerotic condition.

The role of erythropoietin in myocardial protection: potential mechanisms and applications

The glycoprotein erythropoietin was originally discovered as a principal regulator that promotes the survival, proliferation and differentiation of erythroid progenitor cells. Despite potentially detrimental effects, such as increased blood pressure and hyperviscosity, recombinant human erythropoietin has been demonstrated to be a safe drug, as millions of anemia sufferers have received it over the last decade as a form of treatment. Recently, erythropoietin receptors have been discovered in a variety of tissues, including the cardiovascular system, and erythropoietin has been demonstrated to have a beneficial effect in congestive heart failure patients with anemia. The purpose of this review is to summarize the pleiotropic cardioprotective effects of erythropoietin in the cardiovascular system and to evaluate its potential role as a biomarker in these disorders.

Anaemia management and mortality risk in chronic kidney disease

Renal anaemia is a frequent complication in patients with chronic kidney disease (CKD). Severe anaemia (haemoglobin <90 g/l) is associated with increased risks of mortality and cardiac complications, such as left ventricular hypertrophy and cardiovascular disease, and impaired quality of life. Randomized controlled trials have tested the hypothesis that increasing haemoglobin level using erythropoiesis-stimulating agents (ESAs) lowers these risks and improves quality of life. Use of ESAs to normalize haemoglobin levels (to ≥130 g/l) versus the partial correction of anaemia (to haemoglobin levels of 90-110 g/l) has repeatedly been shown to have no cardiac benefit and to be associated with no incremental improvement in outcomes and quality of life (except fatigue), but has been shown to be associated with an increased risk of cardiovascular events and death. Use of more-intense iron dosing has been proposed in order to reduce ESA dosing but liberal intravenous iron therapy is also associated with complications, and its long-term safety has not yet been adequately investigated. For patients with CKD on dialysis, US medication labels recommend administering ESAs at doses sufficient to avoid transfusions, whereas European and Canadian labels recommend targeting haemoglobin levels of 100-120 g/l and 110-120 g/l, respectively. Treatment of anaemia to haemoglobin levels of 90-110 g/l in patients with CKD accomplishes what we want--a reduced need for transfusions and possible reductions in fatigue, while avoiding high doses of ESA or iron in order to achieve a specific haemoglobin goal.

The relationship of serum erythropoietin level with coronary collateral grade

BACKGROUND

Erythropoietin has been shown to induce neovascularization and protect against ischemic vascular injury. We investigated whether a higher serum erythropoietin (EPO) level is related to better coronary collateral vessel grade.

METHODS

Ninety-nine patients with stable angina pectoris who have at least 1 coronary stenosis of equal to or greater than 70% at coronary angiography were prospectively enrolled. Serum EPO and vascular endothelial growth factor (VEGF) levels were studied. Coronary collateral degree was graded according to the Rentrop method. Patients with grade 2-3 collateral degree were included in the good collateral group and formed Group I. The patients with grade 0-1 collateral degree were included in the poor collateral group and formed Group II.

RESULTS

The serum EPO level was significantly higher in the good collateral group (17.3 ± 9.3 mU/mL vs 11.7 ± 5.0 mU/mL; P < 0.001). There was also a positive correlation between serum EPO level and Rentrop score (r = 0.39; P < 0.001). In multivariate analysis, serum EPO level (odds ratio [OR] 1.336; 95% confidence interval [CI], 1.120-1.593; P = 0.001), oxygen saturation (OR 0.638; 95% CI, 0.422-0.963; P = 0.033) and presence of chronic total occlusion (CTO) (OR 26.7; 95% CI, 3.874-184.6; P = 0.001) were independently related to well-developed coronary collaterals.

CONCLUSIONS

Higher serum EPO level is related to better coronary collateral development. Erythropoietin may have a positive effect on the development of collaterals and may provide a new agent for the treatment strategies to enhance coronary collateral vessel development.

Cardiovascular effects of erythropoietin an update

Erythropoietin (EPO) is a therapeutic product of recombinant DNA technology and it has been in clinical use as stimulator of erythropoiesis over the last two decades. Identification of EPO and its receptor (EPOR) in the cardiovascular system expanded understanding of physiological and pathophysiological role of EPO. In experimental models of cardiovascular and cerebrovascular disorders, EPO exerts protection either by preventing apoptosis of cardiac myocytes, smooth muscle cells, and endothelial cells, or by increasing endothelial production of nitric oxide. In addition, EPO stimulates mobilization of progenitor cells from bone marrow thereby accelerating repair of injured endothelium and neovascularization. A novel signal transduction pathway involving EPOR--β-common heteroreceptor is postulated to enhance EPO-mediated tissue protection. A better understanding of the role of β-common receptor signaling as well as development of novel analogs of EPO with enhanced nonhematopoietic protective effects may expand clinical application of EPO in prevention and treatment of cardiovascular and cerebrovascular disorders.

Anemia and anemia correction: surrogate markers or causes of morbidity in chronic kidney disease?

Observational studies have shown a strong positive correlation between the severity of anemia and the risk of poor outcomes in patients with chronic kidney disease (CKD). This observation was initially taken to imply that adverse outcomes in CKD are caused by anemia. However, the assumption of causality ignores the possibility that anemia and adverse outcomes might be unrelated and that both are caused by underlying inflammation, oxidative stress and comorbid conditions. Randomized clinical trials of anemia correction have revealed an increased risk of adverse cardiovascular outcomes in patients assigned to normal, rather than subnormal, hemoglobin targets. As a result, correction of anemia is now considered potentially hazardous in patients with CKD. Notably, individuals who did not reach the target hemoglobin level in the clinical trials, despite receiving high doses of erythropoietin and iron, experienced a disproportionately large share of the adverse outcomes. These observations point to overdose of erythropoietin and iron, rather than anemia correction per se, as the likely culprit. This Review explores the reasons for the apparent contradiction between the findings of observational studies and randomized clinical trials of anemia treatment in CKD. I have focused on data from basic and translational studies, which are often overlooked in the design and interpretation of clinical studies and in the formulation of clinical guidelines.

Erythropoietin after a century of research: younger than ever

In the light of the enthusiasm regarding the use of recombinant human erythropoietin (Epo) and its analogues for treatment of the anaemias of chronic renal failure and malignancies it is worth remembering that today's success has been based on a century of laborious research. The concept of the humoral regulation of haematopoiesis was first formulated in 1906. The term 'erythropoietin' for the erythropoiesis-stimulating hormone was introduced in 1948. Native human Epo was isolated in 1977 and its gene cloned in 1985. During the last 15 yr, major progress has been made in identifying the molecules controlling Epo gene expression, primarily the hypoxia-inducible transcription factors (HIF) that are regulated by specific O2 and oxoglutarate requiring Fe2+-containing dioxygenases. With respect to the action of Epo, its dimeric receptor (Epo-R) has been characterised and shown to signal through protein kinases, anti-apoptotic proteins and transcription factors. The demonstration of Epo-R in non-haematopoietic tissues indicates that Epo is a pleiotropic viability and growth factor. The neuroprotective and cardioprotective potentials of Epo are reviewed with a focus on clinical research. In addition, studies utilising the Epo derivatives with prolonged half-life, peptidic and non-peptidic Epo mimetics, orally active drugs stimulating endogenous Epo production and Epo gene transfer are reviewed.

Indoxyl sulfate stimulates proliferation of rat vascular smooth muscle cells

Vascular smooth muscle cell (VSMC) proliferation is a key event in the progression of arteriosclerosis. Clinical studies show that uremic toxins deteriorate the arteriosclerosis in renal failure patients. Indoxyl sulfate (IS) is a strong protein-bound uremic toxin, but the effect of IS on VSMC proliferation has not been studied. We examined the effect of IS on rat VSMC proliferation, assessed by a cell counting kit (4-[3-[4-lodophenyl]-2-4(4-nitrophenyl)-2H-5-tetrazolio-1,3-benzene disulfonate] assay) and by [(3)H]thymidine incorporation in vitro. We further evaluated a contribution of mitogen-activated protein kinase (MAPK; p44/42 MAPK) to VSMC proliferation by IS. Immunohistochemical staining was performed for VSMCs using antirat organic anion transporter (OAT)3 antibody. The mRNA expressions of platelet-derived growth factor (PDGF)-A and -C chains, and PDGF-beta receptor were evaluated by real-time PCR. IS stimulated the proliferation of VSMCs in a concentration-dependent manner and activated p44/42 MAPK. Concentration of IS needed to stimulate the proliferation of rat VSMC was about 250 microM, which is compatible with that in the serum of end-stage renal failure patients. PD98059 (10 microM), a selective inhibitor of MAPK/extracellular signal-regulated kinase, inhibited the IS-induced (250 microM) VSMC proliferation and phosphorylation of MAPK. Probenecid (0.5 mM), an inhibitor and substrate of OAT, inhibited the IS-induced (250 microM) VSMC proliferation. Rat OAT3 was detected in VSMCs. The mRNA expressions of PDGF-C chain and PDGF-beta receptor were significantly increased by IS. We conclude that IS directly stimulates rat VSMC proliferation and activates MAPK in vitro. This might be one of the mechanisms underlying the progression of atherosclerotic lesions in end-stage renal disease patients.

Calmodulin physically interacts with the erythropoietin receptor and enhances Jak2-mediated signaling

Stimulation of the erythropoietin receptor (EpoR) induces a transient increase in intracellular Ca2+ level as well as activation of the Jak2 tyrosine kinase to stimulate various downstream signaling pathways. Here, we demonstrate that the universal Ca2+ receptor calmodulin (CaM) binds EpoR in a Ca2+-dependent manner in vitro. Binding studies using various EpoR mutants in hematopoietic cells showed that CaM binds the membrane-proximal 65-amino-acid cytoplasmic region (amino acids 258-312) of EpoR that is critical for activation of Jak2-mediated EpoR signaling. Structurally unrelated CaM antagonists, W-13 and CMZ, inhibited activation of Jak2-mediated EpoR signaling pathways, whereas W-12, a W-13 analog, did not show any significant inhibitory effect. Moreover, overexpression of CaM augmented Epo-induced tyrosine phosphorylation of the EpoR. W-13, but not W-12, also inhibited Epo-induced proliferation and survival. Together, these results indicate that CaM binds to the membrane-proximal EpoR cytoplasmic region and plays an essential role in activation of Jak2-mediated EpoR signaling.

Beneficial and ominous aspects of the pleiotropic action of erythropoietin

The primary function of the glycoprotein hormone erythropoietin (Epo) is to promote red cell production by inhibiting apoptosis of erythrocytic progenitors in hemopoietic tissues. However, functional Epo receptors (Epo-R) have recently been demonstrated in various nonhemopoietic tissues indicating that Epo is a more pleiotropic viability and growth factor. Herein, in vitro and in vivo effects of Epo in the brain and the cardiovascular system are reviewed. In addition, the therapeutic impact of Epo in oncology is considered, including the question of whether Epo might promote tumor growth. Convincing evidence is available that Epo acts as a neurotrophic and neuroprotective factor in the brain. Epo prevents neuronal cells from hypoxia-induced and glutamate-induced cell death. Epo-R is expressed by neurons and glia cells in specific regions of the brain. Epo supports the survival of neurons in the ischemic brain. The neuroprotective potential of Epo has already been confirmed in a clinical trial on patients with acute stroke. With respect to the vasculature, Epo acts on both endothelial and smooth muscle cells. Epo promotes angiogenesis and stimulates the production of endothelin and other vasoactive mediators. In addition, Epo-R is expressed by cardiomyocytes. The role of Epo as a myocardial protectant is at the focus of present research. Epo therapy in tumor patients is practiced primarily to maintain the hemoglobin concentration above the transfusion trigger and to reduce fatigue. In addition, increased tumor oxygenation may improve the efficacy of chemotherapy and radiotherapy. However, tumor cells often express Epo-R. Therefore, careful studies are required to fully exclude that recombinant human Epo (rHuEpo) promotes tumor growth.

Modulation of the erythropoietin-induced proliferative pathway by cAMP in vascular smooth muscle cells

We previously reported that erythropoietin (Epo) has a mitogenic effect on rat vascular smooth muscle cells (VSMC) and that activation of the mitogen-activated protein kinase (MAPK) cascade is an important mediator for Epo-induced mitogenesis. An increase in intracellular cAMP has an antiproliferative effect on VSMC. We therefore hypothesized that cAMP effectors inhibit Epo-induced MAPK activation in rat VSMC. When we exposed VSMC to recombinant human Epo (rHuEpo), DNA synthesis was increased. Forskolin (Fsk) or cilostazol (Cil) decreased the DNA synthesis stimulated by rHuEpo. Coincubation with Rp-cAMPS triethylamine canceled the suppression of DNA synthesis and MAPK activity by Fsk. Both rHuEpo and phorbol 12-myristate 13-acetate upregulated phosphorylations of MEK and MAPK. Pretreatment with Fsk inhibited these phosphorylations. Protein kinase C inhibitors also suppressed MEK and MAPK phosphorylations. Moreover, Fsk induced phosphorylation of Raf-1 at serine-259. These results indicated that cAMP inhibited Epo-induced MAPK activation and that this suppression might be regulated upstream or at Raf-1. The results also suggested that these agents, which could accumulate cAMP, might be protective for Epo-stimulated direct action.

Low doses of EPO activate MAP kinases but not JAK2-STAT5 in rat vascular smooth muscle cells

Previous reports have shown a direct effect of erythropoietin (Epo) on vascular smooth muscle cells (VSMCs). Our aim was to assess expression of the Epo receptor (EpoR) on VSMCs and to study the activation of two major signaling cascades activated by Epo, namely JAK2/STAT5 and MAPK pathways. All experiments were performed in parallel using the Epo-responsive UT7 cell line. From semiquantitative RT-PCR experiments, VSMCs were estimated to express approximately 30-fold less EpoR mRNA than UT7 cells. Epo-induced phosphorylation of proteins involved in the EpoR/JAK2/STAT5 cascade could not be detected in VSMCs, even using pharmacological doses of Epo (250 IU/ml). In contrast, a strong activation of MAP kinase pathway was detected with as low as 10 IU/ml Epo. We suggest that MAPK activation reflects a physiologically relevant effect of Epo on VSMCs that may be correlated to cell proliferation.