PARASKEVI TSEKE, MD AND GEORGE TSIRPANLIS, MD

RENAL UNIT, ALEXANDRA GENERAL HOSPITAL,
ATHENS, GREECE

Cardiovascular disease (CVD) is the major cause of morbidity and mortality in patients with end-stage renal failure (ESRF) ¹. Traditional (age, gender, hypertension, diabetes, dyslipidemia) and nontraditional (homocysteine, inflammation, oxidant stress etc) or emerging risk factors are considered to be the causes of atherosclerotic CVD in this high-risk population ².

Anemia in ESRF patients is considered to be an independent risk factor for heart failure upon dialysis initiation, whereas it can also predict the recurrence of heart failure at a later stage ³. Erythropoietin (EPO) deficiency has been recognized as the main cause of anemia in ESRF and the introduction of recombinant human erythropoietin (rHuEpo) in the late 1980s significantly improved its management. Almost 90% of patients receiving rHuEpo respond to treatment and they achieve target hemoglobin (Hb) levels. Nevertheless, a large number of patients show "hyporesponsiveness" or "resistance" to rHuEpo.

Resistance to recombinant human Erythropoietin treatment

According to the European Best Practice Guidelines and the US Guidelines resistance to rHuEpo is defined as inappropriate response to treatment with doses >300 IU/kg/week, when administered subcutaneously, or >400 IU/kg/week and 450 IU/kg/week, respectively, when administered intravenously ^{4, 5}. Among causes of hyporesponsiveness to rHuEpo the most common one is iron deficiency, either functional or absolute. Other conditions involved in rHuEpo resistance are shown in table 1.

Table 1

Haemopoetic factors deficiency Iron deficiency (absolute or functional) Vitamin B12/ folate deficiency Conditions related to uremia/dialysis Chronic inflammation Secondary hyperparathyroidism Haemolysis Aluminium toxicity Inadequate dialysis Malnutrition Other Chronic blood loss ACE inhibitor or AT1 receptor antagonist therapy Marrow dysfunction (including multiple myeloma, myelofibrosis) Infection Haemoglobinopathies Antibodies against rHuEpo

Could rHuEpo resistance be related to atherosclerotic CVD in ESRF patients? We consider that some factors of the former entity are common or related to the later one. We will try to explore these common factors and the probable pathogenetic mechanisms correlating atherosclerotic CVD to rHuEpo resistance in ESRF patients.

RHuEpo resistance conditions related to CVD risk

1. Anemia

As mentioned above anemia is directly related to the development of heart failure in ESRF patients. Anemia is associated with hypoxia, which may lead to left ventricular hypertrophy, myocardial fibrosis, cardiomyopathy and congestive heart failure ⁶

Furthermore, erythrocytes are considered as mobile free radical scavenges, providing not only oxygen, but antioxidant defense systems (eg enzymes of glutathione synthesis and the oxidative pentose phosphate pathway, superoxide dismutase, tocophenols, ascorbic acid) and they are able to reach at any tissue or organ exposed to oxidative stress. In renal failure there is marked reduction in antioxidant and mobile free radical scavenger capacity, due to reduced red cells number and lifespan and increased oxidative stress associated with uremia per se, dialysis procedure and alterations in ATP turnover and neuroaminidase activity ⁷

Thus, anemia per se, the rHuEpo major consequence could contribute to cardiovascular disease and to oxidant stress induction – an emerging CVD risk factor - via antioxidant defense reduction.

2. I.V. iron supplementation

Iron deficiency is the most common cause of rHuEpo resistance ⁴ and iron supplementation is recommended in order to achieve upper limits of ferritin levels in HD patients. In this population, intestinal iron absorption is reduced even in iron-deficient patients and intravenous iron (IV) preparations are usually administered.

IV iron supplementation induces oxidative stress, LDL and other molecules oxidation, endothelial dysfunction and probably hemochromatosis by iron overload. ⁸ Some studies have detected a positive correlation between intravenous iron supplementation and oxidative stress induction. Intravenous iron administration in HD patients has been shown to correlate with increased protein oxidation and inflammation as assessed by elevated CRP and TNF-α levels ⁹. Additionally, a correlation between serum ferritin level and low-density lipoprotein oxidation has been detected, suggesting that iron could promote lipid peroxidation ⁸.

Furthermore, iron seems to correlate with endothelial dysfunction possibly through impairment of nitric oxide activity ⁸.

Oxidative stress induction and its consequences, nitric oxide activity disturbances and endothelial dysfunction are factors, which contributes to the initiation and progression of atherosclerosis.

3. Vitamin B12- Folate - Homocysteine

Homocysteine is a sulfur amino acid whose metabolism depends on Vit B12 and folic acid. Vitamin B12 and folate levels are often low in patients undergoing HD while homocysteine is much more increased than in general population ¹⁰. Homocysteine, is a potential independent risk factor for atherosclerosis in the general population. Serum creatinine is a strong determinant of homocysteine levels ¹¹. High dose folate (5-15 mg) reduces the serum concentration of homocysteine. Some experimental and clinical studies have detected an improvement in endothelial function, by reducing homocysteine with folate supplementation ¹¹. Thus vitamin B12 and folate deficiency via hyperhomocysteinemia are concomitantly risk factors for rHEpo resistance and CVD appearance.

4. Inflammation – Oxidant stress

Inflammatory indices are increased in ESRF patients; the inflammatory state in these patients is due to factors related to uremia itself, to dialysis procedure and possibly to co-morbidities that they present (atherosclerosis, diabetes, sub-clinical infections, advanced age etc). Oxidative stress is another factor possibly interrelated to inflammation in HD patients (^{12, 13}.

Inflammatory cytokines can directly inhibit red cell production by suppressing erythroid colony-forming units and indirectly by impairing iron availability. The second one can be mediated through down regulation of transferring receptors in erythroid cells along with increased expression of lactoferrin receptors, which in turn causes increased iron uptake by tissue macrophages ¹⁴ RhuEpo resistance induced by oxidative stress is possibly mediated through direct suppression of erythroid progenitor cells, oxidative damage of red blood cells (RBC) membrane – reducing RBCs survival- increased lipid peroxidation in erythrocytes and impaired iron availability ^{14, 15, 16, 17}

It is questionable how EPO interferes with oxidative stress. Some studies have shown that EPO has antioxidant properties ^{18, 19, 20, 21} and others that increases the need for antioxidant treatment due to increased consumption of vitamin E, and this is one of the adverse effect correlated with high doses, frequently used in rHuEpo resistance ¹⁵.

Atherosclerosis is an inflammatory process. Recently, many clinical studies showed that low-grade inflammation could predict atherosclerotic CVD. The already existed or by IV iron supplementation or other causes induced inflammation – and oxidant stress - might be a risk index not only for rHuEpo resistance, but also for atherosclerotic CVD. Furthermore, as recent experimental studies showed inflammation might be a risk factor ^{22, 23} rather than only a risk index for atherosclerosis initiation and progression.

5. Secondary Hyperparathyroidism

Secondary hyperparathyroidism is a common complication of ESRF. An inverse relation between intact Parathormone (PTH) levels and rHuEpo resistance has been detected in patients undergoing HD. PTH can directly inhibit erythropoesis via inhibition of EPO synthesis, bone marrow erythroid progenitor’s suppression and indirectly via marrow fibrosis²⁴

Furthermore, there is evidence that secondary hyperparathyroidism in ESRF patients correlates with CVD, especially myocardial infarction and congestive heart failure, mediated through increased intracellular calcium in myocytes, vascular and valvular calcification, impaired vascular reactivity, and direct toxic effects of PTH on myocardium (cardiomyopathy, left ventricular hypertrophy and fibrosis) ^{3, 25}

6. Inadequate dialysis

Although recent conclusions from the HEMO study showed that moderately increasing dialysis adequacy cannot influence morbidity and mortality in HD patients, inadequate dialysis is identified as an important factor of poor response to rHuEpo ²⁶ in HD patients. Dialysis dose and frequency are related to the removal of uremic inhibitors of erythropoesis, whereas bioincompatibility of dialysis membranes plays an important role in anemia and rHuEpo resistance, independently of dialysis adequacy. ²⁶ Secondly, extracorporeal circuit correlates with mechanical injury of red cells and haemolysis. On the other hand, dialysis adequacy is possibly correlated to morbidity and mortality – the cardiovascular one is the more significant – that HD patients present ²⁷. Additionally, bioincompatible dialysis membranes have been associated with cardiovascular risk due to increased monocyte activation and production or release of pro-inflammatory and inflammatory cytokines ²⁷. Thus it seems also that inadequate dialysis and possibly bioincompatibility is another common risk factor for rHEpo and cardiovascular morbidity and mortality in HD patients.

7. Malnutrition

Malnutrition is a common problem in ESRF patients. It is a potential rHEpo factor. The recently described MIA (Malnutrition-Inflammation-Atherosclerosis) syndrome correlates increased inflammatory to decreased nutritional indexes and atherosclerosis in ESRF patients. Thus malnutrition and inflammation could be common and interrelated risk factors both for CVD and rHEpo ²⁸.

Conclusions

Although the hypothesis of the present lecture is paradoxical, literature data seem to confirm this suggestion. Many causative factors of rHEpo are also CVD risk factors. These later are the non-traditional or emerging atherosclerotic CVD factors that probably are more significant for this disease prevalence in populations like ESRF patients than in general population. Fig 1 summarizes correlations between rHEpo – CVD and causative or risk factors. An ESRF patient resistant to rHEpo treatment seems to have also a greater risk for CVD than a responder to this treatment. The cause is the existence of common factors that could provoke both entities.




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