Coronary calcification in hemodialysis patients: The contribution of traditional and uremia-related risk factors

Kidney International, Vol. 67 (2005), pp. 1576–1582

Coronary calcification in hemodialysis patients: The contribution of traditional and uremia-related risk factors DANIELA VEIT BARRETO, FELLYPE CARVALHO BARRETO, ALU´ıZIO BARBOSA CARVALHO, ´ ANTONIO DRAIBE, ROSA MARIE AFONSO MOYSES, LILIAN CUPPARI, MIGUEL CENDOROGLO, SERGIO ´ KATIA RODRIGUES NEVES, VANDA JORGETTI, ANDREW BLAIR, ROBERT GUIBERTEAU, ˆ FERNANDES CANZIANI and MARIA EUGENIA Department of Internal Medicine/Division of Nephrology, Federal University of S˜ao Paulo, S˜ao Paulo, Brazil; Department of Internal Medicine/Division of Nephrology, University of S˜ao Paulo, S˜ao Paulo, Brazil; and Genzyme Corporation, Cambridge, Massachusetts

Coronary calcification in hemodialysis patients: The contribution of traditional and uremia-related risk factors. Background. Coronary artery calcification is a common feature of atherosclerosis, occurring in 90% of angiographically significant lesions. There is recent evidence that coronary artery calcification is frequent in hemodialysis patients and it has been suggested that this increased incidence may be associated to uremia-related factors. The development and progression of coronary artery calcification is similar to osteogenesis. The aim of this study was to evaluate the relationship between coronary artery calcification, uremia-related factors, and bone histomorphometry in hemodialysis patients. Methods. A total of 101 hemodialysis patients were assessed for biochemical markers of inflammation, oxidative stress, and bone metabolism. Subsequently, they were submitted to multislice coronary tomography (MSCT) and transiliac bone biopsy. Results. The median calcium score was 116.2 (range 0 to 5547). Fifty-two percent of the patients showed moderate and severe coronary artery calcification, 20% had calcium scores greater than 1000. In univariate analysis, age (r = 0.57, P < 0.000001), osteoprotegerin (OPG) (r = 0.44, P = 0.00002), and body mass index (BMI) (r = 0.24, P = 0.01) correlated positively with calcium score. Bone trabecular volume and trabecular thickness correlated negatively with calcium score (r = −0.24, P = 0.02; r = −0.22, P = 0.03). There was a correlation of borderline significance between calcium score and C-reactive protein (CRP) (r = 0.18, P = 0.062). The multiple linear regression analysis identified OPG as the only variable independently associated with coronary artery calcification. Conclusion. Coronary artery calcification is highly prevalent in the hemodialysis population and is associated with older age, higher BMI, inflammation and reduced trabecular bone volume. Higher OPG is independently associated with coronary artery calcification and may represent an incomplete self-defensive

Key words: coronary calcification, atherosclerosis, renal failure, bone, inflammation, osteoprotegerin. Received for publication April 29, 2004 and in revised form July 2, 2004, and September 17, 2004 Accepted for publication October 14, 2004
C

2005 by the International Society of Nephrology

response to the progression of atherosclerosis in hemodialysis patients.

Cardiovascular disease is responsible for more than 50% of deaths among patients with end-stage renal disease (ESRD) [1]. In patients treated by hemodialysis, the prevalence of coronary artery disease is approximately 40%, and the risk of dying from a cardiovascular event is 20-fold greater than the risk observed in the general population [2, 3]. The high prevalence of classic cardiovascular risk factors, such as hypertension, dyslipidemia, and diabetes, cannot fully explain this exceptionally increased incidence of coronary artery disease. Recently, factors related to uremia and the hemodialysis treatment itself, like hyperhomocysteinemia, increased oxidative stress, altered mineral metabolism, and chronic inflammation, have been proposed to play a role in the pathogenesis of coronary artery disease in uremic patients [4]. In the last two decades, noninvasive techniques have been introduced for the routine diagnosis of coronary artery disease. These techniques allow the measurement of calcium deposits within the coronary arteries, and include electron-beam tomography (EBT) and multislice coronary tomography (MSCT). Previous studies demonstrated an association between vascular calcification and osteoporosis, which was evaluated through bone mineral density. There is evidence that low bone mineral density in the hip is a marker of advanced atherosclerosis in elderly women [5]. In chronic kidney disease patients, bone mineral density has limitations when compared to bone histomorphometry to evaluate renal osteodystrophy [6, 7]. A recently published study has demonstrated a relationship between bone histomorphometric findings and arterial calcification in uremic patients [8].

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The aim of the present study was to evaluate the prevalence of coronary artery calcification by MSCT in hemodialysis patients, and to address the associations among coronary artery calcification and dyslipidemia, inflammation, oxidative stress, bone biochemical markers, and bone histomorphometry in this population. METHODS Subjects and study design One hundred and one adult patients (age >18 years) on maintenance hemodialysis, for at least 3 months, at four dialysis centers in Sao ˜ Paulo, Brazil, were studied. The present study refers to the baseline data of an ongoing randomized clinical trial comparing two phosphate binders, sevelamer and calcium acetate, in hemodialysis patients. Exclusion criteria included serious gastrointestinal disease, ethanol or drug abuse, active malignancy, human immunodeficiency virus (HIV) infection, chronic inflammatory disease, use of steroids, severe hyperparathyroidism [defined as intact parathyroid hormone (iPTH) >1000 pg/mL], body weight >100 kg, continuous use of antiarrhythmic or seizure drugs, pregnancy or breast-feeding, previous myocardial revascularization, uncontrolled diabetes mellitus, or hypertension (as deemed by the investigator). After screening, subjects underwent a 2-week washout period, in which all phosphate binders were withheld. Patients with hyperphosphatemia (serum phosphorus >5.5 mg/dL) during the washout period were eligible. All patients received hemodialysis three times weekly, for at least 4 hours, using hollow fiber polysulphone or celulose acetate membranes. Native arteriovenous fistula was the vascular access type in 92% of the patients. Regarding renal osteodystrophy treatment, in the month before enrollment, 15 patients were on regular use of oral calcitriol, eight of them intermittently (0.25 to 2 lg/hemodialysis), and seven daily (0.25 to 0.5 lg/day) and 95 patients took calcium-containing phosphate binders (either calcium acetate or carbonate). At this time, no patient was receiving either sevelamer or aluminum hydroxide as phosphate binders. In a cross-sectional design, the demographic, dialysisspecific, and clinical characteristics of the selected patients were obtained. Nutritional status of all patients was evaluated by subjective global assessment and patients were classified as well-nourished, mild to moderately malnourished, or severely malnourished [9]. The risk of developing coronary heart disease according to the Framingham risk index was estimated for each patient, as described elsewhere [10]. Blood samples were drawn in the same period. Subsequently, the patients underwent a MSCT as described below. In order to verify long-term alterations on mineral homeostasis that could be related to coronary artery calcification, a bone biopsy was performed. All patients signed an informed consent.

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The study protocol was reviewed and approved by the local institutional ethics board. Biochemical analysis Whole blood was collected at the respective study sites by vein puncture in a fasting state the morning before the first weekly hemodialysis session. Laboratory evaluation included blood count determination, measurements of ionized calcium, phosphorus, alkaline phosphatase (reference ranges <270 U/L for male and <240 U/L for female), iPTH (chemoluminescent substract, DPC) (Medlab, Los Angeles, CA, USA) (reference range 10 to 65 pg/ mL), 25-(OH) vitamin D (radioimmunoassay) (DiaSorin
) (Stillwater, MN, USA) (reference range 18 to 62 ng/dL), osteoprotegerin (OPG) [enzyme-linked immunosorbent assay (ELISA)] (Immundiagnostik Laboratory, Bensheim, Germany) (reference range 30.45 ± 12.1 pg/mL), soluble RANKL (ELISA) (Immundiagnostik Laboratory) (detection limit 1.5 pg/mL), albumin, C-reactive protein (CRP) (High Sensitive CReactive Protein Immunolite
) (immunometric assay) (functional sensibility <0.2 mg/L), homocysteine [highpressure liquid chromatography (HPLC)] (reference range 5 to 17 mmol/L), and anti-oxized low-density lipoprotein (oxLDL) antibody (Anti-oxLDL antibody ELISA) (IMMCO Diagnostics, Buffalo, NY, USA) (reference range <20 EU/mL = negative, 20 to 25 EU/mL = indeterminate, and >25 EU/mL = positive). Imaging procedure Calcification score was evaluated using MSCT (Somatron Volum Zoom Siemens AG
) (Erlhagen, Germany). A chest radiologic image without contrast was acquired while the subject was in apnea in order to determine the initial and final level of the tomography. Images of each section were then acquired, during a 150 msec exposure, with a distance of 3 mm between each slice. The timing of the image acquisition was coordinated with the diastolic phase of the cardiac cycle, at 60% of the RR interval, with the use of electrocardiograph monitoring. All obtained scans were analyzed with the use of a “Workstation” (Indigo 02
SGI
, Mountain View, CA, USA) in order to determine the calcium score. The software can detect calcified lesions with a density of at least 130 Hounsfield units and minimal 0.5 mm2 area. Total calcium score calculation was based on formulas using measurement of total volume and area of calcified lesions, as well as mean and maximum density. The total score was the sum of each coronary score and was expressed in modified Agatston et al [11] and Janowitz et al [12] units. The classification proposed by Rumberger et al [13] was used for descriptive purposes: calcium scores <10 indicate minimal atherosclerotic plaque, calcium scores between 11 and 100 indicate mild plaque burden, calcium scores between 101 and 400 indicate moderate plaque burden, and

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calcium scores higher than 400 indicate severe and extensive plaque burden.

Table 1. Demographic, clinical and laboratorial characteristics of the patients (N = 101) Age years Race white/nonwhite Etiology of end-stage renal disease Unknown% Hypertension% Chronic glomerulonephritis% Diabetes mellitus% Others% End-stage renal disease diagnosis months Length on hemodialysis months Body mass index kg/m2 Comorbid conditions Hypertension% Diabetes mellitus% Smoking% Hemoglobin g/dL Kt/V Cholesterol mg/dL High-density lipoprotein cholesterol mg/dL Low-density lipoprotein cholesterol mg/dLa Very low-density lipoprotein cholesterol mg/dLa Triglycerides mg/dL Homocysteine mmol/Lb Albumin g/dL C-reactive protein mg/L Antioxidized low-density lipoprotein EU/mLc Ionized calcium mmol/L Phosphorus mg/dL Alkaline phosphatase U/L Intact parathyroid hormone pg/mL 25-(OH) vitamin D 3 ng/dLc Osteoprotegerin pg/mLc sRANK-L pg/mLc

48 ± 13 57/44

(21–81)

Bone biopsy Bone biopsies were carried out in either the right or left iliac crest, using a 7 mm inner diameter electrical trephine (Gauthier Medical, Rochester, MN, USA). All patients were prelabeled with oral tetracycline (20 mg/kg/day for 3 days) administered over two separated periods, 10 days apart. Bone fragments were submitted to the usual processing and histologic studies [14]. Sections were stained with toluidine blue staining. Bone histomorphometry was conducted using the semiautomatic method contained in the software Osteomeasure (Osteometrics Inc., Atlanta, GA, USA). The static and dynamic parameters were analyzed in accordance with the standards of the American Society of Bone and Mineral Research [15]. On the basis of the histomorphometry, the patients were classified into the following groups: (1) predominant hyperparathyroid bone disease, defined as bone formation rate, plus either osteoblast or osteoclast surface more than one SD above normal range, osteoid volume within or above the normal range, and marrow fibrosis > 0.5%; (2) adynamic bone disease, defined as bone formation rate and osteoid volume more than one SD below the normal range and marrow fibrosis <0.5%; (3) osteomalacia, defined as bone formation rate more than one SD below the normal range plus osteoid volume more than one SD above the normal range; (4) mixed renal osteodystrophy, defined as bone formation rate more than one SD below the normal range, osteoid volume and osteoblast surface more than one SD above the normal range, and marrow fibrosis ≥0.5%; and (5) isolated osteopenia, defined as trabecular bone volume more than one SD bellow the normal range with the other parameters within the normal range. The reference ranges for bone formation rate are 0.07 ± 0.03 l3 /l2 /day (female)/0.13 ± 0.07 l3 /l2 /day (male); osteoblast surface are 1.4 ± 3.0% (female)/2.13 ± 3.8% (male); osteoclast surface are 0.04 ± 0.08% (female)/0.07 ± 0.22% (male); osteoid volume are 1.58 ± 1.8% (female)/3.2 ± 3.0% (male); and trabecular bone volume are 22 ± 7.3% (female)/25.5 ± 8.1% (male).

Multiple linear regression analysis was conducted to investigate whether the extent of calcification was related to any demographic, clinical, laboratory, or bone histomorphometric variables. A P value < 0.05 was considered significant. All statistical analysis was carried out using the true Epistat program (Epistat Services, Richardson, TX, USA).

Statistical analysis Demographic characteristics were described using mean ± standard deviation, median (range) and proportions. Regarding renal osteodystrophy, a chi-square test was performed to assess the association between each diagnostic pattern with the presence of coronary artery calcification. Mann-Whitney U test was taken to evaluate the differences in calcium score among these subgroups. The association between calcium score and other variables was assessed using the Spearman correlation coefficient.

RESULTS Demographic, clinical and laboratorial characteristics of the 101 patients studied are listed in Table 1. The patients were young and had been undergoing hemodialysis treatment for an average of 3 years. Although nephrosclerosis was the cause of ESRD in 27% of our patients, 70% had hypertension as a comorbid condition, and 16% had diabetes. The majority of the patients (75%) were considered well nourished as evaluated by the subjective global

28 27 19 9 18 64 ± 65

(6–336)

37 ± 25

(4–103)

25 ± 4.3

(17.4–36.6)

70 16 25 11.3 ± 1.6 1.3 ± 0.2 165.4 ± 41.7 42 ± 10.4

(6.4–15.3) (0.64–1.9) (89–308) (20–71)

91.3 ± 29.6

(28–161)

29.5 ± 15.8

(7–79)

156.5 ± 93.4 41 ± 35.1 3.8 ± 0.2 9.8 ± 16.3 27.3 ± 22.1

(34–489) (10.4–160.2) (3.2–4.5) (0.5–129) (0–126)

1.24 ± 0.08 7.22 ± 1.8 245.3 ± 152.5 351.3 ± 310.1 32.8 ± 14.7 174.7 ± 71.7 5.4 ± 8.7

(0.96–1.45) (5.5–19.2) (88–1155) (9–1266) (11.2–94.4) (42.2–434.8) (1.5–50.6)

sRANK-L is soluble receptor activator of NF-KappaB ligand. Values expressed as mean ± SD (range) and proportions. a N = 98; b N = 99; c N = 87.

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Barreto et al: Coronary calcification in hemodialyis patients Table 2. Histomorphometric characteristics (N = 98) Trabecular bone volume % Osteoid thickness lm Osteoid surface % Osteoblast % Eroded surface % Osteoclast % Trabecular thickness lm Trabecular separation lm Travecular number mm Fibrosis volume % Mineralizing surface % Mineral apposition rate l/day Bone formation rate l2 /l3 /day 0.02 Ajusted apposition rate l/day Mineralization lag time days

16.6 7.65 21.8 2.75 4.0 0.7 111.8 534.25 1.55 0.04 2.1 0.70 (0–0.39) 0.09 45.2

(6.8–41.3) (2.8–18.5) (1.9–71) (0–31.1) (0–25.3) (0–4) (77.3–159.7) (197–1526.8) (0.6–7.31) (0–2.9) (0–20.6) (0–1.9) (0–1) (7–494.5)

Values expressed as median (range).

assessment. Overweight or obesity was observed in 51% [body mass index (BMI) >25] of patients. Total cholesterol levels higher than 200 mg/dL were observed in 17% of the patients, low-density lipoprotein (LDL) cholesterol >100 mg/dL in 33%, high-density lipoprotein (HDL) cholesterol <40 mg/dL in 49%, and triglycerides >150 mg/dL in 39% of them. All but one patient had hyperhomocysteinemia (Hcy) >14 mmol/L. The mean values of CRP and antibody against oxLDL were increased. CRP greater than 1.1 mg/L was observed in 93% of the patients, and 61% had CRP 5.0 mg/L. Antibody against oxLDL was positive in 44% of the patients. As expected, all patients showed hyperphosphatemia. The mean values of alkaline phosphatase, iPTH and 25(OH) vitamin D were close to those normally observed in hemodialysis patients. However, there was great variability in these results. In fact, 33% of the patients had an iPTH <150 pg/mL and 39% had an iPTH greater than 300 pg/mL. Table 2 lists the histomorphometric characteristics. All patients were submitted to bone biopsy, but three of the bone fragments were not adequate for histologic analysis. The distribution of renal osteodystrophy patterns, according to histologic parameters, showed that the majority of the patients (56%) were classified as having adynamic bone disease, 12% osteitis fibrosa, 21% mixed renal osteodystrophy, 3% osteomalacia, 3% isolated osteopenia, and 4% normal bone. It is notable that, in general, patients were asymptomatic regarding the osteoarticular system. The coronary calcium score is described in Table 3. Median calcium score was 116.2 (range 0 to 5545.7). Fiftytwo percent of the patients showed moderate to severe calcification. Twenty percent of the patients had calcium scores greater than 1000 and 28% had calcium score = 0. There was no association between the presence of coronary artery calcification (calcium score >10) and any of the different patterns of renal osteodystrophy.

Table 3. Coronary calcification (N = 101) Rumberger classification

%

10–100 101–400 >400

14 21 31

Values expressed as median (range) and proportions.

Table 4. Coronary calcification correlations Univariate Age Cardiovascular risk (Framingham) Osteoprotegerin Body mass index Bone volume Trabecular thickness C-reactive protein Multivariate Osteoprotegerin

r 0.57 0.56 0.44 0.24 −0.24 −0.22 0.18 b coefficient 7.28

P value <0.000001 <0.000001 0.00002 0.01 0.02 0.03 0.062 P value 0.00018

95% CI 3.59–10.99

Table 4 shows the results of univariate and multivariate analyses. When clinical parameters and blood chemistries were considered as a function of calcification, only age, Framingham risk index, OPG, and BMI were significantly associated with higher calcification scores (Fig. 1). Moreover, the histomorphometric parameters trabecular bone volume and trabecular thickness were negatively correlated with calcification score. There was a correlation of borderline significance between CRP and calcium score. In the multivariate regression analysis, the only independent determinant of calcium score was OPG. No association was observed between calcium score and phosphorus, ionized calcium, calcium-phosphorus product, iPTH, vitamin D, lipid profile, homocysteine, anti-oxLDL antibody, demographic, or clinical characteristics. DISCUSSION This study shows a high prevalence of coronary artery calcification in relatively young ESRD patients undergoing hemodialysis. The importance of this finding relies on the fact that the majority of significant atherosclerotic lesions are calcified [16] and previous studies demonstrated that coronary artery calcification is a predictor of coronary artery disease in persons with and without renal failure [17–19]. Watson et al [20], studying nonuremic individuals with high risk of developing coronary artery disease, concluded that vascular calcification detected by EBT occurred in more than 90% of the patients with significant angiographic lesions. Detrano et al [21] conducted a follow-up study of 30 ± 13 months on 491 patients without renal disease submitted simultaneously to coronary angiography and EBT. Patients with calcium score greater than 75.3 (median) showed sixfold higher

OPG, pg/mL

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500 450 400 350 300 250 200 150 100 50 0

r = 0.44 P = 0.0002

0

1000

2000

3000

4000

5000

6000 Fig. 1. Correlation between osteoprotegerin and calcium score in 87 patients.

risk of suffering a myocardial infarction or cardiac death. Marwick et al [19] followed 1173 patients for an average of 19 months after EBT screening. The odds ratio of cardiac event for a calcium score >160 was 35.4%. In the present study, median calcification score was 116.2 and 31% of the patients had calcium scores higher than 400, which suggest severe and extensive atherosclerotic disease. This is in accordance with the study performed by Braun et al [22] comparing calcium scores detected by EBT in hemodialysis patients and nondialysis patients with suspected or documented coronary artery disease. The hemodialysis patients showed 2.5- to fivefold higher calcium scores and progression of the coronary artery calcification within 1 year. It has also been observed that in young hemodialysis patients (<30 years) coronary artery calcification is a common finding and the rate of progression exceeds the rate observed in older persons with normal renal function [23]. As demonstrated by other authors [22, 24, 25], we observed that older age was associated with higher coronary calcium scores. Other traditional risk factors were highly prevalent in the population studied. Hypertension was a comorbid condition observed in 70% of the patients and 49% of the studied population presented at least one altered lipid fraction. In spite of that, we could not observe significant association between any of these variables and coronary calcification. On the other hand, we found that the Framingham risk index correlated significantly with coronary calcium score. Possibly, the Framingham risk reflects an association between the sum of abovementioned traditional risk factors and coronary calcification in hemodialysis patients. However, because this is a cross-sectional study, one cannot establish a causal relationship. There was also a significant association between BMI and coronary calcium score, suggesting that overweight may contribute to increasing coronary artery calcification in this population. Nontraditional risk factors have been proposed to contribute to the high incidence of cardiovascular disease in hemodialysis patients, including Hcy, oxidative stress,

and inflammation [26–28]. In our study, we observed a high prevalence of Hcy (99.9%), however, it was not correlated with coronary calcification. In previous large epidemiologic studies, elevated serum CRP predicted cardiovascular events and cardiovascular death in both healthy and uremic individuals [29–32]. Recently, it has been suggested that CRP may be one of the key mediators of vessel wall calcification in hemodialysis patients [33]. In the present study, 93% of the patients had CRP levels greater than 1.1 mg/L, which is associated with cardiovascular risk in the general population. We observed that patients with higher CRP levels tended to have higher coronary calcium scores. It is also proposed that disorders of mineral metabolism may play a role in the genesis of vascular calcification in ESRD patients. Epidemiologic studies have demonstrated a direct correlation between serum phosphorus and calcium-phosphorus product and mortality, especially from cardiovascular causes, in hemodialysis patients [34, 35]. In a baseline cross-sectional analysis of a prospective study conducted by Raggi et al [36], serum phosphorus and calcium significantly positively correlated with the severity of coronary artery calcification. Other observational studies in ESRD patients showed variable results concerning disorders of mineral metabolism and coronary artery calcification [24, 37, 38]. Goodman et al [23] observed a significant association between duration of dialysis, body-mass index calciumphosphorus product and the amount of daily-intake of calcium-containing phosphate-binders and coronary calcification score in young dialysis patients. Recently, Chertow et al [39] demonstrated a relation between PTH and the progression of coronary artery calcification in hemodialysis patients, depending on the phosphate binder used. Braun et al [22] found an inverse association between the total coronary artery calcium content and vertebral bone mass, confirming the important relationship between abnormal calcium homeostasis and coronary artery calcification in dialysis patients. In spite of that, they did not observe an association between calcium,

Barreto et al: Coronary calcification in hemodialyis patients

phosphorus, iPTH, or the treatment with vitamin D analogues and the calcium score. Accordingly, in our study, we found no association between coronary calcium score and phosphorus, calcium, iPTH, alkaline phosphatase or 25-(OH) vitamin D. The fact that all the patients studied were hyperphosphatemic and that severe hyperparathyroidism was excluded (iPTH >1000 pg/mL) may have limited our observations about the association between coronary artery calcification and alterations in the calcium-phosphorus homeostasis. In the same way, as this is a cross-sectional analysis, the biochemical data may not reflect the timeaveraged exposure to disorders of mineral metabolism. On the other hand, the bone biopsy is still considered as the gold standard for evaluating bone remodeling alterations. In this study, we found a high prevalence of adynamic bone disease. This may reflect previous overtreatment with vitamin D sterols and calcium containing phosphate binders, or perhaps a high calcium burden from dialysate fluid, as most patients (83%) were treated with a 1.75 mmol/L calcium concentration. It seemed to be a tendency for patients with adynamic bone disease to have higher calcium scores (N = 55) (median calcium score 115.2; 0 to 5545.7 arbitrary units) than patients with OF (N = 12) (median calcium score 19.05; 0 to 1217 arbitrary units), but there was great variability between patients, and this difference did not reach statistical significance. Although in this study we did not find any association between the different bone histological diagnostic patterns and the extent of coronary artery calcification, when we analyzed each histomorphometric parameter, we observed that trabecular bone volume and trabecular bone thickness correlated negatively with coronary calcium score. Previous studies demonstrated the relationship between reduced bone mass and vascular calcification in the elderly with osteoporosis and also in hemodialysis patients [40, 22]. London et al [8] observed an association between low bone activity as well as adynamic bone disease and arterial calcification evaluated through ultrasononagraphy. In fact, vascular calcification seems to be a process similar to bone formation that may involve calcium mobilization from bone, excessive calcium positive balance (from the dialysate fluid and or the calcium-based phosphate binders), or a bone-buffering capacity insufficient relatively to the amount of calcium load [35, 41, 42]. In the present study, we observed that patients had elevated OPG and multiple regression analysis showed that OPG was the only independent variable associated with coronary artery calcification. This is in accordance with the study performed by Nitta et al [43], who concluded that rapid progression of vascular calcification in long-term hemodialysis patients was independently associated with high OPG concentration. Kazama et al [44]

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demonstrated that OPG levels are significantly higher in uremic patients than in general population. In nonrenal population, high serum OPG levels were associated with the presence and severity of coronary artery disease [45]. OPG, a soluble decoy receptor of the tumor necrosis factor family, seems to be a possible link between bone remodeling and vascular calcification. OPG is produced by various tissues (bone, lung, kidney, immune tissues, cardiovascular system) and is also expressed in endothelial and smooth-muscle cells in vitro [46, 47]. This cytokine increases mineral density and bone volume by decreasing the number of active osteoclasts [48]. Transgenic mice that overexpress OPG have osteoporosis [46] and it has been shown that OPG-deficient mice develop severe osteoporosis and medial arterial calcification [49]. The conflicting findings between these experimental observations and the above cited clinical studies could be explained by an incomplete compensatory self-defensive response of OPG to the progression of atherosclerosis [50]. This study evaluated simultaneously the role of traditional and nontraditional cardiovascular risk-factors, like Hcy, increasing in oxidative stress, inflammation, and the alterations on bone and mineral metabolism documented by bone biopsies with the development of coronary artery calcification in hemodialysis patients. We concluded that coronary artery calcification is common and severe in this population and is associated with aging, derangements of mineral metabolism and chronic inflammation. Further studies are needed to evaluate the apparently paradoxical effect of OPG on vascular calcification. Reprint requests to Daniela Veit Barreto. E-mail: [email protected] or [email protected]

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