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NDT Advance Access originally published online on September 27, 2006
Nephrology Dialysis Transplantation 2006 21(12):3354-3357; doi:10.1093/ndt/gfl446
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© The Author [2006]. Published by Oxford University Press on behalf of ERA-EDTA. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Beyond phosphate—role of uraemic toxins in cardiovascular calcification

Igor Nikolov1,2, Nobuhiko Joki1, Tilman Drüeke1 and Ziad Massy2

1Inserm Unit 507 and Division of Nephrology, Necker Hospital, University Paris V, Paris and 2Inserm ERI-12, University of Picardie and Amiens University Hospital, Amiens, France

Correspondence and offprint requests to: Prof. Ziad Massy, MD, PhD, INSERM ERI-12, Division(s) of Clinical Pharmacology and Nephrology, Amiens University Hospital, Av. René Laennec, F-80054, Amiens, France. Email: massy{at}u-picardie.fr

Keywords: bone turnover; cardiovascular mortality; chronic kidney disease; uraemic toxins; vascular calcifications



   Introduction
Top
 Introduction
Lessons from experimental...
 Role of uraemic toxicity
 Conclusion
 Acknowledgement
 References
 
Vascular calcification (VC) is a hallmark of atherosclerosis and has been linked to increased cardiovascular disease and mortality. VC is highly prevalent in patients with chronic kidney disease (CKD), in whom it occurs more frequently and progresses more rapidly than in the general population [1–3]. In CKD patients, cardiovascular mortality increases exponentially with age and is up to 500 times higher than in the general population [4]. The presence of VC is independently predictive of subsequent cardiovascular disease and mortality, beyond established conventional risk factors [5, 6]. VC develops at two sites of the arterial wall: the intima and the media. The survival of CKD patients with arterial media calcification is longer than in patients with arterial intima calcification, whose survival in turn is significantly shorter than that of VC-free patients [7].

Metabolic disturbances in CKD involving calcium, phosphorus, parathyroid hormone and vitamin D, and exogenous factors including excessive vitamin D and calcium intake contribute to the initiation and progression of VC. Disorders of mineral and bone metabolism in CKD patients (CKD-MBD) are associated with an increased risk for cardiovascular calcification, morbidity and mortality [8]. Among them, increased serum phosphorus and serum calcium–phosphorus ion product have been shown to correlate with progressive vascular and/or valvular calcification and mortality in numerous observational studies [2, 9]. However, evidence has been accumulating that soft tissue calcification is not only a passive process involving calcium and phosphate precipitation due to low ion solubility in serum, but also an active process involving numerous players. The major role of circulating and local promoters and inhibitors of extraosseous calcification has been progressively recognized in recent years [10]. The complex interaction between active and passive processes guarantees the prevention of soft tissue calcium phosphate deposition under physiological circumstances. Disturbances of this subtle balance in CKD lead to calcification of blood vessels and other soft tissues. A better understanding of the underlying mechanisms is of great importance for the treatment and prevention of this dramatic complication. It may eventually allow us to improve the poor prognosis of these patients.



   Lessons from experimental studies in vitro and in vivo
Top
 Introduction
 Lessons from experimental...
Role of uraemic toxicity
 Conclusion
 Acknowledgement
 References
 
Vascular smooth muscle cells (VSMCs) maintained in culture can undergo phenotypic changes secondary to an increase in medium phosphate concentration, towards a phenotype of osteoblast-like cells [11]. Increased phosphate and calcium levels in the incubation milieu synergistically and independently induce VSMC phosphate uptake [12]. The relative importance of the two ions in this process remains a matter of debate [13]. Under high incubation medium calcium/phosphate conditions, the surrounding extracellular matrix undergoes mineralization along a process resembling that of bone. However, as aforementioned, high calcium and/or phosphorus concentrations are only part of the game in a complex, multistep process. A large number of factors are involved in the initiation of vascular calcification. Alkaline phosphatase (ALP) is one of the phenotypic markers of osteoblast-like activity within the VSMC layer. Calcifying, VSMC-derived cells express high levels of ALP, and their calcification activity is highly dependent on the degree of ALP activity [14, 15], possibly through degradation of pyrophosphate which is physiologically protective [16]. A higher expression of osteocalcin, osteonectin and bone Gla protein is also associated with the intensity of the transformation of VSMC towards a calcifying phenotype [17, 18]. In contrast, other local or circulating proteins such as osteopontin, matrix-Gla protein, fetuin-A, osteoprotegerin, FGF-23 and klotho function as potent inhibitors of ectopic calcification, as shown in a number of reports based on elegant studies in genetically engineered animals, which have been reviewed elsewhere [19, 20].



   Role of uraemic toxicity
Top
 Introduction
 Lessons from experimental...
 Role of uraemic toxicity
Conclusion
 Acknowledgement
 References
 
CKD is characterized by the retention of uraemic solutes which are normally excreted by healthy kidneys. The uraemic syndrome is attributed to the progressive retention of a large number of compounds, called uraemic toxins, which are thought to interact negatively with numerous biological functions. The uraemic state represents a unique clinical condition in which direct tissue toxicity goes along with indirect toxic effects of retention solutes such as mineral and endocrine disturbances, inflammation and oxidative stress. In patients with advanced stages of CKD, both classical and non-classical cardiovascular risk factors have been considered to be of prime importance [21, 22]. To evaluate the effects of the different classes of uraemic toxins in an optimal manner, they have been recently classified, providing a systematic analytical approach and mapping the relative importance of the enlisted families of toxins [23]. Many protein-bound molecules are small, water-soluble compounds of low-molecular weight (LMW), characterized by reduced removal during standard dialysis using small-pore membranes. Peptidic and non-peptidic substances of middle- and high-molecular weight also accumulate in uraemia, including inflammation and oxidative stress products, and are also considered as uraemic toxins [23]. Uraemic retention solutes may exert toxicity, especially if they are protein-bound [24]. The biological action of such compounds is nevertheless exerted only by their free fraction. Hence, the use of total blood or plasma concentrations for activity assessment in vitro is only adequate if the experimental medium contains appropriate amounts of plasma proteins.

In order to examine the effect of the uraemic state on vascular disease progression, we and others assessed the effects of chronic renal failure on vascular calcification and atherosclerosis in vivo, using the apolipoprotein-E (apoE–/–) knockout mouse model [25–27]. We found enhanced progression of both intimal and medial vessel wall calcification in uraemic mice, as compared with non-uraemic mice. In addition, we also observed accelerated atheromatous plaque formation [27], in agreement with two other groups [25, 26], along with an increase in plaque collagen content. This uraemic mouse model was useful for subsequent experiments aimed at exploring in more detail the contribution of the uraemic state to the progression of atherosclerosis and calcification. We administered sevelamer for this purpose to apoE–/– mice with normal and reduced renal function, respectively [28]. As expected, we found that sevelamer treatment of uraemic apoE–/– mice reduced the progression of arterial calcification, in association with a significant decrease in serum phosphate and calcium phosphate product. Unexpectedly, sevelamer also delayed the progression of atherosclerosis, to become similar to the level of apoE–/– mice with normal renal function. This effect was observed in the absence of a change in serum total cholesterol levels. We could not, however, exclude possible changes of low-density (LDL) and/or high-density lipoproteins (HDL) in this study. We then examined the possible role of uraemic toxins. We failed to observe changes in the serum concentration of five compounds tested, although their serum levels were higher in uraemic than non-uraemic mice. On the other hand, the delayed progression of both vascular calcification and atherosclerosis in response to sevelamer treatment was associated with a significant decrease in the aortic expression of nitrotyrosine, a marker of local oxidative stress. In this study, we did not measure advanced oxidation protein products (AOPP) as markers of systemic oxidative stress. In any case, we believe that assessment of oxidative stress in target vascular tissue, wherever possible, is more relevant than circulating markers. As noted above, CKD is a state of chronic oxidative stress, with excessive production of reactive oxygen species and impaired anti-oxidant defence [29]. This is reflected by increased circulating levels of oxidative stress markers such as AOPP, which can be considered as high-molecular weight uraemic toxins and which are also active players by themselves [30]. Oxidative stress has also been shown to favour vascular calcification in vitro, by stimulating the transformation of VSMC to osteoblast phenotype. Thus, oxidized LDL have been shown to induce the calcification process in vitro [31–35]. This effect can be blocked by HDL [31]. In CKD patients, the reduction by sevelamer of serum total cholesterol, LDL cholesterol and C-reactive protein levels along with the reduced progression of arterial calcification are in favour of a beneficial effect of the induced changes in lipid metabolism and inflammation [36]. In dialysis patients, sevelamer has been shown to reduce serum levels not only of phosphate, but also of other LMW uraemic toxins such as uric acid in patients with CKD [37]. Moreover, in experiments in vitro it was able to absorb uraemic compounds such as indoxyl sulfate, indole and p-cresol [38]. Based on our results with sevelamer in this experimental model and on in vitro and in vivo findings by others, we propose the hypothesis that the effects of this phosphate binder on oxidative stress, and more generally speaking on uraemic toxicity, represents a therapeutic modality for cardiovascular disease, including arterial calcification, beyond the mere control of hyperphosphataemia (Figure 1).


Figure 1
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  		Fig. 1. Hypothetical view on possible interactions between sevelamer, uraemic toxicity, vascular calcification and cardiovascular disease.

 


   Conclusion
Top
 Introduction
 Lessons from experimental...
 Role of uraemic toxicity
 Conclusion
Acknowledgement
 References
 
Numerous in vitro and in vivo experiments have provided valuable novel information on the process of vascular calcification and related cardiovascular disease. They indicate potential targets and point to potential tools for future studies, aimed at halting the ectopic deposition of calcium and phosphate and possibly reversing this process. Among these targets, the uraemic syndrome with its direct and indirect toxicity for practically all tissues and biological systems deserves particular attention. A reduction of the retention of uraemic toxins of any type and molecular class, and of their widespread noxious effects, including substances which enhance oxidative stress and/or impair anti-oxidant defence, may help to decrease the incidence and progression of vascular calcification and related cardiovascular morbidity and mortality. Therefore, we believe that it is time to develop and to explore new pharmacological approaches to clear not only one uraemic toxin but several of them at the same time (Figure 1). This hypothesis, if confirmed by additional solid clinical and experimental evidence, might complement renal replacement therapy procedures in those CKD patients who are already on dialysis treatment, and represent a valuable treatment approach in those who have not yet reached end-stage kidney disease.



   Acknowledgement
Top
 Introduction
 Lessons from experimental...
 Role of uraemic toxicity
 Conclusion
 Acknowledgement
References
 
I.N. was funded by a grant from Egide Foundation, Paris, France.

Conflict of interest statement. T.D. and Z.M. declare having received grant support, lecture fees and honoraria from Genzyme and are members of the European Uraemic Toxin (EUTox) group.



   References
Top
 Introduction
 Lessons from experimental...
 Role of uraemic toxicity
 Conclusion
 Acknowledgement
 References
 

  1. Braun J, Oldendorf M, Moshage W, Heidler R, Zeitler E, Luft PC. (1996) Electron beam computed tomography in the evaluation of cardiac calcification in chronic dialysis patients. Am J Kidney Dis 27:394–401.[Web of Science][Medline]
  2. Goodman WG, Goldin J, Kuizon BD, et al. (2000) Coronary-artery calcification in young adults with end-stage renal disease who are undergoing dialysis. N Engl J Med 342:1478–1483.[Abstract/Free Full Text]
  3. Schwarz U, Buzello M, Ritz E, et al. (2000) Morphology of coronary atherosclerotic lesions in patients with end-stage renal failure. Nephrol Dial Transplant 15:218–223.[Abstract/Free Full Text]
  4. Foley RN, Parfrey PS, Sarnak MJ. (1998) Clinical epidemiology of cardiovascular disease in chronic renal disease. Am J Kidney Dis 32:S112–119.[Web of Science][Medline]
  5. Wilson PW, Kauppila LI, O'Donnell CJ, et al. (2001) Abdominal aortic calcific deposits are an important predictor of vascular morbidity and mortality. Circulation 103:1529–1534.[Abstract/Free Full Text]
  6. Blacher J, Guerin AP, Pannier B, Marchais SJ, London GM. (2001) Arterial calcifications, arterial stiffness, and cardiovascular risk in end-stage renal disease. Hypertension 38:938–942.[Abstract/Free Full Text]
  7. London GM, Guerin AP, Marchais SJ, Metivier F, Pannier B, Adda H. (2003) Arterial media calcification in end-stage renal disease: impact on all-cause and cardiovascular mortality. Nephrol Dial Transplant 18:1731–1740.[Abstract/Free Full Text]
  8. Moe S, Drueke T, Cunningham J, et al. (2006) Definition, evaluation, and classification of renal osteodystrophy: A position statement from Kidney Disease: Improving Global Outcomes (KDIGO). Kidney Int 69:1945–1953.[CrossRef][Web of Science][Medline]
  9. Wang AY, Wang M, Woo J, et al. (2003) Cardiac valve calcification as an important predictor for all-cause mortality and cardiovascular mortality in long-term peritoneal dialysis patients: a prospective study. J Am Soc Nephrol 14:159–168.[Abstract/Free Full Text]
  10. Ketteler M, Brandenburg V, Jahnen-Dechent W, Westerfeld R, Floege J. (2005) Do not be misguided by guidelines: the calcium x phosphate product can be a Trojan horse. Nephrol Dial Transplant 20:673–677.[Free Full Text]
  11. Jono S, McKee MD, Murry CE, et al. (2000) Phosphate regulation of vascular smooth muscle cell calcification. Circ Res 87:E10–17.[Web of Science][Medline]
  12. Reynolds JL, Joannides AJ, Skepper JN, et al. (2004) Human vascular smooth muscle cells undergo vesicle-mediated calcification in response to changes in extracellular calcium and phosphate concentrations: a potential mechanism for accelerated vascular calcification in ESRD. J Am Soc Nephrol 15:2857–2867.[Abstract/Free Full Text]
  13. Lomashvili K, Garg P, O'Neill WC. (2006) Chemical and hormonal determinants of vascular calcification in vitro. Kidney Int 69:1464–1470.[Web of Science][Medline]
  14. Shioi A, Katagi M, Okuno Y, et al. (2002) Induction of bone-type alkaline phosphatase in human vascular smooth muscle cells: roles of tumor necrosis factor-alpha and oncostatin M derived from macrophages. Circ Res 91:9–16.[Abstract/Free Full Text]
  15. Jono S, Nishizawa Y, Shioi A, Morii H. (1998) 1,25-Dihydroxyvitamin D3 increases in vitro vascular calcification by modulating secretion of endogenous parathyroid hormone-related peptide. Circulation 98:1302–1306.[Abstract/Free Full Text]
  16. Lomashvili KA, Cobbs S, Hennigar RA, Hardcaste KI, O'Neil WC. (2004) Phosphate-induced vascular calcification: role of pyrophosphate and osteopontin. J Am Soc Nephrol 15:1392–1401.[Abstract/Free Full Text]
  17. Trion A and van der Laarse A. (2004) Vascular smooth muscle cells and calcification in atherosclerosis. Am Heart J 147:808–814.[CrossRef][Web of Science][Medline]
  18. Steitz SA, Speer MY, Curinga G, et al. (2001) Smooth muscle cell phenotypic transition associated with calcification: upregulation of Cbfa1 and downregulation of smooth muscle lineage markers. Circ Res 89:1147–1154.[Abstract/Free Full Text]
  19. Ketteler M, Westenfeld R, Schlieper G, Brandenburg V, Floege J. (2005) "Missing" inhibitors of calcification: general aspects and implications in renal failure. Pediatr Nephrol 20:383–388.[CrossRef][Web of Science][Medline]
  20. Cozzolino M, Brancaccio D, Gallieni M, Slatopolsky E. (2005) Pathogenesis of vascular calcification in chronic kidney disease. Kidney Int 68:429–436.[CrossRef][Web of Science][Medline]
  21. Fleischmann EH, Bower JD, Salahudeen AK. (2001) Are conventional cardiovascular risk factors predictive of two-year mortality in hemodialysis patients? Clin Nephrol 56:221–230.[Web of Science][Medline]
  22. Kennedy R, Case C, Fathi R, Johnson D, Isbel N, Manvick TH. (2005) Does renal failure cause an atherosclerotic milieu in patients with end-stage renal disease? Am J Med 110:198–204.
  23. Vanholder R, De Smet R, Glorieux G, et al. (2003) Review on uremic toxins: classification, concentration, and interindividual variability. Kidney Int 63:1934–1943.[CrossRef][Web of Science][Medline]
  24. Vanholder R, Massy Z, Argiles A, Spasovski G, Veibexe F, Lameire N. (2005) Chronic kidney disease as cause of cardiovascular morbidity and mortality. Nephrol Dial Transplant 20:1048–1056.[Abstract/Free Full Text]
  25. Bro S, Bentzon JF, Falk E, Andersen CB, Olgaard K, Nialsen LB. (2003) Chronic renal failure accelerates atherogenesis in apolipoprotein E-deficient mice. J Am Soc Nephrol 14:2466–2474.[Abstract/Free Full Text]
  26. Buzello M, Tornig J, Faulhaber J, Ehmke H, Ritz E, Amann K. (2003) The apolipoprotein e knockout mouse: a model documenting accelerated atherogenesis in uremia. J Am Soc Nephrol 14:311–316.[Abstract/Free Full Text]
  27. Massy ZA, Ivanovski O, Nguyen-Khoa T, et al. (2005) Uremia accelerates both atherosclerosis and arterial calcification in apolipoprotein E knockout mice. J Am Soc Nephrol 16:109–116.[Abstract/Free Full Text]
  28. Phan O, Ivanovski O, Nguyen-Khoa T, et al. (2005) Sevelamer prevents uremia-enhanced atherosclerosis progression in apolipoprotein E-deficient mice. Circulation 112:2875–2882.[Abstract/Free Full Text]
  29. Massy ZA, Maziere C, Kamel S, et al. (2005) Impact of inflammation and oxidative stress on vascular calcifications in chronic kidney disease. Pediatr Nephrol 20:380–382.[CrossRef][Web of Science][Medline]
  30. Liu SX, Hou FF, Guo ZJ, et al. (2006) Advanced oxidation protein products accelerate atherosclerosis through promoting oxidative stress and inflammation. Arterioscler Thromb Vasc Biol 26:1156–1162.[Abstract/Free Full Text]
  31. Parhami F, Morrow AD, Balucan J, et al. (1997) Lipid oxidation products have opposite effects on calcifying vascular cell and bone cell differentiation. A possible explanation for the paradox of arterial calcification in osteoporotic patients. Arterioscler Thromb Vasc Biol 17:680–687.[Abstract/Free Full Text]
  32. Tintut Y, Patel J, Parhami F, Demer LL. (2000) Tumor necrosis factor-alpha promotes in vitro calcification of vascular cells via the cAMP pathway. Circulation 102:2636–2642.[Abstract/Free Full Text]
  33. Chen NX, O'Neill KD, Duan D, Moe SM. (2002) Phosphorus and uremic serum up-regulate osteopontin expression in vascular smooth muscle cells. Kidney Int 62:1724–1731.[CrossRef][Web of Science][Medline]
  34. Mody N, Parhami F, Sarafian TA, Demer LL. (2001) Oxidative stress modulates osteoblastic differentiation of vascular and bone cells. Free Radic Biol Med 31:509–519.[CrossRef][Web of Science][Medline]
  35. Yamagishi S, Fujimori H, Yonekura H, Tanaka N, Yamamoto H. (1999) Advanced glycation endproducts accelerate calcification in microvascular pericytes. Biochem Biophys Res Commun 258:353–357.[CrossRef][Web of Science][Medline]
  36. Ferramosca E, Burke S, Chasan-Taber S, et al. (2005) Potential antiatherogenic and anti-inflammatory properties of sevelamer in maintenance hemodialysis patients. Am Heart J 149:820–825.[CrossRef][Web of Science][Medline]
  37. Garg JP, Chasan-Taber S, Blair A, et al. (2005) Effects of sevelamer and calcium-based phosphate binders on uric acid concentrations in patients undergoing hemodialysis: a randomized clinical trial. Arthritis Rheum 52:290–295.[CrossRef][Web of Science][Medline]
  38. DeSmet R, Thermote F, Lameire N, Vanholder R. (2004) Sevelamer hydrochloride (Renagel) adsorbs the uremic compounds indoxyl sulfate, indole and p-cresol [Abstract]. JASN 15:505A.
Received for publication: 23. 6.06
Accepted in revised form: 27. 6.06


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