Cardiovascular risk factors in chronic kidney disease: does phosphate qualify?
Authors: Keith Hruska, Suresh Mathew, Richard Lund, Yifu Fang, Toshifumi Sugatani
Journal: Kidney International (2011)

Risk factors for disease states are rigorously defined. This analysis considers the definition of a risk factor as applied to the question of whether the serum phosphorus level is a risk factor for cardiovascular disease. Observational studies strongly suggest that phosphorus is associated with cardiovascular risk, and definitive prospective animal studies are supportive. A plausible mechanism of action has been discovered demonstrating that phosphorus stimulates osteoblastic transition of cells in the neointima of atherosclerotic plaques, which, if prevented, blocks vascular calcification. However, prospective studies demonstrating that modulation of the putative risk factor affects clinical outcomes are lacking, and phosphorus, as yet, does not qualify as a cardiovascular risk factor. This is a clarion call for additional research. Risk factors for disease states are rigorously defined. This analysis considers the definition of a risk factor as applied to the question of whether the serum phosphorus level is a risk factor for cardiovascular disease. Observational studies strongly suggest that phosphorus is associated with cardiovascular risk, and definitive prospective animal studies are supportive. A plausible mechanism of action has been discovered demonstrating that phosphorus stimulates osteoblastic transition of cells in the neointima of atherosclerotic plaques, which, if prevented, blocks vascular calcification. However, prospective studies demonstrating that modulation of the putative risk factor affects clinical outcomes are lacking, and phosphorus, as yet, does not qualify as a cardiovascular risk factor. This is a clarion call for additional research. Among the 25 million American patients with stage 2–5 chronic kidney disease (CKD), cardiovascular disease causes a disproportionately high mortality risk. Patients with CKD are more likely to die (often of cardiovascular causes) than to progress to dialysis.1.Sarnak M.J. Levey A.S. Cardiovascular disease and chronic renal disease: a new paradigm.Am J Kidney Dis. 2000; 35: S117-S131Abstract Full Text Full Text PDF PubMed Scopus (484) Google Scholar The risk of death is especially high in late-stage kidney disease; a 30-year-old patient with end-stage renal disease faces an equivalent risk of death to a 90-year-old without CKD.2.Foley R.N. Parfrey P.S. Sarnak M.J. Clinical epidemiology of cardiovascular disease in chronic renal disease.Am J Kidney Dis. 1998; 32: S112-S119Abstract Full Text PDF PubMed Scopus (2964) Google Scholar The cardiovascular risk factors generally considered do not explain the heightened cardiovascular risk in CKD.3.Berl T. American Society of Nephrology Presidential Address 2005.J Am Soc Nephrol. 2006; 17: 926-931Crossref PubMed Scopus (2) Google Scholar Vascular calcification4.London G.M. Marchais S.J. Guerin A.P. Metivier F. Adda H. Arterial structure and function in end-stage renal disease.Nephrol Dial Transplant. 2002; 17: 1713-1724Crossref PubMed Scopus (209) Google Scholar, 5.Marchais S.J. Metivier F. Guerin A.P. et al.Association of hyperphosphataemia with haemodynamic disturbances in end-stage renal disease.Nephrol Dial Transplant. 1999; 14: 2178-2183Crossref PubMed Scopus (110) Google Scholar and hyperphosphatemia6.Block G.A. Hulbert-Shearon T.E. Levin N.W. et al.Association of serum phosphorus and calcium x phosphate product with mortality risk in chronic hemodialysis patients: a national study.Am J Kidney Dis. 1998; 31: 607-617Abstract Full Text Full Text PDF PubMed Scopus (2110) Google Scholar drive cardiovascular risk in CKD, and they are related. Risk factors are rigorously defined by four types of evidence: (1) observational studies; (2) definitive prospective translational or clinical studies; (3) mechanism of action studies; and (4) outcome studies showing risk reduction when the putative factor is corrected. Serum phosphorus, at the current state of our knowledge, is associated with the first three of these four lines of evidence. One early observational outcome study has also shown risk reduction benefits with correction of phosphate levels in patients with CKD. This review will summarize current evidence for phosphate as a cardiovascular risk factor in the general population and among patients with CKD. Serum phosphorus and mortality risk have been analyzed in multiple observational studies. In patients with end-stage renal disease on dialysis (40,538 hemodialysis patients), those with serum phosphorus >6.0 to 7.0mg/dl had a relative risk of death 1.25 times that of those with serum phosphorus ≤5mg/dl. The lowest serum phosphate category (<3mg/dl) showed slightly increased risk (1.2 versus 4.5mg/dl).7.Block G.A. Klassen P.S. Lazarus J.M. et al.Mineral metabolism, mortality, and morbidity in maintenance hemodialysis.J Am Soc Nephrol. 2004; 15: 2208-2218Crossref PubMed Scopus (2208) Google Scholar In a retrospective cohort study of 6730 US veterans with CKD not receiving dialysis (those transplanted or without phosphorus measurements were excluded), the adjusted hazard ratio for death rose to 1.90 in patients with serum phosphorus >5.0mg/dl (compared with patients with serum phosphorus <2.5mg/dl).8.Kestenbaum B. Sampson J.N. Rudser K.D. et al.Serum phosphate levels and mortality risk among people with chronic kidney disease.J Am Soc Nephrol. 2005; 16: 520-528Crossref PubMed Scopus (949) Google Scholar In the general population, serum phosphorus has been associated with the risk of cardiovascular disease (Figure 1). The Framingham Offspring Study,9.Dhingra R. Sullivan L.M. Fox C.S. et al.Relations of serum phosphorus and calcium levels to the incidence of cardiovascular disease in the community.Arch Intern Med. 2007; 167: 879-885Crossref PubMed Scopus (674) Google Scholar deleted of participants with histories of CKD, revealed that increasing serum phosphorus was associated with a continuous increasing risk of cardiovascular disease (heart attack, stroke, angina, peripheral vascular disease, or heart failure). In a post hoc analysis of the Cholesterol and Recurrent Events study10.Tonelli M. Sacks F. Pfeffer M. et al.Relation between serum phosphate level and cardiovascular event rate in people with coronary disease.Circulation. 2005; 112: 2627-2633Crossref PubMed Scopus (701) Google Scholar (Figure 1), serum phosphate showed a graded independent relationship to risk of death and new cardiovascular events in patients who had suffered heart attacks previously and had normal kidney function. In both the Cholesterol and Recurrent Events study and the US Third Health and Nutrition Examination Survey populations, elevation of serum alkaline phosphatase and serum phosphate together conferred greater risk than either parameter alone.11.Tonelli M. Curhan G. Pfeffer M. et al.Relation between alkaline phosphatase, serum phosphate, and all-cause or cardiovascular mortality.Circulation. 2009; 120: 1784-1792Crossref PubMed Scopus (204) Google Scholar How might serum phosphorus affect atherosclerosis? In CKD, we associate hyperphosphatemia causally with vascular calcification, which stiffens arteries and leads to cardiovascular events. In young and middle-aged adults without CKD, serum phosphorus levels were associated with vascular stiffness and coronary artery calcium levels in the Multi-Ethnic Study of Atherosclerosis. In middle-aged participants with mild-to-moderate CKD,12Ix J.H. De Boer I.H. Peralta C.A. et al.Serum phosphorus concentrations and arterial stiffness among individuals with normal kidney function to moderate kidney disease in MESA.Clin J Am Soc Nephrol. 2009; 4: 609-615Crossref PubMed Scopus (138) Google Scholar the ankle–brachial index increased in parallel with serum phosphorus levels within the normal range. In the Coronary Artery Risk Development in Young Adults study, which was a 15-year prospective observational study,13.Foley R.N. Collins A.J. Herzog C.A. et al.Serum phosphorus levels associate with coronary atherosclerosis in young adults.J Am Soc Nephrol. 2009; 20: 397-404Crossref PubMed Scopus (291) Google Scholar 10% of the participants with 15-year data experienced significant coronary calcification during follow-up, related to the serum phosphorus level at the beginning of the follow-up period. The complexity of human disease causing cardiovascular risk requires a translational model to dissect the role of phosphorus in vascular calcification and coronary artery disease development in CKD. There are two types of translational studies that address the issue of phosphorus as a cardiovascular risk. In the first, genetic engineering was used to produce deficiency of the fibroblast growth factor-23 skeletal hormone, which is responsible for regulating renal phosphate excretion. Mice with fibroblast growth factor-23 deficiency develop hyperphosphatemia, excess calcitriol and vascular calcification, and have shortened life spans. Feeding these mice low-phosphate diets corrects hyperphosphatemia, eliminates vascular calcification, and lengthens their life span. The second type of study that demonstrates the function of inorganic phosphate as a cardiovascular risk factor comprises studies in animal models of atherosclerosis in which hyperphosphatemia is induced by CKD. We have used the low-density lipoprotein receptor-deficient mouse (ldlr−/−) fed a high-fat diet with ablative CKD. LDL−/− mice fed high-fat diets develop hypercholesterolemia, metabolic syndrome, and vascular calcification. CKD added to this model induces hyperphosphatemia (beginning at stage 3 CKD) and increases vascular, especially aortic, calcification.14.Towler D.A. Bidder M. Latifi T. et al.Diet-induced diabetes activates an osteogenic gene regulatory program in the aortas of low density lipoprotein receptor-deficient mice.J Biol Chem. 1998; 273: 30427-30434Crossref PubMed Scopus (224) Google Scholar Administration of phosphate binders and bone morphogenetic protein (BMP)-7 in this model prevented hyperphosphatemia by reducing phosphate absorption and sending serum phosphate into the skeleton.15.Davies M.R. Lund R.J. Mathew S. et al.Low turnover osteodystrophy and vascular calcification are amenable to skeletal anabolism in an animal model of chronic kidney disease and the metabolic syndrome.J Am Soc Nephrol. 2005; 16: 917-928Crossref PubMed Scopus (136) Google Scholar, 16.Mathew S. Lund R.J. Strebeck F. et al.Reversal of the adynamic bone disorder and decreased vascular calcification in chronic kidney disease by sevelamer carbonate therapy.J Am Soc Nephrol. 2007; 18: 122-130Crossref PubMed Scopus (95) Google Scholar In mice with established vascular calcification (atherogenic diet begun at 10 weeks, CKD induced by 14 weeks, and phosphate binder or BMP-7 administered from 22 to 28 weeks), control of phosphate for weeks 22–28 with sevelamer carbonate or BMP-7 diminished vascular calcification and prevented cardiac hypertrophy.15.Davies M.R. Lund R.J. Mathew S. et al.Low turnover osteodystrophy and vascular calcification are amenable to skeletal anabolism in an animal model of chronic kidney disease and the metabolic syndrome.J Am Soc Nephrol. 2005; 16: 917-928Crossref PubMed Scopus (136) Google Scholar, 16.Mathew S. Lund R.J. Strebeck F. et al.Reversal of the adynamic bone disorder and decreased vascular calcification in chronic kidney disease by sevelamer carbonate therapy.J Am Soc Nephrol. 2007; 18: 122-130Crossref PubMed Scopus (95) Google Scholar The study with BMP-7 originally addressed concerns that BMP-7 might stimulate vascular calcification, but in fact showed BMP-7 to be therapeutic, decreasing aortic calcification below control levels and stimulating bone formation. The stimulation of bone formation was the mechanism of BMP-7-induced correction of hyperphosphatemia, and a component of the action against vascular calcification. A scientific consensus more advanced than mere plausibility has developed with regard to the mechanism by which high serum phosphate leads to vascular calcification and cardiovascular risk. Pathologically, there are two types of large artery calcification stimulated by CKD, calcification of the smooth muscle tunica media and atherosclerotic calcification of smooth muscle cells in plaque neointima (Figure 2). Atherosclerotic calcification is more important in cardiovascular mortality risk, and tunica media calcification is more important in vascular stiffness.17.Hruska K.A. Vascular smooth muscle cells in the pathogenesis of vascular calcification.Circ Res. 2009; 104: 710-711Crossref PubMed Scopus (25) Google Scholar In atherosclerotic calcification, neointimal cells express an osteoblastic phenotype, either by migration and differentiation of pericytes or by dedifferentiation, migration, and redifferentiation of smooth muscle cells. Initial renal injury induces dedifferentiation of vascular smooth muscle cells, which makes them migratory and vulnerable to osteoblastic induction by BMP-2. Additional studies have shown phosphorus to be a molecule capable of stimulating signal transduction.18.Giachelli C.M. Vascular calcification: in vitro evidence for the role of inorganic phosphate.J Am Soc Nephrol. 2003; 14: S300-S304Crossref PubMed Google Scholar, 19.Li X. Yang H.Y. Giachelli C.M. Role of the sodium-dependent phosphate cotransporter, Pit-1, in vascular smooth muscle cell calcification.Circ Res. 2006; 98: 905-912Crossref PubMed Scopus (359) Google Scholar We have shown inorganic phosphate to stimulate expression of osterix both in vivo in our ldlr−/− high-fat fed CKD mice and in vitro in human aortic smooth muscle cells derived from atherosclerotic donors with early CKD20.Mathew S. Tustison K.S. Sugatani T. et al.The mechanism of phosphorus as a cardiovascular risk factor in CKD.J Am Soc Nephrol. 2008; 19: 1092-1105Crossref PubMed Scopus (204) Google Scholar (Figure 3). Osterix is an osteoblast transcription factor required for cellular stimulation of matrix mineralization.21.Nakashima K. Zhou X. Kunkel G. et al.The novel zinc finger-containing transcription factor osterix is required for osteoblast differentiation and bone formation.Cell. 2002; 108: 17-29Abstract Full Text Full Text PDF PubMed Scopus (2784) Google Scholar Primary mouse or human smooth muscle cells in vitro transitioned from normal to high-phosphate culture medium (1 to 2 mmol/l) will mineralize their extracellular matrix in 2–3 weeks.20.Mathew S. Tustison K.S. Sugatani T. et al.The mechanism of phosphorus as a cardiovascular risk factor in CKD.J Am Soc Nephrol. 2008; 19: 1092-1105Crossref PubMed Scopus (204) Google Scholar Blocking of osterix expression in the presence of high phosphorus prevents mineralization.Figure 3Hyperphosphatemia stimulates expression of osterix in the aortas of low-density lipoprotein receptor-deficient mouse (ldlr−/−) with chronic kidney disease (CKD) fed high-fat diets. LaCO3, a non-calcium-containing phosphate binder, reverses osterix expression. GAPDH, glyceraldehyde 3-phosphate dehydrogenase.View Large Image Figure ViewerDownload Hi-res image Download (PPT) Reducing serum phosphorus (for example, with phosphate binders) reverses osteoblastic differentiation of vascular cells and reverses vascular calcification.20.Mathew S. Tustison K.S. Sugatani T. et al.The mechanism of phosphorus as a cardiovascular risk factor in CKD.J Am Soc Nephrol. 2008; 19: 1092-1105Crossref PubMed Scopus (204) Google Scholar Osteoblastic transition and calcification of smooth muscle cells (akin to bone formation) in the atherosclerotic plaque is an active and reversible process. Osteoclasts and large multinucleated macrophages are present in plaques and can actually reabsorb calcification when serum phosphorus is decreased. The phosphate balance diagram is shown in Figure 4. In health, bone formation and resorption balance each other, and there is capacity in the skeletal mineralization fronts to absorb a transient positive phosphate balance, which would be converted into bone. This permits the skeleton to function physiologically as a phosphate and calcium reservoir. In CKD-related mineral and bone disorder, renal osteodystrophy causes excess bone resorption over bone formation in both high- and low-turnover forms of osteodystrophy. This blocks the normal reservoir function of the skeleton. In CKD, increasing fractional excretion of phosphorus (via adaptation of intact nephrons) maintains phosphorus balance in the early stages, but eventually reduction in the tubular reabsorption of phosphate cannot keep pace and phosphate balance becomes positive in stage 3 CKD before actual hyperphosphatemia occurs. The increased bone resorption contributes to the exchangeable phosphate pool, and to hyperphosphatemia. When extra phosphorus is present from a positive balance, in this situation it exits from the exchangeable pool into heterotopic deposition sites. Therefore, the skeleton contributes to heterotopic calcification especially in the vasculature in CKD. The pathophysiology of phosphorus balance in CKD is dependent on a recently discovered skeletal hormone, fibroblast growth factor-23, and its relevant co-receptor, Klotho, which are required for renal phosphate excretion. Their deficiency causes hyperphosphatemia and heterotopic mineralization,22.Razzaque M.S. Lanske B. The emerging role of the fibroblast growth factor-23-klotho axis in renal regulation of phosphate homeostasis.J Endocrinol. 2007; 194: 1-10Crossref PubMed Scopus (185) Google Scholar which can be rescued by a low-phosphate diet. In this translational model, vascular calcification appears to be the mechanism by which high serum phosphorus increases cardiovascular and mortality risk. Low-phosphate diets improved survival along with preventing vascular calcification. Human studies demonstrating that reducing serum phosphorus concentrations decreases cardiovascular risk have not been conducted. However, some studies have been conducted that are supportive of the conclusion that phosphorus is a cardiovascular risk factor. The Accelerated Mortality on Renal Replacement study23.Isakova T. Gutiérrez O.M. Chang Y. et al.Phosphorus binders and survival on hemodialysis.J Am Soc Nephrol. 2009; 20: 388-396Crossref PubMed Scopus (309) Google Scholar (Figure 5) was a prospectively observed cohort study of 10,044 incident hemodialysis patients, comparing those who did with those who did not receive any phosphate-binder treatment during the first 90 days of dialysis. Phosphate binder use was associated with significantly reduced mortality risk on multivariate analysis (on an intent-to-treat or as-treated basis), as well as in a propensity score-matched comparison. Benefits were independent of baseline serum phosphate levels, and thus may reflect lowered levels of the phosphaturic factor fibroblast growth factor-23, which is induced up to 100-fold in untreated hyperphosphatemic dialysis patients. The Renagel in New Dialysis study,24.Block G.A. Spiegel D.M. Ehrlich J. et al.Effects of sevelamer and calcium on coronary artery calcification in patients new to hemodialysis.Kidney Int. 2005; 68: 1815-1824Abstract Full Text Full Text PDF PubMed Scopus (718) Google Scholar an 18-month randomized, controlled trial of sevelamer HCl versus calcium-containing phosphate binder in 109 new hemodialysis patients, showed that among patients with baseline coronary artery calcification, calcium binder use was associated with more rapid and severe progression of calcification than sevelamer HCl use. A pre-specified secondary end-point analysis of the Renagel in New Dialysis study25.Block G.A. Raggi P. Bellasi A. et al.Mortality effect of coronary calcification and phosphate binder choice in incident hemodialysis patients.Kidney Int. 2007; 71: 438-441Abstract Full Text Full Text PDF PubMed Scopus (672) Google Scholar revealed that, in 127 new hemodialysis patients randomized to sevelamer versus calcium-containing phosphate binders and followed up for 44 months, baseline coronary artery calcification score was a predictor of all-cause mortality. Sevelamer was associated with a significant survival benefit versus calcium-containing binders in this study. From the discussion above, the following conclusions are drawn. First, observational studies suggest that serum phosphate is a cardiovascular risk factor in patients with CKD and in the general population. Second, translational studies demonstrate that hyperphosphatemia stimulates atherosclerotic vascular calcification by inducing osteoblastic gene expression in the aorta, and that correction of hyperphosphatemia decreases vascular calcification. Third, studies in vitro demonstrate that medium inorganic phosphate is an active signaling molecule, the receptor may be Pit1, and phosphate directly stimulates osteoblastic transcription factors. Fourth, human studies are consistent in showing the effects of phosphate and its correction on vascular calcification and cardiovascular mortality. However, these studies are not formal outcome studies and are insufficient evidence without the translational data. Fifth, the strong epidemiologic and mechanistic evidence suggesting that serum phosphate is a cardiovascular risk factor needs to be confirmed by human prospective and outcome studies to satisfy the rigorous definition of phosphorus as a cardiovascular risk factor. Finally, in the meantime, before formal outcome studies are completed, interventions to maintain serum phosphate normal in patients with CKD before end-stage renal disease are clearly warranted.