REVIEWS Magnesium and cardiovascular complications of chronic kidney disease Ziad A. Massy and Tilman B. Drüeke Abstract | Cardiovascular complications are the leading cause of death in patients with chronic kidney disease (CKD). Abundant experimental evidence suggests a physiological role of magnesium in cardiovascular function, and clinical evidence suggests a role of the cation in cardiovascular disease in the general population. The role of magnesium in CKD–mineral and bone disorder, and in particular its impact on cardiovascular morbidity and mortality in patients with CKD, is however not well understood. Experimental studies have shown that magnesium inhibits vascular calcification, both by direct effects on the vessel wall and by indirect, systemic effects. Moreover, an increasing number of epidemiologic studies in patients with CKD have shown associations of serum magnesium levels with intermediate and hard outcomes, including vascular calcification, cardiovascular events and mortality. Intervention trials in these patients conducted to date have had small sample sizes and have been limited to the study of surrogate parameters, such as arterial stiffness, vascular calcification and atherosclerosis. Randomized controlled trials are clearly needed to determine the effects of magnesium supplementation on hard outcomes in patients with CKD. Massy, Z. A. & Drüeke, T. B. Nat. Rev. Nephrol. advance online publication 12 May 2015; doi:10.1038/nrneph.2015.74
Introduction
Division of Nephrology, Ambroise Paré University Hospital, University of Versailles St Quentin, 9 Avenue Charles de Gaulle, 92104 Boulogne Billancourt, Paris, France (Z.A.M.). Inserm Unit 1088, UFR of Medicine & Pharmacy, Université of Picardie Jules Vernes, 1 rue des Louvels, 80037 Amiens, France (T.B.D.) Correspondence to: Z.A.M. ziad.massy @apr.aphp.fr
Magnesium is the fourth most abundant cation in the body and the second most abundant cation in the intra cellular fluid compartment. The most important role of magnesium is probably its contribution to chemical energy via the formation of ATP by magnesium-dependent oxi dative phosphorylation. Magnesium is also involved in a variety of enzymatic reactions, glycolysis, DNA syn thesis and transcription, protein synthesis, cellular ion currents and determination of membrane voltage.1 The kidney is a major regulator of magnesium homeo stasis. As proof of concept, a variety of inherited dis orders of magnesium deficiency result from defective renal handling of the cation in the thick ascending limb of the loop of Henle and the distal convoluted tubule.2 In patients with moderate chronic kidney disease (CKD) serum magnesium levels are maintained within normal limits by an increase in fractional excretion to compen sate for the loss of glomerular filtration. As disease sever ity increases this compensatory mechanism becomes insufficient, and overt hypermagnesaemia may develop in patients with end-stage renal disease (ESRD).3 Renal magnesium regulation occurs through glomeru lar filtration and subsequent reabsorption, which takes place in the thick ascending limb via the paracellular pathway, and in the distal convoluted tubule via the transcellular route involving transient receptor potential Competing interests Z.A.M. has received speakers’ honoraria and research grants from Amgen, Genzyme, Fresenius Medical Care and Shire. T.B.D. has received advisor and/or consultancy honoraria from Amgen, Fresenius Medical Care and Sanofi, and speaker honoraria from Amgen, Sanofi and Kirin.
cation channel subfamily M member 6 (TRPM6) chan nels.4 In patients receiving dialysis, serum magnesium levels and magnesium balance largely depend on the magnesium concentration of the dialysate, although dietary intake and use of medications such as laxatives and antacids also contribute. Magnesium status has an important impact on the function of the cardiovascular system. 5 For many decades, clinical interest in the cation was limited to disease states caused by magnesium depletion or excess (Table 1). Magnesium deficiency and/or hypomagnes aemia are often associated with hypocalcaemia and hypokalaemia. Hypomagnesaemia can lead to impaired exercise capacity, sarcopenia and, in more severe forms, general weakness, neuromuscular hyperexcitability with hyper-reflexia, carpopedal spasm and seizure, whereas hypermagnesaemia owing to severe magnesium excess may induce loss of deep tendon reflexes, respira tory paralysis, hypotension and loss of consciousness. Magnesium depletion is associated with cardiovascular disease, probably through several mechanisms, includ ing abnormal cardiac conduction, and might increase the risk of sudden cardiac death.6,7 An association between magnesium depletion and type 2 diabetes mellitus has also been reported.8 On the other hand, magnesium sulphate has an established place in the treatment and prevention of eclampsia.9 Over the past 20 years, numerous prospective obser vational studies and a meta-analysis have shown inverse associations between serum magnesium levels and cardiovascular events in the general population.10 In addi tion, a study of the Atherosclerosis Risk in Communities
NATURE REVIEWS | NEPHROLOGY
ADVANCE ONLINE PUBLICATION | 1 © 2015 Macmillan Publishers Limited. All rights reserved
REVIEWS Key points ■■ Maintenance of normal serum levels of magnesium depends on net absorption in the gut, uptake by and release from bone, and net excretion in the kidney ■■ Patients with advanced chronic kidney disease (CKD) often have high serum magnesium concentrations ■■ Experimental studies suggest that modulation of extracellular magnesium concentrations affects vascular calcification and arterial function via effects on vascular smooth muscle cells and the endothelium ■■ Several different mechanisms exist by which magnesium might inhibit the process of vascular calcification in CKD ■■ Epidemiologic studies indicate possible links between serum magnesium levels, the incidence of CKD, and adverse outcomes, including mortality, in the general population and in patients with CKD ■■ Data from small, preliminary studies suggest beneficial effects of oral magnesium supplementation on cardiovascular calcification and surrogate parameters of atherosclerosis
Table 1 | Manifestations of hypomagnesaemia70,96 and hypermagnesaemia97 Disorder
Serum magnesium level (mmol/l)
Manifestation(s)
Severe hypomagnesemia
5.00
Muscle paralysis Quadriplegia Apnoea Complete heart block Cardiac arrest
Abbreviation: PTH, parathyroid hormone. Modified with permission from Wiley © Navarro-Gonzalez, J. F. et al. Clinical implications of disordered magnesium homeostasis in chronic renal failure and dialysis. Semin. Dial. 22, 37–44 (2009).
(ARIC) cohort, which included >14,000 participants, reported an independent association between low mag nesium levels and incident heart failure.11 Patients with inherited disorders of hypomagnesaemia (mostly owing
to renal tubular defects of magnesium handling) present with a variety of complications, including nephrocalci nosis, mental retardation, epilepsy and diabetes.2 To the best of our knowledge, however, an increased frequency of cardiovascular morbidity and mortality has not been reported in patients with these conditions. Finally, an inverse relationship was observed between dietary mag nesium intake and all-cause mortality in a secondary analysis of a Spanish prospective randomized trial that was conducted in more than 7,000 adults at high risk of cardiovascular disease.12 In patients with CKD, studies of magnesium have long been rare relative to those of calcium and phosphate. Moreover, such studies have focused more often on the effects of various magnesium concentrations in dialysate in patients receiving haemodialysis or peritoneal dialysis than on changes in magnesium metabolism induced by CKD. As a result of the paucity of available evidence, the Kidney Disease: Improving Global Outcomes guideline on CKD–mineral and bone disorder (CKD–MBD) did not elaborate specific diagnostic and therapeutic recom mendations with respect to magnesium, in contrast to those provided for calcium and phosphate.13 The attention paid to this neglected cation in patients with CKD is, however, increasing. Several reasons for this increased interest exist, including epidemiologic studies that suggest links between low serum magne sium levels, the incidence and progression of CKD,14–16 and adverse outcomes in patients with CKD16,17 and/ or acute kidney injury;18 intervention studies showing possible benefits of oral magnesium supplementation in patients with CKD;19,20 and experimental studies report ing inhibitory effects of magnesium on vascular calcifica tion in normal and/or uraemic animals21–23 and in in vitro models.24,25 The introduction of a new oral phosphate binder formulation for patients receiving dialysis that combines a magnesium salt with a calcium salt has also led to increased interest in the effects of magnesium.26,27 In this Review, we examine evidence from experimen tal studies and clinical investigations that suggest a role of magnesium in the high frequency of cardiovascular disease in patients with CKD.
In vitro experimental data
Magnesium exerts direct and indirect actions in cardiac and vascular tissues. Although the best studied effects relate to vascular calcification, direct actions of magne sium on arterial function via effects on vascular smooth muscle cells (VSMCs) and the endothelium are highly likely (Figure 1).
Vascular calcification In mice with CKD, the magnesium content of aortic tissue is reportedly elevated similar to that of calcium and phos phate.28 Magnesium can theoretically interfere with car diovascular calcification by indirect actions via phosphate binding in the intestinal lumen, by systemic effects on CKD–MBD-associated factors, and by direct actions at the level of vascular tissues. Consistent with the latter direct role, in vitro studies have shown that magnesium chloride
2 | ADVANCE ONLINE PUBLICATION
www.nature.com/nrneph © 2015 Macmillan Publishers Limited. All rights reserved
REVIEWS High extracellular P and/or Ca Loss of systemic and/or local mineralization inhibitors (Fetuin A, MGP, PPi, OPN, OPG, BMP7) Mg
Stabilization of Ca–acidic phospholipid–P complexes
Formation of Ca/P nanocrystals
Vitamin D sterol
Mg
Pi Ca
Ca
Pit-1 Pit-2
TRPM7
N
Ca
Ca-channels
Endocytosis Pi Ca
Mg
Vascular smooth muscle cell
CaSR Ca
Formation of matrix vesicles
Intracellular Ca-burst
Mg MGP BMP7 Osteocalcin Inhibitors of calcification BMP2 RUNX2 Promotors of VDR calcification
FGF1R
Mg
Klotho
ase rele op tal ck lo r ys dba oc fee an ive sit Po
Mg
Formation of whitlockite (Ca, Mg, P)
Formation of apatite
Ca/P Ca/P
β-catenin
Formation of apoptotic bodies
LRP 5/6
Frizzled Wnt
Osteogenic differentiation
Apoptosis Mg
Vascular calcification
Figure 1 | The putative inhibitory effects of magnesium on the process of vascular calcification. Abnormalities in mineral Nature Reviews | Nephrology metabolism, particularly hyperphosphataemia, and loss of inhibitors of mineralization leads to the formation and deposition of Ca/P nanocrystals, which are taken up by VSMCs. Lysosomal degradation of the endocytosed crystals results in intracellular release of Ca and Pi. In addition, Pi accumulates in the cell via uptake through Pit‑1 and probably also Pit‑2. To compensate for excess Ca/P, VSMCs form matrix vesicles loaded with Ca/P products and the mineralization inhibitors.95 The intracellular Caburst induced by endocytosed nanocrystals and Pi uptake triggers apoptosis, resulting in the formation of Ca/P-containing apoptotic bodies. Matrix vesicles and apoptotic bodies cause a positive feedback loop through nanocrystal release into the surrounding milieu, thus amplifying the calcification process. Furthermore, Ca/P nanocrystals and Pi induce the expression of genes that promote the calcification–mineralization process and repress the expression of factors that inhibit calcification, resulting in transdifferentiation of VSMCs to osteoblast-like cells and, ultimately, vessel calcification. Mg interferes with the process of vascular calcification by inhibiting transformation of amorphous Ca/P to apatite and by forming Mg-substituted whitlockite crystals,31 which result in smaller, more soluble deposits. Secondly, Mg functions as a Ca-channel antagonist and thus inhibits the entry of Ca into the cells. Thirdly, Mg enters the cell via TRPM7 and restores the balance between expression of calcification promoters and inhibitors by neutralizing phosphate-induced inhibition of MGP and BMP7 and enhanced expression of RUNX2 and BMP2. These effects prevent osteoblastic conversion and calcification of VSMCs. In addition, Mg acts on the CaSR; activation of this receptor by calcimimetics has been shown to inhibit VSMC calcification but the molecular mechanisms have not yet been identified. Abbreviations: BMP, bone morphogenetic protein; Ca, calcium; CaSR, calciumsensing receptor; FGF1R, fibroblast growth factor receptor‑1; LRP 5/6, LDL receptor-related protein 5/6; Mg, magnesium; MGP, matrix gla protein; OPG, osteoprotegerin; OPN, osteopontin; Pi, inorganic phosphate; Pit, sodium-dependent phosphate transporter; PPi, pyrophosphate; RUNX2, runt-related transcription factor 2; TRPM7, transient receptor potential cation channel subfamily M member 7; VDR, vitamin D receptor; VSMC, vascular smooth muscle cell. Permission obtained from Oxford University Press © Massy, Z. A. & Drüeke, T. B. Clin. Kidney J. 5 (Suppl. 1), i52–i61 (2013).
can protect against phosphate-induced calcification of rat aortic rings29 and human aortic VSMCs (Figure 2).22,24,30 The first mode of direct action of magnesium is inhib ition of the formation of calcium phosphate apatite
(CPA) and of calcium–acidic phospholipid–phosphate complexes in metastable calcium phosphate solutions. Such inhibition stabilizes amorphous calcium phosphate and favours the formation of whitlockite crystals, which
NATURE REVIEWS | NEPHROLOGY
ADVANCE ONLINE PUBLICATION | 3 © 2015 Macmillan Publishers Limited. All rights reserved
REVIEWS a
b
1.4
Control
3 mM Pi
1.5 mM Mg2+ + 3 mM Pi
2 mM Mg2+ + 3 mM Pi
Total calcium deposition (μg Ca/mg protein) versus Pi
1.2 1.0 ‡
‡
0.8 0.6 0.4 0.2
*
0.0 Control 3 mM Pi 1.2 mM 1.5 mM 2.0 mM 3.0 mM Mg2+ Mg2+ Mg2+ Mg2+ +3 mM Pi
| Nephrology Figure 2 | Magnesium reduces phosphate-induced calcification in human aortic VSMCs. a | After Nature 14 daysReviews of culture at the indicated Mg2+ concentrations in the or presence or absence of 3 mM Pi, calcium deposition in VSMCs from three different donors was assessed using the o‑cresolphthalein complexone method. Data are represented as the ratio of calcium deposition under the indicated conditions versus calcium deposition in the presence of Pi alone. b | von Kossa staining followed by haematoxylin/Mayer counterstaining confirmed an inhibitory effect of Mg2+ on phosphate-induced calcifications. Nuclei are stained red and granulated calcifications are black (×25 magnification). *P
Introduction
Division of Nephrology, Ambroise Paré University Hospital, University of Versailles St Quentin, 9 Avenue Charles de Gaulle, 92104 Boulogne Billancourt, Paris, France (Z.A.M.). Inserm Unit 1088, UFR of Medicine & Pharmacy, Université of Picardie Jules Vernes, 1 rue des Louvels, 80037 Amiens, France (T.B.D.) Correspondence to: Z.A.M. ziad.massy @apr.aphp.fr
Magnesium is the fourth most abundant cation in the body and the second most abundant cation in the intra cellular fluid compartment. The most important role of magnesium is probably its contribution to chemical energy via the formation of ATP by magnesium-dependent oxi dative phosphorylation. Magnesium is also involved in a variety of enzymatic reactions, glycolysis, DNA syn thesis and transcription, protein synthesis, cellular ion currents and determination of membrane voltage.1 The kidney is a major regulator of magnesium homeo stasis. As proof of concept, a variety of inherited dis orders of magnesium deficiency result from defective renal handling of the cation in the thick ascending limb of the loop of Henle and the distal convoluted tubule.2 In patients with moderate chronic kidney disease (CKD) serum magnesium levels are maintained within normal limits by an increase in fractional excretion to compen sate for the loss of glomerular filtration. As disease sever ity increases this compensatory mechanism becomes insufficient, and overt hypermagnesaemia may develop in patients with end-stage renal disease (ESRD).3 Renal magnesium regulation occurs through glomeru lar filtration and subsequent reabsorption, which takes place in the thick ascending limb via the paracellular pathway, and in the distal convoluted tubule via the transcellular route involving transient receptor potential Competing interests Z.A.M. has received speakers’ honoraria and research grants from Amgen, Genzyme, Fresenius Medical Care and Shire. T.B.D. has received advisor and/or consultancy honoraria from Amgen, Fresenius Medical Care and Sanofi, and speaker honoraria from Amgen, Sanofi and Kirin.
cation channel subfamily M member 6 (TRPM6) chan nels.4 In patients receiving dialysis, serum magnesium levels and magnesium balance largely depend on the magnesium concentration of the dialysate, although dietary intake and use of medications such as laxatives and antacids also contribute. Magnesium status has an important impact on the function of the cardiovascular system. 5 For many decades, clinical interest in the cation was limited to disease states caused by magnesium depletion or excess (Table 1). Magnesium deficiency and/or hypomagnes aemia are often associated with hypocalcaemia and hypokalaemia. Hypomagnesaemia can lead to impaired exercise capacity, sarcopenia and, in more severe forms, general weakness, neuromuscular hyperexcitability with hyper-reflexia, carpopedal spasm and seizure, whereas hypermagnesaemia owing to severe magnesium excess may induce loss of deep tendon reflexes, respira tory paralysis, hypotension and loss of consciousness. Magnesium depletion is associated with cardiovascular disease, probably through several mechanisms, includ ing abnormal cardiac conduction, and might increase the risk of sudden cardiac death.6,7 An association between magnesium depletion and type 2 diabetes mellitus has also been reported.8 On the other hand, magnesium sulphate has an established place in the treatment and prevention of eclampsia.9 Over the past 20 years, numerous prospective obser vational studies and a meta-analysis have shown inverse associations between serum magnesium levels and cardiovascular events in the general population.10 In addi tion, a study of the Atherosclerosis Risk in Communities
NATURE REVIEWS | NEPHROLOGY
ADVANCE ONLINE PUBLICATION | 1 © 2015 Macmillan Publishers Limited. All rights reserved
REVIEWS Key points ■■ Maintenance of normal serum levels of magnesium depends on net absorption in the gut, uptake by and release from bone, and net excretion in the kidney ■■ Patients with advanced chronic kidney disease (CKD) often have high serum magnesium concentrations ■■ Experimental studies suggest that modulation of extracellular magnesium concentrations affects vascular calcification and arterial function via effects on vascular smooth muscle cells and the endothelium ■■ Several different mechanisms exist by which magnesium might inhibit the process of vascular calcification in CKD ■■ Epidemiologic studies indicate possible links between serum magnesium levels, the incidence of CKD, and adverse outcomes, including mortality, in the general population and in patients with CKD ■■ Data from small, preliminary studies suggest beneficial effects of oral magnesium supplementation on cardiovascular calcification and surrogate parameters of atherosclerosis
Table 1 | Manifestations of hypomagnesaemia70,96 and hypermagnesaemia97 Disorder
Serum magnesium level (mmol/l)
Manifestation(s)
Severe hypomagnesemia
5.00
Muscle paralysis Quadriplegia Apnoea Complete heart block Cardiac arrest
Abbreviation: PTH, parathyroid hormone. Modified with permission from Wiley © Navarro-Gonzalez, J. F. et al. Clinical implications of disordered magnesium homeostasis in chronic renal failure and dialysis. Semin. Dial. 22, 37–44 (2009).
(ARIC) cohort, which included >14,000 participants, reported an independent association between low mag nesium levels and incident heart failure.11 Patients with inherited disorders of hypomagnesaemia (mostly owing
to renal tubular defects of magnesium handling) present with a variety of complications, including nephrocalci nosis, mental retardation, epilepsy and diabetes.2 To the best of our knowledge, however, an increased frequency of cardiovascular morbidity and mortality has not been reported in patients with these conditions. Finally, an inverse relationship was observed between dietary mag nesium intake and all-cause mortality in a secondary analysis of a Spanish prospective randomized trial that was conducted in more than 7,000 adults at high risk of cardiovascular disease.12 In patients with CKD, studies of magnesium have long been rare relative to those of calcium and phosphate. Moreover, such studies have focused more often on the effects of various magnesium concentrations in dialysate in patients receiving haemodialysis or peritoneal dialysis than on changes in magnesium metabolism induced by CKD. As a result of the paucity of available evidence, the Kidney Disease: Improving Global Outcomes guideline on CKD–mineral and bone disorder (CKD–MBD) did not elaborate specific diagnostic and therapeutic recom mendations with respect to magnesium, in contrast to those provided for calcium and phosphate.13 The attention paid to this neglected cation in patients with CKD is, however, increasing. Several reasons for this increased interest exist, including epidemiologic studies that suggest links between low serum magne sium levels, the incidence and progression of CKD,14–16 and adverse outcomes in patients with CKD16,17 and/ or acute kidney injury;18 intervention studies showing possible benefits of oral magnesium supplementation in patients with CKD;19,20 and experimental studies report ing inhibitory effects of magnesium on vascular calcifica tion in normal and/or uraemic animals21–23 and in in vitro models.24,25 The introduction of a new oral phosphate binder formulation for patients receiving dialysis that combines a magnesium salt with a calcium salt has also led to increased interest in the effects of magnesium.26,27 In this Review, we examine evidence from experimen tal studies and clinical investigations that suggest a role of magnesium in the high frequency of cardiovascular disease in patients with CKD.
In vitro experimental data
Magnesium exerts direct and indirect actions in cardiac and vascular tissues. Although the best studied effects relate to vascular calcification, direct actions of magne sium on arterial function via effects on vascular smooth muscle cells (VSMCs) and the endothelium are highly likely (Figure 1).
Vascular calcification In mice with CKD, the magnesium content of aortic tissue is reportedly elevated similar to that of calcium and phos phate.28 Magnesium can theoretically interfere with car diovascular calcification by indirect actions via phosphate binding in the intestinal lumen, by systemic effects on CKD–MBD-associated factors, and by direct actions at the level of vascular tissues. Consistent with the latter direct role, in vitro studies have shown that magnesium chloride
2 | ADVANCE ONLINE PUBLICATION
www.nature.com/nrneph © 2015 Macmillan Publishers Limited. All rights reserved
REVIEWS High extracellular P and/or Ca Loss of systemic and/or local mineralization inhibitors (Fetuin A, MGP, PPi, OPN, OPG, BMP7) Mg
Stabilization of Ca–acidic phospholipid–P complexes
Formation of Ca/P nanocrystals
Vitamin D sterol
Mg
Pi Ca
Ca
Pit-1 Pit-2
TRPM7
N
Ca
Ca-channels
Endocytosis Pi Ca
Mg
Vascular smooth muscle cell
CaSR Ca
Formation of matrix vesicles
Intracellular Ca-burst
Mg MGP BMP7 Osteocalcin Inhibitors of calcification BMP2 RUNX2 Promotors of VDR calcification
FGF1R
Mg
Klotho
ase rele op tal ck lo r ys dba oc fee an ive sit Po
Mg
Formation of whitlockite (Ca, Mg, P)
Formation of apatite
Ca/P Ca/P
β-catenin
Formation of apoptotic bodies
LRP 5/6
Frizzled Wnt
Osteogenic differentiation
Apoptosis Mg
Vascular calcification
Figure 1 | The putative inhibitory effects of magnesium on the process of vascular calcification. Abnormalities in mineral Nature Reviews | Nephrology metabolism, particularly hyperphosphataemia, and loss of inhibitors of mineralization leads to the formation and deposition of Ca/P nanocrystals, which are taken up by VSMCs. Lysosomal degradation of the endocytosed crystals results in intracellular release of Ca and Pi. In addition, Pi accumulates in the cell via uptake through Pit‑1 and probably also Pit‑2. To compensate for excess Ca/P, VSMCs form matrix vesicles loaded with Ca/P products and the mineralization inhibitors.95 The intracellular Caburst induced by endocytosed nanocrystals and Pi uptake triggers apoptosis, resulting in the formation of Ca/P-containing apoptotic bodies. Matrix vesicles and apoptotic bodies cause a positive feedback loop through nanocrystal release into the surrounding milieu, thus amplifying the calcification process. Furthermore, Ca/P nanocrystals and Pi induce the expression of genes that promote the calcification–mineralization process and repress the expression of factors that inhibit calcification, resulting in transdifferentiation of VSMCs to osteoblast-like cells and, ultimately, vessel calcification. Mg interferes with the process of vascular calcification by inhibiting transformation of amorphous Ca/P to apatite and by forming Mg-substituted whitlockite crystals,31 which result in smaller, more soluble deposits. Secondly, Mg functions as a Ca-channel antagonist and thus inhibits the entry of Ca into the cells. Thirdly, Mg enters the cell via TRPM7 and restores the balance between expression of calcification promoters and inhibitors by neutralizing phosphate-induced inhibition of MGP and BMP7 and enhanced expression of RUNX2 and BMP2. These effects prevent osteoblastic conversion and calcification of VSMCs. In addition, Mg acts on the CaSR; activation of this receptor by calcimimetics has been shown to inhibit VSMC calcification but the molecular mechanisms have not yet been identified. Abbreviations: BMP, bone morphogenetic protein; Ca, calcium; CaSR, calciumsensing receptor; FGF1R, fibroblast growth factor receptor‑1; LRP 5/6, LDL receptor-related protein 5/6; Mg, magnesium; MGP, matrix gla protein; OPG, osteoprotegerin; OPN, osteopontin; Pi, inorganic phosphate; Pit, sodium-dependent phosphate transporter; PPi, pyrophosphate; RUNX2, runt-related transcription factor 2; TRPM7, transient receptor potential cation channel subfamily M member 7; VDR, vitamin D receptor; VSMC, vascular smooth muscle cell. Permission obtained from Oxford University Press © Massy, Z. A. & Drüeke, T. B. Clin. Kidney J. 5 (Suppl. 1), i52–i61 (2013).
can protect against phosphate-induced calcification of rat aortic rings29 and human aortic VSMCs (Figure 2).22,24,30 The first mode of direct action of magnesium is inhib ition of the formation of calcium phosphate apatite
(CPA) and of calcium–acidic phospholipid–phosphate complexes in metastable calcium phosphate solutions. Such inhibition stabilizes amorphous calcium phosphate and favours the formation of whitlockite crystals, which
NATURE REVIEWS | NEPHROLOGY
ADVANCE ONLINE PUBLICATION | 3 © 2015 Macmillan Publishers Limited. All rights reserved
REVIEWS a
b
1.4
Control
3 mM Pi
1.5 mM Mg2+ + 3 mM Pi
2 mM Mg2+ + 3 mM Pi
Total calcium deposition (μg Ca/mg protein) versus Pi
1.2 1.0 ‡
‡
0.8 0.6 0.4 0.2
*
0.0 Control 3 mM Pi 1.2 mM 1.5 mM 2.0 mM 3.0 mM Mg2+ Mg2+ Mg2+ Mg2+ +3 mM Pi
| Nephrology Figure 2 | Magnesium reduces phosphate-induced calcification in human aortic VSMCs. a | After Nature 14 daysReviews of culture at the indicated Mg2+ concentrations in the or presence or absence of 3 mM Pi, calcium deposition in VSMCs from three different donors was assessed using the o‑cresolphthalein complexone method. Data are represented as the ratio of calcium deposition under the indicated conditions versus calcium deposition in the presence of Pi alone. b | von Kossa staining followed by haematoxylin/Mayer counterstaining confirmed an inhibitory effect of Mg2+ on phosphate-induced calcifications. Nuclei are stained red and granulated calcifications are black (×25 magnification). *P
