Supplement - The Central Role of Phosphate

A supplement to renAl & urology newsnovember 2010

The Central Role of Phosphate50 Years of Research and Discovery in Chronic Kidney Disease and Mineral & Bone Disorder

IntroductIon

research and discovery leading to the naming of chronic kidney disease — mineral and bone dis-order (CKD-MBD) has now celebrated its 50th anniversary. A Phosphate Centric Forum, supported by genzyme Corporation, was held on 24-25 June 2010 at the sheraton west Park Hotel, Munich, germany. Twenty-seven eminent medical and scientific experts met to present and discuss the central role of phosphate in the development of CKD-MBD, first identi-fied 50 years ago, its development over this period, including the pathophysiology, discoveries such as the vitamin D receptor, calcium-sensing receptor (Casr) and fibro - blast growth factor 23 (FgF-23), and available treatment options, as well as future needs regarding research and management of this disorder.

Educational support provided by Genzyme corporation

50 Years of research and discovery in chronic Kidney disease and Mineral & Bone disorder: the central role of Phosphate

co-chaIrs

Eduardo Slatopolsky, MD, FACP, Joseph Friedman Professor of Renal Diseases in Medicine, Washington University School of Medicine, St. Louis, MO, USA

Sharon Moe, MD, Professor of Medicine and Vice-Chair for Research, Department of Medicine, Indiana University School of Medicine, Indianapolis, IN, USA

PrEsEntInG facultY

Roger Bouillon, MD, Laboratory of Experimental Medicine and Endocrinology, Katholieke Universiteit Leuven, Belgium

David Bushinsky, MD, Professor and Associate Chair of Medicine, Nephrology Unit, University of Rochester School of Medicine and Dentistry, Rochester, NY, USA

Cyrille Confavreux, MD, Service de Rhumatologie, Université de Lyon-Sud, France

Tilman Drueke, MD, INSERM Unité 845 and Service de Néphrologie, Hôpital Necker, Assistance Publique-Hôpitaux de Paris, and Faculté de Médecine René Descartes, Paris, France

Keith Hruska, MD, Director of Pediatric Nephrology, Washington University School of Medicine, St. Louis, MO, USA

Harald Jüppner, MD, Associate Professor of Pediatrics, Harvard University Medical School, Boston, MA, USA

Makoto Kuro-o, MD, Associate Professor of Pathology and Center for Mineral Metabolism and Clinical Research, University of Texas Southwestern Medical Center, Dallas, TX, USA

Martin Pollak, MD, Harvard Institutes of Medicine, Department of Genetics, Brigham and Women’s Hospital, Boston, MA, USA

Mariano Rodriguez, MD, Departamento de Medicina, Hospital Universitaria de Reina Sofia, Cordoba, Spain

facultY

Ezequiel Bellorin-Font, MD, Venezuela Tobias Larsson, MD, SwedenJorge Cannata-Andía, MD, Spain Klaas Olgaard, MD, DenmarkAluizio Carvalho, MD, Brazil Anthony Portale, MD, USARicardo Correa-Rotter, MD, Mexico Isidro B. Salusky, MD, USADavid Cunningham, MD, UK José-Vicente Torregrosa, MD, SpainMasafumi Fukugawa, MD, Japan Myles Wolf, MD, USAVanda Jorgetti, MD, Brazil Maria Eugenia Canziani, MD, Brazil

2 NOVEMBER 2010

a supplement to renal & urology news

The Central Role of Phosphate

Professor slatopolsky presented a summary of the last 50 years of research and discovery

starting with the “intact nephron hypothesis:” each individual nephron is either functional or non-function-

al; as the number of functioning nephrons declines in CKD, there is a lower glomerular filtration rate (gFr). surviving nephrons under-go adaptations that maintain renal homeostasis, but with a “trade-off.”

Hormones that reach high levels during uremic nephron adaptation exert adverse effects in other sys-tems; thus, much CKD pathology does not result from reduced renal excretion itself, but from attempts to maintain homeostasis in the face of declining gFr, e.g., the increase in parathyroid hormone secretion, which occurs as a consequence of phosphate retention.

In the 1960s, many factors currently known to be involved in renal phos-phate physiology were unknown, such as calcitriol, vitamin D receptors, FgF-23 and klotho, discussed later in this program (Figure 1). In the early 1970s, Prof. slatopolsky and colleagues showed that alterations in phosphate metabolism led to second-ary hyperparathyroidism (sHPT) and progression of CKD.

Professor slatopolsky showed that a low-phosphate diet reduces para-thyroid hormone levels by reducing the need for nephrons to increase fractional excretion of phosphate; conversely, a high-phosphate diet stimulates adaptation and leads to the down-regulation of calcium receptors in the parathyroid (PT) gland. He also showed that plasma parathyroid hormone is regulated by duodenal phosphate on a time scale of minutes; low-phosphate duode-nal gavage lowers plasma phosphate and parathyroid hormone within 15 minutes, whereas high-phosphate gavage raises plasma parathyroid

NOVEMBER 2010 3

The intact nephron hypothesis: concept to implications ProfEssor Eduardo slatoPolsKY, usa

FiguRE 1: Pathogenesis of secondary hyperparathyroidism in CKD-MBD.



4 NOVEMBER 2010

The genetic pathways of two diseases that were identi-fied decades ago—familial

benign hypercalcemia (FBH) and familial hypocalciuric hypercalcemia (FHH)—have now been linked to

mutations in the calcium-sensing receptor (Casr) gene, thought to control and regulate calcium levels. Casr has been found in numer-ous tissues, including parathyroid glands, kidney, and bone, and is

widely expressed in tissues and cell types without known involvement in mineral ion metabolism. The role of Casr in these tissues is not clear; however, Casr-knockout mice are small and sick (Figure 2).

Casr are now being considered as therapeutic targets for the following two reasons:

Treatment of various disorders 1. of calcium homeostasis/metabo-lism is inadequate (eg, in hemo-dialysis patients, treatment with vitamin D derivatives can con-trol parathyroid hormone but can lead to hypercalcemia and hyperphosphatemia).other potential disease states 2. might also be amenable to treat-ment by activation or inactivation of Casr: kidney stones, osteoporosis,

hormone just as rapidly. Thus, acute regulation of parathyroid hormone in vivo by dietary phosphate may be mediated by a gut-derived hormone— an intestinal ‘phosphatonin.’ Further analysis of this pathway, including the identification of a phosphate sensor and the mediator of the response, will advance our understanding of phosphate homeostasis.

using a rat model, Professor slatopolsky showed that enhanced transforming growth factor alpha (TgFα) and its receptor (egFr) co-expression is a major contributor to parathyroid hyperplasia in early CKD. His group has shown that para-

thyroid Tumor necrosis factor Alpha Converting enzyme (TACe) activity is a key determinant of the severity of TgFα/egFr-driven hyperplasia in sHPT, and dietary phosphate regu-lates TACe expression.

Questions raised by the partici-pants that remain unanswered to date include the relative importance

of a decrease in 1,25(oH)2D levels versus an increase in serum phosphate for hyperplasia of the parathyroid glands; whether there is a phosphate sensor in the parathyroid gland and gut; and the role of phosphate (which can act independently of calcitriol) in modulating vitamin D synthesis in the diseased kidney in early CKD. n

Figure 2: Mice lacking CaSR behave like humans lacking CaSR.

CaSR and calcimimetics dr MartIn PollaK, usa

Acute regulation of parathyroid hormone by dietary phosphate may be mediated by a hormone, possibly derived from the GI tract—an intestinal ‘phosphatonin.’

NOVEMBER 2010 5

Professor Bushinsky presented the timescales involved in the derangement of mineral metabo-

lism in CKD. As kidneys fail, parathyroid hormone increases, levels of vitamin D decrease, and levels of FgF-23 increase substantially; these changes can occur very early in the course of CKD, whereas calcium and phosphate are tightly con-trolled until quite late in the disease progression. Increases in phosphate and FgF-23 have both been shown to be independent predictors of mortality in CKD patients, and calcium and phos-phate increase vascular calcification in CKD patients, which in itself predicts greater mortality.

using the model of calcium metabo-lism (Figure 3) in a healthy person, the 20 mmoles of calcium ingested is excreted via the feces or urine, with the process being buffered by the bone and passing through the extracellular fluid (eCF).

However, as CKD progresses, the amount of calcium excreted by the kidneys decreases to zero, which means that the critical parameter in calcium homeostasis in the CKD patient is the eCF concentration:

net calcium influx into the eCF •promotes calcium retention and may promote vascular calcification.net calcium efflux from the eCF •may worsen sHPT and decrease bone mass.

Parameters that can influence cal-cium balance in CKD patients include hemodialysis, dietary calcium content, calcium-based phosphate binders, and vitamin D, making the determination of calcium balance in CKD patients difficult. Dr. Bushinsky generated assumptions for calculating balance as follows:

Hemodialysis is performed 3 1. times/weekHemodialysis bath calcium = 1.25 2. mM (2.5 meq/l)net calcium flux during hemodial-3. ysis only if there is ultrafiltration3 l of ultrafiltration during each 4. hemodialysis treatmentgastrointestinal calcium absorp-5. tion is 19% of dietary calcium with-out activated vitamin D and 25% of dietary calcium with activated vitamin D, regardless of doseCalcium secretion into the stool 6. = 3.6 mmoles/day

primary hyperparathyroidism (PHPT), etc.Calcimimetics are ligands that act

on the Casr to mimic or potentiate the effects of calcium. There are two types of calcimimetics, distinguished by molecular mechanism of action:

Type I:• conventional agonists: depress parathyroid hormone secretion even in the absence of extracellular calcium. Type II:• Positive allosteric modula-

tors: increase the sensitivity of the Casr to activation by extracellular calcium.Cinacalcet, a Type II calcimimetic, is

the only currently available calcimimetic approved by the FDA for sHPT in patients with CKD undergoing hemo-dialysis and for severe hypercalcemia in parathyroid carcinoma. Cinacalcet treatment results in long-term, sus-tained reduction in serum calcium levels in PHPT patients and reduces parathy-

roid hormone and calcium/phosphate product levels in patients with sHPT; in pivotal trials, most patients were also on vitamin D and a phosphate binder.

Cinacalcet is only approved in stage 5 CKD, not stages 3 and 4, where it has been shown to increase hypocal-cemia and hyperphosphatemia, pos-sibly caused by parathyroid hormone; this indicates that care must be taken when altering the mechanisms that regulate the calcium receptor. n

Calcium myths and calcium needsProfEssor davId BushInsKY, usa

The amount of retained calcium in a CKD patient is significant on a yearly basis.



6 NOVEMBER 2010 www.renalandurologynews.com

All activated vitamin D prepara-7. tions increase intestinal calcium absorption equallyAll calcium from food and binders 8. will be absorbed equallyCalcium loss from sweat = 1.6 9. mmoles/dayurine calcium = 010. no net calcium flux relative to 11. bone

using these assumptions, a patient ingesting 1500 mg calcium/day with

no vitamin D supplementation would accumulate 117 mmoles calcium per year. If the same amount of calcium was ingested with activated vitamin D, this would mean an accumulation of 920 mmoles/year, which is equiva-lent to 2-3% of the total calcium in the bone. For a calcium intake of 2000 mg/day these values increase to 984 mmoles/year and 2,046 mmoles/year, respectively. This final accu-mulation of calcium is equivalent to

6-7% of bone calcium; therefore, the amount of retained calcium in a CKD patient is significant on a yearly basis. Further studies are war-ranted to determine the validity of the assumptions used in this model and, subsequently, the fate of retained calcium.

The forum discussed the fact that hemodialysis would be more ben-eficial if it was carried out 6 times per week as opposed to the stan-dard 4 hours 3 times per week; the need for phosphate binders would greatly decrease. use of the model of calcium metabolism to investi-gate the effects of different types of activated vitamin D on calcium absorption was also discussed, as was the effect of a calcimimetic in this model. Although cinacalcet was shown to reduce calcium absorption in this model, the reduction was not statistically significant. Caution was advised regarding introduction of calcium-raising measures to counter excessive hypocalcemia in hemodi-alysis patients, as patients would then be using large amounts of 1,25 vitamin D and calcium-based phos-phate binders, which may make the situation worse. serum calcium is also not a good measure of calcium physiology because it only represents a small percentage; fluxes into and out of extracellular fluid affect vas-cular calcification and bone health far more. n

Key: αCa = calcium absorption; Brca = bone resorption; frCa = fractional reabsorption; FLca = filtered load; Bfca = bone formation; Uca = urinary calcium

Figure 3: Model of calcium metabolism.

NOVEMBER 2010 7

Increased serum phosphate levels have been shown to be related to the increased risk of

cardiovascular disease in otherwise healthy people, and in patients with CKD requiring or not requiring hemodialysis. Vascular calcification is prevalent in CKD and is com-posed of both plaque (neointimal) and medial calcification. Vascular smooth muscle cells are stimulated by hyperphosphatemia to express genes associated with bone forma-tion and start to lay down mineral deposits. Medial vascular calcifica-tion leads to vessel stiffness, which can result in left ventricular hyper-trophy and cardiac events.

Data from observational studies (the prospective CArDIA study and the population-based cross-sectional MesA study) show that increased serum phosphate lev-els are associated with increased arterial stiffness in healthy young adults (CArDIA) and in middle-aged patients with normal kidney function or mild-to-moderate kid-ney disease (MesA). In a mouse model of CKD and atherosclero-sis, administration of phosphate binders and bone morphogenetic protein-7 (BMP-7) corrected hyper-phosphatemia (Figure 4) and osteo-dystrophy and reversed vascular calcification by improving bone

turnover and increasing phosphate deposition within bone. In this same animal model, sevelamer carbonate reduced serum phosphate levels, reversed adynamic bone disease and prevented aortic calcification and cardiac hypertrophy. The mecha-nism by which phosphate causes vascular calcification seems to be via expression of osterix and other bone transcription factors, which promote vascular mineralization.

The effects of reduced phosphate levels have been investigated in a

prospective cohort study of 10,044 patients initiating hemodialysis in 1056 centers from years 2004 to 2005 (ArMorr). Increasing serum phosphate levels were associated with decreased survival, but this was ameliorated in patients who received any phosphate binder for their first 3 months on hemodialysis. serum phosphate is a cardiovascular risk factor in CKD and the general population. Furthermore, transla-tional studies of human primary aortic cells in vitro have demon-

FiguRE 4: Hyperphosphatemia is central to failure of a multi-organ system in CKD.

The role of phosphate in cardiovascular risks in the general population and in CKD ProfEssor KEIth hrusKa, usa



8 NOVEMBER 2010

Four decades ago, Professor slatopolsky discovered that serum phosphate levels in -

creased as CKD progressed and as the gFr diminished. He also showed that parathyroid hormone increased as gFr declined, but this was improved when phosphate

intake was reduced in dogs with CKD. These data are still relevant today, but we now know more of the mechanisms involved in hyper-phosphatemia in CKD, including FgF-23, klotho, vitamin D, and parathyroid hormone.

There are various phosphate binder

options for the treatment of hyper-phosphatemia, the risks and benefits of each vary (Figure 5):

Aluminum-containing phosphate •binders are very efficacious, but the aluminum can be absorbed by the body, accumulate and cause toxic side effects, such as osteomalacia, encephalopathy and microcytic anemia. Calcium-containing binders are •efficacious, but excessive calcium can be absorbed, leading to vascu-lar calcification. The non-calcium metal-based •binder lanthanum carbonate is absorbed and can accumulate in the body. long-term studies are required to investigate lanthanum’s toxicity and its effect on vascular calcification. The metal-free, non-absorbed phos-•phate binder sevelamer has been shown to reduce the progression of

Four decades of phosphate management ProfEssor tIlMan drüEKE, francE

strated that hyperphosphatemia stimulates vascular calcification by inducing osteoblastic gene expres-sion in the aorta; correction of hyperphosphatemia has been shown to decrease vascular calcification in humans; however, the role of serum phosphate as a validated cardiovas-cular risk factor in CKD needs to be confirmed.

Questions arising from this presen-tation included the role of collagen

as a co-risk factor for vascular calcifi-cation and the role of the phosphate transporter piT-1; phosphate could be used as a marker of vascular cal-cification in the same way blood

pressure is used to predict cardio-vascular mortality in the general population. rCTs are needed to vali-date phosphate as a cardiovascular risk factor. n

Randomized controlled trials are needed to validate phosphate as a cardiovascular risk factor.

FiguRE 5: Risks and benefits of phosphate binders have to be well balanced.

NOVEMBER 2010 9

It is well established that parathy-roid hormone is the main com-ponent of the control system for

modifying phosphate excretion in uremia, and that decreased phos-phate intake can block the increase in parathyroid hormone in animal models of CKD. Phosphate can act via different mechanisms, such as

reducing calcitriol production and release of calcium from bone; it also increases FgF-23 and klotho, which increase parathyroid hormone expres-sion (Figure 6).

Animal studies in moderate CKD demonstrated that elevated parathy-roid hormone levels fail to control serum phosphate, and that high

phosphate levels reduce the calcium response to parathyroid hormone and the inhibition of parathyroid hormone by calcitriol.

In vitro studies have shown that the effect of phosphate on parathy-roid hormone secretion is direct, concentration-dependent, requires cell contact (in whole parathyroid

vascular calcification and to lower mortality in CKD patients in some studies. sevelamer also improves bone quality, reduces inflamma-tion, and lowers lipid levels and oxidative stress. Although dietary phosphate restric-

tion in dogs with CKD reduced parathyroid hormone levels, reduc-ing protein intake to reduce serum phosphate levels increases mortality, probably due to malnutrition. The Treat-To-goal and rInD studies showed that treatment with phosphate binders reduces progression of vas-cular calcification. According to the KDIgo guidelines, the presence of CV calcification strongly predicts CV morbidity and mortality in patients with CKD. Can phosphate binders improve morbidity and mortality by slowing the progression of CV calci-fication? The data are not conclusive yet; more randomized clinical trials are necessary.

Questions raised included the ideal combination of existing phosphate

binders, and whether there was a place for inhibitors of gI sodium/phosphate co-transporters in the management of hyperphosphatemia; both nicotinamide and niacin reduce serum phosphate levels in hemodialy-sis patients. randomized controlled trials are needed to investigate the effects of phosphate binders on hard outcomes in CKD stages 4–5.

some phosphate binders, but not all, have been shown to reduce pro-gression of vascular calcification.

several new therapeutic options have recently been examined. The use of chitosan (a synthetic polymer that can be administered in chewing gum and can drastically lower phosphate

levels in saliva), which results in a 2 mg decrease in serum phosphate, may be of therapeutic use in conjunc-tion with other treatments. Cinacalcet can only be given to patients with sHPT (only approximately one-third of patients with CKD), and it may impact phosphate, especially when used with a phosphate binder. A com-bined calcium/magnesium binder

has recently been marketed in some countries, and although magnesium inhibits vascular calcification, it can also reduce bone mineralization. several studies on iron-containing phosphate binders are ongoing, and results on iron release and accumula-tion are awaited with interest. n

Phosphate and parathyroid hormonedr MarIano rodrIGuEz, sPaIn

Some phosphate binders, but not all, have been shown to reduce progression of vascular calcification.



10 NOVEMBER 2010

The systems controlling phos-phate homeostasis include bone, gI tract, parathyroid

glands, and kidneys, and the hor-mones parathyroid hormone and FgF-23, which act on the kidneys to downregulate the sodium/phosphate co-transporter and thus increase phos-phate excretion (Figure 7). FgF-23 increases as gFr decreases, and is much more sensitive to changes in gFr than phosphate or parathyroid hormone. Increased FgF-23 levels can predict progression of kidney disease in CKD and diabetic patients, with high levels of FgF-23 indica t -ing morbidity, such as left ventricular hypertrophy, and mortality.

In vivo studies conducted to inves-tigate the effects following removal of FgF-23 showed increases in serum phosphate levels to be similar in CKD rats receiving anti-FgF-23 antibodies compared with normal rats. Interestingly, the increase in urinary excretion of phosphate in CKD rats stopped following admin-istration of anti-FgF-23 antibodies. Anti-FgF-23 antibodies increased 1,25(oH)2D production to the same level in CKD rats as in normal rats, while levels in CKD rats not receiv-ing antibodies remained low. These effects were mediated by a correction in the activity of the 1α hydroxylase enzyme, and a reduction in the activ-ity of 24 hydroxylase enzyme. These

glands, slices, or collagen-matrix cultures, not dispersed cells), and is dependent on increased gene expression and enhanced stability of parathyroid hormone mrnA. High phosphate concentrations interfere with the signalling pathway of the calcium-sensing receptors, prevent-ing the release of arachidonic acid from the cell membrane. results from several in vitro experiments have shown that the stimulatory effect of phosphate on parathyroid hormone secretion may be due to phosphate reducing the intracel-lular release of calcium in response to extracellular calcium, thereby reducing the inhibitory arachidonic acid signal.

Discussion on the fact that only intact cells (not dispersed cells) respond to phosphate led to an expla-nation that triggering parathyroid hormone secretion in one cell may cause it to communicate via signals from cell to cell so that more cells are triggered to produce more para-thyroid hormone; phosphate may prevent this from occurring. In vitro studies have shown that 2-4 hours are required to see an effect, which indicates that phosphate may need to enter the cells first and act intracellu-larly. In vivo studies have shown that suppression of parathyroid hormone can occur within 15 minutes, suggest-ing rapid signalling from something other than phosphate. n

FiguRE 6: Phosphate can act via different mechanisms to increase PTH secretion.

Phosphate and FGF-23 ProfEssor harald JüPPnEr, usa

NOVEMBER 2010 11

in vivo results showed that FgF-23 is responsible for renal phosphate reabsorption and normalization of blood phosphate levels in mild CKD, and that FgF-23 acts independently of parathyroid hormone and reduces 1,25(oH)2D production, which leads to low calcium and increased para-thyroid hormone; findings that are consistent with those in humans.

Measurement of biologically active FgF-23 in the circulation can be performed equally well using intact FgF-23 or the C-terminal portion of FgF-23. with reference to the trade-off hypothesis, the actions of phosphate on parathyroid hormone are probably dependent on elevated FgF-23 being secreted from the bone, resulting in lower 1,25(oH)2D levels, consequently triggering the release of parathyroid hormone. Investigation into high levels of FgF-23 in the

circulation and the off-target effects of this molecule are warranted. Interventions need to reduce the absorption of phosphate from the gI tract, and focus on patients with earlier stages of CKD to promote phosphate excretion into urine.

FgF-23 is responsible for renal phos-phate excretion and maintenance of blood phosphate levels in mild CKD

The presentation triggered questions as to the effect of increasing intake of dietary phosphate on FgF-23. when phosphate intake is increased in humans, plasma FgF-23 increases by only a small amount, whereas uri-nary phosphate excretion increases by a large amount. Interestingly, serum phosphate increases follow-ing parathyroidectomy in rats with CKD, despite the powerful effects of FgF-23; increasing FgF-23 does not control serum phosphate in sHPT

or PHPT. one suggestion was that 1,25(oH)2D is low in the absence of parathyroid hormone and is critical for the synthesis of FgF-23. In an organism that has little 1,25(oH)2D and is hypocalcemic, further reduc-ing calcium absorption via FgF-23-mediated inhibition of 1,25(oH)2D would be harmful. In PHPT, FgF-23 is not as high as it should be as it is trying to conserve 1,25(oH)2D and calcium.

To date, klotho has not been mea-sured and FgF-23 levels have not been detected in healthy persons with normal kidney function. The off-tar-get effects of FgF-23 in klotho defi-cient mice include interaction with a variety of receptors or splice variants, although these studies are difficult because these mice also have severe hyperphosphataemia. no difference could be seen in klotho+FgF-23 knockout mice compared with klotho only knockout mice; therefore, an off-target effect of FgF-23 may not be detectable in mice.

regarding the physiological func-tion of FgF-23, some data suggest that, if the signal from the osteocyte tells the kidney to lose phosphate, the signal to the osteocyte comes from the dying kidney, but further studies are needed. A recent study shows that, in very early CKD, patients who have completely nor-mal gFr have very high FgF-23 levels, totally normal parathyroid hormone levels and subtle reduc-tions in serum phosphate; therefore, CKD itself may increase FgF-23. It has also been shown that, follow-ing kidney transplant into a CKD patient, FgF-23 levels decrease by 85% within a day. n

Key: DMP1 = dentin matrix protein; ENP-P1 = ectonucleotide pyrophosphatase/ phosphodiesterase 1; PHEX = phosphate-regulating gene with homologies to endopeptidases on the X-chromosome; FGFR/KL = fibroblast growth factor receptors/Klotho; NaP1 = sodium-coupled phosphate (IIa, IIb, IIc = type); Pit-2 = type III Na+-phosphate cotransporter; PTHR1 = PTH/PTHrP type I receptor; urine Pi = urine phosphate.

FiguRE 7: Adaptation to CKD.



12 NOVEMBER 2010

The klotho gene encodes a single trans-membrane pro-tein that is predominantly

expressed in the kidney, specifically the distal convoluted tubule (DCT), as well as the parathyroid glands, the pituitary, hypothalamus, testes, ovaries and pancreatic β cells; it is associated with endocrine organs. secretion of klotho is stimulated by low extracel-lular calcium and phosphate.

Klotho is an obligate co-receptor for FgF-23, and the phenotype of klotho knockout mice is very similar to that of FgF-23 knockout mice, with both phenotypes having defects associ-ated with advanced aging and disor-dered mineral metabolism. In klotho knockout mice, a low phosphate diet rescues the animals from many of the features of premature aging and reduces hyperphosphatemia; how-ever, it has no effect on hypercalcemia or low levels of vitamin D observed in these animals.

The concept that CKD could be viewed as a state of accelerated aging associated with klotho deficiency and phosphate retention was discussed. As CKD progresses, klotho decreases in the serum and urine, reaching almost zero in CKD stage 5 (Figure 8).

These changes start to occur very early in the course of CKD and result in changes to FgF-23, vitamin D and parathyroid hormone. A decrease in klotho results in an increase in FgF-23 to try to compensate; this leads to

lower levels of calcitriol and higher levels of parathyroid hormone (lead-ing to an increase in FgF-23). The various treatments for CKD need to break this vicious cycle.

As CKD progresses, klotho decreas-es in the serum and urine, reaching almost zero in CKD stage 5

Klotho exists in two forms: mem-brane bound and clipped/secreted. The membrane form of klotho must be expressed to mediate the effects of FgF-23. secreted klotho has been detected in the blood, urine and CsF, and activates calcium chan-nels in the DCT, promoting renal calcium reabsorption, resulting in FgF-23 lowering serum phosphate but not calcium. secreted klotho acts independently of FgF-23, appears to regulate the function of various ion channels and transport-ers, including phosphate transport-ers, and can induce phosphaturia, possibly through the inactivation of sodium/phosphate co-transporters in the PT.

In animals, secreted klotho in blood is a biomarker for decreased expression in the kidney; this has yet to be confirmed in humans. secreted klotho can inhibit Pit 1 and 2 in multiple organs, and can also affect the signalling pathways of TgF-β and IgF-1. Its effects on hydroxylase are unknown, and it is not thought to mediate the phosphaturic effects of FgF-23. It is believed that secreted klotho acts as a paracrine factor within the kidney, acting from DCT to neighboring PT, but not in the general circulation. n

Phosphate and klotho ProfEssor MaKoto Kuro-o, usa

FiguRE 8: CKD is a state of klotho deficiency.

Key: P = phosphate

As CKD progresses, klotho decreases in the serum and urine, reaching almost zero in CKD stage 5.

NOVEMBER 2010 13

Dr. Bouillon shared data and information on the impor-tance of vitamin D. 1,25-dihy-

droxyvitamin D (1,25(oH)2D) and the vitamin D receptor (VDr) (1,25-(oH)2D-VDr) interact directly on the parathyroid glands, kidney, gI tract and bone, and indirectly through its effects on FgF-23, to maintain phosphate and calcium levels in the blood (Figure 9).

A number of studies have been conducted to investigate the opti-mal threshold level of vitamin D for bone health. It has been shown that the risk of sHPT is reduced with serum 25-oH-D levels ≥20 ng/ml,

and supplementation with >400–800 Iu vitamin D3/day decreases risk of fractures and falls by ~20%, according to the results from 12 meta-analyses and 10–20 random-ized clinical trials. It was concluded, therefore, that 25-oH-D >20 ng/ml will safely eliminate most of the potential extra-skeletal risks of vita-min D deficiency.

In order to respond to the world-wide epidemic of vitamin D deficiency and insufficiency, one of three strat-egies could be employed: (1) wait for better evidence-based studies; (2) provide vitamin D supplements (800 Iu/day) for all adults and espe-

cially those in at-risk groups to elimi-nate most extra-skeletal disease risks due to vitamin D deficiency; and (3) recommend better vitamin D status (>32 or >40 ng/ml) for all adults either by higher exposure to uV-B sunlight or by a vitamin D intake of 2000-4000 Iu/day.

Questions raised included the safety of 2000 Iu vitamin D/day, as there are no data from rCTs to date, and the possibility of recommending a target of 30 ng/ml. An rCT investigating 30-40 ng/ml serum levels needs to be conducted in patients with CKD to investigate its safety and efficacy.

It was also noted that, as vitamin D is stored in fat, obese individu-als need more vitamin D than lean individuals.

It was suggested that everyone over 65 years of age should receive 1 g calcium and 800 mg vitamin D daily, although there are no data currently available for younger adults or ado-lescents. The Institute of Medicine will announce its decision regarding vitamin D supplementation in the usA at a later date. n

Worldwide vitamin D deficiency: science or fashion dr roGEr BouIllon, BElGIuM

It was suggested that everyone over 65 years of age should receive 800 mg vitamin D daily

FiguRE 9: Scheme of interaction between 1,25-(OH)2D-VDR system and phosphate homeostasis.

Key: 1a-OHase = 25-hydroxyvitamin D-1-alpha hydroxylase; Ps = phosphate.

14 NOVEMBER 2010

Bone, from a reservoir of minerals to a regulator of energy expenditure dr cYrIllE confavrEux, francE

Bone, appetite, and repro-duction are thought to be regulated by a neuroendo-

crine connection via a common hor-monal system in the brain; leptin (an adipocyte hormone produced by the fat cells of the body) regu-lates appetite and reproduction, and controls bone mass indirectly. leptin deficient mice (ob/ob) are obese and sterile, and have increased bone mass, which can be returned to normal by intracerebroventricular infusion of leptin.

serotonin is a candidate neuro-mediator for leptin activity, and antiserotoninergic antidepressants increase patients’ appetite, which supports the use of serotonin as an appetite regulator. studies have shown that the sympathetic nervous system (sns), via beta2-adrenergic receptor, mediates leptin inhibition of bone formation (Figure 10). If lep-

tin controls bone mass, is this action reciprocated by the bone controlling metabolism?

osteocalcin, an osteoblast-secret-ed hormone, regulates insulin and

adiponectin expression and, conse-quently, metabolism.

Questions following this presen-tation included the suggestion that obese people have increased bone mass, and the need to investigate fur-ther the direct effects of leptin on bone and mineral metabolism, eg, stimulating FgF-23 and reducing 1,25(oH)D; it is possible that leptin’s direct effects on bone are stimulatory where whereas its neuroendocrine effects are inhibitory. n

FiguRE 10: Fat controls bone mass — but does bone regulate energy metabolism?

Key: SNS = sympathetic nervous system.



Further research into the bone-energy endocrine axis and its relationship to other body systems, including the gut and kidneys, is needed.

NOVEMBER 2010 15

Round Table: defining new research priorities in CKD-MBDProfEssor sharon MoE

The meeting concluded with a round table discussion on potential clinical trial designs

and preclinical studies, and develop-ment of other therapeutic targets. The general consensus was that placebo-controlled trials were needed in CKD stages 2-4 to investigate the effect of phosphate and phosphate binders on mortality. Various other endpoints were suggested, such as bone disease, progression of CKD, and cardiovascu-

lar events. ongoing studies investigat-ing a broad range of cardiovascular surrogates are due for completion in 2011. Monitoring methods using the less expensive option of phosphate and creatinine were preferred over FgF-23, which is expensive at present.

The patient group was considered, and patients over 50/55 years of age with high levels of FgF-23 would provide the most valuable data.

Discussion of the development of

other therapeutic targets included biomarkers, such as naPi2a, b or c (to decrease the reabsorption of phos-phate), but these were considered challenging targets; infusion of solu-ble klotho; how the bone senses phos-phate; and why high levels of FgF-23 are found in non-renal organs. It was generally agreed that FgF-23 could be a uremic toxin, which may support development of anti-FgF-23 strate-gies in the future. n

Educational support provided by Genzyme corporation

Editorial support for these proceedings was provided by Envision scientific solutions

Produced by

114 west 26th street, 4th Floornew york, ny 10001

www.renalandurologynews.com

Documents

Supplement - The Central Role of Phosphate