Association between phosphate removal and markers of bone turnover in haemodialysis patients

Crosstalk between bone and muscle in chronic kidney disease

With increasing life expectancy, the related disorders of bone loss, metabolic dysregulation and sarcopenia have become major health threats to the elderly. Each of these conditions is prevalent in patients with chronic kidney disease (CKD), particularly in more advanced stages. Our current understanding of the bone-muscle interaction is beyond mechanical coupling, where bone and muscle have been identified as interrelated secretory organs, and regulation of both bone and muscle metabolism occurs through osteokines and myokines via autocrine, paracrine and endocrine systems. This review appraises the current knowledge regarding biochemical crosstalk between bone and muscle, and considers recent progress related to the role of osteokines and myokines in CKD, including modulatory effects of physical exercise and potential therapeutic targets to improve musculoskeletal health in CKD patients.

Phosphate balance during dialysis and after kidney transplantation in patients with chronic kidney disease

PURPOSE OF REVIEW

In patients with chronic kidney disease (CKD), hyperphosphatemia is associated with several adverse outcomes, including bone fragility and progression of kidney and cardiovascular disease. However, there is a knowledge gap regarding phosphate balance in CKD. This review explores its current state, depending on the stage of CKD, dialysis modalities, and the influence of kidney transplantation.

RECENT FINDINGS

Adequate phosphate control is one of the goals of treatment for CKD-mineral and bone disorder. However, ongoing studies are challenging the benefits of phosphate-lowering treatment. Nevertheless, the current therapy is based on dietary restriction, phosphate binders, and optimal removal by dialysis. In the face of limited adherence, due to the high pill burden, adjuvant options are under investigation. The recent discovery that intestinal absorption of phosphate is mostly paracellular when the intraluminal concentration is adequate might help explain why phosphate is still well absorbed in CKD, despite the lower levels of calcitriol.

SUMMARY

Future studies could confirm the benefits of phosphate control. Greater understanding of the complex distribution of phosphate among the body compartments will help us define a better therapeutic strategy in patients with CKD.

Muscle-Bone Crosstalk in Chronic Kidney Disease: The Potential Modulatory Effects of Exercise

Chronic kidney disease (CKD) is a prevalent worldwide public burden that increasingly compromises overall health as the disease progresses. Two of the most negatively affected tissues are bone and skeletal muscle, with CKD negatively impacting their structure, function and activity, impairing the quality of life of these patients and contributing to morbidity and mortality. Whereas skeletal health in this population has conventionally been associated with bone and mineral disorders, sarcopenia has been observed to impact skeletal muscle health in CKD. Indeed, bone and muscle tissues are linked anatomically and physiologically, and together regulate functional and metabolic mechanisms. With the initial crosstalk between the skeleton and muscle proposed to explain bone formation through muscle contraction, it is now understood that this communication occurs through the interaction of myokines and osteokines, with the skeletal muscle secretome playing a pivotal role in the regulation of bone activity. Regular exercise has been reported to be beneficial to overall health. Also, the positive regulatory effect that exercise has been proposed to have on bone and muscle anatomical, functional, and metabolic activity has led to the proposal of regular physical exercise as a therapeutic strategy for muscle and bone-related disorders. The detection of bone- and muscle-derived cytokine secretion following physical exercise has strengthened the idea of a cross communication between these organs. Hence, this review presents an overview of the impact of CKD in bone and skeletal muscle, and narrates how these tissues intrinsically communicate with each other, with focus on the potential effect of exercise in the modulation of this intercommunication.

Modification and Validation of the Phosphate Removal Model: A Multicenter Study

BACKGROUND

Our research group has previously reported a noninvasive model that estimates phosphate removal within a 4-h hemodialysis (HD) treatment. The aim of this study was to modify the original model and validate the accuracy of the new model of phosphate removal for HD and hemodiafiltration (HDF) treatment.

METHODS

A total of 109 HD patients from 3 HD centers were enrolled. The actual phosphate removal amount was calculated using the area under the dialysate phosphate concentration time curve. Model modification was executed using second-order multivariable polynomial regression analysis to obtain a new parameter for dialyzer phosphate clearance. Bias, precision, and accuracy were measured in the internal and external validation to determine the performance of the modified model.

RESULTS

Mean age of the enrolled patients was 63 ± 12 years, and 67 (61.5%) were male. Phosphate removal was 19.06 ± 8.12 mmol and 17.38 ± 6.75 mmol in 4-h HD and HDF treatments, respectively, with no significant difference. The modified phosphate removal model was expressed as Tpo4 = 80.3 × C45 - 0.024 × age + 0.07 × weight + β × clearance - 8.14 (β = 6.231 × 10-3 × clearance - 1.886 × 10-5 × clearance2 - 0.467), where C45 was the phosphate concentration in the spent dialysate measured at the 45th minute of HD and clearance was the phosphate clearance of the dialyzer. Internal validation indicated that the new model was superior to the original model with a significantly smaller bias and higher accuracy. External validation showed that R2, bias, and accuracy were not significantly different than those of internal validation.

CONCLUSIONS

A new model was generated to quantify phosphate removal by 4-h HD and HDF with a dialyzer surface area of 1.3-1.8 m2. This modified model would contribute to the evaluation of phosphate balance and individualized therapy of hyperphosphatemia.

Pneumatic Compression, But Not Exercise, Can Avoid Intradialytic Hypotension: A Randomized Trial

BACKGROUND

Conventional hemodialysis (HD) is associated with dialysis-induced hypotension (DIH) and ineffective phosphate removal. As the main source of extracellular fluid removed during HD are the legs, we sought to reduce DIH and increase phosphate removal by using cycling and pneumatic compression, which would potentially provide higher venous return, preserving central blood flow and also offering more phosphate to the dialyzer.

METHODS

We evaluated 21 patients in a randomized crossover fashion in which each patient underwent 3 different HD: control; cycling exercise during the first 60 min; and pneumatic compression during the first 60 min. Data obtained included bioelectrical impedance, hourly blood pressure measurement, biochemical parameters, and direct quantification of phosphate through the dialysate. DIH was defined as a drop in mean arterial pressure (MAP) ≥20 mm Hg.

RESULTS

There was no difference in the ultrafiltration rate (p = 0.628), delta weight (p = 0.415), delta of total, intra and extracellular body water among the control, cycling, and pneumatic compression (p = 0.209, p = 0.348, and p = 0.467 respectively). Delta MAP was less changed by pneumatic compression when compared to control, cycling, and pneumatic compression respectively (-4.7 [-17.2, 8.2], -4.7 [-20.5, -0.2], and -2.3 [-8.1, 9.0] mm Hg; p = 0.021). DIH occurred in 43, 38, and 24% of patients in control, cycling, and pneumatic compression respectively (p = 0.014). Phosphate removal did not increase in any intervention (p = 0.486). Higher phosphate removal was dependent on ultrafiltration, pre dialysis serum phosphate, and higher parathyroid hormone.

CONCLUSION

Pneumatic compression during the first hour of dialysis was associated with less DIH, albeit there was no effect on fluid parameters. Neither exercise nor pneumatic compression increased phosphate removal.

Mechanisms of Vascular Calcification: The Pivotal Role of Pyruvate Dehydrogenase Kinase 4

Vascular calcification, abnormal mineralization of the vessel wall, is frequently associated with aging, atherosclerosis, diabetes mellitus, and chronic kidney disease. Vascular calcification is a key risk factor for many adverse clinical outcomes, including ischemic cardiac events and subsequent cardiovascular mortality. Vascular calcification was long considered to be a passive degenerative process, but it is now recognized as an active and highly regulated process similar to bone formation. However, despite numerous studies on the pathogenesis of vascular calcification, the mechanisms driving this process remain poorly understood. Pyruvate dehydrogenase kinases (PDKs) play an important role in the regulation of cellular metabolism and mitochondrial function. Recent studies show that PDK4 is an attractive therapeutic target for the treatment of various metabolic diseases. In this review, we summarize our current knowledge regarding the mechanisms of vascular calcification and describe the role of PDK4 in the osteogenic differentiation of vascular smooth muscle cells and development of vascular calcification. Further studies aimed at understanding the molecular mechanisms of vascular calcification will be critical for the development of novel therapeutic strategies.

Denosumab associated with bone density increase and clinical improvement in a long-term hemodialysis patient. Case report and review of the literature

Denosumab is a human monoclonal antibody representing a novel therapy of osteoporosis. Contrary to always other antiosteoporotic drugs, it is not contraindicated in advanced chronic kidney disease, as its pharmacokinetic does not differ from patients with normal kidney function. However, published case reports in chronic kidney disease (CKD) patients stopped the therapy after single dose because of hypocalcemia. We present a case of successful treatment of osteoporosis in a young hemodialysis patient with repeated denosumab doses.

Age and gender predict OPG level and OPG/sRANKL ratio in maintenance hemodialysis patients

PURPOSE

Cardiovascular disease (CVD) is a major cause of death among chronic hemodialysis (HD) patients. Gender and age belong to its classical risk factors. OPG/RANK/sRANKL (Osteoprotegerin/ Receptor Activator of Nuclear Factor κB/ soluble Receptor Activator of Nuclear Factor κB Ligand) axis constitute a system connecting bone and vascular remodeling.

METHODS

We aimed to evaluate the plasma levels of OPG, sRANKL and OPG/sRANKL ratio in 21 HD patients and 16 healthy volunteers in relation to gender, age and the other clinical parameters.

RESULTS

OPG and OPG/sRANKL ratio were significantly higher in HD patients than in controls whereas sRANKL was similar in both groups. Adjusted for gender, in controls OPG were higher in women whereas sRANKL did not differ between men and women. In HD group OPG and sRANKL were higher in women whereas OPG/sRANKL ratio was similar in both genders. Female patients compared to healthy women revealed 56% higher OPG concentration and 54% higher OPG/ sRANKL ratio. Comparison of male patients and controls revealed 61% higher level of OPG and 75% higher OPG/sRANKL ratio in HD group. Interestingly, OPG and OPG/sRANKL ratio positively correlated with age only in male patients. Contrary, the association between OPG/sRANKL ratio and age was negative in HD women.

CONCLUSION

Higher OPG levels in HD women comparing to age matched HD men indicate the necessity of more careful screening towards the presence of CVD and bone-mineral disorders. The negative association between age and OPG/ sRANKL ratio in HD women warrant in-depth study for thorough understanding of this complex interrelationship.

Aortic valve calcification in chronic kidney disease

Several clinical studies reported an increased prevalence and accelerated progression of aortic valve calcification among patients with end-stage renal disease when compared with subjects with normal kidney function. Recently, mechanisms of calcific valve degeneration have been further elucidated and many of the pathways involved could be amplified in patients with decreased renal function. In particular, calcium-phosphate balance, MGP metabolism, OPG/RANK/RANKL triad, fetuin-A mineral complexes and FGF-23/Klotho axis have been shown to be impaired among patients with advanced chronic kidney disease and could play a role during vascular/valve calcification. The scope of the present review is to summarize the clinical data and the pathophysiological mechanisms potentially involved in the link between renal function decline and the progression of aortic valve disease.

Osteoprotegerin inhibits calcification of vascular smooth muscle cell via down regulation of the Notch1-RBP-Jκ/Msx2 signaling pathway

Objective

Vascular calcification is a common pathobiological process which occurs among the elder population and in patients with diabetes and chronic kidney disease. Osteoprotegerin, a secreted glycoprotein that regulates bone mass, has recently emerged as an important regulator of the development of vascular calcification. However, the mechanism is not fully understood. The purpose of this study is to explore novel signaling mechanisms of osteoprotegerin in the osteoblastic differentiation in rat aortic vascular smooth muscle cells (VSMCs).

Methods and Results

VSMCs were isolated from thoracic aorta of Sprague Dawley rats. Osteoblastic differentiation of VSMCs was induced by an osteogenic medium. We confirmed by Von Kossa staining and direct cellular calcium measurement that mineralization was significantly increased in VSMCs cultured in osteogenic medium; consistent with an enhanced alkaline phosphatase activity. This osteoblastic differentiation in VSMCs was significantly reduced by the addition of osteoprotegerin in a dose responsive manner. Moreover, we identified, by real-time qPCR and western blotting, that expression of Notch1 and RBP-Jκ were significantly up-regulated in VSMCs cultured in osteogenic medium at both the mRNA and protein levels, these effects were dose-dependently abolished by the treatment of osteoprotegerin. Furthermore, we identified that Msx2, a downstream target of the Notch1/RBP-Jκ signaling, was markedly down-regulated by the treatment of osteoprotegerin.

Conclusion

Osteoprotegerin inhibits vascular calcification through the down regulation of the Notch1-RBP-Jκ signaling pathway.

Association of circulating fibroblast growth factor-23 with renal phosphate excretion among hemodialysis patients with residual renal function

BACKGROUND AND OBJECTIVES

High serum levels of fibroblast growth factor-23 (FGF-23) are associated with mortality in patients with ESRD, but whether it still acts as a phosphaturic factor is unknown. This study aimed to explore the role of circulating FGF-23 on urinary phosphate excretion and phosphate balance in maintenance hemodialysis (MHD) patients with residual renal function (RRF).

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

There were 134 MHD patients enrolled in this cross-sectional study from June to July 2010. Demographics, laboratory data, and excretion capacity of phosphate were recorded. Multivariable linear regression was used to analyze the relationship of serum phosphate and the tubular reabsorption rate of phosphate with other factors.

RESULTS

The median age of the patients was 61.0 years and 47.8% were male. Thirty percent of the patients had high urinary output (>200 ml/d) accompanied by lower serum levels of phosphate, calcium, intact parathyroid hormone, and FGF-23 compared with those with low urine output (≤200 ml/d). The independent predictors of serum phosphate were normalized protein nitrogen appearance, intact parathyroid hormone, and FGF-23 in the low urine output group and female sex and GFR in the high urine output group. The tubular reabsorption rate of phosphate decreased to 50% of the normal level in patients with RRF. Elevated circulating FGF-23 was significantly associated with lower tubular phosphate reabsorption after adjusting for GFR.

CONCLUSIONS

RRF is associated with significant capacity to excrete phosphate in MHD patients and high levels of serum FGF-23 may promote phosphate excretion by remnant nephrons.

Phosphate removal model: an observational study of low-flux dialyzers in conventional hemodialysis therapy

Precise assessing phosphate removal by hemodialysis (HD) is important to improve phosphate control in patients on maintenance HD. We reported a simple noninvasive model to estimate phosphate removal within a 4-hour HD. One hundred sixty-five patients who underwent HD 4 hours per session using low-flux dialyzers made of polysulfone (1.2 m(2)) or triacetate (1.3 m(2)) were enrolled. Blood flows varied from 180 to 300 mL/min. Effluent dialysate samples were collected during the 4-hour HD treatment to measure the total phosphate removal. Predialysis levels of serum phosphate, potassium, hematocrit, intact parathyroid hormone, total carbon dioxide (TCO(2)), alkaline phosphatase, clinical and dialysis characteristics were obtained. One hundred thirty-five observations were randomly selected for model building and the remaining 30 for model validation. Total amount of phosphate removal within the 4-hour HD was mostly 15-30 mmol. A primary model (model 1) predicting total phosphate removal was Tpo(4)  = 79.6 × C(45) (mmol/L) - 0.023 × age (years) + 0.065 × weight (kg) - 0.12 × TCO(2) (mmol/L) + 0.05 × clearance (mL/min) - 3.44, where C(45) was phosphate concentration in spent dialysate measured at the 45 minute of HD and clearance was phosphate clearance of dialyzer in vitro conditions offered by manufacturer's data sheet. Since the parameter TCO(2) needed serum sample for measurement, we further derived a noninvasive model (model 2):Tpo(4)  = 80.3 × C(45)  - 0.024 × age + 0.07 × weight + 0.06 × clearance - 8.14. Coefficient of determination, root mean square error, and residual plots showed the appropriateness of two models. Model validation further suggested good and similar predictive ability of them. This study derived a noninvasive model to predict phosphate removal. It applies to patients treated by 4-hour HD under similar conditions.

The circulating soluble TRAIL is a negative marker for inflammation inversely associated with the mortality risk in chronic kidney disease patients

BACKGROUND

Chronic kidney disease (CKD) is associated with accelerated atherosclerosis and an inadequate inflammatory response which may account for the high morbidity and mortality observed in this population. In vitro and preclinical evidence suggests that the tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) might be involved in both the atherosclerosis pathway and modulation of the inflammatory response. The aim of the present study was thus to (i) determine serum levels of soluble TRAIL (sTRAIL) in a cohort of CKD patients, (ii) assess the relationship between sTRAIL and other inflammatory biomarkers (C-reactive protein and albumin) and (iii) evaluate the association between serum sTRAIL levels and the mortality risk.

METHODS

One hundred and thirty patients (mean +/- SD age: 67 +/- 12; 62% males; 8% at CKD stage 2, 26% at stage 3, 27% at stage 4, 8% at stage 5 and 31% at stage 5D) were assayed for sTRAIL and the selected biochemical parameters and then prospectively monitored for mortality.

RESULTS

CKD stage 5D patients had significantly lower serum sTRAIL levels (median: 46 pg/ml) than patients at CKD stages 2 and 3 (median: 62 pg/ml) or stages 4 and 5 (median: 71 pg/ml). There was no correlation between serum sTRAIL and the estimated glomerular filtration rate (GFR) (r(2) = 0.017, P = 0.22) in pre-dialysis patients. In a multivariate regression analysis, the body mass index (beta = 1.48, P = 0.001) and the serum C-reactive protein (CRP) level (beta = -8.841, P < 0.0001) were independently associated with serum sTRAIL. During follow-up (mean: 772 +/- 286 days), 36 patients died (19 from cardiovascular events, 8 from infectious events and 9 from other causes). The lowest sTRAIL levels (first tertile) were associated with the worst all-cause survival (P = 0.010). Cox regression analyses (with non-cumulative models including age, albumin and CRP as covariates) confirmed the low serum sTRAIL level (first tertile) as an independent predictor of all-cause mortality.

CONCLUSIONS

Circulating sTRAIL is a negative marker for inflammation and is inversely associated with the mortality risk in CKD patients. Further studies are needed to better understand the role of sTRAIL as an inflammatory marker and to confirm its protective role in the CKD population.

High alkaline phosphatase levels in hemodialysis patients are associated with higher risk of hospitalization and death

We evaluated risks associated with elevated alkaline phosphatase in hemodialysis patients using longitudinal data from the Dialysis Outcomes and Practice Patterns Study, a prospective observational study of hemodialysis patients in 12 countries. Alkaline phosphatase levels were normalized by the upper limit of the laboratory-reported reference range. Cause-specific hospitalization and mortality risks were evaluated using Cox proportional hazards models, stratified by region and adjusted for phosphorus, calcium, albumin, parathyroid hormone, case mix, and numerous comorbidities. The odds of high normalized alkaline phosphatase were increased twofold in the United States in comparison to Japan. Elevations of normalized alkaline phosphatase were significantly associated with several comorbid conditions, increased fractures, parathyroidectomy, risk of hospitalization due to major adverse cardiac events, higher all-cause cardiovascular, and infection-related mortality risk. Our results also show that elevated serum normalized alkaline phosphatase was associated with higher risks of hospitalization and death in hemodialysis patients, independent of calcium, phosphorus, and parathyroid hormone levels.

Epithelial to mesenchymal transition (EMT) in human prostate cancer: lessons learned from ARCaP model

Androgen refractory cancer of the prostate (ARCaP) cells contain androgen receptor (AR) and synthesize and secrete prostate specific antigen (PSA). We isolated epithelia-like ARCaP(E) from parental ARCaP cells and induced them to undergo epithelial-mesenchymal transition (EMT) by exposing these cells to soluble factors including TGFbeta1 plus EGF, IGF-1, beta2-microglobulin (beta2-m), or a bone microenvironment. The molecular and behavioral characteristics of the resultant ARCaP(M) were characterized extensively in comparison to the parental ARCaP(E) cells. In addition to expressing mesenchymal biomarkers, ARCaP(M) gained 100% incidence of bone metastasis. ARCaP(M) cells express receptor activator of NF-kappaB ligand (RANKL), which was shown to increase tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts in culture, and when metastatic to bone in vivo. We provide evidence that RANKL expression was promoted by increased cell signaling mediated by the activation of Stat3-Snail-LIV-1. RANKL expressed by ARCaP(M) cells is functional both in vitro and in vivo. The lesson we learned from the ARCaP model of EMT is that activation of a specific cell signaling pathway by soluble factors can lead to increased bone turnover, mediated by enhanced RANKL expression by tumor cells, which is implicated in the high incidence of prostate cancer bone colonization. The ARCaP EMT model is highly attractive for developing new therapeutic agents to treat prostate cancer bone metastasis.

Left ventricular hypertrophy: is hyperphosphatemia among dialysis patients a risk factor?

Cardiovascular disease occurs in ESRD patients at rates that are far higher than is seen in the general population, and cardiovascular deaths account for the majority of deaths among dialysis patients. Abnormal mineral metabolism is a novel cardiovascular risk factor among dialysis patients. Recently published results demonstrated that even with good control of BP and anemia, conventional hemodialysis is associated with significant left ventricular hypertrophy (LVH); however, daily hemodialysis was associated with a significant reduction in LV mass index (LVMI). Furthermore, it was shown that control of serum phosphorus correlates with the reduction in LVMI. These data suggest a novel mechanism for the deleterious effect of elevated serum phosphorus on cardiovascular outcomes among hemodialysis patients: LVH. Other investigators have noted an association of hyperphosphatemia and LVH; however, this study was the first to demonstrate that improvement in serum phosphorus is associated with reduction in LVM. In addition, it is shown that daily hemodialysis is an effective modality in improving serum phosphorus through significantly improved phosphorus removal. Elevated serum phosphorus leads to vascular calcification, which can lead to LVH by decreasing vascular compliance. However, our study showed an improvement in LVMI during a 12-mo period. Because vascular calcification is unlikely to remit over this time period, it is proposed that serum phosphorus has a reversible, cardiotoxic effect that leads to LVH that can be reversed successfully with good control of serum phosphorus.

Phosphorus balance and mineral metabolism with 3 h daily hemodialysis

Poor control of mineral metabolism is independently associated with mortality in patients receiving hemodialysis. We analyzed data from a 12-month, prospective, non-randomized, controlled study of daily hemodialysis (DHD) (six sessions/week 3 h each) (n=26) vs conventional hemodialysis (CHD) (three sessions/week 4 h each) (n=51) for achievement of mineral metabolism goals and we performed a substudy of weekly dialytic phosphorus removal in DHD vs CHD. Phosphorus control was superior in the DHD group (% change from baseline to end-of-study -27+/-30% vs +7%+/-35% in the CHD group, P=0.0001). Percentage of patients using phosphate binders decreased from 77 to 40% among subjects on DHD, whereas these parameters did not change (76 vs 77%) in the CHD group (P=0.03 by Breslow-Day test for homogeneity of the odds ratios). Weekly mean phosphorus removal was higher in the DHD group (2452+/-720 mg/week vs 1572+/-366 mg/week, P=0.04). Mean normalized protein catabolic rate increased (0.90+/-0.43-1.22+/-0.26 g/kg/day, P=0.0013). DHD was also associated with an increase in the percent of subjects achieving three or more mineral metabolism goals (for phosphorus, calcium x phosphorus and parathyroid hormone) (15 vs 46%, P=0.046). In conclusion, DHD improves phosphorus control by increasing dialytic phosphorus removal while maintaining nutritional status and reducing the use of phosphate binders. The net effect allows for improved achievement of mineral metabolism goals.