Liesbeth Viaene

BACKGROUND AND OBJECTIVES: Cardiovascular disease is highly prevalent in chronic kidney disease. Traditional risk factors are insufficient to explain the high cardiovascular disease prevalence. Free p-cresol serum concentrations, mainly circulating as its derivative p-cresyl sulfate, are associated with cardiovascular disease in hemodialysis patients. It is not known if p-cresol is associated with cardiovascular disease in patients with chronic kidney disease not yet on dialysis. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: In a prospective observational study in 499 patients with mild-to-moderate kidney disease, we examined the multivariate association between p-cresol free serum concentrations and cardiovascular events. RESULTS: After a mean follow-up of 33 mo, 62 patients reached the primary end point of fatal or nonfatal cardiovascular events. Higher baseline concentrations of free p-cresol were directly associated with cardiovascular events (univariate hazard ratio [HR] 1.79, P<0.0001). In multivariate analysis, p-cresol remained a predictor of cardiovascular events, independent of GFR and independent of Framingham risk factors (full model, HR 1.39, P=0.04). CONCLUSIONS: These findings suggest that p-cresol measurements may help to predict cardiovascular disease risk in renal patients over a wide range of residual renal function, beyond traditional markers of glomerular filtration. Whether p-cresol is a modifiable cardiovascular risk factor in CKD patients remains to be proven.

CONTEXT: Sclerostin, a Wnt antagonist produced by osteocytes, regulates osteoblast activity and is a well-established key player in bone turnover. Recent data indicate that the Wnt pathway may also be involved in vascular calcification. OBJECTIVE: The present study tests the hypothesis that serum sclerostin levels are associated with vascular calcification in patients with chronic kidney disease (CKD) not yet receiving dialysis. DESIGN, SETTING, PARTICIPANTS, AND MEASUREMENTS: We performed a cross-sectional analysis in 154 patients with CKD. Aortic calcification (AC) was assessed by lumbar X-ray and scored with a maximum score of 24. In addition to traditional and nontraditional cardiovascular (CV) risk factors, serum sclerostin levels were assessed (ELISA). Regression analysis was performed to identify determinants of serum sclerostin and AC. RESULTS: AC was present in 59% of patients. Older age (P < .0001), male sex (P = .006), lower estimated glomerular rate (eGFR) (P = .0008), lower bone-specific alkaline phosphatase (P = .03), and the absence of AC (P = .006) were identified as independent determinants of higher serum sclerostin levels. In univariate logistic regression, higher age, diabetes, CV history, higher body mass index, higher serum C-reactive protein and sclerostin levels and lower estimated glomerular rate were all associated with the presence of AC. In multivariate analysis, lower, not higher, sclerostin levels (P = .04, odds ratio [OR] per ng/mL of 0.24), higher age (P < .0001, OR per year of 1.17) and CV history (P = .02, OR for a positive CV history of 3.83) were identified as independent determinants of AC. CONCLUSIONS: In this cohort of patients with CKD, we found that patients with aortic calcifications (ACs) had higher sclerostin levels. However, in multivariate analysis, the association became inverse. Additional clinical and experimental studies are urgently required to clarify whether or not sclerostin protects against progression of vascular calcification.

Indoxyl sulfate and p-cresyl sulfate are two uremic retention solutes implicated in the uremic syndrome. Removal during dialysis is limited, mainly due to protein binding. Binding characteristics to healthy albumin have recently been characterized. Whether uremia alters the binding characteristics of albumin is currently unknown. Moreover, protein binding values previously determined with ultrafiltration are in sharp contrast to recently reported values based on microcalorimetry. In the present study, indoxyl sulfate and p-cresyl sulfate binding were therefore quantified using both equilibrium dialysis and ultrafiltration. Deming regression demonstrated good agreement between equilibrium dialysis and ultrafiltration. Free serum concentrations of indoxyl sulfate (+26.6%) and p-cresyl sulfate (+19.7%) were slightly higher at body temperature compared with at room temperature. To investigate binding kinetics, the plasma of healthy individuals or hemodialysis patients was titrated with albumin solutions. Theoretical models of protein binding were fitted to observed titration curves. Binding coefficients of both toxins were highest in purified albumin, and were reduced from healthy to uremic plasma. In conclusion, the ultrafiltration-HPLC technique reliably measures free serum concentrations of indoxyl sulfate and p-cresyl sulfate. Albumin is the main binding protein, both in health and in advanced stages of chronic kidney disease. Modeling suggests that albumin contains two binding sites for both toxins, a single high affinity binding site and a second low affinity binding site. The high affinity binding site accounts for at least 90% of overall binding. Competition for this binding site could be used to augment free solute concentrations during dialysis, thus improving epuration.

BACKGROUND: Derangements in bone metabolism and vascular calcification (VC) substantially contribute to the accelerated cardiovascular morbidity and mortality in chronic kidney disease (CKD). The Wnt signalling pathway is increasingly recognized to play an important role in bone homeostasis and VC. Circulating levels of the Wnt inhibitor sclerostin are elevated in CKD patients. The present study investigated whether the circulating levels of sclerostin are associated with all-cause mortality in haemodialysis (HD) patients. METHODS: We performed a post-hoc survival analysis in 100 prevalent HD patients (68 ± 13 years, 40 male) recruited in 2006 who were prospectively followed for median 637 (8-1000, range) days. Parameters of mineral metabolism including bone-specific alkaline phosphatase (bsAP) and serum sclerostin were determined in spare blood samples collected at baseline. RESULTS: Serum concentrations of serum sclerostin amounted to 110 (82-151) [median (iqr)] pmol/L. Patients with sclerostin levels above median were characterized by older age, higher haemoglobin and creatinine level and lower bsAP concentration. During a median follow-up of 637 days, 31 patients died. Higher circulating sclerostin levels were associated with decreased mortality in prevalent HD patients: unadjusted hazard ratio (HR) 0.51 (0.24-1.06) (P = 0.06); HR adjusted for age and gender for serum sclerostin levels above versus below median was 0.33 (0.15-0.73) (P = 0.006). When bsAP was entered in the Cox regression analysis, it replaced sclerostin in the final model. CONCLUSIONS: Our data show that high circulating sclerostin levels are associated with improved survival and suggest that a low bsAP activity may be in the causal pathway.

BACKGROUND AND OBJECTIVES: The discovery of fibroblast growth factor-23 (FGF-23) and the elucidation of its function as a phosphaturic and 1,25(OH)2VitD counter-regulatory hormone provides a new conceptual framework for the understanding of the pathogenesis of secondary hyperparathyroidism. This study aims to elucidate the complex associations between FGF-23, parathyroid hormone (PTH), 1,25(OH)2D, and phosphate in patients with early-stage chronic kidney disease (CKD) and to provide clinical evidence in favor of the new phosphate-centric paradigm for the pathogenesis of secondary hyperparathyroidism. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: Serum biointact PTH and FGF-23, 25(OH)D, 1,25(OH)2D, calcium, phosphate, 24-hour urine excretion of phosphate and calcium, and urinary fractional excretion of phosphate were determined in a cross-sectional study including 125 patients with CKD stages 1 to 3. RESULTS: Serum phosphate levels showed an inverse association with estimated GFR (eGFR), but were within the normal range in all but one patient. FGF-23 and PTH were inversely associated with eGFR, even in the subgroup of patients with CKD stages 1 and 2. High FGF-23 levels were significantly more prevalent than high PTH levels. The urinary fractional excretion of phosphate was highest in patients with both a high serum FGF-23 and PTH level. Increased FGF-23 and phosphate and decreased 25(OH)D were independently associated with decreased 1,25(OH)2D. CONCLUSIONS: Our data are in favor of the new paradigm for the pathogenesis of secondary hyperparathyroidism according to which a reduced phosphate excretion capacity is the principal abnormality that initiates secondary hyperparathyroidism.