Björn Meijers

Uremic toxins originating from colonic microbial metabolism

Numerous molecules, which are either excreted or metabolized by the kidney, accumulate in patients with chronic kidney disease (CKD). These uremic retention molecules (URMs), contributing to the syndrome of uremia, may be classified according to their site of origin, that is, endogenous metabolism, microbial metabolism, or exogenous intake. It is increasingly recognized that bacterial metabolites, such as phenols, indoles, and amines, may contribute to uremic toxicity. In vitro studies have implicated bacterial URMs in CKD progression, cardiovascular disease, and bone and mineral disorders. Furthermore, several observational studies have demonstrated a link between serum levels of bacterial URMs and clinical outcomes. Bacterial metabolism may therefore be an important therapeutic target in CKD. There is evidence that besides reduced renal clearance, increased colonic generation and absorption explain the high levels of bacterial URMs in CKD. Factors promoting URM generation and absorption include an increased ratio of dietary protein to carbohydrate due to insufficient intake of fiber and/or reduced intestinal protein assimilation, as well as prolonged colonic transit time. Two main strategies exist to reduce bacterial URM levels: interventions that modulate intestinal bacterial growth (e.g., probiotics, prebiotics, dietary modification) and adsorbent therapies that bind bacterial URMs in the intestines to reduce their absorption (e.g., AST-120, sevelamer). The efficacy and clinical benefit of these strategies are currently an active area of interest.

Free p-cresol is associated with cardiovascular disease in hemodialysis patients

Cardiovascular disease (CVD) is highly prevalent in chronic kidney disease, suggesting that molecules retained in uremia might contribute to this increased risk. We explored the relationship between p-cresol, a protein-bound uremic retention solute, and CVD by comparing the strength of this relationship relative to traditional and novel cardiovascular risk factors. Univariate Cox proportional hazard analysis showed that the free serum p-cresol concentration was significantly associated with CVD when the primary end point was the time to the first cardiovascular event. In multivariate analysis, free p-cresol was significantly associated with CVD in non-diabetics. In diabetic patients, however, a significant relationship between p-cresol and cardiovascular events could not be demonstrated despite their having significantly higher p-cresol levels. Our study shows that free p-cresol is a novel cardiovascular risk factor in non-diabetic hemodialysis patients.

p-Cresol and Cardiovascular Risk in Mild-to-Moderate Kidney Disease

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.

The Uremic Retention Solute p-Cresyl Sulfate and Markers of Endothelial Damage

BACKGROUND Cardiovascular disease is highly prevalent in patients with chronic kidney disease. In hemodialysis patients, the protein-bound uremic retention solute p-cresol is independently associated with cardiovascular disease. The underlying mechanisms have not been elucidated. STUDY DESIGN (1) Prospective observational study of humans and (2) in vitro study in human umbilical vein endothelial cells. SETTING Hemodialysis patients. FACTOR p-Cresol and its main derivative p-cresyl sulfate. OUTCOMES Endothelial dysfunction. MEASUREMENTS We studied: (1) the relation between p-cresol and blood markers of endothelial dysfunction, including soluble P-selectin and endothelial microparticles; and (2) direct effects of p-cresol and p-cresyl sulfate on endothelial cell cultures. RESULTS (1) In a cohort of 100 maintenance hemodialysis patients, free serum p-cresol concentrations (median, 11.7 micromol/L; interquartile range, 15.2) were directly associated with circulating endothelial microparticles (P = 0.007), but not with soluble P-selectin (mean, 37.7 +/- 14.4 [SD] pg/mL). Other independent determinants of the degree of circulating microparticles were greater serum phosphorus (mean, 4.8 +/- 1.5 mg/dL; P = 0.008) and serum calcium concentrations (mean, 9.3 +/- 0.8 mg/dL; P = 0.03), whereas treatment with active vitamin D (P = 0.008) and vintage (median, 25 months; P = 0.04) were inversely associated. (2) In vitro, p-cresyl sulfate induced a dose-dependent increase in the shedding of endothelial microparticles (P < 0.001) by human umbilical vein endothelial cells. Shedding was reduced, but not completely aborted, in the presence of albumin, whereas the selective Rho kinase inhibitor Y-27632 abrogated the p-cresyl sulfate-induced generation of endothelial microparticles. LIMITATIONS The relationship between p-cresyl sulfate and shedding of endothelial microparticles in vivo was not mechanistically explored. CONCLUSION p-Cresyl sulfate induces shedding of endothelial microparticles in the absence of overt endothelial damage in vitro and is independently associated with the number of endothelial microparticles in hemodialysis patients. These findings suggest that p-cresyl sulfate alters endothelial function in hemodialysis patients.

p-Cresyl sulfate serum concentrations in haemodialysis patients are reduced by the prebiotic oligofructose-enriched inulin

INTRODUCTION Protein-bound uraemic retention solutes, including p-cresyl sulfate and indoxyl sulfate, contribute substantially to the uraemic syndrome. These and several other uraemic retention solutes originate from intestinal bacterial protein fermentation. We investigated whether the prebiotic oligofructose-enriched inulin reduced serum concentration of p-cresyl sulfate and indoxyl sulfate, through interference with intestinal generation. METHODS We performed a single centre, non-randomized, open-label phase I/II study in maintenance HD patients with a 4-week, escalating dose regimen of oligofructose-enriched inulin (ORAFTI Synergy 1, Tienen, Belgium) (www.clinicaltrials.gov NCT00695513). Changes in p-cresyl sulfate and indoxyl sulfate serum concentrations as well as changes in p-cresyl sulfate and indoxyl sulfate generation rates were analysed. RESULTS Compliance with therapy was excellent. p-Cresyl sulfate serum concentrations at 4 weeks were significantly reduced by 20% (intention to treat, P = 0.01; per protocol, P = 0.03). Also p-cresyl sulfate generation rates were reduced (P = 0.007). In contrast, neither indoxyl sulfate generation rates (P = 0.9) nor serum concentrations (P = 0.4) were significantly changed. CONCLUSION The prebiotic oligofructose-inulin significantly reduced p-cresyl sulfate generation rates and serum concentrations in haemodialysis patients. Whether reduction of p-cresyl sulfate serum concentrations, an independent predictor of cardiovascular disease in HD patients, will result in improved cardiovascular outcomes remains to be proven.

The gut–kidney axis: indoxyl sulfate, p-cresyl sulfate and CKD progression

The fundamental insights into uraemic toxicity have evolved little since publication of the classical monograph by Cushny nearly a century ago [1]. Organic metabolites are still thought to substantially contribute to uremia (albeit urea might not be the culprit), yet evidence unequivocally demonstrating toxicity of any single uraemic constituent is lacking [2]. To advance research in uraemic toxicity, the European Toxin work group (EUTox) developed a classification of uraemic blood constituents according to characteristics that affect their removal during dialysis [3]. Besides small water-soluble molecules (e.g. urea and creatinine) and peptides/proteins (e.g. β2-microglobulin), they identified a group of solutes that circulate in equilibrium between free solute versus bound to carrier proteins. Tight protein binding severely limits solute clearances by dialysis [4]. Intriguingly, a substantial number of these so-called proteinbound uraemic retention solutes originate from protein fermentation in the large intestine, including p-cresyl sulfate and indoxyl sulfate [5]. Recently, several groups demonstrated direct associations between p-cresol, mainly reflecting p-cresyl sulfate, and overall mortality and cardiovascular disease in end-stage renal disease [6,7] and in chronic kidney disease (CKD) [8,9]. Likewise, direct associations between indoxyl sulfate and overall mortality and cardiovascular disease were reported [10]. While indoxyl sulfate and p-cresyl sulfate are frequently thought of as independent uraemic retention solutes, they share common ground. First, as mentioned before, p-cresyl sulfate and indoxyl sulfate both originate from bacterial protein fermentation in the large intestine. Colonic microbiota degrade tryptophan to indole. Further hydroxylation results in 3-hydroxy-indole, the majority of which is sulfonated to indoxyl sulfate. In parallel, fermentation of tyrosine results in p-cresol and ultimately p-cresyl sulfate [11]. Recently, we reported on sulfate conjugation of p-cresol in CKD [12,13]. Second, most p-cresyl sulfate and indoxyl sulfate circulates noncovalently bound to albumin and competes for the same albumin-binding sites (Sudlow site II) [14] (Figure 1). Indoxyl sulfate and p-cresyl sulfate are interchangeable marker molecules to study behaviour of protein-bound solutes during dialysis [15]. In this issue of Nephrology Dialysis Transplantation, Wu et al. demonstrate that serum indoxyl sulfate is associated with progression of CKD, confirming previous findings. Niwa et al. f irst advanced the hypothesis that accumulation of indoxyl sulfate accelerates glomerular sclerosis and progression of kidney disease [16,17]. Animal and small-scale human studies on CKD patients suggested retardation of CKD progression by adsorption of indole in the large intestine [18,19]. Intriguingly, Wu et al. equally demonstrate that p-cresyl sulfate is associated with CKD progression. Does this indicate that indoxyl sulfate and p-cresyl sulfate be considered equally valid markers for CKD progression? This illustrates one of the key problems we are faced with when investigating uremia. One of the hallmarks of uraemic retention solutes is that they all move more or less in the same direction. When glomerular filtration rate falls, concentrations of the uraemic retention solutes we measure, and most likely a host of solutes that we are not aware of, all rise. Indeed, in the current study, Wu et al. observed a moderate correlation between indoxyl sulfate and estimated glomerular filtration rate (eGFR) (r −0.72, P < 0.001), between p-cresyl sulfate and eGFR (r −0.64, P < 0.001) and between indoxyl sulfate and p-cresyl sulfate (r 0.66, P < 0.001). From a statistical point of view, if nominally related measures actually quantify the same phenomenon, then they are redundant, i.e. collinear. This might lead us to conclude that indoxyl sulfate and p-cresyl sulfate are plain markers of kidney function. The strongpoint of the study by Wu et al. is that they went to great length to correct for residual confounding, including by correction for related protein-bound uraemic retention solutes. They thus demonstrate that, while indoxyl sulfate is independently associated with CKD progression, this association is lost after correction for p-

Recovery of Hyperphosphatoninism and Renal Phosphorus Wasting One Year after Successful Renal Transplantation

BACKGROUND AND OBJECTIVES In the first months after successful kidney transplantation, hypophosphatemia and renal phosphorus wasting are common and related to inappropriately high parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF-23) levels. Little is known about the long-term natural history of renal phosphorus homeostasis in renal transplant recipients. DESIGN, SETTING, PARTICIPANTS We prospectively followed parameters of mineral metabolism (including full-length PTH and FGF-23) in 50 renal transplant recipients at the time of transplantation (Tx), at month 3 (M3) and at month 12 (M12). Transplant recipients were (1:1) matched for estimated GFR with chronic kidney disease (CKD) patients. RESULTS FGF-23 levels (Tx: 2816 [641 to 10665] versus M3: 73 [43 to 111] versus M12: 56 [34 to 78] ng/L, median [interquartile range]) and fractional phosphorus excretion (FE(phos); M3: 45 +/- 19% versus M12: 37 +/- 13%) significantly declined over time after renal transplantation. Levels 1 yr after transplantation were similar to those in CKD patients (FGF-23: 47 [34 to 77] ng/L; FE(phos) 35 +/- 16%). Calcium (9.1 +/- 0.5 versus 8.9 +/- 0.3 mg/dl) and PTH (27.2 [17.0 to 46.0] versus 17.5 [11.7 to 24.4] ng/L) levels were significantly higher, whereas phosphorus (3.0 +/- 0.6 versus 3.3 +/- 0.6 mg/dl) levels were significantly lower 1 yr after renal transplantation as compared with CKD patients. CONCLUSIONS Data indicate that hyperphosphatoninism and renal phosphorus wasting regress by 1 yr after successful renal transplantation.

Intrarenal Resistive Index after Renal Transplantation

BACKGROUND The intrarenal resistive index is routinely measured in many renal-transplantation centers for assessment of renal-allograft status, although the value of the resistive index remains unclear. METHODS In a single-center, prospective study involving 321 renal-allograft recipients, we measured the resistive index at baseline, at the time of protocol-specified renal-allograft biopsies (3, 12, and 24 months after transplantation), and at the time of biopsies performed because of graft dysfunction. A total of 1124 renal-allograft resistive-index measurements were included in the analysis. All patients were followed for at least 4.5 years after transplantation. RESULTS Allograft recipients with a resistive index of at least 0.80 had higher mortality than those with a resistive index of less than 0.80 at 3, 12, and 24 months after transplantation (hazard ratio, 5.20 [95% confidence interval {CI}, 2.14 to 12.64; P<0.001]; 3.46 [95% CI, 1.39 to 8.56; P=0.007]; and 4.12 [95% CI, 1.26 to 13.45; P=0.02], respectively). The need for dialysis did not differ significantly between patients with a resistive index of at least 0.80 and those with a resistive index of less than 0.80 at 3, 12, and 24 months after transplantation (hazard ratio, 1.95 [95% CI, 0.39 to 9.82; P=0.42]; 0.44 [95% CI, 0.05 to 3.72; P=0.45]; and 1.34 [95% CI, 0.20 to 8.82; P=0.76], respectively). At protocol-specified biopsy time points, the resistive index was not associated with renal-allograft histologic features. Older recipient age was the strongest determinant of a higher resistive index (P<0.001). At the time of biopsies performed because of graft dysfunction, antibody-mediated rejection or acute tubular necrosis, as compared with normal biopsy results, was associated with a higher resistive index (0.87 ± 0.12 vs. 0.78 ± 0.14 [P=0.05], and 0.86 ± 0.09 vs. 0.78 ± 0.14 [P=0.007], respectively). CONCLUSIONS The resistive index, routinely measured at predefined time points after transplantation, reflects characteristics of the recipient but not those of the graft. (ClinicalTrials.gov number, NCT01879124 .).

p-Cresyl Sulfate and Indoxyl Sulfate in Hemodialysis Patients

BACKGROUND AND OBJECTIVES Indoxyl sulfate and p-cresyl sulfate are important representatives of the protein-bound uremic retention solutes. Serum levels of p-cresyl sulfate and indoxyl sulfate are linked to cardiovascular outcomes and chronic kidney disease progression, respectively. They share important features such as the albumin-binding site, low dialytic clearance, and both originate from protein fermentation. Whether serum concentrations are related is, however, not known. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS In an observational study in 75 maintenance hemodialysis patients, we studied agreement between indoxyl sulfate and p-cresyl sulfate serum concentrations, dialytic reduction rates, and dialytic clearances. Concentrations were determined by HPLC. Dialytic clearances were determined from total spent dialysate collections. In vitro spiking experiments were performed to explore protein binding characteristics. RESULTS Indoxyl sulfate and p-cresyl sulfate total serum concentrations were not related (r = 0.02, P = 0.9), whereas free serum concentrations were only moderately related (r = 0.53, P < 0.001). Indoxyl sulfate and p-cresyl sulfate share the same albumin binding site, for which they are competitive binding inhibitors. Intriguingly, indoxyl sulfate and p-cresyl sulfate reduction rates (r = 0.91, P < 0.001) and dialytic clearances (r = 0.97, P < 0.001) correlated tightly. CONCLUSIONS Indoxyl sulfate and p-cresyl sulfate serum concentrations are not associated, suggesting different metabolic pathways. Indoxyl sulfate and p-cresyl sulfate are both valid markers to monitor behavior of protein-bound solutes during dialysis. Finally, they are competitive binding inhibitors for the same albumin binding site.

Sclerostin: Another Vascular Calcification Inhibitor?

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.