Arjan J. Kwakernaak

Body Mass Index and Glomerular Hyperfiltration in Renal Transplant Recipients: Cross‐Sectional Analysis and Long‐Term Impact

Obesity is a risk factor for renal graft loss. Higher body mass index (BMI) in native kidneys is associated with glomerular hyperfiltration. Whether higher BMI in renal transplants is associated with hyperfiltration is unknown. We investigated the impact of BMI on renal hemodynamics 1 year post‐transplant. We analyzed glomerular filtration rate (GFR, 125I‐iothalamate) and effective renal plasma flow (ERPF, 131I‐hippurate) in 838 kidney transplants. Data were analyzed for all patients and for the subpopulation without diabetes. Long‐term impact of BMI and renal hemodynamics were explored by Cox‐regression. With higher BMI GFR and filtration fraction (FF) increased significantly. Multivariate analysis supported impact of BMI on GFR (adjusted r2 of the model 0.275) and FF (adjusted r2 of the model 0.158). This association was not explained by diabetes mellitus. On Cox‐regression analysis, lower GFR and higher FF were independent determinants of overall graft loss and graft loss by patient mortality. Lower GFR and higher BMI were determinants of death‐censored graft loss, with borderline contribution of higher FF. In renal transplants higher BMI is independently associated with higher GFR and FF one year posttransplant, suggesting glomerular hyperfiltration with altered afferent–efferent balance. Mechanisms underlying the long‐term prognostic impact of hyperfiltration deserve further exploration.

Effects of sodium restriction and hydrochlorothiazide on RAAS blockade efficacy in diabetic nephropathy: a randomised clinical trial

BACKGROUND Reduction of dietary sodium intake or diuretic treatment increases renin-angiotensin-aldosterone system (RAAS) blockade efficacy in non-diabetic nephropathy. We aimed to investigate the effect of sodium restriction and the diuretic hydrochlorothiazide, separately and in combination, added to RAAS blockade on residual albuminuria in patients with type 2 diabetic nephropathy. METHODS In this multicentre, double-blind, placebo-controlled, crossover randomised trial, we included patients with type 2 diabetic nephropathy. Main entry criteria were microalbuminaria or macroalbuminuria, and creatinine clearance of 30 mL/min or higher with less than 6 mL/min decline in the previous year. We tested the separate and combined effects of sodium restriction (dietary counselling in the outpatient setting) and hydrochlorothiazide (50 mg daily), added to standardised maximal angiotensin-converting enzyme (ACE) inhibition (lisinopril 40 mg daily), on albuminuria (primary endpoint). Patients were given hydrochlorothiazide (50 mg per day) or placebo during four treatment periods of 6 weeks. Both treatments were combined with regular sodium diet or sodium restriction (target sodium intake 50 mmol Na(+) per day). The 6-week treatment periods were done consecutively in a random order. Patients were randomised in blocks of two patients. The trial was analysed by intention to treat. The trial is registered with TrialRegister.nl, number 2366. FINDINGS Of 89 eligible patients, 45 were included in the study. Both sodium restriction and hydrochlorothiazide significantly reduced albuminuria, irrespective of treatment sequence. Residual geometric mean albuminuria with baseline treatment was 711 mg per day (95% CI 485-1043); it was significantly reduced by sodium restriction (393 mg per day [258-599], p=0·0002), by hydrochlorothiazide (434 mg per day [306-618], p=0·0003), and to the greatest extent by their combination (306 mg per day [203-461], p<0·0001). Orthostatic complaints were present in two patients (4%) during baseline treatment, five (11%) during addition of sodium restriction, five (11%) during hydrochlorothiazide treatment, and 12 (27%) during combination treatment. No serious adverse events occurred. INTERPRETATION We conclude that sodium restriction is an effective non-pharmacological intervention to increase RAAS blockade efficacy in type 2 diabetic nephropathy. FUNDING None.

Central Body Fat Distribution Associates with Unfavorable Renal Hemodynamics Independent of Body Mass Index

Central distribution of body fat is associated with a higher risk of renal disease, but whether it is the distribution pattern or the overall excess weight that underlies this association is not well understood. Here, we studied the association between waist-to-hip ratio (WHR), which reflects central adiposity, and renal hemodynamics in 315 healthy persons with a mean body mass index (BMI) of 24.9 kg/m(2) and a mean (125)I-iothalamate GFR of 109 ml/min per 1.73 m(2). In multivariate analyses, WHR was associated with lower GFR, lower effective renal plasma flow, and higher filtration fraction, even after adjustment for sex, age, mean arterial pressure, and BMI. Multivariate models produced similar results regardless of whether the hemodynamic measures were indexed to body surface area. Thus, these results suggest that central body fat distribution, independent of BMI, is associated with an unfavorable pattern of renal hemodynamic measures that could underlie the increased renal risk reported in observational studies.

Plasma proprotein convertase subtilisin–kexin type 9 is predominantly related to intermediate density lipoproteins

OBJECTIVES Proprotein convertase subtilisin-kexin type 9 (PCSK9) is a key regulator of low density lipoprotein (LDL) receptor processing, but the PCSK9 pathway may also be implicated in the metabolism of triglyceride-rich lipoproteins. Here we determined the relationship of plasma PCSK9 with very low density lipoprotein (VLDL) and LDL subfractions. DESIGN AND METHODS The relationship of plasma PCSK9 (sandwich enzyme-linked immunosorbent assay) with 3 very low density lipoprotein (VLDL) and 3 low density lipoprotein (LDL) subfractions (nuclear magnetic resonance spectroscopy) was determined in 52 subjects (30 women). RESULTS In age- and sex-adjusted analysis plasma PCSK9 was correlated positively with total cholesterol, non-high density lipoprotein cholesterol and LDL cholesterol (r=0.516 to 0.547, all p<0.001), as well as with triglycerides (r=0.286, p=0.044). PCSK9 was correlated with the VLDL particle concentration (r=0.336, p=0.017) and with the LDL particle concentration (r=0.362, p=0.010), but only the relationship with the LDL particle concentration remained significant in multivariable linear regression analysis. In an analysis which included the 3 LDL subfractions, PCSK9 was independently related to intermediate density lipoproteins (IDL) (p<0.001), but not to other LDL subfractions. CONCLUSIONS This study suggests that plasma PCSK9 predominantly relates to IDL, a triglyceride-rich LDL subfraction. The PCSK9 pathway may affect plasma triglycerides via effects on the metabolism of triglyceride-rich LDL particles.

Vascular endothelial growth factor C levels are modulated by dietary salt intake in proteinuric chronic kidney disease patients and in healthy subjects

BACKGROUND Recent experimental findings demonstrate vascular endothelial growth factor C (VEGF-C)-mediated water-free storage of salt in the interstitium, which prevents a salt-sensitive blood pressure state. It is unknown whether this mechanism plays a role in salt homeostasis and regulation of blood pressure in humans as well. Therefore, we investigated circulating VEGF-C levels and blood pressure during different well-controlled salt intake in chronic kidney disease (CKD) patients and in healthy subjects. METHODS In two crossover studies, non-diabetic proteinuric CKD patients (n = 32) and healthy subjects (n = 31) were treated with consecutively a low-sodium diet (LS, aim 50 mmol Na(+)/day) and a high-sodium diet (HS, aim 200 mmol Na(+)/day) in random order, during two 6-week (CKD patients) and two 1-week periods (healthy subjects). RESULTS We found that VEGF-C levels are higher during HS than during LS in CKD patients (P = 0.034) with a trend towards higher VEGF-C in healthy subjects as well (P = 0.070). In CKD patients, HS was associated with higher NT-proBNP levels (P = 0.005) and body weight (P = 0.013), consistent with extracellular volume (ECV) expansion and with higher blood pressure (P < 0.001), indicating salt sensitivity. In healthy subjects, blood pressure was not affected by dietary salt (P = 0.14), despite a rise in ECV (P = 0.023). DISCUSSION Our findings support a role for VEGF-C-mediated salt homeostasis in humans. Considering the salt sensitivity of blood pressure, this buffering mechanism appears to be insufficient in proteinuric CKD patients. Future studies are needed to provide causality and to substantiate the clinical and therapeutic relevance of this VEGF-C regulatory mechanism in humans.

The Relationship of the Anti-Oxidant Bilirubin with Free Thyroxine Is Modified by Insulin Resistance in Euthyroid Subjects

Background The strong anti-oxidative properties of bilirubin largely explain its cardioprotective effects. Insulin resistance is featured by low circulating bilirubin. Thyroid hormone affects both bilirubin generation and its biliary transport, but it is unknown whether circulating bilirubin is associated with thyroid function in euthyroid subjects. Aim is to determine relationships of bilirubin with TSH, free T4 and free T3 in euthyroid subjects without type 2 diabetes mellitus (T2DM), and to assess whether such a relationship would be modified by the degree of insulin resistance. Methods Total bilirubin, TSH, free T4, free T3, glucose, insulin, lipids and transaminases were measured in 1854 fasting euthyroid subjects without T2DM, recruited from the general population (PREVEND cohort). Insulin resistance was assessed by homeostasis model assessment. Results Bilirubin was positively related to free T4 (β = 0.116, P<0.001) and free T3 (β = 0.078, P = 0.001), but bilirubin was unrelated to TSH. The relationship of bilirubin with free T4 was modified by insulin resistance with a larger effect in more insulin resistant individuals (adjusted for age and sex: β = 0.043, P = 0.056 for interaction; additionally adjusted for smoking, alcohol intake, transaminases and total cholesterol (β = 0.044, P = 0.044 for interaction). The association of bilirubin with free T4 was also modified by high density lipoprotein cholesterol (age- and sex-adjusted: β = 0.040, P = 0.072). Conclusions Low bilirubin relates to low free T4 in euthyroid non-diabetic subjects. Low normal free T4 may particularly confer low bilirubin in more insulin resistant individuals.

Body mass index and body fat distribution as renal risk factors: a focus on the role of renal haemodynamics.

Weight excess and/or central body fat distribution are associated with increased long-term renal risk, not only in subjects with renal disease or renal transplant recipients, but also in the general population. As the prevalence of weight excess is rising worldwide, this may become a main renal risk factor on a population basis, even more so because the risk extends to the overweight range. Understanding the mechanisms of this detrimental effect of weight excess on the kidneys is needed in order to design preventive treatment strategies. The increased risk associated with weight excess is partly attributed to associated comorbid conditions, such as hypertension, dyslipidaemia, insulin resistance and diabetes; however, current evidence supports a direct pathogenetic role for renal haemodynamics as well. Weight excess is associated with an altered renal haemodynamic profile, i.e. an increased glomerular filtration rate relative to effective renal plasma flow, resulting in an increased filtration fraction (FF). This renal haemodynamic profile is considered to reflect glomerular hyperfiltration and glomerular hypertension, resulting from a dysbalance between afferent and efferent arterial vasomotor balance. This unfavorable renal haemodynamic profile was found to be associated with renal outcome in experimental models and in human renal transplant recipients, and is associated with a blunted sodium excretion, and reversible by weight loss, renin-angiotensin-aldosterone system blockade or by dietary sodium restriction. More recent evidence showed that a central body fat distribution is also associated with an increased FF, even independent of overall weight excess. In this review, we provide an overview on current literature on the impact of weight excess and central body fat distribution on the renal haemodynamic profile in humans, and its possible role in progressive renal damage.

Urinary Plasmin Inhibits TRPV5 in Nephrotic-Range Proteinuria

Urinary proteins that leak through the abnormal glomerulus in nephrotic syndrome may affect tubular transport by interacting with membrane transporters on the luminal side of tubular epithelial cells. Patients with nephrotic syndrome can develop nephrocalcinosis, which animal models suggest may develop from impaired transcellular Ca(2+) reabsorption via TRPV5 in the distal convoluted tubule (DCT). In nephrotic-range proteinuria, filtered plasminogen reaches the luminal side of DCT, where it is cleaved into active plasmin by urokinase. In this study, we found that plasmin purified from the urine of patients with nephrotic-range proteinuria inhibits Ca(2+) uptake in TRPV5-expressing human embryonic kidney 293 cells through the activation of protease-activated receptor-1 (PAR-1). Preincubation with a plasmin inhibitor, a PAR-1 antagonist, or a protein kinase C (PKC) inhibitor abolished the effect of plasmin on TRPV5. In addition, ablation of the PKC phosphorylation site S144 rendered TRPV5 resistant to the action of plasmin. Patch-clamp experiments showed that a decreased TRPV5 pore size and a reduced open probability accompany the plasmin-mediated reduction in Ca(2+) uptake. Furthermore, high-resolution nuclear magnetic resonance spectroscopy demonstrated specific interactions between calmodulin and residues 133-154 of the N-terminus of TRPV5 for both wild-type and phosphorylated (S144pS) peptides. In summary, PAR-1 activation by plasmin induces PKC-mediated phosphorylation of TRPV5, thereby altering calmodulin-TRPV5 binding, resulting in decreased channel activity. These results indicate that urinary plasmin could contribute to the downstream effects of proteinuria on the tubulointerstitium by negatively modulating TRPV5.

Adiposity Blunts the Positive Relationship of Thyrotropin with Proprotein Convertase Subtilisin-Kexin Type 9 Levels in Euthyroid Subjects

BACKGROUND Effects of thyroid function status on lipoprotein metabolism may extend into the euthyroid range. Low-density lipoprotein (LDL) metabolism is governed by proprotein convertase subtilisin-kexin type 9 (PCSK9), which down-regulates LDL receptor expression, resulting in higher LDL cholesterol (LDL-C). Here, we tested whether plasma PCSK9 correlates with thyroid function in nonobese and obese euthyroid subjects. METHODS We assessed the extent to which plasma PCSK9 is determined by thyrotropin (TSH) in 74 euthyroid subjects (31 women; TSH between 0.5 and 4.0 mU/L and free thyroxine [FT4] between 11.0 and 19.5 pM) with varying degrees of obesity (body mass index [BMI] ranging from 20.2 to 40.4 kg/m(2)). RESULTS TSH, FT4, PCSK9, non-high-density lipoprotein cholesterol (non-HDL-C), LDL-C, and apolipoprotein B (apoB) levels were not different between 64 nonobese subjects (BMI<30 kg/m(2)) and 10 obese subjects (BMI≥30 kg/m(2); p>0.20 for each). PCSK9 correlated positively with TSH in nonobese subjects (r=0.285, p=0.023). In contrast, PCSK9 was not associated positively with TSH in obese subjects (r=-0.249, p=0.49). The relationship of PCSK9 with TSH was different between nonobese and obese subjects when taking age, sex, FT4, and the presence of anti-thyroid antibodies into account (multiple linear regression analysis: β=-0.320, p=0.012 for the interaction term between the presence of obesity and TSH on PCSK9), and was also modified by BMI as a continuous trait (β=-0.241, p=0.062 for the interaction term between BMI and TSH on PCSK9). Non-HDL-C, LDL-C, and apoB levels were dependent on PCSK9 in nonobese subjects (p≤0.01 for each), but not in obese subjects (p>0.50), Accordingly, BMI interacted negatively with PCSK9 on non-HDL-C (p=0.028) and apoB (p=0.071). CONCLUSIONS This study suggests that circulating PCSK9 levels correlate with thyroid function even in the normal range. This relationship appears to be blunted by obesity. Thyroid functional status may influence cholesterol metabolism through the PCSK9 pathway.

Fibroblast growth factor 23 and the antiproteinuric response to dietary sodium restriction during renin-angiotensin-aldosterone system blockade.

BACKGROUND Residual proteinuria during renin-angiotensin-aldosterone system (RAAS) blockade is a major renal and cardiovascular risk factor in chronic kidney disease. Dietary sodium restriction potentiates the antiproteinuric effect of RAAS blockade, but residual proteinuria remains in many patients. Previous studies linked high fibroblast growth factor 23 (FGF-23) levels with volume overload; others linked higher serum phosphate levels with impaired RAAS-blockade efficacy. We hypothesized that FGF-23 reduces the capacity of dietary sodium restriction to potentiate RAAS blockade, impairing the antiproteinuric effect. STUDY DESIGN Post hoc analysis of cohort data from a randomized crossover trial with two 6-week study periods comparing proteinuria after a regular-sodium diet with proteinuria after a low-sodium diet, both during background angiotensin-converting enzyme inhibition. SETTING & PARTICIPANTS 47 nondiabetic patients with CKD with residual proteinuria (median protein excretion, 1.9 [IQR, 0.8-3.1] g/d; mean age, 50±13 [SD] years; creatinine clearance, 69 [IQR, 50-110] mL/min). PREDICTOR Plasma carboxy-terminal FGF-23 levels. OUTCOMES Difference in residual proteinuria at the end of the regular-sodium versus low-sodium study period. Residual proteinuria during the low-sodium diet period adjusted for proteinuria during the regular-sodium diet period. RESULTS Higher baseline FGF-23 level was associated with reduced antiproteinuric response to dietary sodium restriction (standardized β=-0.46; P=0.001; model R(2)=0.71). For every 100-RU/mL increase in FGF-23 level, the antiproteinuric response to dietary sodium restriction was reduced by 10.6%. Higher baseline FGF-23 level was a determinant of more residual proteinuria during the low-sodium diet (standardized β=0.27; P=0.003) in linear regression analysis adjusted for baseline proteinuria (model R(2)=0.71). There was no interaction with creatinine clearance (P interaction=0.5). Baseline FGF-23 level did not predict changes in systolic or diastolic blood pressure upon intensified antiproteinuric treatment. LIMITATIONS Observational study, limited sample size. CONCLUSIONS FGF-23 levels are associated independently with impaired antiproteinuric response to sodium restriction in addition to RAAS blockade. Future studies should address whether FGF-23-lowering strategies may further optimize proteinuria reduction by RAAS blockade combined with dietary sodium restriction.