Modifying Phosphate Toxicity in Chronic Kidney Disease

Phosphorus: Chronicles of the epistemology of a vital element

It was in the philosopher's stone quest that the alchemist Hennig Brand isolated chemiluminescent white phosphorus (P), Greek for "light bearer", from urine in 1669. By 1771 phosphorus was isolated from bone, and in 1777 it was identified by Antoine Lavoisier as a highly reactive element that exists predominantly in nature as ionic phosphate (PO43-) and in solution as phosphoric acid (H3PO4). Early 20th century studies revealed phosphorylated biomolecules as essential components of replicative nuclear material (RNA, DNA), a metabolic source of energy (ATP), and structural components of cellular membrane (phospholipid bilayer). Life on earth began as organophosphates of a self-replicating RNA that evolved into DNA and acquired a membrane to form the original eukaryotes, which eventually joined to form multicellular organisms of the deep sea. Tissue mineralization during transition from the ocean to land generated the endoskeleton, the largest phosphorus stores of evolving vertebrates. Subsequent studies of phosphate homeostasis elucidated its complex regulatory system based on the interaction of the kidney, small intestine, bone, and parathyroid glands, orchestrated by hormones (PTH, calcitriol, FGF23, Klotho), and carried out by phosphate-specific transporters (SLC34 and SLC20 families) all to ensure adequate phosphate for survival and health. Paradoxically, kidney replacement therapy in the 1970s, by prolonging the lives of millions of individuals with kidney failure, revealed the hazards of phosphorus excess. "Phosphorus the light bearer" has become in the eyes of many nephrologists "Phosphorus the cardiovascular toxin".

Histidine-Rich Glycoprotein Modulates the Toxic Effects of High-Dose Polyphosphate in Mice

BACKGROUND

Polyphosphate (polyP), a procoagulant released from platelets, activates coagulation via the contact system and modulates cardiomyocyte viability. High-dose intravenous polyP is lethal in mice, presumably because of thrombosis. Previously, we showed that HRG (histidine-rich glycoprotein) binds polyP and attenuates its procoagulant effects. In this study, we investigated the mechanisms responsible for the lethality of intravenous polyP in mice and the impact of HRG on this process.

METHODS

The survival of wild-type or HRG-deficient mice given intravenous synthetic or platelet-derived polyP in doses up to 50 mg/kg or saline was compared. To determine the contribution of thrombosis, the effect of FXII (factor XII) knockdown or enoxaparin on polyP-induced fibrin deposition in the lungs was examined. To assess cardiotoxicity, the ECG was continuously monitored, the levels of troponin I and the myocardial band of creatine kinase were quantified, and the viability of a cultured murine cardiomyocyte cell line exposed to polyP in the absence or presence of HRG was determined.

RESULTS

In HRG-deficient mice, polyP was lethal at 30 mg/kg, whereas it was lethal in wild-type mice at 50 mg/kg. Although FXII knockdown or enoxaparin administration attenuated polyP-induced fibrin deposition in the lungs, neither affected mortality. PolyP induced dose-dependent ECG abnormalities, including heart block and ST-segment changes, and increased the levels of troponin and myocardial band of creatine kinase, effects that were more pronounced in HRG-deficient mice than in wild-type mice and were attenuated when HRG-deficient mice were given supplemental HRG. Consistent with its cardiotoxicity, polyP reduced the viability of cultured cardiomyocytes in a dose-dependent manner, an effect attenuated with supplemental HRG.

CONCLUSIONS

High-dose intravenous polyP is cardiotoxic in mice, and HRG modulates this effect.

Phosphate Homeostasis and Disorders of Phosphate Metabolism

Phosphate is indispensable for human life and evolutionary changes over several millions of years have established tightly regulated mechanisms to ensure phosphate homeostasis. In this process, calcium and phosphate metabolism have come to be intricately linked together. Three hormones (PTH, FGF23 and Calcitriol) maintain the fine balance of calcium and phosphate metabolism through their actions at three sites (the gut, the kidneys and the skeleton). Disorders that disrupt this balance can have serious clinical consequences. Acute changes in serum phosphate levels can result in life threatening complications like respiratory failure and cardiac arrythmias. Chronic hypophosphataemia predominantly affects the musculoskeletal system and presents as impaired linear growth, rickets, osteomalacia and dental problems. Hyperphosphataemia is very common in the setting of chronic kidney disease and can be difficult to manage. A thorough understanding of calcium and phosphate homeostasis is essential to diagnose and treat conditions associated with hypo and hyperphosphataemia. In this review, we will discuss the calcium and phosphate metabolism, aetiologies and management of hypo and hyperphosphataemia.

Redefying the therapeutic strategies against cardiorenal morbidity and mortality: Patient phenotypes

Chronic kidney disease (CKD) patients face an unacceptably high morbidity and mortality, mainly from cardiovascular diseases. Diabetes mellitus, arterial hypertension and dyslipidemia are highly prevalent in CKD patients. Established therapeutic protocols for the treatment of diabetes mellitus, arterial hypertension, and dyslipidemia are not as effective in CKD patients as in the general population. The role of non-traditional risk factors (RF) has gained interest in the last decades. These entail the deranged clinical spectrum of secondary hyperparathyroidism involving vascular and valvular calcification, under the term “CKD-mineral and bone disorder” (CKD-MBD), uremia per se, inflammation and oxidative stress. Each one of these non-traditional RF have been addressed in various study designs, but the results do not exhibit any applied clinical benefit for CKD-patients. The “crusade” against cardiorenal morbidity and mortality in CKD-patients is in some instances, derailed. We propose a therapeutic paradigm advancing from isolated treatment targets, as practiced today, to precision medicine involving patient phenotypes with distinct underlying pathophysiology. In this regard we propose two steps, based on current stratification management of corona virus disease-19 and sepsis. First, select patients who are expected to have a high mortality, i.e., a prognostic enrichment. Second, select patients who are likely to respond to a specific therapy, i.e., a predictive enrichment.

Magnesium to prevent kidney disease-associated vascular calcification: crystal clear?

Vascular calcification is a prognostic marker for cardiovascular mortality in chronic kidney disease (CKD) patients. In these patients, magnesium balance is disturbed, mainly due to limited ultrafiltration of this mineral, changes in dietary intake and the use of diuretics. Observational studies in dialysis patients report that a higher blood magnesium concentration is associated with reduced risk to develop vascular calcification. Magnesium prevents osteogenic vascular smooth muscle cell transdifferentiation in in vitro and in vivo models. In addition, recent studies show that magnesium prevents calciprotein particle maturation, which may be the mechanism underlying the anti-calcification properties of magnesium. Magnesium is an essential protective factor in the calcification milieu, which helps to restore the mineral-buffering system that is overwhelmed by phosphate in CKD patients. The recognition that magnesium is a modifier of calciprotein particle maturation and mineralization of the extracellular matrix renders it a promising novel clinical tool to treat vascular calcification in CKD. Consequently, the optimal serum magnesium concentration for patients with CKD may be higher than in the general population.

Circulating p-Cresyl Sulfate, Non-Hepatic Alkaline Phosphatase and Risk of Bone Fracture Events in Chronic Kidney Disease-Mineral Bone Disease

Patients with chronic kidney disease (CKD), especially those undergoing hemodialysis, are at a considerably high risk of bone fracture events. Experimental data indicate that uremic toxins intricately involved in bone-related proteins exert multi-faced toxicity on bone cells and tissues, leading to chronic kidney disease–mineral and bone disorder (CKD-MBD). Nonetheless, information regarding the association between p-cresyl sulfate (PCS), non-hepatic alkaline phosphatase (NHALP) and skeletal events remains elusive. We aim to explore the association between PCS, NHALP and risk of bone fracture (BF) in patients with hemodialysis. Plasma concentrations of PCS and NHALP were ascertained at study entry. Cox proportional hazard regression analyses were used to determine unadjusted and adjusted hazard ratios (aHRs) of PCS for BF risk. In multivariable analysis, NHALP was associated with incremental risks of BFs [aHR: 1.06 (95% CI: 1.01–1.11)]. The association between the highest PCS tertile and BF risk remained robust [aHR: 2.87 (95% CI: 1.02–8.09)]. With respect to BF events, the interaction between NHALP and PCS was statistically significant (p value for the interaction term < 0.05). In addition to mineral dysregulation and hyperparathyroidism in hemodialysis patients, higher circulating levels of PCS and NHALP are intricately associated with incremental risk of BF events, indicating that a joint evaluation is more comprehensive than single marker. In light of the extremely high prevalence of CKD-MBD in the hemodialysis population, PCS may act as a pro-osteoporotic toxin and serve as a potential surrogate marker for skeletal events.

Emperor's syndrome in the COVID-19 era: Time for patient-centered nephrology?

The coronavirus disease-19 (COVID-19) pandemic has been a wake-up call in which has forced us to react worldwide. Health policies and practices have attracted particular attention in terms of human and financial cost. Before COVID-19, chronic kidney disease was already considered a risk multiplier in patients with diabetes and hypertension, the two now being the major risk factors for COVID-19 infection and adverse outcome. In contrast to the urgent need for action, the nephrology field is considered to be in a state of stagnation regarding the management of chronic kidney disease patients who still experience unacceptably high morbidity and mortality. Ironically and paradoxically in a field lacking robust clinical trials, clinical practice is driven by guidelines-based medicine on weak evidence. The Emperor's syndrome, referring to Hans Christian Andersen's fairy tale, has been described in medicine as voluntary blindness to an obvious truth, being a weak evidence-based therapeutic intervention or weak health care. A promising positive example of improving heart and kidney outcomes is the emerging treatment with sodium-glucose cotransporter 2 inhibitors. COVID-19 could boost actions for patient-centered care as a positive shift in nephrology care.

Retrospective study of factors associated with progression and remission/regression of diabetic kidney disease-hypomagnesemia was associated with progression and elevated serum alanine aminotransferase levels were associated with remission or regression

Aim

This study was aimed at retrospectively investigating some common clinical factors, including the serum level of magnesium (Mg), associated with progression and remission/regression of diabetic kidney disease (DKD).

Methods

The subjects were 690 Japanese patients with type 2 diabetes mellitus who were receiving treatment with oral antidiabetic drugs other than SGLT2 inhibitors. Routine clinical data were collected on the first and last day of the observation period. The prognosis of DKD is categorized into four stages according to the Kidney Disease Improving Global Outcomes classification. Progression was defined as transition from any of the lower three risk categories (LR, MIR, HR) at the start of the observation period, to the VHR stage/category at the end of the observation period. Remission/regression was defined as improvement of the risk category by at least one stage from the start to the end of the observation period. Factors associated with progression and regression/remission were investigated using Cox proportional hazards analysis. Furthermore, the factors associated with the annual decrease in eGFR of 5 ml/min/1.73 m² or more were examined by logistic regression analysis. Factors associated with transition of urinary protein negative to trace or positive, or transition of negative or trace to positive, were investigated by Cox proportional hazard analysis.

Results

The observation period was 2251 ± 1614 days. Age (Exp [B] = 1.10, 95% CI; 1.06-1.14; P < 0.01; 1 year old), serum Mg (Exp [B] = 0.82, 95% CI; 0.71-0.95; P < 0.01); 0.1 mg/dl), and serum HbA1c (Exp [B] = 1.03, 95% CI; 1.01-1.05; P < 0.01: 0.1%) were associated with progression of DKD; on the other hand, serum ALT was associated with the likelihood of remission/regression of DKD (Exp [B] = 1.01, 95% CI; 1.002-1.018; P < 0.05; 1 IU/L). The decline in eGFR was associated with higher HbA1c levels, hypomagnesemia, and lower ALT. The new appearance of trace or overt proteinuria was correlated with higher HbA1c levels, advancing age, hypomagnesemia and hypertriglycemia.

Conclusion

Our findings confirmed previous reports that advancing age and serum HbA1c levels were associated with an increased risk of progression of DKD. Lower serum Mg concentrations were also found to be associated with a high risk of progression of DKD, and interventional studies are needed to confirm a causal relationship. Elevated HbA1c levels and hypomagnesemia were common factors in the decline in eGFR and the appearance of trace or overt proteinuria. Lower serum ALT levels were associated with the decline in eGFR. Since serum ALT is known to decrease as the renal function deteriorates, serum ALT is considered to be a marker of renal function.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13340-020-00483-1.

Impact of β-glycerophosphate on the bioenergetic profile of vascular smooth muscle cells

Abstract

In chronic kidney disease, hyperphosphatemia is a key pathological factor promoting medial vascular calcification, a common complication associated with cardiovascular events and mortality. This active pathophysiological process involves osteo-/chondrogenic transdifferentiation of vascular smooth muscle cells (VSMCs) via complex intracellular mechanisms that are still incompletely understood. Little is known about the effects of phosphate on the bioenergetic profile of VSMCs during the onset of this process. Therefore, the present study explored the effects of the phosphate donor β-glycerophosphate on cellular bioenergetics of VSMCs. Mitochondrial and glycolytic functions were determined utilizing extracellular flux analysis in primary human aortic VSMCs following exposure to β-glycerophosphate. In VSMCs, β-glycerophosphate increased basal respiration, mitochondrial ATP production as well as proton leak and decreased spare respiratory capacity and coupling efficiency, but did not modify non-mitochondrial or maximal respiration. β-Glycerophosphate-treated VSMCs had higher ability to increase mitochondrial glutamine and long-chain fatty acid usage as oxidation substrates to meet their energy demand. β-Glycerophosphate did not modify glycolytic function or basal and glycolytic proton efflux rate. In contrast, β-glycerophosphate increased non-glycolytic acidification. β-Glycerophosphate-treated VSMCs had a more oxidative and less glycolytic phenotype, but a reduced ability to respond to stressed conditions via mitochondrial respiration. Moreover, compounds targeting components of mitochondrial respiration modulated β-glycerophosphate-induced oxidative stress, osteo-/chondrogenic signalling and mineralization of VSMCs. In conclusion, β-glycerophosphate modifies key parameters of mitochondrial function and cellular bioenergetics in VSMCs that may contribute to the onset of phenotypical transdifferentiation and calcification. These observations advance the understanding of the role of energy metabolism in VSMC physiology and pathophysiology of vascular calcification during hyperphosphatemia.

Key messages

β-Glycerophosphate modifies key parameters of mitochondrial respiration in VSMCs.

β-Glycerophosphate induces changes in mitochondrial fuel choice in VSMCs.

β-Glycerophosphate promotes a more oxidative and less glycolytic phenotype of VSMCs.

β-Glycerophosphate triggers mitochondrial-dependent oxidative stress in VSMCs.

Bioenergetics impact β-glycerophosphate-induced VSMC calcification.

Electronic supplementary material

The online version of this article (10.1007/s00109-020-01925-8) contains supplementary material, which is available to authorized users.

Should We Consider the Cardiovascular System While Evaluating CKD-MBD?

Cardiovascular (CV) disease is highly prevalent in the population with chronic kidney disease (CKD), where the risk of CV death in early stages far exceeds the risk of progression to dialysis. The presence of chronic kidney disease-mineral and bone disorder (CKD-MBD) has shown a strong correlation with CV events and mortality. As a non-atheromatous process, it could be partially explained why standard CV disease-modifying drugs do not provide such an impact on CV mortality in CKD as observed in the general population. We summarize the potential association of CV comorbidities with the older (parathyroid hormone, phosphate) and newer (FGF23, Klotho, sclerostin) CKD-MBD biomarkers.