Klotho and calciprotein particles as therapeutic targets against accelerated ageing

The klotho gene, named after a Greek goddess who spins the thread of life, was identified as a putative ‘ageing-suppressor’ gene. Klotho-deficient mice exhibit complex ageing-like phenotypes including hypogonadism, arteriosclerosis (vascular calcification), cardiac hypertrophy, osteopenia, sarcopenia, frailty, and premature death. Klotho protein functions as the obligate co-receptor for fibroblast growth factor-23 (FGF23), a bone-derived hormone that promotes urinary phosphate excretion in response to phosphate intake. Thus, Klotho-deficient mice suffer not only from accelerated ageing but also from phosphate retention due to impaired phosphate excretion. Importantly, restoration of the phosphate balance by placing Klotho-deficient mice on low phosphate diet rescued them from premature ageing, leading us to the notion that phosphate accelerates ageing. Because the extracellular fluid is super-saturated in terms of phosphate and calcium ions, an increase in the phosphate concentration can trigger precipitation of calcium-phosphate. In the blood, calcium-phosphate precipitated upon increase in the blood phosphate concentration is adsorbed by serum protein fetuin-A to form colloidal nanoparticles called calciprotein particles (CPPs). In the urine, CPPs appear in the renal tubular fluid when FGF23 increases phosphate load excreted per nephron. CPPs can induce cell damage, ectopic calcification, and inflammatory responses. CPPs in the blood can induce arteriosclerosis and non-infectious chronic inflammation, whereas CPPs in the urine can induce renal tubular damage and interstitial inflammation/fibrosis. Thus, we propose that CPPs behave like a pathogen that accelerates ageing and should be regarded as a novel therapeutic target against age-related disorders including chronic kidney disease.

Diagnostic Dilemma of Elevated FGF23 in a Patient With Osteomalacia: A Case Report

Introduction: Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome due to FGF23 hypersecretion. It is mostly seen with benign mesenchymal tumors. Establishing the diagnosis could be challenging due to occult nature of the disease. Case: A 54-year-old female presented with right thigh and upper back pain. On presentation, she had hypophosphatemia (1.8 mq/dl), low 25-hydroxyvitamin D (23 ng/ml), normal ionized calcium and normal iPTH (53 pg/mL). CT scan showed healing right 10th rib fracture and right proximal femoral fracture. DEXA scan was remarkable for osteoporosis. 99m-Technetium nuclear bone scan displayed hyperactivity on bilateral femurs and multiple ribs concerning for stress fractures. Patient was thought to have osteomalacia from vitamin D insufficiency and started on ergocalciferol. Two months later, patient was diagnosed with invasive ductal carcinoma. She had unilateral mastectomy and adjuvant chemotherapy. Surveillance studies were negative for recurrence. Patient lost to follow up for 2 years until she returned with right hip and upper back pain. Her labs showed hypophosphatemia (1.2 mg/dL), normocalcemia, elevated iPTH (131 pg/mL), low 25-hydroxyvitamin D (26 ng/ml), low normal 1,25-dihydroxyvitamin D (25 pg/ml, normal: 18 -72) and elevated FGF23 (285 RU/ml, normal level <180). 24-hour urine studies showed phosphate of 330 mg and fractional phosphate excretion (FEPO4) of 12%. Nuclear bone scan reported subacute fractures in left 11th and right 6th ribs and right proximal femur. TIO was entertained as the unifying diagnosis. She was started on calcium, phosphate, ergocalciferol and calcitriol. Localizing studies with a PET scan showed FDG hyperactivity in the right vocal cord. Subsequent MRI showed asymmetric fat tissue between esophagus and left common carotid artery concerning for lipoma. She lost to follow up again during COVID-19 pandemic. Discussion: We present a case of elevated FGF23 with renal phosphate wasting concerning for TIO. This case represents the diagnostic dilemma of elevated FGF23 as well as difficulty in discerning the source in TIO. Although exact tumor source remained unclear in our patient, adipose tissue in the thoracic inlet was a potential culprit. Elevated FGF23 leads to urinary phosphate wasting and inhibition of 1α- hydroxylase. Daily urine phosphate excretion > 100 mg or FEPO4 > 5 % strongly suggest renal phosphate wasting. TIO and iron-induced hypophosphatemia are two most common acquired causes of high FGF23. TIO is mostly caused by benign soft tissue and bone tumors. It can take years to establish the diagnosis due to small size and obscure location of the tumors. Advanced investigation with FDG-PET, 68Ga-DOTATATE scan, octreotide scan and venous sampling of FGF23 can aid in diagnosis. Tumor resection is usually curative. Burosumab (anti-FGF23 monoclonal antibody) can be considered when the tumor is not amenable to resect.

Urinary phosphate-containing nanoparticle contributes to inflammation and kidney injury in a salt-sensitive hypertension rat model

Although disturbed phosphate metabolism frequently accompanies chronic kidney disease (CKD), its causal role in CKD progression remains unclear. It is also not fully understood how excess salt induces organ damage. We here show that urinary phosphate-containing nanoparticles promote kidney injury in salt-sensitive hypertension. In Dahl salt-sensitive rats, salt loading resulted in a significant increase in urinary phosphate excretion without altering serum phosphate levels. An intestinal phosphate binder sucroferric oxyhydroxide attenuated renal inflammation and proteinuria in this model, along with the suppression of phosphaturia. Using cultured proximal tubule cells, we confirmed direct pathogenic roles of phosphate-containing nanoparticles in renal tubules. Finally, transcriptome analysis revealed a potential role of complement C1q in renal inflammation associated with altered phosphate metabolism. These data demonstrate that increased phosphate excretion promotes renal inflammation in salt-sensitive hypertension and suggest a role of disturbed phosphate metabolism in the pathophysiology of hypertensive kidney disease and high salt-induced kidney injury.

P0754FGF23 AND URINARY PHOSPHATE EXCRETION FOR ESTIMATION OF NEPHRON NUMBER: DIFFERENCES BETWEEN CHRONIC KIDNEY FAILURE AND KIDNEY TRANSPLANTATION

Background and Aims Assuming that FGF23 levels correlate with phosphate excretion per nephron, nephron number can be estimated by measuring FGF23 levels and urinary phosphate excretion (FEP). Kuro-O proposed that the ratio of FEP to serum FGF23 levels should correlate with nephron number and is defined as the Nephron Index (NI). The aim of the study is calculating NI as nephron number estimation in patients affected by various degree of chronic kidney disease, both transplanted and not. Method In 147 CRF patients kidney function, mineral metabolism biomarkers and NI were evaluated. Nephron Index was calculated following Kuro-o’s equation (kuro-o 2019): NI=FEP·Ps·eGFRFGF23 Observed patients were divided into two groups: patients with CKD (noTX) and transplanted ones (TX). Results noTX group was made up of 67 patients (40 males and 27 females) affected by CKD stages from G1 to G5. TX group was composed by 80 patient (49 males and 31 females) with various degree of CKD (G1T-G4T) showing mean graft age of 83,2 ± 54,8 months (range: 10,3-268,0). The two groups differed for age (mean age 59 ± 15,6 years in CKD, 55± 10,3years in TX). Mean eGFR did not differ between TX and noTX but NI was higher in TX since FEP was higher despite lower FGF23 levels in TX. The difference in FGF23 levels does not appear to depend on Klotho and PTH whose serum levels were no different between Tx and noTx. (Table1). As far as correlations are concerned, NI correlated with eGFR, FGF23, PTH and 1,25D in both group, while NI correlated with FEP only in the TX group. It's interesting that no correlation existed between FGF23 and sP, FEP, eGFR and sKlotho in TX differently from noTX (Table2). Discussion. NI could not be properly defined as nephron number estimation in TX pts. However it may represent higher function of residual nephrons, since higher FEP did not correlate to FGF23 and could be determined by compensatory hyperfiltration (increased single nephron GFR) in transplanted patients. Conclusion After kidney transplantation, high NI value could have a functional meaning rather than represent residual number of nephrons.

P0894OBESITY IMPAIRS THE ACUTE RESPONSE TO AN ORAL PHOSPHATE CHALLENGE

Background and Aims T Obesity is an increasing health problem world-wide. People who are overweight or obese are at greater risk of developing chronic diseases including cardiovascular disease (CVD). Factors associated with dysregulated phosphate metabolism have been linked to the presence of vascular calcification in people with type 2 diabetes (T2D) with normal kidney function. Insulin resistance and abdominal obesity are associated with increased circulating levels of phosphaturic hormones including fibroblast growth factor 23 (FGF-23) and parathyroid hormone (PTH). Abnormalities in phosphate regulation may not be reflected in single circulating measurements of serum phosphate, but can be revealed by the acute circulating and mineral response to an oral challenge of phosphate. The aim of this study was to determine if obesity and insulin resistance impact the acute capacity to excrete an oral phosphate challenge. Method Community-dwelling people (N=78) free of T2D and symptomatic CVD (∼10 males and ∼10 females from each decade between 40 and 80 years) with normal kidney function were recruited from Kingston, Ontario, Canada. Following a 12-hour fast, participants consumed a 1250 mg phosphate drink (sodium phosphate) where blood and urine were collected at baseline, 1, 2 and 3 hours following the oral challenge. Participants with a high-risk metabolic profile characterized by an elevated waist-to-height ratio (WHtR) (> 0.58) were matched by age and sex to participants with a low risk WHtR (<0.5). Results The results reveal a significant impact of obesity on phosphate excretion in response to an oral phosphate challenge. There was an association between WHtR ratio and the level of iFGF-23 (R=-0.34 p<0.01) but not PTH. After adjustment for age and sex, WHtR ratio was inversely correlated with urinary phosphate excretion in response to the phosphate challenge (R=-0.29, p=0.02) and the change in fractional excretion of phosphate (r=-0.34, p=0.007). From the larger cohort, an age- and sex- matched subset was selected for 12 high risk and 12 low risk metabolic profiles with WHtR of 0.66±0.02 and 0.46±0.01, respectively. Kidney function was the same between the two groups (eGFR 92.3±13.1 versus 95.8±13.6 ml/min/1.73m2 respectively) but high risk participants had significantly higher homeostatic model assessment of insulin resistance (HOMA-IR) (1.61±0.81 versus 0.68±0.3, p<0.01). Participants with a high risk metabolic profile had a greater increase in serum phosphate from baseline (29% increase in the area under the curve, p=0.04) and a significantly blunted increase in the fractional excretion of phosphate in response to the oral phosphate challenge (35% reduction in area under the curve [AUC], p=0.03) compared to the matched low risk profile participants. Conclusion Overweight/obese individuals demonstrate impaired response to an oral phosphate challenge, whereby phosphate excretion was impaired and there was increased exposure to new circulating phosphate. An impaired acute phosphate response may contribute to the initiation or propagation of vascular calcification. Dysregulated phosphate homeostasis may be an under-recognized cardiovascular risk factor in obese people that could be modified by diet and weight loss. Whether insulin enhances renal phosphate reabsorption requires further study.