Marta Christov

Plasma FGF23 levels increase rapidly after acute kidney injury

Emerging evidence suggests that fibroblast growth factor 23 (FGF23) levels are elevated in patients with acute kidney injury (AKI). In order to determine how early this increase occurs we used a murine folic acid nephropathy model and found that plasma FGF23 levels increased significantly from baseline already after 1 hour of AKI, with an 18-fold increase at 24 hours. Similar elevations of FGF23 levels were found when AKI was induced in mice with osteocyte-specific parathyroid hormone receptor ablation or the global deletion of parathyroid hormone or vitamin D receptor, indicating that the increase in FGF23 was independent of parathyroid hormone and vitamin D signaling. Furthermore, FGF23 levels increased to a similar extent in wild-type mice maintained on normal or phosphate-depleted diets prior to induction of AKI, indicating that the marked FGF23 elevation is at least partially independent of dietary phosphate. Bone production of FGF23 was significantly increased in AKI. The half-life of intravenously administered recombinant FGF23 was only modestly increased. Consistent with the mouse data, plasma FGF23 levels rose 15.9-fold by 24 hours following cardiac surgery in patients who developed AKI. The levels were significantly higher than in those without postoperative AKI. Thus, circulating FGF23 levels rise rapidly during AKI in rodents and humans. In mice this increase is independent of established modulators of FGF23 secretion.

Insights from Genetic Disorders of Phosphate Homeostasis

The molecular identification and characterization of genetic defects leading to a number of rare inherited or acquired disorders affecting phosphate homeostasis has added tremendous detail to our understanding of the regulation of phosphate balance. The identification of the key phosphate-regulating hormone, fibroblast growth factor 23 (FGF23), as well as other molecules that control its production, such as the N-acetylgalactosaminyltransferase 3 GALNT3, the endopeptidase phosphate-regulating protein with homologies to endopeptidases on the X chromosome, and the matrix protein dentin matrix protein 1, and molecules that function as downstream effectors of FGF23, such as the longevity factor Klotho and the phosphate transporters NPT2a and NPT2c, has permitted us to understand the elegant and complex interplay that exists between the kidneys, bone, parathyroid, and gut. Such insights from genetic disorders have allowed not only the design of potent targeted therapies for some of these rare genetic disorders, such as using anti-FGF23 antibodies for treatment of X-linked hypophosphatemic rickets, but also have led to clinically relevant observations related to the dysregulation of mineral ion homeostasis in chronic kidney disease. Thus, we are able to leverage our knowledge of rare human disorders affecting only a few individuals, to understand and potentially treat disease processes that affect millions of patients.

Genetic Ablation of Sfrp4 in Mice Does Not Affect Serum Phosphate Homeostasis

Serum phosphate levels are regulated by PTH and the fibroblast growth factor 23 (Fgf23)/Klotho endocrine system, which both affect expression of Npt2a and thus the apical reabsorption of phosphate in the proximal renal tubules. In addition to Fgf23, secreted frizzled-related protein 4 (Sfrp4) has recently been implicated as an additional phosphate regulator in vivo and in vitro. Here we demonstrate that ablation of the Sfrp4 gene in mice does not lead to altered serum or urine phosphate levels. Furthermore, Sfrp4 is unable to compensate for the absence of Fgf23 or Klotho because double knockouts have a similar biochemical profile and phenotype as animals with ablation of Fgf23 or Klotho alone. Taken together, our data suggest that Sfrp4 does not contribute to the long-term regulation of serum phosphate levels in mice.

Fibroblast Growth Factor 23 Levels Associate with AKI and Death in Critical Illness

Elevated plasma levels of the osteocyte-derived hormone fibroblast growth factor 23 (FGF23) have emerged as a powerful biomarker of cardiovascular disease and death in patients with CKD. Whether elevated urinary or plasma FGF23 levels are prospectively associated with AKI and death in critically ill patients is unknown. We therefore conducted a prospective cohort study of 350 critically ill patients admitted to intensive care units at an academic medical center to investigate whether higher urinary FGF23 levels associate with the composite end point of AKI or in-hospital mortality (AKI/death). We measured urinary FGF23 levels within 24 hours of admission to the intensive care unit. In a subcohort (n=131) we also measured plasma levels of FGF23, calcium, phosphate, parathyroid hormone, and vitamin D metabolites. Urinary and plasma FGF23 levels, but not other mineral metabolites, significantly associated with AKI/death. In multivariate analyses, patients in the highest compared with the lowest quartile of urinary FGF23 had a 3.9 greater odds (95% confidence interval, 1.6 to 9.5) of AKI/death. Higher urinary FGF23 levels also independently associated with greater hospital, 90-day, and 1-year mortality; longer length of stay; and several other important adverse outcomes. In conclusion, elevated FGF23 levels measured in the urine or plasma may be a promising novel biomarker of AKI, death, and other adverse outcomes in critically ill patients.

TSH elevations as the first laboratory evidence for pseudohypoparathyroidism type Ib (PHP-Ib).

Hypocalcemia and hyperphosphatemia because of resistance toward parathyroid hormone (PTH) in the proximal renal tubules are the most prominent abnormalities in patients affected by pseudohypoparathyroidism type Ib (PHP‐Ib). In this rare disorder, which is caused by GNAS methylation changes, resistance can occur toward other hormones, such as thyroid‐stimulating hormone (TSH), that mediate their actions through G protein‐coupled receptors. However, these additional laboratory abnormalities are usually not recognized until PTH‐resistant hypocalcemia becomes clinically apparent. We now describe four pediatric patients, first diagnosed with subclinical or overt hypothyroidism between the ages of 0.2 and 15 years, who developed overt PTH‐resistance 3 to 20 years later. Although anti‐thyroperoxidase (anti‐TPO) antibodies provided a plausible explanation for hypothyroidism in one of these patients, this and two other patients revealed broad epigenetic GNAS abnormalities, which included loss of methylation (LOM) at exons AS, XL, and A/B, and gain of methylation at exon NESP55; ie, findings consistent with PHP‐Ib. LOM at GNAS exon A/B alone led in the fourth patient to the identification of a maternally inherited 3‐kb STX16 deletion, a well‐established cause of autosomal dominant PHP‐Ib. Although GNAS methylation changes were not detected in additional pediatric and adult patients with subclinical hypothyroidism (23 pediatric and 39 adult cases), hypothyroidism can obviously be the initial finding in PHP‐Ib patients. One should therefore consider measuring PTH, along with calcium and phosphate, in patients with unexplained hypothyroidism for extended periods of time to avoid hypocalcemia and associated clinical complications.

Renal Interstitial Fibrosis: An Imperfect Predictor of Kidney Disease Progression in Some Patient Cohorts

Background: The extent of interstitial fibrosis on kidney biopsy is regarded as a prognostic indicator and guide to treatment. Patients with extensive fibrosis are assigned to supportive treatments with the expectation that they have advanced beyond the point at which immunosuppressive or other disease-modifying therapies would be of benefit. Our study highlights some of the limitations of using interstitial fibrosis to predict who will develop end-stage renal disease (ESRD). Methods: Analysis of 434 consecutive renal biopsies performed between 2001 and 2012 at a single center. We assessed the influence of various clinical factors along with fibrosis as predictors of ESRD and dialysis-free survival in various patient groups. Results: Interstitial fibrosis performed well overall as a predictor of progression to dialysis. On average, patients with >50% fibrosis progressed more rapidly than those with either 25-49 or 0-24% fibrosis with a median time to dialysis of 1.2, 6.5 and >10 years, respectively. In contrast, interstitial fibrosis was of less value as a predictor of disease progression in a subset of cases that included patients over the age of 70 and those with diabetic nephropathy on biopsy. Surprisingly, 13.9% of patients with normal renal function had 25-49% fibrosis and 5% had more than 50% fibrosis on biopsy, and 5 years after undergoing biopsy 21% of patients with >50% fibrosis still remained dialysis free. Conclusion: Renal fibrosis is an imperfect prognostic indicator for the development of ESRD and caution should be exercised in applying it too rigidly, especially in elderly or diabetic patients.

Acute Parathyroid Hormone Injection Increases C-Terminal but Not Intact Fibroblast Growth Factor 23 Levels

The acute effects of parathyroid hormone (PTH) on fibroblast growth factor 23 (FGF23) in vivo are not well understood. After a single subcutaneous PTH (1-34) injection (50 nmol/kg) in mice, FGF23 levels were assessed in plasma using assays that measure either intact alone (iFGF23) or intact/C-terminal FGF23 (cFGF23). Furthermore, FGF23 messenger RNA (mRNA) and protein levels were assessed in bone. In addition, we examined the effects of PTH treatment on FGF23 production in vitro using differentiated calvarial osteocyte-like cells. cFGF23 levels increased by three- to fivefold within 2 hours following PTH injection, which returned to baseline by 4 hours. In contrast, iFGF23 levels remained unchanged for the first 2 hours, yet declined to ∼60% by 6 hours and remained suppressed before returning to baseline after 24 hours. Using homozygous mice for an autosomal dominant hypophosphatemic rickets-FGF23 mutation or animals treated with a furin inhibitor, we showed that cFGF23 and iFGF23 levels increased equivalently after PTH injection. These findings are consistent with increased FGF23 production in bone, yet rapid cleavage of the secreted intact protein. Using primary osteocyte-like cell cultures, we showed that PTH increased FGF23 mRNA expression through cyclic adenosine monophosphate/protein kinase A, but not inositol triphosphate/protein kinase C signaling; PTH also increased furin protein levels. In conclusion, PTH injection rapidly increases FGF23 production in bone in vivo and in vitro. However, iFGF23 is rapidly degraded. At later time points through an unidentified mechanism, a sustained decrease in FGF23 production occurs.

Fibroblast growth factor 23 in acute kidney injury.

Purpose of reviewTo review the emerging literature on changes in fibroblast growth factor 23 (FGF23) levels in the setting of acute kidney injury (AKI). Recent findingsStudies suggest that FGF23 levels are elevated in patients with AKI and correlate with increased risk of death or need for dialysis in adults, or prolonged ventilation time and higher fluid gain in children. Animal studies have shown that the cause behind this FGF23 increase is multifactorial and includes increased production in bone and decreased clearance, but not vitamin D or parathyroid hormone (PTH) activated pathways. Interestingly, FGF23 levels are found to be mildly elevated even in hospitalized patients without kidney injury, although this observation may be limited to only c-terminal FGF23 fragments. The prognostic implications of an elevated FGF23 value in patients with AKI need to be confirmed in larger cohorts and evaluated for long-term outcomes such as the development of new chronic kidney disease (CKD) or CKD progression, as well as cardiovascular disease, similar to the studies of FGF23 in the prevalent CKD population. SummaryFGF23 levels are elevated in patients with AKI and are associated with morbidity and mortality in small human studies. Mechanistic work in animals suggests that the elevation is independent of PTH or vitamin D signaling pathways. Much work remains to understand the physiology behind FGF23 elevation and the long-term effects of FGF23 in AKI.

Acute blood loss stimulates fibroblast growth factor 23 production

Fibroblast growth factor 23 (FGF23) production is upregulated by iron deficiency and hypoxia. However, the influence of acute blood loss, and the resulting increases in circulating erythropoietin, on FGF23 production is unknown. Using wild-type C57BL/6 mice, we show that acute loss of 10% total blood volume leads to an increase in plasma C-terminal FGF23 (cFGF23) levels within 6 h, while plasma levels of intact FGF23, phosphate, calcium, parathyroid hormone, iron, and ferritin remain similar to control mice without acute blood loss. Volume resuscitation with PBS did not significantly alter these findings. The increase in plasma cFGF23 levels in bled animals was accompanied by increased plasma erythropoietin levels at 6 h. Administration of erythropoietin led to an acute increase in plasma cFGF23 levels similar to that observed in acute blood loss. Fgf23 mRNA expression was increased 20-fold in bone marrow, but not in bone, of bled vs. control mice, suggesting bone marrow as a key source of elevated plasma FGF23 levels following acute blood loss. To extend these findings to humans, we measured plasma cFGF23 levels in 131 critically ill patients admitted to the intensive care unit. In univariate and multivariate models, we found a positive association between number of red blood cell transfusions, an indirect indicator of acute blood loss, and plasma cFGF23 levels. We conclude that FGF23 production is rapidly increased after acute blood loss and that erythropoietin may be the mediator of this increase. Thus erythropoietin may represent a novel physiological regulator of FGF23 production.

Bone biopsy in renal osteodystrophy: continued insights into a complex disease.

Purpose of review The pathogenesis and optimal therapy of renal bone disease remains poorly understood in chronic kidney disease (CKD) and dialysis patients. Bone biopsy is thus far the only window into cellular and molecular events in bone. This review will focus on recent insights into the pathophysiology of renal bone disease, as highlighted by bone biopsy, and discuss implications for treatment. Recent findings Abnormalities in bone physiology start very early in children and adults with CKD, when most clinically measurable mineral metabolism parameters are normal. In addition, racial differences, known to exist in serum markers such as parathyroid hormone, also appear prominent in the bone, suggesting that clinical treatment guidelines may not address the needs of all patient populations. The effects of treatments for secondary hyperparathyroidism on bone may be unexpected. Summary With the help of bone biopsy studies, molecular insights into the pathogenesis of renal osteodystrophy are beginning to emerge. Current therapies may have unexpected effects on bone physiology.