Elevated Plasma Fibroblast Growth Factor 23 Research Articles

Phosphate Restriction Prevents Metabolic Acidosis and Curbs Rise in FGF23 and Mortality in Murine Folic Acid-Induced AKI.

Patients with AKI suffer a staggering mortality rate of approximately 30%. Fibroblast growth factor 23 (FGF23) and phosphate (P i ) rise rapidly after the onset of AKI and have both been independently associated with ensuing morbidity and mortality. This study demonstrates that dietary P i restriction markedly diminished the early rise in plasma FGF23 and prevented the rise in plasma P i , parathyroid hormone, and calcitriol in mice with folic acid-induced AKI (FA-AKI). Furthermore, the study provides evidence for P i -sensitive osseous Fgf23 mRNA expression and reveals that P i restriction mitigated calciprotein particles (CPPs) formation, inflammation, acidosis, cardiac electrical disturbances, and mortality in mice with FA-AKI. These findings suggest that P i restriction may have a prophylactic potential in patients at risk for AKI. In AKI, plasma FGF23 and P i rise rapidly and are independently associated with disease severity and outcome. The effects of normal (NP) and low (LP) dietary P i were investigated in mice with FA-AKI after 3, 24, and 48 hours and 14 days. After 24 hours of AKI, the LP diet curbed the rise in plasma FGF23 and prevented that of parathyroid hormone and calcitriol as well as of osseous but not splenic or thymic Fgf23 mRNA expression. The absence of Pth prevented the rise in calcitriol and reduced the elevation of FGF23 in FA-AKI with the NP diet. Furthermore, the LP diet attenuated the rise in renal and plasma IL-6 and mitigated the decline in renal α -Klotho. After 48 hours, the LP diet further dampened renal IL-6 expression and resulted in lower urinary neutrophil gelatinase-associated lipocalin. In addition, the LP diet prevented the increased formation of CPPs. Fourteen days after AKI induction, the LP diet group maintained less elevated plasma FGF23 levels and had greater survival than the NP diet group. This was associated with prevention of metabolic acidosis, hypocalcemia, hyperkalemia, and cardiac electrical disturbances. This study reveals P i -sensitive FGF23 expression in the bone but not in the thymus or spleen in FA-AKI and demonstrates that P i restriction mitigates CPP formation, inflammation, acidosis, and mortality in this model. These results suggest that dietary P i restriction could have prophylactic potential in patients at risk for AKI.

Comparative analysis of fibroblast growth Factor-23 as a correlate of cardiovascular disease among individuals with chronic kidney disease, hypertensives, and healthy controls.

Chronic kidney disease (CKD) is a global growing public health epidemic with attending morbidity and huge financial cost. Cardiovascular disease (CVD), a major complication of CKD, contributes to its excessive mortality rate. The aetio-pathogenesis of the excess burden of CVD in CKD is a feature yet to be unravelled. Fibroblast growth factor-23 (FGF-23) has been implicated as a risk factor for CVD among patients with CKD. However, most of these studies were predominantly among the Caucasian population. This study aims to determine the correlation between FGF-23 and CVD among Nigerians with CKD. A cross-sectional comparative study composed of three groups: participants with CKD, hypertensives without CKD, and healthy individuals, represented as group 1, 2, and 3, respectively. Information obtained included demographic data and occurrence of risk factors for CVD. Cardiovascular risks were assessed by echocardiography and all the participants had kidney function tests done with plasma FGF-23. The study sample size consisted of 135 participants. The mean (SD) age for participants with CKD and controls were 50.2 (12.7), 54.3 (15.5), and 40.2 (14.1) years, respectively. The median [interquartile range (IQR)] of plasma FGF-23 for participants with CKD 210 (139-304) RU/ml, and controls 124 (86-170) RU/ml, and 71 (38 - 89) RU/ml P < 0.001. Most participants with CKD had left ventricular hypertrophy (LVH) (80.0%), compared to the controls; 28.9% and 6.7% P < 0.001. Similarly, majority of participants with CKD had elevated plasma FGF-23 with LVH (85.7%) compared to controls 55.6% and 11.5%, whereas for aortic valve calcification with elevated plasma FGF-23 among CKD and controls were 53.6% (P = 0.29), 37.0% (P = 0.03), and 19.2% (P = 0.06), respectively. Individuals with CKD had frequencies of elevated plasma FGF-23, LVH, and cardiac valve calcification, which are surrogates of cardiovascular events.

Plasma Biomarkers for Clinical Assessment of Bone Mineral Density in Heart Transplanted Patients-A Single-Center Study at Skåne University Hospital in Lund.

We aimed to identify plasma biomarkers that predict changes in bone mineral density (BMD) and increase the understanding of impaired BMD after heart transplantation (HT). Twenty-eight adult patients were included. Data, including densitometry and 29 plasma proteins, before and 1 year after HT were analyzed. Pre-HT plasma levels of fibroblast growth factor 23 (FGF23) correlated with post-HT T score in lumbar spine, adjusted for age, gender, and BMI (1.72 [95% CI 1.33; 2.22], p = 0.011). Change (∆; post-HT—pre-HT) in plasma levels of melusin correlated to ∆T score from the lumbar spine (p = 0.028). ∆plasma levels of TR-AP, ITGB2, and Stromelysin-1 correlated to ∆T score from the femoral neck (p < 0.05). However, no correlations remained after adjustments for age, gender, and BMI. In conclusion, elevated plasma FGF23 pre-HT predicted an increase in lumbar BMD after HT. However, the results are surprising since FGF23 is known to be inversely correlated with BMD. This may partly be explained by the complex pathophysiology in this particular cohort. Due to the explorative nature of the study and the small sample size, further investigations of biochemical markers on bone metabolism in this patient population are encouraged.

Bone Marrow Sinusoidal Endothelial Cells Are a Site of Fgf23 Upregulation in Iron Deficiency Anemia

Iron deficiency anemia (IDA) has been identified as a potent stimulator of FGF23 (fibroblast growth factor 23), a phosphaturic hormone classically thought to be produced by bone-embedded osteocytes. Recently, both phlebotomy and erythropoietin administration have been shown to upregulate FGF23 production in bone marrow. However, the cell type(s) mediating FGF23 upregulation in states of perturbed erythropoiesis require further clarification.Tmprss6-/- mice exhibit hepcidin elevation leading to systemic iron deficiency and iron-restricted anemia. We previously reported that Tmprss6-/- mice exhibit altered phosphate balance, elevated circulating FGF23, and Fgf23 mRNA upregulation in bone marrow but not cortical bone. Here, we clarify the sites of Fgf23 promoter activity in Tmprss6 -/- bone marrow using a reporter allele in which the enhanced green fluorescent protein (eGFP) coding sequence has been knocked into the endogenous Fgf23 locus. We generated Tmprss6 +/+,Tmprss6 +/-, and Tmprss6 -/- littermates of both sexes that carried either one (Fgf23 +/eGFP) or zero (Fgf23 +/+) copies of the reporter allele. Tmprss6-/- mice showed hyperhepcidinemia, hypoferremia, microcytic anemia, and tissue iron deficiency, which were not altered by heterozygous Fgf23 disruption (Figure 1A-C). By ELISA, Tmprss6-/- Fgf23 +/eGFP mice showed plasma levels of “total” FGF23 (intact, active hormone and C-terminal cleaved fragments) that remained markedly elevated compared to Tmprss6+/+ littermates (Figure 1D). Total FGF23 elevation in Tmprss6-/- Fgf23 +/eGFP mice was slightly less pronounced than Tmprss6-/- Fgf23 +/+ mice, suggesting an effect of Fgf23 gene dosage. In mice with 2 intact Fgf23 alleles, serum erythropoietin showed a strong linear correlation with plasma total FGF23.By confocal imaging, femurs of mice carrying the Fgf23 eGFP allele showed green fluorescence in vascular regions of the bone marrow but not in the bone cortex. Green fluorescence was more intense in Tmprss6-/- Fgf23+/eGFP mice than non-anemic controls. By flow cytometry of enzymatically digested bone marrow, we observed bright green fluorescence in a subset of endothelial cells (CD45 - Ter119 - CD31 +) exclusively in mice carrying the Fgf23 eGFP reporter allele (Figure 1E). The percentage of endothelial cells that were GFP bright was higher in Tmprss6-/- Fgf23 +/eGFP versus non-anemic mice. To clarify the endothelial cell subtype that expresses Fgf23, we mined published transcriptomic datasets from mice of normal iron balance and discovered higher Fgf23 mRNA in bone marrow sinusoidal endothelial cells compared to other bone marrow endothelial cell populations. Accordingly, we used anti-GFP immunohistochemistry in formalin-fixed bone marrow sections to assess Fgf23 eGFP reporter allele expression in the context of tissue architecture. Tmprss6-/- Fgf23 +/eGFP mice showed GFP expression in bone marrow sinusoidal endothelial cells, which was more intense than in non-anemic controls (Figure 1F). GFP reporter expression was also detected in rare cells of the thymus but not in liver, spleen, heart, muscle, or kidney. Collectively, our data reveal that bone marrow sinusoidal endothelial cells are a site of Fgf23 upregulation in chronic IDA.Because IDA in Tmprss6-/- mice results from pathologic hepcidin elevation, we also sought to determine if bone marrow sinusoidal endothelial cells are a site of Fgf23 upregulation in anemic mice with intact hepcidin regulation. We therefore subjected Fgf23 +/eGFP mice (with 2 intact Tmprss6 alleles) to a 500µl phlebotomy regimen (with saline volume replacement) known to induce marked anemia and hepcidin suppression. Compared to non-phlebotomized Fgf23 +/eGFP controls, phlebotomized Fgf23 +/eGFP mice showed severe anemia, elevated serum erythropoietin, and elevated plasma FGF23 18 hours after blood loss. Additionally, immunohistochemistry revealed more intense GFP expression in bone marrow sinusoidal endothelial cells of phlebotomized Fgf23 +/eGFP mice than non-phlebotomized controls. Taken together, our results show for the first time that bone marrow sinusoidal endothelial cells are a site of Fgf23 upregulation in both acute and chronic anemia. Given the serum erythropoietin elevation in both models, our findings suggest that erythropoietin may act directly or indirectly on sinusoidal endothelial cells to promote FGF23 production during anemia. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

High Intakes of Bioavailable Phosphate May Promote Systemic Oxidative Stress and Vascular Calcification by Boosting Mitochondrial Membrane Potential-Is Good Magnesium Status an Antidote?

Chronic kidney disease is characterized by markedly increased risk for cardiovascular mortality, vascular calcification, and ventricular hypertrophy, and is associated with increased systemic oxidative stress. Hyperphosphatemia, reflecting diminished glomerular phosphate (Pi) clearance, coupled with a compensatory increase in fibroblast growth factor 23 (FGF23) secretion are thought to be key mediators of this risk. Elevated serum and dietary Pi and elevated plasma FGF23 are associated with increased cardiovascular and total mortality in people with normal baseline renal function. FGF23 may mediate some of this risk by promoting cardiac hypertrophy via activation of fibroblast growth factor receptor 4 on cardiomyocytes. Elevated serum Pi can also cause a profound increase in systemic oxidative stress, and this may reflect the ability of Pi to act directly on mitochondria to boost membrane potential and thereby increase respiratory chain superoxide production. Moreover, elevated FGF23 likewise induces oxidative stress in vascular endothelium via activation of NADPH oxidase complexes. In vitro exposure of vascular smooth muscle cells to elevated Pi provokes an osteoblastic phenotypic transition that is mediated by increased mitochondrial oxidant production; this is offset dose-dependently by increased exposure to magnesium (Mg). In vivo, dietary Mg is protective in rodent models of vascular calcification. It is proposed that increased intracellular Mg opposes Pi’s ability to increase mitochondrial membrane potential; this model could explain its utility for prevention of vascular calcification and predicts that Mg may have a more global protective impact with regard to the direct pathogenic effects of hyperphosphatemia.

Higher plasma fibroblast growth factor 23 levels are associated with a higher risk profile in pulmonary arterial hypertension.

Metabolic abnormalities are proposed to contribute to pulmonary arterial as well as right ventricular remodelling in pulmonary arterial hypertension. Among the proposed abnormalities are altered glucose and lipid processing, mitochondrial malfunction, oxidative stress as well as vitamin D and iron abnormalities. In the present study, we investigated 11 metabolic plasma biomarkers, with the hypothesis that metabolic proteins may mirror disease severity in pulmonary arterial hypertension. Using proximity extension assays, plasma metabolic biomarkers were measured in 48 pulmonary arterial hypertension patients at diagnosis and, in 33 of them, at an early treatment follow-up, as well as in 16 healthy controls. Among the studied metabolic biomarkers, plasma fibroblast growth factor-23 (p < 0.001), fibroblast growth factor-21 (p < 0.001), fatty acid binding protein 4 (p < 0.001) and lectin-like oxidised low-density lipoprotein receptor 1 (p < 0.001) were increased and paraoxonase-3 was decreased (p < 0.001) in pulmonary arterial hypertension at diagnosis versus controls. Fibroblast growth factor-23 showed the strongest correlations to studied clinical parameters and was therefore selected for further analyses. Fibroblast growth factor-23 correlated specifically to mean right atrial pressure (r = 0.67, p < 0.001), six-min walking distance (r = −0.66, p < 0.001), NT-proBNP (r = 0.64, p < 0.001), venous oxygen saturation (r = −0.61, p < 0.001), cardiac index (r = −0.39, p < 0.007) and pulmonary vascular resistance (r = 0.37, p < 0.01). Fibroblast growth factor-23 correlated moreover to ESC/ERS (r = 0.72, p < 0.001) and the REVEAL risk score (r = 0.61, p < 0.001). Comparing early treatment follow-up with baseline, fibroblast growth factor-23 decreased (p < 0.02), with changes in fibroblast growth factor-23 correlating to changes in six-min walking distance (r = −0.56, p < 0.003), venous oxygen saturation (r = −0.46, p < 0.01), pulmonary vascular resistance (r = 0.43, p < 0.02), mean right atrial pressure (r = 0.38, p < 0.04) and cardiac index (r = −0.39, p < 0.04). Elevated plasma fibroblast growth factor-23 levels at pulmonary arterial hypertension diagnosis were associated with worse haemodynamics and a higher risk profile, and were decreased after the administration of pulmonary arterial hypertension-specific treatment.

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.

Fibroblast Growth Factor-23-A Potential Uremic Toxin.

Fibroblast growth factor-23 (FGF23) is a circulating member of the FGF family produced mainly by the osteocytes and osteoblasts that can act as a hormone. The main action of FGF23 is to lower phosphatemia via the reduction of urinary phosphate reabsorption and the decrease of 1,25(OH)2-D generation in the kidney. In the course of chronic kidney disease (CKD), plasma FGF23 concentration rises early, most probably to compensate the inability of the deteriorating kidneys to excrete an adequate amount of phosphate. However, this comes at the cost of FGF23-related target organ toxicity. Results of clinical studies suggest that elevated plasma FGF23 concentration is independently associated with the increased risk of CKD progression, occurrence of cardio-vascular complications, and mortality in different stages of CKD. FGF23 also contributes to cardiomyocyte hypertrophy, vascular calcification, and endothelial dysfunction. The impact of FGF23 on heart muscle is not dependent on Klotho, but rather on the PLCγ–calcineurin–NFAT (nuclear factor of activated T-cells) pathway. Among the factors increasing plasma FGF23 concentration, active vitamin D analogues play a significant role. Additionally, inflammation and iron deficiency can contribute to the increase of plasma FGF23. Among the factors decreasing plasma FGF23, dietary phosphate restriction, some intestinal phosphate binders, cinacalcet (and other calcimimetics), and nicotinamide can be enumerated. Anti-FGF23 antibodies have also recently been developed to inhibit the action of FGF23 in target organs. Still, the best way to normalize plasma FGF23 in maintenance hemodialysis patients is restoring kidney function by successful kidney transplantation.

Plasma FGF23 Is a Biomarker for Left Ventricular Hypertrophy and Mortality in Adults with Sickle Cell Anemia Chronic Kidney Disease

Sickle cell anemia (SCA) is characterized by end organ damage, especially in the kidney and heart, resulting in a high prevalence of chronic kidney disease (CKD), tricuspid regurgitation and left ventricular hypertrophy (LVH). In the general population, dysregulation of calcium/phosphate homeostasis by elevated plasma fibroblast growth factor 23 (FGF23) directly promotes LVH in end stage CKD without SCA. Accordingly, the high mortality associated with end stage renal disease is primarily cardiac, and is associated with ventricular remodeling and elevated plasma FGF23. However, FGF23 has not been investigated as a biomarker for CKD, LVH and mortality in SCA. Here, we assessed the relationships between CKD, LVH, plasma FGF23 and mortality in adults with SCA.First, we determined that LVH is a common entity in SCA-CKD using a nested case control study of all SS subjects with stage 3-5 CKD (eGFR<60) enrolled in the Bethesda Sickle Cell Cohort Study (ClinicalTrials.gov identifier: NCT00011648) and age and sex-matched SCA subjects with normal renal function. Of those with SCA-CKD (n=42), LVH defined by echocardiogram was present in 74% compared to 41% in those with normal kidney function (odds ratio 3.79; 95%CI 1.57 - 10.06; p=0.0039).We next measured plasma FGF23 levels in normal volunteers (n=37) and age and sex-matched SCA subjects with and without CKD (n=37), using ELISA. We observed a 3.5-fold increase in plasma FGF23 levels among subjects with SCA (p<0.0001) and a 22-fold increase in those with SCA-CKD (p<0.0001). FGF23 levels are elevated at baseline in adults with SCA which has not previously been reported. The much higher fold increase in SCA-CKD was not unexpected, based upon previously reported high plasma FGF23 associated with CKD in the absence of SCA. Spearman correlations between plasma FGF23 levels in adults with SCA were correlated with creatinine (r=0.52, p=0.0011), eGFR (r=-0.57, p=0.0003), left ventricular mass index (LVMI, r=0.48, p=0.0029), and tricuspid regurgitant velocity (TRV; r=0.44, p=0.01). These data suggest that as in CKD without SCA, plasma FGF23 is a biomarker associated with both a decline in renal function and an increase in the prevalence of LVH and tricuspid regurgitation.We next sought to determine if plasma FGF23 levels are a biomarker associated with mortality in the overall SCA patient population. Univariate Kaplan-Meier analyses showed that categorical FGF23 levels (above versus below log mean; hazard ratio=1.74, 95% confidence interval 1.09-2.85, p=0.02] and elevated TRV (≥2.5 m/s versus all others, hazard ratio=4.42, 95% confidence interval 2.60-7.52, p<0.0001) are significantly associated with increased mortality in 478 adults with SCA. LVH defined by sex adjusted echocardiographic thresholds for LVMI was not associated with an increased risk of death (p=0.71). Multivariable Cox proportional hazard analysis indicated that log FGF23 is an independent risk factor for death (hazard ratio=1.46, 95% confidence interval 1.01-2.10, p=0.04), in addition, to the known mortality association with elevated TRV (hazard ratio=4.90, 95% confidence interval 2.86-8.39, p<0.0001). The Cox analysis suggests that plasma FGF23 is a biomarker for mortality in SCA, although the echocardiographic abnormality associated with death in this model is TRV and not LVMI.In conclusion, we observed a high prevalence of LVH in adults with SCA-CKD (70.3%), and SCA-CKD is associated with a 22 fold increase in plasma FGF23 levels. One limitation of our study was the inability to assess associations between plasma FGF23, LVMI, TRV and mortality risk in SCA-CKD due to a limited number of CKD subjects. When FGF23 was investigated in all adults with SCA, plasma FGF23 was a significant risk factor for death, independent of TRV and other known risk factors. Larger SCA-CKD studies will be necessary in the future to better understand the relationships between TRV, LVH, FGF23 and mortality. Overall, plasma FGF23 is a potentially helpful biomarker to further elucidate the pathobiology of SCA-CKD end organ complications. DisclosuresNo relevant conflicts of interest to declare.

Fibroblast Growth Factor-23 and Hypophosphatemia in Chronic Obstructive Pulmonary Disease Patients

Fibroblast Growth Factor-23 and Hypophosphatemia in Chronic Obstructive Pulmonary Disease PatientsImpaired serum phosphate levels may contribute to respiratory muscle weakness that further negatively impacts Chronic Obstructive Pulmonary Disease (COPD) patients. Recently, Fibroblast Growth Factor 23 (FGF-23) has been shown to play an important role in the regulation of body phosphate. The current study includes 2 groups: 70 COPD patients and 34 control subjects. Blood samples were taken for a panel of routine lab tests. FGF-23 was measured using a commercially available ELISA kit. Plasma FGF-23 levels were significantly higher in the patient group compared to the control group (P=0.000). Tubular maximum absorption of phosphate was significantly reduced in COPD patients compared to the control group (P=0.04). Plasma FGF-23 negatively correlated with FEV1 and serum albumin. Elevated plasma FGF-23 levels found in COPD patients correlated with disease severity and may represent an additional factor causing low serum phosphate.

Follow-up study of Gambian children with rickets-like bone deformities and elevated plasma FGF23: possible aetiological factors.

We have previously reported on a case-series of children (n = 46) with suspected calcium-deficiency rickets who presented in The Gambia with rickets-like bone deformities. Biochemical analyses discounted vitamin D-deficiency as an aetiological factor but indicated a perturbation of Ca–P metabolism involving low plasma phosphate and high circulating fibroblast growth factor-23 (FGF23) concentrations.A follow-up study was conducted 5 years after presentation to investigate possible associated factors and characterise recovery. 35 children were investigated at follow-up (RFU). Clinical assessment of bone deformities, overnight fasted 2 h urine and blood samples, 2-day weighed dietary records and 24 h urine collections were obtained. Age- and season-matched data from children from the local community (LC) were used to calculate standard deviation scores (SDS) for RFU children.None of the RFU children had radiological signs of active rickets. However, over half had residual leg deformities consistent with rickets. Dietary Ca intake (SDS-Ca = − 0.52 (0.98) p = 0.04), dietary Ca/P ratio (SDS-Ca/P = − 0.80 (0.82) p = 0.0008) and TmP:GFR (SDS-TmP:GFR = − 0.48 (0.81) p = 0.04) were significantly lower in RFU children compared with LC children and circulating FGF23 concentration was elevated in 19% of RFU children. Furthermore an inverse relationship was seen between haemoglobin and FGF23 (R2 = 25.8, p = 0.004).This study has shown differences in biochemical and dietary profiles between Gambian children with a history of rickets-like bone deformities and children from the local community. This study provided evidence in support of the calcium deficiency hypothesis leading to urinary phosphate wasting and rickets and identified glomerular filtration rate and iron status as possible modulators of FGF23 metabolic pathways.

Regulation of Fibroblast Growth Factor 23 Production in Bone in Uremic Rats

Background: Fibroblast growth factor 23 (FGF23) regulates renal phosphate reabsorption and 1α,25-dihydroxyvitamin D [1,25(OH)₂D₃] metabolism. Patients with chronic kidney disease (CKD) have increased levels of circulating FGF23, but the direct regulation of this elevation of FGF23 is incompletely understood. Method:We measured plasma parameters in uremic rats fed a high-phosphorus diet and then performed parathyroidectomy (PTX) to determine its effect. We also investigated FGF23 mRNA expression in various tissues to identify the major source of circulating FGF23. Result: The uremic rats displayed dramatic changes in plasma FGF23 levels, consistent with increased expression of FGF23 in bone. Elevated FGF23 was associated with phosphate and parathyroid hormone (PTH). After PTX, the elevated FGF23 had decreased, consistent with decreased expression of FGF23 in bone. Significant decreases in plasma FGF23 were associated with PTH and 1,25(OH)₂D₃, but not phosphate. Conclusion: Elevated plasma FGF23 levels in uremic rats reflect the increased expression of FGF23 in bone. The expression of FGF23 in bone may be regulated by a PTH-1,25(OH)₂D₃ axis-dependent pathway and another PTH-dependent and 1,25(OH)₂D₃-independent pathway in uremic rats. The pathway may be decided by the degree of renal dysfunction.