The EPO-FGF23 Signaling Pathway in Erythroid Progenitor Cells: Opening a New Area of Research

Erythropoietin alleviates lung ischemia-reperfusion injury by activating the FGF23/FGFR4/ERK signaling pathway

Background

The purpose of the present study was to investigate the effect of erythropoietin (EPO) on lung ischemia-reperfusion injury (LIRI).

Methods

Sprague Dawley rats and BEAS-2B cells were employed to construct an ischemia-reperfusion (I/R)-induced model in vivo and in vitro, respectively. Afterward, I/R rats and tert-butyl hydroperoxide (TBHP)-induced cells were treated with different concentrations of EPO. Furthermore, 40 patients with LIRI and healthy controls were enrolled in the study.

Results

It was observed that lung tissue damage, cell apoptosis and the expression of BAX and caspase-3 were higher in the LIRI model in vivo and in vitro than in the control group, nevertheless, the Bcl-2, FGF23 and FGFR4 expression level was lower than in the control group. EPO administration significantly reduced lung tissue damage and cell apoptosis while also up-regulating the expression of FGF23 and FGFR4. Rescue experiments indicated that EPO exerted a protective role associated with the FGF23/FGFR4/p-ERK1/2 signal pathway. Notably, the expression of serum EPO, FGF23, FGFR4 and Bcl-2 was decreased in patients with LIRI, while the expression of caspase-3 and BAX was higher.

Conclusion

EPO could effectively improve LIRI, which might be related to the activation of the FGF23/FGFR4/p-ERK1/2 signaling pathway.

Serum sclerostin is associated with recurrent kidney stone formation independent of hypercalciuria

Background

Kidney stones are frequent in industrialized countries with a lifetime risk of 10 to 15%. A high percentage of individuals experience recurrence. Calcium-containing stones account for more than 80% of kidney stones. Diet, environmental factors, behavior, and genetic variants contribute to the development of kidney stones. Osteocytes excrete the 21 kDa glycoprotein sclerostin, which inhibits bone formation by osteoblasts. Animal data suggests that sclerostin might directly or indirectly regulate calcium excretion via the kidney. As hypercalciuria is one of the most relevant risk factors for kidney stones, sclerostin might possess pathogenic relevance in nephrolithiasis.

Methods

We performed a prospective cross-sectional observational controlled study in 150 recurrent kidney stone formers (rKSF) to analyse the association of sclerostin with known stone risk factors and important modulators of calcium-phosphate metabolism. Serum sclerostin levels were determined at the first visit. As controls, we used 388 non-stone formers from a large Swiss epidemiological cohort.

Results

Sclerostin was mildly increased in rKSF in comparison to controls. This finding was more pronounced in women compared to men. Logistic regression indicated an association of serum sclerostin with rKSF status. In hypercalciuric individuals, sclerostin levels were not different from normocalciuric patients. In Spearman correlation analysis we found a positive correlation between sclerostin, age, and BMI and a negative correlation with eGFR. There was a weak correlation with iPTH and intact FGF 23. In contrast, serum sclerostin levels were not associated with 25-OH Vitamin D3, 1,25-dihydroxy-Vitamin D3, urinary calcium and phosphate or other urinary lithogenic risk factors.

Conclusion

This is the first prospective controlled study investigating serum sclerostin in rKSF. Sclerostin levels were increased in rKSF independent of hypercalciuria and significantly associated with the status as rKSF. It appears that mechanisms other than hypercalciuria may be involved and thus further studies are required to elucidate underlying pathways.

Response to oral iron therapy in children with anemia of chronic kidney disease

BACKGROUND

Anemia is a common complication of chronic kidney disease (CKD) and oral iron is recommended as initial therapy. However, response to iron therapy in children with non-dialysis CKD has not been formally assessed.

METHODS

We reviewed medical records of pediatric patients with stages II-IV CKD followed in two New York metropolitan area medical centers between 2010 and 2020 and identified subjects who received oral iron therapy. Response to therapy at follow-up visits was assessed by improvement of hemoglobin, resolution of anemia by the 2012 KDIGO definition, and changes in iron status. Potential predictors of response were examined using regression analyses (adjusted for age, sex, eGFR, and center).

RESULTS

Study criteria were met by 65 children (median age 12 years, 35 males) with a median time between visits of 81 days. Median eGFR was 44 mL/min/1.73 m2, and 40.7% had glomerular CKD etiology. Following iron therapy, hemoglobin improved from 10.2 to 10.8 g/dL (p < 0.001), hematocrit from 31.3 to 32.8% (p < 0.001), serum iron from 49 to 66 mcg/dL (p < 0.001), and transferrin saturation from 16 to 21.4% (p < 0.001). There was no significant change in serum ferritin (55.0 to 44.9 ng/mL). Anemia (defined according to KDIGO) resolved in 29.3% of children. No improvement in hemoglobin/hematocrit was seen in 35% of children, and no transferrin saturation improvement in 26.9%. There was no correlation between changes in hemoglobin and changes in transferrin saturation/serum iron, but there was an inverse correlation between changes in hemoglobin and changes in ferritin. The severity of anemia and alkaline phosphatase at baseline inversely correlated with treatment response.

CONCLUSIONS

Anemia was resistant to 3 months of oral iron therapy in ~ 30% of children with CKD. Children with more severe anemia at baseline had better treatment response, calling for additional studies to refine approaches to iron therapy in children with anemia of CKD and to identify additional predictors of treatment response.

Effect of intravenous iron on endogenous erythropoietin and FGF-23 secretion in patients with chronic kidney disease

INTRODUCTION

It has been observed that intravenous iron administration may suppress endogenous production of erythropoietin (EPO). We postulate that this effect may be mediated by increased FGF-23 secretion.

AIM OF THE STUDY

To evaluate the short-term effect of intravenous iron sucrose administration on endogenous EPO secretion in patients with chronic kidney disease (CKD).

MATERIALS AND METHODS

The cohort comprised 35 nondialysis patients with CKD stages 3-5. All received 100 mg of intravenous iron (III)-hydroxide sucrose complex daily for five consecutive days. Plasma EPO, iFGF-23, cFGF-23, PTH, bone alkaline phosphatase (BAP), phosphorus (PO4), calcium (Ca), and high-sensitive C-reactive protein (CRP) were measured before, and two hours after, the first and third iron infusions, and after completing iron therapy.

RESULTS

EPO concentration at the end of iron treatment was significantly lower than two hours after the first iron infusion (p = 0.0003) and before the third dose (p = 0.0006) (12.6 [10.2, 41.4] mIU/mL. vs. 30.9 [15.9, 54.2] mIU/mL and 33.4 [15.4, 56.7] mIU/mL, respectively). Conversely, plasma iFGF-23 was significantly higher before the third dose (61.1 [18.6, 420.1 4] pg/mL; p = 0.025) and after the course of treatment (92.1 [28.4, 878.1] pg/mL; p = 0.004) compared to pretreatment value (48.4 [16.2, 420] pg/mL). cFGF-23 concentration was significantly lower than baseline after the first iron dose (491.8 [257.7, 1086.3] vs. 339.2 [75.4, 951.2] RU/mL; p = 0.005) and after treatment (398.7 [90.4, 1022.3] RU/mL; p = 0.025). No significant linear correlation was found between changes in plasma EPO and FGF-23.

CONCLUSIONS

Although intravenous iron therapy causes parallel increase of FGF-23 and supression of endogenous EPO, these two effects seem to be independent.

Fibroblast Growth Factor 23 Neutralizing Antibody Ameliorates Abnormal Renal Phosphate Handling in Sickle Cell Disease Mice

We assessed the involvement of fibroblast growth factor 23 (FGF23) in phosphaturia in sickle cell disease (SCD) mice. Control and SCD mice were treated with FGF23 neutralizing antibody (FGF23Ab) for 24 hours. Serum ferritin was significantly increased in SCD mice and was significantly reduced in female but not male SCD mice by FGF23Ab. FGF23Ab significantly reduced increased erythropoietin in SCD kidneys. Serum intact FGF23 was significantly increased in SCD female mice and was markedly increased in SCD male mice; however, FGF23Ab significantly reduced serum intact FGF23 in both genotypes and sexes. Serum carboxy-terminal-fragment FGF23 (cFGF23) was significantly reduced in SCD IgG male mice and was markedly but not significantly reduced in SCD IgG female mice. FGF23Ab significantly increased cFGF23 in both sexes and genotypes. Serum 1,25-dihydroxyvitamin D3 was significantly increased in SCD IgG and was further significantly increased by FGF23Ab in both sexes and genotypes. Significantly increased blood urea nitrogen in SCD was not reduced by FGF23Ab. The urine phosphate (Pi)/creatinine ratio was significantly increased in SCD in both sexes and was significantly reduced by FGF23Ab. Increased SCD kidney damage marker kidney injury molecule 1 was rescued, but sclerotic glomeruli, increased macrophages, and lymphocytes were not rescued by short-term FGF23Ab. FGF23Ab significantly reduced increased phospho-fibroblast growth factor receptor 1, αKlotho, phosphorylated extracellular signal-regulated kinase, phosphorylated serum/glucocorticoid-regulated kinase 1, phosphorylated sodium-hydrogen exchanger regulatory factor-1, phosphorylated janus kinase 3, and phosphorylated transducer and activator of transcription-3 in SCD kidneys. The type II sodium Pi cotransporter (NPT2a) and sodium-dependent Pi transporter PiT-2 proteins were significantly reduced in SCD kidneys and were increased by FGF23Ab. We conclude that increased FGF23/FGF receptor 1/αKlotho signaling promotes Pi wasting in SCD by downregulating NPT2a and PIT2 via modulation of multiple signaling pathways that could be rescued by FGF23Ab.

A Combined Biomarker That Includes Plasma Fibroblast Growth Factor 23, Erythropoietin, and Klotho Predicts Short- and Long-Term Morbimortality and Development of Chronic Kidney Disease in Critical Care Patients with Sepsis: A Prospective Cohort

Acute Kidney Injury (AKI) is a frequent complication in intensive care unit (ICU) patients that increases mortality and chronic kidney disease (CKD) development. AKI is associated with elevated plasma fibroblast growth factor 23 (FGF23), which can be modulated by erythropoietin (EPO) and Klotho. We aimed to evaluate whether a combined biomarker that includes these molecules predicted short-/long-term outcomes. We performed a prospective cohort of ICU patients with sepsis and previously normal renal function. They were followed during their inpatient stay and for one year after admission. We measured plasma FGF23, EPO, and Klotho levels at admission and calculated a combined biomarker (FEK). A total of 164 patients were recruited. Of these, 50 (30.5%) had AKI at admission, and 55 (33.5%) developed AKI within 48 h. Patients with AKI at admission and those who developed AKI within 48 h had 12- and 5-fold higher FEK values than non-AKI patients, respectively. Additionally, patients with higher FEK values had increased 1-year mortality (41.9% vs. 18.6%, p = 0.003) and CKD progression (26.2% vs. 8.3%, p = 0.023). Our data suggest that the FEK indicator predicts the risk of AKI, short-/long-term mortality, and CKD progression in ICU patients with sepsis. This new indicator can improve clinical outcome prediction and guide early therapeutic strategies.

Case Report: Unexplained Mild Hypophosphatemia and Very High Serum FGF23 Concentrations

Fibroblast growth factor (FGF)23 is one of the major regulators of phosphate homeostasis. Hypophosphatemia can lead to muscle weakness, fatigue, and osteomalacia. In the setting of hypophosphatemia, serum FGF23 can be measured to differentiate between FGF23-mediated and non-FGF23-mediated renal phosphate wasting. C-terminal FGF23 (cFGF23) assays detect both cFGF23 and intact FGF23 (iFGF23). Circulating FGF23 is regulated by 1.25-dihydroxy-vitamin D, parathyroid hormone (PTH), serum phosphate, and serum calcium but also by, for example, iron status, inflammation, erythropoietin, and hypoxia-inducible-factor-1-α. We present the case of a 48-year-old woman with unexplained mild hypophosphatemia, very high cFGF23, and normal iFGF23. The patient proved to have an iron deficiency. Iron deficiency alters the iFGF23-to-cFGF23 ratio. After initiation of iron treatment, cFGF23 strongly decreased. This case report illustrates the limitation of cFGF23 assays and urges clinicians to be aware that cFGF23 concentrations do not necessarily reflect iFGF23 concentrations and that alternative causes for its elevation should be considered (eg, iron deficiency). © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC. on behalf of American Society for Bone and Mineral Research.

X-Linked Familial Hypophosphatemia: A Case Report of 27-Year Old Male and Review of Literature

X-linked hypophosphatemia (XLH) associated with short stature during childhood are mostly referred to the hospital and diagnosed as vitamin D deficiency rickets and received vitamin D before adulthood. A case is presented with clinical features of hypophosphatemia from childhood who did not seek medical care for diagnosis and treatment, nor did his mother or two brothers, who have short statures, bone pain, and fractures. The patient was assessed for sociodemographic, hematological, and biochemical parameters together with a genetic assessment. A DEXA scan and X-ray were done to determine the abnormalities and deformities of joints and bones despite clinical examination by an expert physician. All imaging, laboratory parameters, and the genetic study confirmed the diagnosis of XLH. A detailed follow-up of his condition was performed after the use of phosphate tablets and other treatments. X-linked hypophosphatemia needs a good assessment, care, and follow up through a complementary medical team including several specialties. Phosphate tablets in adulthood significantly affects clinical and physical improvement and prevention of further skeletal abnormality and burden on daily activity. The patients should be maintained with an adequate dose of phosphate for better patient compliance. More awareness is needed in society and for health professionals when conducting medical checkups during the presence of stress fractures, frequent dental and gum problems, rickets, short stature, or abnormality in the skeleton or walking to think of secondary causes such as hypophosphatemia. Further investigations including a visit to a specialist is imperative to check for the primary cause of these disturbances.

Vhl deletion in Dmp1 -expressing cells alters MEP metabolism and promotes stress erythropoiesis

In recent years, general hypoxia-inducible factor (HIF)-prolyl hydroxylase (PHD) enzyme inhibitors have been developed for the treatment of anemia due to renal disease and osteoporosis. However, it remains a challenge to target the HIF signaling pathway without dysregulating the skeletal and hematopoietic system. Here, we examined the effects of Vhl deletion in bone by performing longitudinal analyses of Vhl cKO mice at 3, 6, 10, and 24 weeks of age, where at 10 and 24 weeks of age, high bone mass and splenomegaly are present. Using flow cytometry, we observed increased frequency (%) of CD71 lo TER119 hi FSC lo orthochromatophilic erythroblasts and reticulocytes in 10- and 24-week-old Vhl cKO bone marrow (BM), which correlated with elevated erythropoietin levels in the BM and increased number of red blood cells in circulation. The absolute numbers of myeloerythroid progenitors (MEPs) in the BM were significantly reduced at 24 weeks. Bulk RNA-Seq of the MEPs showed upregulation of Epas1 ( Hif1a) and Efnb2 ( Hif2a) in Vhl cKO MEPs, consistent with a response to hypoxia, and genes involved in erythrocyte development, actin filament organization, and response to glucose. Additionally, histological analysis of Vhl cKO spleens revealed red pulp hyperplasia and the presence of megakaryocytes, both of which are features of extramedullary hematopoiesis (EMH). EMH in the spleen was correlated with the presence of mature stress erythroid progenitors, suggesting that stress erythropoiesis is occurring to compensate for the BM microenvironmental irregularities. Our studies implicate that HIF-driven alterations in skeletal homeostasis can accelerate erythropoiesis.

Key Points

• Dysregulation of HIF signaling in Dmp1+ bone cells induces stress erythropoiesis.• Skeletal homeostasis modulates erythropoiesis.

Beneficial effect of roxadustat on early posttransplant anemia and iron utilization in kidney transplant recipients: a retrospective comparative cohort study

Background

Although posttransplant anemia (PTA) is a common complication after kidney transplant, it has not been thoroughly evaluated for appropriate treatment. Roxadustat can stimulate erythropoiesis by increasing erythropoietin (EPO) production and improving the utilization of iron. However, there are currently a few case reports describing its effect on PTA in kidney transplant recipients (KTRs). Our purpose was to evaluate the efficacy and safety of roxadustat in KTRs with PTA.

Methods

In this retrospective study, KTRs with early PTA were divided into a roxadustat group, erythropoiesis-stimulating agent (ESA) group, and untreated group (neither roxadustat nor ESA) according to the treatment prescribed by their physicians. We compared the levels of hemoglobin (Hb), creatinine, lipids, hepcidin, intact fibroblast growth factor 23 (iFGF23) and iron-related indices, at baseline and different time points posttransplant. Outcome was assessed at both month 3 and month 12 posttransplant. Adverse events during the treatment course were also recorded.

Results

A total of 57 KTRs were included (n=22 roxadustat group, n=13 ESA group, n=22 untreated group). There was no difference in age, sex, body mass index, dialysis method and duration, donor type among three groups at baseline. The mean Hb levels at month 3 posttransplant (128.00±19.62 vs. 118.59±11.60 g/L, P=0.048) and the average change in Hb levels from week 2 to month 3 (48.05±22.53 vs. 31.45±12.96 g/L, P=0.005) in the roxadustat group were significantly higher than those in the untreated group. However, there was no significant difference in the above indices between the roxadustat and ESA groups. At month 3, the total iron binding capacity (TIBC) and levels of transferrin were significantly higher while levels of ferritin, hepcidin and iFGF23 were significantly lower in the roxadustat group than in other groups (P<0.05). No significant difference was found in creatinine or estimated glomerular filtration rate (eGFR) levels among the three groups at month 3. During the follow-up, no adverse events related to roxadustat were reported.

Conclusions

Administration of roxadustat in KTRs with early PTA could elevate Hb levels effectively and safely by enhancing endogenous EPO production and improving iron utilization. Further randomized studies with larger sample size are necessary to verify our results.

Interconnections of fibroblast growth factor 23 and klotho with erythropoietin and hypoxia-inducible factor

Bone marrow (BM) hematopoiesis is tightly regulated process and bone components such as osteoblasts, extracellular matrix, and minerals influence hematopoiesis via regulation of hematopoietic stem cell function. Erythropoietin (EPO) secreted mostly by renal EPO producing (REP) cells which employ the hypoxia-inducible factor (HIF) pathway. When tissue hypoxia occurs, HIFs bind to hypoxia response element in the EPO promoter and induce EPO production. EPO binds to the EPO receptor on red cell progenitors in the BM and triggers expansion of red cell mass. Fibroblast growth factor-23 (FGF23) which is secreted mostly by osteoblasts and less by BM impacts hematopoiesis by influencing EPO production. Reciprocally, increases of EPO (acute or chronic) influence both FG23 production and cleavage resulting in variation of c fragment FGF23 (cFGF23) and intact FGF23 (iFGF23) ratios. As HIFs stimulate EPO production, they indirectly affect FGF23. Direct stimulation of FGF23 synthesis by binding of HIF on FGF23 promoter is also suggested. FGF23 cleavage by furin is another potential mechanism affecting FGF23 levels. Klotho is present in membrane-bound (transmembrane) and free (circulating) forms. Transmembrane klotho is the co-receptor of FGF23 and forms complexes with FGF23 receptors in the membrane surface and required for FGF23 actions. Recent evidence showed that klotho is also associated with EPO and HIF production suggesting a complex relationship between FGF23, klotho, EPO, and HIF. In this review, we have summarized the connections between FGF23, klotho, HIF, and EPO and their reflections to hematopoiesis.

Challenges in the management of tumor-induced osteomalacia (TIO)

Tumor-induced osteomalacia (TIO), also known as oncogenic osteomalacia, is a rare acquired paraneoplastic disease, which is challenging to diagnose and treat. TIO is characterized by hypophosphatemia resulting from excess levels of tumor-secreted fibroblast growth factor 23 (FGF23), one of the key physiological regulators of phosphate metabolism. Elevated FGF23 results in renal phosphate wasting and compromised vitamin D activation, ultimately resulting in osteomalacia. Patients typically present with progressive and non-specific symptoms, including bone pain, multiple pathological fractures, and progressive muscle weakness. Diagnosis is often delayed or missed due to the non-specific nature of complaints and lack of disease awareness. Additionally, the disease-causing tumour is often difficult to detect and localize because they are often small, lack localizing symptoms and signs, and dwell in widely variable anatomical locations. Measuring serum/urine phosphate should be an inherent diagnostic component when assessing otherwise unexplained osteomalacia, fractures and weakness. In cases of hypophosphatemia with inappropriate (sustained) phosphaturia and inappropriately normal or frankly low 1,25-dihydroxy vitamin D, differentiation of the potential causes of renal phosphate wasting should include measurement of FGF23, and TIO should be considered. While patients experience severe disability without treatment, complete excision of the tumour is typically curative and results in a dramatic reversal of symptoms. Two additional key current unmet needs in optimizing TIO management are: (1 and 2) the considerable delay in diagnosis and consequent delay between the onset of symptoms and surgical resection; and (2) alternative management. These may be addressed by raising awareness of TIO, and taking into consideration the accessibility and variability of different healthcare infrastructures. By recognizing the challenges associated with the diagnosis and treatment of TIO and by applying a stepwise approach with clear clinical practice guidelines, patient care and outcomes will be improved in the future.

Age and genotype dependent erythropoietin protection in COVID-19

Erythropoietin (EPO) is the main mediator of erythropoiesis and an important tissue protective hormone that appears to mediate an ancestral neuroprotective innate immune response mechanism at an early age. When the young brain is threatened-prematurity, neonatal hyperbilirubinemia, malaria- EPO is hyper-secreted disproportionately to any concurrent anemic stimuli. Under eons of severe malarial selection pressure, neuroprotective EPO augmenting genetic determinants such as the various hemoglobinopathies, and the angiotensin converting enzyme (ACE) I/D polymorphism, have been positively selected. When malarial and other cerebral threats abate and the young child survives to adulthood, EPO subsides. Sustained high ACE and angiotensin II (Ang II) levels through the ACE D allele in adulthood may then become detrimental as witnessed by epidemiological studies. The ubiquitous renin angiotensin system (RAS) influences the α-klotho/fibroblast growth factor 23 (FGF23) circuitry, and both are interconnected with EPO. Here we propose that at a young age, EPO augmenting genetic determinants through ACE D allele elevated Ang II levels in some or HbE/beta thalassemia in others would increase EPO levels and shield against coronavirus disease 2019, akin to protection from malaria and dengue fever. Human evolution may use ACE2 as a “bait” for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) to gain cellular entry in order to trigger an ACE/ACE2 imbalance and stimulate EPO hypersecretion using tissue RAS, uncoupled from hemoglobin levels. In subjects without EPO augmenting genetic determinants at any age, ACE2 binding and internalization upon SARS-CoV-2 entry would trigger an ACE/ACE2 imbalance, and Ang II oversecretion leading to protective EPO stimulation. In children, low nasal ACE2 Levels would beneficially augment this imbalance, especially for those without protective genetic determinants. On the other hand, in predisposed adults with the ACE D allele, ACE/ACE2 imbalance, may lead to uncontrolled RAS overactivity and an Ang II induced proinflammatory state and immune dysregulation, with interleukin 6 (IL-6), plasminogen activator inhibitor, and FGF23 elevations. IL-6 induced EPO suppression, aggravated through co-morbidities such as hypertension, diabetes, obesity, and RAS pharmacological interventions may potentially lead to acute respiratory distress syndrome, cytokine storm and/or autoimmunity. HbE/beta thalassemia carriers would enjoy protection at any age as their EPO stimulation is uncoupled from the RAS system. The timely use of rhEPO, EPO analogs, acetylsalicylic acid, bioactive lipids, or FGF23 antagonists in genetically predisposed individuals may counteract those detrimental effects.

Crosstalk of fibroblast growth factor 23 and anemia-related factors during the development and progression of CKD (Review)

Fibroblast growth factor 23 (FGF23) plays an important role in the development of chronic kidney disease-mineral bone disorder (CKD-MBD). Abnormally elevated levels of 1,25-dihydroxyvitamin D cause osteocytes to secrete FGF23, which subsequently induces phosphaturia. Recent studies have reported that iron deficiency, erythropoietin (EPO) and hypoxia regulate the pathways responsible for FGF23 production. However, the molecular mechanisms underlying the interactions between FGF23 and anemia-related factors are not yet fully understood. The present review discusses the associations between FGF23, iron, EPO and hypoxia-inducible factors (HIFs), and their impact on FGF23 bioactivity, focusing on recent studies. Collectively, these findings propose interactions between FGF23 gene expression and anemia-related factors, including iron deficiency, EPO and HIFs. Taken together, these results suggest that FGF23 bioactivity is closely associated with the occurrence of CKD-related anemia and CKD-MBD.

Erythropoietin treatment and the risk of hip fractures in hemodialysis patients

Erythropoietin (EPO) is the primary regulator of bone marrow erythropoiesis. Mouse models have provided evidence that EPO also promotes bone remodeling and that EPO-stimulated erythropoiesis is accompanied by bone loss independent of increased red blood cell production. EPO has been used clinically for three decades to treat anemia in end-stage renal disease, and notably, although the incidence of hip fractures decreased in the United States generally after 1990, it rose among hemodialysis patients coincident with the introduction and subsequent dose escalation of EPO treatment. Given this clinical paradox and findings from studies in mice that elevated EPO affects bone health, we examined EPO treatment as a risk factor for fractures in hemodialysis patients. Relationships between EPO treatment and hip fractures were analyzed using United States Renal Data System (USRDS) datasets from 1997 to 2013 and Consolidated Renal Operations in a Web-enabled Network (CROWNWeb) datasets for 2013. Fracture risks for patients treated with <50 units of EPO/kg/week were compared to those receiving higher doses by multivariable Cox regression. Hip fracture rates for 747,832 patients in USRDS datasets (1997-2013) increased from 12.0 per 1000 patient years in 1997 to 18.9 in 2004, then decreased to 13.1 by 2013. Concomitantly, average EPO doses increased from 11,900 units/week in 1997 to 18,300 in 2004, then decreased to 8,800 by 2013. During this time, adjusted hazard ratios for hip fractures with EPO doses of 50-149, 150-299, and ≥ 300 units/kg/week compared to <50 units/kg/week were 1.08 (95% confidence interval [CI], 1.01-1.15), 1.22 (95% CI, 1.14-1.31), and 1.41 (95% CI, 1.31-1.52), respectively. Multivariable analyses of 128,941 patients in CROWNWeb datasets (2013) replicated these findings. This study implicates EPO treatment as an independent risk factor for hip fractures in hemodialysis patients and supports the conclusion that EPO treatment may have contributed to changing trends in fracture incidence for these patients during recent decades. Published 2021. This article is a U.S. Government work and is in the public domain in the USA. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Shaping the bone through iron and iron-related proteins

Well-controlled iron levels are indispensable for health. Iron deficiency is the most common cause of anemia, whereas iron overload, either hereditary or secondary due to disorders of ineffective erythropoiesis, causes widespread organ failure. Bone is particularly sensitive to fluctuations in systemic iron levels as both iron deficiency and overload are associated with low bone mineral density and fragility. Recent studies have shown that not only iron itself, but also iron-regulatory proteins that are mutated in hereditary hemochromatosis can control bone mass. This review will summarize the current knowledge on the effects of iron on bone homeostasis and bone cell activities, and on the role of proteins that regulate iron homeostasis, i.e. hemochromatosis proteins and proteins of the bone morphogenetic protein pathway, on bone remodeling. As disorders of iron homeostasis are closely linked to bone fragility, deeper insights into common regulatory mechanisms may provide new opportunities to concurrently treat disorders affecting iron homeostasis and bone.

Controversies in optimal anemia management: conclusions from a Kidney Disease: Improving Global Outcomes (KDIGO) Conference

In chronic kidney disease, anemia and disordered iron homeostasis are prevalent and associated with significant adverse consequences. In 2012, Kidney Disease: Improving Global Outcomes (KDIGO) issued an anemia guideline for managing the diagnosis, evaluation, and treatment of anemia in chronic kidney disease. Since then, new data have accrued from basic research, epidemiological studies, and randomized trials that warrant a re-examination of previous recommendations. Therefore, in 2019, KDIGO decided to convene 2 Controversies Conferences to review the latest evidence, explore new and ongoing controversies, assess change implications for the current KDIGO anemia guideline, and propose a research agenda. The first conference, described here, focused mainly on iron-related issues, including the contribution of disordered iron homeostasis to the anemia of chronic kidney disease, diagnostic challenges, available and emerging iron therapies, treatment targets, and patient outcomes. The second conference will discuss issues more specifically related to erythropoiesis-stimulating agents, including epoetins, and hypoxia-inducible factor-prolyl hydroxylase inhibitors. Here we provide a concise overview of the consensus points and controversies resulting from the first conference and prioritize key questions that need to be answered by future research.

Interplay of erythropoietin, fibroblast growth factor 23, and erythroferrone in patients with hereditary hemolytic anemia

Recently, erythropoietin (EPO) was identified as regulator of fibroblast growth factor 23 (FGF23). Proteolytic cleavage of biologically active intact FGF23 (iFGF23) results in the formation of C-terminal fragments (cFGF23). An increase in cFGF23 relative to iFGF23 suppresses FGF receptor signaling by competitive inhibition. EPO lowers the i:cFGF23 ratio, thereby overcoming iFGF23-mediated suppression of erythropoiesis. We investigated EPO-FGF23 signaling and levels of erythroferrone (ERFE) in 90 patients with hereditary hemolytic anemia (www.trialregister.nl [NL5189]). We show, for the first time, the importance of EPO-FGF23 signaling in hereditary hemolytic anemia: there was a clear correlation between total FGF23 and EPO levels (r = +0.64; 95% confidence interval [CI], 0.09-0.89), which persisted after adjustment for iron load, inflammation, and kidney function. There was no correlation between iFGF23 and EPO. Data are consistent with a low i:cFGF23 ratio. Therefore, as expected, we report a correlation between EPO and ERFE in a diverse set of hereditary hemolytic anemias (r = +0.47; 95% CI, 0.14-0.69). There was no association between ERFE and total FGF23 or iFGF23, which suggests that ERFE does not contribute to the connection between FGF23 and EPO. These findings open a new area of research and might provide potentially new druggable targets with the opportunity to ameliorate ineffective erythropoiesis and the development of disease complications in hereditary hemolytic anemias.

Saccharated ferric oxide attenuates haematopoietic response induced by epoetin beta pegol in patients undergoing haemodialysis

Background

Decreased erythropoietin levels and impaired iron metabolism due to excessive hepcidin levels are responsible for renal anaemia in patients undergoing haemodialysis. Recently, erythroferrone (ERFE) has been identified as a factor that regulates hepcidin. In addition, fibroblast growth factor 23 (FGF23), which has been recognized as a phosphorus-regulating hormone, appears to be involved in haematopoietic regulation. Clarification of the detailed mechanism of haematopoiesis could lead to the improvement of renal anaemia treatment.

Methods

Epoetin beta pegol (CERA) was administered to patients undergoing haemodialysis at week 0, and the same amount of CERA with saccharated ferric oxide (SFO) was administered at week 4. The changes in haematopoiesis-related biomarkers, including ERFE, intact FGF23 (iFGF23), C-terminal FGF23 (cFGF23), and inflammatory markers, were examined.

Results

Administration of CERA increased ERFE levels, decreased hepcidin levels, and stimulated iron usage for haematopoiesis, leading to an increase in reticulocytes (Ret) and haemoglobin (Hb). Simultaneous administration of SFO with CERA (CERA + SFO) significantly attenuated the responses of ERFE, Ret, and Hb compared with CERA alone. Although iFGF23 levels were not affected by either CERA or CERA + SFO, cFGF23 was significantly elevated from baseline after CERA. Since cFGF23 levels were not affected by CERA + SFO, cFGF23 levels after CERA + SFO were significantly lower than those after CERA alone. The ratio of iFGF23 to cFGF23 (i/cFGF23 ratio) was significantly higher after CERA + SFO than that after CERA alone. In addition, high-sensitivity C-reactive protein (hsCRP) levels were significantly higher after CERA + SFO than after CERA alone.

Conclusion

Administration of SFO suppressed haematopoietic responses induced by CERA. Elevation of i/cFGF23 ratio and hsCRP could account for the inhibitory effects of SFO on haematopoiesis.

Trial registration

This study was registered with the University Hospital Medical Information Network (ID UMIN000016552).

Supplementary Information

The online version contains supplementary material available at 10.1186/s12882-021-02320-2.

Serum levels of C-terminal FGF23 (cFGF23) are associated with 1-year-mortality in patients undergoing transcatheter aortic valve replacement (TAVR)

INTRODUCTION

Serum levels of FGF23 have been associated with adverse outcomes in cardiovascular diseases in patients with and without impaired renal function. Hence, this study aimed to explore the prognostic relevance of intact FGF23 (iFGF23) and its derivate C-terminal FGF23 (cFGF23) in patients undergoing transcatheter aortic valve replacement (TAVR) with regard to renal function.

METHODS

A total of 274 patients undergoing transfemoral TAVR were enrolled in this study. Blood samples were obtained preinterventionally and analyzed for iFGF23 and cFGF23 by means of enzyme linked immunosorbent assay (ELISA). Follow-up was obtained for 12 months.

RESULTS

Serum levels of cFGF23 and iFGF23 both correlated positively with serum creatinine and inversely with estimated glomerular filtration rate (eGFR). Cox regression analysis revealed a significant association of cFGF23 with 1-year-mortality in patients with eGFR ≥45ml/min/1.73m², but not in patients with an eGFR <45ml/min/1.73m². A cut-off was calculated for cFGF23 (6.82 pmol/l) and patients with eGFR ≥45ml/min/1.73m² were retrospectively divided into two groups (above/below cut-off). Patients above the cut-off had a significantly worse 1-year-mortality than patients below the cut-off (33.3% vs. 19.6%; OR 2.05 (95%CI 1.03-4.07), p= 0.038). The association of cFGF23 with 1-year-mortality in patients with eGFR ≥45ml/min/1.73m² remained statistically significant even after correction for possible confounders in a multivariate Cox regression analysis.

CONCLUSION

cFGF23 could be an individual risk factor for mortality in patients undergoing TAVR with an eGFR ≥45ml/min/1.73m².

Association of erythropoietin resistance and fibroblast growth factor 23 in dialysis patients: Results from the Japanese Dialysis Outcomes and Practice Patterns Study

BACKGROUND

Fibroblast growth factor 23 (FGF23) plays an important role in chronic kidney disease (CKD)-related mineral and bone disorders. High FGF23 levels are associated with increased risk of anaemia in non-haemodialysis CKD patients. FGF23 also negatively regulates erythropoiesis in mice. We hypothesized that higher FGF23 levels are associated with increased erythropoietin hyporesponsiveness among haemodialysis patients.

METHODS

The study included 1044 patients from the Japanese Dialysis Outcomes and Practice Patterns Study (J-DOPPS) phase 5 (2012-2015). The outcome was erythropoiesis-stimulating agent hyporesponsiveness (ESA-hypo), defined as mean Hgb <10 g/dL and standardized mean ESA dose >6000 u/week over 4 months following FGF23 measurement. The association between ESA-hypo and FGF23 was estimated using multivariable-adjusted logistic generalized estimating equation regression models.

RESULTS

Patients with higher levels of FGF23 were younger and had higher levels of serum albumin, creatinine, albumin-corrected calcium, phosphorus, PTH, 25(OH)-vitamin D, and had higher percentages of intravenous (IV) iron, IV vitamin D and cinacalcet use. ESA-hypo was present in 144 patients (13.8%). Compared with the third quintile of FGF23 levels, the odds ratio (95% CI) of ESA-hypo was 2.14 (0.99, 4.62) and 1.74 (0.74, 4.11) for the first and fifth quintiles, respectively.

CONCLUSION

The lowest and highest levels of FGF23 were associated with higher odds of ESA-hypo in patients on maintenance haemodialysis, although the associations were not statistically significant. The relationship between FGF23 and anaemia, and particularly the increased risks of ESA-hypo at low FGF23 levels which might be the result of energy saving, must be confirmed in larger clinical studies.

Intravenous iron: do we adequately understand the short- and long-term risks in clinical practice?

Intravenous (IV) iron as a therapeutic agent is often administered but not always fully understood. The benefits of IV iron are well proven in many fields, particularly in nephrology. IV iron is beneficial not only for true iron deficiency but also for iron-restricted anaemia (functional iron deficiency). Yet, the literature on intravenous iron has many inconsistencies regarding its adverse effects. Over the last several years, newer forms of iron have been developed, leading to the more regular use of iron and in larger doses. This review will summarize some of the older and newer literature regarding the differences among iron products, including the mechanisms and frequency of their adverse events (AEs). The pathway and frequency of an underrecognized adverse event (hypophosphataemia) will be discussed. Recent insights on infection risk and iron handling by macrophages are examined. Potential but presently unproven risks of iron overload due to IV iron are discussed. The impact of these on the risk:benefit ratio and dosing of intravenous iron are considered in different clinical settings, including pregnancy and cancer. IV iron is an essential component of the therapy of anaemia and understanding these issues will enable more informed treatment decisions and knowledgeable use of these drugs.

Hypoxia Pathway Proteins are Master Regulators of Erythropoiesis

Erythropoiesis is a complex process driving the production of red blood cells. During homeostasis, adult erythropoiesis takes place in the bone marrow and is tightly controlled by erythropoietin (EPO), a central hormone mainly produced in renal EPO-producing cells. The expression of EPO is strictly regulated by local changes in oxygen partial pressure (pO₂) as under-deprived oxygen (hypoxia); the transcription factor hypoxia-inducible factor-2 induces EPO. However, erythropoiesis regulation extends beyond the well-established hypoxia-inducible factor (HIF)-EPO axis and involves processes modulated by other hypoxia pathway proteins (HPPs), including proteins involved in iron metabolism. The importance of a number of these factors is evident as their altered expression has been associated with various anemia-related disorders, including chronic kidney disease. Eventually, our emerging understanding of HPPs and their regulatory feedback will be instrumental in developing specific therapies for anemic patients and beyond.

Hypophosphatemia Is Common After Intravenous Ferric Carboxymaltose Infusion Among Patients With Symptomatic Heart Failure With Reduced Ejection Fraction

Administration of intravenous ferric carboxymaltose (FCM) for iron-deficient patients suffering heart failure with reduced ejection fraction (HFrEF) has been associated with transient hypophosphatemia. We sought to investigate and model the effect of intravenous FCM on phosphate levels in iron-deficient patients with HFrEF. In this single-center retrospective study, serum phosphate levels, recorded for clinical reasons, were collected out to 60 days following intravenous FCM. Hypophosphatemia was defined as a nadir serum phosphate level <0.64 mmol/L. This was further categorized as severe (0.4 to <0.64 mmol/L) and extreme (<0.4 mmol/L). Factors associated with hypophosphatemia and change in serum phosphate over time were explored. Of 173 patients included, 47 (27%) experienced hypophosphatemia, 44 (25%) were classified as severe, and 3 (2%) extreme. Risk of hypophosphatemia was increased for patients with a creatinine clearance between 60 and <90 mL/min (odds ratio, 2.3; 95% confidence interval, 1.0-5.5), while <60 mL/min was protective. The median time to nadir in patients who experienced hypophosphatemia was 8 (interquartile range, 4-16) days, with a return to baseline levels at 6 weeks. Biochemically relevant hypophosphatemia is common following a single dose of intravenous FCM. The median time to nadir was 8 days, and creatinine clearance may influence phosphate levels following intravenous FCM. These observations support the need to increase awareness among clinicians administering intravenous FCM to iron-deficient patients with HFrEF.

Importance of Dietary Phosphorus for Bone Metabolism and Healthy Aging

Inorganic phosphate (Pi) plays a critical function in many tissues of the body: for example, as part of the hydroxyapatite in the skeleton and as a substrate for ATP synthesis. Pi is the main source of dietary phosphorus. Reduced bioavailability of Pi or excessive losses in the urine causes rickets and osteomalacia. While critical for health in normal amounts, dietary phosphorus is plentiful in the Western diet and is often added to foods as a preservative. This abundance of phosphorus may reduce longevity due to metabolic changes and tissue calcifications. In this review, we examine how dietary phosphorus is absorbed in the gut, current knowledge about Pi sensing, and endocrine regulation of Pi levels. Moreover, we also examine the roles of Pi in different tissues, the consequences of low and high dietary phosphorus in these tissues, and the implications for healthy aging.

The Role of Pi, Glutamine and the Essential Amino Acids in Modulating the Metabolism in Diabetes and Cancer

Purpose

Re-examine the current metabolic models.

Methods

Review of literature and gene networks.

Results

Insulin activates Pi uptake, glutamine metabolism to stabilise lipid membranes. Tissue turnover maintains the metabolic health. Current model of intermediary metabolism (IM) suggests glucose is the source of energy, and anaplerotic entry of fatty acids and amino acids into mitochondria increases the oxidative capacity of the TCA cycle to produce the energy (ATP). The reduced cofactors, NADH and FADH2, have different roles in regulating the oxidation of nutrients, membrane potentials and biosynthesis. Trans-hydrogenation of NADH to NADPH activates the biosynthesis. FADH2 sustains the membrane potential during the cell transformations. Glycolytic enzymes assume the non-canonical moonlighting functions, enter the nucleus to remodel the genetic programmes to affect the tissue turnover for efficient use of nutrients. Glycosylation of the CD98 (4F2HC) stabilises the nutrient transporters and regulates the entry of cysteine, glutamine and BCAA into the cells. A reciprocal relationship between the leucine and glutamine entry into cells regulates the cholesterol and fatty acid synthesis and homeostasis in cells. Insulin promotes the Pi transport from the blood to tissues, activates the mitochondrial respiratory activity, and glutamine metabolism, which activates the synthesis of cholesterol and the de novo fatty acids for reorganising and stabilising the lipid membranes for nutrient transport and signal transduction in response to fluctuations in the microenvironmental cues. Fatty acids provide the lipid metabolites, activate the second messengers and protein kinases. Insulin resistance suppresses the lipid raft formation and the mitotic slippage activates the fibrosis and slow death pathways.

Effect of mandibular advancement device treatment on HIF-1α, EPO and VEGF in the myocardium of obstructive sleep apnea-hypopnea syndrome rabbits

The aim of this study was to investigate the effects of mandibular advancement device (MAD) therapy for obstructive sleep apnea-hypopnea syndrome (OSAHS) on hypoxia-inducible factor-1α (HIF-1α), erythropoietin (EPO) and vascular endothelial growth factor (VEGF) in myocardial tissue. New Zealand rabbits were used to develop OSAHS and MAD models. Cone beam computed tomography (CBCT) of the upper airway and polysomnography (PSG) recordings were performed with the animals in the supine position. All of the animals were induced to sleep in a supine position for 4-6 h each day and were observed continuously for 8 weeks. The myocardial tissue of the three groups was dissected to measure the expression of HIF-1α, EPO and VEGF. The results showed that there was higher expression of HIF-1α, EPO and VEGF in the OSAHS group than those in the MAD and control groups. MAD treatment significantly downregulated the expression of HIF-1α, EPO and VEGF in the OSAHS animals. We concluded that MAD treatment could significantly downregulate the increased expression of HIF-1α, EPO and VEGF in OSAHS rabbits, improving their myocardial function.

Fibroblast growth factor 23: a biomarker of fibrosis and prognosis in heart failure with preserved ejection fraction

AIMS

Besides regulating calcium-phosphate metabolism, fibroblast growth factor 23 (FGF-23) has been associated with incident heart failure (HF) and left ventricular hypertrophy. However, data about FGF-23 in HF and preserved ejection fraction (HFpEF) remain limited. The aim of this study was to assess the association between FGF-23 levels, clinical and imaging characteristics, particularly diffuse myocardial fibrosis, and prognosis in HFpEF patients.

METHODS AND RESULTS

We prospectively included 143 consecutive HFpEF patients (78 ± 8 years, 61% female patients) and 31 controls of similar age and gender (75 ± 6 years, 61% female patients). All subjects underwent a complete two-dimensional echocardiography and cardiac magnetic resonance with extracellular volume (ECV) assessment by T1 mapping. FGF-23 was measured at baseline. Among the patients, differences in clinical and imaging characteristics across tertiles of FGF-23 levels were analysed with a trend test across the ordered groups. Patients were followed over time for a primary endpoint of all-cause mortality and first HF hospitalization and a secondary endpoint of all-cause mortality. Median FGF-23 was significantly higher in HFpEF patients compared with controls of similar age and gender (247 [115; 548] RU/mL vs. 61 [51; 68] RU/mL, P < 0.001). Among HFpEF patients, higher FGF-23 levels were associated with female sex, higher incidence of atrial fibrillation, lower haemoglobin, worse renal function, and higher N terminal pro brain natriuretic peptide levels (P for trend < 0.05 for all). Regarding imaging characteristics, patients with higher FGF-23 levels had greater left atrial volumes, worse right ventricular systolic function, and more fibrosis estimated by ECV (P for trend < 0.05 for all). FGF-23 was moderately correlated with ECV (r = 0.46, P < 0.001). Over a mean follow-up of 30 ± 8 months, 43 patients (31%) died and 69 patients (49%) were hospitalized for HF. A total of 87 patients (62%) reached the primary composite endpoint of all-cause mortality and/or first HF hospitalization. In multivariate Cox regression analysis for the primary endpoint, FGF-23 (HR: 3.44 [2.01; 5.90], P < 0.001) and E wave velocities (HR: 1.01 [1.00; 1.02], P = 0.034) were independent predictors of the primary composite endpoint. In multivariate Cox regression analysis for the secondary endpoint, ferritin (HR: 1.02 [1.01; 1.03], P < 0.001), FGF-23 (HR: 2.85 [1.26; 6.44], P = 0.012), and ECV (HR: 1.26 [1.03; 1.23], P = 0.008) were independent predictors of all-cause mortality.

CONCLUSIONS

Fibroblast growth factor 23 (FGF-23) levels were significantly higher in HFpEF patients compared with controls of similar age and gender. FGF-23 was correlated with fibrosis evaluated by ECV. High levels of FGF-23 were significantly associated with signs of disease severity such as worse renal function, larger left atrial volumes, and right ventricular dysfunction. Moreover, FGF-23 was a strong predictor of poor outcome (mortality and first HF hospitalization).

Phospho-PTM proteomic discovery of novel EPO- modulated kinases and phosphatases, including PTPN18 as a positive regulator of EPOR/JAK2 Signaling

The formation of erythroid progenitor cells depends sharply upon erythropoietin (EPO), its cell surface receptor (erythropoietin receptor, EPOR), and Janus kinase 2 (JAK2). Clinically, recombinant human EPO (rhEPO) additionally is an important anti-anemia agent for chronic kidney disease (CKD), myelodysplastic syndrome (MDS) and chemotherapy, but induces hypertension, and can exert certain pro-tumorigenic effects. Cellular signals transduced by EPOR/JAK2 complexes, and the nature of EPO-modulated signal transduction factors, therefore are of significant interest. By employing phospho-tyrosine post-translational modification (p-Y PTM) proteomics and human EPO- dependent UT7epo cells, we have identified 22 novel kinases and phosphatases as novel EPO targets, together with their specific sites of p-Y modification. New kinases modified due to EPO include membrane palmitoylated protein 1 (MPP1) and guanylate kinase 1 (GUK1) guanylate kinases, together with the cytoskeleton remodeling kinases, pseudopodium enriched atypical kinase 1 (PEAK1) and AP2 associated kinase 1 (AAK1). Novel EPO- modified phosphatases include protein tyrosine phosphatase receptor type A (PTPRA), phosphohistidine phosphatase 1 (PHPT1), tensin 2 (TENC1), ubiquitin associated and SH3 domain containing B (UBASH3B) and protein tyrosine phosphatase non-receptor type 18 (PTPN18). Based on PTPN18's high expression in hematopoietic progenitors, its novel connection to JAK kinase signaling, and a unique EPO- regulated PTPN18-pY389 motif which is modulated by JAK2 inhibitors, PTPN18's actions in UT7epo cells were investigated. Upon ectopic expression, wt-PTPN18 promoted EPO dose-dependent cell proliferation, and survival. Mechanistically, PTPN18 sustained the EPO- induced activation of not only mitogen-activated protein kinases 1 and 3 (ERK1/2), AKT serine/threonine kinase 1-3 (AKT), and signal transducer and activator of transcription 5A and 5B (STAT5), but also JAK2. Each effect further proved to depend upon PTPN18's EPO- modulated (p)Y389 site. In analyses of the EPOR and the associated adaptor protein RHEX (regulator of hemoglobinization and erythroid cell expansion), wt-PTPN18 increased high molecular weight EPOR forms, while sharply inhibiting the EPO-induced phosphorylation of RHEX-pY141. Each effect likewise depended upon PTPN18-Y389. PTPN18 thus promotes signals for EPO-dependent hematopoietic cell growth, and may represent a new druggable target for myeloproliferative neoplasms.