FGF-23 from erythroblasts promotes hematopoietic progenitor mobilization

Fibroblast growth factor 23 is pumping iron: C-terminal-fibroblast growth factor 23 cleaved peptide and its function in iron metabolism

PURPOSE OF REVIEW

Iron deficiency regulates the production of the bone-derived phosphaturic hormone fibroblast growth factor 23 (FGF23) but also its cleavage, to generate both intact (iFGF23) and C-terminal (Cter)-FGF23 peptides. Novel studies demonstrate that independently of the phosphaturic effects of iFGF23, Cter-FGF23 peptides play an important role in the regulation of systemic iron homeostasis. This review describes the complex interplay between iron metabolism and FGF23 biology.

RECENT FINDINGS

C-terminal (Cter) FGF23 peptides antagonize inflammation-induced hypoferremia to maintain a pool of bioavailable iron in the circulation. A key mechanism proposed is the down-regulation of the iron-regulating hormone hepcidin by Cter-FGF23.

SUMMARY

In this manuscript, we discuss how FGF23 is produced and cleaved in response to iron deficiency, and the principal functions of cleaved C-terminal FGF23 peptides. We also review possible implications anemia of chronic kidney disease (CKD).

Relationship between Femoral Proximal Bone Quality Assessment by MRI IDEAL-IQ Sequence and Body Mass Index in Elderly Men

BACKGROUND

Bone assessment using the MRI DEAL-IQ sequence may have the potential to serve as a substitute for evaluating bone strength by quantifying the bone marrow hematopoietic region (R2*) and marrow adiposity (proton density fat fraction: PDFF). Higher body mass index (BMI) is associated with increased bone mineral density (BMD) in the proximal femur; however, the relationship between BMI and R2* or PDFF remains unclear. Herein, we investigated the correlation between BMI and MRI IDEAL-IQ based R2* or PDFF of the proximal femur.

METHODS

A retrospective single-cohort study was conducted on 217 patients diagnosed with non-metastatic prostate cancer between September 2019 and December 2022 who underwent MRI. The correlation between BMI and R2* or PDFF of the proximal femur was analyzed using Spearman's rank correlation test.

RESULTS

Among 217 patients (median age, 74 years; median BMI, 23.8 kg/m2), there was a significant positive correlation between BMI and R2* at the right and left proximal femur (r = 0.2686, p < 0.0001; r = 0.2755, p < 0.0001, respectively). Furthermore, BMI and PDFF showed a significant negative correlation (r = -0.239, p = 0.0004; r = -0.2212, p = 0.001, respectively).

CONCLUSION

In elderly men, the increased loading on the proximal femur due to elevated BMI was observed to promote a decrease in bone marrow adiposity in the proximal femur, causing a tendency for a transition from fatty marrow to red marrow with hematopoietic activity. These results indicate that the MRI IDEAL-IQ sequence may be valuable for assessing bone quality deterioration in the proximal femur.

TFPI from erythroblasts drives heme production in central macrophages promoting erythropoiesis in polycythemia

Bleeding and thrombosis are known as common complications of polycythemia for a long time. However, the role of coagulation system in erythropoiesis is unclear. Here, we discover that an anticoagulant protein tissue factor pathway inhibitor (TFPI) plays an essential role in erythropoiesis via the control of heme biosynthesis in central macrophages. TFPI levels are elevated in erythroblasts of human erythroblastic islands with JAK2V617F mutation and hypoxia condition. Erythroid lineage-specific knockout TFPI results in impaired erythropoiesis through decreasing ferrochelatase expression and heme biosynthesis in central macrophages. Mechanistically, the TFPI interacts with thrombomodulin to promote the downstream ERK1/2-GATA1 signaling pathway to induce heme biosynthesis in central macrophages. Furthermore, TFPI blockade impairs human erythropoiesis in vitro, and normalizes the erythroid compartment in mice with polycythemia. These results show that erythroblast-derived TFPI plays an important role in the regulation of erythropoiesis and reveal an interplay between erythroblasts and central macrophages.

Regulation of FGF23 Production in Osteocytes

PURPOSE OF REVIEW

FGF23 is a bone-derived hormone working to reduce serum phosphate level. This review focuses on recent findings regarding regulatory mechanisms of FGF23 expression in osteocytes, FGF23 levels, and activities.

RECENT FINDINGS

Circulatory FGF23 levels reflecting FGF23 biological activities can be regulated by both FGF23 expression and posttranslational modification of FGF23 protein. O-linked glycosylation and phosphorylation of FGF23 protein as well as enzymes that can cleave FGF23 protein are involved in the posttranslational modification. However, precise mechanisms of FGF23 protein processing are not clear. Several extracellular factors have been shown to affect FGF23 levels in kidney injuries. Contribution of these factors may be different depending on the causes and stages of kidney injury. FGF23 activities are regulated by complex mechanisms involving transcriptional and posttranslational modulations. There still remain several questions regarding the regulatory mechanisms of FGF23 expression and FGF23 processing.

Bone marrow sinusoidal endothelial cells are a site of Fgf23 upregulation in a mouse model of iron deficiency anemia

Iron deficiency is a potent stimulator of fibroblast growth factor 23 (FGF23), a hormonal regulator of phosphate and vitamin D metabolism, that is classically thought to be produced by bone-embedded osteocytes. Here, we show that iron-deficient transmembrane serine protease 6 knockout (Tmprss6-/-) mice exhibit elevated circulating FGF23 and Fgf23 messenger RNA (mRNA) upregulation in the bone marrow (BM) but not the cortical bone. To clarify sites of Fgf23 promoter activity in Tmprss6-/- mice, we introduced a heterozygous enhanced green fluorescent protein (eGFP) reporter allele at the endogenous Fgf23 locus. Heterozygous Fgf23 disruption did not alter the severity of systemic iron deficiency or anemia in the Tmprss6-/- mice. Tmprss6-/-Fgf23+/eGFP mice showed green fluorescence in the vascular regions of BM sections and showed a subset of BM endothelial cells that were GFPbright by flow cytometry. Mining of transcriptomic data sets from mice with normal iron balance revealed higher Fgf23 mRNA in BM sinusoidal endothelial cells (BM-SECs) than that in other BM endothelial cell populations. Anti-GFP immunohistochemistry of fixed BM sections from Tmprss6-/-Fgf23+/eGFP mice revealed GFP expression in BM-SECs, which was more intense than in nonanemic controls. In addition, in mice with intact Tmprss6 alleles, Fgf23-eGFP reporter expression increased in BM-SECs following large-volume phlebotomy and also following erythropoietin treatment both ex vivo and in vivo. Collectively, our results identified BM-SECs as a novel site for Fgf23 upregulation in both acute and chronic anemia. Given the elevated serum erythropoietin in both anemic models, our findings raise the possibility that erythropoietin may act directly on BM-SECs to promote FGF23 production during anemia.

Regulation of granulocyte colony-stimulating factor-induced hematopoietic stem cell mobilization by the sympathetic nervous system

PURPOSE OF REVIEW

Granulocyte colony-stimulating factor (G-CSF) is now a standard agent to mobilize hematopoietic stem cells (HSCs) from the bone marrow to circulation. This review introduced mechanistic insights from the aspect of the sympathetic nervous system (SNS).

RECENT FINDINGS

Mobilization efficiency is determined by the balance between promotion and suppression pathways critically regulated by the SNS. G-CSF-induced high catecholaminergic tone promotes mobilization by (1) the strong suppression of osteolineage cells as a hematopoietic microenvironment and (2) fibroblast growth factor 23 production from erythroblasts, which inhibits CXCR4 function in HSCs. Simultaneously, SNS signals inhibit mobilization by (1) prostaglandin E2 production from mature neutrophils to induce osteopontin in osteoblasts to anchor HSCs and (2) angiopoietin-like protein 4 production from immature neutrophils via peroxisome proliferator-activated receptor δ to inhibit BM vascular permeability.

SUMMARY

We now know not only the regulatory mechanisms of G-CSF-induced mobilization but also the leads about unfavorable clinical phenomena, such as low-grade fever, bone pain, and poor mobilizers. Recent understanding of the mechanism will assist clinicians in the treatment for mobilization and researchers in the studies of the hidden potential of BM.

Inhibition of FGF23 is a therapeutic strategy to target hematopoietic stem cell niche defects in β-thalassemia

Clinical evidence highlights a relationship between the blood and the bone, but the underlying mechanism linking these two tissues is not fully elucidated. Here, we used β-thalassemia as a model of congenital anemia with bone and bone marrow (BM) niche defects. We demonstrate that fibroblast growth factor 23 (FGF23) is increased in patients and mice with β-thalassemia because erythropoietin induces FGF23 overproduction in bone and BM erythroid cells via ERK1/2 and STAT5 pathways. We show that in vivo inhibition of FGF23 signaling by carboxyl-terminal FGF23 peptide is a safe and efficacious therapeutic strategy to rescue bone mineralization and deposition in mice with β-thalassemia, normalizing the expression of niche factors and restoring hematopoietic stem cell (HSC) function. FGF23 may thus represent a molecular link connecting anemia, bone, and the HSC niche. This study provides a translational approach to targeting bone defects and rescuing HSC niche interactions, with potential clinical relevance for improving HSC transplantation and gene therapy for hematopoietic disorders.

Serum Phosphate Levels Modify the Impact of FGF23 Levels on Hemoglobin in Chronic Kidney Disease

Anemia is a complication of chronic kidney disease (CKD). Phosphate and fibroblast growth factor-23 (FGF23) have a close relationship, as both are related to the pathogenesis of anemia. However, the possible interplay between them regarding their effect on anemia has not been evaluated. This was a cross-sectional study of 896 participants from the NEFRONA study (273 CKD3, 246 CKD4-5, 282 dialysis and 95 controls). The levels of 25(OH) and 1,25(OH)2 vitamin D, intact FGF23 (iFGF23) and soluble Klotho were measured, together with standard blood biochemistries. Anemia was defined as hemoglobin levels < 13 g/dL in men and <12 g/dL in women. Patients with anemia (407, 45.4%) were younger, mostly men and diabetic; were in advanced CKD stages; had lower calcium, 1,25(OH)2 vitamin D and albumin levels; and had higher ferritin, phosphate, intact PTH, and iFGF23. An inverse correlation was observed between hemoglobin and both iFGF23 and phosphate. The multivariate logistic regression analyses showed that the adjusted risk of anemia was independently associated with higher serum phosphate and LogiFGF23 levels (ORs (95% CIs) of 4.33 (2.11−8.90) and 8.75 (3.17−24.2), respectively (p < 0.001)). A significant interaction between phosphate and iFGF23 (OR of 0.66 (0.53−0.83), p < 0.001) showed that the rise in the adjusted predicted risk of anemia with the increase in iFGF23 was steeper when phosphate levels were low. Phosphate levels acted as modifiers of the effect of iFGF23 concentration on anemia. Thus, the effect of the increase in iFGF23 levels was stronger when phosphate levels were low.

Meteorin links the bone marrow hypoxic state to hematopoietic stem/progenitor cell mobilization

Hematopoietic stem/progenitor cells (HSPCs) are supported and regulated by niche cells in the bone marrow with an important characterization of physiological hypoxia. However, how hypoxia regulates HSPCs is still unclear. Here, we find that meteorin (Metrn) from hypoxic macrophages restrains HSPC mobilization. Hypoxia-induced factor 1α and Yin Yang 1 induce the high expression of Metrn in macrophages, and macrophage-specific Metrn knockout increases HSPC mobilization through modulating HSPC proliferation and migration. Mechanistically, Metrn interacts with its receptor 5-hydroxytryptamine receptor 2b (Htr2b) to regulate the reactive oxygen species levels in HSPCs through targeting phospholipase C signaling. The reactive oxygen species levels are reduced in HSPCs of macrophage-specific Metrn knockout mice with activated phospholipase C signaling. Targeting the Metrn/Htr2b axis could therefore be a potential strategy to improve HSPC mobilization for stem cell-based therapy.

Anemia and fibroblast growth factor 23 elevation in chronic kidney disease: homeostatic interactions and emerging therapeutics

PURPOSE OF REVIEW

Chronic kidney disease (CKD) is a progressive disorder that is associated with development of elevated fibroblast growth factor 23 (FGF23) levels and anemia. Here, we review recent literature that extends our current knowledge on the interactions between FGF23 and anemia in CKD and the impact of anemia-targeting therapeutics on FGF23 elevation in CKD.

RECENT FINDINGS

The anemia of CKD is primarily driven by a lack of erythropoietin (EPO) and iron deficiency. In addition to EPO and iron replacement, novel drug classes to treat anemia have been approved or are in clinical development. A recent observational study provides supportive evidence for the hypothesis that FGF23 elevation in CKD mediates adverse effects of iron deficiency on the cardiovascular system in patients with CKD. Preclinical and clinical studies revealed that ferric citrate (FC), and hypoxia-induced factor-prolyl hydroxylase inhibitor (HIF-PHI) treatment may reduce elevated FGF23 levels in CKD, suggesting that correcting anemia in CKD could potentially lower FGF23 levels. However, as we describe, HIF-PHI have context-dependent effects. Moreover, whether a reduction in FGF23 will improve patient outcomes in patients with CKD remains to be determined.

SUMMARY

With the emergence of novel therapeutics to treat oxygen and iron utilization deficits in CKD, studies have investigated the impact of these new drugs on FGF23. Several of these drugs, including FC and HIF-PHIs, alleviate iron homeostasis alterations in CKD and are associated with FGF23 reduction. Herein, we review the relationships between oxygen/iron sensing and FGF23 in CKD, recent findings which link FGF23 with cardiac dysfunction, as well as future translational and clinical avenues.

Hematopoietic stem cell mobilization

Hematopoietic stem cell (HSC) transplantation has been used to treat hematopoietic diseases for over 50 years. HSCs can be isolated from bone marrow (BM), umbilical cord blood, or peripheral blood. Because of lower costs, shorter hospitalization, and faster engraftment, peripheral blood has become the predominant source of HSCs for transplantation. The major factors determining the rate of successful HSC transplantation include the degree of human leukocyte antigen matching between the donor and recipient and the number of HSCs for transplantation. Administration of granulocyte colony-stimulating factor (G-CSF) alone or combined with plerixafor (AMD3100) are clinical used methods to promote HSC mobilization from BM to the peripheral blood for HSC transplantations. However, a significant portion of healthy donors or patients may be poor mobilizers of G-CSF, resulting in an insufficient number of HSCs for the transplantation and necessitating alternative strategies to increase the apheresis yield. The detailed mechanisms underlying G-CSF-mediated HSC mobilization remain to be elucidated. This review summarizes the current research on deciphering the mechanism of HSC mobilization.

Regulation of FGF23: Beyond Bone

PURPOSE OF REVIEW

Fibroblast growth factor 23 (FGF23) is a bone- and bone marrow-derived hormone that is critical to maintain phosphate homeostasis. The principal actions of FGF23 are to reduce serum phosphate levels by decreasing kidney phosphate reabsorption and 1,25-dihydroxyvitamin D synthesis. FGF23 deficiency causes hyperphosphatemia and ectopic calcifications, while FGF23 excess causes hypophosphatemia and skeletal defects. Excess FGF23 also correlates with kidney disease, where it is associated with increased morbidity and mortality. Accordingly, FGF23 levels are tightly regulated, but the mechanisms remain incompletely understood.

RECENT FINDINGS

In addition to bone mineral factors, additional factors including iron, erythropoietin, inflammation, energy, and metabolism regulate FGF23. All these factors affect Fgf23 expression, while some also regulate FGF23 protein cleavage. Conversely, FGF23 may have a functional role in regulating these biologic processes. Understanding the bi-directional relationship between FGF23 and non-bone mineral factors is providing new insights into FGF23 regulation and function.

Glycerol-3-phosphate and fibroblast growth factor 23 regulation

PURPOSE OF REVIEW

Both classical and nonclassical factors regulate fibroblast growth factor 23 (FGF23), with impacts on gene expression and proteolytic cleavage. Here, we review recent publications that extend current knowledge on these factors.

RECENT FINDINGS

Emerging nonclassical FGF23 regulators such as erythropoietin cause a balanced increase in FGF23 expression and cleavage, with minimal or no increase in biologically active intact FGF23 (iFGF23) in blood. However, circulating FGF23 profiles may not reflect the bone marrow microenvironment. For example, granulocyte colony-stimulating factor increases local marrow iFGF23 levels without impacting circulating iFGF23 levels. The view that phosphate does not increase bone FGF23 production also warrants reconsideration, as phosphate can reduce iFGF23 cleavage and phosphate-containing calciprotein particles increase FGF23 expression. Finally, a screen of renal venous plasma identifies glycerol-3-phosphate as a kidney-derived molecule that circulates to bone and bone marrow, where it is converted to lysophosphatidic acid and signals through a G-protein coupled receptor to increase FGF23 synthesis.

SUMMARY

FGF23 regulation is complex, requiring consideration of known and emerging stimuli, expression and cleavage, and circulating and local levels. Recent work identifies glycerol-3-phosphate as an FGF23 regulator derived from the injured kidney; whether it participates in FGF23 production downstream of classical or nonclassical factors requires further study.