Reduced hepcidin level features osteoporosis

Screening of superior anti-osteoporotic flavonoids from Epimedii Folium with dual effects of reversing iron overload and promoting osteogenesis

Iron overload is a risk factor for postmenopausal osteoporosis (PMOP) and lowering iron levels to regulate the labile plasma iron is the preferred therapy. Icariin (ICA), baohuoside I (BHS) and icaritin (ICT) are three flavonoids obtained from Epimedii Folium that are efficient in facilitating osteogenesis. In this study, an active flavonoid with dual effects of reversing iron overload and promoting osteogenesis was screened based on pharmacokinetics, iron complexation properties and the potential to downregulate iron overload, reversing PMOP. As a result, the in vivo absorption of three compounds was ICA > ICT > BHS, while the exposure in muscle and bone was BHS > ICT > ICA. In vitro complexation showed that only ICT complexed with Fe (III) at a 1:1 ratio on 3-OH and the ICT-Fe (III) complex (m/z 424.3750) was identified by UPLC-Q-TOF-MS. In vivo dynamic detection also showed that the concentration of ICT-Fe (III) complexes varied with the concentration of ICT in plasma. The behavioral blunting and bone loss in zebrafish induced by Fe (III) were significantly reversed by ICT in a dose-dependent manner. Pharmacokinetic-pharmacodynamic analysis showed that ICT was negatively correlated with serum ferritin and positively correlated with osteogenic markers including alkaline phosphatase, osteocalcin and osteoprotegerin. Bone loss in ovariectomized rats was significantly altered after ICT intervention, with reduced serum ferritin levels and improved osteogenic marker levels. These results demonstrated that ICT had favorable musculoskeletal penetration and iron complexation capability to shrink labile plasma iron, showing superior performance in anti-PMOP through the dual effects of reversing iron overload and promoting osteogenesis.

Role of Iron Accumulation in Osteoporosis and the Underlying Mechanisms

Osteoporosis is prevalent in postmenopausal women. The underlying reason is mainly estrogen deficiency, but recent studies have indicated that osteoporosis is also associated with iron accumulation after menopause. It has been confirmed that some methods of decreasing iron accumulation can improve the abnormal bone metabolism associated with postmenopausal osteoporosis. However, the mechanism of iron accumulation-induced osteoporosis is still unclear. Iron accumulation may inhibit the canonical Wnt/β-catenin pathway via oxidative stress, leading to osteoporosis by decreasing bone formation and increasing bone resorption via the osteoprotegerin (OPG)/receptor activator of nuclear factor kappa-B ligand (RANKL)/receptor activator of nuclear factor kappa-B (RANK) system. In addition to oxidative stress, iron accumulation also has been reported to inhibit either osteoblastogenesis or osteoblastic function as well as to stimulate either osteoclastogenesis or osteoclastic function directly. Furthermore, serum ferritin has been widely used for the prediction of bone status, and nontraumatic measurement of iron content by magnetic resonance imaging may be a promising early indicator of postmenopausal osteoporosis.

No genetic causal association between iron status and osteoporosis: A two-sample Mendelian randomization

OBJECTIVE

To explore the genetic causal association between osteoporosis (OP) and iron status through Mendelian randomization (MR).

METHODS

Publicly available genome-wide association study (GWAS) summary data were used for MR analysis with four iron status-related indicators (ferritin, iron, total iron binding capacity, and transferrin saturation) as exposures and three different types of OP (OP, OP with pathological fracture, and postmenopausal OP with pathological fracture) as outcomes. The inverse-variance weighted (IVW) method was used to analyze the genetic causal association between the four indicators of iron status and OP. The heterogeneity of MR results was determined using IVW and MR-Egger methods. The pleiotropy of MR results was determined using MR-Egger regression. A leave-one-SNP-out test was performed to determine whether the MR results were affected by a single nucleotide polymorphism (SNP). The weighted median method was conducted to further validate our results.

RESULTS

Based on IVW, MR-Egger and weighted median models, we found no causal association between iron status (ferritin, iron, total iron binding capacity, or transferrin saturation) and OP (P_beta > 0.05 in all models). IVW and MR-Egger analysis of OP with pathological fracture and iron status indicators showed no potential genetic causal association (P_beta> 0.05 in the two analyses). The results of the weighted median were consistent with those of IVW (P_beta> 0.05 in all analyses). There was no potential genetic causal association between iron status and postmenopausal OP with pathological fracture based on serum iron (P_beta>0.05 in all models). No heterogeneity or horizontal pleiotropy was found in any of the analyses. None of the leave-one-out tests in the analyses found any SNP that could affect the results of MR.

CONCLUSION

Our results demonstrate that there is no genetic causal association between OP and iron status, but the effects of other factors were not excluded.

Serum hepcidin level, iron metabolism and osteoporosis in patients with rheumatoid arthritis

Hepcidin, a major regulator of iron metabolism and homeostasis, is regulated by inflammation. Recent studies have suggested that hepcidin and iron metabolism are involved in osteoporosis, and the aim of this study was to determine whether serum hepcidin levels are correlated with the degree of osteoporosis in patients with rheumatoid arthritis (RA). A total of 262 patients with RA (67.5 ± 11.4 years; 77.5% female) were enrolled. Serum iron, ferritin, and hepcidin levels were positively correlated each other. Multiple regression analyses revealed that the serum iron level was positively correlated with femoral T and Z scores, whereas the serum hepcidin level was not. Serum hepcidin level was correlated with the serum 25-hydroxy vitamin D level, which was in turn positively related to the femoral Z score. Serum hepcidin and serum iron were indirectly and directly related to osteoporosis in patients with RA.

Hepcidin deficiency causes bone loss through interfering with the canonical Wnt/β-catenin pathway via Forkhead box O3a

Objective

Hepcidin deficiency is known to cause body iron accumulation and bone microarchitecture defects, but the exact underlying mechanisms of hepcidin deficiency-induced bone loss remain unclear. Our objective was to understand the molecular mechanism of hepcidin deficiency-induced bone loss.

Methods

The bone phenotypes of wild type (WT) and hepcidin knockout (Hepcidin-KO) mice were measured by microcomputed tomography. The osteoclastic marker of the bone was measured by tartrate-resistant acid phosphatase staining. The osteoblastic marker of the bone was measured by immunostaining of osteocalcin. Primary osteoblastic and osteoclastic differentiation was performed using bone marrow cells. The mature osteoclast was determined by tartrate-resistant acid phosphatase staining, pit formation assay and relative gene expression. The mature osteoblast was determined by alkaline phosphatase activity, alkaline phosphatase staining, Alizarin Red staining and relative gene expression. The protein expression of β-catenin, TCF4/TCF7L2 and Forkhead box O3a (FOXO3a) was measured by Western blot and their combination by co-immunoprecipitation. In vivo study was performed by tail vein administration of FOXO3a-RNAi using an adeno-associated virus in Hepcidin-KO mice.

Results

We found that Hepcidin-KO mice exhibited iron accumulation and bone loss compared with WT mice. The osteoclastic differentiation of bone marrow-derived macrophages from Hepcidin-KO mice was not significantly different from that of bone marrow–derived macrophages from WT mice. However, the osteoblastic differentiation of bone marrow–derived mesenchymal stem cells from Hepcidin-KO mice was obviously decreased compared with that of bone marrow–derived mesenchymal stem cells from WT mice. Furthermore, it was confirmed in this study that upon hepcidin deficiency, β-catenin, TCF4/TCF7L2 and FOXO3a expression in bone tissues was not altered, but β-catenin combination with TCF4/TCF7L2 was strongly inhibited by β-catenin combination with FOXO3a, indicating that the canonical Wnt/β-catenin pathway was affected. Tail vein administration of FOXO3a-RNAi using an adeno-associated virus in Hepcidin-KO mice resulted in bone mass recovery.

Conclusion

These findings suggested that hepcidin deficiency might cause bone loss by interfering with the canonical Wnt/β-catenin pathway via FOXO3a, and FOXO3a inhibition would be a possible approach to treat hepcidin deficiency-induced bone loss.

The translational potential of this article

Hepcidin deficiency, as well as iron accumulation, has been considered as a risk factor for osteoporosis. For this kind of osteoporosis, inhibition of FOXO3a either by neutralized antibody or AAV-mediated RNAi, represents an effective and promising method.