Use of bone morphogenetic protein among patients undergoing fusion for degenerative diagnoses in the United States, 2002 to 2012

BACKGROUND CONTEXT

Use of bone morphogenetic protein (BMP) as an adjunct to spinal fusion surgery proliferated after Food and Drug Administration (FDA) approval in 2002. Major safety concerns emerged in 2008.

PURPOSE

The purpose of this study was to examine whether published concerns about the safety of BMP altered clinical practice.

STUDY DESIGN/SETTING

The study design involved the analysis of the National Inpatient Sample from 2002 through 2012.

PATIENT SAMPLE

Adults (older than 20 years) undergoing an elective fusion operation for common degenerative diagnoses were identified using codes from the International Classification of Diseases, ninth revision, Clinical Modification.

OUTCOME MEASURES

Outcome measures were proportion of cervical and lumbar fusion operations, over time, that involved BMP.

METHODS

We aggregated the data into a monthly time series and reported the proportion of cervical and lumbar fusion operations, over time, that involved BMP. Autoregressive Integrated Moving Average, a regression model for time series data, was used to test whether there was a statistically significant change in the overall rate of BMP use after an FDA Public Health Notification in 2008.

RESULTS

Use of BMP in spinal fusion procedures increased rapidly until 2008, involving up to 45.2% of lumbar and 13.5% of cervical fusions. Bone morphogenetic protein use significantly decreased after the 2008 FDA Public Health Notification and revelations of financial payments to surgeons involved in the pivotal FDA-approved trials. For lumbar fusion, the average annual increase was 7.9 percentage points per year from 2002 to 2008, followed by an average annual decrease of 11.7 percentage points thereafter (p≤.001). Use of BMP in cervical fusion increased 2.0% per year until the FDA Public Health Notification, followed by a 2.8% per year decrease (p=.035).

CONCLUSIONS

Use of BMP in spinal fusion surgery declined subsequent to published safety concerns and revelations of financial conflicts of interest for investigators involved in the pivotal clinical trials.

Growth factor treatment to overcome Alzheimer's dysfunctional signaling

The number of people suffering from Alzheimer's disease (AD) will increase as the world population ages, creating a huge socio-economic burden. The three pathophysiological hallmarks of AD are the cholinergic system dysfunction, the β-amyloid peptide deposition and the Tau protein hyperphosphorylation. Current treatments have only transient effects and each tends to concentrate on a single pathophysiological aspect of AD. This review first provides an overall view of AD in terms of its pathophysiological symptoms and signaling dysfunction. We then examine the therapeutic potential of growth factors (GFs) by showing how they can overcome the dysfunctional cell signaling that occurs in AD. Finally, we discuss new alternatives to GFs that help overcome the problem of brain uptake, such as small peptides, with evidence from some of our unpublished data on human neuronal cell line.

N-methyl pyrrolidone/bone morphogenetic protein-2 double delivery with in situ forming implants

Bone morphogenetic proteins (BMPs) are growth and differentiation factors involved during development in morphogenesis, organogenesis and later mainly in regeneration processes, in particular in bone where they are responsible for osteoinduction. For more than a decade, recombinant human (rh)BMP-2 has been used in the clinic for lumbar spinal fusion at non-physiological high dosages that appear to be causative for side effects, like male sterility. A possible strategy to reduce the effective amount of rhBMP-2 in the clinic is the co-delivery with an enhancer of BMPs' activity. In an earlier study, we showed that N-methylpyrrolidone (NMP) enhances BMP activity in vitro and in vivo. Here we report on the development of a slow and sustained double delivery of rhBMP-2 and NMP via an in situ forming implant based on poly(lactide-co-glycolide). The results showed that the release of NMP can be adjusted by varying the lactide/glycolide ratio and the polymer's molecular weight. The same applied to rhBMP-2, with release rates that could be sustained from two to three weeks. In the in vivo model of a critical size defect in the calvarial bone of rabbits, the implant containing 50mol% lactide performed better than the one having 75mol% lactide in terms of defect bridging and extent of bony regenerated area. In situ forming implants for the double delivery of the BMP enhancer NMP and rhBMP-2 appear to be promising delivery systems in bone regeneration.

BMPR2 loss in fibroblasts promotes mammary carcinoma metastasis via increased inflammation

Bone Morphogenetic Protein (BMP) receptors mediate a diverse range of signals to regulate both development and disease. BMP activity has been linked to both tumor promoting and suppressive functions in both tumor cells and their surrounding microenvironment. We sought to investigate the requirement for BMPR2 in stromal fibroblasts during mammary tumor formation and metastasis. We utilized FSP1 (Fibroblast Specific Protein-1) promoter driven Cre to genetically delete BMPR2 in mice expressing the MMTV.PyVmT mammary carcinoma oncogene. We found that abrogation of stromal BMPR2 expression via FSP1 driven Cre resulted in increased tumor metastasis. Additionally, similar to epithelial BMPR2 abrogation, stromal loss of BMPR2 results in increased inflammatory cell infiltration. We proceeded to isolate and establish fibroblast cell lines without BMPR2 and found a cell autonomous increase in inflammatory cytokine secretion. Fibroblasts were co-implanted with syngeneic tumor cells and resulted in accelerated tumor growth and increased metastasis when fibroblasts lacked BMPR2. We observed that the loss of BMPR2 results in increased chemokine expression, which facilitates inflammation by a sustained increase in myeloid cells. The chemokines increased in BMPR2 deleted cells correlated with poor outcome in human breast cancer patients. We conclude that BMPR2 has tumor suppressive functions in the stroma by regulating inflammation.

Experimental in vitro infection of rat osteoblasts with measles virus stimulates osteogenic differentiation

In this work we characterized the infection of a primary culture of rat osteoblastic lineage cells (OBCs) with measles virus (MeV) and the effect of infection on cell differentiation and maturation. Infection of OBCs with MeV led to high titers of infectivity released early after infection. Also, analysis of mRNAs corresponding to osteogenic differentiation markers like alkaline phosphatase (ALP), bone sialo-protein (BSP) and bone morphogenetic proteins (BMPs) 1-4-5-7 in OBCs revealed higher values (2-75-fold of increment) for infected cells in comparison with uninfected controls. Differentiation of OBCs in osteogenic medium prior to infection influenced the level of stimulation induced by MeV. Furthermore, treatment of OBCs with Ly294002, a PI3K/AKT inhibitor, increased viral titers, whereas treatment with 10μM or 100μM ATPγS diminished MeV multiplication. In addition, increments of osteogenic differentiation markers induced by MeV infection were not modified either by treatment with Ly294002 or ATPγS. These data provide the first evidence demonstrating that MeV can infect osteoblasts in vitro leading to osteoblastic differentiation, a key feature in bone pathogenic processes like otosclerosis.

Induction of bone formation in abdominal implants constituted by collagen sponges embedded with plant-based human transforming growth factor family proteins in ectopic dog model

Background

Trauma, osteomyelitis, bone tumour resections and congenital deformities are the main causes of bone deficiency in which autologous graft is the preferred treatment, but usually the bone supplies are limited.

Methods

An experimental model of heterotopic bone formation in the subcutaneous abdominal area of dogs was developed. This model consists in omentum wrapped implants constituted by collagen type 1 sponges embedded with demineralized bone powder, calcium cloride, thrombin and platelet rich plasma; the implant is totally converted in trabecular bone after four months of implantation. This model was improved by accelerating bone production, after the isolation of the most conspicuous histological constituents (inflammatory, bone and adipose tissues) by laser microdisection and purified from them RNA that was used to determine by RT-PCR the gene expression kinetics of the most important growth bone factors. Then, the most abundant and rapidly synthesized factors were produced by genetic engineering in tobacco plants.

Results

Bone morphogenetic proteins 2 and 7 and transforming growth factor-β1were the most rapidly and highly synthesized factors, and they were efficiently produced in a genetic engineering plant based system in tobacco leaves. Their incorporation as recombinant proteins in the scaffold collagen sponge induced in just one month mature heterotopic bone.

Conclusion

This study demonstrates for the first time that this plant system is able to produce recombinant bone growth factors in high amount and at low cost, and they were highly efficient to rapidly induce bone formation in abdominal implants potentially useful for autotransplantation.

Bone morphogenetic proteins and secreted frizzled related protein 2 maintain the quiescence of adult mammalian retinal stem cells

Rare retinal stem cells (RSCs) within the ciliary epithelium at the retinal margin of the adult mouse and human eyes can divide in vitro in the absence of growth factors to generate clonal, self-renewing spheres which can generate all the retinal cell types. Since no regenerative properties are seen in situ in the adult mammalian eye, we sought to determine the factors that are involved in the repression of endogenous RSCs. We discovered that factors secreted by the adult lens and cornea block the proliferation of adult RSCs in vitro. Bone morphogenetic protein (BMP)2, BMP4, and secreted frizzled related protein 2 were identified as principal effectors of the anti-proliferative effects on RSCs. As a similar induced quiescence was observed in vitro on both mouse and human RSCs, targeting these molecules in vivo may reactivate RSCs directly in situ in the eyes of the blind.

Bone morphogenetic protein-focused strategies to induce cytotoxicity in lung cancer cells

BACKGROUND

High bone morphogenetic protein (BMP)-2 expression in lung carcinoma correlates with poor patient prognosis. The present study explored strategies to repress BMP signaling.

MATERIALS AND METHODS

The cytotoxicity of BMP2-knockdown, dorsomorphin derivatives, and microRNAs was tested in transformed and non-transformed lung cells. Microarray analyses of 1,145 microRNAs in A549 lung adenocarcinoma cells and two other transformed lung cell types relative to BEAS-2B bronchial epithelial cells were performed.

RESULTS

Reduced BMP2 synthesis inhibited A549 cell growth. The dorsomorphin derivative LDN-193189, but not DMH1 or DMH4, was strongly cytotoxic towards A549 cells, but not towards BEAS-2B cells. Microarray analysis revealed that 106 miRNAs were down-regulated and 69 miRNAs were up-regulated in the three transformed lines. Three down-regulated miRNAs, hsa-mir-34b, hsa-mir-34c-3p, and hsa-miR-486-3p, repressed a BMP2 reporter gene and were cytotoxic in A549 cells, but not towards BEAS-2B cells.

CONCLUSION

The observed cytotoxicity suggests that reducing BMP signaling is a useful line of attack for therapy of lung cancer.

Autocrine activation of canonical BMP signaling regulates PD-L1 and PD-L2 expression in human dendritic cells

Bone morphogenetic proteins (BMPs) are multifunctional growth factors regulating differentiation and proliferation in numerous systems including the immune system. Previously, we described that the BMP signaling pathway is functional in human monocyte-derived dendritic cells (MoDCs), which were found to express both the specific receptors and the Smad proteins required for signal transduction. In this study, we provide evidence that human MoDCs produce BMP-4 and that this production is increased over the maturation process as is BMP signal transduction. When DCs are matured in the presence of an inhibitor of the BMP pathway, the expression of the maturation markers PD-L1 and PD-L2 is reduced, while cytokine production is not affected. As a result, these mature DCs present an augmented ability to stimulate both T cells and NK cells. Eventually, the inhibition of BMP signaling during maturation causes a reduced expression of IRF-1, a transcription factor that positively regulates the expression of PD-L1 and PD-L2. The present study indicates that the BMP signaling pathway regulates PD-L1 and PD-L2 expression in human MoDCs during the maturation process, probably through the IRF-1 transcription factor, and also points out that the manipulation of BMP signaling might considerably improve the immunogenicity of MoDCs used in immunotherapy.

Mechanisms of scaling in pattern formation

Many organisms and their constituent tissues and organs vary substantially in size but differ little in morphology; they appear to be scaled versions of a common template or pattern. Such scaling involves adjusting the intrinsic scale of spatial patterns of gene expression that are set up during development to the size of the system. Identifying the mechanisms that regulate scaling of patterns at the tissue, organ and organism level during development is a longstanding challenge in biology, but recent molecular-level data and mathematical modeling have shed light on scaling mechanisms in several systems, including Drosophila and Xenopus. Here, we investigate the underlying principles needed for understanding the mechanisms that can produce scale invariance in spatial pattern formation and discuss examples of systems that scale during development.

Spatial patterning of BMP-2 and BMP-7 on biopolymeric films and the guidance of muscle cell fate

In the cellular microenvironment, growth factor gradients are crucial in dictating cell fate. Towards developing materials that capture the native microenvironment we engineered biomimetic films that present gradients of matrix-bound bone morphogenetic proteins (BMP-2 and BMP-7). To this end layer-by-layer films composed of poly(L-lysine) and hyaluronan were combined in a simple microfluidic device enabling spatially controlled growth factor diffusion along the film. Linear long-range gradients of both BMPs induced the trans-differentiation of C2C12 myoblasts towards the osteogenic lineage in a dose dependent manner with a different signature for each BMP. The osteogenic marker alkaline phosphatase (ALP) increased in a linear manner for BMP-7 and non-linearly for BMP-2. Moreover, an increased expression of the myogenic marker troponin T was observed with decreasing matrix-bound BMP concentration, providing a substrate that it is both osteo- and myo-inductive. Lastly, dual parallel matrix-bound gradients of BMP-2 and -7 revealed a complete saturation of the ALP signal. This suggested an additive or synergistic effect of the two BMPs. This simple technology allows for determining quickly and efficiently the optimal concentration of matrix-bound growth factors, as well as for investigating the presentation of multiple growth factors in their solid-phase and in a spatially controlled manner.

Bone Morphogenetic Protein-2 Desensitizes MC3T3-E1 Osteoblastic Cells to Estrogen Through Transcriptional Downregulation of Estrogen Receptor 1

Background

Estrogens exert preferable effects on bone metabolism through two estrogen receptors (ERs), ER1 and ER2, which activate the transcription of a set of genes as ligand-dependent transcription factors. Thus, growth factors and hormones which modulate ER expression in the bone, if any, may possibly modulate the effect of estrogens on bone metabolism. However, research as to which of these molecules regulate the expression of ERs in osteoblasts has not been well documented.

Methods

A reporter assay system developed in this study was used to explore molecules that modulate ER1 expression in MC3T3-E1 osteoblastic cells. Gene expression was analyzed by reverse transcription-polymerase chain reaction.

Results

A pilot study using the reporter system revealed that bone morphogenetic protein (BMP)-2 negatively regulated ER1, but not ER2, expression in MC3T3-E1 cells. Consistently, estradiol-induced reporter activity via an estrogen responsive element was strongly suppressed in MC3T3-E1 cells pretreated with BMP-2.

Conclusions

BMP-2 desensitizes osteoblastic cells to estrogen through downregulation of ER1 expression.

Regulation of epithelial to mesenchymal transition by bone morphogenetic proteins

Epithelial to mesenchymal transition (EMT) is a process in which fully differentiated epithelial cells lose many of their epithelial characteristics and adopt features typical of mesenchymal cells, thus allowing cells to become migratory and invasive. EMT is a critical process in development and its role in cancer and fibrosis is becoming increasingly recognised. It is also becoming apparent that EMT is not just restricted to embryonic development and disease in adults, but in fact may be an important process for the maintenance and regeneration of adult tissue architecture. While transforming growth factor-β (TGF-β) is considered a prototypic inducer of EMT, relatively little is known about other signalling molecules that regulate EMT. Bone morphogenic proteins (BMPs) are members of the TGF-β superfamily and 20 different human BMPs have been identified. Originally named for their effects on bone, these proteins are now considered to be key morphogenetic signals that orchestrate tissue architecture throughout the body. BMP2, -4 and -7 are the best studied to date. There are disparate reports of the roles of BMPs in EMT during development, cancer and fibrosis. Here, we present an overview of this literature as well as the emerging role of EMT in tissue regeneration and the involvement of BMPs in regulating this process.

Bone morphogenetic proteins: periodontal regeneration

Periodontitis is an infectious inflammatory disease that results in attachment loss and bone loss. Regeneration of the periodontal tissues entails de novo formation of cementum, periodontal ligament, and alveolar bone. Several different approaches are currently being explored to achieve complete, reliable, and reproducible regeneration of periodontal tissues. The therapeutic management of new bone formation is one of the key issues in successful periodontal regeneration. Bone morphogenetic proteins form a unique group of proteins within the transforming growth factor superfamily of genes and have a vital role in the regulation in the bone induction and maintenance. The activity of bone morphogenetic proteins was first identified in the 1960s, but the proteins responsible for bone induction were unknown until the purification and cloning of human bone morphogenetic proteins in the 1980s, because of their osteoinductive potential. Bone morphogenetic proteins have gained a lot of interest as therapeutic agents for treating periodontal defects. A systematic search for data related to the use of bone morphogenetic proteins for the regeneration of periodontal defects was performed to recognize studies on animals and human (PUBMED, MEDLINE, COCHRANE, and Google search). All the studies included showed noticeable regeneration of periodontal tissues with the use of BMP.

Masquelet's procedure and bone morphogenetic protein in congenital pseudarthrosis of the tibia in children: a case series and meta-analysis

PURPOSE

A type 2 recombinant human bone morphogenetic protein (rhBMP2) and Masquelet's procedure were used in three children presenting with congenital pseudarthrosis of the tibia (CPT). Recent studies on CPT suggested the presence in situ of pathologic tissues promoting pseudarthrosis. The authors hypothesized that large segmental resection of pseudarthrosis could improve prognosis of the CPT. Masquelet's procedure and rhBMP2 have been advocated for the treatment of long bone defect.

METHOD

The authors report three cases of CPT in children treated with Masquelet's procedure and application of rhBMP2. They analyzed all published cases of CPT similarly treated.

RESULTS

In the present study, Masquelet's procedure did not improve the results in the treatment of CPT, but segmental bone reconstruction was possible. Bone healing was obtained in three out of the five applications of rhBMP2. In one case, the patient's parents asked for leg amputation. Analysis of the 33 published cases with the application of BMP in CPT points to a 62 % healing rate in this pathology.

CONCLUSION

The authors confirmed that segmental bone reconstruction is possible in CPT using Masquelet's procedure. In the literature, the success rate of the application of rhBMP in CPT appears to be lower than the healing rate usually reported without BMP. Nevertheless, the strict selection of patients, limited number of cases, and their heterogeneity make interpreting the results difficult. However, the theoretical risk which the children are exposed to during the use of BMP makes rigorous selection of the indications necessary. Finally, the interest of rhBMP2 application in Masquelet's procedure remained to be proven.

Future Prospects for Periodontal Bioengineering Using Growth Factors

Polypeptide growth factors have demonstrated strong potential to repair defects associated with teeth and dental implants. Over the past two decades, intense research efforts have led to the clinical development of several growth factors or biologic agents, including bone morphogenetic proteins, platelet-derived growth factor, fibroblast growth factors, and enamel matrix proteins. Several of these growth factors are now being used clinically for a variety of applications, such as the promotion of periodontal regeneration, sinus floor augmentation, and root coverage procedures. Although clinical results have been promising and growth factors add another dimension to clinical care, optimization of growth factor targeting approaches to periodontal wounds remains a challenge. Enhancement of growth factor local application to improve bioavailability, bioactivity, and allowance of three-dimensional reconstruction of complex anatomic defects is a goal. This article will highlight developments for growth factor delivery to better stimulate the wound healing response for periodontal and bone regeneration in the maxillofacial region.

Bone morphogenetic protein signaling: implications in urology

The bone morphogenetic proteins (BMPs), as members of the transforming growth factor-beta (TGF-beta) superfamily, not only control bone formation, but also regulate multiple key steps during embryonic development and differentiation. Furthermore, BMPs play critical roles in maintaining the homeostasis of the cardiovascular, pulmonary, reproductive, urogenital, and nervous systems in adult life. Like all members of the TGF-beta superfamily, BMP signaling is mediated through a heteromeric complex of type I and type II transmembrane serine/threonine kinase receptors. The subsequent signal transduction cascade includes either the canonical Smad-dependent or non-canonical Smad-independent pathways. Reflecting the critical function of BMPs, BMP signaling is tightly regulated at multiple steps by various mechanisms including extracellular endogenous antagonists, neutralizing antibodies/extracellular soluble receptor domains, small molecule inhibitors, cytoplasmic inhibitory Smads, and transcriptional co-repressors. Recently, dorsomorphin, the first small molecule inhibitor of BMP signaling, was identified and suggested as a useful tool for dissecting the mechanisms of signaling pathways and for developing novel therapeutics for diverse human diseases that are related to the BMP signaling pathways. In this article, we discuss various mechanisms involved in regulating BMP signaling pathways and their implications for urology.

Soluble and insoluble signals sculpt osteogenesis in angiogenesis

The basic tissue engineering paradigm is tissue induction and morphogenesis by combinatorial molecular protocols whereby soluble molecular signals are combined with insoluble signals or substrata. The insoluble signal acts as a three-dimensional scaffold for the initiation of de novo tissue induction and morphogenesis. The osteogenic soluble molecular signals of the transforming growth factor-β (TGF-β) supergene family, the bone morphogenetic/osteogenic proteins (BMPs/OPs) and, uniquely in the non-human primate Papio ursinus (P. ursinus), the three mammalian TGF-β isoforms induce bone formation as a recapitulation of embryonic development. In this paper, I discuss the pleiotropic activity of the BMPs/OPs in the non-human primate P. ursinus, the induction of bone by transitional uroepithelium, and the apparent redundancy of molecular signals initiating bone formation by induction including the three mammalian TGF-β isoforms. Amongst all mammals tested so far, the three mammalian TGF-β isoforms induce endochondral bone formation in the non-human primate P. ursinus only. Bone tissue engineering starts by erecting scaffolds of biomimetic biomaterial matrices that mimic the supramolecular assembly of the extracellular matrix of bone. The molecular scaffolding lies at the hearth of all tissue engineering strategies including the induction of bone formation. The novel concept of tissue engineering is the generation of newly formed bone by the implantation of "smart" intelligent biomimetic matrices that per se initiate the ripple-like cascade of bone differentiation by induction without exogenously applied BMPs/OPs of the TGF-β supergene family. A comprehensive digital iconographic material presents the modified tissue engineering paradigm whereby the induction of bone formation is initiated by intelligent smart biomimetic matrices that per se initiate the induction of bone formation without the exogenous application of the soluble osteogenic molecular signals. The driving force of the intrinsic induction of bone formation by bioactive biomimetic matrices is the shape of the implanted substratum. The language of shape is the language of geometry; the language of geometry is the language of a sequence of repetitive concavities, which biomimetizes the remodelling cycle of the primate osteonic bone.

Expression of bone morphogenetic protein 2 and 6 and clinical significance in liver carcinoma

AIM: To investigate the expression of bone morphogenetic protein 2 and 6 (BMP-2, 6) mRNA in human liver cancer, and to explore its relationship with biologic action in liver cancer.

METHODS: The expressions of BMP-2 and BMP-6 in 30 carcinoma liver tissue samples and 30 normal liver tissue samples were detected and analyzed by reverse transcriptase PCR and Western blot.

RESULTS: The relative level of BMP-2 and BMP-6 mRNA was significantly lower in the normal liver tissue samples than in the liver carcinoma tissue samples (0.3245 ± 0.1127 vs 0.8298 ± 0.1187, 0.2947 ± 0.1853 vs 0.7145 ± 0.1373, both P < 0.05). The relative level of BMP-2 and BMP-6 mRNA in III, IV stages of liver cancer was significantly higher than that in I, II stages (0.92431 ± 0.1234 vs 0.69355 ± 0.1925, 0.8354 ± 0.1423 vs 0.6043 ± 0.1234, both P < 0.05). The expression level of BMP-2 and BMP-6 protein were significantly higher in the metastatic liver cancer compared with that without metastasis (0.9854 ± 0.2888 vs 0.6244 ± 0.3087, 0.9076 ± 0.1276 vs 0.5678 ± 0.2493, both P < 0.01).

CONCLUSION: Up-regulated expressions of BMP-2 and BMP-6 play an important role in liver cancer. It may have the active role in invasion and metastasis of liver cancer.

Function of the hemochromatosis protein HFE: Lessons from animal models

Hereditary hemochromatosis (HH) is caused by chronic hyperabsorption of dietary iron. Progressive accumulation of excess iron within tissue parenchymal cells may lead to severe organ damage. The most prevalent type of HH is linked to mutations in the HFE gene, encoding an atypical major histocompatibility complex classImolecule. Shortly after its discovery in 1996, the hemochromatosis protein HFE was shown to physically interact with transferrin receptor 1 (TfR1) and impair the uptake of transferrin-bound iron in cells. However, these findings provided no clue why HFE mutations associate with systemic iron overload. It was later established that all forms of HH result from misregulation of hepcidin expression. This liver-derived circulating peptide hormone controls iron efflux from duodenal enterocytes and reticuloendothelial macrophages by promoting the degradation of the iron exporter ferroportin. Recent studies with animal models of HH uncover a crucial role of HFE as a hepatocyte iron sensor and upstream regulator of hepcidin. Thus, hepatocyte HFE is indispensable for signaling to hepcidin, presumably as a constituent of a larger iron-sensing complex. A working model postulates that the signaling activity of HFE is silenced when the protein is bound to TfR1. An increase in the iron saturation of plasma transferrin leads to displacement of TfR1 from HFE and assembly of the putative iron-sensing complex. In this way, iron uptake by the hepatocyte is translated into upregulation of hepcidin, reinforcing the concept that the liver is the major regulatory site for systemic iron homeostasis, and not merely an iron storage depot.