Endoplasmic reticulum stress controls iron metabolism through TMPRSS6 repression and hepcidin mRNA stabilization by RNA-binding protein HuR

Effects of genetic ablation and pharmacological inhibition of HuR on gene expression, iron metabolism, and hormone levels

BACKGROUND

HuR/ELAV1, a ubiquitous RNA-binding protein, belongs to the RNA-binding protein family and is crucial for stabilizing and regulating the translation of various mRNA targets, influencing gene expression. Elevated HuR levels are associated with multiple disorders, including cancer and neurodegenerative diseases. Despite the identification of small molecule inhibitors targeting HuR, their detailed characterization remains limited. Recently, Eltrombopag, an FDA-approved drug for immune thrombocytopenic purpura and chemotherapy-induced thrombocytopenia, emerged as a potential HuR inhibitor. However, the specific molecular pathways influenced by both HuR and Eltrombopag are not fully understood.

RESULTS

Our study demonstrates that Eltrombopag operates via HuR inhibition, affecting gene expression regulation at the posttranscriptional level. We show that both HuR knockout and Eltrombopag treatment modulate iron metabolism by decreasing ferritin heavy chain (FTH1) and light chain (FTL) synthesis while increasing the expression of iron-regulatory protein 2 (IRP2), a key regulator of ferritin translation. Additionally, HuR inhibition reduces the levels of glycoprotein hormones, alpha polypeptide (CGA), a marker associated with hormone-induced tumors, suggesting a potential use of Eltrombopag in treatment of cancers overexpressing CGA. We observed that the main of control is manifested at the level of translation inhibition, with proteasome-mediated regulation also playing an important role.

CONCLUSIONS

These findings uncover novel posttranscriptional mechanisms governed by HuR and its inhibitor, elucidating pathways relevant to HuR-mediated regulation and molecular therapies aimed at targeting this protein.

Spotlight on iron and ferroptosis: research progress in diabetic retinopathy

Iron, as the most abundant metallic element within the human organism, is an indispensable ion for sustaining life and assumes a pivotal role in governing glucose and lipid metabolism, along with orchestrating inflammatory responses. The presence of diabetes mellitus (DM) can induce aberrant iron accumulation within the corporeal system. Consequentially, iron overload precipitates a sequence of important adversities, subsequently setting in motion a domino effect wherein ferroptosis emerges as the utmost pernicious outcome. Ferroptosis, an emerging variant of non-apoptotic regulated cell death, operates independently of caspases and GSDMD. It distinguishes itself from alternative forms of controlled cell death through distinctive morphological and biochemical attributes. Its principal hallmark resides in the pathological accrual of intracellular iron and the concomitant generation of iron-driven lipid peroxides. Diabetic retinopathy (DR), established as the predominant cause of adult blindness, wields profound influence over the well-being and psychosocial strain experienced by afflicted individuals. Presently, an abundance of research endeavors has ascertained the pervasive engagement of iron and ferroptosis in the microangiopathy inherent to DR. Evidently, judicious management of iron overload and ferroptosis in the early stages of DR bears the potential to considerably decelerate disease progression. Within this discourse, we undertake a comprehensive exploration of the regulatory mechanisms governing iron homeostasis and ferroptosis. Furthermore, we expound upon the subsequent detriments induced by their dysregulation. Concurrently, we elucidate the intricate interplay linking iron overload, ferroptosis, and DR. Delving deeper, we engage in a comprehensive deliberation regarding strategies to modulate their influence, thereby effecting prospective interventions in the trajectory of DR's advancement or employing them as therapeutic modalities.

Protective Role for Smooth Muscle Cell Hepcidin in Abdominal Aortic Aneurysm

BACKGROUND

Hepcidin is a liver-derived hormone that controls systemic iron homeostasis, by inhibiting the iron exporter ferroportin in the gut and spleen, respective sites of iron absorption and recycling. Hepcidin is also expressed ectopically in the context of cardiovascular disease. However, the precise role of ectopic hepcidin in underlying pathophysiology is unknown. In patients with abdominal aortic aneurysm (AAA), hepcidin is markedly induced in smooth muscle cells (SMCs) of the aneurysm wall and inversely correlated with the expression of LCN2 (lipocalin-2), a protein implicated in AAA pathology. In addition, plasma hepcidin levels were inversely correlated with aneurysm growth, suggesting hepcidin has a potential disease-modifying role.

METHODS

To probe the role of SMC-derived hepcidin in the setting of AAA, we applied AngII (Angiotensin-II)-induced AAA model to mice harbouring an inducible, SMC-specific deletion of hepcidin. To determine whether SMC-derived hepcidin acted cell-autonomously, we also used mice harboring an inducible SMC-specific knock-in of hepcidin-resistant ferroportinC326Y. The involvement of LCN2 was established using a LCN2-neutralizing antibody.

RESULTS

Mice with SMC-specific deletion of hepcidin or knock-in of hepcidin-resistant ferroportinC326Y had a heightened AAA phenotype compared with controls. In both models, SMCs exhibited raised ferroportin expression and reduced iron retention, accompanied by failure to suppress LCN2, impaired autophagy in SMCs, and greater aortic neutrophil infiltration. Pretreatment with LCN2-neutralizing antibody restored autophagy, reduced neutrophil infiltration, and prevented the heightened AAA phenotype. Finally, plasma hepcidin levels were consistently lower in mice with SMC-specific deletion of hepcidin than in controls, indicating that SMC-derived hepcidin contributes to the circulating pool in AAA.

CONCLUSIONS

Hepcidin elevation in SMCs plays a protective role in the setting of AAA. These findings are the first demonstration of a protective rather than deleterious role for hepcidin in cardiovascular disease. They highlight the need to further explore the prognostic and therapeutic value of hepcidin outside disorders of iron homeostasis.

EPA and DHA confer protection against deoxynivalenol-induced endoplasmic reticulum stress and iron imbalance in IPEC-1 cells

This study assessed the molecular mechanism of EPA or DHA protection against intestinal porcine epithelial cell line 1 (IPEC-1) cell damage induced by deoxynivalenol (DON). The cells were divided into six groups, including the CON group, the EPA group, the DHA group, the DON group, the EPA + DON group and the DHA + DON group. RNA sequencing was used to investigate the potential mechanism, and qRT-PCR was employed to verify the expression of selected genes. Changes in ultrastructure were used to estimate pathological changes and endoplasmic reticulum (ER) injury in IPEC-1 cells. Transferrin receptor 1 (TFR1) was tested by ELISA. Fe²⁺ and malondialdehyde (MDA) contents were estimated by spectrophotometry, and reactive oxygen species (ROS) was assayed by fluorospectrophotometry. RNA sequencing analysis showed that EPA and DHA had a significant effect on the expression of genes involved in ER stress and iron balance during DON-induced cell injury. The results showed that DON increased ER damage, the content of MDA and ROS, the ratio of X-box binding protein 1s (XBP-1s)/X-box binding protein 1u (XBP-1u), the concentration of Fe²⁺ and the activity of TFR1. However, the results also showed that EPA and DHA decreased the ratio of XBP-1s/XBP-1u to relieve DON-induced ER damage of IPEC-1 cells. Moreover, EPA and DHA (especially DHA) reversed the factors related to iron balance. It can be concluded that EPA and DHA reversed IPEC-1 cell damage induced by DON. DHA has the potential to protect IPEC-1 cells from DON-induced iron imbalance by inhibiting ER stress.

20 years of Hepcidin: How far we have come

Twenty years ago the discovery of hepcidin deeply changed our understanding of the regulation of systemic iron homeostasis. It is now clear that hepcidin orchestrates systemic iron levels by controlling the amount of iron exported into the bloodstream through ferroportin. Hepcidin expression is increased in situations where systemic iron levels should be reduced, such as in iron overload and infection. Conversely, hepcidin is repressed during iron deficiency, hypoxia or expanded erythropoiesis, to increase systemic iron availability and sustain erythropoiesis. In this review, we will focus on molecular mechanisms of hepcidin regulation and on the pathological consequences of their disruption.

Physiological and pathophysiological mechanisms of hepcidin regulation: clinical implications for iron disorders

The discovery of hepcidin has provided a solid foundation for understanding the mechanisms of systemic iron homeostasis and the aetiologies of iron disorders. Hepcidin assures the balance of circulating and stored iron levels for multiple physiological processes including oxygen transport and erythropoiesis, while limiting the toxicity of excess iron. The liver is the major site where regulatory signals from iron, erythropoietic drive and inflammation are integrated to control hepcidin production. Pathologically, hepcidin dysregulation by genetic inactivation, ineffective erythropoiesis, or inflammation leads to diseases of iron deficiency or overload such as iron-refractory iron-deficiency anaemia, anaemia of inflammation, iron-loading anaemias and hereditary haemochromatosis. In the present review, we discuss recent insights into the molecular mechanisms governing hepcidin regulation, how these pathways are disrupted in iron disorders, and how this knowledge is being used to develop novel diagnostic and therapeutic strategies.

Matriptase-2 and Hemojuvelin in Hepcidin Regulation: In Vivo Immunoblot Studies in Mask Mice

Matriptase-2, a serine protease expressed in hepatocytes, is a negative regulator of hepcidin expression. The purpose of the study was to investigate the interaction of matriptase-2 with hemojuvelin protein in vivo. Mice lacking the matriptase-2 proteolytic activity (mask mice) display decreased content of hemojuvelin protein. Vice versa, the absence of hemojuvelin results in decreased liver content of matriptase-2, indicating that the two proteins interact. To further characterize the role of matriptase-2, we investigated iron metabolism in mask mice fed experimental diets. Administration of iron-enriched diet increased liver iron stores as well as hepcidin expression. Treatment of iron-overloaded mask mice with erythropoietin increased hemoglobin and hematocrit, indicating that the response to erythropoietin is intact in mask mice. Feeding of an iron-deficient diet to mask mice significantly increased spleen weight as well as the splenic content of erythroferrone and transferrin receptor proteins, indicating stress erythropoiesis. Liver hepcidin expression was decreased; expression of Id1 was not changed. Overall, the results suggest a complex interaction between matriptase-2 and hemojuvelin, and demonstrate that hepcidin can to some extent be regulated even in the absence of matriptase-2 proteolytic activity.

The Hepatitis C virus NS5A and core proteins exert antagonistic effects on HAMP gene expression: the hidden interplay with the MTF-1/MRE pathway

Hepcidin, a 25-amino acid peptide encoded by the HAMP gene and produced mainly by hepatocytes and macrophages, is a mediator of innate immunity and the central iron-regulatory hormone. Circulating hepcidin controls iron efflux by inducing degradation of the cellular iron exporter ferroportin. HCV infection is associated with hepatic iron overload and elevated serum iron, which correlate with poor antiviral responses. The HCV nonstructural NS5A protein is known to function in multiple aspects of the HCV life cycle, probably exerting its activity in concert with cellular factor(s). In this study, we attempted to delineate the effect of HCV NS5A on HAMP gene expression. We observed that transient transfection of hepatoma cell lines with HCV NS5A resulted in down-regulation of HAMP promoter activity. A similar effect was evident after transduction of Huh7 cells with a recombinant baculovirus vector expressing NS5A protein. We proceeded to construct an NS5A-expressing stable cell line, which also exhibited down-regulation of HAMP gene promoter activity and significant reduction of HAMP mRNA and hepcidin protein levels. Concurrent expression of HCV core protein, a well-characterized hepcidin inducer, revealed antagonism between those two proteins for hepcidin regulation. In attempting to identify the pathways involved in NS5A-driven reduction of hepcidin levels, we ruled out any NS5A-induced alterations in the expression of the well-known hepcidin inducers SMAD4 and STAT3. Further analysis linked the abundance of intracellular zinc ions and the deregulation of the MTF-1/MRE/hepcidin axis with the observed phenomenon. This effect could be associated with distinct phases in HCV life cycle.