Insulin stimulates cellular iron uptake and causes the redistribution of intracellular transferrin receptors to the plasma membrane

The Functions of SARS-CoV-2 Receptors in Diabetes-Related Severe COVID-19

Angiotensin-converting enzyme 2 (ACE2) is considered a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) receptor of high importance, but due to its non-ubiquitous expression, studies of other proteins that may participate in virus internalisation have been undertaken. To date, many alternative receptors have been discovered. Their functioning may provide an explanation for some of the events observed in severe COVID-19 that cannot be directly explained by the model in which ACE2 constitutes the central point of infection. Diabetes mellitus type 2 (T2D) can induce severe COVID-19 development. Although many mechanisms associated with ACE2 can lead to increased SARS-CoV-2 virulence in diabetes, proteins such as basigin (CD147), glucose-regulated protein 78 kDa (GRP78), cluster of differentiation 4 (CD4), transferrin receptor (TfR), integrins α5β1/αvβ3, or ACE2 co-receptors neuropilin 2 (NRP2), vimentin, and even syalilated gangliosides may also be responsible for worsening the COVID-19 course. On the other hand, some others may play protective roles. Understanding how diabetes-associated mechanisms can induce severe COVID-19 via modification of virus receptor functioning needs further extensive studies.

Exploring Potential Mechanisms Accounting for Iron Accumulation in the Central Nervous System of Patients with Alzheimer's Disease

Elevated levels of iron occur in both cortical and subcortical regions of the CNS in patients with Alzheimer's disease. This accumulation is present early in the disease process as well as in more advanced stages. The factors potentially accounting for this increase are numerous, including: (1) Cells increase their uptake of iron and reduce their export of iron, as iron becomes sequestered (trapped within the lysosome, bound to amyloid β or tau, etc.); (2) metabolic disturbances, such as insulin resistance and mitochondrial dysfunction, disrupt cellular iron homeostasis; (3) inflammation, glutamate excitotoxicity, or other pathological disturbances (loss of neuronal interconnections, soluble amyloid β, etc.) trigger cells to acquire iron; and (4) following neurodegeneration, iron becomes trapped within microglia. Some of these mechanisms are also present in other neurological disorders and can also begin early in the disease course, indicating that iron accumulation is a relatively common event in neurological conditions. In response to pathogenic processes, the directed cellular efforts that contribute to iron buildup reflect the importance of correcting a functional iron deficiency to support essential biochemical processes. In other words, cells prioritize correcting an insufficiency of available iron while tolerating deposited iron. An analysis of the mechanisms accounting for iron accumulation in Alzheimer's disease, and in other relevant neurological conditions, is put forward.

A New Concept in Antidiabetic Therapeutics: A Concerted Removal of Labile Iron and Intracellular Deposition of Zinc

BACKGRUOUND

The inflammatory process is known to be an integral part of the pathophysiology of type 2 diabetes mellitus (T2DM). The "labile," redox-active iron, serving as a catalyst in Fenton reaction, producing the deleterious reactive oxygen species, triggering and maintaining inflammation, is hypothesized to play a causative role in this process. Concenter Biopharma continued the development of a new platform of iron chelators (Zygosids), first initiated at the Hebrew University of Jerusalem, Israel (HUJI), acting via the novel mechanism, based on a sequestration of the labile redox-active iron and its substitution by zinc or gallium. The mode of action of Zygosids is based on the higher affinity of the metal-binding moiety of the complex to Fe3+ in comparison to already bound ion, leading to rapid release of the ion of another metal and chelation of Fe3+. Concomitantly, zinc ion, released by the complex, is known for its antidiabetic and anti-inflammatory role.

METHODS

The therapeutic effect of zinc-desferrioxamine (Zygosid-50) and gallium-desferrioxamine, was tested on fat sand rat (Psammomys obesus) model of diet-induced T2DM and on Leprdb transgenic diabetic mice.

RESULTS

Zygosids demonstrated an ability to noticeably reduce blood glucose and insulin levels and improve the lipid profile. Moreover, an ability to mitigate insulin resistance by >90% was shown on the sand rat model. In addition, a potent anti-inflammatory effect, expressed as a diminishment of the proinflammatory cytokines in tissue levels, was demonstrated.

CONCLUSION

Zygosids demonstrated robust therapeutic efficacy in treatment of T2DM. Importantly, no adverse effects were detected, in all the experiments, indicating high safety profile.

High-throughput proteomics uncovers exercise training and type 2 diabetes-induced changes in human white adipose tissue

White adipose tissue (WAT) is important for metabolic homeostasis. We established the differential proteomic signatures of WAT in glucose-tolerant lean and obese individuals and patients with type 2 diabetes (T2D) and the response to 8 weeks of high-intensity interval training (HIIT). Using a high-throughput and reproducible mass spectrometry-based proteomics pipeline, we identified 3773 proteins and found that most regulated proteins displayed progression in markers of dysfunctional WAT from lean to obese to T2D individuals and were highly associated with clinical measures such as insulin sensitivity and HbA1c. We propose that these distinct markers could serve as potential clinical biomarkers. HIIT induced only minor changes in the WAT proteome. This included an increase in WAT ferritin levels independent of obesity and T2D, and WAT ferritin levels were strongly correlated with individual insulin sensitivity. Together, we report a proteomic signature of WAT related to obesity and T2D and highlight an unrecognized role of human WAT iron metabolism in exercise training adaptations.

Iron, glucose and fat metabolism and obesity: an intertwined relationship

A bidirectional relationship exists between adipose tissue metabolism and iron regulation. Total body fat, fat distribution and exercise influence iron status and components of the iron-regulatory pathway, including hepcidin and erythroferrone. Conversely, whole body and tissue iron stores associate with fat mass and distribution and glucose and lipid metabolism in adipose tissue, liver, and muscle. Manipulation of the iron-regulatory proteins erythroferrone and erythropoietin affects glucose and lipid metabolism. Several lines of evidence suggest that iron accumulation and metabolism may play a role in the development of metabolic diseases including obesity, type 2 diabetes, hyperlipidaemia and non-alcoholic fatty liver disease. In this review we summarise the current understanding of the relationship between iron homoeostasis and metabolic disease.

Nutritional Aspects of Iron in Health and Disease

Dietary iron assimilation is critical for health and essential to prevent iron-deficient states and related comorbidities, such as anemia. The bioavailability of iron is generally low, while its absorption and metabolism are tightly controlled to satisfy metabolic needs and prevent toxicity of excessive iron accumulation. Iron entry into the bloodstream is limited by hepcidin, the iron regulatory hormone. Hepcidin deficiency due to loss-of-function mutations in upstream gene regulators causes hereditary hemochromatosis, an endocrine disorder of iron overload characterized by chronic hyperabsorption of dietary iron, with deleterious clinical complications if untreated. The impact of high dietary iron intake and elevated body iron stores in the general population is not well understood. Herein, we summarize epidemiological data suggesting that a high intake of heme iron, which is abundant in meat products, poses a risk factor for metabolic syndrome pathologies, cardiovascular diseases, and some cancers. We discuss the clinical relevance and potential limitations of data from cohort studies, as well as the need to establish causality and elucidate molecular mechanisms.

Ferritin, transferrin, and transferrin receptor in relation to metabolic obesity phenotypes: Findings from the China Health and Nutrition Survey

Background

This study aimed to explore the relationship between iron markers and metabolic obesity phenotypes and the role of age.

Methods

Data were from the China Health and Nutrition Survey 2009. Metabolic obesity phenotypes included metabolically healthy with normal weight (MHNW), metabolically unhealthy with normal weight (MUNW), metabolically healthy with overweight/obesity (MHO), and metabolically unhealthy with overweight/obesity (MUO). Iron markers including ferritin, transferrin, and soluble transferrin receptor were calculated as Log and quartered. The linear regression and multinomial logistic regression were used to explore the association of iron markers with age and metabolic obesity phenotypes, respectively.

Results

Ferritin was linearly related with age, with β (95% confidence interval, CI) of 0.029 (0.027 to 0.032) and -0.005 (-0.007 to -0.002) for women and men. Transferrin was negatively associated with age in both men and women (β < -0.011). Furthermore, compared with participants in the quartile 1 ferritin group, those in the quartile 4 had increased odds of MUNW, MHO, and MUO, with odds ratio and 95% confidence interval (OR, 95% CI) of 3.06 (2.20 to 4.25), 1.66 (1.35 to 2.05), and 5.27 (4.17 to 6.66). Transferrin showed similar relationships with MUNW, MUO, and MHO; whereas transferrin receptor showed no significance. We also found joint associations of ferritin and transferrin with MUNW, MUO, and MHO. The interactive effect of ferritin and transferrin on MUO was significant (P = 0.015).

Conclusion

Increased ferritin and transferrin were associated with MUNW, MHO, and MUO. Age should be considered when investigating iron.

Increased Serum Soluble Transferrin Receptor Levels Were Associated With High Prevalence of Cardiovascular Diseases: Insights From the National Health and Nutrition Examination Survey 2017–2018

Background: Iron deficiency is common in cardiovascular diseases (CVD), e.g., heart failure and coronary heart disease. Soluble transferrin receptor (sTfR) is a promising marker representing unmet cellular iron demands. However, whether higher serum sTfR is associated with increased risk of CVDs needs further investigation.

Methods: In the present cross-sectional study, we analyzed data of 4,867 adult participants of the National Health and Nutrition Examination Survey (NHANES) 2017–2018. Linear regression models were employed to identify possible correlations between sTfR and other characteristics. The association between sTfR and CVDs was assessed with univariable and multivariable logistics regression models.

Results: The prevalence of CVDs was 9.5% among participants, and higher sTfR levels were found in participants with CVDs (p < 0.001). Linear regression models revealed positive associations between sTfR and age, body mass index, systolic blood pressure, glycated hemoglobulin A1c, and insulin resistance (all p < 0.001). In the multivariable logistics regression model, the adjusted odds ratio of sTfR for CVDs was 2.05 (per 1 log₂ mg/L, 95% confidence interval: 1.03∼4.05, p = 0.046). Further subgroup analysis identified the associations of sTfR and CVDs were only significant in participants ≥60 years old, or with hypertension (all p < 0.05).

Conclusion: Our study demonstrated that increased serum sTfR levels were associated with a high prevalence of cardiovascular diseases.

Trace elements concentration in adipose tissue and the risk of incident type 2 diabetes in a prospective adult cohort

The aim of this work was to study the associations of adipose tissue trace element concentrations with type 2 diabetes (T2DM) incidence over a 16-year follow-up period in an adult cohort from Southern Spain. 16-year T2DM incidence was gathered from hospital records. Chemical analyses of Cr, V, Zn, Fe, Cu and Se in adipose tissue were performed using inductively coupled plasma mass spectrometry. Multivariable Cox-regression models were used. Complementary cross-sectional analyses with markers of glucose homeostasis at recruitment were performed by multivariable linear regression. Out of 214 participants, 39 developed T2DM during the follow-up. Adipose tissue concentrations of Fe (HR = 1.97, 95% CI: 0.99 to 2.58, p = 0.057), Cr (HR = 1.58, 95% CI: 1.07-2.33, p = 0.022) and Cu (HR = 1.61, 95% CI: 1.01-2.58, p = 0.046) were individually associated with T2DM incidence. When Fe, Cr and Cu were simultaneously entered in a model, only Cr was significantly associated with T2DM incidence (HR = 1.68, 95% CI: 1.02-2.76, p = 0.041). Furthermore, adipose tissue V (β = 0.283, p = 0.004) and Zn (β = 0.217, p = 0.028) concentrations were positively associated with β-pancreatic cell function (HOMA-β), while Se showed an inverse association (β = -0.049, p = 0.027). Although further research is warranted on the potential mechanisms of action, our results suggest that adipose tissue concentrations of certain trace elements (particularly Fe, Cr and Cu) are associated with the risk of incident T2DM, while V and Zn might have a protective effect. These biomarkers might complement prediction algorithms and contribute to identify patients with an increased risk of T2DM.

Impact of Environmental and Lifestyle Use of Chromium on Male Fertility: Focus on Antioxidant Activity and Oxidative Stress

Male reproductive tissues are strongly susceptible to several environmental and lifestyle stressors. In general, male reproductive health is highly sensitive to oxidative stress, which results in reversible and/or irreversible changes in testosterone-producing cells, spermatogenesis, and sperm quality. Chromium compounds are widely used in the +3 and +6 valence states, as food supplements, and in the industrial field, respectively. Chromium (III) compounds, i.e., Cr(III)-tris-picolinate, [Cr(pic)₃], known as chromium picolinate, are used as nutritional supplements for the control of diabetes, body weight, and muscular growth. However, previous studies showed that animal models exposed to chromium picolinate experienced degenerative changes in spermatogenesis. Contradictory results are documented in the literature and deserve discussion. Furthermore, the long-term effects of chromium picolinate on the antioxidant system of treated subjects have not been properly studied. Comprehensive studies on the role of this compound will help to establish the safe and useful use of chromium supplementation. On the other hand, chromium (VI) compounds are widely used in several industries, despite being well-known environmental pollutants (i.e., welding fumes). Chromium (VI) is known for its deleterious effects on male reproductive health as toxic, carcinogenic, and mutagenic. Previous studies have demonstrated severe lesions to mouse spermatogenesis after exposure to chromium (VI). However, workers worldwide are still exposed to hexavalent chromium, particularly in electronics and military industries. Data from the literature pinpoints mechanisms of oxidative stress induced by chromium compounds in somatic and germ cells that lead to apoptosis, thus underlining the impairment of fertility potential. In this review, we analyze the benefits and risks of chromium compounds on male fertility, as well as the mechanisms underlying (in)fertility outcomes. Although supplements with antioxidant properties may maximize male fertility, adverse effects need to be investigated and discussed.

Dietary iron to total energy intake ratio and type 2 diabetes incidence in a longitudinal 12-year analysis of the Korean Genome and Epidemiology Cohort Study

PURPOSE

Recent study found iron consumption has been associated with an increased risk of type 2 diabetes (T2DM). Even though, high iron intake is correlated with total caloric intake, most studies have evaluated the individual effect of iron and total caloric intake. The aim of this study was to investigate the effect of iron intake, in conjunction with total energy intake, on developing T2DM. We also investigated the interactions between dietary iron and energy ratios (IERs) and iron-related single nucleotide polymorphisms (SNPs) in the development of T2DM.

METHODS

The study was carried out in Ansan and Ansung, Korea, between March 2001 and December 2014. A total of 6413 participants (3073 men and 3340 women), aged 40-69 years, were enrolled in this study. The mean follow-up period was 8.4 years. The study population was divided into quartiles based on IERs with cut-off points at 4.54, 5.41, and 6.29. The odds ratios (ORs) for new-onset T2DM were calculated across each quartile of IERs and a random forest model was constructed using the default settings to predict new-onset T2DM. To confirm the interaction among IERs, SNPs, and the incidence of T2DM, we measured the predictive power of new-onset T2DM using IER and six SNPs in genes related to iron metabolism [rs855791 (TPMRSS6), rs38116479 (TF), rs1799852 (TF), rs2280673, rs1799945 (HFT), rs180562 (HFE)].

RESULTS

The prevalence of T2DM was 762 (11.8%). IERs showed a positive association with T2DM. The ORs were 1.30 (95% CI 1.02-1.67), 1.20 (95% CI 0.94-1.56), and 1.43 (95% CI 1.11-1.86) across the IER quartiles after adjusting for non-dietary and dietary metabolic risk factors. When the IER was 1.89-fold higher than the reference group, the risk of developing T2DM increased by 43% (OR 1.43; 95% CI 1.11-1.86).

CONCLUSION

A higher IER was positively associated with developing T2DM independent of dietary or non-dietary risk factors. We also found the possible interactions between the identified SNPs and iron intake in relations to T2DM.

Diabesity negatively affects transferrin saturation and iron status. The DICARIVA study

AIMS

The relationship between iron status, obesity and type 2 diabetes mellitus (T2DM) has scarcely been tested. This study hypothesizes that patients with obesity and T2DM have altered iron metabolism.

METHODS

537 T2DM patients were selected from the cross-sectional DICARIVA study excluding patients with high-sensitivity-C-reactive-protein (hs-CRP) ≥  10 mg/L. Three groups according to body mass index (BMI) and waist perimeter (WP) were analysed: a) BMI < 30 kg/m², non-high WP (n = 105); b) BMI < 30 kg/m², high WP (n = 202); and c) diabesity, BMI ≥  30 kg/m², high WP (n = 230). Group differences on cardiometabolic and iron status markers were tested.

RESULTS

Women had significantly lower iron, ferritin, and transferrin saturation (TSAT) but higher transferrin and total iron binding capacity than men. Triglycerides/HDL-c ratio, as insulin-resistance (IR) marker, was higher in men while hs-CRP in women. TSAT was inversely related to BMI and hs-CRP. The diabesity group showed the highest hs-CRP (p < 0.001) and IR (p < 0.001) with the lowest TSAT (p = 0.003).

CONCLUSIONS

Low TSAT was highly prevalent in diabesity, mainly in women, suggesting that IR, inflammation, and abdominal adiposity alter iron transport and accumulation. The convenience of iron supplementation in diabesity patients with low TSAT should be urgently assessed, due the pro-oxidant effects of excess iron.

Regulatory Connections between Iron and Glucose Metabolism

Iron is essential for energy metabolism, and states of iron deficiency or excess are detrimental for organisms and cells. Therefore, iron and carbohydrate metabolism are tightly regulated. Serum iron and glucose levels are subjected to hormonal regulation by hepcidin and insulin, respectively. Hepcidin is a liver-derived peptide hormone that inactivates the iron exporter ferroportin in target cells, thereby limiting iron efflux to the bloodstream. Insulin is a protein hormone secreted from pancreatic β-cells that stimulates glucose uptake and metabolism via insulin receptor signaling. There is increasing evidence that systemic, but also cellular iron and glucose metabolic pathways are interconnected. This review article presents relevant data derived primarily from mouse models and biochemical studies. In addition, it discusses iron and glucose metabolism in the context of human disease.

Iron metabolism and the exocrine pancreas

Although the pathophysiological mechanisms and consequences of gross derangements in iron metabolism are well known, little is known about the pathophysiological mechanisms underlying mild-to-moderate alterations in iron metabolism and their consequences. Growing evidence indicates that the exocrine pancreas has a bidirectional relationship with iron metabolism. Studies have shown alterations in circulating markers of iron metabolism, iron absorption, and intra-pancreatic iron deposition in pancreatitis. At the same time, exocrine pancreatic dysfunction has been shown in iron overload disorders. These observations reveal a compelling connection between the exocrine pancreas and iron metabolism, which are further elucidated by observations of therapeutic benefits of iron chelating agents and pancreatic enzyme replacement therapy. While the pancreas is not a major reservoir of iron in the body, better understanding of its relationship with iron metabolism may yield unexpected insights.

Iron metabolism and type 2 diabetes mellitus: A meta-analysis and systematic review

AIMS/INTRODUCTION

Iron metabolism can directly or indirectly affect the occurrence and development of type 2 diabetes. This meta-analysis and systematic review aimed to analyze the association between serum iron metabolism indicators and type 2 diabetes.

MATERIALS AND METHODS

The databases PubMed and Embase were searched for studies on the correlations between serum iron metabolism indicators (iron, ferritin, transferrin, hepcidin and soluble transferrin receptor) and type 2 diabetes since January 2006. Relevant data were extracted from the included studies, and meta-analysis was carried out.

RESULTS

A total of 12 case-control and cohort studies were analyzed. Of the 12 studies, 11 described the correlation between serum ferritin levels and type 2 diabetes. The median and high serum ferritin concentrations were significantly associated with the risks of type 2 diabetes (odds ratio [OR] 1.20, 95% confidence interval [CI] 1.08-1.33 and OR 1.43, 95% CI 1.29-1.59, respectively). However, the low concentration was not correlated with the risk of type 2 diabetes (OR 0.99, 95% CI 0.89-1.11). No significant association was observed between serum soluble transferrin receptor and type 2 diabetes, whereas the soluble transferrin receptor-to-ferritin ratio was significantly inversely related to the risk of type 2 diabetes in the median and high ratio subgroups (OR 0.71, 95% CI 0.51, 0.99 and OR 0.65, 95% CI 0.45-0.95).

CONCLUSIONS

The elevated serum ferritin was one of the risk factors for type 2 diabetes, and soluble transferrin receptor-to-ferritin ratio was inversely related to the risk of type 2 diabetes. A systematic review showed that serum transferrin and hepcidin might be directly or indirectly related to the development of diabetes.

Hepcidin and diabetes are independently related with soluble transferrin receptor levels in chronic dialysis patients

Background: Soluble transferrin receptor (sTfR) is a biomarker of erythropoiesis, which is often impaired in dialysis patients. The aim of our study was to evaluate sTfR levels in chronically dialyzed patients and assess potential determinants of its levels. Methods: We performed a cross-sectional study by evaluating 246 end-stage renal disease patients undergoing dialysis and 32 healthy controls. Circulating levels of interleukin (IL)-6, C-reactive protein (CRP), tumor necrosis factor (TNF)-α, hepcidin, sTfR, growth differentiation factor 15 (GDF15), and traditional iron metabolism markers were measured, as well as hemogram parameters. Clinical data was obtained from all patients. Results: Compared to controls, patients presented similar values of sTfR, reticulocytes and reticulocyte production index (RPI), and significantly higher levels of IL-6, CRP, ferritin, hepcidin, TNF-α, and GDF15. Iron, transferrin, hemoglobin levels, erythrocyte count, mean cell hemoglobin (MCH), and mean cell hemoglobin concentration (MCHC) values were significantly lower in dialysis group. Within patients, sTfR values were higher in diabetic patients and were positively and significantly correlated with reticulocytes and erythrocytes, RPI, and therapeutic doses of erythropoiesis stimulating agents (ESA) and intravenous iron; and inversely and significantly correlated with circulating iron, ferritin, transferrin saturation, hepcidin, MCH, and MCHC. In multiple linear regression analysis, ESA dose, RPI, serum iron, diabetes, and hepcidin levels were independently associated with sTfR levels in dialysis patients and, thus, with erythropoiesis. Conclusion: Our data suggest that, besides RPI and ESA dose, diabetes and hepcidin are closely related to erythropoiesis in dialysis patients. The influence of diabetes on sTfR levels deserves further investigation.

Adipose Tissue Expansion by Overfeeding Healthy Men Alters Iron Gene Expression

CONTEXT

Iron overload has been associated with greater adipose tissue (AT) depots. We retrospectively studied the potential interactions between iron and AT during an experimental overfeeding in participants without obesity.

METHODS

Twenty-six participants (mean body mass index ± SD, 24.7 ± 3.1 kg/m2) underwent a 56-day overfeeding (+760 kcal/d). Serum iron biomarkers (ELISA), subcutaneous AT (SAT) gene expression, and abdominal AT distribution assessed by MRI were analyzed at the beginning and the end of the intervention.

RESULTS

Before intervention: SAT mRNA expression of the iron transporter transferrin (Tf) was positively correlated with the expression of genes related to lipogenesis (lipin 1, ACSL1) and lipid storage (SCD). SAT expression of the ferritin light chain (FTL) gene, encoding ferritin (FT), an intracellular iron storage protein, was negatively correlated to SREBF1, a gene related to lipogenesis. Serum FT (mean, 92 ± 57 ng/mL) was negatively correlated with the expression of SAT genes linked to lipid storage (SCD, DGAT2) and to lipogenesis (SREBF1, ACSL1). After intervention: Overfeeding led to a 2.3 ± 1.3-kg weight gain. In parallel to increased expression of lipid storage-related genes (mitoNEET, SCD, DGAT2, SREBF1), SAT Tf, SLC40A1 (encoding ferroportin 1, a membrane iron export channel) and hephaestin mRNA levels increased, whereas SAT FTL mRNA decreased, suggesting increased AT iron requirement. Serum FT decreased to 67 ± 43 ng/mL. However, no significant associations between serum iron biomarkers and AT distribution or expansion were observed.

CONCLUSION

In healthy men, iron metabolism gene expression in SAT is associated with lipid storage and lipogenesis genes expression and is modulated during a 56-day overfeeding diet.

Effect of treatment with vitamin D plus calcium on oxidative stress in streptozotocin-induced diabetic rats

Background

In diabetes mellitus, uncontrolled hyperglycemia has been reported to induce oxidative stress, which may lead to health complications. Vitamin D, however, acts as a non-enzymatic antioxidant to protect cells against oxidative stress and damage.

Objective

To investigate the antioxidative effect of vitamin D combined with calcium in streptozotocin (STZ)-induced diabetic rats.

Methods

Rats were divided into four groups (ten rats in each group). The first group (control) received a normal diet and water. The second group, including STZ-induced diabetic rats (diabetic controls), received a normal diet and water. The third group, also including STZ-induced diabetic rats, received vitamin D (2000 IU/day) with calcium (500 mg/kg/day) orally for 28 consecutive days. The fourth group consisted of STZ-induced diabetic rats that received insulin treatment for 28 consecutive days. Activities of superoxide dismutase (SOD), glutathione peroxidase (GPO) and catalase were measured in the liver tissues. The level of malonaldehyde (MDA) was measured in the plasma.

Results

Diabetic rats showed a significant decrease in the activities of SOD, GPO and catalase compared to normal rats. Oral administration of vitamin D with calcium to diabetic rats caused a significant increase in the activities of SOD, GPO and catalase compared with the untreated group. Furthermore, the plasma level of MDA was significantly elevated in diabetic rats compared to normal rats. Diabetic rats treated with vitamin D and calcium had a significantly reduced level of MDA, suggesting that vitamin D with calcium played a vital role in the protection of tissues from damage by free radicals.

Conclusion

Oral supplementation with vitamin D and calcium may be a useful treatment for diabetic patients to reduce/prevent the pathological complications of diabetes.

Iron suppresses ovarian granulosa cell proliferation and arrests cell cycle through regulating p38 mitogen-activated protein kinase/p53/p21 pathway

Iron is an essential nutrient that may exert toxic effects when it accumulates in tissues. Little is known regarding its effects on gonadal function. Both Fe2+ and Fe3+ could be released from iron deposition. We employed mouse nonluteinized granulosa cell for in vitro studies and human ovarian tissues for Prussian blue and immunohistochemical staining to identify the iron deposition and effect in vivo. After treatment with FeSO4-7H2O or FeCl3 in granulosa cell cultured with follicle-stimulating hormone (FSH) for 48 h, we found that Fe2+ significantly suppressed FSH-induced granulosa cell proliferation and arrested the cell cycle at the G2/M phase by cell proliferation assay and flow cytometry. Fe2+ significantly increased intracellular reactive oxygen species (ROS) and ferritin levels of mouse granulosa cells. The increases in p21 and p53 messenger RNA and protein expression facilitated by Fe2+ treatment in mouse granulosa cells were significantly suppressed by separate treatments with p53 small interfering RNA and p38 mitogen-activated protein kinase (MAPK) inhibitors. An ROS inhibitor downregulated Fe2+-induced increases in p38MAPK expression in mouse granulosa cells. Quantitative analysis of immunohistochemical staining revealed that human ovarian tissue sections with positive Prussian blue staining had lower levels of proliferating cell nuclear antigen expression, but higher levels of p21, p53, and CDC25C expression than those with negative Prussian blue staining. Conclusively, Fe2+ could directly arrest the cell cycle and inhibit granulosa cell proliferation by regulating the ROS-mediated p38MAPK/p53/p21 pathway. Therefore, iron can directly affect female gonadal function.

Hepatic iron is the major determinant of serum ferritin in NAFLD patients

BACKGROUND AND AIMS

Elevated serum ferritin is common in NAFLD, and is associated with more advanced disease and increased mortality. Hyperferritinaemia in NAFLD is often attributed to inflammation, while in other conditions ferritin closely reflects body iron stores. The aim of this study was to clarify the underlying cause of hyperferritinaemia in NAFLD.

METHODS

Ferritin levels were examined with markers of iron status, inflammation and liver injury across the clinical spectrum of NAFLD using blood, tissue and magnetic resonance (MR) imaging. A separate larger group of NAFLD patients with hepatic iron staining and quantification were used for validation.

RESULTS

Serum ferritin correlated closely with the iron regulatory hormone hepcidin, and liver iron levels determined by MR. Furthermore, ferritin levels reflected lower serum adiponectin, a marker of insulin resistance, and liver fat, but not cytokine or CRP levels. Ferritin levels differed according to fibrosis stage, increasing from early to moderate disease, and declining in cirrhosis. A similar pattern was found in the validation cohort of NAFLD patients, where ferritin levels were highest in those with macrophage iron deposition. Multivariate analysis revealed liver iron and hepcidin levels as the major determinants of serum ferritin.

CONCLUSIONS

While hyperferritinaemia is associated with markers of liver injury and insulin resistance, serum hepcidin and hepatic iron are the strongest predictors of ferritin levels. These findings highlight the role of disordered iron homeostasis in the pathogenesis of NAFLD, suggesting that therapies aimed at correcting iron metabolism may be beneficial.

Diabetes in HFE Hemochromatosis

Diabetes in whites of European descent with hemochromatosis was first attributed to pancreatic siderosis. Later observations revealed that the pathogenesis of diabetes in HFE hemochromatosis is multifactorial and its clinical manifestations are heterogeneous. Increased type 2 diabetes risk in HFE hemochromatosis is associated with one or more factors, including abnormal iron homeostasis and iron overload, decreased insulin secretion, cirrhosis, diabetes in first-degree relatives, increased body mass index, insulin resistance, and metabolic syndrome. In p.C282Y homozygotes, serum ferritin, usually elevated at hemochromatosis diagnosis, largely reflects body iron stores but not diabetes risk. In persons with diabetes type 2 without hemochromatosis diagnoses, serum ferritin levels are higher than those of persons without diabetes, but most values are within the reference range. Phlebotomy therapy to achieve iron depletion does not improve diabetes control in all persons with HFE hemochromatosis. The prevalence of type 2 diabetes diagnosed today in whites of European descent with and without HFE hemochromatosis is similar. Routine iron phenotyping or HFE genotyping of patients with type 2 diabetes is not recommended. Herein, we review diabetes in HFE hemochromatosis and the role of iron in diabetes pathogenesis in whites of European descent with and without HFE hemochromatosis.

Metabolomic profiling identifies potential pathways involved in the interaction of iron homeostasis with glucose metabolism

Objective

Elevated serum ferritin has been linked to type 2 diabetes (T2D) and adverse health outcomes in subjects with the Metabolic Syndrome (MetS). As the mechanisms underlying the negative impact of excess iron have so far remained elusive, we aimed to identify potential links between iron homeostasis and metabolic pathways.

Methods

In a cross-sectional study, data were obtained from 163 patients, allocated to one of three groups: (1) lean, healthy controls (n = 53), (2) MetS without hyperferritinemia (n = 54) and (3) MetS with hyperferritinemia (n = 56). An additional phlebotomy study included 29 patients with biopsy-proven iron overload before and after iron removal. A detailed clinical and biochemical characterization was obtained and metabolomic profiling was performed via a targeted metabolomics approach.

Results

Subjects with MetS and elevated ferritin had higher fasting glucose (p < 0.001), HbA1c (p = 0.035) and 1 h glucose in oral glucose tolerance test (p = 0.002) compared to MetS subjects without iron overload, whereas other clinical and biochemical features of the MetS were not different. The metabolomic study revealed significant differences between MetS with high and low ferritin in the serum concentrations of sarcosine, citrulline and particularly long-chain phosphatidylcholines. Methionine, glutamate, and long-chain phosphatidylcholines were significantly different before and after phlebotomy (p < 0.05 for all metabolites).

Conclusions

Our data suggest that high serum ferritin concentrations are linked to impaired glucose homeostasis in subjects with the MetS. Iron excess is associated to distinct changes in the serum concentrations of phosphatidylcholine subsets. A pathway involving sarcosine and citrulline also may be involved in iron-induced impairment of glucose metabolism.

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This metabolomic study focuses on pathways linking iron status to insulin resistance.

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Metabolomic differences in Metabolic Syndrome with/without iron overload are shown.

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Phlebotomy changes methionine, glutamate and long-chain phosphatidylcholines levels.

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Phosphatidylcholines are involved in the interaction of iron and glucose homeostasis.

Effects of iron supplementation in mice with hypoferremia induced by obesity

Iron is an important micronutrient, but it can also act as a dangerous element by interfering with glucose homeostasis and inflammation, two features that are already disturbed in obese subjects. In this work, we study the effects of systemic iron supplementation on metabolic and inflammatory responses in mice with hypoferremia induced by obesity to better characterize whether iron worsens the parameters that are already altered after 24 weeks of a high-fat diet (HFD). Mice were maintained on a control diet or a HFD for 24 weeks and received iron-III polymaltose (50 mg/kg/every 2 days) during the last two weeks. Glucose homeostasis (basal glucose and insulin test tolerance) and systemic and visceral adipose tissue (VAT) inflammation were assessed. Iron levels were measured in serum. The Prussian blue reaction was used in isolated macrophages to detect iron deposition. Iron supplementation resulted in an increased number of VAT macrophages that were positive for Prussian blue staining as well as increased serum iron levels. Systemic hepcidin, leptin, resistin, and monocyte chemoattractant protein-1 (MCP-1) levels were not altered by iron supplementation. Local adipose tissue inflammation was also not made worse by iron supplementation because the levels of hepcidin, MCP-1, leptin, and interleukin (IL)-6 were not altered. In contrast, iron supplementation resulted in an increased production of IL-10 by adipose tissue and VAT macrophages. Leukocytosis and VAT plasminogen activator inhibitor-1 (PAI-1) level were reduced, but insulin resistance was not altered after iron supplementation. In conclusion, systemic iron supplementation in mice with hypoferremia induced by obesity did not worsen inflammatory marker or adipose tissue inflammation or the metabolic status established by obesity. Iron deposition was observed in adipose tissue, mainly in macrophages, suggesting that these cells have mechanisms that promote iron incorporation without increasing the production of inflammatory mediators.

Relationships of iron metabolism with insulin resistance and glucose levels in young and healthy adults

AIMS

Several biomarkers within the iron metabolism pathway have been related to the occurrence of diabetes mellitus, but underlying mechanisms are unknown. The aim of our study was to investigate the differential relationships of iron metabolism with a broad range of diabetes markers in young and healthy adults.

DESIGN

2160 participants aged 25 to 41years were enrolled in a population-based study. Established cardiovascular disease, diabetes or a body mass index >35kg/m(2) were exclusion criteria. Multivariable linear regression models were built to assess the associations of ferritin and transferrin saturation (TSAT) with blood levels of glucagon-like peptide-1 (GLP-1), insulin, homeostatic model assessment-insulin resistance (HOMA-IR), fasting plasma glucose (FPG) and hemoglobin A1c (HbA1c).

RESULTS

Median (interquartile range) age was 37 (31, 40) years. In multivariable linear regression analyses, β-coefficients (95% confidence intervals) per 1-SD increase in ferritin were 0.04 (0.02; 0.07, p=0.0008) for GLP-1, 0.06 (0.04; 0.08, p<0.0001) for insulin, 0.07 (0.04; 0.09, p<0.0001) for HOMA-IR, 0.004 (-0.00; 0.01, p=0.07) for FPG and -0.003 (-0.01; -0.00, p=0.07) for HbA1c. β-coefficients (95% CI) per 1-SD increase in TSAT were -0.07 (-0.09; -0.05, p<0.0001) for GLP-1, -0.06 (-0.08; -0.04, p<0.0001) for insulin, -0.07(-0.09; -0.05, p<0.0001) for HOMA-IR, -0.01 (-0.01; -0.00, p<0.0001) for FPG and -0.01 (-0.01; -0.00, p=0.0004) for HbA1c.

CONCLUSIONS

Markers of insulin resistance are strongly related with markers of iron metabolism in healthy subjects. These relationships were inconsistent and weaker for short-term and long-term glucose levels. These results may provide insights in the relationships between iron metabolism and diabetes occurrence.

Iron Status and Metabolic Syndrome in Patients with Non-Alcoholic Fatty Liver Disease

BACKGROUND

A hypothesis has been presented about the role of serum iron, ferritin and transferrin saturation among patients with non-alcoholic fatty liver disease (NAFLD) and resistance to insulin (metabolic syndrome [MetS]), but there is much controversy. This study aimed at investigating the level of serum iron and demographic characteristics in patients with NAFLD with or without MetS.

METHODS

A case-control study was conducted on patients with elevated liver enzymes referring to Baqiyatallah clinic, Tehran, Iran during 2010-2011. After ruling out other causes of increased aminotransferases and approving the diagnosis of NAFLD, the patients were divided into two groups of with or without MetS. Then, the individuals’ demographic, sonographic, and laboratory characteristics were recorded.

RESULTS

This research included 299 patients suffering from NAFLD who were divided into MetS (n=143; 47.8%) and non-MetS (n=156; 52.2%) groups. The age, systolic and diastolic blood pressure, body mass index, waist/hip ratio, glucose tolerance test, serum insulin, C. peptide, triglyceride, and HB A1c were different between MetS and non-MetS groups (p<0.05). There was no significant difference in serum iron and ferritin levels between the two groups, however, a significant correlation was found between serum ferritin and alanine transaminase (p=0.005) and also aspartate aminotransferase (p=0.032).

CONCLUSION

Our findings did not show a significant relationship between iron, in free or storage form, and the presence of MetS among patients with NAFLD, but serum ferritin can correlate with hepatocytes injuries indicated by raised aminotransferases. Nevertheless, to clarify this relationship further molecular, genomic, and histopathological studies are required.

Association of Serum Ferritin Level with Risk of Incident Abnormal Glucose Metabolism in Southwestern China: a Prospective Cohort Study

This prospective cohort study aimed to analyze the association between serum ferritin levels and the risk of abnormal glucose metabolism (AGM) in Southwestern Chinese population. The 383 subjects who are aged ≥20 years and free of AGM at baseline between in 2007 and in 2008 were included in Southwestern China, and their baseline serum ferritin levels were measured. Among these subjects, 140 subjects were developed into AGM during the follow-up (2008-2012). In logistic regression models, the relative risk in the top versus that in the lowest quartile of serum ferritin levels was 2.86 (p = 0.013) in females and 3.50 (p = 0.029) in males after adjusting the age, gender, family history of diabetes, current smoking, and alcohol; however, serum ferritin levels were not significantly associated with incident of AGM after controlling for metabolic factors (waist circumference, systolic pressure (SBP), triglyceride (TG), and homeostasis model assessment formula insulin resistance (HOMA-IR)). Elevated serum ferritin levels are associated with AGM but not an independent risk factor.

Mechanisms Linking Glucose Homeostasis and Iron Metabolism Toward the Onset and Progression of Type 2 Diabetes

OBJECTIVE

The bidirectional relationship between iron metabolism and glucose homeostasis is increasingly recognized. Several pathways of iron metabolism are modified according to systemic glucose levels, whereas insulin action and secretion are influenced by changes in relative iron excess. We aimed to update the possible influence of iron on insulin action and secretion and vice versa.

RESEARCH DESIGN AND METHODS

The mechanisms that link iron metabolism and glucose homeostasis in the main insulin-sensitive tissues and insulin-producing β-cells were revised according to their possible influence on the development of type 2 diabetes (T2D).

RESULTS

The mechanisms leading to dysmetabolic hyperferritinemia and hepatic overload syndrome were diverse, including diet-induced alterations in iron absorption, modulation of gluconeogenesis, heme-mediated disruption of circadian glucose rhythm, impaired hepcidin secretion and action, and reduced copper availability. Glucose metabolism in adipose tissue seems to be affected by both iron deficiency and excess through interaction with adipocyte differentiation, tissue hyperplasia and hypertrophy, release of adipokines, lipid synthesis, and lipolysis. Reduced heme synthesis and dysregulated iron uptake or export could also be contributing factors affecting glucose metabolism in the senescent muscle, whereas exercise is known to affect iron and glucose status. Finally, iron also seems to modulate β-cells and insulin secretion, although this has been scarcely studied.

CONCLUSIONS

Iron is increasingly recognized to influence glucose metabolism at multiple levels. Body iron stores should be considered as a potential target for therapy in subjects with T2D or those at risk for developing T2D. Further research is warranted.

Serum ferritin and glucose homeostasis: change in the association by glycaemic state

BACKGROUND

Data on the association between body iron and glucose homeostasis by the three glycaemic states are scarce. Thus, we investigated the association between body iron as assessed by a serum ferritin concentration and glucose homeostasis using homeostasis model assessment (HOMA) of insulin resistance (HOMA-IR) and beta cell function (HOMA-BcF) in different glycaemic states.

METHODS

A cross-sectional analysis was conducted in 2541 men aged 42-60 years in 1984-1989 in the Kuopio Ischemic Heart Disease Risk Factor Study. Subjects were classified into the three glycaemic states, normoglycaemia, prediabetes and type 2 diabetes (T2D), by fasting plasma glucose measurements and the information collected at study visit. The association between serum ferritin quartiles and HOMA-IR and HOMA-BcF for each glycaemic state was examined by analysis of covariance and linear regression analysis.

RESULTS

The mean age and serum ferritin concentrations were 53.1 years (standard deviation = 5.7, range = 42.0-61.3 years) and 166.2 µg/L (standard deviation = 141.7, range = 11-960 µg/L), respectively. After multivariable adjustments, a weak and direct association was observed between serum ferritin quartiles and HOMA-IR in normoglycaemia (P-trend = 0.001) but a direct association in prediabetes (P-trend = 0.007) and in T2D (P-trend = 0.078). In HOMA-BcF, the association was weak and direct in normoglycaemia (P-trend = 0.003), direct in prediabetes (P-trend = 0.005) and inverse in T2D (P-trend = 0.105). Strongest associations were observed in prediabetes (β = 0.25, 95% confidence interval = 0.14-0.36 and P = 0.004 in HOMA-IR; β = 0.23, 95% confidence interval = 0.15-0.31 and P = 0.008 in HOMA-BcF) after a 100-µg/L increase in serum ferritin (log-transformed).

CONCLUSIONS

These data suggest that both the strength and the direction of the association between body iron stores and glucose homeostasis are dependent on the glycaemic state of the population.

Increased inflammation and decreased insulin sensitivity indicate metabolic disturbances in zoo-managed compared to free-ranging black rhinoceros (Diceros bicornis)

Black rhinoceros (rhinos) living in zoos express a host of unusual disease syndromes that are associated with increased morbidity and mortality, including hemolytic anemia, rhabdomyolysis, hepatopathy and ulcerative skin disease, hypophosphatemia and iron overload. We hypothesized that iron overload is a consequence and indicator of disturbances related to inflammation and insulin/glucose metabolism. The objectives of this study were to: (1) generate the first baseline information on biomarkers of inflammation (tumor necrosis factor alpha [TNFα], serum amyloid A [SAA]), insulin sensitivity (insulin, glucose and proxy calculations of insulin sensitivity), phosphate and iron stores (ferritin) using banked serum from free-ranging black rhinos; and (2) then compare serum biomarkers between zoo-managed (n=86 individuals) and free-ranging (n=120) animals. Enzyme immunoassays were validated for serum and then biomarker levels analyzed using mixed models while controlling for sex, age and year of sample collection. Concentrations of TNFα, SAA, insulin and insulin-to glucose ratio were higher (P<0.05) in black rhinos managed in ex situ conditions compared to free-living counterparts. Findings indicate that the captive environment is contributing to increased inflammation and decreased insulin sensitivity in this endangered species.

Fasting serum levels of ferritin are associated with impaired pancreatic beta cell function and decreased insulin sensitivity: a population-based study

AIMS/HYPOTHESIS

Elevated serum ferritin levels are associated with an increased risk of type 2 diabetes, but the nature of this association remains elusive. The aim of this study was to test the hypothesis that an elevated fasting serum ferritin level is associated with an increased risk of type 2 diabetes due to its association with impaired beta cell function and decreased insulin sensitivity.

METHODS

We investigated 6,392 individuals from the Danish general population. Surrogate measures of beta cell function and insulin sensitivity were calculated for approximately 6,100 individuals based on OGTT examinations.

RESULTS

The ORs for type 2 diabetes were 4.2 (95% CI 2.4, 7.2) for the highest vs the lowest quintile of serum ferritin, and 17 (95% CI 8.9, 33) for serum ferritin levels ≥97.5th percentile vs <20th percentile. Elevated serum ferritin levels were associated with elevated plasma glucose levels at 0, 30 and 120 min (p < 0.001), elevated serum insulin levels at 0 and 120 min (p = 0.02 and p < 0.001), decreased beta cell function estimated as the insulinogenic index and corrected insulin response (p < 0.001), and decreased insulin sensitivity estimated by the Matsuda index of insulin sensitivity and HOMA-IR (p < 0.001). Whereas the association with impaired beta cell function was present in both men and women, the association with decreased insulin sensitivity was observed among men and older women but not among younger women.

CONCLUSIONS/INTERPRETATION

Elevated fasting serum ferritin levels are associated with surrogate measures of both impaired beta cell function and decreased insulin sensitivity. Menopause seems to modify the association with insulin sensitivity.

Quantitative analysis of transferrin cycling by automated fluorescence microscopy

Surface receptors are transported between the plasma membrane and intracellular compartments by various endocytic mechanisms and by recycling via different pathways from sorting or recycling endosomes. The analysis of cellular components involved in mediating or regulating these transport steps is of high current interest and requires quantitative methods to determine rates of endocytosis and/or recycling. Various biochemical procedures to measure uptake of labeled ligand molecules or internalization and reappearance of surface-labeled receptors have been developed. Here, we describe a quantitative method based on fluorescence microscopy of adherent cells taking advantage of the transferrin (Tf) receptor as the prototype of cycling transport receptors. Tf is endocytosed with bound Fe(3+) and, upon release of the iron ion in endosomes, recycled as apo-Tf together with the receptor. To follow the ligand-receptor complex, fluorescently labeled Tf is used and detected microscopically with or without releasing Tf from cell surface receptors by acid stripping. To go beyond the observation of a few individual cells, automated fluorescence microscopy is employed to image thousands of cells at different time points and in parallel with different treatments (such as chemical inhibitors, siRNA silencing, or transfection of candidate genes) in a 96-well format. Computer-assisted image analysis allows unbiased quantitation of Tf content of each cell and to distinguish between different cell populations.

Hyperinsulinemia induces hepatic iron overload by increasing liver TFR1 via the PI3K/IRP2 pathway

Dysmetabolic iron overload syndrome (DIOS) is frequently observed, but the underlying mechanism remains unclear. We propose the hypothesis that hyperinsulinemia, a common characteristic of DIOS, may stimulate liver transferrin receptor 1 (TFR1) expression via the PI3K/iron regulatory protein 2 (IRP2) pathway, leading to the occurrence of DIOS. The hepatic iron content, serum iron parameters, and expressions of TFRs and IRPs in the liver were determined in rats with temporary or long-lasting hyperinsulinemia induced by acute or chronic administration of insulin. The effect of insulin on TFR1 expression and its molecular mechanism were determined in HL-7702 cells in vitro. It was found that long-lasting hyperinsulinemia significantly increased TFR1 expression in the liver and induced mild-to-moderate hepatic iron overload, which was accompanied by a normal level of serum iron. Insulin markedly upregulated both protein and mRNA levels of TFR1 in HL-7702 cells. The stability of TFR1 mRNA stability, together with expression of IRPs expression, were both significantly increased by insulin treatment. Insulin-induced TFR1 expression was blocked by IRP2, but not by IRP1 interference, and disappeared when HL-7702 cells were pretreated with LY294002, triciribine hydrate, or rapamycin. In conclusion, the findings of this study indicate that hyperinsulnemia could induce hepatic iron overload by upregulating liver TFR1 via the PI3K/AKT/mTOR/IRP2 pathway, which may be one of the main reasons for the occurrence of DIOS.

Excess body iron and the risk of type 2 diabetes mellitus: a nested case–control in the PREDIMED (PREvention with MEDiterranean Diet) study

A prospective nested case–control study within the PREvention with MEDiterranean Diet (PREDIMED) was conducted to evaluate the relationship between excess body Fe (measured as serum ferritin (SF), soluble transferrin receptor (sTfR) and sTfR:ferritin ratio) and the risk of type 2 diabetes mellitus (T2DM) in a Mediterranean population at a high risk of CVD, without T2DM at the start of the study. The study contained 459 subjects, 153 with incident T2DM (cases) and 306 without incident T2DM (controls). The follow-up period was for 6·0 (interquartile range 3·9–6·5) years. For each incident diabetic subject, two subjects were selected as controls who were matched broadly for age as well as for sex, intervention group and BMI. We observed a relationship between SF values >257 μg/l in males and >139 μg/l in females and the risk of T2DM, following adjustment in the conditional logistic regression model for high-sensitivity C-reactive protein, fasting glucose and other components of the metabolic syndrome (OR 3·62, 95 % CI 1·32, 19·95; P= 0·022). We also found an association between low sTfR:ferritin ratio levels and the incidence of T2DM (OR 3·02, 95 % CI 1·09, 8·39; P= 0·042), but no association with sTfR (OR 1·29, 95 % CI 0·51, 3·23; P= 0·722). Oxidative stress has been hypothesised to contribute to the development of insulin resistance and β-cell dysfunction, the two key events in the clinical development of T2DM. Following adjustment for other risk factors for T2DM, excess body Fe (measured as SF and sTfR:ferritin ratio) was associated with an increased risk of developing T2DM in a Mediterranean population at a high risk of CVD.

Iron homeostasis in peripheral nervous system, still a black box?

SIGNIFICANCE

Iron is the most abundant transition metal in biology and an essential cofactor for many cellular enzymes. Iron homeostasis impairment is also a component of peripheral neuropathies.

RECENT ADVANCES

During the past years, much effort has been paid to understand the molecular mechanism involved in maintaining systemic iron homeostasis in mammals. This has been stimulated by the evidence that iron dyshomeostasis is an initial cause of several disorders, including genetic and sporadic neurodegenerative disorders.

CRITICAL ISSUES

However, very little has been done to investigate the physiological role of iron in peripheral nervous system (PNS), despite the development of suitable cellular and animal models.

FUTURE DIRECTIONS

To stimulate research on iron metabolism and peripheral neuropathy, we provide a summary of the knowledge on iron homeostasis in the PNS, on its transport across the blood-nerve barrier, its involvement in myelination, and we identify unresolved questions. Furthermore, we comment on the role of iron in iron-related disorder with peripheral component, in demyelinating and metabolic peripheral neuropathies.

Relationship between Serum Levels of Body Iron Parameters and Insulin Resistance and Metabolic Syndrome in Korean Children

OBJECTIVES

An increase in serum ferritin and levels of the cleaved soluble form of transferrin receptor (sTfR) are related to several metabolic conditions. We evaluated the relationship between body iron status indicators, including ferritin and sTfR, and insulin resistance and metabolic syndrome (MetS) in Korean children.

METHODS

A cross-sectional study was conducted on 1350 children in Korea. Anthropometrical parameters; lipid profiles; levels of glucose, insulin, and leptin; and iron status indicators, including sTfR, serum ferritin, serum iron, total iron-binding capacity (TIBC), and transferrin saturation (TS), were analyzed.

RESULTS

Although serum sTfR levels were significantly higher in boys than in girls (2.20 vs. 2.06 mg/L, p < 0.0001), serum iron and TS were higher in girls than in boys (101.38 vs. 95.77 mg/L, p = 0.027 and 30.15 vs. 28.91%, p = 0.04, respectively). Waist circumference (WC) and leptin were most significantly associated with body iron indicators when adjusted for age and sex. After adjusting for age, sex, and WC, sTfR levels showed the strongest positive association with leptin levels (p = 0.0001). Children in the highest tertile for homeostasis model assessment-insulin resistance (HOMA-IR) had higher TIBC (p = 0.0005) and lower serum iron (p = 0.0341), and the lowest TS (p < 0.0001) after adjustment for confounders. Children with higher sTfR were most significantly associated with risk of MetS compared with those lower sTfR (p = 0.0077).

CONCLUSION

The associations of serum levels of iron metabolism markers with leptin levels, HOMA-IR, and MetS suggest that iron-related factors may involve insulin resistance and MetS.

Effects of iron overload on chronic metabolic diseases

Iron can affect the clinical course of several chronic metabolic diseases such as type 2 diabetes, obesity, non-alcoholic fatty liver disease, and atherosclerosis. Iron overload can affect major tissues involved in glucose and lipid metabolism (pancreatic β cells, liver, muscle, and adipose tissue) and organs affected by chronic diabetic complications. Because iron is a potent pro-oxidant, fine-tuned control mechanisms have evolved to regulate entry, recycling, and loss of body iron. These mechanisms include the interplay of iron with transferrin, ferritin, insulin, and hepcidin, as well as with adipokines and proinflammatory molecules. An imbalance of these homoeostatic mechanisms results in systemic and parenchymal siderosis that contributes to organ damage (such as β-cell dysfunction, fibrosis in liver diseases, and atherosclerotic plaque growth and instability). Conversely, iron depletion can exert beneficial effects in patients with iron overload and even in healthy frequent blood donors. Regular assessment of iron balance should be recommended for patients with chronic metabolic diseases, and further research is needed to produce guidelines for the identification of patients who would benefit from iron depletion.

Insulin resistance modulates iron-related proteins in adipose tissue

OBJECTIVE Circulating markers of iron overload are associated with insulin resistance. Less is known about the impact of iron overload on adipose tissue (AT). We hypothesized that gene expression markers of iron metabolism in AT could be associated with insulin action. RESEARCH DESIGN AND METHODS The AT expression of ferroportin (SLC40A1), transferrin (TF), TF receptor (TFRC), ferritin (FT) heavy polypeptide 1 (FTH1), and FT light polypeptide (FTL) was analyzed cross-sectionally in three independent cohorts and also after weight loss-induced changes in insulin sensitivity (clamp M value) in an independent fourth cohort. RESULTS In human AT, TF mRNA and protein levels were decreased with obesity and insulin resistance in the three cohorts and were positively associated with adipogenic mRNAs and insulin action. Otherwise, FTL mRNA and protein and SLC40A1 transcripts were positively associated with BMI and negatively linked to adipogenic genes and insulin action. Bariatric surgery-induced weight loss led to increased TF and decreased TFRC, FTH1, FTL, and SLC40A1 in subcutaneous AT in parallel to improved insulin action. CONCLUSIONS These results suggest that iron overload impacts on AT in association with insulin resistance.

Obesity alters adipose tissue macrophage iron content and tissue iron distribution

Adipose tissue (AT) expansion is accompanied by the infiltration and accumulation of AT macrophages (ATMs), as well as a shift in ATM polarization. Several studies have implicated recruited M1 ATMs in the metabolic consequences of obesity; however, little is known regarding the role of alternatively activated resident M2 ATMs in AT homeostasis or how their function is altered in obesity. Herein, we report the discovery of a population of alternatively activated ATMs with elevated cellular iron content and an iron-recycling gene expression profile. These iron-rich ATMs are referred to as MFe(hi), and the remaining ATMs are referred to as MFe(lo). In lean mice, ~25% of the ATMs are MFe(hi); this percentage decreases in obesity owing to the recruitment of MFe(lo) macrophages. Similar to MFe(lo) cells, MFe(hi) ATMs undergo an inflammatory shift in obesity. In vivo, obesity reduces the iron content of MFe(hi) ATMs and the gene expression of iron importers as well as the iron exporter, ferroportin, suggesting an impaired ability to handle iron. In vitro, exposure of primary peritoneal macrophages to saturated fatty acids also alters iron metabolism gene expression. Finally, the impaired MFe(hi) iron handling coincides with adipocyte iron overload in obese mice. In conclusion, in obesity, iron distribution is altered both at the cellular and tissue levels, with AT playing a predominant role in this change. An increased availability of fatty acids during obesity may contribute to the observed changes in MFe(hi) ATM phenotype and their reduced capacity to handle iron.

Associations between red meat intake and biomarkers of inflammation and glucose metabolism in women

BACKGROUND

Greater red meat intake is associated with an increased type 2 diabetes and cardiovascular disease risk. However, the relation of red meat intake to biomarkers of inflammation and glucose metabolism has not been investigated thoroughly.

OBJECTIVE

We hypothesized that greater red meat intake would be associated with biomarkers of inflammation and glucose metabolism, which would be partly explained by body mass index (BMI).

DESIGN

We analyzed cross-sectional data from diabetes-free female participants in the Nurses' Health Study (n = 3690). Multiple linear regression was conducted to assess the associations of total, unprocessed, and processed red meat intakes (quartile categories) with plasma C-reactive protein (CRP), ferritin, adiponectin, fasting insulin, and hemoglobin A1c (Hb A1c).

RESULTS

Greater total, unprocessed, and processed red meat intakes were associated with higher plasma CRP, ferritin, fasting insulin, and Hb A1c and lower adiponectin after adjustment for demographic information (P-trend ≤ 0.03 for all). Adiponectin was not associated with any type of red meat intake when further adjusted for medical and lifestyle factors. After adjustment for BMI, most of these associations with inflammatory and glucose metabolic biomarkers were substantially attenuated and no longer significant. BMI accounted for a statistically significant proportion of associations with CRP, Hb A1c, and fasting insulin (P-contribution ≤ 0.02 for all) but not with ferritin. Substituting a serving of total red meat intake with alternative protein food in a combination of poultry, fish, legumes, and nuts was associated with significantly lower CRP (β ± SE: -0.106 ± 0.043), ferritin (-0.212 ± 0.075), Hb A1c (-0.052 ± 0.015), and fasting insulin (-0.119 ± 0.036) (all P ≤ 0.02 for comparison of extreme quartiles for all).

CONCLUSIONS

Greater red meat intake is associated with unfavorable plasma concentrations of inflammatory and glucose metabolic biomarkers in diabetes-free women. BMI accounts for a significant proportion of the associations with these biomarkers, except for ferritin. Substituting red meat with another protein food is associated with a healthier biomarker profile of inflammatory and glucose metabolism.

Increased visfatin in hemodialysis patients is associated with decreased demands for recombinant human erythropoietin

BACKGROUND

Studies detected an association between visfatin and markers of iron metabolism in patients with insulin resistance. In this study, such a relation was evaluated in hemodialysis (HD) patients. Also relations between visfatin and hepcidin, demands for recombinant human erythropoietin (rHuEpo), inflammation, and situations characterized by insulin resistance were evaluated.

METHODS

After a four-week washout period from iron treatment, 33 HD patients and 20 healthy volunteers enrolled in the study. Serum visfatin, hepcidin, and interleukin-6 (IL-6) were assessed by means of enzyme-linked immunosorbent assay. Hemoglobin, serum iron, ferritin, and transferrin saturation (TSAT) were also measured.

RESULTS

Visfatin was markedly increased in HD patients. Visfatin levels did not differ between diabetics and non-diabetics. No relation was detected between visfatin and body mass index or IL-6 in HD patients. From the markers of iron metabolism, the hepcidin included, visfatin was related only to TSAT. A strong positive relation was revealed between visfatin and hemoglobin, whereas visfatin was inversely related to rHuEpo dose. Resistance to rHuEpo index was inversely and independently of TSAT related to visfatin.

CONCLUSION

Visfatin is increased in HD patients and it is associated with decreased demands for rHuEpo.

Liver steatosis correlates with iron overload but not with HFE gene mutations in chronic hepatitis C

BACKGROUND

Liver steatosis and iron overload, which are frequently observed in chronic hepatitis C (CHC), may contribute to the progression of liver injury. This study aimed to evaluate the correlation between liver steatosis and iron overload in Polish patients with CHC compared to non-alcoholic fatty liver disease (NAFLD) and HFE-hereditary hemochromatosis (HH) patients.

METHODS

A total of 191 CHC patients were compared with 67 NAFLD and 21 HH patients. Liver function tests, serum markers of iron metabolism, cholesterol and triglycerides were assayed. The inflammatory activity, fibrosis, iron deposits and steatosis stages were assessed in liver specimens. HFE gene polymorphisms were investigated by PCR-RFLP.

RESULTS

Liver steatosis was associated with obesity and diabetes mellitus. This disease was confirmed in 76/174 (44%) CHC patients, most of whom were infected with genotype 1. The average grade of steatosis was higher in NAFLD patients. CHC patients had significantly higher iron concentrations and transferrin saturations than NAFLD patients. Compared with CHC patients, HH patients had higher values of serum iron parameters and more intensive hepatocyte iron deposits without differences in the prevalence and intensity of liver steatosis. In the CHC group, lipids accumulation in hepatocytes was significantly associated with the presence of serum markers of iron overload. No correlation between the HFE gene polymorphism and liver steatosis in CHC patients was found.

CONCLUSIONS

Liver steatosis was diagnosed in nearly half of CHC patients, most of whom were infected with genotype 1. The intensity of steatosis was lower in CHC patients than that in NAFLD patients because of a less frequent diagnosis of metabolic syndrome. Only in CHC patients were biochemical markers of iron accumulation positively correlated with liver steatosis; these findings were independent of HFE gene mutations.

Iron and diabetes risk

Iron overload is a risk factor for diabetes. The link between iron and diabetes was first recognized in pathologic conditions-hereditary hemochromatosis and thalassemia-but high levels of dietary iron also impart diabetes risk. Iron plays a direct and causal role in diabetes pathogenesis mediated both by β cell failure and insulin resistance. Iron also regulates metabolism in most tissues involved in fuel homeostasis, with the adipocyte in particular serving an iron-sensing role. The underlying molecular mechanisms mediating these effects are numerous and incompletely understood but include oxidant stress and modulation of adipokines and intracellular signal transduction pathways.

Iron metabolism is associated with adipocyte insulin resistance and plasma adiponectin: the Cohort on Diabetes and Atherosclerosis Maastricht (CODAM) study

OBJECTIVE

Adipocyte insulin resistance (IR) is a key feature early in the pathogenesis of type 2 diabetes mellitus (T2DM), and although scarce, data in the literature suggest a direct role for iron and iron metabolism-related factors in adipose tissue function and metabolism. Serum ferritin and transferrin were shown to be associated with muscle insulin resistance (IR) and T2DM, but little is known about the role of iron metabolism on adipose tissue. We therefore investigated whether markers of iron metabolism were associated with adipocyte IR and plasma adiponectin.

RESEARCH DESIGN AND METHODS

Serum ferritin, transferrin, total iron, non-transferrin-bound iron (NTBI), transferrin saturation, and plasma adiponectin were determined in 492 individuals. Adipocyte IR was defined by the product of fasting insulin and nonesterified fatty acids (NEFAs). Using linear regression analyses, we investigated the difference in adipocyte IR or adiponectin (in %) according to differences in iron metabolism markers.

RESULTS

Serum ferritin (β = 1.00% increase in adipocyte IR per 10 μg/L [95% CI 0.66-1.34]), transferrin (4.18% per 0.1 g/L [2.88-5.50]), total iron (1.36% per μmol/L [0.61-2.12]), and NTBI (5.14% per μmol/L [1.88-8.52]) were associated with adipocyte IR after adjustment for several covariates, including inflammatory markers. All markers of iron metabolism were also associated with NEFAs (all P < 0.01). In addition, ferritin and transferrin were inversely associated with adiponectin (both P < 0.01).

CONCLUSIONS

The observed associations of several markers of iron metabolism with adipocyte IR and adiponectin suggest that factors related to iron and iron metabolism may contribute to adipocyte IR early in the pathogenesis of T2DM.

Insulin promotes iron uptake in human hepatic cell by regulating transferrin receptor-1 transcription mediated by hypoxia inducible factor-1

Hepatic iron is known to regulate insulin signaling pathways and to influence insulin sensitivity in insulin resistance (IR) patients. However, the role of insulin on hepatic iron homeostasis remains unexplored. Here, we report that insulin promotes transferrin-bound iron uptake but shows no influence on non transferrin-bound iron uptake in human hepatic HepG2 cells. As a mechanism we detected increased transferrin receptor-1 (TfR1) expression both at protein and mRNA levels. Unaltered stability of protein and transcript of TfR1 suggested the regulation at transcriptional level that was confirmed by promoter activity. Involvement of transcription factor hypoxia inducible factor-1 (HIF-1) was shown by mutational analyses of the TfR1 promoter region and by electrophoretic mobility shift assay. When HepG2 cells were transfected with specific siRNA targeted to 3'UTR of HIF-1α, the regulatory subunit of HIF-1; insulin-induced TfR1 expression and iron uptake were inhibited. Transfection of cDNA expressing stable form of HIF-1α reversed the increased TfR1 expression and iron uptake. These results suggest a novel role of insulin in hepatic iron uptake by a HIF-1 dependent transcriptional regulation of TfR1.

Adipocyte iron regulates adiponectin and insulin sensitivity

Iron overload is associated with increased diabetes risk. We therefore investigated the effect of iron on adiponectin, an insulin-sensitizing adipokine that is decreased in diabetic patients. In humans, normal-range serum ferritin levels were inversely associated with adiponectin, independent of inflammation. Ferritin was increased and adiponectin was decreased in type 2 diabetic and in obese diabetic subjects compared with those in equally obese individuals without metabolic syndrome. Mice fed a high-iron diet and cultured adipocytes treated with iron exhibited decreased adiponectin mRNA and protein. We found that iron negatively regulated adiponectin transcription via FOXO1-mediated repression. Further, loss of the adipocyte iron export channel, ferroportin, in mice resulted in adipocyte iron loading, decreased adiponectin, and insulin resistance. Conversely, organismal iron overload and increased adipocyte ferroportin expression because of hemochromatosis are associated with decreased adipocyte iron, increased adiponectin, improved glucose tolerance, and increased insulin sensitivity. Phlebotomy of humans with impaired glucose tolerance and ferritin values in the highest quartile of normal increased adiponectin and improved glucose tolerance. These findings demonstrate a causal role for iron as a risk factor for metabolic syndrome and a role for adipocytes in modulating metabolism through adiponectin in response to iron stores.