An unrestrained proinflammatory M1 macrophage population induced by iron impairs wound healing in humans and mice

Uncontrolled macrophage activation is now considered to be a critical event in the pathogenesis of chronic inflammatory diseases such as atherosclerosis, multiple sclerosis, and chronic venous leg ulcers. However, it is still unclear which environmental cues induce persistent activation of macrophages in vivo and how macrophage-derived effector molecules maintain chronic inflammation and affect resident fibroblasts essential for tissue homeostasis and repair. We used a complementary approach studying human subjects with chronic venous leg ulcers, a model disease for macrophage-driven chronic inflammation, while establishing a mouse model closely reflecting its pathogenesis. Here, we have shown that iron overloading of macrophages--as was found to occur in human chronic venous leg ulcers and the mouse model--induced a macrophage population in situ with an unrestrained proinflammatory M1 activation state. Via enhanced TNF-α and hydroxyl radical release, this macrophage population perpetuated inflammation and induced a p16(INK4a)-dependent senescence program in resident fibroblasts, eventually leading to impaired wound healing. This study provides insight into the role of what we believe to be a previously undescribed iron-induced macrophage population in vivo. Targeting this population may hold promise for the development of novel therapies for chronic inflammatory diseases such as chronic venous leg ulcers.

The overlapping of local iron overload and HFE mutation in venous leg ulcer pathogenesis

Chronic venous stasis determines red blood cell extravasation and either dermal hemosiderin deposits or iron-laden phagocytes. Several authors have suspected that iron could play a role in the pathogenesis of venous leg ulcers. They hypothesized that local iron overload could generate free radicals or activate a proteolytic hyperactivity on the part of metalloproteinases (MMPs) or else down-regulate tissue inhibitors of MMPs. However, they were unable to explain why iron deposits, visible in the legs of patients with chronic venous disease (CVD), cause lesions in only some individuals, whereas in others they do not. We hypothesized that such individual differences could be genetically determined and investigated the role of the C282Y and H63D mutations of the HFE gene. C282Y mutation significantly increases the risk of ulcer in primary CVD more than six times (OR = 6.69; 1.45-30.8; p = 0.01). Patients carrying the H63D variant have an earlier age of ulcer onset, by almost 10 years (p > 0.004). The increased risk of skin lesion and the early age of onset of the disease in HFE carriers confirm in a clinical setting that intracellular iron deposits of mutated macrophages have less stability than those of the wild type. We hypothesize that the physiologic iron protective mechanisms are affected by the HFE mutations and should be investigated in all diseases characterized by the combination of iron overload and inflammation.

Oxidative stress in chronic venous leg ulcers

Venous leg ulcers are common and cause considerable morbidity in the population. As healing may be slow or may never be achieved, ulcers create persistent and substantial demands on clinical resources. Great efforts have been made to accelerate tissue repair in chronic venous leg ulcers with limited success. This may at least be partly due to the limited knowledge on the microenvironment of chronic wounds. In fact, the tremendous impact of the microenvironmental conditions on the outcome of wound healing has increasingly become apparent. Oxidative stress as a consequence of an imbalance in the prooxidant-antioxidant homeostasis in chronic wounds is thought to drive a deleterious sequence of events finally resulting in the nonhealing state. The majority of reactive oxygen species are most likely released by neutrophils and macrophages and to an unknown extent from resident fibroblasts and endothelial cells. As the inflammatory phase does not resolve in chronic wounds, the load of reactive oxygen species persists over a long period of time with subsequent continuous damage and perpetuation of the inflammation. In this article, we will critically discuss recent findings that support the role of oxidative stress in the pathophysiology of nonhealing chronic venous leg ulcers.

Extent of iron pick-up in deforoxamine-coupled polyurethane materials for therapy of chronic wounds

Polyurethane net substrates (PNS) coupled with deferoxamine (DFO) have been studied to determine the extent of Fe2+ pick-up for use in chronic wound therapy. A m solution of ferrous sulphate (FeSO4) was used to generate ferrous ions similar to those found in chronic wounds. The concentration of Fe as a function of position through the dressings was evaluated using a variety of techniques. Atomic force microscopy (AFM) and energy-filtered transmission electron microscopy (EFTEM) revealed a rough precipitated layer at the surface of activated PNS exposed to FeSO4 solution. Optical microscopy (OM) and backscattered environmental scanning electron microscopy (ESEM) showed a clear layer of Fe(3+)-enriched material in the surface regions exposed to DFO. The penetration depth of DFO into activated dressings was found to be 20-30 microm. Energy-dispersive X-ray (EDX) analysis was used to approximate the distribution of bound- and unbound-Fe as a function of position within BPNS and DFO-activated dressings after immersing them in a FeSO4 solution for various times. These studies have shown the activity of iron with respect to ionic state in DFO-activated PNS for potential using as dressing for chronic wounds.

Endothelial cell apoptosis is accelerated by inorganic iron and heat via an oxygen radical dependent mechanism

BACKGROUND

Iron participates in diverse pathologic processes by way of the Fenton reaction, which catalyzes the formation of reactive oxygen species (ROS). To test the hypothesis that this reaction accelerates apoptosis, we used human umbilical vein endothelial cells (HUVECs) as surrogates for the microvasculature in vivo.

METHODS

HUVECs were loaded with Fe [III](ferric chloride and ferric ammonium citrate) with 8-hydroxyquinoline as carrier and were then challenged with two stimuli of the heat shock response, authentic heat or sodium arsenite. Iron dependence was tested with two chelators, membrane-impermeable deferoxamine and membrane-permeable o-phenanthroline. The role of ROS was assessed with superoxide dismutase, catalase, and the reporter compound dichlorofluorescein diacetate. The mechanism of cell death was assessed with three complementary techniques, Annexin V/propidium iodide labeling, the TUNEL stain, and electron microscopy.

RESULTS

Iron-loaded HUVECs executed apoptosis after a heat shock stimulus. Iron-catalyzed formation of ROS appeared to be a critical mechanism, because both chelation of iron and enzymatic detoxification of ROS attenuated this apoptosis.

CONCLUSIONS

Inorganic iron, in concert with chemical and physical inducers of the heat shock response, may trigger apoptosis. The accumulation of iron in injured tissue may thereby predispose to accelerated apoptosis and account, in part, for poor wound healing and organ failure.

Reactive oxygen species and iron--a dangerous partnership in inflammation

Cells of nearly all forms of life require well-defined amounts of iron for survival, replication and expression of differentiated processes. It has a central role in erythropoiesis but is also involved in many other intracellular processes in the tissues of the body. It is the fourth most abundant element in the Earth's crust and the most abundant transition metal in living organisms for which its characteristic chemistry endows it with a series of properties enabling it to fulfil certain biological reactions especially those involving redox mechanisms. It is involved in the transport of oxygen, in electron transfer, in the synthesis of DNA, in oxidations by oxygen (O2) and hydrogen peroxide (H2O2) and in many other processes maintaining normal structure and function of virtually all mammalian cells. Because an iron atom can exist in two valency states, ferrous and ferric, iron became the primordial partner of oxygen in evolution. However, as de Sousa et al. (1989) state, such long standing partnerships have to use protective devices to ensure that the toxicity of neither partner is expressed in the presence of the other. Here, we discuss this dangerous partnership and its relevance to inflammation. The main themes of this review are the known roles of iron in the generation of reactive oxygen intermediates and new developments, including iron and transcription and the reaction of iron with nitric oxide. We also consider the widening recognition of the importance of oxygen metabolites in hypoxia-reperfusion injury and disease of the skin and joint.

The role of iron in an acute model of skin inflammation induced by reactive oxygen species (ROS)

The effect of iron was studied in rats in a ROS-initiated model of acute skin inflammation. Iron dextran was administered i.v. 24 h before the induction of the inflammatory response by intradermal injection of glucose oxidase attached to polyethylene glycol (GOD-PEG). Iron exacerbated the response at 24 and 48 h (P greater than 0.001). Histologically, a similar picture was seen to that without iron except for an increase in tissue oedema and matrix destruction including the skin glands. Associated with iron loading was an increase in Perls stainable iron in the skin (P greater than 0.025) and liver (P greater than 0.001). However, skin inflammation without iron loading also increased skin iron levels (P greater than 0.025). Total serum iron was decreased in iron-loaded and GOD-PEG animals (P greater than 0.01) and the unbound iron binding capacity (UIBC) increased (P greater than 0.01).