The proinflammatory vasculotoxic effects of intravascular hemolysis are modulated by plasma hemoglobin and heme clearance via the haptoglobin/CD163 system and the hemopexin/CD91 system, respectively, and detoxification through the heme oxygenase/ferritin system. However, sudden or excessive hemolysis can overwhelm these protective systems leading to heme interacting with cells of the vasculature. Heme presents a damage-associated molecular pattern to the innate immune system. Heme is an extracellular inflammatory signaling molecule with strict binding specificity for TLR4 on monocyte/macrophages, endothelial, and other cells. The resulting TLR4 signaling cascade rapidly leads to intracellular oxidative stress and an inflammatory response. Heme also induces a cytoprotective response that includes Nrf2 responsive genes such as heme oxygenase-1, ferritin, haptoglobin, hemopexin, and other antioxidant response genes. It is the balance between the pro-inflammatory/vasculotoxic effects of plasma hemoglobin/heme and the cytoprotective responses that ultimately determines the pathophysiologic outcome in patients. 1. Introduction When hemoglobin (Hb) is released from red blood cells (RBCs) into plasma, it has the potential to release free heme that can trigger severe oxidative, proinflammatory, and pro-thrombotic injury. Heme has several proinflammatory activities, including leukocyte activation and migration, upregulation of adhesion molecules, reactive oxygen species (ROS) production, and induction of cytokine and chemokine expression [1–4]. Organisms have evolved intricate systems to defend against free heme. The term “free” heme will be used loosely in this review, as heme is amphipathic, mostly insoluble in aqueous solutions at neutral pH, and likely bound to proteins and/or lipids in vivo. This review will focus on the proinflammatory and anti-inflammatory molecular signaling events that are activated by cells in response to free heme. 2. Hemolysis and Plasma Defenses Intravascular hemolysis releases Hb from an antioxidant rich environment inside the RBC into the plasma. Once free in plasma the Hb tetramer is in equilibrium with the α/β Hb dimer favored at low plasma Hb concentrations. The Hb dimers bind to haptoglobin which can safely carry Hb to CD163 receptors on macrophages where the Hb is degraded [5–7]. However, during massive or chronic hemolysis, the haptoglobin/CD163 system can be overwhelmed, leaving free Hb in plasma. When not bound to haptoglobin, oxyHb in plasma can react rapidly with nitric oxide (NO) to form nitrate ( ) and ferric Hb
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