FIG. 3.

Molecular mechanisms underlying heme- and iron-driven EC and macrophage activation. NHBH and NTBI mediate the proinflammatory activation of ECs and macrophages by producing ROS and activating the TLR4/MyD88/MAPK/ERK/NFkB and inflammasome signaling pathways. Heme and iron induce ROS generation non-enzymatically, through the Fenton reaction and by converting organic hydroperoxides into free radicals, and enzymatically, through the activation of the p-Syk/PKC/NOX pathway. TLR4 activation by heme/iron promotes MAPKs and NFkB activation, which in turn induce the transcription of inflammation-related genes, including inflammatory cytokines and chemokines (e.g., IL-6, TNFα, IL-1β, IL-8) and adhesion molecules (e.g., E-selectin, P-selectin, ICAM-1, VCAM-1). Although heme and iron trigger ROS formation independently of TLR4 activation, ROS production is required for full MAPK activation and cytokine induction, suggesting a synergistic action of the two pathways. By inducing ROS, heme and iron also contribute to NLRP3/inflammasome activation, leading to the maturation of pro-IL1β to IL1β via caspase-1-mediated cleavage. TLR4 inhibitors and antioxidants by blocking the major heme/iron-induced pathways, and chelators (transferrin, deferoxamine, deferasirox, deferiprone, hemopexin) by scavenging heme/iron prevent the activation of ECs and macrophages in conditions where free heme and iron are elevated. ERK, extracellular-signal-regulated kinase; ICAM-1, intercellular adhesion molecule 1; IL, interleukin; MAPK, mitogen-activated protein kinase; MyD88, myeloid differentiation primary response 88; NFkB, nuclear factor kappa-light-chain-enhancer of activated B cells; NHBH, nonhemopexin-bound iron; NLRP3, NLR Family Pyrin Domain Containing 3; NOX, NADPH oxidase; PKC, protein kinase C; p-Syk = phosphorylated spleen tironase kinase; ROS, reactive oxygen species; TLR4, toll-like receptor 4; TNFα, tumor necrosis factor alpha; VCAM-1, vascular adhesion molecule 1. Color images are available online.