Key Points Heme and iron induce macrophage phenotypic switching toward an M1 proinflammatory phenotype. By scavenging free heme, hemopexin reverts heme-induced proinflammatory activation of macrophages in a mouse model of sickle cell disease.

Hemolysis, which occurs in many disease states, can trigger a diverse pathophysiologic cascade that is related to the specific biochemical activities of free Hb and its porphyrin component heme. Normal erythropoiesis and concomitant removal of senescent red blood cells (RBC) from the circulation occurs at rates of approximately 2 × 106 RBCs/second. Within this physiologic range of RBC turnover, a small fraction of hemoglobin (Hb) is released into plasma as free extracellular Hb. In humans, there is an efficient multicomponent system of Hb sequestration, oxidative neutralization and clearance. Haptoglobin (Hp) is the primary Hb-binding protein in human plasma, which attenuates the adverse biochemical and physiologic effects of extracellular Hb. The cellular receptor target of Hp is the monocyte/macrophage scavenger receptor, CD163. Following Hb-Hp binding to CD163, cellular internalization of the complex leads to globin and heme metabolism, which is followed by adaptive changes in antioxidant and iron metabolism pathways and macrophage phenotype polarization. When Hb is released from RBCs within the physiologic range of Hp, the potential deleterious effects of Hb are prevented. However, during hyper-hemolytic conditions or with chronic hemolysis, Hp is depleted and Hb readily distributes to tissues where it might be exposed to oxidative conditions. In such conditions, heme can be released from ferric Hb. The free heme can then accelerate tissue damage by promoting peroxidative reactions and activation of inflammatory cascades. Hemopexin (Hx) is another plasma glycoprotein able to bind heme with high affinity. Hx sequesters heme in an inert, non-toxic form and transports it to the liver for catabolism and excretion. In the present review we discuss the components of physiologic Hb/heme detoxification and their potential therapeutic application in a wide range of hemolytic conditions.

Heme is required for cell respiration and survival. Nevertheless, its intracellular levels need to be finely regulated to avoid heme excess, which may catalyze the production of reactive oxygen species (ROS) and promote cell death. Here, we show that alteration of heme homeostasis in endothelial cells due to the loss of the heme exporter FLVCR1a, results in impaired angiogenesis. In vitro, FLVCR1a silencing in endothelial cells causes defective tubulogenesis and poor viability due to intracellular heme accumulation. Consistently, endothelial-specific Flvcr1a knockout mice show aberrant angiogenesis responsible for hemorrhages and embryonic lethality. Importantly, we demonstrate that impaired heme export leads to endothelial cell death by paraptosis and provide evidence that endoplasmic reticulum (ER) stress precedes heme-induced paraptosis. These findings highlight a crucial role for the cytosolic heme pool in the control of endothelial cell survival and in the regulation of the angiogenic process. Interfering with endothelial heme export represents a valuable model for a deeper understanding of the molecular mechanisms underlying heme-triggered paraptosis and, in the future, might provide a novel tool for the modulation of angiogenesis in pathophysiologic conditions.

Background & AimsThe liver has one of the highest rates of heme synthesis of any organ. More than 50% of the heme synthesized in the liver is used for synthesis of P450 enzymes, which metabolize exogenous and endogenous compounds that include natural products, hormones, drugs, and carcinogens. Feline leukemia virus subgroup C cellular receptor 1a (FLVCR1a) is plasma membrane heme exporter that is ubiquitously expressed and controls intracellular heme content in hematopoietic lineages. We investigated the role of Flvcr1a in liver function in mice.MethodsWe created mice with conditional disruption of Mfsd7b, which encodes Flvcr1a, in hepatocytes (Flvcr1afl/fl;alb-cre mice). Mice were analyzed under basal conditions, after phenylhydrazine-induced hemolysis, and after induction of cytochromes P450 synthesis. Livers were collected and analyzed by histologic, quantitative real-time polymerase chain reaction, and immunoblot analyses. Hepatic P450 enzymatic activities were measured.ResultsFlvcr1afl/fl;alb-cre mice accumulated heme and iron in liver despite up-regulation of heme oxygenase 1, ferroportin, and ferritins. Hepatic heme export activity of Flvcr1a was closely associated with heme biosynthesis, which is required to sustain cytochrome induction. Upon cytochromes P450 stimulation, Flvcr1afl/fl;alb-cre mice had reduced cytochrome activity, associated with accumulation of heme in hepatocytes. The expansion of the cytosolic heme pool in these mice was likely responsible for the early inhibition of heme synthesis and increased degradation of heme, which reduced expression and activity of cytochromes P450.ConclusionsIn livers of mice, Flvcr1a maintains a free heme pool that regulates heme synthesis and degradation as well as cytochromes P450 expression and activity. These findings have important implications for drug metabolism.