Significance of Heme and Heme Degradation in the Pathogenesis of Acute Lung and Inflammatory Disorders

Abstract

The heme molecule serves as an essential prosthetic group for oxygen transport and storage proteins, as well for cellular metabolic enzyme activities, including those involved in mitochondrial respiration, xenobiotic metabolism, and antioxidant responses. Dysfunction in both heme synthesis and degradation pathways can promote human disease. Heme is a pro-oxidant via iron catalysis that can induce cytotoxicity and injury to the vascular endothelium. Additionally, heme can modulate inflammatory and immune system functions. Thus, the synthesis, utilization and turnover of heme are by necessity tightly regulated. The microsomal heme oxygenase (HO) system degrades heme to carbon monoxide (CO), iron, and biliverdin-IXα, that latter which is converted to bilirubin-IXα by biliverdin reductase. Heme degradation by heme oxygenase-1 (HO-1) is linked to cytoprotection via heme removal, as well as by activity-dependent end-product generation (i.e., bile pigments and CO), and other potential mechanisms. Therapeutic strategies targeting the heme/HO-1 pathway, including therapeutic modulation of heme levels, elevation (or inhibition) of HO-1 protein and activity, and application of CO donor compounds or gas show potential in inflammatory conditions including sepsis and pulmonary diseases.

Keywords: acute lung injury; carbon monoxide; heme; heme oxygenase; inflammation; lung disease; sepsis.

Figure 1

Mammalian Heme Synthesis and Utilization Pathways. Heme synthesis requires eight sequential enzymatic steps, which begin and end in the mitochondria, with intermittent cytosolic steps. (Step 1), mitochondrial 5-aminolevulinic acid synthase (ALAS; EC 2.3.1.37) condenses succinyl-Co-A and glycine to form 5-aminolevulinic acid (ALA). (Step 2) Two mol ALA are condensed to porphobilinogen (PBG) by ALA dehydratase (ALAD, EC 4.2.1.24, porphobilinogen synthase). (Step 3) Porphobilinogen deaminase (PBGD, EC 2.5.1.61, hydroxymethylbilane synthase) condenses four mol PBG, to generate hydroxymethylbilane (HMB). (Step 4) Uroporphyrinogen-III synthase (UROS, EC 4.2.1.75, uroporphyrinogen III cosynthase) catalyzes the cyclization of hydroxymethylbilane and inversion the D ring to form uroporphyrinogen III (UPGIII). (Step 5) Uroporphyrinogen III is decarboxylated by uroporphyrinogen decarboxylase (UROD, EC 4.1.1.37) to generate coproporphyrinogen III (CPGIII). (Step 6) Coproporphyrinogen III is imported into mitochondria and then decarboxylated by coproporphyrinogen oxidase (CPO, EC 1.3.3.3) to form protoporphyrinogen IX (PPGIX). (Step 7) Protoporphyrinogen IX is converted to protoporphyrin IX (PPIX) by protoporphyrinogen oxidase (PPO, EC 1.3.3.4, protoporphyrinogenase). In the final step (Step 8), ferrous iron is incorporated into the PPIX ring within the mitochondria to form heme-b by the enzyme ferrochelatase (FECH, EC 4.99.1.1, protoheme ferrolyase). Heme is used systemically for oxygen transport and storage functions. In eukaryotic cells, heme is utilized for peroxidase, monooxygenase, and dioxygenase activities, and for cytochromes, including cytochrome p-450s involved in drug metabolism and mitochondrial respiratory chain components.

Figure 2

Heme degradation and cytoprotective effects of the reaction products. The heme molecule is a potent transcriptional activator of heme oxygenase-1 via repression of the transcriptional inhibitor Bach1, resulting in increased synthesis of enzymatically active. The heme oxygenase (HO, EC 1:14:14:18) reaction oxidizes heme, which serves as substrate and co-factor in its degradation, at the α-methene bridge carbon. The reaction, which requires O₂, NADPH and the reductase component of cytochrome p450, produces carbon monoxide (CO), biliverdin-IXα and ferrous iron (Fe II). In the second step of heme degradation biliverdin-IXα is reduced to bilirubin-IXα by NAD(P)H: biliverdin reductase (BVR; EC 1.3.1.24). Both BV and BR are implicated as cellular and circulating antioxidants. Iron released from HO activity is sequestered in a complex with ferritin, which serves as a cellular antioxidant. Excess iron may drive pathological processes including free radical generation and ferroptotic cell death. CO generated from the HO reaction can exert multiple cellular effects, which may be beneficial at low concentrations. Namely, these include inhibition of apoptosis and inflammatory pathways, as well as inhibition of cell proliferation. CO forms a tight binding complex with hemoglobin in circulation to form carboxyhemoglobin (CO-Hb). CO is eliminated by diffusion at the alveolus and exhaled.

Figure 3

Pathological consequences of heme release. Free heme can promote hemolysis of red cells. Heme released into the circulation under hemolytic conditions poses a risk to the vascular endothelial cells, including the promotion of membrane damage, and necrotic and apoptotic cell death. Injured endothelial cells may compromise vascular function, and release DAMPs into the circulation. The toxicity of free heme may be limited by scavenging with hemopexin. Heme can act as a catalyst for pro-oxidant reactions, including LDL oxidation, and the peroxidation of lipids. Iron released from heme degradation may, in excess, further propagate pro-oxidant reactions and trigger ferroptotic cell death. Heme may also promote inflammation by activating TLR4-dependent and inflammasome-dependent pathways in inflammatory cells, leading to the production of pro-inflammatory cytokines.