Heme Oxgenase-1, a Cardinal Modulator of Regulated Cell Death and Inflammation

Abstract

Heme oxygenase catalyzes the rate-limiting step in heme degradation in order to generate biliverdin, carbon monoxide (CO), and iron. The inducible form of the enzyme, heme oxygenase-1 (HO-1), exerts a central role in cellular protection. The substrate, heme, is a potent pro-oxidant that can accelerate inflammatory injury and promote cell death. HO-1 has been implicated as a key mediator of inflammatory cell and tissue injury, as validated in preclinical models of acute lung injury and sepsis. A large body of work has also implicated HO-1 as a cytoprotective molecule against various forms of cell death, including necrosis, apoptosis and newly recognized regulated cell death (RCD) programs such as necroptosis, pyroptosis, and ferroptosis. While the antiapoptotic potential of HO-1 and its reaction product CO in apoptosis regulation has been extensively characterized, relatively fewer studies have explored the regulatory role of HO-1 in other forms of necrotic and inflammatory RCD (i.e., pyroptosis, necroptosis and ferroptosis). HO-1 may provide anti-inflammatory protection in necroptosis or pyroptosis. In contrast, in ferroptosis, HO-1 may play a pro-death role via enhancing iron release. HO-1 has also been implicated in co-regulation of autophagy, a cellular homeostatic program for catabolic recycling of proteins and organelles. While autophagy is primarily associated with cell survival, its occurrence can coincide with RCD programs. This review will summarize the roles of HO-1 and its reaction products in co-regulating RCD and autophagy programs, with its implication for both protective and detrimental tissue responses, with emphasis on how these impact HO-1 as a candidate therapeutic target in disease.

Keywords: apoptosis; autophagy; carbon monoxide; cell death; ferroptosis; heme oxygenase; inflammasome; inflammation; necroptosis; pyroptosis.

Figure 1

HO activity and role in cytoprotection. Heme oxygenase (HO: heme, hydrogen-donor:oxygen oxidoreductase (α-methene-oxidizing, hydroxylating), EC: 1:14:99:3) is the rate-limiting step in heme degradation. HO catalyzes the oxidative cleavage of heme at the α-methene bridge carbon, released as carbon monoxide (CO), to generate biliverdin-IXα (BV), while releasing the central heme iron chelate as ferrous iron (Fe II). Enzymatic heme degradation requires three moles of molecular oxygen (O₂) and electrons derived from NADPH-cytochrome p450 reductase (EC: 1.6.2.4). The BV generated in the HO reaction is subsequently reduced by NAD(P)H: biliverdin reductase (BVR; EC: 1.3.1.24) to generate the lipid-soluble bile pigment bilirubin-IXα (BR). The source of heme for the HO reaction is derived from the turnover of hemoglobin and cellular hemoproteins. Free unbound heme released from hemolysis may represent a pro-oxidant hazard to vascular endothelium and may initiate pro-inflammatory reactions. BV and BR are known antioxidants, with circulating BR implicated as a mitigator of cardiovascular disease risk. Iron released from HO activity is equilibrated into ferritin storage, whereas unbound iron may propagate injury via catalysis of free radical-generating reactions. CO evolving from HO activity may modulate cellular function, via regulation of vascular function, immune system function, inflammation, apoptosis, and cellular proliferation. Abbreviations: CO: carbon monoxide; Cyt p450: cytochrome p450; iNOS: inducible nitric oxide synthase; NF-κB: nuclear factor-kappa-B; NOX: NADPH oxidase isoforms; p38 MAPK: p38 mitogen-activated protein kinase; sGC: soluble guanylate cyclase; TLR4: Toll-like receptor-4; UDP: uridine 5’-diphosphate.

Figure 2

Subcellular localization and non-canonical roles of HO-1. Endoplasmic reticulum (microsomal)-associated HO-1 induction by chemical and physical stress represents the classical form of HO-1 regulation and is implicated in cellular homeostasis and cytoprotection. Additional compartment-specific subcellular localization of HO-1 has been described including stress activated translocation of HO-1 to the mitochondria. The HO-1 likely functions in regulating heme bioavailability in this compartment/and or localized CO production. HO-1 has also been shown to localize to plasma membrane caveolae, where it forms an inhibitory complex with caveolin-1. HO-1 may also migrate to the nucleus in a COOH-terminal truncated nuclear form (NHO-1) that is devoid of enzyme activity. This NHO-1 has been implicated in transcription factor regulation, including NF-κB, AP-1, and Nrf2, the latter which regulates the antioxidant response, including HO-1 gene expression. Abbreviations: AP-1: activator protein-1; ER: endoplasmic reticulum; HO-1: heme oxygenae-1; miR: microRNA, NF-κB: nuclear factor-kappa-B; NHO-1: nuclear HO-1, Nrf2: NF-E2-related factor-2; ROS: reactive oxygen species.

Figure 3

Positioning of HO-1 in relation to cellular homeostatic, apoptotic, and regulated cell death (RCD) programs. Heme oxygenase-1 and/or its reaction product CO can modulate the cellular autophagy program, which degrades cytosolic proteins and organelles, and is itself implicated in cellular survival/protection. HO-1 via its reaction product CO has been identified as a cellular antiapoptotic mediator via regulation of p38 MAPK and other factors. Induction of HO-1 via either heme clearance and/or CO production may act as a mediator of inflammatory RCD programs. Among these, HO-1-derived CO may inhibit NLRP3-ASC-mediated caspase-1 activation, which in term regulate gasdermin-D (GSDMD)-initiated pyroptosis. HO-1 dependent heme clearance may play a protective role in regulating RIPK3/MLKL-dependent necroptosis. Finally, HO-1-derived iron may promote lipid peroxidation, leading to ferroptotic cell death which implicates HO-1 as a pro-death regulator in the context of iron overproduction. This pathway is counter-regulated by ferritin, which sequesters iron, and promoted by autophagy-dependent degradation of ferritin (ferritinophagy). Abbreviations: ASC: apoptosis-associated speck-like protein containing caspase-activation and recruitment doman [CARD]; GSDMD: gasdermin-D; GSH: glutathione (reduced form); GSSG: glutathione disulfide (oxidized form); GPX4: glutathione peroxidase-4; GR: glutathione reductase; LH: lipid (reduced form); LOOH: lipid hydroperoxide; MLKL: mixed-lineage kinase domain-like pseudokinase (p-, denotes phospho- form); NLRP3: nucleotide-binding domain, leucine-rich-containing family, pyrin domain-containing-3; RIPK1: receptor-interacting serine/threonine-protein kinase 1; RIPK3: receptor-interacting serine/threonine-protein kinase-3 (p-, denotes phospho- form); SXc-: cystine/glutamate transporter, System Xc-.