Heme Oxygenase-1: An Anti-Inflammatory Effector in Cardiovascular, Lung, and Related Metabolic Disorders

Abstract

The heme oxygenase (HO) enzyme system catabolizes heme to carbon monoxide (CO), ferrous iron, and biliverdin-IXα (BV), which is reduced to bilirubin-IXα (BR) by biliverdin reductase (BVR). HO activity is represented by two distinct isozymes, the inducible form, HO-1, and a constitutive form, HO-2, encoded by distinct genes (HMOX1, HMOX2, respectively). HO-1 responds to transcriptional activation in response to a wide variety of chemical and physical stimuli, including its natural substrate heme, oxidants, and phytochemical antioxidants. The expression of HO-1 is regulated by NF-E2-related factor-2 and counter-regulated by Bach-1, in a heme-sensitive manner. Additionally, HMOX1 promoter polymorphisms have been associated with human disease. The induction of HO-1 can confer protection in inflammatory conditions through removal of heme, a pro-oxidant and potential catalyst of lipid peroxidation, whereas iron released from HO activity may trigger ferritin synthesis or ferroptosis. The production of heme-derived reaction products (i.e., BV, BR) may contribute to HO-dependent cytoprotection via antioxidant and immunomodulatory effects. Additionally, BVR and BR have newly recognized roles in lipid regulation. CO may alter mitochondrial function leading to modulation of downstream signaling pathways that culminate in anti-apoptotic, anti-inflammatory, anti-proliferative and immunomodulatory effects. This review will present evidence for beneficial effects of HO-1 and its reaction products in human diseases, including cardiovascular disease (CVD), metabolic conditions, including diabetes and obesity, as well as acute and chronic diseases of the liver, kidney, or lung. Strategies targeting the HO-1 pathway, including genetic or chemical modulation of HO-1 expression, or application of BR, CO gas, or CO donor compounds show therapeutic potential in inflammatory conditions, including organ ischemia/reperfusion injury. Evidence from human studies indicate that HO-1 expression may represent a biomarker of oxidative stress in various clinical conditions, while increases in serum BR levels have been correlated inversely to risk of CVD and metabolic disease. Ongoing human clinical trials investigate the potential of CO as a therapeutic in human disease.

Keywords: bilirubin; carbon monoxide; cardiovascular disease; heme oxygenase; inflammation; kidney disease; lung disease; metabolic disease.

Figure 1

Enzymatic heme catabolism. Heme is a vital molecule used as a prosthetic group for diverse cellular heme-containing proteins, including oxygen carriers (i.e., hemoglobin, myoglobin), mitochondrial cytochromes, and enzymes. In free form, heme can catalyze pro-oxidant reactions and promote inflammation. Heme oxygenase (HO, E.C. 1:14:14:18) catalyzes the oxidative degradation of heme to biliverdin-IXα (BV). This reaction requires 3 mol O₂ and NADPH: cytochrome p-450 reductase as the electron source. During heme cleavage, the α-methene bridge carbon of heme is released as carbon monoxide (CO), while the central heme iron is liberated as ferrous iron (Fe-II). HO activity is represented by an inducible form (HO-1) and a constitutively expressed form (HO-2). BV is converted to bilirubin-IXα (BR) by NAD(P)H: biliverdin reductase. Both BV and BR have potent cellular antioxidant activities. Additionally, BR has immunomodulatory effects, and a newly recognized role as a regulator of lipid metabolism. Iron released from HO activity may act as a pro-oxidant or trigger the synthesis of ferritin, which can act as a co-cytoprotectant by sequestering heme-derived iron. HO-1-derived CO can regulate cellular processes including inflammation, apoptosis, and cell proliferation. CO and BV/BR can confer protection in experimental models of ALI, AKI, CVD and related metabolic disorders, and organ IRI. Ferritin has also been implicated in vascular protection.

Figure 2

Regulation of HO-1 gene expression. The Hmox1 gene contains distal enhancer regions located at −4 kb and −10 kb and additional cis-elements in the proximal promoter region. HO-1 responds to transcriptional regulation by heme, electrophilic compounds, ROS, heavy metals and other diverse stimuli. HO-1 gene expression in response to diverse agents is regulated by the nuclear factor erythroid 2–related factor 2 (Nrf2). Nrf2 forms stable heterodimers with small Maf proteins, which target antioxidant response elements (ARE) within the enhancers. The Kelch-like ECH-associated protein (Keap1), serves as a cytoplasmic anchor for Nrf2 under basal conditions, and facilitates its proteasomal degradation. When cells are exposed to inducing stimuli, Keap1 disengages from Nrf2, enabling Nrf2 to translocate to the nucleus where it can transactivate Hmox1 gene expression. Nrf2 can also regulate other targets such as NAD(P)H: Quinone oxidoreductase 1 (NQO1) and glutathione S-transferase (GSTA1), and the autophagy substrate protein p62^SQSTM1. The BTB and CNC homology-1 (Bach-1) protein acts as a transcriptional repressor of HO-1 and other Nrf2 target genes. Bach1 heterodimerizes with small Maf proteins to compete with Nrf2 for binding to ARE sequences in target gene promoters. Heme binding to Bach-1 inhibits DNA binding activity and promotes its nuclear export via the Crm1 transporter, leading to its proteasomal degradation. Additional transcription factors implicated in HO-1 transcriptional regulation include activator protein (AP)-1, hypoxia-inducible factor-1 (HIF-1), and nuclear factor-kappa-B (NF-κB).

Figure 3

Role of HO-1/CO system in lung, vascular and kidney diseases. HO serves primarily a detoxification role in the removal of heme, which in free form can propagate inflammatory injury. Displacement of heme in exchange for labile iron may have toxic sequelae such as promotion of membrane lipid peroxidation and ferroptotic cell death. Labile iron triggers ferritin synthesis, which can contribute to vascular protection in CVD. Experimental models have suggested that BV/BR generation may contribute to protection in vascular injury, IRI, AKI, and ALI. BV and BR are known antioxidants. BR has a newly defined role as a regulator of lipid metabolism via activation of PPARα. BVR, which converts BV to BR, can modulate signaling pathways via intrinsic kinase activity. These effects may be important for regulation of lipid levels in the pathogenesis of liver and metabolic disorders. HO-derived CO generation (or exogenous CO application) triggers complex downstream signaling events with mitochondria believed to represent the proximal target. Among these include p38 MAPK and various other known signaling intermediates. CO-dependent signaling can trigger anti-inflammatory processes such as downregulation of neutrophil migration and macrophage pro-inflammatory cytokines production, that may provide protection in diverse conditions including vascular injury, liver injury, AKI, and ALI, and IRI in general. Additionally, CO can modulate smooth muscle proliferation, which can contribute to vascular protection. CO can also modulate fibroblast proliferation/activation, which may confer protection in organ fibrosis.