The Diverse Roles of Heme Oxygenase-1 in Tumor Progression

Abstract

Heme oxygenase-1 (HO-1) is an inducible intracellular enzyme that is expressed in response to a variety of stimuli to degrade heme, which generates the biologically active catabolites carbon monoxide (CO), biliverdin and ferrous iron (Fe²⁺). HO-1 is expressed across a range of cancers and has been demonstrated to promote tumor progression through a variety of mechanisms. HO-1 can be expressed in a variety of cells within the tumor microenvironment (TME), including both the malignant tumor cells as well as stromal cell populations such as macrophages, dendritic cells and regulatory T-cells. Intrinsically to the cell, HO-1 activity provides antioxidant, anti-apoptotic and cytoprotective effects via its catabolites as well as clearing toxic intracellular heme. However, the catabolites of heme degradation can also diffuse outside of the cell to extrinsically modulate the wider TME, influencing cellular functionality and biological processes which promote tumor progression, such as facilitating angiogenesis and metastasis, as well as promoting anti-inflammation and immune suppression. Pharmacological inhibition of HO-1 has been demonstrated to be a promising therapeutic approach to promote anti-tumor immune responses and inhibit metastasis. However, these biological functions might be context, TME and cell type-dependent as there is also conflicting reports for HO-1 activity facilitating anti-tumoral processes. This review will consider our current understanding of the role of HO-1 in cancer progression and as a therapeutic target in cancer.

Keywords: angiogenesis; cancer; cytoprotection; heme oxygenase-1 (HO-1); metastasis; tumor associated macrophages (TAMs); tumor immunology.

Figure 1

The oxidative degradation of heme by HO-1. In the first step of heme degradation, the ER membrane bound HO-1 interacts with the electron donor NADPH-cytochrome P450 reductase, and an oxygen molecule. The complex degrades heme to biliverdin, carbon monoxide (CO), and a ferrous iron (Fe²⁺). NADPH-biliverdin reductase competitively binds to HO-1 to reduce biliverdin to bilirubin, by using NADPH as an electron donor (1, 33).

Figure 2

The diverse range of signals in the TME which could induce HMOX1 expression. HMOX1 mRNA expression is induced by a range of molecular and physical signals. The figure highlights common physical characteristics (hypoxia), metabolic by-products (ROS) and cytokines commonly associated with the TME which could induce the expression of HO-1 in cancer. In normal conditions, the transcription factor Nrf2 is inhibited by Keap1 which prevents nuclear translocation and promotes its proteasomal degradation. ROS generated by oxidative stress inhibit the interaction between Keap1 and Nrf2, allowing Nrf2 to translocate to the nucleus and bind to the ARE of the HMOX1 gene. In the presence of heme, Bach1 is prevented from inhibiting Nrf2 access to the ARE allowing the expression of HMOX1. Under conditions of hypoxia, the transcription factor HIF-1α escapes proteasomal degradation and translocates into the nucleus where it forms a complex with HIF-1β and binds to HRE to up-regulate HMOX1 gene expression. Cytokines such as IL-10 and IL-6 can induce expression via the JAK/STAT3 pathway, while IL-1α and TNF-α induced HMOX1 expression through a PKC, Ca²⁺ signaling, and PLA2 dependent pathway and AP-1 element in the promoter region.

Figure 3

The diverse range of possible modulatory effects of the heme catabolites generated by the enzyme activity of HO-1 in the TME. This figure summarizes the possible modulatory effects of the heme catabolites in the TME. However, the relative utilization and biological importance of each pathway in disease progression or control is TME-dependent and key observations relating to these pathways are discussed within the text. CO and biliverdin/bilirubin mediate their anti-apoptotic effects via the modulation of signal transduction, and the inhibition of K⁺ influx. Their antioxidant effects are mediated via the inhibition of iNOS. Both CO and biliverdin/bilirubin inhibit the activity of pro-inflammatory cytokines, the NF-κB pathway and increase production of IL-10, and CO also activates sGC. CO induces the maturation of tolerogenic DCs and inhibits the activity and proliferation of CD8⁺ T-cells, while bilirubin inhibits the activation of CD4⁺ T-cells. Ferrous iron [Fe (II)] enacts antioxidant effects via ferritin, however, an imbalance between ROS levels and iron overload can lead to lipid oxidation and ferroptosis.

Figure 4

The effects of HO-1 activity on immune cells in the TME. HO-1 activity may affect several immune cell types to suppress the anti-tumor immune response. HO-1 reduces the effector functions of CD8⁺ T-cells by suppressing their expression of TNF-α, granzyme-B and IFN-γ. Specifically, CO suppresses the effector functions mediated via TCR engagement. HO-1 has also been implicated in suppressing CD8⁺ T-cell proliferation, tumor infiltration and increasing their activation threshold. In CD4⁺ T-cells CO can block TCR-dependent IL-2 production, thus influencing the proliferation of CD8⁺ and CD4⁺ T-cells. Bilirubin/Biliverdin can block CD4⁺ T-cell activity by inducing apoptosis, inhibiting proliferation and suppressing co-stimulatory molecule activities. In Tregs, HO-1 has been associated with the expansion of the population and HO-1-derived CO can improve Treg survival in conditions of hypoxia. HO-1 can also cause DCs to secrete IL-10 which maintains these cells in a tolerogenic state alongside up-regulating their expression of HO-1. HO-1 activity also down-regulates the expression of pro-inflammatory cytokines in these cells. Additionally, both CO and bilirubin can down-regulate MHCII expression in DCs. In TAMs, CO inhibits activation while also suppressing the release of pro-inflammatory cytokines and concurrently eliciting an increase in IL-10 production. In endothelial cells, CO can suppress the expression of adhesion molecules, affecting the ability of neutrophils to attach. HO-1 may also inhibit neutrophil activation itself and CO and biliverdin can inhibit migration and infiltration of both neutrophils and TAMs. Additionally, in NK cells HO-1 activity can decrease their cytotoxicity by suppressing their ability to secrete pro-inflammatory cytokines and express activatory cell receptors. Image was created using BioRender software.