Haem oxygenase-1 (HO-1) catalyses the rate-limiting step in haem degradation. All three metabolites resulting
from haem degradation (carbon monoxide (CO), biliverdin and free iron) have anti-inflammatory and anti-apoptotic
properties. HO-1 is a stress-inducible enzyme found extensively expressed in a vast variety of both human and murine
cancers, where it serves as an essential survival molecule by modulating expression of molecules regulating apoptosis and
stimulating angiogenesis. In addition, HO-1 contributes in a critical manner to inhibition or termination of inflammation.
Consequently, several anticancer strategies aim at targeting HO-1. The inhibition of HO-1 may cause tumour cells to
become more sensitive to chemotherapy and radiation therapy. The water-soluble forms of the HO-1 inhibitor Zinc
protoporphyrin (ZnPP) have seemed promising in different in-vivo models, in which it has induced growth arrest in
tumour cells with few, if any, side effects. Studies have suggested that HO-1 may also function to disrupt the tumour
metastasising process, since the expression of the metalloprotease MMP9 is inversely correlated with HO-1 expression.
Additionally, HO-1 has anti-inflammatory functions which play a very important role in the negative regulation of the
immune system. Immunological targeting of HO-1 might represent an interesting approach, as epitopes derived from HO-
1 have been found exclusively on tumour tissue. Natural HO-1-specific T-cell responses have been identified in cancer
patients. Hence, recently HO-1-specific, CD8+ regulatory T cells were described in cancer patients, which in concert with
HO-1 expression might be responsible for a highly immunosuppressive tumour microenvironment.
Here, we summarise current knowledge of the role of HO-1 in cancer, report the different results of the targeting of HO-1
in preclinical and clinical settings, and discuss future opportunities.