Heme oxygenase-1 (HO-1) catalyzes the oxidation of heme to biologically active products: carbon monoxide (CO), biliverdin, and ferrous iron. It participates in maintaining cellular homeostasis and plays an important protective role in the tissues by reducing oxidative injury, attenuating the inflammatory response, inhibiting cell apoptosis, and regulating cell proliferation. HO-1 is also an important proangiogenic mediator. Most studies have focused on the role of HO-1 in cardiovascular diseases, in which its significant, beneficial activity is well recognized. A growing body of evidence indicates, however, that HO-1 activation may play a role in carcinogenesis and can potently influence the growth and metastasis of tumors. HO-1 is very often upregulated in tumor tissues, and its expression is further increased in response to therapies. Although the exact effect can be tissue specific, HO-1 can be regarded as an enzyme facilitating tumor progression. Accordingly, inhibition of HO-1 can be suggested as a potential therapeutic approach sensitizing tumors to radiation, chemotherapy, or photodynamic therapy.
HEME OXYGENASE-1 (HO-1)HEME OXYGENASE (HO) is a microsomal enzyme catalyzing the first, rate-limiting step in degradation of heme and playing an important role in recycling of iron (103). It cleaves the α-meso carbon bridge of heme, yielding equimolar quantities of carbon monoxide (CO) and iron ions Fe 2+ and biliverdin (156). CO is then exhaled from the organisms through the lung (50). Free iron induces the expression of the iron-sequestering ferritin and activates FeATPase, an iron transporter, which decrease intracellular Fe 2+ content. Finally, biliverdin is converted by biliverdin reductase to bilirubin (144), which can be oxidized by cytochrome P450 enzymes, such as Cyp1A1, Cyp2B1, or Cyp2a5, or glucoronidated by UDPglucuronyl-transferase and subsequently eliminated as bilirubin glucoronides by the biliaryfecal pathway (25).The enzymatic activity of HO results in decreased oxidative stress, attenuated inflammatory response, and a lower rate of apoptosis (Fig. 1). This is due to removal of heme, a potent prooxidant and proinflammatory agent, but also because of generation of biologically active products. Among them, CO is an important cellular messenger, with the signaling function resembling that of nitric oxide (NO). Like NO, CO induces soluble guanylyl cyclase (sGC) and thereby inhibits platelet aggregation, decreases leukocyte adhesion, and reduces endothelial cell apoptosis. In addition, it exerts antiinflammatory effects by inhibition of tumor necrosis factor (TNF), interleukin-1β (IL-1β), and macrophage inflammatory protein-1β (MIP-1β), or by upregulation of interleukin-10 (IL-10) (123). Ferrous iron, the © Mary Ann Liebert, Inc. Address reprint requests to: Alicja Jozkowicz, Ph.D., D.Sc., Department of Medical Biotechnology, Faculty of Biophysics, Biochemistry and Biotechnology, Jagiellonian University, Gronostajowa 7, 30-387 Krakow, Poland, E-mail:alicia@mol.uj.edu.pl.
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