Haem oxygenase shows pro-oxidant activity in microsomal and cellular systems: implications for the release of low-molecular-mass iron

Abstract

Haem oxygenase-1 (HO-1) is a highly inducible stress protein that removes haem from cells with the release of biliverdin, carbon monoxide and low-molecular-mass iron (LMrFe). Several antioxidant functions have been ascribed to HO; its induction is considered to be a protective event. However, LMrFe produced during haem catabolism might elicit a pro-oxidant response, with deleterious consequences. We therefore investigated the delicate balance between pro-oxidant and antioxidant events with the use of a microsomal lipid peroxidation (LPO) system. By using microsomal-bound HO in an NADPH-dependent LPO system, we assessed the pro-oxidant nature of the released LMrFe and the antioxidant effect of the released bilirubin. Hb, a biologically relevant substrate for HO, was included with the microsomes to supplement the source of haem iron and to promote LPO. We found significant increases in microsomal LPO, by using the thiobarbituric acid (TBA) test, after incubation with Hb. This Hb-stimulated peroxidation was inhibited by HO inhibitors and by iron chelators, suggesting a HO-driven, iron-dependent mechanism. GLC-MS was employed to measure the specific LPO product 4-hydroxy-2-nonenal and to confirm our TBA test results. A HO inhibitor attenuated an increase in intracellular LMrFe that occurred after treatment of rat pulmonary artery smooth-muscle cells with Hb. Additionally, exogenously added bilirubin at an equimolar concentration to the LMrFe present in both microsomal and liposomal systems was unable to prevent the pro-oxidant effect of the iron. Under certain circumstances HO can act as a pro-oxidant and seems to have a role in stimulating microsomal LPO.

A Dual Role of Heme Oxygenase-1 in Cancer Cells

“…Ferroptosis induction by HO-1 is mediated by iron augmentation and lipid peroxidation [15,16,17]. The induction is tightly associated with oxidative stress, due to its predominant sensitivity to oxidative inducers, such as redox-active iron, heme, hemoglobin, and heme-containing proteins [61,62]. The upregulation of HO-1 can enhance heme degradation and ferritin synthesis and change the intracellular iron distribution [57].…”

Modulation of Inflammation and Immune Responses by Heme Oxygenase-1: Implications for Infection with Intracellular Pathogens

“…Despite its well-known antioxidant effects, HO-1 activity, when upregulated, can act in the opposite direction inducing intracellular labile iron accumulation leading to ROS production [152,153], effects that were shown to contribute to the cellular dysfunction observed in LPS-induced endotoxic shock [154]. Induction of HO-1 expression by hemin or genetically induced HO-1 overexpression was found to induce ferroptosis, a necrotic form of cell death dependent on iron accumulation and peroxidation of cell membrane lipids [155].…”

Role of Heme Oxygenase as a Modulator of Heme-Mediated Pathways

“…An increased HO activity transiently augments the intracellular free iron pool [96] (or loosely bound or labile iron pool [97]) and leads to an increased ROS formation [74,98]. Although ROS stimulates ferritin, synthesis and sequestration of iron, the increased iron deposition may lead to iron overload, as has been shown following sustained HO overexpression [74,99]. Iron overload is generally associated with a higher risk of oxidative damage [100,101].…”