The Nuclear Form of Heme Oxygenase (HO)-1 Preferentially Promotes Glycolysis in Hyperoxia But Fails to Protect Against Heme Toxicity

Heme oxygenase-1 (HO-1) is a stress-inducible enzyme that catalyzes the degradation of heme, generating carbon monoxide and antioxidant bile pigments. Cytoplasmic HO-1 is tethered to the smooth endoplasmic reticulum via a C-terminal trans-membrane segment. Cleavage of this anchor allows for localization of a truncated, catalytically inactive form of HO-1 to the nucleus, as has been observed in fetal lung cells in hyperoxia and in several cancer tissues. While cytoplasmic HO-1 is associated with cytoprotection directly via its antioxidant activities, nuclear HO-1 also plays a role in protection against oxidative injury via its interaction with other nuclear proteins and transcription factors, resulting in altered gene expression of several rate-limiting enzymes in key metabolic pathways including glycolysis. Interestingly, nuclear HO-1 is also associated with a proliferative phenotype. Although HO-1 has been shown to play a role in glucose metabolism and altered mitochondrial dynamics, the mechanism by which this occurs is not known. In an effort to investigate this, wild type and HO-1 -/- mouse embryonic fibroblast (MEF) cells as well as MEF expressing only the nuclear, truncated HO (HO-TR), were evaluated for glycolysis and oxidative phosphorylation (oxphos) after hyperoxic exposure (95% O 2 /5% CO 2 for 24 hours) and controls were exposed to air/5% CO 2 for 24 hours. The HO-TR had increased glycolytic activity compared to HO +/+ or HO -/- cells. In contrast, oxphos, in particular spare respiratory capacity, was no different between any of the groups in hyperoxia. However, the presence of excess heme (0-50 micromolar) inhibited oxphos activity in the MEF lacking HO-1 catalytic activity, namely, HO-TR and HO -/- , in a dose dependent fashion. Our results suggest a role of nuclear HO-1 in enhancing glycolysis during hyperoxic stress thereby protecting against injury and promoting cell proliferation. They also suggest that catalytically active HO-1 is required for homeostasis of mitochondial function. Taken together our results suggest that nuclear HO-1 is cytoprotective under hyperoxic stress via a metabolic shift toward glycolysis and highlight a putative role of HO-1 in mitochondrial dynamics and metabolic regulation.