Enhanced Translation of Heme Oxygenase-2 Preserves Human Endothelial Cell Viability during Hypoxia

Abstract

Heme oxygenases (HOs) -1 and -2 catalyze the breakdown of heme to release carbon monoxide, biliverdin, and ferrous iron, which may preserve cell function during oxidative stress. HO-1 levels decrease in endothelial cells exposed to hypoxia, whereas the effect of hypoxia on HO-2 expression is unknown. The current study was carried out to determine if hypoxia alters HO-2 protein levels in human endothelial cells and whether this enzyme plays a role in preserving their viability during hypoxic stress. Human umbilical vein endothelial cells (HUVECs), human aortic endothelial cells (HAECs), and human blood outgrowth endothelial cells were exposed to 21% or 1% O 2 for 48 or 16 h in the presence or absence of tumor necrosis factor-␣ (10 ng/ml) or H 2 O 2 (100 M). In all three endothelial cell types HO-1 mRNA and protein levels were decreased following hypoxic incubation, whereas HO-2 protein levels were unaltered. In HUVECs HO-2 levels were maintained during hypoxia despite a 57% reduction in steady-state HO-2 mRNA level and a 43% reduction in total protein synthesis. Polysome profiling revealed increased HO-2 transcript association with polysomes during hypoxia consistent with enhanced translation of these transcripts. Importantly, inhibition of HO-2 expression by small interference RNA increased oxidative stress, exacerbated mitochondrial membrane depolarization, and enhanced caspase activation and apoptotic cell death in cells incubated under hypoxic but not normoxic conditions. These data indicate that HO-2 is important in maintaining endothelial viability and may preserve local regulation of vascular tone, thrombosis, and inflammatory responses during reductions in systemic oxygen delivery.

Effects of Ellagic Acid on Angiogenic Factors in Prostate Cancer Cells

“…HO-1 expression has been demonstrated to be associated with prostate cancer progression [56]. In addition, it has been shown that selective inhibition of HO-2 by siRNA increases reactive oxygen species and activates caspases inducing apoptotic cell death [57], but up to date, the role of HO-2 in cancer is almost unexplored. Results reported in the present study evidenced, for the first time, the ability of EA to decrease the expressions of HO-1 and HO-2, both representative of one of the most important cytoprotective system in the cell.…”

Functioning of an arteriovenous fistula requires heme oxygenase-2

“…HO-2, for example, is a survival factor in the endothelium as shown by the observations that HO-2 deficiency exacerbates apoptosis of endothelial cells incurred by either glutamate or TNF-␣ (3,34), whereas increased amounts of HO-2 protein reduce the lethal effects of hypoxia in endothelial cells (18). Deficiency of HO-2 promotes the appearance of an activated phenotype in aortic endothelial cells (4), and following the wounding of the corneal epithelium, deficiency of HO-2 enhances inflammatory responses and impairs reparative responses (5,17).…”

Age sensitizes the kidney to heme protein-induced acute kidney injury

“…HO-2 deficiency, for example, leads to an activated phenotype in aortic endothelial cells (7), and promotes glutamate-induced or TNF-␣-induced apoptosis in cerebral endothelial cells (5,27); increased amounts of HO-2 protein protects endothelial cells against hypoxia (17); HO-2 deficiency adversely affects inflammatory and reparative responses following corneal wounding (16,34); finally, genetic deficiency of HO-2 exacerbates diabetes-induced renal injury in the streptozotocin murine model (15). However, prior studies demonstrate that cortical neuronal cultures and striatal cells from HO-2 Ϫ/Ϫ mice are relatively resistant to the cytotoxic effect of hemoglobin (31,32), while intracerebral injection of blood induces less cytotoxicity and oxidative stress in HO-2 Ϫ/Ϫ mice (30); in contrast, genetic deficiency of HO-2 increases the sensitivity of astrocytes to hemin (9).…”