Exacerbation of chronic renovascular hypertension and acute renal failure in heme oxygenase-1-deficient mice

Heme oxygenase (HO) is a cytoprotective enzyme that degrades heme (a potent oxidant) to generate carbon monoxide (a vasodilatory gas that has anti-inflammatory properties), bilirubin (an antioxidant derived from biliverdin), and iron (sequestered by ferritin). Because of properties of HO and its products, we hypothesized that HO would be important for the regulation of blood pressure and ischemic injury. We studied chronic renovascular hypertension in mice deficient in the inducible isoform of HO (HO-1) using a one kidney-one clip (1K1C) model of disease. Systolic blood pressure was not different between wild-type (HO-1(+/+)), heterozygous (HO-1(+/-)), and homozygous null (HO-1(-/-)) mice at baseline. After 1K1C surgery, HO-1(+/+) mice developed hypertension (140+/-2 mm Hg) and cardiac hypertrophy (cardiac weight index of 5.0+/-0.2 mg/g) compared with sham-operated HO-1(+/+) mice (108+/-5 mm Hg and 4.1+/-0.1 mg/g, respectively). However, 1K1C produced more severe hypertension (164+/-2 mm Hg) and cardiac hypertrophy (6.9+/-0.6 mg/g) in HO-1(-/-) mice. HO-1(-/-) mice also experienced a high rate of death (56%) within 72 hours after 1K1C surgery compared with HO-1(+/+) (25%) and HO-1(+/-) (28%) mice. Assessment of renal function showed a significantly higher plasma creatinine in HO-1(-/-) mice compared with HO-1(+/-) mice. Histological analysis of kidneys from 1K1C HO-1(-/-) mice revealed extensive ischemic injury at the corticomedullary junction, whereas kidneys from sham HO-1(-/-) and 1K1C HO-1(+/-) mice appeared normal. Taken together, these data suggest that chronic deficiency of HO-1 does not alter basal blood pressure; however, in the 1K1C model an absence of HO-1 leads to more severe renovascular hypertension and cardiac hypertrophy. Moreover, renal artery clipping leads to an acute increase in ischemic damage and death in the absence of HO-1.

Endotoxin-induced mortality is related to increased oxidative stress and end-organ dysfunction, not refractory hypotension, in heme oxygenase-1-deficient mice

BACKGROUND

Heme oxygenase (HO)-1 is an enzyme that degrades heme to generate CO (a vasodilatory gas), iron, and the potent antioxidant bilirubin. A disease process characterized by decreases in vascular tone and increases in oxidative stress is endotoxic shock. Moreover, HO-1 is markedly induced in multiple organs after the administration of endotoxin (lipopolysaccharide [LPS]) to mice.

METHODS AND RESULTS

To determine the role of HO-1 in endotoxemia, we administered LPS to mice that were wild-type (+/+), heterozygous (+/-), or homozygous null (-/-) for targeted disruption of HO-1. LPS produced a similar induction of HO-1 mRNA and protein in HO-1(+/+) and HO-1(+/-) mice, whereas HO-1(-/-) mice showed no HO-1 expression. Four hours after LPS, systolic blood pressure (SBP) decreased in all the groups. However, SBP was significantly higher in HO-1(-/-) mice (121+/-5 mm Hg) after 24 hours, compared with HO-1(+/+) (96+/-7 mm Hg) and HO-1(+/-) (89+/-13 mm Hg) mice. A sustained increase in endothelin-1 contributed to this SBP response. Even though SBP was higher, mortality was increased in HO-1(-/-) mice, and they exhibited hepatic and renal dysfunction that was not present in HO-1(+/+) and HO-1(+/-) mice. The end-organ damage and death in HO-1(-/-) mice was related to increased oxidative stress.

CONCLUSIONS

These data suggest that the increased mortality during endotoxemia in HO-1(-/-) mice is related to increased oxidative stress and end-organ (renal and hepatic) damage, not to refractory hypotension.