Induction of heme oxygenase 1 attenuates placental ischemia-induced hypertension

Abstract

Recent in vitro studies have reported that heme oxygenase 1 (HO-1) downregulates the angiostatic protein soluble fms-like tyrosine kinase 1 from placental villous explants and that the HO-1 metabolites CO and bilirubin negatively regulate endothelin 1 and reactive oxygen species. Although soluble fms-like tyrosine kinase 1, endothelin 1, and reactive oxygen species have been implicated in the pathophysiology of hypertension during preeclampsia and in response to placental ischemia in pregnant rats, it is unknown whether chronic induction of HO-1 alters the hypertensive response to placental ischemia. The present study examined the hypothesis that HO-1 induction in a rat model of placental ischemia would beneficially affect blood pressure, angiogenic balance, superoxide, and endothelin 1 production in the ischemic placenta. To achieve this goal we examined the effects of cobalt protoporphyrin, an HO-1 inducer, in the reduced uterine perfusion pressure (RUPP) placental ischemia model and in normal pregnant rats. In response to RUPP treatment, mean arterial pressure increases 29 mm Hg (136±7 versus 106±5 mm Hg), which is significantly attenuated by cobalt protoporphyrin (118±5 mm Hg). Although RUPP treatment causes placental soluble fms-like tyrosine kinase 1/vascular endothelial growth factor ratios to alter significantly to an angiostatic balance (1.00±0.10 versus 1.27±0.20), treatment with cobalt protoporphyrin causes a significant shift in the ratio to an angiogenic balance (0.68±0.10). Placental superoxide increased in RUPP (952.5±278.8 versus 243.9±70.5 relative light units/min per milligram) but was significantly attenuated by HO-1 induction (482.7±117.4 relative light units/min per milligram). Also, the preproendothelin message was significantly increased in RUPP, which was prevented by cobalt protoporphyrin. These data indicate that HO-1, or its metabolites, is a potential therapeutic for the treatment of preeclampsia.

Figure 1

CoPP induces HO-1 production in RUPP rats. Placental HO-1 was measured by western blot (A) and quantitation was performed by comparison to β-actin levels (B). HO-1 was increased in the placentas of both RUPP and RUPP-treated animals. HO activity was assayed in placental and liver lysates (C-D). CoPP induced increased HO activity in the liver of control and RUPP animals, while significantly increasing activity in the placenta of control animals. Error bars demonstrate standard error.

Figure 2

HO-1 induction attenuates RUPP induced hypertension but has no significant effect on pup size. (A) RUPP treatment led to a significant 29 mmHg increase in MAP, which was significantly attenuated by HO-1 induction. (B-C) HO-1 induction had no significant effect on either pup or placental mass. Error bars demonstrate standard error.

Figure 3

HO-1 induction normalized placental angiogenic balance in RUPP animals and promotes circulating VEGF. (A) Placental levels of sFlt-1 and VEGF were determined by ELISA, and molar ratio determined. In RUPP animals sFlt-1/VEGF was increased, indicating an angiostatic phenotype. In response to HO-1induction, the sFlt-1/VEGF ratio decreased significantly, indicating a net angiogenic balance. (B) Circulating levels of VEGF were decreased significantly in response to RUPP treatment, but were fully restored by induction of HO-1 by CoPP. Error bars demonstrate standard error.

Figure 4

HO-1 induction decreases oxidative stress in the ischemic placenta. RUPP treatment induced significant increases in baseline placental superoxide (A) and NADPH oxidase activity (B) in the placenta. While administration of CoPP raised the baseline superoxide and NADPH oxidase activity in control animals, it also attenuated both in the RUPP group. Error bars demonstrate standard error.

Figure 5

HO-1 induction attenuates placental ischemia induced increases in vascular preproendothelin message production. Aorta from RUPP rats had a significantly increased expression of preproendothelin as assayed by qRT-PCR. RUPP rats also demonstrated an increase in preproendothelin message, but the increase was markedly less and failed to meet statistical significance. Error bars represent standard error.