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1 Department of Physiology, Vascular Biology Center, University of Tennessee Health Science Center, Memphis, TN, USA
2 Department of Emergency Medicine, Thomas Jefferson University, Philadelphia, PA, USA
* To whom correspondence should be addressed. E-mail: hparf{at}physio1.utmem.edu.
In cerebral circulation, epileptic seizures associated with excessive release of the excitatory neurotransmitter glutamate cause endothelial injury. Heme oxygenase (HO), which metabolizes heme to a vasodilator, CO, and anti-oxidants, biliverdin/bilirubin, is highly expressed in cerebral microvessels as a constitutive isoform, HO-2, while the inducible HO-1 is not detectable. Using cerebral vascular endothelial cells from newborn pigs and HO-2 knock-out mice, we addressed the hypotheses that: 1) glutamate induces oxidative stress-related endothelial death by apoptosis, and 2) HO-1 and HO-2 are protective against glutamate cytotoxicity. In cerebral endothelial cells, glutamate (0.1-2.0 mM) increased formation of reactive oxygen species (ROS), including superoxide radicals, and induced major keystone events of apoptosis such as NF
B nuclear translocation, caspase-3 activation, DNA fragmentation, and cell detachment. Glutamate-induced apoptosis was greatly exacerbated in HO-2 gene-deleted murine cerebrovascular endothelial cells and in porcine cells with pharmacologically inhibited HO-2 activity. Glutamate toxicity was prevented by superoxide dismutase, suggesting apoptotic changes are oxidative stress-related. When HO-1 was pharmacologically upregulated by cobalt protoporphyrin, apoptotic effects of glutamate in cerebral endothelial cells were completely prevented. Glutamate-induced ROS production and apoptosis were blocked by a CO-releasing compound, CORM-A1 (50 µM), and by bilirubin (1 µM), consistent with the anti-oxidant and cytoprotective roles of the end products of HO activity. We conclude that both HO-1 and HO-2 have anti-apoptotic effects against oxidative stress-related glutamate toxicity in cerebral vascular endothelium. Although HO-1, when induced, provides powerful protection, HO-2 is an essential endogenous anti-apoptotic factor against glutamate toxicity in cerebral vascular endothelium.
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