Role of hemin in the modulation of H2O2-mediated endothelial cell injury

Heme oxygenase (HO) has been primarily regarded as the rate-limiting enzyme in the degradation of heme. However it has recently been proposed that the inducible isoform, HO-1 (EC 1.14.99.3), functions as a stress-responsive antioxidant enzyme, with the capacity to protect against oxidant-mediated vascular injury. This study used an in vitro model of endothelial permeability to determine the effects of the HO-1-inducing agent hemin on noncytotoxic endothelial injury mediated by acute oxidant stress. Effects of hemin on oxidant-mediated cytotoxicity in a number of endothelial cell types were also investigated. A 20-min exposure of human umbilical vein endothelial cell (HUVEC) monolayers to H(2)O(2) resulted in a significant concentration-dependent increase in permeability, which was reversible 48 h later. Pretreatment of monolayers with hemin for 2 h followed by 18 h in complete medium resulted in HO-1 induction and the attenuation of H(2)O(2)-mediated increases in endothelial permeability, and significantly improved the restoration of endothelial barrier function 48 h later. In HUVEC and in the human microvascular endothelial cell line HMEC-1, hemin treatment as above resulted in protection against cytotoxicity, but not in bovine aortic endothelial cells (BAECs), where such toxicity was potentiated. This potentiation was inhibited by incubation with the HO inhibitor tin protoporphyrin IX, supporting a role for HO-1 in the potentiation of the cytotoxic response. When the exposure time of BAEC to hemin was extended to 24 h, H(2)O(2)-mediated cytotoxicity was attenuated. We conclude that hemin treatment is cytoprotective against noncytotoxic endothelial injury in vitro, under conditions that may not offer global protection against cytotoxic injury to vascular endothelium. This would indicate that HO-1 induction associated with cytotoxic injury in vivo is not always beneficial and therefore that the use of hemin as a therapeutic agent to offset oxidant injury in vascular endothelium should be undertaken with caution.