Survival Response by HO-1 and Its Products 199 Fig. 1. Signaling pathways leading to HO-1 expression by phytochemicals and drugs through MAPK-dependent Nrf2 activation. Besides MAPK, protein kinase C (PKC), phosphatidylinositol 3-kinase (PI3K), and RNA- dependant protein kinase-like endoplasmic reticulum kinase (PERK), depending on cell types and agent properties, have been also implicated in Nrf2 activation. Other transcription factors that also regulate HO-1 expression include heat-shock factor (HSF), nuclear factor-erythroid 2 (NF-E2), activator protein-1 (AP-1), and nuclear factor-κB (NF-κB). Upon external stimuli, the active forms of these transcription factors translocate to the nucleus where they bind to the specific DNA sequence leading to the transcription of HO-1 gene. Some phytochemicals directly activate Nrf2 nuclear translocation or indirectly through ROS generation. protective HO-1 expression [24]. Integrative Survival Response Evoked by HO-1 and Heme Metabolites Although the protective effects of HO-1 have been con- firmed in a number of experimental models, the mechanism of its action has not been completely elucidated. Degrada- tion of the pro-oxidant heme by HO-1 appears to first aid in tissue protection; however, recent evidence suggests that one or more of the by-products of its heme catabolism (CO, free iron or BV/BR) mediates the cytoprotective effects of HO-1 [4] (Fig. 2). Heme degradation Heme is inherently dangerous when released from intra- cellular heme-containing proteins [1]. It offers toxic effects to tissues and organs through oxidative stress by generating Vol. 42, No. 3, 2008 reactive oxygen species (ROS) via Fenton reaction [25], which may promote membrane lipid peroxidation and inflammation. Thus, heme catabolism by HO-1 is the first important antioxidant mechanism for survival from the redox environment induced by free heme. It should be noted that the insufficient catabolism of free heme by HO-1 may induce disturbances of the iron-reuse system, causing iron- deficiency anemia as already evidenced in HO-1-deficient human and mice [8]. CO Although the gaseous CO is classically thought of as a toxic molecule and cellular asphyxiate, recent studies have revealed that CO has profound effects on the intracellular signaling processes, culminating in anti-inflammatory, anti- proliferative, and anti-apoptotic effects (excellently reviewed in Ref. 4). The pleiotropic effects of CO may involve two general mechanisms; CO can activate soluble guanylyl cyclase (sGC), with the resultant production of cyclic guanine monophosphate (cGMP), and CO can modulate MAPK pathway. CO, like NO, binds to the heme moiety of sGC, leading to the stimulation of sGC and subsequent elevation of cGMP levels that causes protective vascular relaxation [26]. Other cGMP-mediated effects of CO include the inhibition of platelet aggregation [27] and vascular smooth muscle proliferation [26], and the protection of pancreatic β cells from apoptosis [28]. Most interestingly, CO induces a general down-regulation of pro-inflammatory cytokine production through MAPK-dependent pathway, leading to anti-inflammatory tissue protection [29, 30]. Also, both anti-apoptotic and antiproliferative actions of CO involve the MAPK pathway [31, 32]. Iron and ferritin Although ferrous iron, an extremely pro-oxidative mole- cule, is released during the breakdown of free heme by HO- 1, the iron is rapidly removed by ferritin, a ubiquitously existing intracellular protein that is able to effectively sequester intracellular iron and, hence, limit its pro-oxidant capacity. Thus, it is not surprising that ferritin expression is enhanced in conjunction with HO-1 expression [33]. Ferritin synthesized as a consequence of HO-1 activity has been implicated as a cytoprotective molecule in a number of experimental models [34]. BV/BR In cellular system, the soluble greenish pigment BV is rapidly converted by BV reductase into the hydrophobic yellowish pigment BR. BR is now recognized as being a potent antioxidant manufactured by the body, because BR can directly scavenge ROS and reduce ROS production [35]. By virtue of its ROS-scavenging capacity, BR confers cardiovascular, hepatic, and neural protection [35]. More-