Vol. 52 No. 2/2005, 273–284 Review on line at www.actabp.pl

Heme oxygenase-1 (HO-1) is an enzyme which catalyzes the rate-limiting step in heme degradation resulting in the formation of iron, carbon monoxide and biliverdin, which is subsequently converted to bilirubin by biliverdin reductase. The biological effects exerted by the products of this enzymatic reaction have gained much attention. The anti-oxidant, anti-inflammatory and cytoprotective functions associated with HO-1 are attributable to one or more of its degradation products. Induction of HO-1 occurs as an adaptive and beneficial response to several injurious stimuli including heme and this inducible nature of HO-1 signifies its importance in several pathophysiological disease states. The beneficial role of HO-1 has been implicated in several clinically relevant disease states involving multiple organ systems as well as significant biological processes such as ischemia-reperfusion injury, inflammation/immune dysfunction and transplantation. HO-1 has thus emerged as a key target molecule with therapeutic implications.


ENZYMATIC REACTION CATALYZED BY HEME OXYGENASE-1
Heme oxygenase is the rate limiting enzyme in the degradation of heme and results in the release of equimolar quantities of biliverdin, iron and carbon monoxide (CO) (Fig 1 .)(Maines, 1997).Biliverdin reductase subsequently converts biliverdin to bilirubin.Amongst the two reported isoforms of heme oxygenase, HO-1 is the highly inducible enzyme by heme and various other stimuli including oxidative stress (Alam et al., 1989;Camhi et al., 1995;Durante et al., 1997;Agarwal et al., 1998;Camhi et al., 1998;Alam et al., 2000;Alcaraz et al., 2001;Sikorski et al., 2004).HO-2 is the constitutively expressed isoform.A third isoform HO-3 has also been described (McCoubrey et al., 1997) but has recently been shown to be a pseudogene (Hayashi et al., 2004).Although 45% amino-acid homology exists between HO-1 and HO-2, (Maines, 1997) they are differentially regulated and expressed in tissues.HO-1 is ubiquitously induced in mammalian tissues and is localized to the endoplasmic reticu-274 2005 J. Deshane and others have been recently reviewed (Tomaro & Batlle, 2002;Kapitulnik, 2004;Ryter & OĴerbein, 2004).The focus of this article is to provide a comprehensive review of the current literature on the functional role of HO-1 gene expression in different disease states.The molecular regulation of HO-1 gene expression has been reviewed elsewhere (Sikorski et al., 2004).

DISEASES ASSOCIATED WITH HO-1
The expression of HO-1 has been implicated in several disease states including atherosclerosis, hypertension, transplant rejection, acute renal injury hyperoxia and hypoxia-induced lung injury, cancer, as well as others (Table 1).More importantly, the expression of HO-1 modulates several critical biological processes such as ischemia-reperfusion injury, inflammation/immune dysfunction and transplantation in multiple organ systems.

HO-1 IN ISCHEMIA-REPERFUSION INJURY
Ischemia and reperfusion constitute a major mechanism of organ failure and tissue injury.HO-1 has been associated with a tissue protective role in ischemia-reperfusion injury in the heart, kidney, liver, brain and lung.One possible mechanism for this cytoprotection is perhaps by the modulation of the pro-and anti-apoptotic pathways by HO-1 (Tsuchihashi et al., 2004).Pachori and colleagues have shown that an adenoviral vector system containing the erythropoietin hypoxia response element for ischemia-regulated expression of the human HO-1 gene, conferred tissue protection in the heart, liver and skeletal muscle (Pachori et al., 2004).Both CO and bilirubin have been reported to mediate the protective effects of HO-1 expression in ischemiareperfusion injury.Inhalation of CO is protective in ischemia-reperfusion injury in the heart, lung, kidney and liver (Fujita et al., 2001;Nakao et al., 2003;Neto et al., 2004;Nakao et al., 2005).Studies using exogenous bilirubin have also shown that the protective effects of HO-1 activity in ischemia-reperfusion injury in the heart, liver and kidney are mediated through bilirubin (Clark et al., 2000;Kato et al., 2003;Adin et al., 2005).Thus, HO-1 expression serves as a protective response in ischemia-reperfusion, effects mediated via CO and/or bilirubin.

HO-1 IN INFLAMMATION
HO-1 plays an important role in the inflammatory response (Willis et al., 1996;Yet et al., 1997;Wang et al., 1998;OĴerbein et al., 1999b;Ishikawa et al., 2001b;Kapturczak et al., 2004).The beneficial effects of HO-1 in inflammation were first reported by Willis and colleagues in a model of pleural inflammation (Willis et al., 1996).Inhibition of HO-1 using tin protoporphyrin (SnPP), significantly increased inflammatory infiltrate, while prior induction with hemin resulted in a significant reduction of inflammation suggesting that HO-1 activity modulates the inflammatory response.Similar findings have been reported in other models of inflammation as well (Vogt et al., 1996;Siow et al., 1999).Vogt and co-workers demonstrated a novel phenomenon of acquired resistance to renal tubular injury in glomerular inflammation that was dependent on the induction of HO-1 in renal tubules (Vogt et al., 1996).Induction of HO-1 by its inducer hemin has been shown to reduce inflammation of the gut and decreases mucosal injury in an animal model of small bowel ischemia (AĴuwaybi et al., 2004).
The importance of HO-1 in inflammation is supported by findings in HO-1 knockout mice and the human HO-1 deficient child, both exhibiting a pro-inflammatory phenotype (Poss & Tonegawa, 1997a;1997b;Yachie et al., 1999;Kapturczak et al., 2004).In addition, several pro-inflammatory mediators are activated in HO-1 deficiency (Kapturczak et al., 2004) and overexpression of HO-1 or its byproducts are anti-inflammatory.Furthermore, anti-inflammatory mediators such as IL-10 have been shown to confer protection through upregulation of HO-1 in a murine model of sepsis (Lee & Chau, 2002).IL-13, an immunoregulatory cytokine that is a key mediator in allergic inflammation, has also been shown to induce HO-1 (Ke et al., 2002).Similar to the effects of IL-10 in sepsis, HO-1 induction has been suggested to mediate the effects of IL-13 in vivo in rat cardiac allograĞs (Ke et al., 2002).
Although preinduction of HO-1 inhibits inflammation, pro-inflammatory mediators like TNFα, IL-1, LPS and oxidized lipids are potent inducers of HO-1 expression in endothelial cells and macro- Heme is cleaved by heme oxygenase to generate equimolar quantities of iron, carbon monoxide and biliverdin.Biliverdin is then converted by biliverdin reductase to bilirubin.
V o l .5 2 2 7 5 Heme oxygenase-1 and disease states phages (Camhi et al., 1998;Terry et al., 1999;Wagener et al., 2003;Chen & Kunsch, 2004).In addition, several adhesion molecules that are key mediators of inflammation such as ICAM-1, VCAM-1 and selectins are activated by inducers of HO-1 (Wagener et al., 1997;Soares et al., 2004).In the context of vascular disorders and transplant rejection, activation, survival or apoptosis and differentiation of monocytes are crucial factors which determine fate of the disease.Recent studies by Lang and colleagues have demonstrated that the dose and time dependent induction of HO-1 by hemin inhibited apoptosis in monocytes despite the upregulation of caspase-3 pathways (Lang et al., 2005).HO-1 induction has also been shown to inhibit microvascular endothelial cell leukocyte adhesion through the action of its metabolites, bilirubin and CO (Morisaki et al., 2002;Zampetaki et al., 2003;Keshavan et al., 2005).

TRANSPLANTATION
Perhaps the most significant area that has generated research interest involving HO-1 is in the field of transplantation.HO-1 is induced in several models of acute transplant rejection and localizes predominantly to infiltrating cells (Agarwal et al., 1996b;Avihingsanon et al., 2002;Souza et al., 2005).Such induction is functionally relevant, since the absence of HO-1 leads to accelerated graĞ rejection in cardiac allo-and xenotransplantation (Soares et al., 1998;Holweg et al., 2004).The functional significance of HO-1 in transplantation has been corroborated in other organ transplant models as well.In addition to transplant rejection, HO-1 induction also aĴenuates ischemia/reperfusion injury that affects donor organ quality and subsequent transplantation (Amersi et al., 1999;Nath, 1999).Amersi and colleagues have demonstrated that overexpression of HO-1, using either cobalt protoporphyrin (CoPP) or adenoviral HO-1 gene transfer aĴenuated ischemia-reperfusion injury and prolonged survival, following cold ischemia/isotransplantation of livers (Amersi et al., 1999).A recent study evaluating the effect of HO-1 upregulation showed that peritransplant upregulation of HO-1 by administration of CoPP significantly aĴenuated chronic rejection of renal allograĞs (Bedard et al., 2005).
HO-1 and its byproduct CO prevent ischemiareperfusion injury associated with heart transplantation (Sato et al., 2001;Beltowski et al., 2004;Braudeau et al., 2004).Akamatsu and co-authors have shown that exposure of the donor and the graĞ to CO confers a protective effect in cardiac transplant associated ischemia-reperfusion injury.In addition CO (250 ppm) improves function of renal graĞs and imparts significant protective effects against renal ischemiareperfusion injury (Akamatsu et al., 2004;Neto et al., 2004a).RDP1258, a novel peptide derived from the HLA class I heavy chain, has been shown to possess immunoregulatory function via modulation of HO-1 enzyme activity (Cuturi et al., 1999;Magee et al., 1999).These recent developments provide new therapeutic approaches in the overall success of organ transplantation and prolongation of graĞ survival.

ATHEROSCLEROSIS
The expression of HO-1 in atherosclerosis is a protective response.This is supported by the following findings.First, an abundance of HO-1 (mRNA and protein) has been identified in human atherosclerotic plaques, providing in vivo relevance to this enzyme in atherosclerosis (Wang et al., 1998).Increased HO-1 expression is also present in advanced lesions in animal models of atherosclerosis (Wang et al., 1998).Secondly, overexpression of HO-1 in the vasculature in apolipoprotein E (apoE)-deficient mice aĴenuates the development of atherosclerosis (Juan et al., 2001).Thirdly, inhibition of HO enzyme  (Ishikawa et al., 2001a).Hoekstra and coworkers have also reported that differences in susceptibility to atherosclerosis between resistant and susceptible strains of Japanese quail may be due to differences in endothelial HO and anti-oxidant components (Hoekstra et al., 2003).Fourth, transgenic mice deficient in HO-1 in an apoE knockout background develop significantly more atherosclerosis compared to wild-type mice (Yet et al., 2003).Finally, atherogenic lipoproteins like oxidized LDL that have been implicated in the pathogenesis of atherosclerosis (Shi et al., 2000;Furnkranz et al., 2005) are potent inducers of HO-1 in vascular cells and renal tubular epithelial cells (Agarwal et al., 1996a).More importantly, oxidized LDL-mediated HO-1 induction inhibits monocyte chemotaxis (Ishikawa et al., 1997), a key inflammatory event in the pathogenesis of atherosclerosis.
The major stimulus for the induction of HO-1 in atherosclerotic plaques is oxidized LDL (Agarwal et al., 1996a;Ishikawa et al., 1997) and more specifically, its faĴy acid component, linoleyl hydroperoxide (Agarwal et al., 1998).13-HPODE, one of the major components of oxidized LDL induces HO-1 via transcriptional mechanisms (Agarwal et al., 1998).Our laboratory has identified a distal cis-acting region in the human HO-1 promoter that regulates this response in human aortic endothelial cells (Hill- Kapturczak et al., 2003).Studies to further delineate this region are in progress.In murine macrophages, OxLDL causes nuclear accumulation of Nrf2, which in turn activates HO-1 (Ishii et al., 2004).Bach-1 has recently been identified as a potential transcriptional repressor for HO-1.Although HO-1 has been implicated in the protective response against atherosclerosis, the functional role of Bach-1 in modulating this response is not well understood.In a recent study involving cuff injury in Bach-1 deficient mice, Bach-1 was shown to play a critical role in the regulation of HO-1 expression, macrophage function, smooth muscle cell proliferation and neointima formation (Omura et al., 2005).In smooth muscle cells derived from Bach-1 deficient mice, HO-1 expression was increased and associated with decreased proliferation compared with wild type cells (Omura et al., 2005).Thus during inflammation or atherogenesis, Bach-1 may regulate HO-1 gene expression and this hypothesis requires further investigation.

VASCULAR RESTENOSIS AND OTHER CARDIOVASCULAR DISEASES
Several lines of evidence suggest that upregulation of HO-1 may be an important protective factor aĞer balloon angioplasty in cardiovascular diseases such as vascular restenosis (Ishikawa, 2003;Schill-inger et al., 2004).Prior induction of HO-1 by chemical and genetic manipulation aĴenuates vascular neointimal proliferation following balloon injury, while inhibition of HO enzyme activity, leads to worsening of the lesion (Aizawa et al., 1999;Tulis et al., 2001a;2001b).HO-1 knockout mice demonstrate exaggerated vascular neointimal proliferation following wireinduced injury (Duckers et al., 2001).In recent work, Visner and coworkers have suggested that the antiproliferative effects of rapamycin in vascular smooth muscle cells are mediated through the induction of HO-1 (Visner et al., 2003).Rapamycin-coated stents have been used to prevent restenosis following angioplasty and these findings implicate HO-1 as the underlying mechanism for the beneficial effects of rapamycin in vascular injury.
Recent studies focusing on a (GT)n repeat region in the proximal human HO-1 promoter have yielded interesting results in vascular restenosis.A study investigating the association of length polymorphisms of the human HO-1 promoter and peripheral vascular restenosis showed significantly reduced level of inflammation following balloon angioplasty in patients with short (GT)n repeats (< 25) when compared to longer (GT)n repeats (reviewed in Exner et al., 2004;Schillinger et al., 2004).These findings were confirmed in coronary artery restenosis wherein the carriers of longer (GT)n repeats had a 3.74 fold higher risk for restenosis compared with those with shorter (GT)n repeats.Significant association was also observed between HO-1 (GT)n polymorphisms and abdominal aortic aneurysms (Schillinger et al., 2002).On the other hand, no association has been found between HO-1 (GT)n repeat polymorphism and Kawasaki disease and systemic vasculitis in Japanese children (Kanai et al., 2003).

RENAL DISEASES
Studies utilizing chemical inducers and inhibitors as well as HO-1 knockout mice have shown that the expression of HO-1 is cytoprotective in heme and non-heme mediated models of renal injury (Nath et al., 1992;Agarwal et al., 1995;Shiraishi et al., 2000).A detailed review of this area is summarized in a recent article from our group (Hill- Kapturczak et al., 2002).

HYPOXIA/HYPEROXIA LUNG INJURY AND EMPHYSEMA
HO-1 is protective in both hyperoxia as well as hypoxia-induced lung injury (Choi & Alam, 1996;Taylor et al., 1998;OĴerbein et al., 1999a;Christou et al., 2000;Zampetaki et al., 2003).The generation of CO appears to be the mechanism involved in these V o l .5 2 2 7 7 Heme oxygenase-1 and disease states models since exogenous administration of CO protects against lung injury (OĴerbein et al., 1999b), results that are similar to HO-1 gene delivery studies (OĴerbein et al., 1999a).In mouse lung ischemiareperfusion injury models as well as primary rat pulmonary artery endothelial cells, overexpression of HO-1 aĴenuates apoptosis and knockdown of HO-1 by siRNA in endothelial cells increases anoxia-reoxygenation induced apoptosis (Zhang et al., 2004).
Studies carried out in patients with emphysema (Yamada et al., 2000) suggests that long (GT)n repeats reduces HO-1 inducibility in response to smoking and thus perhaps a much higher risk for development of chronic obstructive pulmonary disease.On the contrary, a study constituting 621 smokers found no link between HO-1 promoter genotype and loss of lung function (He et al., 2002).

PRE-ECLAMPSIA AND INTRA-UTERINE GROWTH RETARDATION
Endothelial oxidative stress plays a significant role in the pathophysiology of preeclampsia, a hypertensive disorder in pregnancy (Lum & Roebuck, 2001).Critical inflammatory processes like increased leukocyte-endothelial interaction/endothelial dysfunction, associated upregulation of cellular adhesion molecules and endothelial permeability by reactive oxygen species are involved in the development of this condition.HO-1 has been proposed to be involved in these processes.In a study investigating the effect of HO-1 activation on TNF-α induced placental damage and feto-placental circulation, induction of HO-1 significantly aĴenuated the inflammatory response mediated cellular damage in placental villous explants (Ahmed et al., 2000).Recent studies have also shown that large amounts of peroxynitrite are generated in the maternal vasculature (Zhao et al., 2004) suggesting a possible role for peroxynitrite in the pathogenesis in preeclampsia.Endothelial oxidative stress induced by peroxynitrite upregulated adhesion molecule expression and induced HO-1.Treatment of endothelial cells with either peroxynitrite scavenger or HO-1 inhibitor abolished the increased expression of adhesion molecules.Therefore, the modulation of expression of adhesion molecules may be mediated by HO-1 regulation (Zhao et al., 2004).
Damage to the endothelium and impaired microvascularization are commonly linked with recurrent miscarriages.HO-1 protein levels were significantly lower in placentae from cases with preeclampsia, compared with gestationally matched normal pregnancies (Lash et al., 2003).A study investigating 162 women with recurrent miscarriages compared to a group of postmenopausal healthy women showed a significant association between HO-1 (GT)n repeat polymorphisms and incidence of miscarriages (Denschlag et al., 2004).In pregnant women who had a fetus with IUGR, levels of HO-1 expression in placental trophoblasts were significantly reduced when compared to a group of normal pregnant women (Wang & Yu, 2002).On the contrary, an earlier study trying to correlate expression of HO-1 and HO-2 to preeclampsia and fetal growth restriction, showed that reduced expression of HO-2 in endothelial cells under these abnormal conditions may be responsible for reduced placental blood flow (Barber et al., 2001).However, no significant difference in HO-1 expression levels was noted in endothelial cells and in the placental bed in preeclampsia or fetal growth restriction.McLaughlin and coauthors on the other hand, have found increased HO-1 expression in chorionic villi and fetal membranes from preeclamptic pregnancies compared to normotensive controls (McLaughlin et al., 2003).
CO, one of the products of heme degradation by HO-1, has been considered as a vascular relaxant (McFaul & McGrath, 1987).Studies of inhibition of HO-1 in isolated perfused placentae showed increase in placental perfusion pressure suggesting that CO levels are perhaps crucial for maintenance of blood flow in the placenta which is of vital importance for a healthy pregnancy (Lyall et al., 2000).In preconstricted placental arteries, hemin reduced vascular tension significantly and hemin induced vascular relaxation as well as production of CO, was inhibited by SnPP (Ahmed et al., 2000) suggesting a role for HO-1 as an endogenous placental factor conferring cytoprotection and placental blood vessel relaxation.

HYPERTENSION
Johnson and coworkers demonstrated in Dahl salt sensitive (DS) rats, that coronary arterial HO-1 expression was increased with salt induced hypertension, and cardioprotection was provided by promoting coronary vasodilation (Johnson et al., 2004b).On the other hand, endothelium dependent vasodilator responses were aĴenuated in arterioles from another severely salt sensitive model of hypertension, deoxycorticosterone acetate (DOCA) rats and not in the spontaneously hypertensive (SHR) rat model (Johnson et al., 2004a).Using an inhibitor, which abolishes endogenous CO production, they show data which suggests that DOCA-salt hypertension is associated with increased generation of endogenous CO which may play a role in endothelial dysfunction.Yang and coauthors have demonstrated that overexpression of HO-1 leads to a reduction in pressor responsiveness to angiotensin II (Yang et al., 2004).This is most likely due to the increased generation of one of the HO-1 metabolites, presumably CO, which has the ability to inhibit vascular reactivity to constrictor stimuli.Several studies have 278 2005 J. Deshane and others documented the induction of vascular, cardiac and renal HO-1 in response to angiotensin II both in vitro and in vivo (Aizawa et al., 2000;Haugen et al., 2000;Das et al., 2004).MoĴerlini and coworkers have also shown previously that HO-1 derived CO plays a role in the suppression of an acute hypertensive response in vivo (MoĴerlini et al., 1998).

DIABETES
Oxidative stress and generation of reactive oxygen species, specifically superoxide anion has been implicated in the cardiovascular complications seen in patients with diabetes (Giugliano et al., 1995;Mohamed et al., 1999).Hyperglycemia has been shown to mediate endothelial dysfunction, delayed cell replication and enhanced apoptosis (Lorenzi et al., 1987;Baumgartner-Parzer et al., 1995;Zou et al., 2002).These events seem to be reversible by increased expression of anti-oxidant enzymes such as HO-1 (Lorenzi et al., 1985;Curcio & Ceriello, 1992).Cosso and coauthors have shown that diabetes induces an increase in oxidative stress and results in upregulation of HO-1 in liver (Cosso et al., 2001).Increased HO-1 expression has also been observed in glomerular cells of diabetic rats (Agarwal & Nick, 2000;Hayashi et al., 2001).Quan and coworkers have reported a decrease in HO activity in the early stages of diabetes and an increase in number of circulating endothelial cells in streptozotocin-induced diabetic rats (Quan et al., 2004).Overexpression HO-1 in diabetic rats resulted in increased serum bilirubin, reduced production of reactive oxygen species and aĴenuated sloughing of endothelial cells (Abraham et al., 2004;Quan et al., 2004).Interestingly, hyperglycemia per se represses HO-1 gene expression (Abraham et al., 2003) while low glucose induces HO-1 gene expression (Chang et al., 2003).
In rodent models of islet transplantation induction of HO-1 in islet cells resulted in a protective response from pro-apoptotic stimuli and improved islet function (Pileggi et al., 2001;Tobiasch et al., 2001).Studies conducted in diabetic and non diabetic HO-1-/-and +/+ mice have shown that animals lacking HO-1 are more susceptible to damage from myocardial ischemia-reperfusion injury and the presence of diabetes worsens the injury (Liu et al., 2005).Myocardial infarct size was significantly higher in HO-1 deficient mice, whereas, overexpression of HO-1 conferred protection against myocardial injury in diabetic rats (Liu et al., 2005).

CANCER
It is well known that HO-1 is expressed in a variety of tumors (Goodman et al., 1997;Doi et al., 1999;Tsuji et al., 1999;Deininger et al., 2000;Fang et al., 2003) and that HO-1 directly contributes to rapid tumor growth via its anti-oxidative and anti-apoptotic effects (Doi et al., 1999;Tanaka et al., 2003).The antiapoptotic action of HO-1 is believed to be mediated by multiple mechanisms including decreased levels of intracellular pro-oxidants and increased bilirubin and CO levels.CO exerts its anti-apoptotic effect by inhibiting expression of the tumor suppressor protein, p53, and release of mitochondrial cytochrome c (Liu et al., 2002).In a study investigating the relationship between expression levels of HO-1 and cervical lymph node metastasis of tongue squamous cell carcinoma, low HO-1 expression was associated with lymph node metastasis (Yanagawa et al., 2004) and hence suggested to be a possible clinical marker for the disease.Fang and coworkers have shown in human colon carcinoma cells that treatment with a HO inhibitor, ZnPP, enhanced the chemotherapeutic response of tumor cells and reduced tumor growth suggesting that HO-1 may be an aĴractive target for chemotherapeutic intervention (Fang et al., 2003).Chen and coauthors have demonstrated in papillary thyroid carcinoma cells that induction of HO-1 markedly reduces the sensitivity of the cells to apoptotic stimuli (Chen et al., 2004).Thus HO-1 may be an effective target for anti-cancer therapy.
However, HO-1 has also been shown to have a protective effect in cancer which is contradictory to its tumorigenic properties.Results from a study conducted to establish an association between incidence of lung adenocarcinoma and HO-1 polymorphisms among Japanese patients compared to controls showed that large (GT)n repeats in the HO-1 gene promoter may be directly correlated with the development of the disease (Kikuchi et al., 2005).Recent studies also demonstrate an association between risk of oral squamous cell carcinoma amongst areca chewers and longer (GT)n repeat alleles in the HO-1 promoter and suggests that shorter (GT)n repeats may in fact confer protection against oral carcinogenesis (Chang et al., 2004).Further studies will delineate the dual role played by HO-1 in cancer and the underlying mechanisms.

CEREBROVASCULAR ACCIDENT
Studies on focal cerebral ischemia in rats showed that treatment with an HO inhibitor, ZnPP before ischemia significantly reduced the infarct size and edema following the event (Kadoya et al., 1995).Recent evidence indicates that prolonged expression of HO-1 in glial cells in human brains following focal cerebral infarctions or traumatic brain injury helps in the recovery of neuronal tissue following these insults (Beschorner et al., 2000).In a study involving 399 patients with ischemic cerebrovascular V o l .5 2 2 7 9 Heme oxygenase-1 and disease states events, and 398 healthy control subjects, short < 25 (GT)n repeats in the HO-1 promoter conferred a reduced risk for cerebrovascular events in people with normal plasma lipid levels (Funk et al., 2004).These studies also show a contradictory role for HO-1 in this disease context.Since specificity of HO inhibitors is questionable, studies using genetic manipulation of HO-1 would provide more insight into the underlying mechanisms.

DRUGS
Several important therapeutic agents have been shown to induce HO-1 expression and mediate their beneficial effects, at least in part, through the induction of HO-1.For example, rapamycin, an immunosuppressive drug which has significant antiproliferative actions is a potent inducer of HO-1 expression in vascular cells (Visner et al., 2003).Such induction is functionally important since HO inhibition with tin protoporphyrin leads to a loss of the antiproliferative effect of rapamycin in smooth muscle cells.Several studies have shown the beneficial effects of statins in reducing the mortality rate in patients with coronary heart disease (LaRosa, 2000;Vaughan et al., 2000).Mechanisms beyond the lipid-lowering effects per se significantly contribute to the antiatherogenic and tissue protective properties of statins.Recent studies have shown that statins, albeit at relatively high concentrations, are potent inducers of HO-1 in vitro and in vivo (Grosser et al., 2004a;2004b;Lee et al., 2004).It has also been suggested that the anti-inflammatory as well as the antiproliferative actions of statins are mediated through the induction of HO-1.
Probucol, a cholesterol lowering drug which inhibits atherosclerosis and vascular restenosis has been shown to protect against smooth muscle cell proliferation by inducing expression of HO-1 (Deng et al., 2004).On the other hand, treatment with antioxidants such as probucol, completely normalized the HO-1 induction observed in diabetic glomeruli (Gorogawa et al., 2002;Koya et al., 2003).Other therapeutic agents such as aspirin and dopamine have also been shown to induce HO-1 (Berger et al., 2000;Grosser et al., 2003).

SUMMARY
In summary, induction of HO-1 plays an important role in the pathophysiology of several diseases such as atherosclerosis, hypertension, acute renal injury, lung injury, cancer as well as others involving multiple organ systems.Upregulation of HO-1 by various stimuli also modulates key biological processes including inflammation, ischemic injury and transplant rejection.Evaluation of the role played by products of the HO-1 catalyzed reaction in mediating the protective response will provide further insight into the underlying mechanisms of the cytoprotective effect elicited by HO-1.