Inflammation ( # 2014) DOI: 10.1007/s10753-014-9908-z

Therapeutic Potential of HO-1 in Autoimmune Diseases Bao-Zhu Li,1,2 Biao Guo,3 Hai-Yan Zhang,4 Juan Liu,1,2 Sha-Sha Tao,1,2 Hai-Feng Pan,1,2 and Dong-Qing Ye1,2,5

Abstract—Heme oxygenase-1 (HO-1), the inducible isoform of heme oxygenase (HO), has raised a lot of concerns in recent years due to its multiple functions. HO-1 was found to be a pivotal cytoprotective, antioxidant, anti-apoptotic, immunosuppressive, as well as anti-inflammatory molecule. Recent studies have clarified its significant functions in many diseases with substantial findings. In autoimmune diseases, HO-1 may have promising therapeutic potential. Here, we briefly reviewed recent advances in this field, aiming at hopefully exploring the potential therapeutic roles of HO-1, and design HO-1-based strategies for the treatment of autoimmune diseases. KEY WORDS: heme oxygenase-1; carbon monoxide; autoimmune diseases; anti-inflammation; immunomodulation.

INTRODUCTION Heme oxygenase-1 (HO-1) was considered an especially charming molecule for the prevention and management of immune-mediated injuries and diseases [1]. HO-1 catalyzed the first and rate-limiting step in the metabolism of heme, in which carbon monoxide (CO), ferrous iron, and biliverdin were generated [2, 3]. Subsequently, biliverdin was promptly converted to bilirubin, whereas free iron was quickly sequestered by ferritin and recycled for heme synthesis [3]. Both bilirubin and ferritin could act as antioxidants [4, 5]. CO, a gas molecule that has drawn increasing attentions recently with similar functions as nitric oxide (NO), could initiate cytoprotective signaling cascades via Bao-Zhu Li, Biao Guo, and Hai-Yan Zhang have equally contributed to this work and should be considered as co-first authors. 1

Department of Epidemiology and Biostatistics, School of Public Health, Anhui Medical University, 81 Meishan Road, Hefei, Anhui 230032, China 2 Anhui Provincial Laboratory of Population Health & Major Disease Screening and Diagnosis, Anhui Medical University, Hefei, Anhui, China 3 School of Health Service Management, Anhui Medical University, 81 Meishan Road, Hefei, Anhui 230032, China 4 Department of Hygiene Analysis and Detection, School of Public Health, Anhui Medical University, 81 Meishan Road, Hefei, Anhui 230032, China 5 To whom correspondence should be addressed at Department of Epidemiology and Biostatistics, School of Public Health, Anhui Medical University, 81 Meishan Road, Hefei, Anhui 230032, China. E-mail: [email protected]

triggering the activity of guanylate cyclase [6]; suppress apoptosis and inflammation; restrain the synthesis of inflammatory mediators, for instance, proinflammatory cytokines and NO; and regulate immune cells [4]. Therefore, heme degradation products and their metabolic derivatives could have multiple biochemical actions and render HO-1 with the ability to exert cytoprotective functions [4]. As a pivotal cytoprotective, antioxidant, anti-apoptotic, immunosuppressive, as well as anti-inflammatory molecule, the functional mechanisms of HO-1 were multifactorial (Fig. 1). This review focused on the functions of HO1 and its roles in autoimmune diseases, proposing to make a comprehensive understanding of HO-1 and its promising therapeutic potential in these diseases.

ANTI-INFLAMMATORY AND IMMUNOREGULATORY ROLES OF HO-1 Roles of HO-1 in anti-inflammation and immunoregulation have evoked worldwide interests of researchers. HO-1 was proved to be induced by a variety of cellular stimuli, including agents involved in oxidative stress, proinflammatory cytokines, and some anti-inflammatory stimuli [7, 8]. Increasing evidence indicated that HO-1 played anti-inflammatory and immunoregulatory roles. Downregulation of HO-1 was involved in many diseases [7, 8]. Moreover, an array of researches suggested that HO1 inhibited the initiation of inflammation via directly

0360-3997/14/0000-0001/0 # 2014 Springer Science+Business Media New York

Li, Guo, Zhang, Liu, Tao, Pan, and Ye

Fig. 1. Heme oxygenase-1 (HO-1) as a therapeutic target.

modulating immune cells activation, such as antigen-presenting cells (APCs) or lymphocytes [9]. One of the pioneer studies reported that HO-1 knockout mice performed a chronic inflammatory state featured by an increased peripheral blood lymphocyte count, an enhanced immunoglobin level, and an accumulation of polymorphonuclear cells and monocyte/macrophage in the spleen [9, 10]. These HO-1 knockout mice lacked the ability to reuse iron [1, 10, 11]. These mice were featured by progressive anemia, tissue iron deposition, chronic inflammatory reactions such as splenomegaly, lymphadenopathy, hepatic and renal inflammation, and growth delay [1, 10, 11]. Besides, high CD4+:CD8+ ratios could also be observed in spleen and lymph node [10]. Later, a study of human HO-1 deficiency confirmed as well that HO deletion could cause chronic inflammation [11]. Kapturczak et al. also found marked splenomegaly and fibrosis in HO-1 knockout mice [1]. In view of the roles of HO-1 in immune regulation, Kapturczak et al. found relatively decreased CD3- and B220-positive cells in lymph nodes of HO-1 deficient mice [1]. Remarkably higher baseline serum IgM levels could also be observed in HO-1 knockout mice [1]. This indicated a possible abnormality in the activation of B cell with hampered immunoglobulin isotype switching [1]. Furthermore, under mitogen stimulation with lipopolysaccharide (LPS) or anti-CD3/anti-CD28, HO-1 knockout splenocytes secreted higher levels of proinflammatory T helper (Th)1 cytokines such as interleukin (IL)-1, IL-6, interferon (IFN)-

γ, and tumor necrosis factor (TNF)-α [1]. Anti-CD3/antiCD28 exposure also resulted in a remarkable increase in granulocyte-macrophage colony-stimulating factor in HO1 knockout splenocytes, indicating a general proinflammatory tendency related to HO-1 deficiency [1]. This study suggested that HO-1 activity was significant in more downstream stages of immune response like the modulation of lymphocyte activation [1]. Pae et al. demonstrated that HO-1/CO suppressed T cell proliferation and IL-2 secretion, suggesting that HO-1 might be critical in regulating T cell activation and homeostasis [12, 13]. This notion was further supported by a separate study which proved that HO-1 could control the activation, maturation, and proliferation of human naïve T cells in vitro [13]. Administration of a recombinant adenoassociated virus-bearing HO-1 gene could significantly lower the levels of the Th1-type cytokines IL-2 and IFNγ in circulation and in activated splenocytes with no effect on the levels of the Th2-type cytokines IL-4 and IL-10 [14], which indicated an effect of HO-1 on the Th1/Th2 cytokine profile and on the preferential suppression of Th1 effector cell [14]. Xia et al. found that HO-1 had antiinflammatory effects probably through the improvement of the secretion of IL-10 and enhancement of the percentage of CD4+CD25high regulatory T cell (Treg) [15]. However, Gu et al. suggested that HO-1 did not affect the number of Tregs [16]. Instead, it inhibited the expression of activation markers CD62L and CD44 [16].

Therapeutic Potential of HO-1 in Autoimmune Diseases Moreover, suppressed 5-bromodeoxyuridine incorporation and lower expression of IFN-γ were found in the remaining CD4+ T cells [16]. These data suggested that inhibited activation, proliferation, and Th1 polarization of CD4+ T cells were not accomplished via increasing Tregs [16]. Besides, this study illustrated that HO-1 might restrict activation, proliferation, and Th1 polarization of liverinfiltrating CD4+ T cells via inhibiting the dendritic cell (DC) maturation [16]. Human and rat immature DCs were proved to express HO-1 [17]. In addition, HO-1 expression was downregulated by maturation stimuli [17]. Induction of HO-1 expression made DCs refractory to LPS-induced maturation, whereas it preserved the secretion of IL-10, suggesting that HO-1 could modulate DC functions [17]. Another study also showed that in vitro treatment of mice DCs with HO-1 or CO inhibited both LPS-induced maturation in vitro and their immunogenic capacities in vivo [18]. HO-1−/−mice performed a remarkably higher percentage of Foxp3expressing cells among total CD4+ and CD4+CD25+ cells compared with HO-1+/+mice [19]. Besides, HO-1−/−Treg cells were as effective as HO-1+/+Treg cells in inhibiting proliferation of effector T cells in vitro from either HO-1+/+or HO-1−/−mice [19]. Nevertheless, the deficiency of HO-1 in APCs abrogated the prohibitive activity of Treg cells on effector T cells [19]. These findings demonstrated that HO-1 activity in APCs was crucial for Treg-mediated suppression, providing an explanation for the apparent defect in immune regulation in HO-1 deficient mice [19]. Furthermore, it was possible that the absence in expression of CD8+ DCs could be associated with the immunological abnormalities observed in the deficient of HO-1 expression [20]. A recent study found that HO-1 and CO could suppress the ability of LPS-treated DCs to present exogenous soluble antigens to naïve T cells via blocking antigen trafficking at the level of late endosome–lysosome fusion [21]. The protective effect of HO-1 was also proved to be related to the suppression of major histocompatibility complex (MHC) II expression by APCs as well as the suppression of T helper cells and CD8 T cells accumulation and function in the organ [7, 22]. Concerning the relationship between HO-1 and Th17, upregulation of HO-1 was found to suppress IL-17 in vivo [23]. An earlier study showed that the upregulation of HO1 inhibited the production of IL-6, a pivotal cytokine priming Th17 cell differentiation and a downstream mediator of IL-17 action [4, 23–26]. Therefore, HO-1 could block the differentiation of Th17 cell and the activity of IL17 in part by inhibiting IL-6 [23]. The latest study using an ovalbumin-induced non-eosinophilic asthma model

showed that the induction of HO-1 could suppress Th17 response and upregulate Tregs [27]. Moreover, transforming growth factor-β (TGF-β) plus IL-6-induced Th17 cell differentiation in vitro were likely to be restrained by HO-1 induction through the downregulation of phosphorylated signal transducer and activator of transcription 3 [27]. Recent studies demonstrated that CO was the key molecule that mediated the protective role of HO-1 [28, 29]. As discussed above, CO could downmodulate immune responses in many pathogenic processes and was suggested to mediate most of the immunomodulatory parts attributed to HO-1 [30]. CO also served as a potent antiinflammatory molecule both in vitro and in vivo through different signal pathways (Fig. 2) [31–36]. A study revealed that CO selectively suppressed the LPS-induced expression of the proinflammatory cytokines TNF-α, IL1β, and macrophage inflammatory protein (MIP)-1β, while at the same time it enhanced the production of the anti-inflammatory cytokine like IL-10 [31]. CO mediated these anti-inflammatory effects not via a guanylyl cyclase– cyclic guanosine monophosphate or NO pathway, but through the MAP kinase kinase (MKK) 3/p38 pathway instead [31]. CO alone could generate mitochondrial reactive oxygen species (ROS) that, in turn, helped to upregulate peroxisome proliferator-activated receptors (PPAR)γ, and further, CO blocked the expression of early growth response protein (Egr)-1, a key transcription factor for the generation of the inflammatory response in macrophages [32]. These inhibitory effects of CO on Egr-1 were PPARγ dependent: the upregulation of PPARγ by CO was crucial for the inhibition of Egr-1 by CO [32]. CO was also found to mediate anti-inflammatory effects through activating extracellular signal-regulated kinases (ERK)5/PPARδ and ERK5-mediated CO and HO-1-induced PPARδ activation through its interaction with PPARδ [33]. As reported, CO negatively regulated Toll-like receptor (TLR) signaling pathways by inhibiting translocation of TLR4 signaling to lipid rafts through suppressing NADPH oxidase (NOX)-dependent ROS generation [34]. Moreover, caveolin-1 (cav-1), the principle structural protein of plasmalemmal caveolae, could regulate inflammatory signaling processes deriving at the membrane [35]. Cav-1 exerted its anti-inflammatory effects via direct binding of TLR4, which prevented TLR4 association with myeloid differentiation primary response gene 88 (MyD88) and Toll/IL-1resistance domain-containing adaptor-inducing IFN-β (TRIF), and downstream activation of the nuclear factor κB (NF-κB) pathway [35]. CO augmented the cav-1/TLR4 interaction to perform its anti-inflammatory part [35]. Exposure of macrophage to CO could lead to a remarkable

Li, Guo, Zhang, Liu, Tao, Pan, and Ye

Fig. 2. Potential signaling pathways influenced by CO, the main byproduct of HO-1, leading to anti-inflammation.

and transient burst of ROS arising from the mitochondria [36]. The ROS enhanced rapid activation and stabilization of the transcription factor hypoxia-inducible factor 1α (HIF-1α) [36]. HIF-1α had the ability to regulate expression of genes involved in inflammation [36]. The increase of HIF-1α expression induced by CO-regulated expression of antiinflammatory cytokine like TGF-β [36]. Through this way, CO might also play protective role [36]. Studies found that the anti-inflammatory effects of HO-1 and its byproducts such as CO could in part be the result of the immunoregulation of the expansion of immune cells, which would in turn reduce the secretion of inflammatory cytokines and suppress inflammation [30]. HO-1 and CO activity could play a crucial role in immune system homeostasis by inducing pleiotropic anti-inflammatory effects, which could be considered a potential therapy target for diseases like autoimmune diseases [30].

HO-1 AND AUTOIMMUNE DISEASES The aforementioned properties of HO-1 provoked researchers’ interest to investigate the effect of HO-1 on autoimmune diseases, and increasing evidence suggested that HO-1 may play a significant role in autoimmune diseases such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and multiple sclerosis (MS). In the following sections, we will discuss in detail the latest advancement in studying the role of HO-1 in autoimmune diseases (Table 1).

Systemic Lupus Erythematosus SLE is a chronic systemic autoimmune disease characterized by the production of multiple autoantibodies, complement activation, and immune-complex deposition, resulting in tissue and organ damages [37]. HO-1 expression both in transcription and protein levels was reported notably reduced in monocytes from patients with SLE [38]. This finding suggested that unbalanced monocyte functions were possibly linked to diminished HO-1 activity in SLE [38]. However, there were no significant differences in HO-1 expression in DCs or CD4+ T cells from SLE patients [38]. Reduced expression and activity of HO-1 in monocytes could not only weaken the tolerogenic capacity of monocytes, but also improve their immunogenicity [38]. Moreover, the reduced HO-1 expression in monocytes did not significantly relate with disease activity [38]. These data indicated that HO-1 abnormality might participate in the initial steps of SLE development rather than in the progression of the disease [38]. One possible mechanism of the reduction of HO-1 could be the high circulating levels of type 1 IFN and IFN-γ in the blood of patients with SLE [38]. Besides, increased HO-1 expression in circulating inflammatory cells was reported to augment their phagocytic capacity [38, 39]. Thus, reduced levels of HO-1 might contribute to the initiation and maintenance of an immune response against autoantigens and the defect in clearing apoptotic cells by monocytes [38]. Similarly, the levels of HO-1 were also found decreased in lupus nephritis (LN) [40]. Takeda et al. demonstrated that chemical induction of HO-1 could ameliorate

Therapeutic Potential of HO-1 in Autoimmune Diseases Table 1. Main Findings of Roles and Mechanisms of HO-1 Performed in Autoimmune Diseases Autoimmune diseases

Ref no.

HO-1 inducer/inhibitor/ injection/deletion

Results and mechanisms

SLE

[41]

Hemin

[30]

CoPP/CO treatment/SnPP

RA

[45]

CoPP/SnPP

MS

[22]

CoPP

[55] [58]

Hemin EPO

[59]

RDP58

HO-1 ameliorated lupus nephritis. HO-1 suppressed iNOS expression in the kidney. HO-1 reduced of circulating levels of IgG anti-dsDNA antibody, glomerular immunodeposits and serum IFN-γ. CO reduced the expansion of CD11b+ cells, autoantibody levels, but increased the frequency of regulatory T cells in the spleen of lupus mice. HO-1 induction and CO exposure ameliorated renal damage of lupus mice. HO-1 could inhibited cartilage erosion together with extensive fibrosis in the joint and reduced the incidence of CIA. CoPP treatment suppressed levels of TNF-α, IL-2, and IL-10 while increased levels of VEGF. Treatment SnPP alleviated the severity of CIA, with restraint of joint inflammation and cartilage damage. The levels of PGE2, IL-1β, and TNF-α were also significantly decreased by SnPP treatment. CoPP acted via HO-1 to inhibit EAE progression. CoPP treatment suppressed major histocompatibility complex II expression by APCs as well as the suppression of T helper cells and CD8 T cells accumulation and function in the organ. Administration of hemin inhibited the onset of the disease and reduced the disease duration. EAE was attenuated. Administration of EPO inhibited Th1 and Th17 responses. Administration of EPO enhanced systemic Th2 and Tregs populations. Administration of RDP58 effectively prevented EAE.

[60]

Deletion of HO-1

[61] [14]

SnPP AAV-HO-1

[65]

CoPP/SnMP

[67] [68]

Curcumin NCD

TID

HO-1 deficiency lowered IFN-β production. HO-1 deficiency increased APCs, Th17 cells infiltration, and the reactivity of a nonregressing myelin-specific T cell. SnPP suppressed clinical scores, weight loss, and certain signs of pathology in this model. AAV-HO-1 treatment significantly suppressed the progression of destructive insulitis and the occurrence of diabetes. Usage of AAV-HO-1 lowered the levels of the Th1-type cytokines IL-2 and IFN-γ in circulation. AAV-HO-1 treatment lowered CD11c + MHC II+ dendritic cell population. Administration of CoPP could attenuate β cell destruction and inhibit the development of diabetes. Administration of CoPP prevented CD11c + DC infiltration into pancreatic tissues, increased phosphorylated AKT (pAKT) and B-cell lymphoma (BcL)-xL activation, and improved the level of insulin secretion. Administration of SnPP reversed CD11c+ DC infiltration phosphorylated AKT (pAKT) and B-cell lymphoma (BcL)-xL activation, the level of insulin secretion preformed in the precess of CoPP treatment. Insulin secretion was found significantly increased. Small dose of NCD possessed antidiabetic actions.

LN in MRL/lpr mice and was involved in the inhibition of inducible nitric oxide synthase expression in the kidney and the decrease of circulating levels of IgG anti-dsDNA antibody and IFN-γ [41]. Hence, pharmacological induction of HO-1 might be a novel therapeutic strategy for lupus nephritis and even other autoimmune diseases [41]. More recently, a study found that administration of CO could decline the expansion of CD11b+ cells, prevent

the reduction of Tregs, and lessen antihistone antibodies in untreated Fcγ receptors (FcγR)IIb knockout mice [30]. In addition, animals treated by CO or with modulating HO-1 manifested less kidney damages [30]. These findings strongly suggest the potential of HO-1 as a new target for autoimmune treatment, and CO- and HO-1-inducing agents may thus be candidates for the treatment of these diseases [30].

Li, Guo, Zhang, Liu, Tao, Pan, and Ye Rheumatoid Arthritis RA is a chronic inflammatory disease featured by synovial hyperplasia, with excessive inflammatory cell infiltration in the joints, resulting in the erosion of the articular cartilage and marginal bone, and subsequent joint destruction [42–44]. As reported, HO-1 was highly expressed in animal models of arthritis [45]. In a collagen-induced arthritis (CIA) model with a predominant Th1 response at the onset of arthritis, HO-1 was significantly induced in inflamed tissues [45]. Besides, HO-1 was reported to be highly expressed in synovial tissues [4], peripheral blood monocytes [46], and synovial fluid [47] of patients with RA. HO-1 level in synovial fluid was correlated with serum C-reactive protein level in patients with RA [47], which was known as one of the indicators of active inflammation. In addition, the levels of HO-1 protein might probably also correlate with matrix metalloproteinase-3 (MMP-3) [47]. Serum MMP-3 level was recognized to be a marker of joint destruction and could reflect synovial tissue proliferation [47]. Therefore, this study indicated that HO-1 might be a useful marker of joint inflammation in RA patients [47]. Regarding the role of HO-1 played in RA, there were discrepancies. In a murine CIA model, HO-1, strongly induced by cobalt protoporphyrin IX (CoPP), could lead to the inhibition of cartilage erosion together with extensive fibrosis in the joint and reduction of the incidence of CIA [45]. However, levels of TNF-α, IL-2, and IL-10 were suppressed by CoPP treatment while those of vascular endothelial growth factor (VEGF) were increased [45]. Besides, the progression of the disease failed to be slowed [45]. Treatment with tin protoporphyrin IX (SnPP), which could inhibit HO-1, remarkably alleviated the severity of CIA, with restraint of joint inflammation and cartilage damage [45]. The levels of prostaglandin E2, IL-1β, and TNF-α were also remarkably decreased by SnPP treatment, which did not change cyclooxygenase 2 (COX-2) protein expressions [45]. Therefore, this study made a conclusion that the overexpression of HO-1 indeed caused some beneficial effects [45]. However, HO-1 overexpression could not slow the progression of this disease, whereas treatment with SnPP presented prophylactic and therapeutic influences [45]. Another study in which a rat adjuvant arthritis model was used showed that HO-1 was upregulated during the maintenance of chronic inflammation in this model and might play a part in the inflammatory characteristics of this model [48]. Particularly, HO-1

activity was correlated with VEGF production and angiogenesis in time of chronic inflammation [48]. Therapeutic use of the HO-1 inhibitor SnPP could control the symptoms of arthritis [48]. However, in some studies, HO-1 was proved to play a regulatory role and could be an effective therapeutic target of RA. Suppression of endogenously expressed HO-1 could lead to significant increase of spontaneous TNF-α and LPS-induced IL-6 and IL-8 synthesis, whereas induction of HO-1 could result in reduced synthesis of the proinflammatory cytokines and expression of COX-2 [4, 46]. Therefore, HO-1 might play a regulatory role in the development of inflammation in RA. Such contradictions in the roles of HO-1 between human in vitro studies using RA synovial cell lines, and rat adjuvant arthritis and mouse arthritis studies might arise from variations in the HO-1 expression level between these systems [4]. Consequently, with respect to the therapeutic application of HO-1, it was critical to optimize the expression level through pharmacologic induction or gene therapy [4]. The suppressive effects of auranofin, which was known to suppress proinflammatory cytokines, were partially reversed by the addition of HO-1-specific siRNA, suggesting that some antirheumatic effects of auranofin depended on HO-1 induction [4]. It was well known that inflammatory synovium expressed inflammatory cytokines like IL-1β, which contributed to further joint destruction [47]. CoPP-induced expression of HO-1 in joints and liver was associated with marked decrease in IL-1β [49]. In RA synovial fibroblasts, IL-1β was also proved to stimulate the activation of p42/p44 mitogen-activated protein kinases and c-Jun amino-terminal kinases-1/2, which caused the production of NOX-dependent ROS, the activation of AP-1, as well as gene expression of cytosolic phospholipase A2 (cPLA2) [50]. Overexpression of HO-1 attenuated IL-1β induced cPLA2 protein, mRNA expression, ROS generation, and NOX activity through inhibiting these signaling components [50]. A study further elucidated the therapeutic potential of the two heme degradation products of HO-1, CO, and biliverdin in the CIA model and found CO and biliverdin could ameliorate clinical symptoms of arthritis in CIA model [51]. In addition, CO treatment resulted in a decrease in anticollagen II antibody levels and histomorphological signs of bone destruction, while at the same time CO and biliverdin diminished cartilage degradation [51]. All suggested that HO-1 was a novel therapeutic strategy for RA.

Therapeutic Potential of HO-1 in Autoimmune Diseases Due to the existing conflicting results of the role of HO-1 in RA, more carefully designed and in-depth studies are still needed. MULTIPLE SCLEROSIS MS is an immune-mediated, chronic inflammatory perivascular demyelinating disease of the central nervous system (CNS) [52]. HO-1 has been proved neuroprotective [53]. Schluesener et al. found that HO-1 activity substantially increased with the progress of experimental autoimmune encephalomyelitis (EAE) [54, 55], which was a CD4+ T cell-mediated inflammatory demyelinating disease of the CNS and was a commonly applied model of MS [56, 57]. Liu et al. clearly verified that HO-1 was significantly expressed in EAE, mainly in the macrophages of the lesions, and fulfilled the protective effect [55]. Induction of EAE in Hmox1−/−C57BL/6 mice led to enhanced CNS demyelination, paralysis, and even mortality compared with Hmox1+/+mice [22]. Endogenous HO-1 alleviated the severity and the mortality related with the development of this disease [22]. Hemin, an inducer of HO-1, prominently attenuated the pathological changes of EAE, inhibited the onset of the disease and reduced the disease duration [55]. Besides, HO-1 induced by CoPP administration after EAE reversed paralysis in C57BL/6 mice [22]. This protective effect could be abolished in Hmox1−/−C57BL/6 mice, which indicated that CoPP inhibited EAE progression via HO-1 [22]. The cytoprotective effect of HO-1 induced by erythropoietin (EPO) was also been proved [58]. Exogenous EPO promoted the induction of endogenous HO-1, which inhibited Th1 and Th17 responses in situ, and enhanced systemic Th2 and Tregs populations to attenuate EAE [58]. Administration of RDP58, which is an immunomodulatory peptide and an effective upregulator of HO-1, was also found effective in preventing this disease in the Lewis rat acute EAE model [59]. On the contrary, rats treated with HO-1 inhibitor tin mesoporphyrin IX (SnMP), deteriorated the pathological changes of EAE and increased severity of this disease [55]. Moreover, it was recently reported that HO-1 expression was reduced in PBMCs of MS patients and that during the exacerbation of the disease there was a notable downregulation of this enzyme [8]. In addition, treatment with corticosteroids resulted in a significant upregulation of HO-1 [8]. This suggested that the anti-inflammatory and immunosuppressive impacts of steroids on MS and perhaps other autoimmune diseases might be mediated by the upregulation of HO-1 [8]. If it were true, one could consider the possibility to use HO-1 inducers as a therapeutic tool either

alone or with steroids for the therapy of acute and chronic immunoinflammatory diseases [8]. All these indicated that HO-1 played important protective roles in this disease. Targeted induction of HO-1 might serve as a novel therapy for the treatment of multiple sclerosis. With regard to the mechanism of HO-1 in MS, a previous research reported that HO-1 formed a complex with interferon regulatory factor (IRF)3, the transcription factor downstream of both TLR3/4 and retinoic acid-inducible gene-I/melanoma differentiation-associated gene 5 and was essential for IRF3 activation and subsequent gene expression in response to TLR3/TLR4 stimulation, and thus it modulated the type I IFN production in macrophages. [60]. HO-1-deficient macrophages had significantly lower IFN-β production and of primary IRF3 target genes encoding regulated on activation normal T cell expressed and secreted, IFN-γ-inducible protein-10, and monocyte chemoattractant protein-1 as compared with control cells [8, 60]. Besides, the amount of IFN-β produced after LPS stimulation of HO-1-deficient macrophages was significantly hampered [8, 60]. In addition, after induction of EAE, mice with myeloid-specific HO-1 deletion developed a higher incidence and an exacerbated, unremitting clinical disease associated with continuous activation of APCs, increased Th17 cells infiltration, as well as the reactivity of a nonregressing myelin-specific T cell [60]. It is worth noting that these defects were rectified via exogenous administration of IFN-β, which testified that HO-1 functioned on upstream of this pivotal immune pathway directly [60]. Though most studies suggested the protective effect of HO-1 in MS, Chakrabarty et al. reported that HO-1 could promote the pathology of experimental allergic encephalomyelitis [61]. SnPP, a putative inhibitor of HO-1 was given to SJL mice during active disease in this study [61]. SnPP suppressed clinical scores, weight loss, and certain signs of pathology in this model [61]. The exact reason for discrepancies in the role of HO-1 played in MS was not clear yet. Further investigations are thus needed to clarify the roles and mechanisms of HO-1 in MS, and to develop new therapeutic strategy by targeting HO-1 for MS. TYPE 1 DIABETES Type 1 diabetes (T1D) is a chronic autoimmune disorder characterized by the progressive destruction of pancreatic insulin-producing β-cells [14, 62]. Studies on experimental diabetes found lower levels of HO-1 expression and HO activity in streptozotocin-induced diabetic rats [63, 64].

Li, Guo, Zhang, Liu, Tao, Pan, and Ye Hu et al. performed in vivo HO-1 gene transfer in nonobese diabetic (NOD) mice, which significantly suppressed the progression of destructive insulitis and the occurrence of diabetes [14]. AAV-HO-1 treatment lowered the Th1-type cytokines and CD11c+ MHC II+ dendritic cell population [14]. Besides, Li et al. demonstrated that upregulation of HO-1 expression by CoPP in the early development of diabetes in NOD mice led to acquisition of a new pancreatic phenotype, attenuation of β cell destruction, and inhibition of the development of diabetes [65]. This was associated with the prevention of CD11c+ DC infiltration into pancreatic tissues, an increase in phosphorylated AKT (pAKT) and B-cell lymphoma (BcL)-xL activation, and an improvement in the level of insulin secretion [65]. Furthermore, when NOD mice were treated with the HO inhibitor, SnMP, the euglycemic effects of HO-1 induction, prevention of CD11c+ DC infiltration, and pAKT and BcL-xL activation were completely reversed [65]. To assess the protective effect of β cell-specific HO-1 in autoimmune diabetes, Huang et al. found less severity of insulitis and a lower diabetes incidence, using an insulin promoter-driven murine HO-1 construct transgenic NOD mouse [66]. Islets from transgenic mice expressed lower levels of proinflammatory cytokines or chemokines, pro-apoptotic gene expression, and ROS/reactive nitrogen species, and were more resistant to apoptosis induced by TNF-α and IFN-γ [66]. In addition, the protective effect of HO-1 on β cells was dependent on the IFN-γ-signal transducer and activator of transcription-1 pathway [66]. In another experiment, insulin secretion was significantly increased in islets cultured with curcumin that was known to induce HO-1, and insulin secretion was significantly decreased by incubation of islets with stannus mesoporphyrin (HO activity inhibitor), confirming the role of HO-1 in insulin secretion in pancreatic islets [67]. Similarly, Aziz et al. applied a novel water-soluble curcumin derivative (NCD) to induce HO-1 and found that small dose of NCD possessed antidiabetic actions and heme oxygenase induction seemed to play a critical role in its antidiabetic effects [68]. These findings clearly indicated a new therapeutic approach for T1D by induction of HO-1.

HO-1 GENE PROMOTER POLYMORPHISM AND AUTOIMMUNE DISEASES The length of a guanine-thymidine (GT)n-repeat polymorphism in the promoter region of the HMOX1 gene was

confirmed to determine the level of HO-1 induction [69, 70]. When given the same stimulus, people who were homozygous for a (GT)n repeat where n

Therapeutic potential of HO-1 in autoimmune diseases.

Heme oxygenase-1 (HO-1), the inducible isoform of heme oxygenase (HO), has raised a lot of concerns in recent years due to its multiple functions. HO-...
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