Chinese Herbal Medicine Targets Mitochondrial Dysfunction in the Treatment of Psoriasis

DONG Rui (董 芮), DAI Dan (代 丹), SONG Ping (宋 坪)

1. Department of Dermatology, Guang'anmen Hospital, China Academy of Chinese Medical Sciences, Beijing 100053,China

2. Graduate College, Beijing University of Chinese Medicine, Beijing 100029, China

Correspondence to: SONG Ping, E-mail: songping@vip.126.com.

Supported by: National Natural Science Foundation of China (Grant No.82074448)

ABSTRACT Psoriasis is a chronic skin disease that affects 2%-3% of the worldwide population. Western medicine is poorly tolerated and ineffective in the treatment of psoriasis. In comparison to conventional methods, traditional Chinese medicine has been proved to be safe and effective in the treatment of psoriasis by a number of high-quality clinical randomized controlled trials. Previous studies have found that mitochondrial dysfunction may play a critical role in the development of psoriasis. Mitochondrial dysfunction can lead to cell damage caused by excessive generation of mitochondrial reactive oxygen species, induce abnormal apoptosis mainly through mitochondria pathway and affect cell proliferation and differentiation by affecting energy supply. More than that, it releases several inflammatory cytokines and activates inflammatory cells and finally affect the development of psoriasis. The active ingredients in herbal medicine will regulate mitochondrial dysfunction. Therefore, the treatment of psoriasis by regulating mitochondrial dysfunction can provide a new idea for the treatment of psoriasis.

KEYWORDS Mitochondrial dysfunction; Chinese herbal medicine; Psoriasis

INTRODUCTION

Psoriasis is a common, chronic inflammatory,relapsing, refractory skin disease with a global incidence of 2-3%[1], which is characterized by sharply demarcated, erythematous papules covered with silvery scales，accompanied with different degrees of pruritus[2]. Pathologic features are hyperproliferation and abnormal differentiation of keratinocytes，neovascularization and infiltration of lymphocytes[3].

The etiology of psoriasis is complex and the specific mechanism is still being explored. Currently,it is generally considered that psoriasis caused by a combination of environmental, metabolism,emotion and immunologic factors under polygenic background, involving T-lymphocytes (T cells),dendritic cells (DC), keratinocytes, natural killer cells,macrophages[4]. The importance of immune system in the pathogenesis of psoriasis has been widely accepted. Mitochondria are essential organelles in eukaryotic organisms that crucial in determining cell fate, involved in cellular metabolic homeostasis,signal transduction, proliferation and differentiation.At present, many studies have showed that mitochondrial dysfunction tightly related to the development and treatment of psoriasis, which is an important therapeutic target.

At present, western medicine targeting antiinflammation, inhibit the proliferation of immune cells and keratinocytes, such as methotrexate, retinoic acid and biological agents, has certain curative effect in the treatment of psoriasis, but due to the high price and some side effects[5], it is restricted in patient use. Therefore, it is extremely urgent to find safe and effective therapeutics. Traditional Chinese medicine (TCM) have been applied in the treatment of psoriasis for thousands of years. Molecular mechanistic studies have illustrated that many traditional Chinese medicine monomers can not only modulate immune responses, but also regulate mitochondrial dysfunction, thereby increasing the efficacy of treatment. Therefore, mitochondria dysfunction could be used as a potential therapeutic target for psoriasis. In this review, we mainly focus on the mechanism of Chinese medicines in the treatment of psoriasis by regulating mitochondrial dysfunction

THE RELATIONSHIP BETWEEN MITOCHONDRIA AND PSORIASIS

Mitochondrial DNA Promotes Inflammatory Responses

Mitochondrial DNA (mtDNA) is a 16,569 bp,double-stranded circular molecule encoding 13 polypeptides, all of which are involved in oxidative phosphorylation as well as two ribosomal RNAs(rRNAs) and 22 transfer RNAs (tRNAs) for mitochondrial respiration[6]. MtDNA can serve as a critical activator of the immune system. The innate immune system recognizes pathogen-associated molecular patterns through pattern-recognition receptors (PRRs), which is the first line of host defense[7]. As damage-associated molecular patterns(DAMPs), mtDNA activate the cytokine signaling pathway to modulate the immune response.

Experimental studies have shown that mtDNA levels are high in serum of psoriatic patients[8].MtDNA may derive from apoptotic keratinocytes or activated immune cells, especially mast cells.MtDNA breaks innate tolerance to self-DNA by forming a complex, subsequentlytrigger Tolllike receptor-9(TLR9) and induce interferon (IFN)production[9]. When mtDNA forms a complex with cathelicidin LL37, it can escape from autophagy and degradation by deoxyribonuclease II, thus triggering Toll-like receptor (TLR)9-dependent responses,activating plasmacytoid dendritic cells (pDCs) and initiating an inflammatory cascade[10].

MtDNA activates several immune pathways involving TLR9, inflammasome, and stimulator of interferon genes (STING) signaling to modulate immune functions. Previous studies have shown that mtDNA interacts with the TLR9 receptor and initiates a cascade of immuno-stimulatory signaling events.TLR9 becomes activated and recruitsmyeloid differentiation factor88 (MyD88), leading to the production of Interleukin(IL)-6, tumor necrosis factor α (TNF-α) and IL-12[11]. The overexpression of heparin-binding epidermal growth factor-like growth factor(HB-EGF) and transforming growth factorα(TGF-α) were detected in psoriasis keratinocytes,Luo et al.[12]found these mitogenic factors heavily relied on the release of TNF-α. More than that,microRNA-31 induced by NF-κB can promote keratinocyte proliferation by targeting protein phosphatase 6[13].

Studies have demonstrated that nucleotidebinding domain (NOD)-, Leucine-rich repeat (LRR)-and pyrin domain-containing protein 3（NLRP3）inflammasome-mediated IL-1β production accounts for psoriasis development[14]. MtDNA is the endogenous agonists for NALP3 inflammasome.Mitochondrial reactive oxygen species[15]or oxidized mtDNA[16]are required for NLRP3 inflammasome activation, leading to activate caspase-1and secrete elevated levels of IL-18 and IL-1β[17]. MtDNA also triggers cyclic GMP-AMP synthase (cGAS)/STING signaling cascade, activates inflammatory mediators and promotes type I IFN production[18].

Aberrant Mitochondrial Metabolism Promotes Th17/Treg Imbalance

Mitochondria are vital energy-generating organelles in eukaryotic cells, which produce most of the cellular adenosine triphosphate (ATP)through oxidative phosphorylation (OXPHOS).Under normoxic conditions, pyruvate produced by glycolysis is oxidized by pyruvate dehydrogenase to form acetyl-coenzyme A (Acetyl-CoA) which is further metabolized via tricarboxylic acid cycle(TCA cycle)[19]. Reduced nicotinamide adenine dinucleotide (NADH) and reduced flavin adenine dinucleotide (FADH2) are generated during reactions of the oxidative TCA cycle, which donate electrons for the electron transport chain (ETC)[20].In the ETC, the electrons are passed to molecular oxygen, creating an electrochemical gradient across the inner mitochondrial membrane which is utilized to synthesize ATP by FoF1 ATP synthase[21]. Xie et al.[22]found that inhibition of F1F0-ATP synthase blocked human immortalized keratinocytes(HaCaT) cell differentiation, suggesting F1F0-ATP synthase expression associated with the process of keratinocyte differentiation.

There are a large number of infiltrating immune cells dominated by T lymphocytes in psoriatic lesions. In addition to affect keratinocytes,Mitochondria also influence T cell proliferation,differentiation and function. T cell differentiation is accompanied by a switch of the ATP production pathway from OXPHOS to glycolysis. Na?ve T cells mostly rely on OXPHOS for ATP production[23]. Upon activation, effector T cells exhibit enhanced glucose uptake and decreased oxidative phosphorylation to produce ATP[23]. In contrast to naive T cells, memory T cells exhibit elevated mitochondrial mass[25]. High spare respiratory capacity is believed to be a feature of memory T cells that produce extra amounts of ATP through oxidative phosphorylation in case of antigenic stimulation[26].

Studies have indicated that the balance of regulatory T (Treg) cells and T helper 17 (Th17)cells is a key factor in the pathogenesis of psoriasis.Th17 cells depend mainly on glycolysis to obtain energy, whereas Treg cells rely more on the fatty acid oxidation(FAO) and OXPHOS than on glycolysis[27]. Thus, Na?ve T cell differentiation can be controlled by the metabolic mode of cells.Enhanced the FAO or suppressed glycolytic leads to a shift in the Th17/Treg ratio in the favor of Treg cells[28]. Additionally, Treg cells maintain immune homeostasis by inhibiting abnormal immune responses and suppressing inflammation, which requires mitochondrial complex III to exert[29].

Abnormal Mitochondrial Apoptosis Pathway Leads Reduced Keratinocyte Apoptosis

The stimuli such as DNA damage, activate the cleavage of the BH3 (Bcl-2 homology domain 3)-only proteins and trigger the insertion of Bcl-2-asslciated protein X (Bax) into the mitochondrial outer membrane, causing mitochondrial outer membrane permeabilization (MOMP) and the release of cytochrome c. Cytochrome c binds to caspase 9 and initiates the cascade of cell death[30].After the release of cytochrome c into the cytosol,apoptosome formed by apoptotic protease activating factor-1 (APAF-1) induce procaspase-9 to be cleaved and activate downstream caspase-3, thus causing an apoptosis cascade reaction[31].

It is reported that psoriatic keratinocytes exhibit enhanced ability to resist apoptosis. When psoriatic keratinocytes and healthy control cells were exposed to UVB at the same time, the survival rate of psoriatic keratinocytes was higher. Further study found the reason for the imbalance of apoptosis in psoriatic keratinocytes located in the upstream of cytochrome c released, which may be related to the up-regulated expression of B-cell lymphoma/leukemia-2 (Bcl-2), Mucin-like 1 (MUCL1) and Fibronectin (FN)[32]. Researchers have observed an increase in the expression of B-cell lymphoma-extra large (Bcl-xL) in the skin of patients with psoriasis,which may be one of the reasons for preventing the release of cytochrome c into the cytoplasm of psoriatic skin[33]. Therefore, induce keratinocytes through mitochondrial pathway of apoptosis leading to inhibit keratinocyte proliferation. Methotrexate is a commonly used drug in clinical treatment of psoriasis, which induces increased the expression of cytochrome c and caspase-9 resulting in apoptosis of psoriatic keratinocytes[34].

Mitochondrial ROS Induce the Differentiation of Dendritic Cells and Mediate Inflammatory Responses

Reactive oxygen species (ROS) are highly chemically reactive species derived from molecular oxygen, such as singlet oxygen, hydrogen peroxide(H2O2), hydroxyl radicals (OH·) and superoxide(O2-)[35]. The main source of ROS generation is the ETC in mitochondria. ROS in cells are mainly produced by the electron transport complex on the electron transport chain (ETC). ROS play a key role in cell multiple biological processes，including cellular proliferation, gene expression and cellular homeostasis. Furthermore, ROS regulate cytokine synthesis and release through NF-κB and mitogenactivated protein kinase (MAPK) signaling pathways.

On the one hand, previous studies have demonstrated that mitochondrial ROS (mtROS)promote DC differentiation, which increase the number of mitochondria and enhance respiratory capacity during monocyte-DC differentiation.Following the use of ETC inhibitor，the number of mitochondria and ATP levels drop but cell viability remain unaffected[36]. This suggests that impaired mitochondrial respiration inhibits DC differentiation.On the other hand, mitochondrial ROS has been shown to play an important role in keratinocyte differentiation. The functions of autophagy and lysosome are necessary for the differentiation of human skin keratinocytes. Lysosomes support mitochondrial metabolism and mtROS production,while mtROS trigger the degradation necessary for epidermal differentiation mediated by autophagy and lysosomes. Additionally, exogenous ROS treatment rescued the differentiation defect of keratinocytes caused by lysosome inhibition in psoriasis[37].

Oxidative stress is defined as a dysregulation between the production of ROS and the endogenous antioxidant defense mechanisms[38]. An excess of ROS leads to DNA mutation and lipid peroxidation oxidative damage biomarkers increased such as malondialdehyde (MDA), resulting in irreversible damage and cell apoptotic[39]. An imbalance between oxidants and antioxidants can result in skin diseases, especially psoriasis. It has been found the antioxidant activity decreased and antioxidant activity increased in psoriatic patients, showing decreased total antioxidant status (TAS), increased activity of malondialdehyde (MDA), nitric oxide (NO),superoxide dismutase (SOD) and catalase (CAT).The levels of MDA, NO and cytochrome c were significantly negatively correlated with SOD and CAT, and positively correlated with the severity of psoriasis[40].

ADVANTAGES OF TRADITIONAL CHINESE MEDICINE IN THE TREATMENT OF PSORIASIS

It is known that the etiologies of psoriasis are complex, TCM regulates systemic status and offers an opportunity for personalized treatment. According to traditional Chinese medicine theories, psoriasis is classified into three categories: blood heat, blood dryness, and blood stasis based on lesions. In the latter stages of the disease, there may be a series of diseases and symptoms related to kidney essence deficiency, such as fatigue. At the initial stage,psoriasis usually manifests itself as the blood-heat syndrome, which is characterized by increased skin lesions, flushing, new rashes continue to appear, the original rash continues to expand, usually treated with antipyretic drugs such as Cortex Moutan Radicis, Radix Rehmanniae and Radix Sophorae Flavescentis. The blood stasis syndrome of psoriasis is characterized by dark complexion or purple lip nail, hypertrophy and infiltration of skin lesions,rough, dark red or purplish red plaques，usually treated with drug for activating blood and eliminating stasis, such as Radix Salviae Miltiorrhizae. As the disease progressed, stubborn psoriatic lesions accompanied with the manifestations of kidney essence deficiency, usually treated with Herba Epimedii tonifying the kidney, Recent studies found that TCM could relieve the inflammatory reaction,suppress angiogenesis and inhibit keratinocyte proliferation. Apart from this, TCM can also reduce oxidative stress and promote apoptosis through regulating mitochondrial pathway in psoriasis.

Dihuang (Radix Rehmanniae Recens)

Catalpol, the extract of iridoid glycosides from Rehmannia glutinosa, is one of the main effective components of Rehmannia glutinosa. Liu et al.[41]revealed that catalpol may reduce oxidative stress and inflammation by inhibiting NF-κB and MAPK signaling pathways, thus achieving the therapeutic effect of psoriasis. Catalpol can not only enhance endogenous antioxidation, as evidenced by increased the levels of GSH, SOD and glutathione peroxidase (GSH-Px), but also elevate the activities of mitochondrial Na+-K+-ATP and Ca2+-Mg2+ATPase in senescent cells to improve energy metabolism[42].In addition to alleviating oxidative stress and suppresses inflammation，Radix Rehmanniae Recens could also inhibit proliferation and induce apoptosis. Shraibom et al.[43]prepared an aqueous solution mainly contained Radix Rehmanniae Recens, which significantly inhibited the proliferation and induced apoptosis of HaCaT cells. Catalpol can also induce apoptosis by changing the expression of Bcl-2 and Bax and activating mitochondrialdependent caspase pathway[44].

Yinyanghuo (Herba Epimedii)

Icariin is one of the important monomer compounds isolated in Herba Epimedii, which has the functions of immune regulation, antiinflammation, antioxidation and so on. After treatment with icariin, dynamin-related protein 1 (Drp1) decreased, while mitofusion 2 fusion protein 2 (Mfn2) increased, suggesting that icariin can promote mitochondrial transport and protect mitochondria from fragmentation[45]. Icariin can inhibit the production of ROS, increase the activity of SOD and enhance the antioxidant capacity of cardiomyocytes during left ventricular remodeling.Icariin also controls the mitochondrial apoptosis pathway through Bax/Bcl-2 signal, and then affects cell apoptosis[46].

Danshen (Radix Salviae Miltiorrhizae)

The active ingredients of Radix Salviae Miltiorrhizae are divided into two groups, one of which is lipophilic components including tanshinone I, tanshinone IIA, cryptotanshinone, and the other is hydrophilic compounds such as salvianolic acids A and B, salvianic acid A (Danshensu). Tanshinone IIA significantly inhibits the proliferation of keratinocytes in a dose-and time-dependent manner. Li et al.[47]found the decrease in mitochondrial membrane potential and the release of cytochrome c into the cytoplasm when treated with Tanshinone IIA.In addition, Danshensu reduced the epidermal dysplasia of psoriasis in vivo and in vitro. Hu et al.[48]showed that Danshensu activated nuclear factor erythroid 2-related factor 2(Nrf2) signaling pathway to clear ROS by Akt/ERK pathway, decreased the content of MDA in a rat model of myocardial ischemia/reperfusion, and increased the activities of CAT, GSH-Px and SOD in serum.

Kushen (Radix Sophorae Flavescentis)

Matrine and oxymatrine are alkaloid extracted from Sophora flavescens, inducing proliferation by mitochondrial apoptosis pathway. Matrine increased levels of intracellular ROS and the ratio of Bax to Bcl-2, up-regulated the activities of caspase-3, 8 and 9 in a dose-dependent manner[49], indicating that mitochondria play an important role in in matrine-induced apoptosis. Homocysteine induced endothelial damage through ROS accumulation,NF-κB activation and nitric oxide production, while oxymatrine inhibited the increase of ROS, MDA and lactate dehydrogenase levels[50].

Mudanpi (Cortex Moutan Radicis)

Paeonol is a kind of natural phenolic compound with biological activity isolated from Cortex moutan,which has anti-inflammatory and antioxidant effects.Paeonol had protective effects on mitochondrial damage caused by hypoxia both in vivo and in vitro,and can eliminate mitochondrial damage caused by hypoxia, including inhibition of ATP synthesis,abnormal morphology of mitochondria and increase of ROS[51]. The extract of Cortex moutan has been validated to exert cardioprotective effects. The cellular antioxidant defense system was enhanced by increasing the activities of SOD and GSH and decreasing the activity of MDA[52].

Xingren (Semen Armeniacae Amarum)

Amygdalin is a natural vitamin found in bitter almond, which promotes apoptosis through mitochondrial-mediated pathway and cell cycle arrest during G2/M phase[53]. After HaCaT cells were treated with bitter apricot essential oil, the expression of Bax increased, the expression of Bcl-2 decreased, and caspases-3/8/9 increased.Beside this, the apoptosis was significantly inhibited by the addition of caspase inhibitor, indicating that the apoptosis induced by bitter apricot essential oil through mitochondrial pathway[54].

CONCLUSION AND PROSPECT

Above all, some investigators have found that mitochondrial dysfunction is one of the key factors affecting the occurrence and development of psoriasis in exploring the pathogenesis of psoriasis. Mitochondria are the energy factory of cells and provide ATP, and they provide energy for cell activity. Insufficient supply of ATP caused by mitochondrial dysfunction will affect the proliferation,differentiation and maintenance of daily function of immune cells. In addition, oxidative stress caused by excessive release of mtROS and damage of antioxidant system in mitochondria, abnormal apoptosis mediated by the mitochondrial pathway,and increased expression of mtDNA also affect the proliferation and differentiation of keratinocytes,stimulate the release of multiple inflammatory factors and the activation of inflammatory cells, lead to the aggravation of the inflammatory reaction in psoriasis.

A number of high-quality clinical randomized controlled trials have confirmed that traditional Chinese medicine is an effective and safe method for the treatment of psoriasis[55]. The results show that TCM herbs and their active components can regulate mitochondrial dysfunction. First of all,traditional Chinese medicine induce apoptosis and inhibit cell proliferation through mitochondrialmediated apoptosis. Secondly, traditional Chinese medicine enhance antioxidant ability by up-regulating the expression and activity of antioxidant enzymes,reducing the content of oxidants and enhance the antioxidant capacity of cells. Finally, traditional Chinese medicine also has the function of protecting mitochondria. The new discovery of pathological mechanism targets provides a research basis for accurate medical treatment and the mechanism of curative effect. Mitochondrial dysfunction plays an important role in the pathogenesis of psoriasis,but there are relatively few studies on its specific targets and upstream regulation mechanism. More comprehensive and in-depth research is needed.