Signaling pathways and targets of natural products in psoriasis treatment

Aim: Psoriasis is a common chronic inflammatory skin disorder, which has adverse effects on patients’ quality of life. Natural products exhibit significant therapeutic capacities with small side effects and might be preferable alternative treatments for patients with psoriasis. This study summarizes the signaling pathways with the potential targets of natural products and their efficacy for psoriasis treatment. Methods: The literature for this article was acquired from PubMed and Web of Science, from January 2010 to December 2020. The keywords for searching included “psoriasis” and “natural product”, “herbal medicine”, “herbal therapy”, “medicinal plant”, “medicinal herb” or “pharmaceutical plant”. Results: Herbal extracts, natural compounds, and herbal prescriptions could regulate the signaling pathways to alleviate psoriasis symptoms, such as T helper 17 (Th17) differentiation, Janus kinase (JAK)/signal transducer and activator of transcription (STAT), nuclear factor-kappa B (NF-κB), mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR), and other signaling pathways, which are involved in the inflammatory response and keratinocyte hyperproliferation. The anti-psoriatic effect of natural products in clinical trials was summarized. Conclusions: Natural products exerted the anti-psoriatic effect by targeting multiple signaling pathways, providing evidence for the investigation of novel drugs. Further experimental research should be performed to screen and characterize the therapeutic targets of natural products for application in psoriasis treatment.


Materials and methods
PubMed and Web of Science databases were used for searching the literature published from January 2010 to December 2020 for this review article. The keywords included "psoriasis" and "natural product", "herbal medicine", "herbal therapy", "medicinal plant", "medicinal herb" or "pharmaceutical plant".
Inclusion criteria include clinical studies using natural products (herbs, natural compounds, herbal formula) with placebo or drug control treatment and preclinical studies demonstrated effects and target signaling pathways of natural products in psoriasis treatment. The flow chart of this study is shown in Figure 1.

Signaling pathways and targets of natural products related to psoriasis
Various signaling pathways have been demonstrated to play a role in the development of psoriasis. The effects of natural products, including herbs, natural compounds, and herbal formulas on psoriasis-related signaling pathways are shown in Tables 1-3.    BMDCs: bone-marrow derived dendritic cells; CXCL8: chemokine ligand 8; HUVEC: human umbilical vein endothelial cell; MAPK: mitogen-activated protein kinase; MMP-9: matrix metalloproteinase-9; MPO: myeloperoxidase; mTOR: mammalian target of rapamycin; NHEK: normal human epidermal keratinocyte; Nrf2: nuclear factor-erythroid 2-related factor 2; PBMCs: peripheral blood mononuclear cells; VEGF: vascular endothelial growth factor; VEGFR2: vascular endothelial growth factor receptor 2; -: not applicable

Lobelia chinensis
Aerial part

Glycyrrhiza uralensis Root
Atg7: autophagy related 7; PAMs: plant antimicrobial solutions; TGF-β: transforming growth factor-beta; -: not applicable Classical signaling pathways Th17 cell differentiation pathway Psoriasis has been considered an immune-mediated skin disease. Emerging evidence suggested the crucial role of Th17 cells in the pathogenesis of psoriasis. TGF-β in combination with proinflammatory cytokines, including IL-23, IL-6, and IL-1β, can drive the differentiation of naive T cells to Th17 cells. IL-23 further promotes the survival and proliferation of Th17 cells, as well as the migration of these cells into psoriatic skin lesions [23]. Th17 cells are considered a distinct subset of CD4 + Th cells by the ability to secrete IL-17, however, Th17 cells can also produce various inflammatory cytokines, such as IL-22, IL-21, IL-6, and TNF-α to promote inflammation and keratinocyte proliferation in psoriatic skin lesions [24]. Previous studies demonstrated that the serum levels of TGF-β, IL-17, IL-22, and IL-6 were significantly higher in patients with psoriasis compared with healthy subjects [25,26].
Water-processed rosin from Pinus massoniana significantly reduced the proportion of Th17 cells in the spleen and inhibited the expression of Th17-related cytokines, including IL-17, IL-22, IL-23, and TNF-α in imiquimod (IMQ)-induced psoriasis-like mouse model [27]. Lavandula angustifolia essential oil and its component linalool showed significant decreases in IL-17 and IL-22 levels in IMQ-induced skin lesions [28]. Treatment with an ethanolic extract of Solanum xanthocarpum stem inhibited the skin expression of IL-17, IL-1β, IL-6, and TNF-α in the psoriasis mouse model [29]. Antrodia cinnamomea extract exerted inhibitory effects on Th17 cell differentiation and the production of IL-17, IL-22, and TNF-α in IMQ-treated mice [30]. Indigo naturalis, an extract from leaves of Baphicacanthus cusia (Ness) Bremek significantly decreased the expression of IL-1β, TNF-α, and IL-23 in keratinocytes, as well as inhibited the production of IL-17 and IL-22 in Jurkat T cells [31]. A methanolic extract of Euphorbia kansui root alleviated psoriasis symptoms by inhibiting the production of IL-23, IL-17, and IL-22 in lymph nodes from psoriatic mice [32].
Gold lotion, an ethanolic extract of a mixture from peels of six citrus fruits, including Citrus sinensis (navel oranges), Citrus hassaku, Citrus limon, Citrus natsudaidai, Citrus miyauchi Iyo, and Citrus unshiu (Satsuma) decreased the ratio of Th17 cells in the spleen and reduced the expression of IL-23, IL-6, TNF-α, IL-17, and IL-22 at mRNA levels in skin lesions in IMQ-induced mouse model of psoriasis [39]. Bai Xuan Xia Ta Re Pian, a traditional herbal formula consisting of six herbs (Euphorbia Humifusae Herba, Chebula Fructus, Terminalia bellirica Fructus, Chebula Fructus Immaturus, Aloe, and Resina Scammoniae) significantly suppressed the expression of IL-23 and IL-17 in the skin of psoriatic mice [40].

NF-κB signaling pathway
NF-κB is a key transcription factor involved in the regulation of various cellular biological processes, including inflammatory responses [41]. Clinical studies in adult patients with moderate to severe psoriasis indicated that the level of the active form of NF-κB was significantly upregulated in psoriatic plaques, compared with non-lesional psoriatic skin and normal skin [42]. NF-κB transcription factor is a homodimer or heterodimer of NF-κB subunits (NFKBs), including p65 (RELA), RELB, p50, p52, and c-REL. In the baseline state, NF-κB dimers form a complex with the inhibitor of NF-κB (IκB) in the cytosol. Upon stimuli such as TNF-α, IκB kinase (IKK) phosphorylates IκB, leading to proteasomal degradation of IκB and releasing of NF-κB from the complex. Free NF-κB dimers translocate from the cytosol into the nucleus and bind to the promoter regions to regulate the transcription of various target genes [43]. NF-κB transcription factor is involved in the pathogenesis of psoriasis by regulating the expression of numerous cytokines, chemokines, and adhesion molecules to modulate inflammation, as well as keratinocyte proliferation and differentiation [42].
Paeoniflorin, the major bioactive compound from Paeonia lactiflora Pall, alleviated psoriasis-like skin symptoms in IMQ-induced mice and inhibited hyperproliferation by suppressing phosphorylation of IκB-α and NF-κB in psoriatic keratinocytes [49]. cis-Khellactone, a common pyranocoumarin, reduced IMQ-induced psoriasis-like skin inflammation and decreased LPS-induced production of pro-inflammatory cytokines in macrophages by inhibiting phosphorylation of IKKα/β and NF-κB p65 [50]. Aloe polysaccharide, the main constituent of Aloe vera, decreased TNF-α-induced inflammation and proliferation in HaCaT cells by inhibiting phosphorylation of p65 and increasing the expression of IκB-α [51]. Luteolin, a common flavone, showed inhibitory effects on TNF-α-induced production of IL-6, IL-8, and VEGF, as well as hyperproliferation in HaCaT cells and NHEKs by decreasing mRNA levels of two genes (NFKB1 and RELA) and inhibiting nuclear translocation of NF-κB [52]. Chebulanin, a natural polyphenol derived from Terminalia chebula Retz., ameliorated IMQ-induced psoriatic skin lesions in mice and reduced inflammation and proliferation in HaCaT cells by decreasing phosphorylation of p65 at both mRNA and protein levels [53].
PAMs, a mixture of ethanolic extracts from Carthamus tinctorius, Lithospermum erythrorhizon, Solanum indicum, and Cymbopogon distans reduced skin symptoms in a psoriatic mouse model and inhibited the production of inflammatory cytokines and chemokines in HaCaT cells by suppressing nuclear translocation of NF-κB [54].
JAK/STAT signaling pathway JAK/STAT pathway plays an important role in immune diseases by mediating various cytokine signalings to regulate inflammation and cell proliferation. JAK protein family includes four tyrosine kinases (TYKs): JAK1-3 and TYK2. The STAT family consists of seven members: STAT1-4, STAT5A, STAT5B, and STAT6. Upon binding of type I and II cytokines to their corresponding receptors, JAKs are activated and phosphorylated, leading to the recruitment and phosphorylation of STATs. Phosphorylated STATs can form dimers and translocate to the nucleus to regulate the transcription of various target genes involved in immune responses [55,56]. Upregulated expression of JAK1 and STAT3 has been reported in skin lesions from patients with psoriasis, compared with normal skin. In addition, STAT3 expression had a positive correlation with the severity of psoriasis [57]. A variety of inflammatory cytokines related to psoriasis, such as IL-6, IL-23 can activate JAK/STAT signaling pathway to promote the development of psoriasis by triggering inflammatory response as well as keratinocyte proliferation in skin lesions [58]. Inhibition of JAK/STAT pathway by JAK inhibitors such as tofacitinib improved disease severity in patients with moderate-to-severe psoriasis [59], suggesting that modulation of JAK/STAT signaling pathway might a potential approach for psoriasis treatment.
A methanolic extract of root bark of Dictamnus dasycarpus Turcz. improved scaly skin lesions, reduced the number of inflammatory cell infiltration, and decreased epidermal thickness in IMQ-induced psoriasis mice by inhibiting STAT3 signaling pathway and reducing the number of Th17 cells as well as IL-17 production [72]. 9,19-cycloartenol glycosides G3, the main component of Cimicifuga simplex exhibited anti-psoriatic effects by suppressing the differentiation of CD4 + T cell into Th17 phenotype and inhibiting IFN-γ-induced JAK/STAT activation [73]. Withasteroid B isolated from Datura metel L. showed the inhibitory effects on JAK/STAT signaling pathway and reduced the ratio of Th17 cells as well as the production of Th17-related inflammatory cytokines [74]. Total glucosides extracted from Paeonia lactiflora Pall alleviated IMQ-induced psoriasis-like skin symptoms in mice, inhibited Th17 differentiation, and suppressed phosphorylation of STAT1 and STAT3 [75].

NF-κB and related signaling pathways
NF-κB pathway can be activated by several upstream pathways, including PI3K/Akt and MAPK signalings to regulate inflammation [79,80]. Nrf2 signaling can attenuate NF-κB activation, and in contrast, NF-κB could suppress Nrf2 activity [81]. NF-κB and JAK/STAT signaling pathways are involved in the regulation of inflammatory response in psoriasis. A previous study demonstrated that JAK/STAT signaling synergized with NF-κB to activate the transcription of various inflammatory genes in response to stimuli [82]. NF-κB activation also modulates many downstream signaling pathways which are involved in the pathogenesis of psoriasis. NF-κB signaling shows intrinsic and extrinsic effects on Th17 differentiation [83]. VEGF signaling, which is important in regulating angiogenesis (a hallmark of psoriasis), is also a downstream pathway of NF-κB [84]. Moreover, NF-κB pathway was suggested to be involved in NLRP3 inflammasome activation in psoriasis [85].
Chrysin, a common flavone found in various natural sources, such as honey, passion flowers, or propolis, alleviated IMQ-induced psoriasis symptoms in mice and inhibited the production of inflammatory cytokines, chemokines, and antimicrobial peptides in keratinocytes by suppressing NF-κB, MAPK, and JAK/STAT signaling pathways [91]. Glycyrrhizin, a major component of Glycyrrhiza glabra, improved psoriasis-like skin lesions in IMQ-induced mice and reduced ICAM-1 production in TNF-α-stimulated HaCaT cells by inhibiting NF-κB/MAPK signaling [92]. Tussilagonone, a natural compound derived from Tussilago farfara, alleviated psoriasis symptoms in IMQ-treated mice and TNF-α-treated keratinocytes via Nrf2 activation and NF-κB/STAT3 inhibition [93]. Curcumin, a main compound of Curcuma longa, attenuated psoriasis pathology by inhibiting and NF-κB and Th17 differentiation pathways in keratinocytes and immune cells [94][95][96]. Honokiol (a lignan isolated from Magnolia officinalis) and gambogic acid (a xanthone from Garcinia harburyi) showed the anti-psoriatic effect by suppressing NF-κB/VEGF signaling pathway [97,98]. Saikosaponin A, a component of Bupleurum chinense, reduced psoriasis area and severity index (PASI) scores and epidermal hyperplasia in IMQ-induced mice and attenuated cytokine-induced inflammation in human keratinocytes by suppressing the phosphorylation of NF-κB and the expression of NLRP3 [99].

Other signaling pathways
An ethanolic extract of Artemisia capillaris ameliorated IMQ-induced psoriasis-like symptoms in mice and showed antiproliferative effect by promoting apoptosis in keratinocytes [103]. The leaf extract of Vitis vinifera L. alleviated psoriatic inflammation by inhibiting the activation of AIM2 inflammasome signaling [104]. Salvia miltiorrhiza extract [also known as danshensu in traditional Chinese medicine (TCM)] suppressed epidermal hyperplasia in IMQ-induced mice and hyperproliferation in cytokine-stimulated keratinocytes by reducing the expression of yes-associated protein (YAP, an important component of Hippo signaling pathway) [105]. Andrographolide, a major component from Andrographis paniculata, exerted the anti-psoriatic effect by inhibiting TLR/MyD88 signaling in DCs [106]. Ar-turmerone, a sesquiterpenoid from Curcuma longa, suppressed TNF-α-induced inflammation and proliferation in HaCaT cells by inhibiting Hedgehog signaling pathway [107]. Fisetin (a common flavonol) and imperatorin (a furocoumarin derived from Angelica hirsutiflora) alleviated psoriasis-like symptoms by suppressing PI3K/Akt/mTOR and MAPK signaling pathways in keratinocytes and immune cells, respectively [108,109].

Clinical efficacy of natural products in psoriasis treatment
Clinical studies demonstrated the anti-psoriatic effects of natural products are shown in Table 4. Topical application of extract from sea buckthorn, Indigo naturalis, Hypericum perforatum, and Gynura pseudochina showed significant reductions in skin severity compared with placebo in patients with mild to moderate psoriasis [110][111][112][113]. Sea buckthorn has been traditionally used for thousands of years for treatment of skin diseases due to its anti-inflammatory and immunomodulatory effects [114]. Indigo naturalis has been widely used for psoriasis treatment in TCM for over 50 years [115]. Hypericum perforatum and Gynura pseudochina are also common medicinal plants used to treat skin symptoms [116,117]. Herbal formula Pulian ointment (consisting of two herbs: Phellodendron amurense and Scutellaria baicalensis) and Shi Du Ruan Gao (a mixture of six herbs: Indigo naturalis, Cortex Phellodendri, Gypsum fibrosum preparatum, Calamine, Galla chinensis) also exerted anti-psoriatic activity by decreasing PASI scores without any severe adverse events after four weeks and eight weeks of topical treatment, respectively [118,119]. Pulian ointment has been approved by Beijing Food and Drug Administration as a prescription use in hospitals in China for treatment of psoriasis [119]. Shi Du Ruan Gao has been developed and used for psoriasis treatment in hospital for over 60 years [118].   [120,121]. Liang xue huo xue was used for psoriasis treatment due to its anti-proliferative effects on keratinocytes [122]. Yinxieling has been applied in clinical and exerted effectiveness in treatment of psoriasis [123]. Treatment with Liangxue Jiedu decoction (10 herbs: Rhizoma Smilacis Chinae, Flos Sophorae, Radix Lithospermi, Rhizoma Paridis, Radix Rehmanniae, Cortex Dictamni Radicis, Radix Paeoniae Rubra, Flos Lonicerae, Rhizoma Imperatae, Radix Sophorae Flavescentis) showed significant improvements in skin symptoms in comparison with Western medicine (cetirizine hydrochloride, vitamin C, and vitamin B complex) after eight weeks [124]. Liangxue Jiedu decoction was used for treatment of psoriasis in TCM due to its immunomodulatory activity [125].
All the nine clinical studies mentioned in Table 1 were randomized studies with single-blind or double-blind, single-center or multi-center, and placebo-controlled or positive-controlled observation. These studies were conducted in small groups of patients (10-50 patients) or larger groups (100-300 patients). Participants were included in clinical studies consisting of both men and women, aged from 18 years old to 80 years old with skin symptoms from mild to severe. Some studies specifically targeted the patients with the blood heat syndrome based on TCM diagnosis. According to TCM, blood heat is the most common syndrome (53.8%) in patients with psoriasis, compared with blood-dryness (27.4%) and blood-stasis syndrome (18.1%) [126]. Hence, the number of blood heat type psoriasis patients might be large enough for studies rather than other types. Moreover, blood heat type patients also exhibited typical features of psoriasis with elevated levels of Th1/Th17-related cytokines, IFN-γ, IL-17, IL-23, and TNF-α [127]. Duration of treatments ranged from four weeks to eight weeks for topical application and from six weeks to eight weeks for oral administration. Both oral and topical treatment showed therapeutic effects on psoriasis in comparison with placebo or positive control drugs with no significant adverse events.
Herbal products were also used in combination with other therapies in the treatment of psoriasis. Oral administration of Curcuma longa extract combined with ultraviolet A (UVA) therapy showed higher effects on skin severity compared with psoralen plus UVA [128]. Treatment with total glucosides of paeony, a bioactive component derived from dry paeony root in combination with acitretin significantly improved PASI50 (50% reduction of PASI scores) in patients with moderate-to-severe plaque psoriasis, in comparison with placebo plus acitretin [129]. Oral treatment of a Korean herbal formula Yangdokbagho-tang (a mixture of six ingredients: Gypsum fibrosum, Rehmanniae Radix Crudus, Anemarrhenae Rhizoma, Schizonepetae Spica, Saposhnikoviae Radix, Arctii Semen) combined with acupuncture, probiotics, and phototherapy reduced PASI scores in two cases of moderate and severe psoriasis [130].
Several clinical trials also demonstrated that there were no significant differences between the effects of natural products and placebo or drug treatment on psoriasis symptoms. Application of ointment with silver fir (Abies alba) bark showed no significant effects compared with placebo [131]. The anti-psoriatic effects of Tripterygium wilfordii extract were not significantly different in comparison with acitretin [132]. Oral treatment with a TCM formula consisting of 16 herbs (Herba ephedrae, Radix aconiti lateralis preparate, Semen sinapis, Cortex cinnamomi, Rhizoma zingiberis, Cornu cervi degelatinatum, Radix rehmanniae preparate, Rhizoma Smilacis Glabrae, Cortex Dictamni, Rhizoma Imperatae, Radix salviae miltiorrhizae, Caulis spatholobi, Arnebiae Radix, Flos Sophorae, Radix glycyrrhizae, Indigo naturalis) for six months showed less effective outcomes compared with both placebo and methotrexate [133].

Discussion
Natural products have been used to treat psoriasis for centuries with significant effectiveness and few adverse events. In the aspect of TCM, psoriasis includes three phenotypes: blood heat in the active stage, blood dryness in the regression stage, and blood stasis in the resting stage. Blood heat phenotype is characterized by the continuous appearance of spot-like skin rash and skin itching. Blood dryness features include coin-like skin rash with light red color. The symptoms of blood stasis are thickened dark red skin lesions. Among these three phenotypes, blood heat is the most common with over 50% of patients suffering from this syndrome [126]. Several herbal formulas listed in Table 4 exerted efficacy on the treatment of blood heat type of psoriasis. Among the clinical trials listed in Table 4, studies investigating the clinical efficacy of Liangxue Jiedu and Pulian might be considered most valuable due to the large numbers of participants in these studies. However, since the scientific evidence for traditional herbal medicines or natural products is still limited, large-scale clinical trials (1,000 participants or more) to examine their efficacies have not been conducted. This review organized and summarized the underlying mechanisms for anti-psoriatic effects of natural products, which support the scientific base for future clinical trials.
Medicinal herbs and traditional herbal formulas consist of various active compounds with multiple targets and multiple related signaling pathways. This characteristic might lead to the higher effects of natural products but might be an obstacle to investigate their mechanisms of action for psoriasis treatment. Moreover, the chemical composition of a plant might vary in the number of compounds, as well as the amount of each compound under different growth environments, leading to the difficulty in the repetition of experiments. Therefore, clarification of the major component in the plants is important in the study of herbal medicines. In this review, we summarized the anti-psoriatic effects of natural products, including natural compounds, herb extracts, and herbal prescriptions. These natural products target numerous psoriasis-related signaling pathways, such as Th17 differentiation, JAK/STAT, NF-κB, MAPK, PI3K/Akt/mTOR, and other signaling pathways to alleviate inflammatory response and reducing keratinocyte hyperproliferation, thus improving psoriasis symptoms. Based on the current review, inhibiting Th17 differentiation pathway as well as related targets, such as IL-17, IL-22, IL-23, and TNF-α was the most common mechanism of action of natural products against psoriasis. These targets might be considered the most credible targets and might be applied in psoriasis studies using natural products.
Most animal studies utilized IMQ-induced mice as a model of psoriasis. Application of IMQ (a ligand of TLR7/8) to mouse skin can induce inflammatory skin lesions, resembling psoriasis symptoms in humans with activation of IL-23/IL-17 axis [134]. After the first report in 2009, IMQ-induced psoriasis-like skin inflammation in mice was widely used to investigate new underlying mechanisms in the pathogenesis of psoriasis, as well as to examine the therapeutic effects of potential agents. However, this model has certain limitations. There are several critical differences between mouse and human skin, including permeability, thickness, cutaneous immunity, and renewal process of epidermis and hair follicles, leading to the differences in drug absorption and immune response in the mouse model, compared with the human [135]. Therefore, the use of other models, which more resemble human skin, such as human three-dimensional skin equivalents is necessary and appropriate to investigate the anti-psoriatic effect of natural products.
In conclusion, natural products show promising application in the treatment of psoriasis. The underlying mechanisms of action for the anti-psoriatic effect of natural compounds and herbal products are complex with the involvement of multiple signaling pathways ( Figure 2). Further studies to evaluate the therapeutic effects of natural products in more relevant psoriasis models and larger-scale clinical trials should be conducted in the future. The author contributed solely to the work.

Conflicts of interest
The author declares that there are no conflicts of interest.

Ethical approval
Not applicable.