Serine protease inhibitors (Serpins) is a new family proposed by Hunt et al. (1, 2) in 1980 on the basis of the conserved primary structure of ovalbumin, α1 antitrypsin, and human antithrombin. Many members of this family can inhibit the activity of serine proteases such as chymotrypsin, so they are named serine protease inhibitors (Serpins) (3, 4). Currently, approximately 1500 serpin sequences have been identified in animals, poxvirus, plants, bacteria and archaea (5, 6). The Serpins family contains a group of structurally similar but functionally distinct proteins, making it the largest and most functionally diverse family of protease inhibitors. Serpins are involved in inflammation, immunity, tumorigenesis, blood coagulation, blood pressure and cancer metastasis (7).

Psoriasis is a chronic, recurrent and inflammatory immune-mediated skin disease induced by genetic and environmental interaction (8). Psoriasis is characterized by excessive proliferation and poor differentiation of keratinocytes, immune cell infiltration, skin inflammation as well as impaired epidermal barrier (8–10). According to the clinical characteristics of psoriasis, psoriasis can be divided into: psoriasis vulgaris, psoriasis arthropathica, psoriasis erythrodermic and psoriasis pustulosa (11), of which vulgaris is the most common type, and other types are mostly converted from psoriasis vulgaris (12, 13). Pustular psoriasis can be further divided into localized (e.g. palmoplantar pustulosis) and generalized pustular psoriasis (GPP) (14). In the skin, the epidermis is an abundant source of proteases and protease inhibitors (15). Endogenous and exogenous proteases, such as caspases, cathepsins, kallikreins and proteases from microorganisms, play important roles in the desquamation and defense regulation of the stratum corneum. Protease inhibitors contribute to skin integrity and protective barrier function by regulating their proteolytic activity (16). An imbalance of proteases/protease inhibitors in the skin can lead to changes in protease hydrolysis of target proteins such as filaginins, cytokines, and receptors, inducing skin inflammatory responses and barrier dysfunction (16). Serpins are the most diverse superfamily of protease inhibitors (1). Serpins are abundant in skin and play an important role in maintaining skin homeostasis. In recent years, a growing body of literature have shown that some serpins are differentially expressed genes in psoriasis lesions (17, 18), gene polymorphisms and mutations of some serpins are also involved in psoriasis pathogenesis (19, 20). These genetic changes in serpins lead to changes in their target protease activity and proteolysis of target proteins, which contribute to psoriatic inflammation and skin barrier dysfunction. Here we briefly introduce the structure, functional characteristics, expression and distribution of serpins in skin and focus on the regulation of serpins in the epidermal barrier and the pathogenic role of serpins in psoriasis.

Serpin is a single chain protein, consisting of 350 ~ 400 amino acids. The serpins have a core structure of about 380 amino acids, which folds into a very conservative, typical three-dimensional structure in a metastable state. This structure consists of 8 ~ 9 α helices (hA ~ hI) and 3 β sheets (A-β, B-β, C-β) (21) ( Figure 1 ). The region of interaction with the target enzyme is a bare ring motif, also known as the reaction center ring (RCL) ( Figure 1 ), which is spread on the serpin scaffold and contains the protease recognition site P1 (5). The Serpin protein superfamily is divided into groups called clades based on their sequence similarity. Clades are classified into A-P, and human serpins are phylogenetically classified into clades A-I (21, 22). In the human genome, there are 36 protein-coding genes of serpins (21) and the two largest clades of the 36 identified serpins are extracellular “clade A” (12 members located on chromosomes 14, and X) and intracellular “clade B” (13 members located on chromosomes 6 and 18) (22–24). Clade C (serpinC1) is located on chromosome 1. Clade D (serpinD1) is located on chromosome 22. Clade E (3 members) are located on chromosome 2, 7 and 13. Clade F (2 members) are located on chromosome 17. Clade G (serpinG1) and Clade H (serpinH1) are both located on chromosome 11. Clade I (2 members) are located on chromosome 3.

Although Serpins have similar structural characteristics, their modes of action are very different. According to these characteristics, Serpin superfamily members are divided into inhibitory and non-inhibitory serpins. The inhibitory serpins can suppress the serine proteases, caspases and papain-like cysteine proteases by an irreversible suicidal mechanism (21, 25, 26) and we summarized the target enzymes inhibited by each human serpin member in Table 1 . However, the non-inhibitory serpins exhibit functions unrelated to inhibiting catalytic activity, such as hormone transport or blood pressure regulation (5, 6, 21, 107). For example, SerpinA8 is a blood pressure regulator (108). SerpinA6 and A7 are responsible for the transport of cortisol (109) and thyroid hormone (46), respectively. In humans, most (24 out of 36) serpins are inhibitory ( Table 1 ). The inhibitory serpins plays an important role both intracellular and extracellular, including coagulation regulation (110), inflammatory response, complement activation (102) and immune regulation. A number of serpins play vital roles in the inflammation and immunity. In Clade A, there are some inhibitory serpins containing anti-inflammatory molecules (SerpinA1, A3, A4, A12) (111–115) and non-inhibitory serpins containing inflammatory molecule(SerpinA6, 8) (20, 47, 109, 170). The inhibitory SerpinA9 is involved in maturation and maintenance of naïve B cells (50). In Clade B, there are some inhibitory serpins containing anti-inflammatory molecules (SerpinB1, B2, B6) (116, 117) and pro-inflammatory molecules (SerpinB3/B4, B10) (78, 118) ( Table 1 ). Act as potent inhibitors of proteases in neutrophils, SerpinA1 (21), A3 (33, 34), B1 (116) and B6 (80) are thought to protect monocytes, neutrophils, and onlookers from ectopic neutrophil-derived proteases during inflammation (82, 119). SerpinB1 is a protective immunomodulatory that protects the host’s antimicrobial defenses and prevents tissue damage (61, 120). SerpinB9 is a potent inhibitor of granzyme B, which is located in the same subcellular compartment of cytotoxic lymphocytes as granzyme B, and can protect cytotoxic lymphocytes from the destruction of granzyme B (89). SerpinB9 also inhibits IL-1β maturation by inhibiting Caspase-1. Mutations in the SerpinB9 gene may contribute to the development of autoinflammatory diseases (121). Some serpins are important hemostatic regulators, including SerpinA5 (42), SerpinA10 (51), SerpinC1 (98), SerpinD1(heparin cofactor II) (122), SerpinE1(PAI1) (122), SerpinE2 (100) and SerpinF2 (122). SerpinC1 not only has anticoagulant and anti-inflammatory effects (98), but also has the effect of inhibiting liver cancer (123) ( Table 1 ). Human Serpins also contain some tumor suppressor genes, including A5 (124), A11 (52), and B5 (125), which may inhibit tumor growth by inhibiting tumor metabolism.

Some serpins are commonly expressed in various human tissues and cells and some serpins are expressed specifically in certain tissues, which we have summarized in Table 1 . Serpins are abundant in expression and distribution in the skin. In the Clade A, SerpinA12 is mainly expressed in the skin and keratinocytes in the skin has been identified as a rich source of vaspin (serpinA12) (55). AGT is an important part of the renin-angiotensin system (RAS). Steckelings UM et al. found that the complete renin-angiotensin system exists in human skin (48). The majority of serpins containing(B1-8, B12, B13) in Clade B are expressed in the skin (35). SerpinB2 is expressed in keratinocytes of the skin and one study suggested that SerpinB2 (cross-linked to the cornified envelope) was present in the stratum corneum (71). In skin, SerpinB3 is expressed in the spinous and granular layers of normal epithelium (126). In normal skin, SerpinB13 expression is mainly confined to the basal layer, while in diseased skin, it is mainly distributed in the outermost layers of granular and upper spinous layers (127). In other branches, SerpinE3 and SerpinG1 are also expressed in the skin (35).

The exposed epidermis is a laminated squamous epithelium composed of multiple layers of keratinocytes. Epidermis is the physical barrier of our human skin, protecting us from moisture loss and mechanical damage. It is complemented by a chemical barrier composed of antimicrobial peptides and proteins to protect the host from the surrounding microbiota (16). Profilaggrin metabolism, the formation of cornified envelope, desmosomes, intercellular lipid lamellae and zonula occludens, desquamation play important roles in the integrity of the epidermal barrier (128–130). The differentiation of keratinocytes affects the formation of the cornified envelope that is an insoluble protein and lipid structure with barrier functional properties (131, 132). As mentioned earlier, many members of the serpin family are expressed in the skin, which play a an important role in regulating epidermal barrier and maintain skin homeostasis. Reduced expression of SerpinA12 leads to decreased expression of the desmosomal proteins, the cornified envelope proteins, and keratins (55) that results in reduced impaired skin barrier. As a predominantly expressed adipokine in the skin (55, 115), SerpinA12 may alter its expression or functional activity by inhibiting kallikrein 7 (KLK7), which KLK7 controls desquamation and is a key molecule in the maintenance of skin barrier function (53, 133–135). Reduced expression of SerpinA12 in keratinocyte has been reported to decreased expression of the desmosomal proteins, cornified envelope proteins, and keratins (55) which are proteins that maintain the integrity of the epidermal barrier. The decrease of SerpinA12 expression may weaken the inhibition of KLK7 activity. Abnormal KLK activity impairs epidermal barrier function (136). So we hypothesized that reduced SerpinA12 expression may lead to impaired epidermal barrier by inhibiting KLK7 activity.

SerpinB2 is considered to be one of the precursors of the envelope and is involved in the formation of the cornified envelope (64, 71). SerpinB2 deficiency was found to cause stratum corneum defect and more susceptible to topical application of inflammatory agents. The function of SerpinB2 in keratinocytes is to protect the stratum corneum from proteolysis by inhibiting urokinase, thereby maintaining integrity of the stratum corneum and its barrier effect, especially during skin inflammation (71). Epidermis with high SerpinB3 expression level increase sensitivity to barrier destruction by external stimuli, suggesting that SerpinB3 plays an important role in inducing epidermal barrier disruption (137). As a molecule elevated in both psoriasis and atopic dermatitis patients, SerpinB3/4 has been shown to cause epidermal barrier dysfunction in experimental mouse models of atopic dermatitis (78). As a common target of SerpinB3 and SerpinB13, cathepsin L is the elusive enzyme that processes and activates transglutaminase 3(TGM3) (138). The TGM is involved in formation of cornified envelope in the epidermis and plays an important role in epidermal barrier (138). Both SerpinB3 and SerpinB13 are highly expressed in psoriasis lesions (18, 139). Psoriasis showed impaired epidermal barrier function (140). So we hypothesize that increased SerpinB3/B13 expression suppress TGM3 activation by inhibiting cathepsin L activity, thereby affecting cross-linking of the stratum corneum and resulting in impaired skin barrier. SerpinB7 deficiency can cause epidermal barrier dysfunction in IMQ-induced psoriasis like models (85). SerpinB12 is a gene associated with epidermal permeability barrier, and its expression is decreased in atopic dermatitis model mice (95). Impaired epidermal function is a significant feature of atopic dermatitis (78). However, it is unclear whether SerpinB12 is a key molecule and specific mechanism that regulates impaired epidermal barrier function in atopic dermatitis, and further research is needed.

At present, the majority of serpins focus on the pathogenesis of psoriasis vulgaris. SerpinA12 and SerpinG1 is significantly elevated in the serum of patients with psoriatic arthritis (141, 142), but its specific mechanism of action in psoriatic arthritis has not been reported. Next, we summarized the pathogenic role of serpins in psoriasis.

GPP is a severe form of psoriasis, which is characterized by large, visually visible pustules on the non-extremity skin, with or without systemic symptoms such as fever, neutrophilism, and elevated serum C-reactive protein levels (12).

Kantaputra P et al. (72) found that the skin of GPP patients with SerpinB3 mutation showed high expression of SerpinB3. The mutations in both SerpinA1 and SerpinA3 are likely to be predisposing risk factors of generalized pustular psoriasis (GPP) (19). A heterozygous missense mutation of SerpinA1 occurs in patients with GPP (19). SepinA1 can inhibit the activity of elastase in neutrophils (28), and it has been found that SepinA1 mutation can cause overactivity of elastase (29). The overactivity of neutrophil elastase can promote the activation of IL-36α, and the activated IL-36α can enhance the ability of keratinocytes to produce chemokines (30). The inhibitory capacity of SerpinA1 is reduced in symptom-free and in those with stationary lesions patients with psoriasis (31). Barszcz D et al. (31) suggested that the abnormal function of SerpinA1 may be related to the pathogenesis of psoriasis. Frey S, et al. found a rare loss of function mutation in the entire reactive center loop of SerpinA3 in a small number of patients with GPP (36, 37). Liu Y, et al. (38) identified four novel mutations in SerpinA3 in seventy children with GPP and demonstrated that three of these mutations lead to loss of function of ACT (the protein encoded by SerpinA3), thereby reduce the inhibition of ACT on Cathepsin G. Cathepsin G (a protease produced by infiltrating neutrophils in the epidermis) can activate IL-36γ in the epidermis of patients with GPP, and activated IL-36γ induces an increased ability of keratinocytes to produce chemokines (CXCL1, CXCL2 and CXCL8) and enhance recruitment of neutrophils, thereby exacerbating skin inflammation (30, 39). In the skin of GPP patients, the expression and activity of IL-36α and IL-36γ is elevated in keratinocytes located around neutrophil microabscesses (30). Therefore, we speculate that similar to SerpinA3, SerpinA1 is involved in the pathogenesis of GPP through loss-of function mutations, but further studies are needed to confirm it ( Figure 2 ).

In summary, the changes in the expression of some serpins in the skin participate in the pathogenesis of psoriasis by regulating the inflammation, differentiation and interaction with immune cells of keratinocytes. When the expression of some serpins increases, their transcriptional regulators that inhibit serpin expression or corresponding antibodies can be developed to reduce the expression of serpins. When the expression of some serpins reduces, psoriasis can be treated with its recombinant protein. For example, in an animal model of psoriasis, the application of recombinant vaspin (SerpinA12) reduced infiltration by myeloid cells into the skin (55).

Mutations and polymorphisms of some serpins are also involved in the pathogenesis of psoriasis, and we can use their downstream product related antagonists to treat psoriasis. For example, losartan act as an angiotensin receptor (AT1R) antagonist, attenuates imiquimod-induced psoriasis-like inflammation (171).

Serpins play an complex role in regulation of the epidermal barrier and the development of psoriasis. Some serpins, including SerpinA12, SerpinB2/3//7 play multiple roles in skin barrier function and pathogenesis of psoriasis. The decrease in the expression of SerpinA12, SerpinB7 deficiency and increase in expression of SerpinB3/4 in the skin can promote inflammation, poor differentiation of keratinocyte and damaged skin barrier. Pso p27, derived from SerpinB3/B4, is an autoantigen that can enhanced immune response in psoriasis. However, whether SerpinB3 is participating in the pathogenesis of psoriasis by itself or by hydrolyzing into Pso p27 has not been investigated. The expression of SerpinB7 in psoriasis is controversial, which requires further studies to confirm. SerpinB2 plays a role in maintaining epidermal barrier integrity and inhibiting keratinocyte proliferation. Some anti-inflammatory serpins(SerpinB1, B2) are highly expressed in psoriasis, which may be the body’s response to the development of the disease. Some serpins(SerpinA1, A3, B3) cause GPP through a genetic mutation that triggers skin inflammation. AGT(SerpinA8) and SerpinB8 are susceptibility gene for psoriasis, but the specific mechanism has not been studied. Based on the relevant literatures, we summarized the possible mechanism of AGT and SerpinB8 to provide reference for future research. The underlying mechanisms of serpins have not been fully elucidated and needs to be further explored. The study of serpins in the pathogenesis of psoriasis may provide a novel therapeutic target for the treatment of psoriasis.