In this study, we analysed proteomic differences in skin lesions between males and females with moderate-to-severe psoriasis using DIA-MS, then used the new 4D-PRM technique to simultaneously validate 26 screened proteins (In fact, we simultaneously validated up to 96 proteins selected from three different studies). Our research identified 416 proteins significantly changed in males compared to females. Consistently with the previous study, the KEGG enrichment of these proteins observed some crucial pathways closely related to psoriases, such as the IL − 17 signalling pathway, T cell receptor signalling pathway, Th17 cell differentiation, Oxidative phosphorylation, PI3K − Akt signalling pathway, MAPK signalling pathway, and numerous pathways associated with various infection11.
Our dataset found that CHUK (IKKA), IKBKB (IKKB), MALT1, SRC, TLR2, and CASP3 are associated with IL-17 or TNF, which suggests that these proteins may play a critical role in the maintenance or progression of psoriatic inflammation. Inhibitor of nuclear factor kappa-B kinase subunit alpha (CHUK) is a serine kinase that participates in the core IkB kinase (IKK) complex in the canonical Nuclear factor kappa B (NF-κB) pathway and noncanonical NF-κB pathway, which regulate B cell survival, or coding for chemokines and cytokines12. Furthermore, CHUK has some unique functions independent of NF-κB13–15 and can negatively regulate canonical NF-kB signals to limit inflammation16. In our study, CHUK is down-regulated and negatively correlated with PASI in the male group, suggesting it may play a negative regulatory role in psoriatic inflammation. Interestingly, CHUK was involved in estrogen receptor (ER)-mediated gene transcription17 and regulates cell cycle progression18. Therefore, it can act as a multifunctional signalling protein19.
Inhibitor of nuclear factor kappa-B kinase subunit beta (IKBKB) shares structural and biochemical similarities with CHUK and acts as a part of the IKK complex in NF-kappa-B to activate genes involved in immune response20. Interestingly, IKBKB and CHUK were all down-regulated in our study. The reason may be the regulation of other proteins, such as IKK-related kinases21,22, prolonged stimulation with TNFα23, or mutual regulation among them24,25. One study revealed that the canonical IKKs and IKK-related kinases regulate each other by a complicated network to regulate their activities in innate immunity critically26. Our PPI network also found that TLR2 and TNF are connected and indirectly affect cytokines such as IL-17 and IL-22. The specific relationship among them and their role in psoriasis requires further study.
Another well-known protein closely associated with the NF-kappa-B signalling pathway and TNF is MALT1. It is a protease that is involved in the activation of NF-kB as well as the MAP signalling27 by the formation of CARD14-BCL10-MALT1 (CBM) signalling complex, which is similar to that formed in antigen receptor-stimulated lymphocytes along with the recruitment of TRAF6, MALT1, and BCL1028,29. Furthermore, a distinct role for MALT1 was found in inflammatory signalling in keratinocytes. For example, stimuli such as Staphylococcus aureus induced MALT1 proteolytic activity in keratinocytes30. Inhibition of MALT1 in keratinocytes can reduce proinflammatory cytokines (TNF, IL-1b, and IL-17C), chemokines, and antimicrobial peptides30. Moreover, A20 and CYLD, associated with psoriasis and as substrates of MALT131–33, can inhibit CARD14-mediated signalling30. These findings imply that MALT1 may play a role in the pathogenesis of psoriasis and provide a basis that MALT1 inhibitors can be used as a treatment for psoriasis34.
Notably, the CBM complex activates the NF-kB signalling in two ways. In addition to acting as a scaffold to recruit and activate the IKK complex and the nuclear translocation of NF-kB, it can promote optimal NF-kB activation by MALT1-mediated cleavage of RelB and A2034. In our data, MALT1 is upregulated while CHUK and IKBKB are down-regulated. We speculate that the CBM complex can compete with the IKK complex to activate the NF-kB signalling pathway. These results explain the higher severity in men and the lower treatment response than in women6. Remarkably, one study found that estradiol acts as a protective factor against psoriatic inflammation in mice7, and our study also found 9 DEPs involved in estrogen signalling; however, this pathway was predicted to be activated in males but not in women. Whether hormonal hormones affect the onset and severity of psoriasis needs more research.
TLR2 is part of the TLR family expressed on various cells, such as DCs, macrophages, and lymphocytes35–37. Although TLR2 signalling can induce an innate immune response to microbial lipoproteins and other cell components38 through canonical or noncanonical NF-kappaB cascade39–41, one study contradicts that TLR2 signalling suppress psoriasis-like skin inflammation42. Consistent with Nakao M et al., our dataset showed that TLR2 was down-regulated in the male group which means it acts as an immune suppressant. The down-regulation of TLR2 is likely related to the downregulation of receptor-mediated endocytosis. Saturated fatty acids can activate TLR2. However, psoriatic patients are not advised to eat foods rich in saturated fatty acids because they can lead to the expression of proinflammatory genes. Omega-3 polyunsaturated fatty acids and eicosapentaenoic acids, which downregulate the TLR2 signal, are often used to treat psoriasis43. The reason may be that TLR2-induced cytokines are preferred over the induction of an anti-inflammatory cytokine in humans42. Furthermore, as our results shown, they may interact with other proteins to increase the inflammatory response.
Our study has several limitations. Firstly, there is a small number of cases. Secondly, DIA-mass did not detect some proteins, such as IL-17 and TNF, as discussed in the previous study. Thirdly, little differences in severity between men and women (Student's t-test, t = 2.21, P = 0.045). These confounding factors may affect the accuracy of the result.