Upregulated E3 ligase tripartite motif‐containing protein 21 in psoriatic epidermis ubiquitylates nuclear factor‐κB p65 subunit and promotes inflammation in keratinocytes*.

Summary: Background: Tripartite motif‐containing protein 21 (Trim21) is an E3 ubiquitin‐protein ligase that plays pivotal roles in various diseases. However, its role in mediating keratinocyte inflammation, which is a hallmark of psoriasis, has not been thoroughly elucidated. Objectives: To clarify whether Trim21 plays a pivotal role in regulating keratinocyte inflammation in psoriasis, while focusing on identifying key Trim21 substrates involved in mediating proinflammatory cytokine and chemokine production. Materials and methods: Cytokine and chemokine secretion was examined by quantitative real‐time polymerase chain reaction (qPCR) in Trim21‐knockdown human keratinocytes. Downstream pathways and substrates of Trim21 were evaluated using immunoblotting, immunoprecipitation and immunofluorescence. The influence of Trim21 ubiquitination on its substrates was tested by in vitro ubiquitination assay, immunoprecipitation and immunofluorescence. The effectiveness of targeting Trim21 for psoriasis treatment was assessed in vivo with haematoxylin and eosin staining, immunofluorescence and qPCR. Results: Knocking down Trim21 expression alleviated keratinocyte inflammation. Trim21 colocalized with p65/nuclear factor (NF)‐κB in the cytosol and physically bound and ubiquitinated p65 via a lysine 63 (K63) linkage. Instead of changing p65 protein stability, Trim21 enhanced the interaction of p65 with IκB kinase, which promoted p65 phosphorylation, nuclear transport and downstream gene activation. Finally, both in vitro and in vivo experiments verified that topical application of Trim21‐specific small interfering RNA markedly ameliorated imiquimod‐induced psoriasis‐like lesions. Conclusions: Our study confirms that upregulated Trim21 in psoriatic epidermis ubiquitylates p65 and activates the NF‐κB pathway, which promotes keratinocyte inflammation. Hence, Trim21 represents a potential target for psoriasis treatment. What is already known about this topic? Tripartite motif‐containing protein 21 (Trim21) is implicated in the inflammatory response.Trim21 expression is upregulated in psoriatic epidermis.Keratinocytes from psoriatic skin secrete proinflammatory cytokines such as interleukin (IL)‐1β, IL‐8, IL‐15, IL‐23 and interferon‐γ, which evoke and promote the inflammatory process. What does this study add? The p65 subunit of nuclear factor (NF)‐κB is a newly recognized substrate of Trim21.Upregulated Trim21 in psoriatic epidermis ubiquitinates p65 and facilitates the activation of the NF‐κB pathway, promoting inflammation in keratinocytes.Trim21 is a potential target for the treatment of psoriasis. What is the translational message? Ubiquitination is involved in the inflammatory process in psoriasis.Targeting the ubiquitination system for future psoriasis treatment should be explored. Linked Comment:O'Shaughnessy. Br J Dermatol 2021; 184:8–9. Plain language summary available online [ABSTRACT FROM AUTHOR]