Regulation of interleukin-17 signaling at the receptor level and implications in the pathogenesis of skin diseases
Description
Interleukin-17 (IL-17) is a pro-inflammatory cytokine, participating in innate and adaptive immune responses, that plays an important role in various physiological and pathological conditions, including host defense against pathogens (e.g., Klebsiella pneumoniae, Candida albicans, and Mycobacterium tuberculosis ), autoimmune diseases (e.g., rheumatoid arthritis, multiple sclerosis, systemic lupus erythematosus, psoriasis, and inflammatory bowel diseases), tissue regeneration, metabolic regulation, and tumor progression in some organs (e.g., prostate, colon, breast, liver, and skin). There are six IL-17 cytokines, including IL-17A, IL-17B, IL-17C, IL-17D, IL-17E (also known as IL-25), and IL-17F. Five canonical IL-17 receptors (IL-17Rs), named from IL-17RA to IL-17RE, are single transmembrane proteins with conserved structure features, containing fibronectin ІІ-like domains and a similar expression to fibroblast growth factor genes and IL-17R (SEFIR) domain. The mechanisms of how IL-17 signaling pathway participate in development of autoimmune diseases and host defense are well-documented. Recently, researchers have deepened their functional studies of individual IL-17 family members in allergic inflammation and tissue regeneration. So far, the most thoroughly studied IL-17 signaling is the one mediated by IL-17A and IL-17F through interacting with IL-17RA and IL-17RC. Upon binding of IL-17 to its receptor, a signaling cascade is activated, leading to activation of nuclear factor κB (NF-κB), mitogen-activated protein kinase (MAPK) and CCAAT-enhancer binding protein β (C/EBP-β), which subsequently induces expression of pro-inflammatory cytokines and chemokines as well as other immune-related genes. Post-translational modifications (PTMs) are crucial for protein function, stability, cellular localization, cellular transduction, and cell death. IL-17-signaling pathway is also precisely modulated by PTMs. However, the studies at IL-17 cytokines and IL-17 receptor level are restricted to various binding forms, dimerization conditions and interaction with downstream molecules. IL-17RA is unique among the family members due to its extra-long intracellular tail and it is a common subunit dimerizing with other receptors. Therefore, in the current dissertation study, we aim to investigate phosphorylation and ubiquitylation of IL-17RA. We used a lupus-prone mouse model to investigate the effects of cyclin-dependent kinase 4/6 (CDK4/6) inhibitor Palbociclib on inflammatory responses. From this study, we learned that CDK4/6 inhibitor Palbociclib reduced inflammatory responses in a gender- and organ-specific manner. We further used bioinformatics tools and biochemistry techniques to determine that F-box and WD repeats domain containing 11 (FBXW11) ubiquitylated IL-17RA through a lysine 27-linked polyubiquitin chain, targeting IL-17RA for proteasomal degradation. Domain 665-804 of IL-17RA is critical for the interaction with FBXW11. During E3 ligase screening, we unexpectedly found that FBXW5 was firmly bound to IL-17RA. We hypothesized that FBXW5 ubiquitylates IL-17RA and targets it for degradation. FBXW5 is reported to be phosphorylated by polo-like kinase 4 (PLK4), which leads to ubiquitylation and degradation. To determine the effects of FBXW5 on IL-17-mediated inflammatory responses, we used wild-type (WT), Il-17ra(T779A)-knockin (KI), and Il-17rc-knockout (KO) C57BL/6J mouse strains to establish a psoriasis model and a skin papilloma model for studying the effects of PLK4 selective inhibitor centrinone on skin lesion formation. We showed that Il-17rc-KO prevented skin lesion formation in psoriasis and skin papilloma models but centrinone had no effects in both models, while Il-17ra(T779A)-KI only significantly inhibited skin papilloma formation.