Inflammation-dependent secretion and splicing of IL-32γ in rheumatoid arthritis

Abstract

Different splice variants of the proinflammatory cytokine IL-32 are found in various tissues; their putative differences in biological function remain unknown. In the present study, we report that IL-32γ is the most active isoform of the cytokine. Splicing to one less active IL-32β appears to be a salvage mechanism to reduce inflammation. Adenoviral overexpression of IL-32γ (AdIL-32γ) resulted in exclusion of the IL-32γ-specific exon in vitro as well as in vivo, primarily leading to expression of IL-32β mRNA and protein. Splicing of the IL-32γ-specific exon was prevented by single-nucleotide mutation, which blocked recognition of the splice site by the spliceosome. Overexpression of splice-resistant IL-32γ in THP1 cells or rheumatoid arthritis (RA) synovial fibroblasts resulted in a greater induction of proinflammatory cytokines such as IL-1β, compared with IL-32β. Intraarticular introduction of IL-32γ in mice resulted in joint inflammation and induction of several mediators associated with joint destruction. In RA synovial fibroblasts, overexpression of primarily IL-32β showed minimal secretion and reduced cytokine production. In contrast, overexpression of splice-resistant IL-32γ in RA synovial fibroblasts exhibited marked secretion of IL-32γ. In RA, we observed increased IL-32γ expression compared with osteoarthritis synovial tissue. Furthermore, expression of TNFα and IL-6 correlated significantly with IL-32γ expression in RA, whereas this was not observed for IL-32β. These data reveal that naturally occurring IL-32γ can be spliced into IL-32β, which is a less potent proinflammatory mediator. Splicing of IL-32γ into IL-32β is a safety switch in controlling the effects of IL-32γ and thereby reduces chronic inflammation.

Role of Interleukin-32 in Helicobacter pylori-Induced Gastric Inflammation

“…IL-32␤ is reportedly a major isoform in endothelial cells, and it has a critical function in vascular inflammation and sepsis (25). The longest IL-32 isoform, IL-32␥, is the most abundant in rheumatoid arthritis synovial tissue (14). IL-32␥ exerts the greatest induction of TNF-␣ in human peripheral blood mononuclear cells (4).…”

“…IL-32 does not share sequence homology with other cytokines, and no homolog has been found in rodents. Previous reports showed that IL-32 expression is increased in various inflammatory diseases, and it is involved in the pathogenesis of rheumatoid arthritis and Crohn's disease (14,44,45). IL-32 expression is induced by hepatitis B virus, hepatitis C virus, and Mycobacterium tuberculosis (3,32,35,41).…”

Cytokines in the pathogenesis of rheumatoid arthritis: new players and therapeutic targets

“…IL-32β, δ, and γ mRNA overexpression in RA FLS is primarily induced by TNF-α, IFN-γ and toll-like receptor (TLR)-2, −3, and −4 ligands, and the overexpression of IL-32 seems to stabilize the mRNA transcripts of other cytokines, in particular TNF-α, IL-1β and IL-8 [110,126]. In FLS, TNF-α-activates Syk/PKC-d/JNK/c-Jun pathway to induce IL-32 (isoforms α, β, δ, and γ) [127], suggesting a splicing of IL-32γ into IL-32β [110]; interestingly, IL-32β is associated with lower inflammation and less severity of RA when compared with IL-32γ. IL-32 stimulates the synthesis of prostaglandin E2, an important mediator of cartilage and bone destruction in RA [128].…”

“…IL-32 was originally described as an mRNA called NK cell transcript 4 (NK4), which encoded a protein with many characteristics of a cytokine, derived from IL-2 activated natural killer cells [108]. NK cells, monocytes/macrophages, T lymphocytes, as well as epithelial cells, endothelial cells, fibroblasts, and hepatocytes, express IL-32 [109], mainly intracellularly, although some reports suggest that the IL-32γ isoform, could be secreted in limited amounts [110]. However, depending on the cell type and stimulus, IL-32 may be released after necrotic cell death or in vesicles such as exosomes [111,112].…”

IL32 expression in peripheral blood CD3+ cells from myelodysplastic syndromes patients

“…Apart from the proinflammatory role of IL-32, the association of IL-32α with PKCε and STAT3 [14] or with FAK1 has been reported [15], suggesting a possible role of this cytokine as an intracellular signaling mediator. Furthermore, the role of IL-32 in the production of regulatory cytokines has been described: IL-32β interacts with PKCδ and C/EBPα, which results in the production of IL-10 [16], and IL-32γ is correlated with enhanced production of proinflammatory cytokines, such as IL-1β and IL-6 [17]. IL-32γ is also implicated in HIV immunosuppression [18] and tumoral growth inhibition [19].…”