The pro-inflammatory cytokine IL-18 is expressed in human adipose tissue and strongly upregulated by TNFα in human adipocytes

White adipocytes have been examined as a potential source of interleukin-18 (IL-18), the circulating levels of which are increased in obesity. IL-18 gene expression was evident in human subcutaneous and visceral adipose tissue, and expression occurred in mature adipocytes and the stromal-vascular fraction. Expression of the IL-18 receptor complex (IL-18Rα and IL-18Rβ) and the IL-18 binding protein (IL-18BP) genes was also observed, mirroring that of IL-18. IL-18 mRNA level increased rapidly (within 2 h) and dramatically (>900-fold) in response to TNFα in human adipocytes differentiated in culture. IL-18 protein was detected in lysates of cultured adipocytes, though not in the medium. There was a small increase in IL-18 in lysates of adipocytes treated with TNFα, but the protein was again undetectable in the medium. IL-18 may be part of the inflammatory cascade within adipose tissue; however, human adipocytes do not appear to secrete significant amounts of IL-18.