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-18R alpha and IL-18R beta) 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 TNF alpha 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 TNF alpha, 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. (c) 2005 Elsevier Inc. All rights reserved.