Elevated circulating fatty acid concentration is a hallmark of insulin resistance and is at least in part attributed to the action of adipose tissue-derived tumor necrosis factor-alpha (TNF-alpha) on lipolysis. Cell death-inducing DFFA (DNA fragmentation factor-alpha)-like effector A (CIDEA) belongs to a family of proapoptotic proteins that has five known members in humans and mice. The action of CIDEA is unknown, but CIDEA-null mice are resistant to obesity and diabetes. We investigated CIDEA in adipose tissue of obese and lean humans and mice. The mRNA was expressed in white human fat cells and in brown mouse adipocytes. The adipose mRNA expression of CIDEA in mice was not influenced by obesity. However, CIDEA expression was decreased twofold in obese humans and normalized after weight reduction. Low adipose CIDEA expression was associated with several features of the metabolic syndrome. Human adipocyte depletion of CIDEA by RNA interference stimulated lipolysis and increased TNF-alpha secretion by a posttranscriptional effect. Conversely, TNF-alpha treatment decreased adipocyte CIDEA expression via the mitogen-activated protein kinase c-Jun NH(2)-terminal kinase. We propose an important and human-specific role for CIDEA in lipolysis regulation and metabolic complications of obesity, which is at least in part mediated by cross-talk between CIDEA and TNF-alpha.