To investigate the effects of cytokines on adipocyte lipolysis, a macrophage cell line (RAW 264.7) was treated with Escherichia coli lipopolysaccharide (1 microgram/ml) for 18 h to induce cytokine release. Conditioned medium (5%, vol/vol) from these cells was added to rat epididymal adipocytes isolated and incubated under sterile conditions. After incubation, the adipocytes were washed, and the rate of lipolysis (glycerol release) was determined after a further 1-h incubation. The conditioned medium caused an approximately 2.7-fold increase in lipolysis, detectable after 6-12 h, maximal by 24 h, and reversible by 48 h after washing the cells. The effect of conditioned medium was reversed by a neutralizing antibody to mouse tumor necrosis factor-alpha (TNF alpha), and the direct addition of recombinant human TNF alpha (0.1-50 ng/ml) reproduced the effect, with a half-maximally effective concentration of approximately 3 ng/ml. The effect of TNF on the expression of hormone-sensitive lipase (HSL; the rate-limiting enzyme for lipolysis) was investigated by Western immunoblots using an antibody raised to a bacterially expressed 96-amino acid portion of the HSL enzyme. TNF treatment did not alter the concentration of immunoreactive HSL. From these data we conclude that 1) macrophages release a cytokine(s) in response to lipopolysaccharide that stimulates lipolysis in freshly isolated adipocytes; 2) TNF alpha can account for most, or perhaps all, of this effect; 3) TNF alpha increases the rate of lipolysis by a mechanism that does not involve increased expression of HSL. Based on the time-dependent aspects of TNF alpha stimulation and the lack of change in immunoreactive HSL, the findings suggest a TNF-induced posttranslational modification of the enzyme.