Continuous autotropic signaling by membrane-expressed tumor necrosis factor

Tumor necrosis factor (TNF) exists in two bioactive forms, the membrane-int egrated form and the proteolytically derived soluble cytokine. Cells that p roduce TNF are often responsive to TNF, allowing autocrine/juxtacrine feedb ack loops. However, whether the membrane form of TNF is involved in such re gulatory circuits is unclear. Here we demonstrate that HeLa cells, expressi ng a permanently membrane-integrated mutant form of TNF, constitutively exp ress TNF.TNF receptor complexes at their cell surface. These cells show a p ermanent activation of the transcription factor NF-kappa B, exert constitut ive p38 mitogen-activated protein kinase activity, and produce high amounts of interleukin-6. In parallel, transmembrane TNF-expressing HeLa cells dis play high sensitivity to cycloheximide or interferon-gamma, similar to untr ansfected cells treated with these agents in combination with sTNF, Moreove r, cycloheximide-induced apoptosis in transmembrane TNF transfectants can b e blocked by the caspase inhibitor zVAD-fmk and does not necessarily need c ell to cell contact, indicating a critical role of constitutive autotropic signaling of TNF . TNF receptor complexes. These data demonstrate that auto tropic signaling loops of membrane TNF can exist, which may be of importanc e for cells that express both TNF and TNF receptors, such as T lymphocytes, macrophages, and endothelial cells.