Involvement of FADD and caspase-8 signalling in detachment-induced apoptosis

Detachment of most untransformed adherent cells from the extracellular matr ix promotes apoptosis, in a process termed anoikis [1,2]. The death signall ing mechanisms involved in this process are not known, although adhesion or transformation by ras oncogenes have been shown to protect epithelial cell s from apoptosis through activation of phosphatidylinositol 3-kinase and pr otein kinase B (PKB/Akt) [3]. Here we show that detachment-induced apoptosi s (anoikis) is blocked by the expression of a dominant-negative form of FAS -associated death domain protein (FADD) in a number of untransformed epithe lial cell lines. Because the soluble extracellular domains of the death rec eptors CD95, DR4 and DR5 failed to block anoikis, we conclude that ligand-d ependent activation of these death receptors is not involved in this proces s. Detachment induced strong activation of caspase 8 and caspase 3. Detachm ent-induced caspase-8 activation did not require the function of downstream caspases but was blocked by overexpression of the anti-apoptotic proteins Bcl-2 or Bcl-X-L. We propose that caspase-8 activation is the initiating ev ent in anoikis, which is subsequently subject to a positive-feedback loop i nvolving mitochondrial events.