Futile caspase-8 activation during the apoptotic cell death induced by DNA damaging agents in human B-lymphoblasts.

Caspase-8 plays an essential role in apoptosis induced by Fas activation. Moreover, caspase-8 can be processed also in response to exposure to genotoxic agents. To decipher the role of caspase-8 in DNA damaging agent (DDA)-induced apoptosis as well as the pathway(s) leading to its activation in response to genotoxic stress, we investigated caspase-8 processing induced by ionizing radiation (IR) or mitomycin C (MMC) treatment in human B-lymphoblasts. Altogether, our observations establish that caspase-8 is actively processed in both receptor-mediated and DDA-induced cell death. However, while Fas-dependent apoptosis absolutely required caspase-8 activity, it is not necessary for completion of the apoptotic program induced by IR and MMC. Experiments performed to understand the molecular pathway(s) of the caspase-8 activation after DDA demonstrated that for both IR and MMC, the Fas/Fas-L interaction is dispensable. Data obtained from caspase inhibitors and from lymphoblasts carrying mutations in ATM and FANCC proteins, involved in DDA response, clearly showed that distinct mechanisms are responsible for caspase-8 activation by IR and MMC in B-lymphoblasts. IR-dependent processing of caspase-8 involves ATM, mitochondrial collapse, FANCC, and caspase-3 activation. Caspase-8 activation by MMC evokes the mitochondrial pathways involving FANCC but not ATM. Collectively, our data indicate that caspase-8 activation is essentially a bystander effect and not a major determinant of the behavior of DDA-exposed cells.