The Drosophila retinoblastoma protein, Rbf1, induces a Debcl- and Drp1-dependent mitochondrial apoptosis.

In accordance with its tumor suppressor role, the retinoblastoma protein pRb can ensure pro-apoptotic functions. Rbf1, the Drosophila homolog of Rb, also displays a pro-apoptotic activity in proliferative cells. We have previously shown that the Rbf1 pro-apoptotic activity depends on its ability to decrease the level of anti-apoptotic proteins such as the Bcl-2 family protein Buffy. Buffy often acts in an opposite manner to Debcl, the other Drosophila Bcl-2-family protein. Both proteins can localize at the mitochondrion, but the way they control apoptosis still remains unclear. Here, we demonstrate that Debcl and the pro-fission gene Drp1 are necessary downstream of Buffy to trigger a mitochondrial fragmentation during Rbf1-induced apoptosis. Interestingly, Rbf1-induced apoptosis leads to a Debcl- and Drp1-dependent reactive oxygen species production, which in turn activates the Jun Kinase pathway to trigger cell death. Moreover, we show that Debcl and Drp1 can interact and that Buffy inhibits this interaction. Notably, Debcl modulates Drp1 mitochondrial localization during apoptosis. These results provide a mechanism by which Drosophila Bcl-2 family proteins can control apoptosis, and shed light on a link between Rbf1 and mitochondrial dynamics in vivo.

Secreted autotransporter toxin (Sat) triggers autophagy in epithelial cells that relies on cell detachment.

The secreted autotransporter toxin, Sat, which belongs to the subfamily of serine protease autotransporters of Enterobacteriaceae, acts as a virulence factor in extraintestinal and intestinal pathogenic strains of Escherichia coli. We observed that HeLa cells exposed to the cell-free culture supernatant of recombinant strain AAEC185p(Sat-IH11128) producing the Sat toxin (CFCS(Sat) ), displayed dramatic disorganization of the F-actin cytoskeleton before loosening cell-to-cell junctions and detachment. Examination of the effect of Sat on GFP-microtubule-associated protein light chain 3 (LC3) HeLa cells revealed that CFCS(Sat) -induced autophagy follows CFCS(Sat) -induced F-actin cytoskeleton rearrangement. The induced autophagy shows an acceleration of the autophagy flux soon after Sat treatment, followed later by a blockade of the flux leading to the accumulation of large GFP-LC3-positive vacuoles in the cell cytoplasm. CFCS(Sat) did not induce cell detachment in autophagy-deficient mouse embryonic fibroblasts in contrast with wild-type mouse embryonic fibroblasts. The CFCS(Sat) -induced large GFP-LC3 dots do not display the characteristics of autophagolysosomes including expression of cathepsin D and Lamp-1 and 2 proteins, and Lysotracker Red- and DQ-BSA-positive labelling. We provide evidences that CFCS(Sat) -induced autophagy is not a cell response intended to get rid of the intracellular toxin. By a pharmacological blockers approach, we found that the blockade of Erk1/2 and p38 MAPKs, but not JNK, inhibited the CFCS(Sat) -induced autophagy and cell detachment whereas phosphatidylinositol-3 kinase blockers inhibiting canonical autophagy were inactive. When attached CFCS(Sat) -treated cells start to detach they showed caspase-independent cell death and rearrangements of the focal adhesion-associated vinculin and paxillin. Collectively, our results support that Sat triggers autophagy in epithelial cells that relies on its cell-detachment effect.