Pro-apoptotic Bid induces membrane perturbation by inserting selected lysolipids into the bilayer

Abstract

Bid is a BH3-only member of the Bcl-2 family that regulates cell death at the level of mitochondrial membranes. Bid appears to link the mitochondrial pathway with the death receptor-mediated pathway of cell death. It is generally assumed that the f.l. (full-length) protein becomes activated after proteolytic cleavage, especially by apical caspases like caspase 8. The cleaved protein then relocates to mitochondria and promotes membrane permeabilization, presumably by interaction with mitochondrial lipids and other Bcl-2 proteins that facilitate the release of apoptogenic proteins like cytochrome c. Although the major action may reside in the C-terminus part, tBid (cleaved Bid), un-cleaved Bid also has pro-apoptotic potential when ectopically expressed in cells or in vitro. This pro-apoptotic action of f.l. Bid has remained unexplained, especially at the biochemical level. In the present study, we show that f.l. (full-length) Bid can insert specific lysolipids into the membrane surface, thereby priming mitochondria for the release of apoptogenic factors. This is most effective for lysophosphatidylcholine species that we report to accumulate in mitochondria during apoptosis induction. A Bid mutant that is not pro-apoptotic in vivo is defective in lysophosphatidylcholine-mediated membrane perturbation in vitro. Our results thus provide a biochemical explanation for the pro-apoptotic action of f.l. Bid.

Lysophosphatidylcholine as a death effector in the lipoapoptosis of hepatocytes

“…Bid in the mitochondrial fraction was increased by PA treatment, which was inhibited by BEL (Fig. 4C), suggesting a possible role for full-length Bid in cell death by LPC, as reported (35). Probably because of the decrease in cardiolipin content and the increase in Bid in the mitochondria, cytochrome c was translocated from mitochondria to cytoplasm and mitochondrial potential was decreased between 3 and 24 h after PA treatment (Fig.…”

“…4B). We also studied whether LPC could directly induce changes in Bid apart from its receptor-mediated effect, because previous papers reported that Bid could perturb mitochondrial membrane in association with LPC (35). Bid in the mitochondrial fraction was increased by PA treatment, which was inhibited by BEL (Fig.…”

Methods for the assessment of mitochondrial membrane permeabilization in apoptosis

“…tBid acts at mitochondria through the cardiolipin-facilitated interaction with other pro-apoptotic members of the Bcl-2 family (e.g. Bax, Bak) [28], by destabilizing mitochondrial membranes through the insertion of selected lysolipids [29] or by promoting the propagation of Ca 2+ signals to the mitochondria [30].…”

“…Bak) to promote the formation of large homoor hetero-multimeric channels, through which IMS proteins are released [40][41][42][43]. (2) Pro-apoptotic factors may act directly on the lipidic component of the OM, or at the lipidprotein interface, thus favoring the formation of pores though which IMS proteins may exit to the cytosol [29,[44][45][46]. (3) Activated pro-apoptotic members of the Bcl-2 family may interact with components of the permeability transition pore complex (PTPC), thus favoring (or de-inhibiting) the permeability transition (PT) of the IM, that in turn leads to the physical rupture of the OM [47][48][49].…”

Mitochondria and cancer: is there a morphological connection?

“…Modifications of lipid composition and bilayer curvature of the mitochondrial membranes seem to be crucial for permeabilization. Recent work has highlighted the importance of tBid, which is able to insert into specific lysolipids of the OMM (Esposti et al, 2001;Goonesinghe et al, 2005) and to promote a negative membrane curvature of synthetic lipidic membranes (Epand et al, 2002a). These changes presumably prepare the ground for insertion and oligomerization of Bax and Bak.…”