Apoptogenic Ganglioside GD3 Directly Induces the Mitochondrial Permeability Transition

Abstract

Early events in apoptotic cascades initiated by ceramides or by activation of the surface receptor CD95 (Fas/APO-1) include the formation of ganglioside GD3. GD3 appears to be both necessary and sufficient to propagate this lipid-mediated apoptotic pathway. Later events common to many apoptotic pathways include induction of the mitochondrial permeability transition (PT) and cytochrome c release, which in turn triggers downstream caspases and cell death. The links between GD3 formation and downstream stages of apoptosis are unknown. We report that ganglioside GD3 directly induces the PT in isolated rat liver mitochondria at 30-100 microM in the presence of exogenous substrate (succinate) and at approximately 3 microM in the absence of exogenous substrate. In contrast, other gangliosides tested (e.g. GM1) have only weak stimulatory effects in the presence of succinate and protect against PT induction in the absence of respiratory substrates. GD3-mediated induction of PT was antagonized by known PT inhibitors, namely cyclosporin A, ADP, trifluoperazine, and Mg(2+). GD3 induced PT even in the presence of submicromolar Ca(2+); GD3 is therefore the first biological PT inducer identified that does not require elevated Ca(2+). Exposure to GD3 also led to mitochondrial cytochrome c release. In contrast, C(2)-ceramide, which can initiate the lipid-mediated apoptotic cascade in susceptible cells, failed to either induce PT or release cytochrome c. These observations suggest that GD3 propagates apoptosis by inducing the PT and cytochrome c release. This model provides a mechanistic link between the earlier and later stages of CD95-induced/ceramide-mediated apoptosis.