Schema 5.

Cross-roads of mitophagy and apoptosis.

Autophagy and apoptosis are two processes that may be mutually inhibitory; autophagy usually precedes apoptosis while triggering of apoptosis is associated with blocked autophagy. It is likely that these two pathways are intrinsically interconnected via molecular switches that turn on the autophagy process (with still inhibited apoptosis) followed by activation of apoptosis (with turned off autophagy). Both processes function as parts of an essential combined mechanism of elimination of irreparably damaged cells. Phospholipid signaling, particularly deregulation of characteristic for normal cells asymmetry of phospholipids, has been discovered as one of important factors in both autophagy and apoptosis. Collapse of cardiolipin asymmetry in mitochondria and covalent association of phosphatidylethanolamine (or phosphatidylserine) with LC3₁ are the two major events in signaling, culminating in apoptotic cell death and mitophagy, respectively. It is possible that changes of cardiolipin asymmetry in mitochondria are at the center of the chain of events leading to cell death. This chain includes several consecutive levels of regulation:

1. Synthesis of cardiolipin (CL) and its molecular speciation with a balance of poly- and mono-unsaturated molecular forms as well as saturated CLs).

2. Scramblase-3 (SCR-3) is inactive; maintenance of asymmetry of CL between the inner and outer mitochondrial membranes; CL and cyt c are spatially separated.

3. Regulation of cyt c/CL interactions via cyt c phosphorylation hindering binding of negatively charged CL to cyt c.

4. Low levels of H₂O₂ production leading to insufficiency of oxidizing equivalents.

5. Cyt c can undergo phosphorylation of its Tyr 97 (in the heart) [168] Tyr 48 (in the liver) [169] likely via a cAMP dependent pathway. Phosphorylation of Y97 is associated with changes of the absorbance at 695 nm which suggests subtle structural changes in the heme environment [168]. Peroxidase activity of cyt c/CL complex involves formation of Tyr radicals [20]. It is possible that phosphorylation of Y97 and Y48 affects binding of cyt c with CL as well as its peroxidase activity.

6. During initiation of autophagy, SCR phosphorylation (resulting in its activation) moves CL to the outer mitochondrial membrane and stimulates (turns on) mitophagy. There is no CL oxidation at this time, because cyt c may be phosphorylated and H₂O₂ is still unavailable. As damage develops, cyt c can be dephosphorylated and more avidly binds with CL. This disrupts electron transport and stimulates H₂O₂ production. As a result, CL gets oxidized, thus initiating apoptosis and the end of autophagy (mitophagy).