Lihua He

Mitochondrial permeability transition: a common pathway to necrosis and apoptosis

Opening of high conductance permeability transition pores in mitochondria initiates onset of the mitochondrial permeability transition (MPT). The MPT is a causative event, leading to necrosis and apoptosis in hepatocytes after oxidative stress, Ca(2+) toxicity, and ischemia/reperfusion. CsA blocks opening of permeability transition pores and protects cell death after these stresses. In contrast to necrotic cell death which is a consequence of ATP depletion, ATP is required for the development of apoptosis. Reperfusion and the return of normal pH after ischemia initiate the MPT, but the balance between ATP depletion after the MPT and ATP generation by glycolysis determines whether the fate of cells will be apoptotic or necrotic death. Thus, the MPT is a common pathway leading to both necrotic and apoptotic cell death after ischemia/reperfusion.

Regulated and unregulated mitochondrial permeability transition pores: a new paradigm of pore structure and function?

Cyclosporin A (CsA) inhibits the mitochondrial permeability transition (MPT), but not always. To characterize the CsA‐sensitive and ‐insensitive MPT, rat liver mitochondria were exposed to low and high doses of various MPT inducers. Mitochondrial swelling, cyclophilin D membrane binding and permeability transition (PT) pore diameter were measured. The results indicate two conductance modes for the PT pore: one activated by Ca2+ and inhibited by CsA and Mg2+ and the other unregulated. We propose a new model of pore formation and gating in which PT pores form by aggregation of misfolded integral membrane proteins damaged by oxidant and other stresses. Chaperone‐like proteins initially block conductance through these misfolded protein clusters; however, increased Ca2+ opens these regulated PT pores, an effect blocked by CsA. When protein clusters exceed chaperones available to block conductance, unregulated pore opening occurs.

Role of Mitochondrial Inner Membrane Permeabilization in Necrotic Cell Death, Apoptosis, and Autophagy

Inhibition of mitochondrial oxidative phosphorylation progresses to uncoupling when opening of cyclosporin A-sensitive permeability transition pores increases permeability of the mitochondrial inner membrane to small solutes. Involvement of the mitochondrial permeability transition (MPT) in necrotic and apoptotic cell death is implicated by demonstrations of protection by cyclosporin A against oxidative stress, ischemia/reperfusion, tumor necrosis factor-alpha exposure, Fas ligation, calcium overload, and a variety of toxic chemicals. Confocal microscopy directly visualizes the MPT in single mitochondria within living cells from the translocation of impermeant fluorophores, such as calcein, across the inner membrane. Simultaneously, mitochondria release potential-indicating fluorophores. Subsequently, mitochondria swell, causing outer membrane rupture and release of cytochrome c and other proapoptotic proteins from the intermembrane space. In situ a sequence of decreased NAD(P)H, increased free calcium, and increased reactive oxygen species formation within mitochondria promotes the MPT and subsequent cell death. Necrotic and apoptotic cell death after the MPT depends, in part, on ATP levels. If ATP levels fall profoundly, glycine-sensitive plasma membrane permeabilization and rupture ensue. If ATP levels are partially maintained, apoptosis follows the MPT. The MPT also signals mitochondrial autophagy, a process that may be important in removing damaged mitochondria. Cellular features of necrosis, apoptosis, and autophagy frequently occur together after death signals and toxic stresses. A new term, necrapoptosis, describes such death processes that begin with a common stress or death signal, progress by shared pathways, but culminate in either cell lysis (necrosis) or programmed cellular resorption (apoptosis), depending on modifying factors such as ATP.

Liver regeneration is suppressed in small-for-size liver grafts after transplantation: involvement of c-Jun N-terminal kinase, cyclin D1, and defective energy supply.

Background. Small-for-size liver grafts have decreased survival compared to full-size grafts. This study investigated mechanisms of suppression of liver regeneration in small-for-size grafts. Methods. Rat liver explants were reduced in size to 50% and implanted into recipients of different body weights, resulting in graft weight/standard liver weights of ∼50% (half-size) and ∼25% (quarter-size). Results. Hepatic cellular 5-bromo-2′-deoxyuridine (BrdU) incorporation increased from 0.2% after sham operation to 2%, 18%, and 1.2% in full-size, half-size, and quarter-size grafts, respectively. Graft weight did not increase in full- and quarter-size grafts but increased 40% in half-size grafts. By contrast, apoptosis remained low (≤0.7%) and stem cells did not increase in all conditions. Phospho-c-Jun increased 27-fold in half-size grafts but only sevenfold in quarter-size grafts. Activating protein-1 activation increased 14-fold in half-size grafts but only fivefold in quarter-size grafts. Cyclin D1 (CyD1), which was barely detectable in full- and quarter-size grafts, increased 8.3-fold in half-size grafts. Adenosine 5′-triphosphate (ATP) per gram tissue decreased 70% in quarter-size grafts. Treatment of quarter-size grafts with radical scavenging C. sinenesis polyphenols (20 μg/ml) increased BrdU labeling and weight gain to 35% and 56%, respectively, reversed inhibition of CyD1 expression, c-Jun phosphorylation, and AP-1 activation in quarter-size grafts compared to half-size grafts, and restored ATP levels to 75%. Conclusions. Liver regeneration is stimulated in half-size grafts but suppressed in quarter-size grafts. Defective liver regeneration in small grafts is associated with an inhibition of the c-Jun N-terminal kinase/c-Jun and CyD1 pathways and compromised energy production.