Induction of autophagy, a promising approach for treating liver injury

Abstract

Recent investigations have demonstrated a complex interrelationship between autophagy and cell death. A common mechanism of cell death in liver injury is tumor necrosis factor (TNF) cytotoxicity. To better delineate the in vivo function of autophagy in cell death, we examined the role of autophagy in TNF-induced hepatic injury. Atg7Δhep mice with a hepatocyte-specific knockout of the autophagy gene atg7 were generated and cotreated with D-galactosamine (GalN) and lipopolysaccharide (LPS). GalN/LPS-treated Atg7Δhep mice had increased serum alanine aminotransferase levels, histological injury, numbers of TUNEL (terminal deoxynucleotide transferase-mediated deoxyuridine triphosphate nick end-labeling)-positive cells and mortality as compared with littermate controls. Loss of hepatocyte autophagy similarly sensitized to GalN/TNF liver injury. GalN/LPS injury in knockout animals did not result from altered production of TNF or other cytokines. Atg7Δhep mice had accelerated activation of the mitochondrial death pathway and caspase-3 and -7 cleavage. Increased cell death did not occur from direct mitochondrial toxicity or a lack of mitophagy, but rather from increased activation of initiator caspase-8 causing Bid cleavage. GalN blocked LPS induction of hepatic autophagy, and increased autophagy from beclin 1 overexpression prevented GalN/LPS injury. Autophagy, therefore, mediates cellular resistance to TNF toxicity in vivo by blocking activation of caspase-8 and the mitochondrial death pathway, suggesting that autophagy is a therapeutic target in TNF-dependent tissue injury.

Figure 1. The intracellular death signalings regulate cross talk of apoptosis and autophagy in liver injury

TNF-α binds to its receptor to stimulate receptor trimerization and the assembly of the death-inducing signaling complex. In this complex, FADD recruits caspase-8 (Casp-8) to promote Casp-8 oligomerization and self-activation, which is inhibited by c-FLIP. p62 binds to polyubiquitinated Casp-8 and recruits Casp-8 to the autophagosomal membranes through its direct interaction with LC3-II to promote Casp-8 activation. Activated Casp-8 cleaves Bid to tBid to trigger the mitochondrial release of cytochrome c and SMAC resulting in Casp-9 and its downstream Casp-3/6/7 activation and apoptosis. Deleting Atg7 in the mouse liver leads to the overactivation of JNK in LPS/D-GalN-treated mice, which promotes c-FLIP degradation and promotes Casp-8 activation. In contrast, overexpression of an autophagy protein Beclin-1 inhibits LPS/D-GalN-induced Casp-8 activation and liver injury.