Mechanisms of lipotoxicity in NAFLD and clinical implications

Abstract

With the epidemic of childhood obesity, nonalcoholic fatty liver disease (NAFLD) has become the most common cause of chronic liver disease in pediatrics. NAFLD is strongly associated with insulin resistance and increased level of serum free fatty acids (FFAs). FFAs have direct hepatotoxicity through the induction of an endoplasmic reticulum stress response and subsequently activation of the mitochondrial pathway of cell death. FFAs may also result in lysosomal dysfunction and alter death receptor gene expression. Lipoapoptosis is a key pathogenic process in NAFLD, and correlates with progressive inflammation, and fibrosis. Accumulation of triglyceride in the liver results from uptake and esterification of FFAs by the hepatocyte, and is less likely to be hepatotoxic per se. To date, there are no proven effective therapies that halt NAFLD progression or unfortunately improve prognosis in children. The cellular mechanisms of lipotoxicity are complex but provide potential therapeutic targets for NAFLD. In this review we discuss several potential therapeutic opportunities in detail including inhibition of apoptosis, c-Jun-N-terminal kinase, and endoplasmic reticulum stress pathways.

Figure 1. Bcl-2 Protein Family

Saturated FFAs activate the BH3-only proteins (BIM and PUMA), resulting in inactivation of the antiapoptotic Bcl-2 family members (Mcl-1 and Bcl-xL), releasing Bax and Bak from the inhibitory effect of the antiapoptotic Bcl-2 proteins, and causing their activation. Bim and PUMA can also activate Bax and/or Bak directly. Once activated, Bax and Bak promote mitochondrial dysfunction, leading to the activation of the caspase cascade and apoptosis.

Figure 2. ER stress

In the setting of ER stress the three trans-membrane biosensors PERK, ATF6, and IRE1-a are activated. PERK activation induces the expression of the proapoptotic transcription factor CHOP. CHOP in turn, mediates apoptosis through several pathways including generation of ROS. IRE1-a activates JNK a key player in apoptosis. IRE1-a also generates a spliced form of XBP (s-XBP) that promotes degradation of misfolded proteins. ATF6 contributes in CHOP induction, and heterodimerizes with XBP, enhancing protein degradation.

Figure 3. Integrated model of lipoapotosis by saturated FFAs and potential antiapoptotic agents

Saturated FFAs induce ER stress which in turn activates JNK and CHOP. JNK leads to the upregulation of the pro-apoptotic BH3-only proteins PUMA. CHOP enhances the expression of the proapoptotic BH3-only protein Bim, contributes to PUMA upregulation and mediates the generation of ROS. Bim in cooperation with PUMA induces the activation of the multi-domain executioner proapoptotic protein Bax. Bax activation results in mitochondrial dysfunction, activation of the caspase cascade, and cell death. Saturated FFAs also cause Bax dependent lysosomal permeabilization and release of cathepsin B into the cytosol, cathepsin B mediates down stream mitochondrial permeabilization and apoptosis. Therapeutic strategies to prevent cell death in the setting of saturated FFA-induced apoptosis include as outlined: PUFA, ER Chaperons, GSK3-inhibition, JNK-inhibition, antioxidants, cathepsin B-inhibition, and caspase-inhibition.