Glial Cell Line–Derived Neurotrophic Factor Enhances Autophagic Flux in Mouse and Rat Hepatocytes and Protects Against Palmitate Lipotoxicity

Abstract

Glial cell line–derived neurotrophic factor (GDNF) is a protein that is required for the development and survival of enteric, sympathetic, and catecholaminergic neurons. We previously reported that GDNF is protective against high fat diet (HFD)‐induced hepatic steatosis in mice through suppression of hepatic expression of peroxisome proliferator activated receptor‐γ and genes encoding enzymes involved in de novo lipogenesis. We also reported that transgenic overexpression of GDNF in mice prevented the HFD‐induced liver accumulation of the autophagy cargo‐associated protein p62/sequestosome 1 characteristic of impaired autophagy. Here we investigated the effects of GDNF on hepatic autophagy in response to increased fat load, and on hepatocyte mitochondrial fatty acid β‐oxidation and cell survival. GDNF not only prevented the reductions in the liver levels of some key autophagy‐related proteins, including Atg5, Atg7, Beclin‐1 and LC3A/B‐II, seen in HFD‐fed control mice, but enhanced their levels after 12 weeks of HFD feeding. In vitro, GDNF accelerated autophagic cargo clearance in primary mouse hepatocytes and a rat hepatocyte cell line, and reduced the phosphorylation of the mechanistic target of rapamycin complex downstream‐target p70S6 kinase similar to the autophagy activator rapamycin. GDNF also enhanced mitochondrial fatty acid β‐oxidation in primary mouse and rat hepatocytes, and protected against palmitate‐induced lipotoxicity. Conclusion: We demonstrate a role for GDNF in enhancing hepatic autophagy and in potentiating mitochondrial function and fatty acid oxidation. Our studies show that GDNF and its receptor agonists could be useful for enhancing hepatocyte survival and protecting against fatty acid–induced hepatic lipotoxicity.

Hepatic Autonomic Nervous System and Neurotrophic Factors Regulate the Pathogenesis and Progression of Non-alcoholic Fatty Liver Disease

“…As compared to HFD-fed WT mice, GDNF transgenic mice displayed reduced expression of autophagic marker p62/sequestosome-1 and increased expression of autophagy related 5 (Atg5), Beclin 1 and microtubule-associated protein-1 light chain 3 (20). This in vivo effect of GDNF was further confirmed by in vitro analysis of autophagic flux in primary mouse hepatocytes and secondary rat cell line (74).…”

“…Autophagy maintains insulin sensitivity, regulates lipid stores, and protects against the development of inflammation (4). Work in our lab has demonstrated a stimulatory effect of GDNF on hepatic macroautophagy (20,74). As compared to HFD-fed WT mice, GDNF transgenic mice displayed reduced expression of autophagic marker p62/sequestosome-1 and increased expression of autophagy related 5 (Atg5), Beclin 1 and microtubule-associated protein-1 light chain 3 (20).…”

Glial cell line-derived neurotrophic factor (GDNF) mediates hepatic stellate cell activation via ALK5/Smad signalling

“…It was recently reported that GDNF is protective in HFD-induced insulin resistance, hyperlipidemia and obesity in mice. 32, 33 In their experiments, Mwangi et al used transgenic GDNF expression in mice, and investigated GDNF responses of hepatocytes in experimental NAFLD. They convincingly showed that in the setting of hepatocyte steatosis, GDNF is protective through the downregulation of peroxisome proliferator-activated receptor- γ (PPAR-γ) expression.…”

RORα Enhances Lysosomal Acidification and Autophagic Flux in the Hepatocytes

“…To date, many studies showed defects in autophagy processes in NAFLD pathogenesis, in particular, in the early and intermediate steps of autophagy (i.e., autophagosome formation and autophagosome−lysosome fusion), which are under the control of mTOR and of TFEB. ( 2 , 7 , 8 , 35 , 36 , 37 ) We observed that RORα also functions to regulate autophagosome formation, as the number of autophagosome puncta was decreased in the hepatocytes of RORα‐LKO mice (Fig. 3D ).…”

“…(5,6) In experimental animals, the enhancement of the autophagic flux protected against palmitate lipotoxicity in murine hepatocytes. (7) In addition, liver injury and inflammation were increased when hepatic autophagy was reduced by genetic knockout of autophagy-related 5 in a mouse model. (8) The degradative ability of the autophagic pathway is afforded by lysosomes.…”