Ethanol exposure inhibits hepatocyte lipophagy by inactivating the small guanosine triphosphatase Rab7

Abstract

Alcohol consumption is a well‐established risk factor for the onset and progression of fatty liver disease. An estimated 90% of heavy drinkers are thought to develop significant liver steatosis. For these reasons, an increased understanding of the molecular basis for alcohol‐induced hepatic steatosis is important. It has become clear that autophagy, a catabolic process of intracellular degradation and recycling, plays a key role in hepatic lipid metabolism. We have shown that Rab7, a small guanosine triphosphatase known to regulate membrane trafficking, acts as a key orchestrator of hepatocellular lipophagy, a selective form of autophagy in which lipid droplets (LDs) are specifically targeted for turnover by the autophagic machinery. Nutrient starvation results in Rab7 activation on the surface of the LD and lysosomal compartments, resulting in the mobilization of triglycerides stored within the LDs for energy production. Here, we examine whether the steatotic effects of alcohol exposure are a result of perturbations to the Rab7‐mediated lipophagic pathway. Rats chronically fed an ethanol‐containing diet accumulated significantly higher levels of fat in their hepatocytes. Interestingly, hepatocytes isolated from these ethanol‐fed rats contained juxtanuclear lysosomes that exhibited impaired motility. These changes are similar to those we observed in Rab7‐depleted hepatocytes. Consistent with these defects in the lysosomal compartment, we observed a marked 80% reduction in Rab7 activity in cultured hepatocytes as well as a complete block in starvation‐induced Rab7 activation in primary hepatocytes isolated from chronic ethanol‐fed animals. Conclusion: A mechanism is supported whereby ethanol exposure inhibits Rab7 activity, resulting in the impaired transport, targeting, and fusion of the autophagic machinery with LDs, leading to an accumulation of hepatocellular lipids and hepatic steatosis. (Hepatology Communications 2017;1:140‐152)

Hepatic lipophagy: New insights into autophagic catabolism of lipid droplets in the liver

“…(92,93) Chronic ethanol exposure may interfere with the nuclear localization of transcriptional lipophagic regulators, such as TFEB (as above), (94) or cause the harmful modulation of key mediators of LD interactions with the lipophagic machinery (as with Rab7). (74) Together, these data serve as strong evidence that lipophagy normally serves a hepatoprotective role in response to alcohol consumption. Disruption of this pathway over time, however, may provide an explanation for the prevalence of steatosis that persists with chronic alcohol abuse.…”

“…This role of Rab7 is essential for starvation‐induced lipophagy as loss of Rab7 function after siRNA‐mediated knockdown or expression of a dominant‐negative mutant impaired LD breakdown . In a follow‐up study, it was observed that chronic ethanol exposure inhibits Rab7 activity, disrupts degradative compartment morphology and motility, and causes reduced LD turnover during the process of lipophagy . These findings provided some additional insights into how alcohol might exacerbate the progression of fatty liver.…”

“…Livers from patients with a history of chronic alcohol abuse are often identifiable as having microvesicular steatosis, a histologic observation that can indicate the potential for cirrhotic progression . Chronic ethanol exposure may interfere with the nuclear localization of transcriptional lipophagic regulators, such as TFEB (as above), or cause the harmful modulation of key mediators of LD interactions with the lipophagic machinery (as with Rab7) . Together, these data serve as strong evidence that lipophagy normally serves a hepatoprotective role in response to alcohol consumption.…”

Breaking fat: The regulation and mechanisms of lipophagy

“…This result suggests that Rab7 may be an important LD-localized node for the regulation of hepatic lipophagy. Indeed, a very recent report shows that Rab7 is also a primary target for ethanol-induced hepatic steatosis [41]. The authors found that rats chronically fed an alcohol-containing diet are resistant to starvation-induced lipophagy, which ultimately results in liver steatosis.…”

Impaired Rab7 and dynamin2 block fat turnover by autophagy in alcoholic fatty livers

“…In the first study, Schulze et al investigated the role of Rab7 in hepatocyte lipophagy under chronic ethanol exposure (7). Rab7 belongs to the Rab family of guanosine triphosphate (GTP) binding proteins that regulate intracellular vesicular trafficking.…”