A molecular mechanism for lipophagy regulation in the liver
Authors: Didac Carmona‐Gutierrez, Andreas Zimmermann, Frank Madeo
Journal: Hepatology (2015)

Supported by the Austrian Science Fund FWF (grants LIPOTOX, I1000‐B20, P23490‐B12, and P24381‐B20 to F.M.). We gratefully acknowledge support from NAWI Graz and BioTechMed Graz. Also supported by the Austrian Research Association and the Faculty of Natural Sciences, University of Graz (to A.Z.). Potential conflict of interest: Nothing to report. See Article on Page 1896 Macroautophagy (hereafter termed "autophagy") is a process of cellular self‐digestion that targets superfluous or damaged cell components for lysosomal degradation. Thereby, cytosolic portions containing the target molecules/organelles are enveloped by double‐membrane vesicles (autophagosomes), which then fuse with lysosomes. There, the cargo is degraded and the resulting compounds (e.g., amino acids, fatty acids) are released into the cytoplasm for recycling and/or energy production. Accordingly, lipophagy defines the autophagic turnover of cytosolic lipid droplets (LDs), the main neutral lipid stores in the cell: LDs are engulfed by autophagosomes and, after lysosomal fusion, degraded by acid lipases. Together with lipolysis, which involves the direct docking of cytoplasmic lipases to the LD surface for hydrolysis, lipophagy is key for the regulation of LD catabolism. Imbalanced LD homeostasis, as may result from genetic factors, infection, or a particular lifestyle (e.g., high‐fat diet, excessive alcohol consumption), may lead to multiple afflictions. Obesity, for example, can cause the so‐called metabolic syndrome, a cluster of metabolic disorders that increases the risk of cardiovascular diseases, type 2 diabetes, and liver pathologies. In fact, the liver, the central transfer site for lipid trafficking, is particularly susceptible to such deregulation, which may result in hepatic disorders such as nonalcoholic fatty liver disease and alcohol‐induced hepatic steatosis. Interestingly, autophagy has been associated with several pathophysiological scenarios in the liver. For instance, inhibition of autophagy at both the cellular and the organismal level promotes hepatocellular steatosis.1 Still, many mechanistic details underlying the regulation of autophagic LD turnover in the liver remain unknown. The study by Schroeder et al. in this issue of hepatology2 gives detailed insight into how LDs interact with the autophagic pathway, determining the Rab7 guanosine triphosphatase as a key regulator in this process. It is known that Rab7 is associated with LD membranes and a regulator of transport and maturation of the late endocytic and autophagic compartments. Using nutrient starvation conditions, under which autophagy is strongly induced, the authors demonstrate increased Rab7 activation and recruitment to LDs of hepatoma cells. This is a prerequisite specifically for LD breakdown because genetic and pharmacological inhibition of Rab7 impairs the ability to degrade LDs (but does not alter LD formation).2 Thus, it seems as if active Rab7 would "prime" LDs for autophagic degradation. Likewise, nutrient deprivation also activates Rab7 at diverse degradative compartments of the autophagic pathway, including autophagosomes. This mediates their recruitment through a physical interaction with Rab7‐primed LDs. Activation of Rab7 also occurs at multivesicular bodies and is necessary for lipophagy.2 Multivesicular bodies usually form an intermediate compartment (amphisome) with autophagosomes before fusion with the lysosome. Whether the formation of an amphisome is required or this association represents a molecular platform to ease an interaction with lysosomes remains to be explored. In addition, the authors show a direct interaction between autophagosomal LDs and lysosomes.2 Using fluorescent microscopy analyses on fixed and live cells, they show a kiss‐and‐run mechanism: (1) recruitment of lysosomes to Rab7‐primed autophagosomal LDs, (2) docking for a short period of time, and (3) departing of the lysosome. They interpret that LD breakdown might be facilitated through such a sequential and repeated "nibbling off" by different lysosomes. This regulatory capacity of Rab7 depends on the ability to associate to membranes because the expression of a prenylation‐defective Rab7 mutant in Rab7 knockdown cells is not able to restore the lipophagy defect.2 In fact, this mutation precludes Rab7 from associating with LDs as well as actual guanosine triphosphatase activity. Interestingly, a further Rab7 mutant unable to bind to Rab7‐interacting lysosomal protein (RILP), a downstream effector of Rab7 involved in endolysosomal trafficking, remains associated to LDs and maintains its guanosine triphosphatase activity but still fails to restore lipophagy when Rab7 is knocked down.2 Importantly, Rab7 is known to regulate microtubule minus end–directed transport through simultaneous binding of RILP and oxysterol‐binding protein–related protein 1L in order to recruit the motor protein complex dynein/dynactin.3 At the same time, FYVE and coiled‐coil domain containing 1 (FYCO1), another Rab7‐interacting protein, regulates plus end–directed transport of autophagosomes, probably through interaction with LC3 (Atg8), phosphoinositol‐3‐phosphate, and the kinesin motor.4 Thus, Rab7 seems to directly control the recruitment and docking events between primed LDs and degradative compartments through microtubule‐assisted regulation. Further analysis of other Rab7 downstream effectors, like the homotypic fusion and protein‐sorting (HOPS) complex, which is required for autophagosome–lysosome fusion, may shed further light on lipophagic execution. Interestingly, the HOPS complex has been recently shown to directly interact with Pleckstrin homology domain containing protein family member 1, which contains an LC3‐interacting region that mediates its binding to autophagosomal membranes.5 Besides such downstream effectors, it will also be interesting to explore how prolipophagic upstream signals feed into Rab7 activation and how the Rab7 network might integrate into the regulation of different autophagy types. Altogether, the article by Schroeder et al. provides exciting mechanistic insights into Rab7 as the executory and regulatory factor during lipophagy. The significance of unveiling the regulatory network governing the lipophagic process in general and hepatic LD turnover in particular is evident given the medical implications of LD imbalance. This importance is further underscored by the increasing indications of the pleiotropic nature of autophagy for lipid degradation in different tissues.6 In the liver, lipophagy induction may prevent adverse conditions resulting from LD accumulation. For this purpose, different strategies that could counteract the aging phenomenon through autophagy induction may be applied, among them lifestyle (e.g., fasting, exercise) and pharmacological strategies (e.g., rapamycin, spermidine).7 Of note, polyphenols from coffee have recently been identified as proautophagic compounds.8 Epidemiologic studies further support the idea that drinking coffee reduces the risk for diverse afflictions in nonalcoholic fatty liver disease patients. In addition, caffeine protects against fatty liver by coordinating the induction of lipophagy and mitochondrial β‐oxidation.9 Nevertheless, gross autophagy activation in the liver might not be appropriate under specific conditions. For instance, in hepatocarcinogenesis, autophagic induction suppresses early tumorigenesis through its intracellular cleaning function. However, in established tumors, this cytoprotective effect may contribute to chemotherapy resistance and counteract the adversity of the hypoxic environment around the tumor. A recent study using mice with hepatocyte‐specific knockout of the autophagy‐essential Atg5 also suggests that after hepatocarcinogenesis initiation, autophagy might restrain the expression of tumor suppressors and thus fuel the progression of hepatocellular carcinoma.10 Similarly, autophagy might have an ambivalent impact on liver fibrosis, a typical wound‐healing reaction to apoptosis and necrosis‐mediated chronic liver injury that is associated with inflammation. Among other effects, this leads to the transdifferentiation of hepatic stellate cells to hepatic myofibroblasts that drive the fibrogenic process. In this context, autophagy exerts antifibrogenic effects through its cytoprotective and anti‐inflammatory properties but at the same time has profibrogenic properties due to its direct contribution to the process of hepatic stellate cell activation.11 These examples show that cell‐specific activation of autophagy is crucial to effectively prevent and/or counteract at least some hepatic pathologies. In sum, a deeper understanding of the molecular mechanisms of hepatic autophagy in general and lipophagy in particular is necessary for pathophysiological and therapeutical reasons. With the characterization of Rab7 as the lipophagic network orchestrator, a big step has been accomplished toward revealing the complexity of and allowing an intervention in how the liver controls its lipid balance. Author names in bold designate shared co‐first authorship.