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Lipid Droplets Guard Mitochondria during Autophagy
Developmental Cell
Previews Lipid Droplets Guard Mitochondria during Autophagy Till Klecker,1,* Ralf J. Braun,1 and Benedikt Westermann1
1Zellbiologie, Universita €t Bayreuth, 95440 Bayreuth, Germany *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2017.06.018
In this issue of Developmental Cell, Nguyen et al. (2017) show that lipid droplets serve a dual purpose during starvation. First, they act as an energy source by supplying fatty acids for mitochondrial b oxidation. Second, they sequester toxic lipids that arise during autophagic degradation of membranous organelles, thereby protecting mitochondria. Life depends on a constant supply of free energy, which is provided by metabolic degradation of nutrients. However, cells frequently endure periods of starvation when external energy sources are scarce. Thus, mechanisms evolved that help to maintain essential cellular functions before cells starve to death. Among these is the ability of cells to gain energy by degrading dispensable organelles and other cellular components. This process of cellular self-eating, or autophagy, includes the formation of autophagosomes, double-membrane bounded structures that engulf cytoplasmic cargo (Figure 1). Autophagosomes subsequently fuse with the lysosome, where the cargo is dismantled into its molecular building blocks, mostly amino acids and fatty acids (FAs). These are either recycled or utilized for energy generation (Kaur and Debnath, 2015). Furthermore, cells prepare for starvation by synthesizing lipids as energy-rich storage compounds under conditions of ample nutrient supply. These can be stocked within specialized organelles, lipid droplets (LDs), which consist of a core of storage lipids encased by a phospholipid monolayer. Upon nutrient depletion, cells survive by converting these internal energy supplies into FAs, which are used for ATP production by b oxidation in mitochondria (Walther and Farese, 2012). In this issue of Developmental Cell, Nguyen et al. (2017) report that LDs not only fuel mitochondrial energy generation during starvation but also protect mitochondria by sequestering excess FAs that are released by autophagy and might damage organellar membranes. The cellular starvation response shifts the cell’s metabolism toward mitochondrial FA oxidation and shuts down most energy-consuming anabolic processes.
Therefore, it was surprising to find that mouse embryonic fibroblasts (MEFs) exhibit a marked increase in the amount and size of LDs during starvation (Rambold et al., 2015). Nguyen et al. (2017) now show that the ER-residing enzyme diacylglycerol acyltransferase 1 (DGAT1) is a key mediator of starvation-induced LD biosynthesis. They report that during nutrient depletion, DGAT1 catalyzes the formation of storage lipids, triacylglycerols (TAGs), from FAs that are released during autophagic degradation of organellar membranes (Figure 1). Their results also explain why starvation-induced LD biogenesis was previously found to depend on autophagy and why fluorescently labeled lipids initially accumulate in LDs in starved MEFs (Rambold et al., 2015). At first glance it seems counterintuitive that the amount of cellular storage lipids increases while the cells suffer from nutrient deprivation. Indeed, TAGs serve as an important cellular energy source during starvation and are lipolytically broken down into FAs, which are imported into mitochondria and consumed by b oxidation to produce ATP. This has been visualized by tracking the transfer of fluorescently labeled FAs from LDs to mitochondria under nutrient deprivation (Rambold et al., 2015). Nguyen et al. (2017) now report that TAGs from preexisting and newly formed LDs are subjected to lipolytic breakdown during starvation. Following the fate of isotopically labeled palmitate, the authors observed that inhibition of DGAT1 activity causes a massive re-routing of FAs into lipid species other than TAGs, including a strong increase of acylcarnitines, intermediates of mitochondrial FA import (Figure 1). This indicates that the LD conduit is not mandatory for efficient FA transport from autophagosomes to mitochondria. So
why do cells convert autophagy-released FAs first into TAGs, which in turn have to be broken down into FAs for energy generation? FAs are known to be precarious molecules because they damage cellular membranes when present in excess amounts. Thus, when cells remobilize FAs from TAG stores or scavenge them during autophagy, they have to avoid the release of excess FAs into the cytosol to avert cytotoxicity. LDs have been assigned an important protective role against different types of lipotoxicity (Walther and Farese, 2012; Listenberger et al., 2003). Nguyen et al. (2017) now report that inhibition of DGAT1 prevents starvation-induced LD biosynthesis and causes mitochondrial damage and dysfunction. They tracked this toxic effect down to the accumulation of acylcarnitines and conclude that LDs guard mitochondria during starvation by sequestering potentially lipotoxic FAs and storing them as non-cytotoxic TAGs. Thus, LDs balance intracellular FA concentrations by storage or release, depending on cellular demands. Additionally, storage of autophagically produced FAs as TAGs in LDs may serve to regulate energy generation by mitochondrial FA oxidation. Autophagic degradation of organellar membranes presumably releases FAs in amounts that far exceed the immediate cellular need as energy source. When uncontrolled, this could produce significant bursts of energy that might interfere with the cellular starvation response. To avert this scenario, LDs might function as a buffer that allows fine-tuning of TAG lipolysis and FA degradation. Consistent with this, Nguyen et al. (2017) observed that two of the major regulators of cellular energy balance, mTORC1 and AMPK, regulate LD dynamics during nutrient
Developmental Cell 42, July 10, 2017 ª 2017 Elsevier Inc. 1
Developmental Cell
Previews
deprivation. However, we are mains to be identified. It will only beginning to underbe interesting to see in the stand how cells orchestrate future how further studies will multiple signaling pathways complete our understanding to balance lipolysis and of how autophagy, LD biosynTAG biogenesis to maintain thesis, lipolysis, mitochondrial optimal cellular energy levels FA import, lipotoxicity, and b and to avoid the accumulaoxidation are entangled and tion of lipotoxic FAs. orchestrated to cope with the How FAs are transported challenge of nutrient deprivaspecifically to mitochondria tion in starving cells. after their release from LDs remains unknown. LDs REFERENCES are found in close proximity Gao, Q., and Goodman, J.M. (2015). to mitochondria in different Front. Cell Dev. Biol. 3, 49. cell types (Goodman, 2008). Goodman, J.M. (2008). J. Biol. Intriguingly, these intimate Chem. 283, 28005–28009. contacts are increasingly formed under starvation conHerms, A., Bosch, M., Reddy, B.J., Schieber, N.L., Fajardo, A., Rupe´rez, ditions (Herms et al., 2015; C., Ferna´ndez-Vidal, A., Ferguson, Rambold et al., 2015; Nguyen C., Rentero, C., Tebar, F., et al. et al., 2017). Such junc(2015). Nat. Commun. 6, 7176. Figure 1. Avoiding Lipotoxicity: Fatty Acids Are Forced to Make a Pit tions could enable a direct, Stop in Lipid Droplets on Their Way from Autophagosomes to Kaur, J., and Debnath, J. (2015). Nat. Mitochondria vesicle-free lipid transport beRev. Mol. Cell Biol. 16, 461–472. In starved cells, fatty acids (FAs) are released during the autophagic degratween both organelles, as it dation of membranous organelles. Instead of being directly transported to Klecker, T., Bo¨ckler, S., and Westerhas been observed between mitochondria for energy generation by b oxidation, FAs are rerouted to the ER, mann, B. (2014). Trends Cell Biol. where they are used by diacylglycerol acyltransferase 1 (DGAT1) to synthesize 24, 537–545. other organelles, e.g., mitotriacylglycerols (TAGs), which are stored in ER-derived lipid droplets (LDs). At chondria and the ER (Klecker Listenberger, L.L., Han, X., Lewis, the same time, TAG stores within LDs are used by the lipolytic enzyme adipose et al., 2014). Thus, close S.E., Cases, S., Farese, R.V., Jr., triglyceride lipase (ATGL) to generate FAs. These are subsequently converted Ory, D.S., and Schaffer, J.E. juxtaposition of LDs and mitoto acylcarnitine (AC) by carnitine palmitoyltransferase 1 (CPT1) and trans(2003). Proc. Natl. Acad. Sci. USA ported into mitochondria, where they fuel ATP production by b oxidation. ACs chondria could facilitate FA 100, 3077–3082. pose a major threat to the cell because they can damage mitochondria, retransport, thereby preventing sulting in mitochondrial dysfunction. Strikingly, LDs and mitochondria are Nguyen, T.B., Louie, S.M., Daniele, a systemic rise of cytosolic frequently found in close proximity, most likely because they form contact sites J.R., Tran, Q., Dillin, A., Zoncu, R., to facilitate efficient FA transport. The molecular constituents of the tethering FA levels. However, this hypoNomura, D.K., and Olzmann, J.A. complex that juxtaposes LDs and mitochondria are currently unknown. (2017). Dev. Cell 42, this thetical model awaits experiissue, 9–21. mental validation. To date, the only identified candidate molecular also negatively regulates TAG lipolysis on Rambold, A.S., Cohen, S., and Lippincottconstituent of the LD-mitochondria con- LDs (Gao and Goodman, 2015), revealing Schwartz, J. (2015). Dev. Cell 32, 678–692. tact site is the LD-associated protein a potential interconnection and co-regula- Walther, T.C., and Farese, R.V., Jr. (2012). Annu. PLIN5 (Gao and Goodman, 2015). Expres- tion between FA remobilization from LDs Rev. Biochem. 81, 687–714. sion of this protein has been reported to and the establishment of LD-mitochondria Wang, H., Sreenivasan, U., Hu, H., Saladino, A., Polster, B.M., Lund, L.M., Gong, D.W., Stanley, recruit mitochondria to the surface of LDs contact sites. However, a PLIN5 binding W.C., and Sztalryd, C. (2011). J. Lipid Res. 52, partner on the mitochondrial surface re2159–2168. (Wang et al., 2011). Of interest, PLIN5
2 Developmental Cell 42, July 10, 2017
Previews Lipid Droplets Guard Mitochondria during Autophagy Till Klecker,1,* Ralf J. Braun,1 and Benedikt Westermann1
1Zellbiologie, Universita €t Bayreuth, 95440 Bayreuth, Germany *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2017.06.018
In this issue of Developmental Cell, Nguyen et al. (2017) show that lipid droplets serve a dual purpose during starvation. First, they act as an energy source by supplying fatty acids for mitochondrial b oxidation. Second, they sequester toxic lipids that arise during autophagic degradation of membranous organelles, thereby protecting mitochondria. Life depends on a constant supply of free energy, which is provided by metabolic degradation of nutrients. However, cells frequently endure periods of starvation when external energy sources are scarce. Thus, mechanisms evolved that help to maintain essential cellular functions before cells starve to death. Among these is the ability of cells to gain energy by degrading dispensable organelles and other cellular components. This process of cellular self-eating, or autophagy, includes the formation of autophagosomes, double-membrane bounded structures that engulf cytoplasmic cargo (Figure 1). Autophagosomes subsequently fuse with the lysosome, where the cargo is dismantled into its molecular building blocks, mostly amino acids and fatty acids (FAs). These are either recycled or utilized for energy generation (Kaur and Debnath, 2015). Furthermore, cells prepare for starvation by synthesizing lipids as energy-rich storage compounds under conditions of ample nutrient supply. These can be stocked within specialized organelles, lipid droplets (LDs), which consist of a core of storage lipids encased by a phospholipid monolayer. Upon nutrient depletion, cells survive by converting these internal energy supplies into FAs, which are used for ATP production by b oxidation in mitochondria (Walther and Farese, 2012). In this issue of Developmental Cell, Nguyen et al. (2017) report that LDs not only fuel mitochondrial energy generation during starvation but also protect mitochondria by sequestering excess FAs that are released by autophagy and might damage organellar membranes. The cellular starvation response shifts the cell’s metabolism toward mitochondrial FA oxidation and shuts down most energy-consuming anabolic processes.
Therefore, it was surprising to find that mouse embryonic fibroblasts (MEFs) exhibit a marked increase in the amount and size of LDs during starvation (Rambold et al., 2015). Nguyen et al. (2017) now show that the ER-residing enzyme diacylglycerol acyltransferase 1 (DGAT1) is a key mediator of starvation-induced LD biosynthesis. They report that during nutrient depletion, DGAT1 catalyzes the formation of storage lipids, triacylglycerols (TAGs), from FAs that are released during autophagic degradation of organellar membranes (Figure 1). Their results also explain why starvation-induced LD biogenesis was previously found to depend on autophagy and why fluorescently labeled lipids initially accumulate in LDs in starved MEFs (Rambold et al., 2015). At first glance it seems counterintuitive that the amount of cellular storage lipids increases while the cells suffer from nutrient deprivation. Indeed, TAGs serve as an important cellular energy source during starvation and are lipolytically broken down into FAs, which are imported into mitochondria and consumed by b oxidation to produce ATP. This has been visualized by tracking the transfer of fluorescently labeled FAs from LDs to mitochondria under nutrient deprivation (Rambold et al., 2015). Nguyen et al. (2017) now report that TAGs from preexisting and newly formed LDs are subjected to lipolytic breakdown during starvation. Following the fate of isotopically labeled palmitate, the authors observed that inhibition of DGAT1 activity causes a massive re-routing of FAs into lipid species other than TAGs, including a strong increase of acylcarnitines, intermediates of mitochondrial FA import (Figure 1). This indicates that the LD conduit is not mandatory for efficient FA transport from autophagosomes to mitochondria. So
why do cells convert autophagy-released FAs first into TAGs, which in turn have to be broken down into FAs for energy generation? FAs are known to be precarious molecules because they damage cellular membranes when present in excess amounts. Thus, when cells remobilize FAs from TAG stores or scavenge them during autophagy, they have to avoid the release of excess FAs into the cytosol to avert cytotoxicity. LDs have been assigned an important protective role against different types of lipotoxicity (Walther and Farese, 2012; Listenberger et al., 2003). Nguyen et al. (2017) now report that inhibition of DGAT1 prevents starvation-induced LD biosynthesis and causes mitochondrial damage and dysfunction. They tracked this toxic effect down to the accumulation of acylcarnitines and conclude that LDs guard mitochondria during starvation by sequestering potentially lipotoxic FAs and storing them as non-cytotoxic TAGs. Thus, LDs balance intracellular FA concentrations by storage or release, depending on cellular demands. Additionally, storage of autophagically produced FAs as TAGs in LDs may serve to regulate energy generation by mitochondrial FA oxidation. Autophagic degradation of organellar membranes presumably releases FAs in amounts that far exceed the immediate cellular need as energy source. When uncontrolled, this could produce significant bursts of energy that might interfere with the cellular starvation response. To avert this scenario, LDs might function as a buffer that allows fine-tuning of TAG lipolysis and FA degradation. Consistent with this, Nguyen et al. (2017) observed that two of the major regulators of cellular energy balance, mTORC1 and AMPK, regulate LD dynamics during nutrient
Developmental Cell 42, July 10, 2017 ª 2017 Elsevier Inc. 1
Developmental Cell
Previews
deprivation. However, we are mains to be identified. It will only beginning to underbe interesting to see in the stand how cells orchestrate future how further studies will multiple signaling pathways complete our understanding to balance lipolysis and of how autophagy, LD biosynTAG biogenesis to maintain thesis, lipolysis, mitochondrial optimal cellular energy levels FA import, lipotoxicity, and b and to avoid the accumulaoxidation are entangled and tion of lipotoxic FAs. orchestrated to cope with the How FAs are transported challenge of nutrient deprivaspecifically to mitochondria tion in starving cells. after their release from LDs remains unknown. LDs REFERENCES are found in close proximity Gao, Q., and Goodman, J.M. (2015). to mitochondria in different Front. Cell Dev. Biol. 3, 49. cell types (Goodman, 2008). Goodman, J.M. (2008). J. Biol. Intriguingly, these intimate Chem. 283, 28005–28009. contacts are increasingly formed under starvation conHerms, A., Bosch, M., Reddy, B.J., Schieber, N.L., Fajardo, A., Rupe´rez, ditions (Herms et al., 2015; C., Ferna´ndez-Vidal, A., Ferguson, Rambold et al., 2015; Nguyen C., Rentero, C., Tebar, F., et al. et al., 2017). Such junc(2015). Nat. Commun. 6, 7176. Figure 1. Avoiding Lipotoxicity: Fatty Acids Are Forced to Make a Pit tions could enable a direct, Stop in Lipid Droplets on Their Way from Autophagosomes to Kaur, J., and Debnath, J. (2015). Nat. Mitochondria vesicle-free lipid transport beRev. Mol. Cell Biol. 16, 461–472. In starved cells, fatty acids (FAs) are released during the autophagic degratween both organelles, as it dation of membranous organelles. Instead of being directly transported to Klecker, T., Bo¨ckler, S., and Westerhas been observed between mitochondria for energy generation by b oxidation, FAs are rerouted to the ER, mann, B. (2014). Trends Cell Biol. where they are used by diacylglycerol acyltransferase 1 (DGAT1) to synthesize 24, 537–545. other organelles, e.g., mitotriacylglycerols (TAGs), which are stored in ER-derived lipid droplets (LDs). At chondria and the ER (Klecker Listenberger, L.L., Han, X., Lewis, the same time, TAG stores within LDs are used by the lipolytic enzyme adipose et al., 2014). Thus, close S.E., Cases, S., Farese, R.V., Jr., triglyceride lipase (ATGL) to generate FAs. These are subsequently converted Ory, D.S., and Schaffer, J.E. juxtaposition of LDs and mitoto acylcarnitine (AC) by carnitine palmitoyltransferase 1 (CPT1) and trans(2003). Proc. Natl. Acad. Sci. USA ported into mitochondria, where they fuel ATP production by b oxidation. ACs chondria could facilitate FA 100, 3077–3082. pose a major threat to the cell because they can damage mitochondria, retransport, thereby preventing sulting in mitochondrial dysfunction. Strikingly, LDs and mitochondria are Nguyen, T.B., Louie, S.M., Daniele, a systemic rise of cytosolic frequently found in close proximity, most likely because they form contact sites J.R., Tran, Q., Dillin, A., Zoncu, R., to facilitate efficient FA transport. The molecular constituents of the tethering FA levels. However, this hypoNomura, D.K., and Olzmann, J.A. complex that juxtaposes LDs and mitochondria are currently unknown. (2017). Dev. Cell 42, this thetical model awaits experiissue, 9–21. mental validation. To date, the only identified candidate molecular also negatively regulates TAG lipolysis on Rambold, A.S., Cohen, S., and Lippincottconstituent of the LD-mitochondria con- LDs (Gao and Goodman, 2015), revealing Schwartz, J. (2015). Dev. Cell 32, 678–692. tact site is the LD-associated protein a potential interconnection and co-regula- Walther, T.C., and Farese, R.V., Jr. (2012). Annu. PLIN5 (Gao and Goodman, 2015). Expres- tion between FA remobilization from LDs Rev. Biochem. 81, 687–714. sion of this protein has been reported to and the establishment of LD-mitochondria Wang, H., Sreenivasan, U., Hu, H., Saladino, A., Polster, B.M., Lund, L.M., Gong, D.W., Stanley, recruit mitochondria to the surface of LDs contact sites. However, a PLIN5 binding W.C., and Sztalryd, C. (2011). J. Lipid Res. 52, partner on the mitochondrial surface re2159–2168. (Wang et al., 2011). Of interest, PLIN5
2 Developmental Cell 42, July 10, 2017