Autophagy links lipid metabolism to longevity in C. elegans

Louis R. Lapierre,1 Alicia Meléndez2 and Malene Hansen1,*1Sanford-Burnham Medical Research Institute; Del E. Webb Neuroscience, Aging and Stem Cell Research Center; Program of Development and Aging; La Jolla,CA USA; 2Queens College; Department of Biology; and The Graduate Center; City University of New York; Flushing, NY USA

Keywords: autophagy, aging, lipolysis,lipophagy, TOR, PHA-4, DAF-16,LIPL-4, C. elegans

Submitted: 10/05/11

Revised: 10/12/11

Accepted: 11/09/11

http://dx.doi.org/10.4161/auto.8.1.18722*Correspondence to: Malene Hansen;Email: mhansen@sanfordburnham.org

Punctum to: Lapierre LR, Gelino S, Melendez A,Hansen M. Autophagy and lipid metabolismcoordinately modulate life span in germline-lessC. elegans. Curr Biol 2011; 21:1507–14;PMID:21906946; http://dx.doi.org/10.1016/j.cub.2011.07.042

The cellular recycling process ofautophagy is emerging as a centralplayer in many of the conserved longevitypathways in C. elegans, but the under-lying mechanisms that link autophagyand life span remain unclear. In a recentstudy, we provided evidence to suggestthat autophagy modulates aging throughan effect on lipid homeostasis. Specifi-cally, we identified a role for autophagyin a longevity model in which germlineremoval in C. elegans extends life span.Life-span extension in these animalsis achieved, at least in part, throughincreased expression of the lipase LIPL-4.We found that autophagy and LIPL-4–dependent lipolysis are both upregulatedin germline-less animals and work inter-dependently to prolong life span. Whilethese genetic results lend further supportto a growing link between autophagy andlipid metabolism, our findings are thefirst to suggest a possible molecularmechanism by which autophagy modu-lates organismal aging.

Over the past two decades, the nematodeC. elegans has played a central role inidentifying the molecular components ofseveral nutrient-sensing signaling pathwaysthat extend life span. The most well-studied example is the conserved insulin/IGF-1 signaling pathway, in whichinactivation of the insulin/IGF-1 receptorDAF-2 acts through the FoxO transcrip-tion factor DAF-16 to regulate theexpression of longevity genes and extendlife span. More recent research has focusedon understanding the cellular mechanismsby which such pathways modulate lifespan, and has highlighted a direct role forthe cellular recycling process of autophagy

in aging. Specifically, long-lived C. elegansmutants with perturbations in nutrient-sensing pathways induce autophagy, andrequire autophagy to live long (Fig. 1A).However, it is not yet known how theautophagy process modulates aging, whichpresumably occurs through the beneficialturnover of as-yet-unidentified cargo.

Removal of the germline representsanother nutrient-responsive longevityparadigm, which, similar to the insulin/IGF-1 signaling pathway, relies on DAF-16/FoxO to extend life span in bothworms and flies. Interestingly, lipid meta-bolism is critically required for the long lifespan of germline-less C. elegans. Germlineremoval induces the expression of apredicted lipase called LIPL-4 throughthe transcription factor DAF-16/FoxO,and lipl-4 is required for these animals tolive long. Importantly, overexpression ofLIPL-4 alone is sufficient to extend lifespan, a finding that provides one of thefirst direct links between lipid metabolismand longevity. Because recent reports havesuggested that autophagy can hydrolyzelipids through a process termed lipophagy,we investigated whether autophagy mightbe important for the long life spanobserved in germline-less mutants, and ifso, whether there was a role for lipidmetabolism in that process. Indeed,demonstrating an autophagy–lipid home-ostasis–longevity connection would notjust have an impact upon the longevityfield but may also likely have implicationsfor human diseases like metabolic syn-drome and diabetes.

We provided evidence for this model(Fig. 1B) in the following ways. We foundthat germline-deficient animals haveincreased numbers of autophagic eventsin their intestines as well as in the

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http://dx.doi.org/10.4161/auto.8.1.18722http://dx.doi.org/10.4161/auto.8.1.18722http://www.ncbi.nlm.nih.gov/pubmed/21906946http://dx.doi.org/10.1016/j.cub.2011.07.042http://dx.doi.org/10.1016/j.cub.2011.07.042

hypodermis, as assessed by electron micro-scopy. These two tissues were specificallyexamined because they are both sites of fatstorage in the worm, and they are tissuesin which the lipase LIPL-4 is expressed.Consistent with these observations, wedetected increased levels of the autophagymarker LGG-1/LC3 in these same tissues.Interestingly, we discovered that inductionof autophagy was likely transcriptionallyregulated, because germline-less animalsshowed upregulated expression of severalautophagy genes, including unc-51/ULK1,bec-1/BECN1, and lgg-1/LC3. Inductionof these autophagy genes was regulated bythe FoxA transcription factor PHA-4, andwe next asked if autophagy genes andPHA-4/FoxA were required for the longlife span of germline-less animals. Wefound this was indeed the case. The lifespan of germline-less, but not wild-type,animals was significantly shortened byfeeding worms RNAi of several genes func-tioning at different steps in the autophagypathway. From these results, we concludedthat increased autophagy is required forgermline-less animals to live long.

Having established a role for autophagyin germline-less animals, we next sought todetermine if autophagy could be linkedto lipid metabolism. Because the lipaseLIPL-4 was required for the long life spanof germline-less animals, similar to theautophagy genes, we asked if lipolysis andautophagy might be connected, and if so,whether such a link was important for life-span extension. We used an in vitro lipaseassay to show that autophagy genes areimportant for the increased lipase activityobserved in germline-less animals. Con-versely, we found that overexpression ofLIPL-4 was sufficient to increase auto-phagic activity. To make the final link tolongevity, we asked if long-lived animalsoverexpressing the lipase LIPL-4 requiredthe activity of autophagy genes. In sup-port of this idea, we found that multipleautophagy genes as well as PHA-4 wererequired for LIPL-4–overexpressing ani-mals to live long, just like we observed ingermline-less animals. Importantly, addi-tional results suggested that the conservedautophagy regulator TOR (Target ofRapamycin) plays an important role in

the link between LIPL-4 and autophagy,as TOR likely functions as a commonupstream regulator of both LIPL-4and autophagy in germline-less animals(Fig. 1B). Taken together, these resultssuggest that autophagy and lipolysis func-tion interdependently to extend the lifespan of germline-less C. elegans, andfurther imply that partitioning of lipidsthrough lipophagy is a possible mechan-ism by which autophagy modulates orga-nismal aging.

As such, our new findings underscorethe importance of autophagy for life-span extension in C. elegans, and provideadditional support for autophagy playingan influential role in lipid homeostasis.Our results also raise many interestingquestions. It will be important to probethe link between autophagy and lipidmetabolism in more detail; for example,by asking how autophagy and the lipaseLIPL-4 might function interdependentlyto affect longevity, and in which tissuesthese changes may be important forlife-span extension. Moreover, it willbe critical to examine how autophagy

Figure 1. Role of autophagy in nutrient-sensitive longevity pathways in the nematode C. elegans. (A) Multiple conserved longevity pathways extend lifespan in C. elegans by upregulating autophagy. Highlighted in blue is the germline signaling pathway, in which removal of germline stem cells extendsthe life span of adult worms. Germline loss can be achieved by laser ablation of germline precursor cells in the worm larvae, or by genetic mutation.(B) Model of the mechanism by the germline signaling pathway extends life span in C. elegans by coordinately regulating LIPL-4–mediated lipolysis andautophagy. These processes are likely regulated by the common upstream regulator TOR through the forkhead transcription factors DAF-16/FoxOand PHA/FoxA. These two transcription factors regulate lipid metabolism and autophagy, respectively. This model is modified from Lapierre et al.,Curr Biol 2011.

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affects lipid metabolism, and how deregu-lation of autophagy influences lipidprofiles, including those of membranelipids, in long-lived germline-less animals.Another priority will be to investigate thisnovel link in additional longevity mutantsthat show increased autophagy levels.These questions constitute important

avenues for future research, and theresults will no doubt shed light on thefascinating role of autophagy in the agingprocess.

Acknowledgments

We thank Drs. Hannes Bülow andAnne O’Rourke for comments on the

manuscript. MH is supported by twoR01 grants from the National Instituteof Aging, and AM is supported by aNational Science Foundation ResearchInitiation Award and a National Instituteof Aging supplement award. MH andAM are both Ellison Medical FoundationNew Scholars in Aging.

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