Lipases in lysosomes, what for?

866 Autophagy 2009; Vol. 5 Issue 6 We have all learned in textbooks that “lysosomes contain hydrolases able to degrade all types of intracellular molecules which include proteases, glycosidases, nucleotidases and lipases.” To date, the only logical explanation for the presence of lipases inside of lysosomes was for the degradation of lipoproteins internalized by endocytosis, and for the breakdown of intralysosomal vesicles derived from fusion with autophagosomes or multivesicular bodies. However, in our recent work we found a novel role for lysosomal lipases in the basic cellular process that regulates intracellular lipid stores that we have named “macrolipophagy.” Cells store lipids as cholesterol and triglycerides in the form of lipid droplets from which the lipids can be mobilized when nutrients are scarce, to provide cells with energy and essential elements to preserve biosynthesis. Lipolysis, or the breakdown of triglycerides into free fatty acids, has traditionally been attributed to the actions of both neutral and acidic lipases. However, very little has been known about how intracellular lipids could reach lysosomes, the only acidic compartment in the cell and presumably the preferential location for acid lipase(s). The similarities in the activation of lipolysis and macroautophagy—both are upregulated during nutrient deprivation and repressed by insulin—made us consider a potential link between these two catabolic processes and the possibility that macroautophagy delivers intracellular lipids to lysosomes for hydrolysis. In our recent work, we show that functional macroautophagy is required for starvation-induced lipolysis in hepatocytes, the main repository for the increased lipids circulating in blood as the result of the mobilization of adipocyte fat stores under these conditions. Furthermore, macroautophagy continually contributes to the degradation of intracellular lipids because blockage of macroautophagy even under basal conditions results in abnormally high levels of triglycerides and cholesterol, and increased lipid droplet size and number. Basal macrolipophagy may thus downregulate the lipid content of hepatocytes, fibroblasts and presumably other cell types, except for the specialized lipid-storing cell, the adipocyte. Using both biochemical and morphological procedures, we confirmed the presence of integral components of lipid droplets (both lipids themselves and structural proteins associated with the lipid droplets) in autophagosomes and lysosomes. As predicted, conditions resulting in an upregulation of macroautophagy increased the association between lipid droplet components and autophagic compartments, both in cultured hepatocytes and in vivo in the livers of mice starved for increasing periods of time. We further confirmed this essential role of basal macroautophagy in the maintenance of hepatocyte lipid content using a mouse model with impaired autophagy in liver (conditional Atg7 knockout mice). The livers of these mice, even under basal conditions, displayed massive accumulation of lipid droplets and abnormally high levels of intracellular cholesterol and triglyceride. Contrary to our preconceived idea that the limiting membrane of the autophagosome sequestered whole lipid droplets and delivered them to lysosomes for hydrolysis, we found that in most instances only a portion of the lipid droplet was engulfed at one time. In fact, conjugation of LC3 to form the limiting membrane occurred on the lipid droplet surface, resulting in a penetrating membrane that seals on itself, sequestering both lipids and lipid droplet structural proteins (Fig. 1). As with conventional macroautophagy, basal macrolipophagy does not seem to be a selective process, because portions of lipid droplets are engulfed along with other cytosolic cargo giving rise to autophagic vacuoles with heterogeneous content. However, as starvation persists, macroautophagy becomes somewhat selective as autophagosomes with only lipid droplet content in their lumens constitute the most abundant type of autophagic vacuole under this condition. Macrolipophagy is not only upregulated with nutritional deprivation, but also to some extent when cells are exposed to challenges that increase cellular lipid levels (such as extracellular supplementation with free fatty acids or cholesterol). Activation of macrolipophagy in these conditions is likely required to prevent *Correspondence to: Mark J. Czaja; Albert Einstein College of Medicine; Department of Medicine; Marion Bessin Liver Research Center; 1300 Morris Park Ave.; Bronx, NY 10461 USA; Tel.: 718.430.4255; Fax: 718.430.8975; Email: [email protected]/ Ana Maria Cuervo; Albert Einstein College of Medicine; Department of Developmental and Molecular Biology; 1300 Morris Park Ave.; Ullmann B. 614; Bronx, NY 10461 USA; Tel.: 718.430.2689; Fax: 718.430.8975; Email: [email protected]