Biogenesis and Breakdown of Lipid Droplets in Pathological Conditions

Abstract

Lipid droplets (LD) have long been considered as mere fat drops; however, LD have lately been revealed to be ubiquitous, dynamic and to be present in diverse organelles in which they have a wide range of key functions. Although incompletely understood, the biogenesis of eukaryotic LD initiates with the synthesis of neutral lipids (NL) by enzymes located in the endoplasmic reticulum (ER). The accumulation of NL leads to their segregation into nanometric nuclei which then grow into lenses between the ER leaflets as they are further filled with NL. The lipid composition and interfacial tensions of both ER and the lenses modulate their shape which, together with specific ER proteins, determine the proneness of LD to bud from the ER toward the cytoplasm. The most important function of LD is the buffering of energy. But far beyond this, LD are actively integrated into physiological processes, such as lipid metabolism, control of protein homeostasis, sequestration of toxic lipid metabolic intermediates, protection from stress, and proliferation of tumours. Besides, LD may serve as platforms for pathogen replication and defense. To accomplish these functions, from biogenesis to breakdown, eukaryotic LD have developed mechanisms to travel within the cytoplasm and to establish contact with other organelles. When nutrient deprivation occurs, LD undergo breakdown (lipolysis), which begins with the LD-associated members of the perilipins family PLIN2 and PLIN3 chaperone-mediated autophagy degradation (CMA), a specific type of autophagy that selectively degrades a subset of cytosolic proteins in lysosomes. Indeed, PLINs CMA degradation is a prerequisite for further true lipolysis, which occurs via cytosolic lipases or by lysosome luminal lipases when autophagosomes engulf portions of LD and target them to lysosomes. LD play a crucial role in several pathophysiological processes. Increased accumulation of LD in non-adipose cells is commonly observed in numerous infectious diseases caused by intracellular pathogens including viral, bacterial, and parasite infections, and is gradually recognized as a prominent characteristic in a variety of cancers. This review discusses current evidence related to the modulation of LD biogenesis and breakdown caused by intracellular pathogens and cancer.

Keywords: LD biogenesis; LD breakdown; cancer; lipid droplet (LD); protozoans; viral infection.

FIGURE 1

LD Biogenesis. From a biophysical perspective, LD biogenesis can be described as a four-step process. (A) Nucleation occurs when the ER bilayer saturates with NL, reaching a concentration in the bilayer to form lenses of the size in the order of nanometers. The PL composition and seipin among other proteins modulates this process through bilayer hydration, bending and curvature. (B) The individual lens growth takes place as newly synthesized NL are incorporated and by ripening and fusion between lenses; seipin prevents NL from shedding. (C) Budding is a spontaneous process (dewetting) driven by the interfacial tensions that come into play surrounding the droplet. The bilayer tension and the phospholipid asymmetry determine the sphericity and direction of budding, respectively. FIT has been recently proposed to be involved in the maintenance of the appropriate PL composition in each ER hemi-layer. (D) Although detachment is not to be expected for all LD, it is a reversible process. COPI has been proposed to be involved in the detachment/re-attachment process to the ER membrane. (E) Metabolic reactions more closely affecting NL synthesis and its modulating pathway SREBP. Note: the scheme represents a simplification of the metabolic vias according to the available evidence regarding the revised pathologies. For a more detailed description, see Pol et al. (2014).

FIGURE 2

LD Breakdown. (A) Given that PLINs are gatekeepers of LD breakdown and further lipolysis, they must be degraded for breakdown to proceed. PLIN1 and PLIN2 are substrates of the ubiquitin-proteasome system (UPS), and PLIN2, PLIN3 and PLIN5 are substrates of chaperone mediated autophagy (CMA) under conditions of lipid demand. For PLIN2 and PLIN3, a KFERQ peptide has been demonstrated to mediate CMA. (B) The degradation of LD TAG and SE occurs by the sequential action of PNPLA2, LIPE and MGLL to produce glycerol (G) and free FA for further β-oxidation. Concomitantly, PNPLA2 and LIPE located on the LD surface can interact with LC3-II on phagophore membranes through their LC3 interacting regions (LIR) to promote lipophagy-mediated LD breakdown.

FIGURE 3

LD in the biological cycles of protozoan parasites. Trypanosoma cruzi displays reservosomes (R) in epimastigotes and cytoplasmic LD in metacyclic trypomastigotes (MT) that mainly store cholesterol and SE, respectively. While degradation of R occurs during metacyclogenesis, the contact with the host cell induces the production of LD and PGE2 in MT and in the host cell favoring T. cruzi infection and replication. Toxoplasma gondii expresses TgDGAT1, TgACAT1 and TgACAT2, which are the enzymes responsible for the LD synthesis in the parasite. An increased number of host LD was observed in cells containing vacuoles with tachyzoites, which is the characteristic parasite stage of the acute infection. These LD are also observed in the vacuole lumen and inside the parasite cytoplasm, thus evidencing the transport of host lipids to the parasite. Leishmania spp. increases the LD number during the evolution from procyclic to metacyclic promastigotes, which also contain PGF2α, suggesting a role of this eicosanoid as a parasite virulence factor. Host’s LD are also increased in macrophages infected with Leishmania. Like T. gondii, these host LD have been observed inside the Leishmania vacuole and even in the parasite’s cytoplasm. Plasmodium was found to store NL in the food vacuole. NL are important to prevent heme toxicity by production of the malaria pigment hemozoin. Infected red-blood cells increase the number and size of LD when they evolve from the trophozoite to squizont form, whereas LD breakdown characterizes merozoite maturation and release together with FA.

FIGURE 4

Flaviviridae effects in LD metabolism: (A) HCV, ZIKV/DENV stimulate the SREBP pathway through the transcription of genes involved in LD and lipids biosynthesis in order to cover the extra membrane requirement that virus replication generates. (B) DENV/ZIKV stimulates lipophagy by recruitment of deubiquitinated AUP1 from LD membrane to the LC3-positive autophagosome; this process generates FA that after catabolism inside the mitochondria (β-oxidation) produce energy (ATP) to accomplish viral replication. Data on the HCV effect on lipophagy are controversial: some authors report a stimulation of this process while other suggest that inhibition of lipophagy may occur. (C) Some authors hypothesize that there is a putative secretion of HCV or DENV/ZIKV virions, viral proteins or infectious viral RNA mediated by autophagy of LC3-positive LD vesicles (eLD) to spread the infection. This phenomenon may support the bystander effect proposed for DENV/ZIKV infections. (D) Apoptosis is inhibited (HCV) or stimulated (DENV/ZIKV) by viral and nLD interaction with PML nuclear bodies.

FIGURE 5

The regulatory mechanism of LD in cancer progression. Different signaling pathways of lipid acquisition may drive LD biogenesis in stressed cancer cells. Increased LD contents could expand the source of lipid substrates and energy to meet the metabolic needs of proliferating cancer cells. In the tumor microenvironment, LD could act as an energy reservoir for an aggressive cancer to trigger metastatic cloning. LD accumulation extensively mediates proliferation, invasion, metastasis, and oxidative stress and chemotherapy resistance in multiple types of cancers.