Nuclear-lipid-droplet proteome: carboxylesterase as a nuclear lipase involved in lipid-droplet homeostasis

Nuclear-lipid droplets (nLD)—a dynamic cellular organelle that stores neutral lipids, within the nucleus of eukaryotic cells—consists of a hydrophobic triacylglycerol –cholesterol-ester core enriched in oleic acid (OA) surrounded by a monolayer of polar lipids, cholesterol, and proteins. nLD are probably involved in nuclear-lipid homeostasis serving as an endonuclear buffer that provides or incorporates lipids and proteins participating in signaling pathways, as transcription factors and enzymes of lipid metabolism and nuclear processes. In the present work, we analyzed the nLD proteome and hypothesized that nLD-monolayer proteins could be involved in processes similar as the ones occurring in the cLD including lipid metabolism and other cellular functions. We evaluated the rat-liver–nLD proteome under physiological and nonpathological conditions by GeLC-MS². Since isolated nLD are highly diluted, a protein-concentrating isolation protocol was designed. Thirty-five proteins were identified within the functional categories: cytoskeleton and structural, transcription and translation, histones, protein-folding and posttranslational modification, cellular proliferation and/or cancer, lipid metabolism, and transport. Purified nLD contained an enzyme from the lipid-metabolism pathway, carboxylesterase 1d (Ces1d/Ces3). Nuclear Carboxylesterase localization was confirmed by Western blotting. By in-silico analyses rat Ces1d/Ces3 secondary and tertiary structure predicted would be equivalent to human CES1. These results—the first nLD proteome—demonstrate that a tandem-GeLC-MS²-analysis protocol facilitates studies like these on rat-liver nuclei. A diversity of cellular-protein function was identified indicating the direct or indirect nLD participation and involving Ces1d/Ces3 in the LD-population homeostasis.

Reversible Nuclear-Lipid-Droplet Morphology Induced by Oleic Acid: A Link to Cellular-Lipid Metabolism

Neutral lipids—involved in many cellular processes—are stored as lipid droplets (LD), those mainly cytosolic (cLD) along with a small nuclear population (nLD). nLD could be involved in nuclear-lipid homeostasis serving as an endonuclear buffering system that would provide or incorporate lipids and proteins involved in signalling pathways as transcription factors and as enzymes of lipid metabolism and nuclear processes. Our aim was to determine if nLD constituted a dynamic domain. Oleic-acid (OA) added to rat hepatocytes or HepG2 cells in culture produced cellular-phenotypic LD modifications: increases in TAG, CE, C, and PL content and in cLD and nLD numbers and sizes. LD increments were reversed on exclusion of OA and were prevented by inhibition of acyl-CoA synthetase (with Triacsin C) and thus lipid biosynthesis. Under all conditions, nLD corresponded to a small population (2–10%) of total cellular LD. The anabolism triggered by OA, involving morphologic and size changes within the cLD and nLD populations, was reversed by a net balance of catabolism, upon eliminating OA. These catabolic processes included lipolysis and the mobilization of hydrolyzed FA from the LD to cytosolic-oxidation sites. These results would imply that nLD are actively involved in nuclear processes that include lipids. In conclusion, nLD are a dynamic nuclear domain since they are modified by OA through a reversible mechanism in combination with cLD; this process involves acyl-CoA-synthetase activity; ongoing TAG, CE, and PL biosynthesis. Thus, liver nLD and cLD are both dynamic cellular organelles.

Nuclear lipid droplets: a novel nuclear domain

We investigated nuclear neutral-lipid (NL) composition and organization, as NL may represent an alternative source for providing fatty acids and cholesterol (C) to membranes, signaling paths, and transcription factors in the nucleus. We show here that nuclear NL were organized into nonpolar domains in the form of nuclear-lipid droplets (nLD). By fluorescent confocal microscopy, representative nLD were observed in situ within the nuclei of rat hepatocytes in vivo and HepG2 cells, maintained under standard conditions in culture, and within nuclei isolated from rat liver. nLD were resistant to Triton X-100 and became stained with Sudan Red, OsO4, and BODIPY493/503. nLD and control cytosolic-lipid droplets (cLD) were isolated from rat-liver nuclei and from homogenates, respectively, by sucrose-gradient sedimentation. Lipids were extracted, separated by thin-layer chromatography, and quantified. nLD were composed of 37% lipids and 63% proteins. The nLD lipid composition was as follows: 19% triacylglycerols (TAG), 39% cholesteryl esters, 27% C, and 15% polar lipids; whereas the cLD composition contained different proportions of these same lipid classes, in particular 91% TAG. The TAG fatty acids from both lipid droplets were enriched in oleic, linoleic, and palmitic acids. The TAG from the nLD corresponded to a small pool, whereas the TAG from the cLD constituted the main cellular pool (at about 100% yield from the total homogenate). In conclusion, nLD are a domain within the nucleus where NL are stored and organized and may be involved in nuclear lipid homeostasis.

Incorporation of arachidonic and stearic acids bound to L-FABP into nuclear and endonuclear lipids from rat liver cells

The incorporation of exogenous fatty acids bound to L-FABP into nuclei was studied. Rat liver cell nuclei and nuclear matrices (membrane depleted nuclei) were incubated in vitro with [1-(14)C]18:0 and 20:4n-6 either free or bound to L-FABP, ATP and CoA. FA esterification in whole nuclei and endonuclear lipids was ATP-CoA-dependent, and with specificity regarding fatty acid type and lipid class. 18:0 and 20:4n-6, free or L-FABP bound, showed the same incorporation and esterification pattern in lipids of whole nuclei. Only 20:4n-6 L-FABP bound was less incorporated into TAG with respect to free 20:4n-6. In the nuclear matrix, 18:0 free or L-FABP bound was esterified with a higher specific activity (SA) into: PtdEtn > PtdIns, PtdSer > PtdCho. 20:4n-6 free or L-FABP bound was esterified into: PtdIns > PtdEtn > PtdCho. 20:4n-6:L-FABP was esterified in endonuclear total-PL and PtdIns with a greater SA with respect to free 20:4n-6 and with a minor one as FFA. To summarize, trafficking of FA to nuclei includes esterification of 18:0 and 20:4n-6 either free or L-FABP-bound, into nuclear and endonuclear lipids by an ATP-CoA-dependent pathway. Endonuclear fatty acid esterification was more active than that in whole nuclei, and independent of the nuclear membrane. Esterification patterns of fatty acids L-FABP-bound or free into whole nuclear lipids were the same whereas in the nuclear matrix, L-FABP could play an important role in the mobilization of 20:4n-6 into specific sites of utilization such as the PtdIns pools.