Nuclear carboxylesterase is a lipase involved in lipid-droplets homeostasis

LAGRUTTA, LC.; TREJO, SEBASTIÁN A; VES LOSADA, ANA

Congreso; LVII Reunión Anual de SAIB y el XVI Congreso Anual de la Asociación Civil de Microbiología General (SAMIGE); 2021

SAIB y SAMIGE

In eukaryotic cells under normal conditions, hydrophobic lipids (triacylglycerol: TAG; cholesterol-ester: CE; cholesterol: C) are stored and organized as Lipid Droplets (LD). LD are mainly located in the cytosol (cLD), and within the nucleus we have already identified a small LD population (nLD). nLD consists of a hydrophobic TAG-CE-C core enriched in oleic acid 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. We analyzed the nLD proteome and hypothesized that nLD-monolayer proteins could be involved in lipid metabolism and other cellular functions. We evaluated the rat-liver?nLD proteome under physiological/nonpathological conditions by GeLC-MS2. Since isolated nLD are highly diluted, a protein-concentrating isolation protocol was designed. 35 proteins were identified within the functional categories: cytoskeleton/structural, transcription/translation, histones, protein-folding/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 and immunohistochemistry. Ces1d/Ces3 belongs to the large, highly conserved carboxylesterase multigene superfamily of the carboxyl-esterase hydrolases (EC 3.1.1.1), and is a class of serine hydrolases that catalyze the hydrolysis of esters, thioesters, and amide bonds in a wide variety of molecules. By in silico analyses, the three-dimensional structure predicted for rat Ces1d/Ces3 shows a high similarity with the 3-D structure of the human orthologous CES1, and we propose that the secondary and tertiary structures of rat Ces1d/Ces3 would be similar to human CES1. In this model of the tertiary structure of rat Ces1d/Ces3, potential key residues that would contribute to the catalytic activity, subcellular localization and structure of the protein were identified and localized. In particular, the spatial orientation of those residues that would participate in the catalytic activity (catalytic triad, side door and Z-site) of rat Ces1d/Ces3 correlates almost perfectly with those previously identified in human CES1.In conclusion, a diversity of cellular-protein function was identified indicating the direct or indirect nLD participation and involving Ces1d/Ces3. By in-silico analyses, rat Ces1d/Ces3 secondary and tertiary structure predicted would be equivalent to human CES1. The enzymatic activity of LD carboxylesterase could generate molecules in situ that in addition to being oxidized could constitute lipidic second messengers to regulate the nuclear- and/or cellular-lipid homeostasis.