The Proteome of Cholesteryl-Ester-Enriched Versus Triacylglycerol-Enriched Lipid Droplets

Abstract

Within cells, lipids are stored in the form of lipid droplets (LDs), consisting of a neutral lipid core, surrounded by a phospholipid monolayer and an outer layer of protein. LDs typically accumulate either triacylglycerol (TAG) and diacylglycerol or cholesteryl ester (CE), depending on the type of tissue. Recently, there has been an increased interest in the proteins that surround LDs. LD proteins have been found to be quite diverse, from structural proteins to metabolic enzymes, proteins involved in vesicular transport, and proteins that may play a role in LD formation. Previous proteomics analyses have focused on TAG-enriched LDs, whereas CE-enriched LDs have been largely ignored. Our study has compared the LD proteins from CE-enriched LDs to TAG-enriched LDs in steroidogenic cells. In primary rat granulosa cells loaded with either HDL to produce CE-enriched LDs or fatty acids to produce TAG-enriched LDs, 61 proteins were found to be elevated in CE-enriched LDs and 40 proteins elevated in TAG-enriched LDs with 278 proteins in similar amounts. Protein expression was further validated by selected reaction monitoring (SRM) mass spectrometry (MS). SRM verified expression of 25 of 27 peptides that were previously detected by tandem mass tagging MS. Several proteins were confirmed to be elevated in CE-enriched LDs by SRM including the intermediate filament vimentin. This study is the first to compare the proteins found on CE-enriched LDs with TAG-enriched LDs and constitutes the first step in creating a better understanding of the proteins found on CE-enriched LDs in steroidogenic cells.

Dephosphorylation of MAP2D enhances its binding to vimentin in preovulatory ovarian granulosa cells

“…Lipid droplets from adrenal cells and adipocytes are surrounded by a vimentin-enriched phospholipid monolayer (Almahbobi et al, 1992;Brasaemle et al, 2004;Franke et al, 1987;Lieber and Evans, 1996). A recent report comparing the proteome of cholesterol-ester-versus triacylglycerolenriched lipid droplets from rat granulosa cells demonstrated the enrichment of a number of proteins in cholesterol-ester-enriched lipid droplets that included vimentin (4.92-fold enrichment; Khor et al, 2014). Moreover, the genetic ablation of vimentin resulted in a 70% decrease in hCG-stimulated progesterone production, compared to wild-type mice, by preovulatory granulosa cells (Shen et al, 2012), although the mice remain fertile (Colucci-Guyon et al, 1994).…”

The why, when and how of lipid droplet diversity

“…In terms of the lipid-dropletassociated proteins, a core set of about 50 proteins has been found to decorate lipid droplets in various cellular systems (reviewed in Hodges and Wu, 2010). However, a diverse and large number of additional proteins has been identified across many studies (a selection of studies is given in Beilstein et al, 2013;Beller et al, 2006;Cermelli et al, 2006;Chen et al, 2016;Currie et al, 2014;Dahlhoff et al, 2015;Ivashov et al, 2013;Khan et al, 2015;Khor et al, 2014;Krahmer et al, 2013;Li et al, 2016;Na et al, 2013;Schmidt et al, 2013;Su et al, 2014;Zhang et al, 2011;Zhu et al, 2015), suggesting a significant amount of variability in terms of the lipid droplet protein composition between lipid droplets isolated from different cell types.…”

Lipid droplets and steroidogenic cells

“…While the intracellular LDs in adipocytes, liver, and muscle cells consist primarily of TAG and diacylglycerol (DAG), the LDs in steroidogenic cells, such as ovarian granulosa and adrenocortical cells, as well as testicular Leydig cells, primarily accumulate CE (3–5). Over the past decade, the employment of genetic and proteomic approaches have allowed the identification of many proteins on the LD surface and have provided clues to their function in cellular and LD physiology (6–10). Here we aim at providing a review of the findings related to LD function in steroidogenic cells.…”