Cell Biology Symposium: imaging the organization and trafficking of lipolytic effectors in adipocytes

Vacuolar protein sorting 13C is a novel lipid droplet protein that inhibits lipolysis in brown adipocytes

Objective

Brown adipose tissue (BAT) thermogenesis depends on the mobilization and oxidation of fatty acids from intracellular lipid droplets (LD) within brown adipocytes (BAs); however, the identity and function of LD proteins that control BAT lipolysis remain incomplete. Proteomic analysis of mouse BAT subcellular fractions identified vacuolar protein sorting 13C (VPS13C) as a novel LD protein. The aim of this work was to investigate the role of VPS13C on BA LDs.

Methods

Biochemical fractionation and high resolution confocal and immuno-transmission electron microscopy (TEM) were used to determine the subcellular distribution of VPS13C in mouse BAT, white adipose tissue, and BA cell culture. Lentivirus-delivered shRNA was used to determine the role of VPS13C in regulating lipolysis and gene expression in cultured BA cells.

Results

We found that VPS13C is highly expressed in mouse BAT where it is targeted to multilocular LDs in a subspherical subdomain. In inguinal white adipocytes, VPS13C was mainly observed on small LDs and β3-adrenergic stimulation increased VPS13C in this depot. Silencing of VPS13C in cultured BAs decreased LD size and triglyceride content, increased basal free fatty acid release, augmented the expression of thermogenic genes, and enhanced the lipolytic potency and efficacy of isoproterenol. Mechanistically, we found that BA lipolysis required activation of adipose tissue triglyceride lipase (ATGL) and that loss of VPS13C greatly increased the association of ATGL to LDs.

Conclusions

VPS13C is present on BA LDs where is targeted to a distinct subdomain. VPS13C limits the access of ATGL to LD and loss of VPS13C elevates lipolysis and promotes oxidative gene expression.

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VPS13C is highly expressed on mouse brown adipose tissue lipid droplets.

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VPS13C is present on lipid droplets in a unique subspheric pattern.

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VPS13C silencing in BAs decreases lipid droplet size and triglyceride content.

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VPS13C silencing in BAs increases lipolysis and oxidative gene expression.

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VPS13C silencing in BAs enhances ATGL but not HSL targeting to LDs.

Mature adipocytes in bone marrow protect myeloma cells against chemotherapy through autophagy activation

A major problem in patients with multiple myeloma is chemotherapy resistance, which develops in myeloma cells upon interaction with bone marrow stromal cells. However, few studies have determined the role of bone marrow adipocytes, a major component of stromal cells in the bone marrow, in myeloma chemotherapy resistance. We reveal that mature human adipocytes activate autophagy and upregulate the expression of autophagic proteins, thereby suppressing chemotherapy-induced caspase cleavage and apoptosis in myeloma cells. We found that adipocytes secreted known and novel adipokines, such as leptin and adipsin. The addition of these adipokines enhanced the expression of autophagic proteins and reduced apoptosis in myeloma cells. In vivo studies further demonstrated the importance of bone marrow-derived adipocytes in the reduced response of myeloma cells to chemotherapy. Our findings suggest that adipocytes, adipocyte-secreted adipokines, and adipocyte-activated autophagy are novel targets for combatting chemotherapy resistance and enhancing treatment efficacy in myeloma patients.

Characterization of the bovine gene LIPE and possible influence on fatty acid composition of meat

LIPE is an intracellular neutral lipase, which is capable of hydrolyzing a variety of esters and plays a key role in the mobilization of fatty acids from diacylglycerols. The objectives of this study were to characterize the genetic polymorphism of bovine LIPE gene and to evaluate the possible association between three SNPs in the coding regions of this gene with the fatty acid composition of meat in a cattle population. Forty-three unrelated animals from different cattle breeds were re-sequenced and 21 SNPs were detected over approximately 2600 bp, five of these SNPs were novel. Three SNPs were selected, on the basis of evolutionary conservation, to perform validation and association studies in a crossbred cattle population. Our results may suggest a possible association of SNP1 with contents of oleic acid and total monounsaturated fatty acids (p < 0.01), and SNP2 and SNP3 with Heneicosylic acid content (p < 0.01), may be helpful to improve the quality of meat and improve health.

Adipose depots differ in cellularity, adipokines produced, gene expression, and cell systems

The race to manage the health concerns related to excess fat deposition has spawned a proliferation of clinical and basic research efforts to understand variables including dietary uptake, metabolism, and lipid deposition by adipocytes. A full appreciation of these variables must also include a depot-specific understanding of content and location in order to elucidate mechanisms governing cellular development and regulation of fat deposition. Because adipose tissue depots contain various cell types, differences in the cellularity among and within adipose depots are presently being documented to ascertain functional differences. This has led to the possibility of there being, within any one adipose depot, cellular distinctions that essentially result in adipose depots within depots. The papers comprising this issue will underscore numerous differences in cellularity (development, histogenesis, growth, metabolic function, regulation) of different adipose depots. Such information is useful in deciphering adipose depot involvement both in normal physiology and in pathology. Obesity, diabetes, metabolic syndrome, carcass composition of meat animals, performance of elite athletes, physiology/pathophysiology of aging, and numerous other diseases might be altered with a greater understanding of adipose depots and the cells that comprise them-including stem cells-during initial development and subsequent periods of normal/abnormal growth into senescence. Once thought to be dormant and innocuous, the adipocyte is emerging as a dynamic and influential cell and research will continue to identify complex physiologic regulation of processes involved in adipose depot physiology.

Identification of diverse lipid droplet targeting motifs in the PNPLA family of triglyceride lipases

Members of the Patatin-like Phospholipase Domain containing Protein A (PNPLA) family play key roles in triglyceride hydrolysis, energy metabolism, and lipid droplet (LD) homoeostasis. Here we report the identification of two distinct LD targeting motifs (LTM) for PNPLA family members. Transient transfection of truncated versions of human adipose triglyceride lipase (ATGL, also known as PNPLA2), PNPLA3/adiponutrin, or PNPLA5 (GS2-like) fused to GFP revealed that the C-terminal third of these proteins contains sequences that are sufficient for targeting to LDs. Furthermore, fusing the C-termini of PNPLA3 or PNPLA5 confers LD localization to PNPLA4, which is otherwise cytoplasmic. Analyses of additional mutants in ATGL, PNPLA5, and Brummer Lipase, the Drosophila homolog of mammalian ATGL, identified two different types of LTMs. The first type, in PNPLA5 and Brummer lipase, is a set of loosely conserved basic residues, while the second type, in ATGL, is contained within a stretch of hydrophobic residues. These results show that even closely related members of the PNPLA family employ different molecular motifs to associate with LDs.