Lipid Droplet-Derived Monounsaturated Fatty Acids Traffic via PLIN5 to Allosterically Activate SIRT1.

Lipid droplets (LDs) provide a reservoir for triacylglycerol storage and are a central hub for fatty acid trafficking and signaling in cells. Lipolysis promotes mitochondrial biogenesis and oxidative metabolism via a SIRT1/PGC-1α/PPARα-dependent pathway through an unknown mechanism. Herein, we identify that monounsaturated fatty acids (MUFAs) allosterically activate SIRT1 toward select peptide-substrates such as PGC-1α. MUFAs enhance PGC-1α/PPARα signaling and promote oxidative metabolism in cells and animal models in a SIRT1-dependent manner. Moreover, we characterize the LD protein perilipin 5 (PLIN5), which is known to enhance mitochondrial biogenesis and function, to be a fatty-acid-binding protein that preferentially binds LD-derived monounsaturated fatty acids and traffics them to the nucleus following cAMP/PKA-mediated lipolytic stimulation. Thus, these studies identify the first-known endogenous allosteric modulators of SIRT1 and characterize a LD-nuclear signaling axis that underlies the known metabolic benefits of MUFAs and PLIN5.

Understanding the Role of Perilipin 5 in Non-Alcoholic Fatty Liver Disease and Its Role in Hepatocellular Carcinoma: A Review of Novel Insights.

Consumption of high-calorie foods, such as diets rich in fats, is an important factor leading to the development of steatohepatitis. Several studies have suggested how lipid accumulation creates a lipotoxic microenvironment for cells, leading cells to deregulate their transcriptional and translational activity. This deregulation induces the development of liver diseases such as non-alcoholic fatty liver disease (NAFLD) and subsequently also the appearance of hepatocellular carcinoma (HCC) which is one of the deadliest types of cancers worldwide. Understanding its pathology and studying new biomarkers with better specificity in predicting disease prognosis can help in the personalized treatment of the disease. In this setting, understanding the link between NAFLD and HCC progression, the differentiation of each stage in between as well as the mechanisms underlying this process, are vital for development of new treatments and in exploring new therapeutic targets. Perilipins are a family of five closely related proteins expressed on the surface of lipid droplets (LD) in several tissues acting in several pathways involved in lipid metabolism. Recent studies have shown that Plin5 depletion acts protectively in the pathogenesis of liver injury underpinning the importance of pathways associated with PLIN5. PLIN5 expression is involved in pro-inflammatory cytokine regulation and mitochondrial damage, as well as endoplasmic reticulum (ER) stress, making it critical target of the NAFLD-HCC studies. The aim of this review is to dissect the recent findings and functions of PLIN5 in lipid metabolism, metabolic disorders, and NAFLD as well as the progression of NAFLD to HCC.

Lipid Droplets' Role in the Regulation of ß-Cell Function and ß-Cell Demise in Type 2 Diabetes.

During development of type 2 diabetes (T2D), excessive nutritional load is thought to expose pancreatic islets to toxic effects of lipids and reduce ß-cell function and mass. However, lipids also play a positive role in cellular metabolism and function. Thus, proper trafficking of lipids is critical for ß cells to maximize the beneficial effects of these molecules while preventing their toxic effects. Lipid droplets (LDs) are organelles that play an important role in the storage and trafficking of lipids. In this review, we summarize the discovery of LDs in pancreatic ß cells, LD lifecycle, and the effect of LD catabolism on ß-cell insulin secretion. We discuss factors affecting LD formation such as age, cell type, species, and nutrient availability. We then outline published studies targeting critical LD regulators, primarily in rat and human ß-cell models, to understand the molecular effect of LD formation and degradation on ß-cell function and health. Furthermore, based on the abnormal LD accumulation observed in human T2D islets, we discuss the possible role of LDs during the development of ß-cell failure in T2D. Current knowledge indicates that proper formation and clearance of LDs are critical to normal insulin secretion, endoplasmic reticulum homeostasis, and mitochondrial integrity in ß cells. However, it remains unclear whether LDs positively or negatively affect human ß-cell demise in T2D. Thus, we discuss possible research directions to address the knowledge gap regarding the role of LDs in ß-cell failure.

Perilipin 5 Reduces Oxidative Damage Associated With Lipotoxicity by Activating the PI3K/ERK-Mediated Nrf2-ARE Signaling Pathway in INS-1 Pancreatic ß-Cells.

Oxidative stress induced by free fatty acid overload in pancreatic ß-cells is a potential contributory factor to dysfunction of insulin secretion and apoptotic cell death. Perilipin 5 (Plin5) has been reported to ameliorate oxidative stress-mediated damage in non-insulin-secreting tissues. We tested the hypothesis that Plin5 plays a similar role in pancreatic ß-cells, which are extremely sensitive to oxidative stress. Here, our in vitro data showed that Plin5-mediated alleviation of palmitate-triggered apoptosis involves the mitochondrial pathway. And the protective role of Plin5 on ß-cells was partially dependent on its modulation in oxidative stress. Upregulation of Plin5 in INS-1 cells decreased reactive oxygen species production, enhanced cellular glutathione levels, and induced expression of antioxidant enzymes glutamate-cysteine ligase catalytic subunit and heme oxygenase-1. However, knocking out of Plin5 abolished all of these beneficial effects. Furthermore, by using the O2- scavenger MnTMPyP, we verified that altering Plin5 expression impacted lipotoxic cell death partially via modulating oxidative stress. Mechanistic experiments revealed that Plin5 induced Nrf2-ARE system, a master regulator in the cellular adaptive response to oxidative stress, by activating PI3K/Akt and ERK signal pathways, contributing to the increase of antioxidant defense and consequently improving ß-cell function and survival in the presence of lipotoxic oxidative stress. Overall, our findings indicate that Plin5 abrogates oxidative damage in INS-1 ß-cells during lipotoxic stress partially through the enhancement of antioxidant defense involving the PI3K/Akt and ERK mediated Nrf2-ARE system.

Targeting PLIN2/PLIN5-PPARγ: Sulforaphane Disturbs the Maturation of Lipid Droplets.

SCOPE: The effects of sulforaphane (SFN) on the maturation of lipid droplets (LDs)-the storage units for free fatty acids and sterols as triacylglycerides (TAG) and cholesterol esters (CE)-are far from being understood, despite the fact that SFN is known to be beneficial for ameliorating lipid metabolism disorders. METHODS AND RESULTS: High-fat-intake models are established in both HHL-5 hepatocytes and rodents. The numbers and sizes of LDs are decreased by SFN. The accumulation of lipid core components (TAG & CE) is reduced and the expression of their key synthetases, acyl-coenzyme A: diacylglycerol acyltransferases 2 (DGAT2) and acyl-coenzyme A: cholesterol acyltransferases 1 (ACAT1), is also inhibited. Moreover, SFN decreases LD-associated protein PLIN2 and PLIN5 expression, but not that of PLIN1 and PLIN3, both in vivo and in vitro. Furthermore, over-expression of peroxisome proliferator-activated receptor gamma (PPARγ) induces the accumulation of TAG and the up-regulation of PLIN2 and PLIN5, which are not reversed by SFN. These results suggest that PPARγ may be a target of SFN in lipid metabolism. CONCLUSION: SFN disturbs LD maturation by inhibiting the formation of the neutral lipid core and decreases PLIN2 and PLIN5 via down-regulation of PPARγ.