Perilipin 5 protects the mitochondrial oxidative functions and improves the alcoholic liver injury in mice.

BACKGROUND AND AIMS: Alcohol consumption is a well-established risk factor for the onset and progression of hepatic steatosis. Perilipin 5 (Plin5), a lipid droplet protein, is an important protective factor against hepatic lipotoxicity induced by excessive lipolysis, but its role and molecular mechanism in alcoholic liver disease (ALD) are not fully elucidated. METHODS: The optimized National Institute on Alcohol Abuse and Alcoholism model was used to construct ALD model mice. Automatic biochemical analyser was used for Biochemical Parameters. The primary hepatocytes and Plin5-overexpressed HepG2 cells (including full-length Plin5 and Plin5 deleting 444-464 aa) were used for in vitro experiment. Haematoxylin and Eosin staining, Oil Red O staining, Bodipy 493/503 staining, Periodic Acid-Schiff staining, immunohistochemistry and JC-1 staining were used to evaluate cell morphology, lipids, glycogen, inflammation and membrane potential. Commercially kits are used to detect glycolipid metabolites, such as triglycerides, glycogen, glucose, reactive oxygen species, lactic acids, ketone bodies. Fluorescently labelled deoxyglucose, NBDG, was used for glucose intake. An XF96 extracellular flux analyser was used to determinate oxygen consumption rate in hepatocytes. The morphological and structural damage of mitochondria was evaluated by electron microscopy. Classical ultracentrifugation is used to separate the subcellular organelles of tissues and cells. Immunoblotting and qPCR were used to detect changes in mRNA and protein levels of related genes. RESULTS: Our results showed that the expression of Plin5 in mouse livers was enhanced by alcohol intake, and Plin5 deficiency aggravated the alcohol-induced liver injury. To clarify the mechanism, we found that Plin5 deficiency significantly elevated the hepatic NADH levels and ketone body production in the alcohol-treated mice. As NADH elevation could promote the reduction of pyruvate into lactate and then inhibit the gluconeogenesis, alcohol-treated Plin5-deficient mice exhibited more lactate production and severer hypoglycemia. These results implied that Plin5 deficiency impaired the mitochondrial oxidative functions in the presence of alcohol. In addition, we demonstrated that Plin5 could be recruited onto mitochondria by alcohol, while Plin5 without mitochondrial targeting sequences lost its mitochondrial protection functions. CONCLUSION: Collectively, this study demonstrated that the mitochondrial Plin5 could protect the alcohol-induced mitochondrial injury, which provides an important new insight on the roles of Plin5 in highly oxidative tissues.

Perilipin 5 deficiency aggravates cardiac hypertrophy by stimulating lactate production in leptin-deficient mice.

BACKGROUND: Perilipin 5 (Plin5) is well known to maintain the stability of intracellular lipid droplets (LDs) and regulate fatty acid metabolism in oxidative tissues. It is highly expressed in the heart, but its roles have yet to be fully elucidated. METHODS: Plin5-deficient mice and Plin5/leptin-double-knockout mice were produced, and their histological structures and myocardial functions were observed. Critical proteins related to fatty acid and glucose metabolism were measured in heart tissues, neonatal mouse cardiomyocytes and Plin5-overexpressing H9C2 cells. 2-NBDG was employed to detect glucose uptake. The mitochondria and lipid contents were observed by MitoTracker and BODIPY 493/503 staining in neonatal mouse cardiomyocytes. RESULTS: Plin5 deficiency impaired glucose utilization and caused insulin resistance in mouse cardiomyocytes, particularly in the presence of fatty acids (FAs). Additionally, Plin5 deficiency increased the NADH content and elevated the expression of lactate dehydrogenase (LDHA) in cardiomyocytes, which resulted in increased lactate production. Moreover, when fatty acid oxidation was blocked by etomoxir or LDHA was inhibited by GSK2837808A in Plin5-deficient cardiomyocytes, glucose utilization was improved. Leptin-deficient mice exhibited myocardial hypertrophy, insulin resistance and altered substrate utilization, and Plin5 deficiency exacerbated myocardial hypertrophy in leptin-deficient mice. CONCLUSION: Our results demonstrated that Plin5 plays a critical role in coordinating fatty acid and glucose oxidation in cardiomyocytes, providing a potential target for the treatment of metabolic disorders in the heart.

Effect of an acute long-duration exercise bout on skeletal muscle lipid droplet morphology, GLUT 4 protein, and perilipin protein expression.

PURPOSE: Smaller lipid droplet morphology and GLUT 4 protein expression have been associated with greater muscle oxidative capacity and glucose uptake, respectively. The main purpose of this study was to determine the effect of an acute long-duration exercise bout on skeletal muscle lipid droplet morphology, GLUT4, perilipin 3, and perilipin 5 expressions. METHODS: Twenty healthy men (age 24.0 ± 1.0 years, BMI 23.6 ± 0.4 kg/m2) were recruited for the study. The participants were subjected to an acute bout of exercise on a cycle ergometer at 50% VO2max until they reached a total energy expenditure of 650 kcal. The study was conducted after an overnight fast. Vastus lateralis muscle biopsies were obtained before and immediately after exercise for immunohistochemical analysis to determine lipid, perilipin 3, perilipin 5, and GLUT4 protein contents while GLUT 4 mRNA was quantified using RT-qPCR. RESULTS: Lipid droplet size decreased whereas total intramyocellular lipid content tended to reduce (p = 0.07) after an acute bout of endurance exercise. The density of smaller lipid droplets in the peripheral sarcoplasmic region significantly increased (0.584 ± 0.04 to 0.638 ± 0.08 AU; p = 0.01) while larger lipid droplets significantly decreased (p < 0.05). GLUT4 mRNA tended to increase (p = 0.05). There were no significant changes in GLUT 4, perilipin 3, and perilipin 5 protein levels. CONCLUSION: The study demonstrates that exercise may impact metabolism by enhancing the quantity of smaller lipid droplets over larger lipid droplets.

PLIN5 interacts with FATP4 at membrane contact sites to promote lipid droplet-to-mitochondria fatty acid transport.

Cells adjust their metabolism by remodeling membrane contact sites that channel metabolites to different fates. Lipid droplet (LD)-mitochondria contacts change in response to fasting, cold exposure, and exercise. However, their function and mechanism of formation have remained controversial. We focused on perilipin 5 (PLIN5), an LD protein that tethers mitochondria, to probe the function and regulation of LD-mitochondria contacts. We demonstrate that efficient LD-to-mitochondria fatty acid (FA) trafficking and ß-oxidation during starvation of myoblasts are promoted by phosphorylation of PLIN5 and require an intact PLIN5 mitochondrial tethering domain. Using human and murine cells, we further identified the acyl-CoA synthetase, FATP4 (ACSVL4), as a mitochondrial interactor of PLIN5. The C-terminal domains of PLIN5 and FATP4 constitute a minimal protein interaction capable of inducing organelle contacts. Our work suggests that starvation leads to phosphorylation of PLIN5, lipolysis, and subsequent channeling of FAs from LDs to FATP4 on mitochondria for conversion to fatty-acyl-CoAs and subsequent oxidation.

Adverse cardiac remodeling augments adipose tissue ß-adrenergic signaling and lipolysis counteracting diet-induced obesity.

Cardiac triacylglycerol accumulation is a common characteristic of obesity and type 2 diabetes and strongly correlates with heart morbidity and mortality. We have previously shown that cardiomyocyte-specific perilipin 5 overexpression (Plin5-Tg) provokes significant cardiac steatosis via lowering cardiac lipolysis and fatty acid (FA) oxidation. In strong contrast to cardiac steatosis and lethal heart dysfunction in adipose triglyceride lipase deficiency, Plin5-Tg mice do not develop heart dysfunction and show a normal life span on chow diet. This finding prompted us to study heart function and energy metabolism in Plin5-Tg mice fed high-fat diet (HFD). Plin5-Tg mice showed adverse cardiac remodeling on HFD with heart function only being compromised in one-year-old mice, likely due to reduced cardiac FA uptake, thereby delaying deleterious cardiac lipotoxicity. Notably, Plin5-Tg mice were less obese and protected from glucose intolerance on HFD. Changes in cardiac energy catabolism in Plin5-Tg mice increased ß-adrenergic signaling, lipolytic, and thermogenic protein expression in adipose tissue ultimately counteracting HFD-induced obesity. Acute cold exposure further augmented ß-adrenergic signaling in Plin5-Tg mice, whereas housing at thermoneutrality did not protect Plin5-Tg mice from HFD-induced obesity albeit blood glucose and insulin levels remained low in transgenic mice. Overall, our data suggest that the limited capacity for myocardial FA oxidation on HFD increases cardiac stress in Plin5-Tg mice, thereby stimulating adipose tissue ß-adrenergic signaling, triacylglycerol catabolism, and thermogenesis. However, long-term HFD-mediated metabolic stress causes contractile dysfunction in Plin5-Tg mice, which emphasizes the importance of a carefully controlled dietary regime in patients with cardiac steatosis and hypertrophy.

Lipid-Independent Regulation of PLIN5 via IL-6 through the JAK/STAT3 Axis in Hep3B Cells.

Perilipin 5 (PLIN5) is a lipid droplet coat protein that is highly expressed in oxidative tissues such as those of muscles, the heart and the liver. PLIN5 expression is regulated by a family of peroxisome proliferator-activated receptors (PPARs) and modulated by the cellular lipid status. So far, research has focused on the role of PLIN5 in the context of non-alcoholic fatty liver disease (NAFLD) and specifically in lipid droplet formation and lipolysis, where PLIN5 serves as a regulator of lipid metabolism. In addition, there are only limited studies connecting PLIN5 to hepatocellular carcinoma (HCC), where PLIN5 expression is proven to be upregulated in hepatic tissue. Considering that HCC development is highly driven by cytokines present throughout NAFLD development and in the tumor microenvironment, we here explore the possible regulation of PLIN5 by cytokines known to be involved in HCC and NAFLD progression. We demonstrate that PLIN5 expression is strongly induced by interleukin-6 (IL-6) in a dose- and time-dependent manner in Hep3B cells. Moreover, IL-6-dependent PLIN5 upregulation is mediated by the JAK/STAT3 signaling pathway, which can be blocked by transforming growth factor-ß (TGF-ß) and tumor necrosis factor-α (TNF-α). Furthermore, IL-6-mediated PLIN5 upregulation changes when IL-6 trans-signaling is stimulated through the addition of soluble IL-6R. In sum, this study sheds light on lipid-independent regulation of PLIN5 expression in the liver, making PLIN5 a crucial target for NAFLD-induced HCC.

Endocrine modulation of brain-skeleton axis driven by neural stem cell-derived perilipin 5 in the lipid metabolism homeostasis for bone regeneration.

Factors released from the nervous system always play crucial roles in modulating bone metabolism and regeneration. How the brain-driven endocrine axes maintain bone homeostasis, especially under metabolic disorders, remains obscure. Here, we found that neural stem cells (NSCs) residing in the subventricular zone participated in lipid metabolism homeostasis of regenerative bone through exosomal perilipin 5 (PLIN5). Fluorescence-labeled exosomes tracing and histological detection identified that NSC-derived exosomes (NSC-Exo) could travel from the lateral ventricle into bone injury sites. Homocysteine (Hcy) led to osteogenic and angiogenic impairment, whereas the NSC-Exo were confirmed to restore it. Mecobalamin, a clinically used neurotrophic drug, further enhanced the protective effects of NSC-Exo through increased PLIN5 expression. Mechanistically, NSC-derived PLIN5 reversed excessive Hcy-induced lipid metabolic imbalance and aberrant lipid droplet accumulation through lipophagy-dependent intracellular lipolysis. Intracerebroventricular administration of mecobalamin and/or AAV-shPlin5 confirmed the effects of PLIN5-driven endocrine modulations on new bone formation and vascular reconstruction in hyperhomocysteinemic and high-fat diet models. This study uncovered a novel brain-skeleton axis that NSCs in the mammalian brain modulated bone regeneration through PLIN5-driven lipid metabolism modulation, providing evidence for lipid- or bone-targeted medicine development.

Cardiac Plin5 interacts with SERCA2 and promotes calcium handling and cardiomyocyte contractility.

The adult heart develops hypertrophy to reduce ventricular wall stress and maintain cardiac function in response to an increased workload. Although pathological hypertrophy generally progresses to heart failure, physiological hypertrophy may be cardioprotective. Cardiac-specific overexpression of the lipid-droplet protein perilipin 5 (Plin5) promotes cardiac hypertrophy, but it is unclear whether this response is beneficial. We analyzed RNA-sequencing data from human left ventricle and showed that cardiac PLIN5 expression correlates with up-regulation of cardiac contraction-related processes. To investigate how elevated cardiac Plin5 levels affect cardiac contractility, we generated mice with cardiac-specific overexpression of Plin5 (MHC-Plin5 mice). These mice displayed increased left ventricular mass and cardiomyocyte size but preserved heart function. Quantitative proteomics identified sarcoplasmic/endoplasmic reticulum Ca2+ ATPase 2 (SERCA2) as a Plin5-interacting protein. In situ proximity ligation assay further confirmed the Plin5/SERCA2 interaction. Live imaging showed increases in intracellular Ca2+ release during contraction, Ca2+ removal during relaxation, and SERCA2 function in MHC-Plin5 versus WT cardiomyocytes. These results identify a role of Plin5 in improving cardiac contractility through enhanced Ca2+ signaling.

Hepatic PLIN5 Deficiency Impairs Lipogenesis through Mitochondrial Dysfunction.

Regulation of lipid droplets (LDs) metabolism is the core of controlling intracellular fatty acids (FAs) fluxes, and perilipin 5 (PLIN5) plays a key role in this process. Our previous studies have found that hepatic PLIN5 deficiency reduces LDs accumulation, but the trafficking of FAs produced from this pathway and the interaction between mitochondria and LDs in this process are largely unknown. Here, we found that the deficiency of PLIN5 decreases LDs accumulation by increasing FAs efflux. In addition, the decreased lipogenesis of PLIN5-deficient hepatocytes is accompanied by mitochondrial dysfunction, suggesting that PLIN5 plays an important role in mediating the interaction between LDs and mitochondria. Importantly, PLIN5 ablation negates oxidative capacity differences of peri-droplet and cytosolic mitochondria. In summary, these data indicate that PLIN5 plays a vital role in maintaining mitochondrial-mediated lipogenesis, which provides an important new perspective on the regulation of liver lipid storage and the relationship between PLIN5 and mitochondria.

Effects of perilipin-5 on lipid metabolism and high-sensitivity cardiac troponin I.

OBJECTIVE: Heart attack is one of the most common causes of sudden death in adults. Therefore, early detection of heart attack and investigation of potential new biomarkers are of great importance. We investigated whether perilipin-5 is a potential biomarker by examining changes in perilipin-5 serum levels along with high-sensitivity cardiac troponin I during a heart attack. METHODS: The subjects were divided into two groups: (1) control group and (2) patients with heart attack, with 150 people in each group. High-sensitivity cardiac troponin I, perilipin-5, total oxidant status, malondialdehyde, reduced glutathione, and superoxide dismutase levels in serum samples were measured. In addition, perilipin-5 mRNA expressions and protein levels were analyzed. RESULTS: There was no overall statistical difference between the demographic characteristics of the groups. However, high-density lipoprotein, creatine kinase, Creatine kinase myocardial band, aspartate amino transferase, lactate dehydrogenase, and calcium levels were higher in the heart attack group compared to the control group. We found that the high-sensitivity cardiac troponin I and perilipin-5 levels increased in the patients with heart attack (p<0.0001) compared to control. Although there was an insignificant increase in malondialdehyde levels in the heart attack group (p>0.05), there was a 35.9% increase in total oxidant status levels and a 33.5 and 24.1% decrease in glutathione and superoxide dismutase levels, respectively (p<0.01), compared to control. Perilipin-5 mRNA and protein levels in heart attack patients increased by 48.2 and 23.6%, respectively, compared to the control group (p<0.01). CONCLUSION: Our results showed that perilipin-5 together with high-sensitivity cardiac troponin I could be a promising biomarker in heart attack.

Zeaxanthin remodels cytoplasmic lipid droplets via ß3-adrenergic receptor signaling and enhances perilipin 5-mediated lipid droplet-mitochondrion interactions in adipocytes.

Cytoplasmic lipid droplets (LDs), which are remarkably dynamic, neutral lipid storage organelles, play fundamental roles in lipid metabolism and energy homeostasis. Both the dynamic remodeling of LDs and LD-mitochondrion interactions in adipocytes are effective mechanisms to ameliorate obesity and related comorbidities. Zeaxanthin (ZEA) is a natural carotenoid and has beneficial effects on anti-obesity. However, the underlying mechanisms of ZEA on LD modulation are still unclear. In the present study, ZEA efficiently inhibited LD accumulation and attenuated adipocyte proliferation by arresting the cell cycle. ZEA drove transcriptional alterations to reprogram a lipid oxidative metabolism phenotype in mature 3T3-L1 adipocytes. ZEA significantly decreased the TAG and FA content and modulated the dynamic alterations of LDs by upregulating the expression of lipases and the LD-mitochondrion contact site protein, perilipin 5 (PLIN5), and downregulating the LD fusion protein, fat-specific protein 27 (FSP27). Mechanistically, ZEA stimulated LD remodeling and ameliorated mitochondrial defects caused by large and unilocular LD accumulation by activating ß3-adrenergic receptor (ß3-AR) signaling. Furthermore, the knockdown of PLIN5 impaired the LD-mitochondrion interactions, thereby disrupting the role of ZEA in promoting mitochondrial fatty acid oxidation and respiratory chain operation. Collectively, the present study demonstrates that ZEA induces LD structural and metabolic remodeling by activating ß3-AR signaling and enhances PLIN5-mediated LD-mitochondrion interactions in hypertrophic white adipocytes, thereby enhancing oxidative capacity, and has the potential as a nutritional intervention for the prevention and treatment of obesity and associated metabolic syndrome.

Plin5, a New Target in Diabetic Cardiomyopathy.

Abnormal lipid accumulation is commonly observed in diabetic cardiomyopathy (DC), which can create a lipotoxic microenvironment and damage cardiomyocytes. Lipid toxicity is an important pathogenic factor due to abnormal lipid accumulation in DC. As a lipid droplet (LD) decomposition barrier, Plin5 can protect LDs from lipase decomposition and regulate lipid metabolism, which is involved in the occurrence and development of cardiovascular diseases. In recent years, studies have shown that Plin5 expression is involved in the pathogenesis of DC lipid toxicity, such as oxidative stress, mitochondrial dysfunction, endoplasmic reticulum (ER) stress, and insulin resistance (IR) and has become a key target of DC research. Therefore, understanding the relationship between Plin5 and DC progression as well as the mechanism of this process is crucial for developing new therapeutic approaches and exploring new therapeutic targets. This review is aimed at exploring the latest findings and roles of Plin5 in lipid metabolism and DC-related pathogenesis, to explore possible clinical intervention approaches.

Plin5 inhibits proliferation and migration of vascular smooth muscle cell through interacting with PGC-1α following vascular injury.

Abnormal proliferation and migration of vascular smooth muscle cell (VSMC) is a hallmark of vascular neointima hyperplasia. Perilipin 5 (Plin5), a regulator of lipid metabolism, is also confirmed to be involved in vascular disorders, such as microvascular endothelial dysfunction and atherosclerosis. To investigate the regulation and function of plin5 in the phenotypic alteration of VSMC, -an animal model of vascular intima hyperplasia was established in C57BL/6 J and Plin5 knockdown (Plin5±) mice by wire injure. Immunohistochemical staining was used to analyze neointima hyperplasia in artery. Ki-67, dihydroethidium immunofluorescence staining and wound healing assay were used to measure proliferation, reactive oxygen species (ROS) generation and migration of VSMC, respectively. Plin5 was downregulated in artery subjected to vascular injury and in VSMC subjected to platelet-derived growth factor (PDGF)-BB. Plin5 knockdown led to accelerated neointima hyperplasia, excessive proliferation and migration of VSMC after injury. In vitro, we observed increased ROS content in VSMC isolated from Plin5± mice. Antioxidative N-acetylcysteine (NAC) inhibited VSMC proliferation and migration induced by PDGF-BB or plin5 knockdown. More importantly, plin5-peroxlsome proliferator-activated receptor-γ coactivator (PGC)-1α interaction was also attenuated in VSMC after knockdown of plin5. Overexpression of PGC-1α suppressed PDGF-BB-induced ROS generation, proliferation, and migration in VSMC isolated from Plin5± mice. These data suggest that plin5 serves as a potent regulator of VSMC proliferation, migration, and neointima hyperplasia by interacting with PGC-1α and affecting ROS generation.

Plin5 Bidirectionally Regulates Lipid Metabolism in Oxidative Tissues.

Cytoplasmic lipid droplets (LDs) can store neutral lipids as an energy source when needed and also regulate the key metabolic processes of intracellular lipid accumulation, which is associated with several metabolic diseases. The perilipins (Plins) are a family of proteins that associate with the surface of LDs. As a member of Plins superfamily, perilipin 5 (Plin5) coats LDs in cardiomyocytes, which is significantly related to reactive oxygen species (ROS) production originated from mitochondria in the heart, consequently determining the progression of diabetic cardiomyopathy. Plin5 may play a bidirectional function in lipid metabolism which is in a state of dynamic balance. In the basic state, Plin5 inhibited the binding of comparative gene identification-58 (CGI-58) to adipose triglyceride lipase (ATGL) by binding CGI-58, thus inhibiting lipolysis. However, when the body is under stress (such as cold, fasting, exercise, and other stimuli), protein kinase A (PKA) phosphorylates and activates Plin5, which then causes Plin5 to release the binding site of CGI-58 and ATGL, prompting CGI-58 to bind to ATGL and activate ATGL activity, thus accelerating the lipolysis process, revealing the indispensable role of Plin5 in lipid turnover. Here, the purpose of this review is to summarize the present understanding of the bidirectional regulation role of Plin5 in oxidative tissues and to reveal its potential role in diabetic cardiomyopathy protection.

Perilipin 5 protects against oxygen-glucose deprivation/reoxygenation-elicited neuronal damage by inhibiting oxidative stress and inflammatory injury via the Akt-GSK-3ß-Nrf2 pathway.

BACKGROUND: Perilipin 5 (Plin5) acts as a pivotal mediator of oxidative stress and inflammation and is associated with the progression of relevant diseases. Cerebral ischemic stroke is a severe pathological condition that involves excess oxidative stress and inflammation. However, whether Plin5 plays a role in the progression of cerebral ischemic stroke remains unaddressed. This work focused on the investigation of Plin5 in oxygen-glucose deprivation/reoxygenation (OGD/R)-injured neurons, an in vitro model for studying cerebral ischemic stroke. METHODS: The primary neuronal cells were isolated from the hippocampus of newborn mice. Neurons were subjected to OGD/R treatment to establish an in vitro model for studying cerebral ischemic stroke. Neurons were infected with recombinant adenovirus expressing Plin5 to upregulate Plin5 expression. The mRNA levels were measured by real-time quantitative PCR (RT-qPCR). Protein levels were determined by immunoblotting. Cell viability was assessed via cell counting kit-8 (CCK-8) assay. Cell apoptosis was evaluated via terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) and Annexin V-Allophycocyanin/7-Amino Actinomycin D (Annexin V-APC/7-AAD) apoptotic assays. Oxidative stress was monitored by dichlorofluorescein diacetate (DCFH-DA) probe. Inflammatory cytokine release was detected by enzyme-linked immunosorbent assay (ELISA). RESULTS: A decreased level of Plin5 was observed in neurons challenged with OGD/R. Plin5 overexpression remarkably subdued OGD/R-elicited apoptosis, oxidative stress and proinflammatory response. Plin5 overexpression led to an enhancement of the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway associated with regulation of the Akt-glycogen synthase kinase-3ß (GSK-3ß). The blocking of Akt was able to reverse the enhancing effect of Plin5 on Nrf2 activation. The restraining of Akt or silencing of Nrf2 diminished the protective effects of Plin5 in OGD/R-injured neurons. CONCLUSIONS: Plin5 confers neuroprotection for neurons against OGD/R damage via effects on the Nrf2-Akt-GSK-3ß pathway. This work indicates a possible role of Plin5 in cerebral ischemic stroke and the up-regulation of Plin5 is a sort of survival strategy for neurons suffering from ischemic injury.

Enlarged PLIN5-uncoated lipid droplets in inner regions of skeletal muscle type II fibers associate with type 2 diabetes.

Skeletal muscle physiology remains of paramount importance in understanding insulin resistance. Due to its high lipid turnover rates, regulation of intramyocellular lipid droplets (LDs) is a key factor. Perilipin 5 (PLIN5) is one of the most critical agents in such regulation, being often referred as a protector against lipotoxicity and consequent skeletal muscle insulin resistance. We examined area fraction, size, subcellular localization and PLIN5 association of LDs in two fiber types of type 2 diabetic (T2D), obese (OB) and healthy (HC) individuals by means of fluorescence microscopy and image analysis. We found that T2D type II fibers have a significant sub-population of large and internalized LDs, uncoated by PLIN5. Based on this novel result, additional hypotheses for the pathophysiology of skeletal muscle insulin resistance are formulated, together with future research directions.

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.

Lipid droplet-mitochondria coupling via perilipin 5 augments respiratory capacity but is dispensable for FA oxidation.

Disturbances in lipid homeostasis can cause mitochondrial dysfunction and lipotoxicity. Perilipin 5 (PLIN5) decorates intracellular lipid droplets (LDs) in oxidative tissues and controls triacylglycerol (TG) turnover via its interactions with adipose triglyceride lipase and the adipose triglyceride lipase coactivator, comparative gene identification-58. Furthermore, PLIN5 anchors mitochondria to the LD membrane via the outermost part of the carboxyl terminus. However, the role of this LD-mitochondria coupling (LDMC) in cellular energy catabolism is less established. In this study, we investigated the impact of PLIN5-mediated LDMC in comparison to disrupted LDMC on cellular TG homeostasis, FA oxidation, mitochondrial respiration, and protein interaction. To do so, we established PLIN5 mutants deficient in LDMC whilst maintaining normal interactions with key lipolytic players. Radiotracer studies with cell lines stably overexpressing wild-type or truncated PLIN5 revealed that LDMC has no significant impact on FA esterification upon lipid loading or TG catabolism during stimulated lipolysis. Moreover, we demonstrated that LDMC exerts a minor if any role in mitochondrial FA oxidation. In contrast, LDMC significantly improved the mitochondrial respiratory capacity and metabolic flexibility of lipid-challenged cardiomyocytes, which was corroborated by LDMC-dependent interactions of PLIN5 with mitochondrial proteins involved in mitochondrial respiration, dynamics, and cristae organization. Taken together, this study suggests that PLIN5 preserves mitochondrial function by adjusting FA supply via the regulation of TG hydrolysis and that LDMC is a vital part of mitochondrial integrity.

PNPLA3(I148M) Inhibits Lipolysis by Perilipin-5-Dependent Competition with ATGL.

The single nucleotide polymorphism I148M of the lipase patatin-like phospholipase domain containing 3 (PNPLA3) is associated with an unfavorable prognosis in alcoholic and non-alcoholic steatohepatitis (ASH, NASH), with progression to liver cirrhosis and development of hepatocellular carcinoma. In this study, we investigated the mechanistic interaction of PNPLA3 with lipid droplet (LD)-associated proteins of the perilipin family, which serve as gatekeepers for LD degradation. In a collective of 106 NASH, ASH and control liver samples, immunohistochemical analyses revealed increased ballooning, inflammation and fibrosis, as well as an accumulation of PNPLA3-perilipin 5 complexes on larger LDs in patients homo- and heterozygous for PNPLA3(I148M). Co-immunoprecipitation demonstrated an interaction of PNPLA3 with perilipin 5 and the key enzyme of lipolysis, adipose triglyceride lipase (ATGL). Localization studies in cell cultures and human liver showed colocalization of perilipin 5, ATGL and PNPLA3. Moreover, the lipolytic activity of ATGL was negatively regulated by PNPLA3 and perilipin 5, whereas perilipin 1 displaced PNPLA3 from the ATGL complex. Furthermore, ballooned hepatocytes, the hallmark of steatohepatitis, were positive for PNPLA3 and perilipins 2 and 5, but showed decreased perilipin 1 expression with respect to neighboured hepatocytes. In summary, PNPLA3- and ATGL-driven lipolysis is significantly regulated by perilipin 1 and 5 in steatohepatitis.

Role of Plin5 Deficiency in Progression of Non-Alcoholic Fatty Liver Disease Induced by a High-Fat Diet in Mice.

Characterized by steatosis, inflammation and fibrosis, non-alcoholic fatty liver disease (NAFLD) is a metabolic disorder. As a major lipid droplet-binding protein, Plin5 has been reported to have multiple effects on metabolism, but the effect of Plin5 deficiency on NAFLD is unknown. Plin5 knockout mice and wild-type mice were used to investigate the role of Plin5 in the progression of NAFLD by feeding a high-fat diet (HFD) for 20 weeks. Plin5 deficiency improved obesity induced by the HFD and altered glucose tolerance. Histological examination revealed that Plin5 deficiency alleviated hepatic steatosis and fibrosis induced by the HFD. Plin5 deficiency was also associated with a significant change in lipid metabolism-associated molecules. Further studies of these molecules indicated that Plin5 deficiency activated the expression of AMP-activated protein kinase and inhibited the core regulator of lipogenesis, sterol regulatory element binding protein 1 and its downstream lipid synthesis-related genes. These findings suggest that Plin5 deficiency ameliorates NAFLD by regulating lipid metabolism and inhibiting lipogenesis, and may provide a new strategy for the treatment of NAFLD.