Subcellular localization of skeletal muscle lipid droplets and PLIN family proteins OXPAT and ADRP at rest and following contraction in rat soleus muscle

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.

State of Knowledge on Molecular Adaptations to Exercise in Humans: Historical Perspectives and Future Directions

For centuries, regular exercise has been acknowledged as a potent stimulus to promote, maintain, and restore healthy functioning of nearly every physiological system of the human body. With advancing understanding of the complexity of human physiology, continually evolving methodological possibilities, and an increasingly dire public health situation, the study of exercise as a preventative or therapeutic treatment has never been more interdisciplinary, or more impactful. During the early stages of the NIH Common Fund Molecular Transducers of Physical Activity Consortium (MoTrPAC) Initiative, the field is well-positioned to build substantially upon the existing understanding of the mechanisms underlying benefits associated with exercise. Thus, we present a comprehensive body of the knowledge detailing the current literature basis surrounding the molecular adaptations to exercise in humans to provide a view of the state of the field at this critical juncture, as well as a resource for scientists bringing external expertise to the field of exercise physiology. In reviewing current literature related to molecular and cellular processes underlying exercise-induced benefits and adaptations, we also draw attention to existing knowledge gaps warranting continued research effort. © 2021 American Physiological Society. Compr Physiol 12:3193-3279, 2022.

Distinct Roles of Perilipins in the Intramuscular Deposition of Lipids in Glutamine-Supplemented, Low-, and Normal-Birth-Weight Piglets

Piglets with low birth weight (LBW) usually have reduced muscle mass and increased lipid deposition compared with their normal-birth-weight (NBW) littermates. Supplementation of piglets with amino acids during the first days of life may improve muscle growth and simultaneously alter the intramuscular lipid deposition. The aim of the current study was to investigate the influence of glutamine (Gln) supplementation during the early suckling period on lipid deposition in the longissimus muscle (MLD) and the role of different perilipin (PLIN) family members in this process. Four groups were generated consisting of 72 male LBW piglets and 72 NBW littermates. Piglets were supplemented with either 1 g Gln/kg body weight or an isonitrogenous amount of alanine (Ala) between days post natum (dpn) 1 and 12. Twelve piglets per group were slaughtered at 5, 12, and 26 dpn, and muscle tissue was collected. Perilipins were localized by immunohistochemistry in muscle sections. The mRNA and protein abundances of PLIN family members and related lipases were quantified by quantitative RT-PCR (qPCR) and western blots, respectively. While PLIN1 was localized around lipid droplets in mature and developing adipocytes, PLIN2 was localized at intramyocellular lipid droplets, PLIN3 and 4 at cell membranes of muscle fibers and adipocytes, and PLIN5 in the cytoplasm of undefined cells. The western blot results indicated higher protein abundances of PLIN2, 3, 4, and 5 in LBW piglets (p < 0.05) at 5 dpn compared with their NBW littermates independent of supplementation, while not directly reflecting the mRNA expression levels. The mRNA abundance of PLIN2 was lower while PLIN4 was higher in piglets at 26 dpn in comparison with piglets at 5 dpn (p < 0.01). Relative mRNA expression of LPL and CGI-58 was lowest in piglets at 5 dpn (p < 0.001). However, ATGL mRNA was not influenced by birth weight or supplementation, but the Spearman correlation coefficient analysis revealed close correlations with PLIN2, 4, and 5 mRNA at 5 and 26 dpn (r > 0.5, p < 0.001). The results indicated the importance of birth weight and age for intramuscular lipid deposition and different roles of PLIN family members in this process, but no clear modulating effect of Gln supplementation.

Skeletal muscle lipid droplets are resynthesized before being coated with perilipin proteins following prolonged exercise in elite male triathletes

Intramuscular triglycerides (IMTG) are a key substrate during prolonged exercise, but little is known about the rate of IMTG resynthesis in the postexercise period. We investigated the hypothesis that the distribution of the lipid droplet (LD)-associated perilipin (PLIN) proteins is linked to IMTG storage following exercise. Fourteen elite male triathletes (27 ± 1 yr, 66.5 ± 1.3 mL·kg^-1·min^-1) completed 4 h of moderate-intensity cycling. During the first 4 h of recovery, subjects received either carbohydrate or H₂O, after which both groups received carbohydrate. Muscle biopsies collected pre- and postexercise and 4 and 24 h postexercise were analyzed using confocal immunofluorescence microscopy for fiber type-specific IMTG content and PLIN distribution with LDs. Exercise reduced IMTG content in type I fibers (-53%, P = 0.002), with no change in type IIa fibers. During the first 4 h of recovery, IMTG content increased in type I fibers (P = 0.014), but was not increased more after 24 h, where it was similar to baseline levels in both conditions. During recovery the number of LDs labeled with PLIN2 (70%), PLIN3 (63%), and PLIN5 (62%; all P < 0.05) all increased in type I fibers. Importantly, the increase in LDs labeled with PLIN proteins only occurred at 24 h postexercise. In conclusion, IMTG resynthesis occurs rapidly in type I fibers following prolonged exercise in highly trained individuals. Furthermore, increases in IMTG content following exercise preceded an increase in the number of LDs labeled with PLIN proteins. These data, therefore, suggest that the PLIN proteins do not play a key role in postexercise IMTG resynthesis.

Disorder in milk proteins: adipophilin and TIP47, important constituents of the milk fat globule membrane

Milk fat globules (MFGs), which are secreted by the epithelial cells of the lactating mammary glands, account for the most of the nutritional value of milk. They are enveloped by the milk fat globule membrane (MFGM), a complex structure consisting of three phospholipid membrane monolayers and containing various lipids. Depending on the origin of milk, specific proteins accounts for 5-70% of the MFGM mass. Proteome of MFGMs includes hundreds of proteins, with nine major components being adipophilin, butyrophilin, cluster of differentiation 36, fatty acid binding protein, lactadherin, mucin 1, mucin 15, tail-interacting protein 47 (TIP47), and xanthine oxidoreductase. Two of the MFGM components, adipophilin and TIP47, belong to the five-member perilipin family of lipid droplet proteins. Adipophilin is involved in the formation of cytoplasmic lipid droplets and secretion of MFGs. This protein is also related to the formation of other lipid droplets that exist in most cell types, playing an important role in the transport of lipids from ER to the surface of lipid droplets. TIP47 acts as a cytoplasmic sorting factor for mannose 6-phosphate receptors and is recruited to the MFGM. Therefore, both adipophilin and TIP47 are moonlighting proteins, each possessing several unrelated functions. This review focuses on the main functions and specific structural features of adipophilin and TIP47, analyzes similarities and differences of these proteins among different species, and describes these proteins in the context of other members of the perilipin family.Communicated by Ramaswamy H. Sarma.

Intramyocellular Lipid Droplet Size Rather Than Total Lipid Content is Related to Insulin Sensitivity After 8 Weeks of Overfeeding

OBJECTIVE

Intramyocellular lipid (IMCL) is inversely related to insulin sensitivity in sedentary populations, yet no prospective studies in humans have examined IMCL accumulation with overfeeding.

METHODS

Twenty-nine males were overfed a high-fat diet (140% caloric intake, 44% from fat) for 8 weeks. Measures of IMCL, whole-body fat oxidation from a 24-hour metabolic chamber, muscle protein extracts, and muscle ceramide measures were obtained before and after the intervention.

RESULTS

Eight weeks of overfeeding did not increase overall IMCL. The content of smaller lipid droplets peripherally located in the myofiber decreased, while increases in larger droplets correlated inversely with glucose disposal rate. Overfeeding resulted in inhibition of Akt activity, which correlated with the reductions in smaller, peripherally located lipid droplets and drastic increases in ceramide content. Additionally, peripherally located lipid droplets were associated with more efficient lipid oxidation. Finally, participants who maintained a greater number of smaller, peripherally located lipid droplets displayed a better resistance to weight gain with overfeeding.

CONCLUSIONS

These results show that lipid droplet size and location rather than mere IMCL content are important to understanding insulin sensitivity.

Training alters the distribution of perilipin proteins in muscle following acute free fatty acid exposure

KEY POINTS

The lipid droplet (LD)-associated perilipin (PLIN) proteins promote intramuscular triglyceride (IMTG) storage, although whether the abundance and association of the PLIN proteins with LDs is related to the diverse lipid storage in muscle between trained and sedentary individuals is unknown. We show that lipid infusion augments IMTG content in type I fibres of both trained and sedentary individuals. Most importantly, despite there being no change in PLIN protein content, lipid infusion did increase the number of LDs connected with PLIN proteins in trained individuals only. We conclude that trained individuals are able to redistribute the pre-existing pool of PLIN proteins to an expanded LD pool during lipid infusion and, via this adaptation, may support the storage of fatty acids in IMTG.

ABSTRACT

Because the lipid droplet (LD)-associated perilipin (PLIN) proteins promote intramuscular triglyceride (IMTG) storage, we investigated the hypothesis that differential protein content of PLINs and their distribution with LDs may be linked to the diverse lipid storage in muscle between trained and sedentary individuals. Trained (n = 11) and sedentary (n = 10) subjects, matched for age, sex and body mass index, received either a 6 h lipid or glycerol infusion in the setting of a concurrent hyperinsulinaemic-euglycaemic clamp. Sequential muscle biopsies (0, 2 and 6 h) were analysed using confocal immunofluorescence microscopy for fibre type-specific IMTG content and PLIN associations with LDs. In both groups, lipid infusion increased IMTG content in type I fibres (trained: +62%, sedentary: +79%; P < 0.05) but did not affect PLIN protein content. At baseline, PLIN2 (+65%), PLIN3 (+105%) and PLIN5 (+53%; all P < 0.05) protein content was higher in trained compared to sedentary individuals. In trained individuals, lipid infusion increased the number of LDs associated with PLIN2 (+27%), PLIN3 (+73%) and PLIN5 (+40%; all P < 0.05) in type I fibres. By contrast, in sedentary individuals, lipid infusion only increased the number of LDs not associated with PLIN proteins. Acute free fatty acid elevation therefore induces a redistribution of PLIN proteins to an expanded LD pool in trained individuals only and this may be part of the mechanism that enables fatty acids to be stored in IMTG.

Muscle Lipid Metabolism: Role of Lipid Droplets and Perilipins

Skeletal muscle is one of the main regulators of carbohydrate and lipid metabolism in our organism, and therefore, it is highly susceptible to changes in glucose and fatty acid (FA) availability. Skeletal muscle is an extremely complex tissue: its metabolic capacity depends on the type of fibers it is made up of and the level of stimulation it undergoes, such as acute or chronic contraction. Obesity is often associated with increased FA levels, which leads to the accumulation of toxic lipid intermediates, oxidative stress, and autophagy in skeletal fibers. This lipotoxicity is one of the most common causes of insulin resistance (IR). In this scenario, the "isolation" of certain lipids in specific cell compartments, through the action of the specific lipid droplet, perilipin (PLIN) family of proteins, is conceived as a lifeguard compensatory strategy. In this review, we summarize the cellular mechanism underlying lipid mobilization and metabolism inside skeletal muscle, focusing on the function of lipid droplets, the PLIN family of proteins, and how these entities are modified in exercise, obesity, and IR conditions.

Perilipin 4 in human skeletal muscle: localization and effect of physical activity

Perilipins (PLINs) coat the surface of lipid droplets and are important for the regulation of lipid turnover. Knowledge about the physiological role of the individual PLINs in skeletal muscle is limited although lipid metabolism is very important for muscle contraction. To determine the effect of long-term exercise on PLINs expression, 26 middle-aged, sedentary men underwent 12 weeks combined endurance and strength training intervention. Muscle biopsies from m. vastus lateralis and subcutaneous adipose tissue were taken before and after the intervention and total gene expression was measured with deep mRNA sequencing. PLIN4 mRNA exhibited the highest expression of all five PLINs in both tissues, and the expression was significantly reduced after long-term exercise in skeletal muscle. Moreover, PLIN4 mRNA expression levels in muscle correlated with the expression of genes involved in de novo phospholipid biosynthesis, with muscular content of phosphatidylethanolamine and phosphatidylcholine, and with the content of subsarcolemmal lipid droplets. The PLIN4 protein was mainly located at the periphery of skeletal muscle fibers, with higher levels in slow-twitch as compared to fast-twitch skeletal muscle fibers. In summary, we report reduced expression of PLIN4 after long-term physical activity, and preferential slow-twitch skeletal muscle fibers and plasma membrane-associated PLIN4 location.

Genetic Disruption of Protein Kinase STK25 Ameliorates Metabolic Defects in a Diet-Induced Type 2 Diabetes Model

Understanding the molecular networks controlling ectopic lipid deposition, glucose tolerance, and insulin sensitivity is essential to identifying new pharmacological approaches to treat type 2 diabetes. We recently identified serine/threonine protein kinase 25 (STK25) as a negative regulator of glucose and insulin homeostasis based on observations in myoblasts with acute depletion of STK25 and in STK25-overexpressing transgenic mice. Here, we challenged Stk25 knockout mice and wild-type littermates with a high-fat diet and showed that STK25 deficiency suppressed development of hyperglycemia and hyperinsulinemia, improved systemic glucose tolerance, reduced hepatic gluconeogenesis, and increased insulin sensitivity. Stk25(-/-) mice were protected from diet-induced liver steatosis accompanied by decreased protein levels of acetyl-CoA carboxylase, a key regulator of both lipid oxidation and synthesis. Lipid accumulation in Stk25(-/-) skeletal muscle was reduced, and expression of enzymes controlling the muscle oxidative capacity (Cpt1, Acox1, Cs, Cycs, Ucp3) and glucose metabolism (Glut1, Glut4, Hk2) was increased. These data are consistent with our previous study of STK25 knockdown in myoblasts and reciprocal to the metabolic phenotype of Stk25 transgenic mice, reinforcing the validity of the results. The findings suggest that STK25 deficiency protects against the metabolic consequences of chronic exposure to dietary lipids and highlight the potential of STK25 antagonists for the treatment of type 2 diabetes.

Changes in mitochondrial perilipin 3 and perilipin 5 protein content in rat skeletal muscle following endurance training and acute stimulated contraction

NEW FINDINGS

What is the central question of this study? The aim was to determine whether mitochondrial protein content of perilipin 3 (PLIN3) and perilipin 5 (PLIN5) is increased following endurance training and whether mitochondrial PLIN5 protein is increased to a greater extent in endurance-trained rats when compared with sedentary rats following acute contraction. What is the main finding and its importance? Mitochondrial PLIN3 but not PLIN5 protein was increased in endurance-trained compared with sedentary rats, suggesting a mitochondrial role for PLIN3 due to chronic exercise. Contrary to our hypothesis, acute mitochondrial PLIN5 protein was similar in both sedentary and endurance-trained rats. Endurance training results in an increased association between skeletal muscle lipid droplets and mitochondria. This association is likely to be important for the expected increase in intramuscular fatty acid oxidation that occurs with endurance training. The perilipin family of lipid droplet proteins, PLIN(2-5), are thought to play a role in skeletal muscle lipolysis. Recently, results from our laboratory demonstrated that skeletal muscle mitochondria contain PLIN3 and PLIN5 protein. Furthermore, 30 min of stimulated contraction induces an increased mitochondrial PLIN5 content. To determine whether mitochondrial content of PLIN3 and PLIN5 is altered with endurance training, Sprague-Dawley rats were randomized into sedentary or endurance-trained groups for 8 weeks of treadmill running followed by an acute (30 min) sciatic nerve stimulation to induce lipolysis. Mitochondrial PLIN3 protein was ∼1.5-fold higher in red gastrocnemius of endurance-trained rats compared with sedentary animals, with no change in mitochondrial PLIN5 protein. In addition, there was an increase in plantaris intramuscular lipid storage. Acute electrically stimulated contraction in red gastrocnemius from sedentary and endurance-trained rats resulted in a similar increase of mitochondrial PLIN5 between these two groups, with no net change in PLIN3 in either group. Plantaris intramuscular lipid content decreased to a similar extent in sedentary and endurance-trained rats. These results suggest that while total mitochondrial PLIN5 content is not altered by endurance training, PLIN5 does have an acute role in the mitochondrial fraction during muscle contraction. Conversely, mitochondrial PLIN3 does not change acutely with muscle contraction, but PLIN3 content was increased following endurance training, indicating a role in chronic adaptations of skeletal muscle.

Unraveling the roles of PLIN5: linking cell biology to physiology

The discovery of perilipin (PLIN) 1 provided a major conceptual shift in the understanding of adipose tissue lipolysis and generated intense interest in lipid droplet biology research. The subsequent discovery of other PLIN proteins revealed unique tissue distribution profiles, subcellular locations, and lipid-binding properties and divergent cellular functions. PLIN5 is highly expressed in oxidative tissues such as skeletal muscle, liver, and heart and is central to lipid homeostasis in these tissues. Studies in cell systems have ascribed several metabolic roles to PLIN5 and demonstrated interactions with other proteins that are requisite for these functions. We examine recent in vivo studies and ask whether the evidence from the cell biology approaches is consistent with the physiological roles of PLIN5.

Piecing together the puzzle of perilipin proteins and skeletal muscle lipolysis

The regulation of skeletal muscle lipolysis and fat oxidation is a complex process involving multiple proteins and enzymes. Emerging work indicates that skeletal muscle PLIN proteins likely play a role in the hydrolysis of triglycerides stored in lipid droplets and the passage of fatty acids to the mitochondria for oxidation. In adipocytes, PLIN1 regulates lipolysis by interacting with comparative gene identification-58 (CGI-58), an activator of adipose triglyceride lipase (ATGL). Upon lipolytic stimulation, PLIN1 is phosphorylated, releasing CGI-58 to activate ATGL and initiate triglyceride breakdown. The absence of PLIN1 in skeletal muscle leads us to believe that other PLIN family members undertake this role. The focus of this review is on the PLIN family proteins expressed in skeletal muscle: PLIN2, PLIN3, and PLIN5. To date, most studies involving these PLIN proteins have used nonmuscle tissues and cell cultures to determine their potential roles. Results from work in these models support a role for PLIN proteins in sequestering lipases during basal conditions and in potentially working together for lipase translocation and activity during lipolysis. In skeletal muscle, PLIN2 tends to mirror the lipid content and may play a role in lipid droplet growth and stability through lipase interactions on the lipid droplet surface, whereas the skeletal muscle roles of both PLIN3 and PLIN5 seem to be more complex because they are found not only on the lipid droplet, but also at the mitochondria. Clearly, further work is needed to fully understand the intricate mechanisms by which PLIN proteins contribute to skeletal muscle lipid metabolism.

A maternal high fat diet has long-lasting effects on skeletal muscle lipid and PLIN protein content in rat offspring at young adulthood

A maternal high fat diet (HFD) can have adverse effects on skeletal muscle development. Skeletal muscle PLIN proteins (PLIN2, 3 and 5) are thought to play critical roles in lipid metabolism, however effects of HFD on PLIN and lipases (HSL, ATGL, CGI-58) in mothers as well as their offspring have yet to be investigated. The primary objective of this study was to determine whether maternal HFD would influence skeletal muscle lipase and PLIN protein content in offspring at weaning (19 d) and young adulthood (3 mo). Female rats (28 d old, n = 9/group) were fed control (CON, AIN93G, 7% soybean oil) or HFD (AIN93G, 20% lard) for 10 weeks prior to mating and throughout pregnancy and lactation. All offspring were weaned to CON [n = 18/group, 1 female and 1 male pup per litter were studied at weaning (19 d) and 3 mo of age]. There was no effect of sex for the main outcomes measured in plantaris, therefore male and female data was combined. Maternal HFD resulted in higher triacylglycerol content in pups at 3 mo (p < 0.05), as well as in the dams (p = 0.015). Maternal HFD resulted in higher PLIN5 content in pups at weaning and 3 mo (p = 0.05). PLIN2 and PLIN5 content decreased at 3 mo versus weaning (p < 0.001). HFD dams had a higher PLIN3 content (p = 0.016). Diet had no effect on ATGL, CGI-58, or HSL content. In conclusion, exposure to a maternal HFD resulted in higher skeletal muscle lipid and PLIN5 content in plantaris of offspring through to young adulthood.

Higher PLIN5 but not PLIN3 content in isolated skeletal muscle mitochondria following acute in vivo contraction in rat hindlimb

Contraction-mediated lipolysis increases the association of lipid droplets and mitochondria, indicating an important role in the passage of fatty acids from lipid droplets to mitochondria in skeletal muscle. PLIN3 and PLIN5 are of particular interest to the lipid droplet-mitochondria interaction because PLIN3 is able to move about within cells and PLIN5 associates with skeletal muscle mitochondria. This study primarily investigated: 1) if PLIN3 is detected in skeletal muscle mitochondrial fraction; and 2) if mitochondrial protein content of PLIN3 and/or PLIN5 changes following stimulated contraction. A secondary aim was to determine if PLIN3 and PLIN5 associate and whether this changes following contraction. Male Long Evans rats (n = 21; age, 52 days; weight = 317 ± 6 g) underwent 30 min of hindlimb stimulation (10 msec impulses, 100 Hz/3 sec at 10-20 V; train duration 100 msec). Contraction induced a ~50% reduction in intramuscular lipid content measured by oil red-O staining of red gastrocnemius muscle. Mitochondria were isolated from red gastrocnemius muscle by differential centrifugation and proteins were detected by western blotting. Mitochondrial PLIN5 content was ~1.6-fold higher following 30 min of contraction and PLIN3 content was detected in the mitochondrial fraction, and unchanged following contraction. An association between PLIN3 and PLIN5 was observed and remained unaltered following contraction. PLIN5 may play a role in mitochondria during lipolysis, which is consistent with a role in facilitating/regulating mitochondrial fatty acid oxidation. PLIN3 and PLIN5 may be working together on the lipid droplet and mitochondria during contraction-induced lipolysis.

Cellular localization and associations of the major lipolytic proteins in human skeletal muscle at rest and during exercise

Lipolysis involves the sequential breakdown of fatty acids from triacylglycerol and is increased during energy stress such as exercise. Adipose triglyceride lipase (ATGL) is a key regulator of skeletal muscle lipolysis and perilipin (PLIN) 5 is postulated to be an important regulator of ATGL action of muscle lipolysis. Hence, we hypothesized that non-genomic regulation such as cellular localization and the interaction of these key proteins modulate muscle lipolysis during exercise. PLIN5, ATGL and CGI-58 were highly (>60%) colocated with Oil Red O (ORO) stained lipid droplets. PLIN5 was significantly colocated with ATGL, mitochondria and CGI-58, indicating a close association between the key lipolytic effectors in resting skeletal muscle. The colocation of the lipolytic proteins, their independent association with ORO and the PLIN5/ORO colocation were not altered after 60 min of moderate intensity exercise. Further experiments in cultured human myocytes showed that PLIN5 colocation with ORO or mitochondria is unaffected by pharmacological activation of lipolytic pathways. Together, these data suggest that the major lipolytic proteins are highly expressed at the lipid droplet and colocate in resting skeletal muscle, that their localization and interactions appear to remain unchanged during prolonged exercise, and, accordingly, that other post-translational mechanisms are likely regulators of skeletal muscle lipolysis.

Perilipin 5, a lipid droplet protein adapted to mitochondrial energy utilization

PURPOSE OF REVIEW

We summarize recent mechanistic and physiological studies related to the role of perilipin 5 (Plin5) in regulating lipid droplet accumulation and protection to fatty acids in tissues with high lipid oxidative metabolism.

RECENT FINDINGS

Plin5 is a lipid droplet targeting protein that promotes association of lipid droplets with mitochondria and is most highly expressed in oxidative tissues, including cardiac and skeletal muscle. Recent in-vivo and in-vitro data indicate an important role for Plin5 in the regulation of cardiac lipid storage and function. Targeted overexpression of Plin5 in heart causes steatosis, mild mitochondria dysfunction, and hypertrophy in cardiac tissue, but without affecting cardiac function. In contrast, whole body ablation of Plin5 (Plin5  mice) reduces cardiac lipid droplet formation, increases cardiac fatty acid oxidation, and promotes cardiac dysfunction; cardiac defects can be prevented with antioxidative therapy. These data suggest a cytoprotective role for Plin5 to promote lipid storage but to limit fatty acid toxicity, parameters critical for tissues with high lipid oxidative metabolism.

SUMMARY

In-vivo and in-vitro data suggest that Plin5 is part of a cell-adaptive response to high lipid oxidative metabolism to protect lipid droplet storage against neutral lipases and, so, limit fatty acid accumulation. Although the specific mechanisms that underlie Plin5 lipid droplet storage protection in oxidative tissues remain to be fully elucidated, Plin5 provides a basis for the novel cytoprotective nature of lipid droplets.

Skeletal muscle perilipin 3 and coatomer proteins are increased following exercise and are associated with fat oxidation

Lipid droplet-associated proteins such as perilipin 3 (PLIN3) and coatomer GTPase proteins (GBF1, ARF1, Sec23a, and ARFRP1) are expressed in skeletal muscle but little is known so far as to their regulation of lipolysis. We aimed here to explore the effects of lipolytic stimulation in vitro in primary human myotubes as well as in vivo following an acute exercise bout. In vitro lipolytic stimulation by epinephrine (100 μM) or by a lipolytic cocktail (30 μM palmitate, 4 μM forskolin, and 0.5 μM ionomycin, PFI) resulted in increases in PLIN3 protein content. Coatomer GTPases such as GBF1, ARF1, Sec23a, and ARFRP1 also increased in response to lipolytic stimuli. Furthermore, a long duration endurance exercise bout (20 males; age 24.0±4.5 y; BMI 23.6±1.8 kg/m²) increased PLIN3 protein in human skeletal muscle (p = 0.03) in proportion to ex vivo palmitate oxidation (r = 0.45, p = 0.04) and whole body in vivo fat oxidation (r = 0.52, p = 0.03). Protein content of ARF1 was increased (p = 0.04) while mRNA expression was increased for several other coatomers (GBF1, ARF1, and Sec23a, all p<0.05). These data provide novel observational insight into the possible relationships between lipolysis and PLIN3 along with these coatomoer GTPase proteins in human skeletal muscle.

Mitochondrial dysfunction induces formation of lipid droplets as a generalized response to stress

Lipid droplet (LD) formation is a hallmark of cellular stress. Cells attempt to combat noxious stimuli by switching their metabolism from oxidative phosphorylation to glycolysis, sparing resources in LDs for generating cellular reducing power and for anabolic biosynthesis. Membrane phospholipids are also a source of LDs. To elucidate the formation of LDs, we exposed mice to hyperoxia, hypoxia, myocardial ischemia, and sepsis induced by cecal ligation and puncture (CLP). All the above-mentioned stressors enhanced the formation of LDs, as assessed by transmission electron microscopy, with severe mitochondrial swelling. Disruption of mitochondria by depleting mitochondrial DNA ( ρ 0 cells) significantly augmented the formation of LDs, causing transcriptional activation of fatty acid biosynthesis and metabolic reprogramming to glycolysis. Heme oxygenase (HO)-1 counteracts CLP-mediated septic shock in mouse models. In HO-1-deficient mice, LD formation was not observed upon CLP, but a concomitant decrease in "LD-decorating proteins" was observed, implying a link between LDs and cytoprotective activity. Collectively, LD biogenesis during stress can trigger adaptive LD formation, which is dependent on mitochondrial integrity and HO-1 activity; this may be a cellular survival strategy, apportioning energy-generating substrates to cellular defense.

Skeletal muscle PLIN3 and PLIN5 are serine phosphorylated at rest and following lipolysis during adrenergic or contractile stimulation

In adipose tissue, access of adipose triglyceride and hormone-sensitive lipases (ATGL and HSL) to the lipid droplet depends on PLIN1 phosphorylation, however, PLIN1 is not expressed in skeletal muscle and the phosphorylation of the expressed PLINs has yet to be investigated. Further, direct interactions between skeletal muscle PLINs and HSL are unknown. We investigated the isolated and combined effects of epinephrine and contraction on PLIN-to-lipase interactions as well as phosphorylation. Isolated rat solei were assigned to one of four 30 min in vitro conditions (25°C): (1) rest; (2) intermittent tetanic stimulation (60 Hz for 150 msec; train rate 20/min); (3) 5 nmol/L epinephrine; (4) intermittent tetanic stimulation and 5 nmol/L epinephrine. Immunoprecipitation of serine phosphorylated proteins followed by Western blotting for PLIN2, PLIN3, PLIN5, revealed that only PLIN2 is not phosphorylated under any of the experimental conditions. This is the first study to show that in whole rat skeletal muscle PLIN3 and PLIN5 are serine phosphorylated. The degree of serine phosphorylation remained unchanged following adrenergic and/or contractile stimulation. Oil red O staining of muscle sections for lipid content shows a significant decrease following each condition, confirming lipolysis occurred (P < 0.05). PLIN2, 3, and 5 all interact with HSL and ATGL, but these interactions were unchanged following treatments. Our results show that in skeletal muscle, PLIN2 is not serine phosphorylated at rest or with lipolytic stimulation and that while PLIN3, PLIN5 are serine phosphorylated at rest, the degree of phosphorylation does not change with lipolytic stimulation.

Skeletal muscle PLIN proteins, ATGL and CGI-58, interactions at rest and following stimulated contraction

Evidence indicates that skeletal muscle lipid droplet-associated proteins (PLINs) regulate lipolysis through protein-protein interactions on the lipid droplet surface. In adipocytes, PLIN1 is thought to regulate lipolysis by directly interacting with comparative gene identification-58 (CGI-58), an activator of adipose triglyceride lipase (ATGL). Upon lipolytic stimulation, PLIN1 is phosphorylated, releasing CGI-58 to fully activate ATGL and initiate triglyceride breakdown. The absence of PLIN1 in skeletal muscle leads us to believe that other PLIN family members undertake this role. Our purpose was to examine interactions between PLIN2, PLIN3, and PLIN5, with ATGL and its coactivator CGI-58 at rest and following contraction. Isolated rat solei were incubated for 30 min at rest or during 30 min of intermittent tetanic stimulation [150-ms volleys at 60 Hz with a train rate of 20 tetani/min (25°C)] to maximally stimulate intramuscular lipid breakdown. Results show that the interaction between ATGL and CGI-58 increased 128% following contraction (P = 0.041). Further, ATGL interacts with PLIN2, PLIN3, and PLIN5 at rest and following contraction. The PLIN2-ATGL interaction decreased significantly by 21% following stimulation (P = 0.013). Both PLIN3 and PLIN5 coprecipitated with CGI-58 at rest and following contraction, while there was no detectable interaction between PLIN2 and CGI-58 in either condition. Therefore, our findings indicate that in skeletal muscle, during contraction-induced muscle lipolysis, ATGL and CGI-58 strongly associate and that the PLIN proteins work together to regulate lipolysis, in part, by preventing ATGL and CGI-58 interactions at rest.

Sprint interval and traditional endurance training increase net intramuscular triglyceride breakdown and expression of perilipin 2 and 5

Intramuscular triglyceride (IMTG) utilization is enhanced by endurance training (ET) and is linked to improved insulin sensitivity. This study first investigated the hypothesis that ET-induced increases in net IMTG breakdown and insulin sensitivity are related to increased expression of perilipin 2 (PLIN2) and perilipin 5 (PLIN5). Second, we hypothesized that sprint interval training (SIT) also promotes increases in IMTG utilization and insulin sensitivity. Sixteen sedentary males performed 6 weeks of either SIT (4-6, 30 s Wingate tests per session, 3 days week(-1)) or ET (40-60 min moderate-intensity cycling, 5 days week(-1)). Training increased resting IMTG content (SIT 1.7-fold, ET 2.4-fold; P < 0.05), concomitant with parallel increases in PLIN2 (SIT 2.3-fold, ET 2.8-fold; P < 0.01) and PLIN5 expression (SIT 2.2-fold, ET 3.1-fold; P < 0.01). Pre-training, 60 min cycling at ∼65% pre-training decreased IMTG content in type I fibres (SIT 17 ± 10%, ET 15 ± 12%; P < 0.05). Following training, a significantly greater breakdown of IMTG in type I fibres occurred during exercise (SIT 27 ± 13%, ET 43 ± 6%; P < 0.05), with preferential breakdown of PLIN2- and particularly PLIN5-associated lipid droplets. Training increased the Matsuda insulin sensitivity index (SIT 56 ± 15%, ET 29 ± 12%; main effect P < 0.05). No training × group interactions were observed for any variables. In conclusion, SIT and ET both increase net IMTG breakdown during exercise and increase in PLIN2 and PLIN5 protein expression. The data are consistent with the hypothesis that increases in PLIN2 and PLIN5 are related to the mechanisms that promote increased IMTG utilization during exercise and improve insulin sensitivity following 6 weeks of SIT and ET.

Perilipin family (PLIN) proteins in human skeletal muscle: the effect of sex, obesity, and endurance training

Proteins that coat the lipid droplets (also known as PAT proteins or perilipin (PLIN) family proteins) have diverse functions that are not well elucidated in many tissues. In skeletal muscle, there is even less known about the functions or characteristics of these proteins or how they might change in response to perturbations that alter both intramyocellular lipid (IMCL) content and fat utilization and oxidation. Therefore, the purpose of this study was to examine the human muscle content and gene expression of the four skeletal muscle PLIN proteins in both lean and obese men and women and how this was changed following a 12-week endurance training protocol. PLIN2–PLIN5 proteins were all more abundant in women than in men (p = 0.037 and p < 0.0001, respectively), consistent with higher IMCL content observed in female skeletal muscle. PLIN5 (previously known as OXPAT) is of particular interest because it has previously been associated primarily with oxidative tissues that rely heavily on fat oxidation for energy production. Although PLIN5 was not different between lean and obese subjects, it was the only PLIN protein to increase in response to endurance training in both sexes. PLIN5 correlated with IMCL volume (p < 0.0001), but in general, the other PLIN proteins did not correlate well with IMCL volume, suggesting that the relationship between lipid accumulation and PLIN family protein content is not a simple one. Although more work is necessary, it is clear that PLIN5 likely plays an important role in IMCL accumulation and oxidation, both of which increase with endurance training in human skeletal muscle.

Role of intramyocelluar lipids in human health

Intramyocellular lipid (IMCL) is predominantly stored as intramuscular triglyceride (IMTG) in lipid droplets and is utilized as metabolic fuel during physical exercise. IMTG is also implicated in muscle insulin resistance (IR) in type 2 diabetes. However, it has become apparent that lipid moieties such as ceramide and diacylglycerol are the likely culprits of IR. This article reviews current knowledge of IMCL-mediated IR and important areas of investigation, including myocellular lipid transport and lipid droplet proteins. Several crucial questions remain unanswered, such as the identity of specific ceramide and diacylglycerol species that mediate IR in human muscle and their subcellular location. Quantitative lipidomics and proteomics of targeted subcellular organelles will help to better define the mechanisms underlying pathological IMCL accumulation and IR.