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.

PLIN5 deletion remodels intracellular lipid composition and causes insulin resistance in muscle.

Defective control of lipid metabolism leading to lipotoxicity causes insulin resistance in skeletal muscle, a major factor leading to diabetes. Here, we demonstrate that perilipin (PLIN) 5 is required to couple intramyocellular triacylglycerol lipolysis with the metabolic demand for fatty acids. PLIN5 ablation depleted triacylglycerol stores but increased sphingolipids including ceramide, hydroxylceramides and sphingomyelin. We generated perilipin 5 (Plin5)(-/-) mice to determine the functional significance of PLIN5 in metabolic control and insulin action. Loss of PLIN5 had no effect on body weight, feeding or adiposity but increased whole-body carbohydrate oxidation. Plin5 (-/-) mice developed skeletal muscle insulin resistance, which was associated with ceramide accumulation. Liver insulin sensitivity was improved in Plin5 (-/-) mice, indicating tissue-specific effects of PLIN5 on insulin action. We conclude that PLIN5 plays a critical role in coordinating skeletal muscle triacylglycerol metabolism, which impacts sphingolipid metabolism, and is requisite for the maintenance of skeletal muscle insulin action.

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.

Insulin-mediated suppression of lipolysis in adipose tissue and skeletal muscle of obese type 2 diabetic men and men with normal glucose tolerance.

AIMS/HYPOTHESIS: Impaired regulation of lipolysis and accumulation of lipid intermediates may contribute to obesity-related insulin resistance and type 2 diabetes mellitus. We investigated insulin-mediated suppression of lipolysis in abdominal subcutaneous adipose tissue (AT) and skeletal muscle (SM) of obese men with normal glucose tolerance (NGT) and obese type 2 diabetic men. METHODS: Eleven NGT men and nine long-term diagnosed type 2 diabetic men (7 ± 1 years), matched for age (58 ± 2 vs 62 ± 2 years), BMI (31.4 ± 0.6 vs 30.5 ± 0.6 kg/m(2)) and [Formula: see text] (28.9 ± 1.5 vs 29.5 ± 2.4 ml kg(-1) min(-1)) participated in this study. Interstitial glycerol concentrations in AT and SM were assessed using microdialysis during a 1 h basal period and a 6 h stepwise hyperinsulinaemic-euglycaemic clamp (8, 20 and 40 mU m(-2) min(-1)). AT and SM biopsies were collected to investigate underlying mechanisms. RESULTS: Hyperinsulinaemia suppressed interstitial SM glycerol concentrations less in men with type 2 diabetes (-7 ± 6%, -13 ± 9% and -27 ± 9%) compared with men with NGT (-21 ± 7%, -38 ± 8% and -53 ± 8%) (p = 0.014). This was accompanied by increased circulating fatty acid and glycerol concentrations, a lower glucose infusion rate (21.8 ± 3.1 vs 30.5 ± 2.0 µmol kg body weight(-1) min(-1); p < 0.05), higher hormone-sensitive lipase (HSL) serine 660 phosphorylation, increased saturated diacylglycerol (DAG) lipid species in the muscle membrane and increased protein kinase C (PKC) activation in type 2 diabetic men vs men with NGT. No significant differences in insulin-mediated reduction in AT interstitial glycerol were observed between groups. CONCLUSIONS/INTERPRETATION: Our results suggest that a blunted insulin-mediated suppression of SM lipolysis may promote the accumulation of membrane saturated DAG, aggravating insulin resistance, at least partly mediated by PKC. This may represent an important mechanism involved in the progression of insulin resistance towards type 2 diabetes. TRIAL REGISTRATION: ClinicalTrials.gov NCT01680133.

Phosphorylation of adipose triglyceride lipase Ser(404) is not related to 5'-AMPK activation during moderate-intensity exercise in humans.

Intramyocellular triacylglycerol provides fatty acid substrate for ATP generation in contracting muscle. The protein adipose triglyceride lipase (ATGL) is a key regulator of triacylglycerol lipolysis and whole body energy metabolism at rest and during exercise, and ATGL activity is reported to be enhanced by 5'-AMP-activated protein kinase (AMPK)-mediated phosphorylation at Ser(406) in mice. This is a curious observation, because AMPK activation reduces lipolysis in several cell types. We investigated whether the phosphorylation of ATGL Ser(404) (corresponding to murine Ser(406)) was increased during exercise in human skeletal muscle and with pharmacological AMPK activation in myotubes in vitro. In human experiments, skeletal muscle and venous blood samples were obtained from recreationally active male subjects before and at 5 and 60 min during exercise. ATGL Ser(404) phosphorylation was not increased from rest during exercise, but ATGL Ser(404) phosphorylation correlated with myosin heavy chain 1 expression, suggesting a possible fiber type dependency. ATGL Ser(404) phosphorylation was not related to increases in AMPK activity, and immunoprecipitation experiments indicated no interaction between AMPK and ATGL. Rather, ATGL Ser(404) phosphorylation was associated with protein kinase A (PKA) signaling. ATGL Ser(406) phosphorylation in C(2)C(12) myotubes was unaffected by 5-aminoimidazole-4-carboxaminde-1-ß-d-ribofuranoside, an AMPK activator, and the PKA activator forskolin. Our results demonstrate that ATGL Ser(404) phosphorylation is not increased in mixed skeletal muscle during moderate-intensity exercise and that AMPK does not appear to be an activating kinase for ATGL Ser(404/406) in skeletal muscle.

Is 8 weeks of side-alternating whole-body vibration a safe and acceptable modality to improve functional performance in multiple sclerosis?

PURPOSE: To examine whether an 8-week period of side-alternating whole-body vibration (WBV) exercise is an acceptable and effective exercise intervention to improve and maintain functional performance in multiple sclerosis people. METHODS: A total of 15 participants with MS (11 women [mean age 50.2 ± 6.9 years; body mass 65.7 ± 19.2 kg; height 165.3 ± 6.1 cm; EDSS 3.5 ± 0.9] and 4 males [mean age 50.5 ± 5.2 years; body mass 85.3 ± 16.0 kg; height 175.3 ± 3.2 cm; EDSS 3.4 ± 0.5]) were selected for this study. Quality of life, timed up-and-go, functional reach, standing balance and 10-m walk test were performed prior to and after 4 and 8 weeks of vibration exercise, and 2 weeks after cessation of vibration exercise. RESULTS: There was no evidence of vibration exercise producing any anxiety or discomfort. Compared with baseline measurements, the 10-m walk test showed significant improvements in 2, 8 and 10 m times at 8 week (p < 0.05) and 2 week post-vibration (p < 0.05). Timed up-and-go demonstrated a significant and positive time effect (p < 0.05). Standing balance showed significant improvements from baseline, at 4- (p < 0.05) and 2-weeks post-vibration (p < 0.05). CONCLUSION: This is the first study to investigate side-alternating WBV as an exercise training modality for MS people. From an active MS population, this study has shown that WBV training not only improved the standing balance and walking time but there were also no adverse effects from using this modality.