Insulin Regulates Adipocyte Lipolysis via an Akt-Independent Signaling Pathway

After a meal, insulin suppresses lipolysis through the activation of its downstream kinase, Akt, resulting in the inhibition of protein kinase A (PKA), the main positive effector of lipolysis. During insulin resistance, this process is ineffective, leading to a characteristic dyslipidemia and the worsening of impaired insulin action and obesity. Here, we describe a noncanonical Akt-independent, phosphoinositide-3 kinase (PI3K)-dependent pathway that regulates adipocyte lipolysis using restricted subcellular signaling. This pathway selectively alters the PKA phosphorylation of its major lipid droplet-associated substrate, perilipin. In contrast, the phosphorylation of another PKA substrate, hormone-sensitive lipase (HSL), remains Akt dependent. Furthermore, insulin regulates total PKA activity in an Akt-dependent manner. These findings indicate that localized changes in insulin action are responsible for the differential phosphorylation of PKA substrates. Thus, we identify a pathway by which insulin regulates lipolysis through the spatially compartmentalized modulation of PKA.The storage and mobilization of nutrients from specialized tissues requires the spatial organization of both signaling functions and energy stores. Nowhere is this more evident than in mammalian adipose tissue, which maintains the most efficient repository for readily available energy. Here, fuel is segregated into lipid droplets, once thought to be inert storehouses but now recognized as complex structures that represent a regulatable adaptation of a ubiquitous organelle (5, 40). The synthesis and maintenance of functional lipid droplets requires numerous proteins, not only fatty acid binding proteins and enzymes of lipid synthesis but also molecules critical to constitutive and specialized membrane protein trafficking (23).During times of nutritional need, triglycerides within the adipocyte lipid droplet are hydrolyzed into their components, fatty acids, acyl-glycerides, and, ultimately, glycerol. This process, termed lipolysis, is controlled dynamically by multiple hormonal signals that respond to the nutrient status of the organism. During fasting, catecholamines such as norepinephrine stimulate lipolysis via beta-adrenergic receptor activation, promoting adenylyl cyclase activity and the production of cyclic AMP (cAMP) (17). cAMP binds to the regulatory subunits of its major effector, protein kinase A (PKA), triggering the dissociation of these subunits and the subsequent activation of the catalytic subunits (62, 63). PKA is frequently sequestered into multiple parallel, intracellular signaling complexes, though such structures have not been studied in hormone-responsive adipocytes (68). Two targets of activated PKA important for lipolysis are hormone-sensitive lipase (HSL) and perilipin, the major lipid droplet coat protein (17). The phosphorylation of HSL on Ser 559/660 is crucial for its activation and translocation to the lipid droplet, where HSL catalyzes the hydrolysis of diglycerides to monoglycerides (26, 55). Another lipase, adipose triglyceride lipase (ATGL), carries out the initial cleavage of triglycerides to diglycerides and most likely is rate limiting for lipolysis, but it does not appear to be regulated directly via PKA phosphorylation (24, 73). Perilipin under basal conditions acts as a protective barrier against lipase activity; upon stimulation, the phosphorylation of least six PKA consensus sites triggers a conformational change in perilipin, permitting access to the lipid substrates in the droplet, the recruitment of HSL, and possibly the activation of ATGL (7, 8, 21, 41, 46, 58, 60, 61). Perilipin, therefore, possesses dual functions, both blocking lipolysis in the basal state as well as promoting lipolysis upon its phosphorylation (5, 58, 60).Following the ingestion of a meal, insulin stimulates the uptake of nutrients such as glucose into specialized tissues and also potently inhibits lipolysis in adipocytes (17). Insulin signaling in the adipocyte involves the activation of the insulin receptor tyrosine kinase, the phosphorylation of insulin receptor substrates, the activation of PI3K, and the subsequent production of specific phosphoinositides at the plasma membrane (59). These phosphoinositides then recruit Akt, via its pleckstrin homology domain, to the plasma membrane, where Akt becomes phosphorylated and activated by two upstream kinases. Akt stimulates the translocation of the glucose transporter GLUT4 to the plasma membrane, thereby promoting the uptake of glucose into the cell (2). The mechanism by which insulin inhibits lipolysis has been proposed to involve the reduction of cAMP levels and thus PKA activity. In this model, insulin signaling activates phosphodiesterase 3b (PDE3b) via the Akt-mediated phosphorylation of Ser273 (14, 32). Upon activation by Akt, PDE3b catalyzes the hydrolysis of cAMP to 5′AMP, thereby attenuating PKA activity and lipolysis. Recent studies of PDE3b knockout mice have highlighted the importance of PDE3b activity in the regulation of lipolysis but were uninformative regarding the mechanism of insulin action (12). Adipocytes isolated from these mice exhibit reduced responses to insulin with respect to lipolysis, but it is not clear whether this is due to the loss of the critical target enzyme or a normal mechanism being overwhelmed by supraphysiological concentrations of cAMP (12). Biochemical studies using dominant-inhibitory Akt have demonstrated that Akt can regulate PDE3b activity, and other studies also have suggested that Akt interacts directly with PDE3b, implying a direct connection to lipolysis regulation (1, 32). Nevertheless, the actual requirement for Akt in insulin action with regard to the lipolysis itself has not been demonstrated directly in, for example, genetic loss-of-function experiments.There now is substantial evidence implicating elevated free fatty acid levels as a consequence of inappropriate lipolysis as a major etiological factor for insulin resistance and type 2 diabetes mellitus (T2DM) (51). Conditions such as obesity and diabetes are characterized by a pathophysiological state in which these tissues become unresponsive to insulin, which contribute to the adverse long-term sequelae of diseases such as T2DM and the metabolic syndrome (4, 44). Thus, understanding in detail the mechanism by which insulin suppresses fat cell lipolysis is critical to identifying the underlying defect in resistant adipose tissue and ultimately developing effective therapeutics. In the present study, we investigated both Akt-dependent and -independent modes of insulin action toward lipolysis. We found the latter to predominate at low, physiological levels of adrenergic stimulation, acting via a pathway dependent on the preferential phosphorylation of downstream PKA substrates.     

Early B Cell Factor 1 Regulates Adipocyte Morphology and Lipolysis in White Adipose Tissue

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The Immune Receptor NOD1 and Kinase RIP2 Interact with Bacterial Peptidoglycan on Early Endosomes to Promote Autophagy and Inflammatory Signaling

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Effects of Genetic Loci Associated with Central Obesity on Adipocyte Lipolysis

ObjectivesNumerous genetic loci have been associated with measures of central fat accumulation, such as waist-to-hip ratio adjusted for body mass index (WHRadjBMI). However the mechanisms by which genetic variations influence obesity remain largely elusive. Lipolysis is a key process for regulation of lipid storage in adipocytes, thus is implicated in obesity and its metabolic complications. Here, genetic variants at 36 WHRadjBMI-associated loci were examined for their influence on abdominal subcutaneous adipocyte lipolysis.ResultsThe WHRadjBMI-associated loci CMIP, PLXND1, VEGFA and ZNRF3-KREMEN1 demonstrated nominal associations with spontaneous and/or stimulated lipolysis. Candidate genes in these loci have been reported to influence NFκB-signaling, fat cell size and Wnt signalling, all of which may influence lipolysis.SignificanceThis report provides evidence for specific WHRadjBMI-associated loci as candidates to modulate adipocyte lipolysis. Additionally, our data suggests that genetically increased central fat accumulation is unlikely to be a major cause of altered lipolysis in abdominal adipocytes.     

Dermal Adipocyte Lipolysis and Myofibroblast Conversion Are Required for Efficient Skin Repair

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Relation of Cheddar Cheese Ripening to Bacterial Lipolysis

This investigation was undertaken to find the relationship between fat hydrolysis and lipolytic activities of lactic acid bacteria participated in Cheddar cheese ripening. Increases in titratable acidities due to lactic fermentation were completed at early stage of ripening. Ripening indices (ratio of water-soluble nitrogen to total nitrogen) increased rapidly until 90 days and thereafter gradually up to 150 days. Considerable amounts of free fatty acids were released from cheese fat throughout the ripening period. Cheese bacteria were enumerated on the media of tomato-glucose-agar and acetate-agar. About 70% of bacteria isolated from cheese at age of 150 days were classified into Lactobacillus casei and L. plantarum. Lipolytic activities of lactobacilli isolated were detected definitely on double-layered agar plates containing Victoria blue-stained olive oil. Lipase activities were determined in cheese extracts during ripening.     

Bacterial membrane vesicles deliver peptidoglycan to NOD1 in epithelial cells

Gram‐negative bacterial peptidoglycan is specifically recognized by the host intracellular sensor NOD1, resulting in the generation of innate immune responses. Although epithelial cells are normally refractory to external stimulation with peptidoglycan, these cells have been shown to respond in a NOD1‐dependent manner to Gram‐negative pathogens that can either invade or secrete factors into host cells. In the present work, we report that Gram‐negative bacteria can deliver peptidoglycan to cytosolic NOD1 in host cells via a novel mechanism involving outer membrane vesicles (OMVs). We purified OMVs from the Gram‐negative mucosal pathogens: Helicobacter pylori, Pseudomonas aeruginosa and Neisseria gonorrhoea and demonstrated that these peptidoglycan containing OMVs upregulated NF‐κB and NOD1‐dependent responses in vitro. These OMVs entered epithelial cells through lipid rafts thereby inducing NOD1‐dependent responses in vitro. Moreover, OMVs delivered intragastrically to mice‐induced innate and adaptive immune responses via a NOD1‐dependent but TLR‐independent mechanism. Collectively, our findings identify OMVs as a generalized mechanism whereby Gram‐negative bacteria deliver peptidoglycan to cytosolic NOD1. We propose that OMVs released by bacteria in vivo may promote inflammation and pathology in infected hosts.     

Regulation of Visceral and Subcutaneous Adipocyte Lipolysis by Acute AICAR‐induced AMPK Activation

This study investigated the role of adenosine monophosphate–activated protein kinase (AMPK) in the regulation of lipolysis in visceral (VC) and subcutaneous (SC) rat adipocytes and the molecular mechanisms involved in this process. VC (epididymal and retroperitoneal) and SC (inguinal) adipocytes were isolated from male Wistar rats (160–180 g). Adipocytes were incubated either in the absence or in the presence of the AMPK agonist 5‐aminoimidazole‐4‐carboxamide‐1‐β‐d‐ribofuranoside (AICAR, 0–500 µmol/l). AMPK and acetyl‐CoA carboxylase (ACC) phosphorylation, basal and epinephrine‐stimulated (100 nmol/l) glycerol release, and hormone‐sensitive lipase (HSL) phosphorylation and activity were determined. AICAR‐induced (500 µmol/l) AMPK activation inhibited basal glycerol release by ～42, 41, and 44% in epididymal, retroperitoneal, and inguinal adipocytes, respectively. Epinephrine‐stimulated glycerol release was almost completely prevented by AICAR treatment in adipocytes from all fat depots. The AMPK inhibitor compound C (20 µmol/l) prevented AICAR‐induced phosphorylation of AMPK and significantly increased basal (～1.3‐, 1.4‐, and 1.7‐fold) and epinephrine‐stimulated (～1.3‐, 1.2‐, 1.4‐fold) glycerol release in epididymal, retroperitoneal, and inguinal adipocytes, respectively. AICAR increased phosphorylation of HSL_Ser565 and inhibited epinephrine‐induced phosphorylation of HSL_Ser563 and HSL_Ser660. This was also accompanied by a 73% reduction in epinephrine‐stimulated HSL activity. Compound C prevented the phosphorylation of HSL_Ser565 induced by AICAR and partially prevented the inhibitory effect of this drug on basal and epinephrine‐stimulated lipolysis in adipocytes in VC and SC fat depots. In summary, despite different fat depots eliciting distinct rates of lipolysis, acute AICAR‐induced AMPK activation suppressed HSL phosphorylation/activation and exerted similar antilipolytic effects on both VC and SC adipocytes.     

Resveratrol Antagonizes Antimicrobial Lethality and Stimulates Recovery of Bacterial Mutants

Reactive oxygen species (ROS; superoxide, peroxide, and hydroxyl radical) are thought to contribute to the rapid bactericidal activity of diverse antimicrobial agents. The possibility has been raised that consumption of antioxidants in food may interfere with the lethal action of antimicrobials. Whether nutritional supplements containing antioxidant activity are also likely to interfere with antimicrobial lethality is unknown. To examine this possibility, resveratrol, a popular antioxidant dietary supplement, was added to cultures of Escherichia coli and Staphylococcus aureus that were then treated with antimicrobial and assayed for bacterial survival and the recovery of mutants resistant to an unrelated antimicrobial, rifampicin. Resveratrol, at concentrations likely to be present during human consumption, caused a 2- to 3-fold reduction in killing during a 2-hr treatment with moxifloxacin or kanamycin. At higher, but still subinhibitory concentrations, resveratrol reduced antimicrobial lethality by more than 3 orders of magnitude. Resveratrol also reduced the increase in reactive oxygen species (ROS) characteristic of treatment with quinolone (oxolinic acid). These data support the general idea that the lethal activity of some antimicrobials involves ROS. Surprisingly, subinhibitory concentrations of resveratrol promoted (2- to 6-fold) the recovery of rifampicin-resistant mutants arising from the action of ciprofloxacin, kanamycin, or daptomycin. This result is consistent with resveratrol reducing ROS to sublethal levels that are still mutagenic, while the absence of resveratrol allows ROS levels to high enough to kill mutagenized cells. Suppression of antimicrobial lethality and promotion of mutant recovery by resveratrol suggests that the antioxidant may contribute to the emergence of resistance to several antimicrobials, especially if new derivatives and/or formulations of resveratrol markedly increase bioavailability.     

Diversity of Innate Immune Recognition Mechanism for Bacterial Polymeric meso-Diaminopimelic Acid-type Peptidoglycan in Insects

In Drosophila, the synthesis of antimicrobial peptides in response to microbial infections is under the control of the Toll and immune deficiency (Imd) signaling pathway. The Toll signaling pathway responds mainly to the lysine-type peptidoglycan of Gram-positive bacteria and fungal β-1,3-glucan, whereas the Imd pathway responds to the meso-diaminopimelic acid (DAP)-type peptidoglycan of Gram-negative bacteria and certain Gram-positive bacilli. Recently we determined the activation mechanism of a Toll signaling pathway biochemically using a large beetle, Tenebrio molitor. However, DAP-type peptidoglycan recognition mechanism and its signaling pathway are still unclear in the fly and beetle. Here, we show that polymeric DAP-type peptidoglycan, but not its monomeric form, formed a complex with Tenebrio peptidoglycan recognition protein-SA, and this complex activated the three-step proteolytic cascade to produce processed Spätzle, a Toll receptor ligand, and induced Drosophila defensin-like antimicrobial peptide in Tenebrio larvae similarly to polymeric lysine-type peptidoglycan. Monomeric DAP-type peptidoglycan induced Drosophila diptericin-like antimicrobial peptide in Tenebrio hemocytes. In addition, both polymeric and monomeric DAP-type peptidoglycans induced expression of Tenebrio peptidoglycan recognition protein-SC2, which is DAP-type peptidoglycan-selective N-acetylmuramyl-l-alanine amidase that functions as a DAP-type peptidoglycan scavenger, appearing to function as a negative regulator of the DAP-type peptidoglycan signaling by cleaving DAP-type peptidoglycan in Tenebrio larvae. Taken together, these results demonstrate that molecular recognition mechanism for polymeric DAP-type peptidoglycan is different between Tenebrio larvae and Drosophila adults, providing biochemical evidences of biological diversity of innate immune responses in insects.     

Signalling pathways and molecular interactions of NOD1 and NOD2

The NOD (nucleotide-binding oligomerization domain) proteins NOD1 and NOD2 have important roles in innate immunity as sensors of microbial components derived from bacterial peptidoglycan. The importance of these molecules is underscored by the fact that mutations in the gene that encodes NOD2 occur in a subpopulation of patients with Crohn's disease, and NOD1 has also been shown to participate in host defence against infection with Helicobacter pylori. Here, we focus on the molecular interactions between these NOD proteins and other intracellular molecules to elucidate the mechanisms by which NOD1 and NOD2 contribute to the maintenance of mucosal homeostasis and the induction of mucosal inflammation.