Hormone-sensitive lipase functions as an oligomer

Photo-Induced Cross-Linking of Unmodified α-Synuclein Oligomers

Photo-induced cross-linking of unmodified proteins (PICUP) has been used in the past to study size distributions of protein assemblies. PICUP may, for example, overcome the significant experimental challenges related to the transient nature, heterogeneity, and low concentration of amyloid protein oligomers relative to monomeric and fibrillar species. In the current study, a reaction chamber was designed, produced, and used for PICUP reaction optimization in terms of reaction conditions and lighting time from ms to s. These efforts make the method more reproducible and accessible and enable the use of shorter reaction times compared to previous studies. We applied the optimized method to an α-synuclein aggregation time course to monitor the relative concentration and size distribution of oligomers over time. The data are compared to the time evolution of the fibril mass concentration, as monitored by thioflavin T fluorescence. At all time points, the smaller the oligomer, the higher its concentration observed after PICUP. Moreover, the total oligomer concentration is highest at short aggregation times, and the decline over time follows the disappearance of monomers. We can therefore conclude that these oligomers form from monomers.

Hormone sensitive lipase ablation promotes bone regeneration

There is an inverse relationship between the differentiation of mesenchymal stem cells (MSCs) along either an adipocyte or osteoblast lineage, with lineage differentiation known to be mediated by transcription factors PPARγ and Runx2, respectively. Endogenous ligands for PPARγ are generated during the hydrolysis of triacylglycerols to fatty acids through the actions of lipases such as hormone sensitive lipase (HSL). To examine whether reduced production of endogenous PPARγ ligands would influence bone regeneration, we examined the effects of HSL knockout on fracture repair in mice using a tibial mono-cortical defect as a model. We found an improved rate of fracture repair in HSL-ko mice documented by serial μCT and bone histomorphometry compared to wild-type (WT) mice. Similarly, accelerated rates of bone regeneration were observed with a calvarial model where implantation of bone grafts from HSL-ko mice accelerated bone regeneration at the injury site. Further analysis revealed improved MSC differentiation down osteoblast and chondrocyte lineage with inhibition of HSL. MSC recruitment to the injury site was greater in HSL-ko mice than WT. Finally, we used single cell RNAseq to understand the osteoimmunological differences between WT and HSL-ko mice and found changes in the pre-osteoclast population. Our study shows HSL-ko mice as an interesting model to study improvements to bone injury repair. Furthermore, our study highlights the potential importance of pre-osteoclasts and osteoclasts in bone repair.

The acceleration of yellow lupine flower abscission by jasmonates is accompanied by lipid-related events in abscission zone cells

Yellow lupine is an economically important crop. This species has been used as a great model for abscission processes for several years due to extreme flower abortion, which takes place in the abscission zone (AZ). AZ activation involves modifications of cell walls, membranes, and cellular structure. In this paper, we applied physiological, molecular, biochemical, and instrumental methods to explore lipid-associated changes and the possible involvement of lipid-derived phytohormones - jasmonates (JAs) - in flower AZ activation. Our comprehensive analyses revealed that natural abscission is accompanied by the upregulation of peroxidase, which reflects a disruption of redox balance and/or lipids peroxidation in AZ cell membranes. Redox imbalance was confirmed by appearance of malondialdehyde. Lipid-related processes involved the specific localization and increased level and activity of lipase and LOX, enzymes associated with cell membrane rupture, and JA biosynthesis. Lipid-hydrolyzing phospholipase D, implicated previously in abscission, is also found in naturally active AZs. Observed changes are accompanied by the accumulation of jasmonates, both free jasmonic acid and its methyl ester. The JA derivative exhibited higher biological activity than the nonconjugated form. Overall, our study shed new light on the lipid and phytohormonal regulation of AZ functioning supporting a role of JAs during abscission-associated events.

Hormone-sensitive lipase: sixty years later

Hormone-sensitive lipase (HSL) was initially characterized as the hormonally regulated neutral lipase activity responsible for the breakdown of triacylglycerols into fatty acids in adipose tissue. This review aims at providing up-to-date information on structural properties, regulation of expression, activity and function as well as therapeutic potential. The lipase is expressed as different isoforms produced from tissue-specific alternative promoters. All isoforms are composed of an N-terminal domain and a C-terminal catalytic domain within which a regulatory domain containing the phosphorylation sites is embedded. Some isoforms possess additional N-terminal regions. The catalytic domain shares similarities with bacteria, fungus and vascular plant proteins but not with other mammalian lipases. HSL singularity is provided by regulatory and N-terminal domains sharing no homology with other proteins. HSL has a broad substrate specificity compared to other neutral lipases. It hydrolyzes acylglycerols, cholesteryl and retinyl esters among other substrates. A novel role of HSL, independent of its enzymatic function, has recently been described in adipocytes. Clinical studies revealed dysregulations of HSL expression and activity in disorders, such as lipodystrophy, obesity, type 2 diabetes and cancer-associated cachexia. Development of specific inhibitors positions HSL as a pharmacological target for the treatment of metabolic complications.

The Lipolysome-A Highly Complex and Dynamic Protein Network Orchestrating Cytoplasmic Triacylglycerol Degradation

The catabolism of intracellular triacylglycerols (TAGs) involves the activity of cytoplasmic and lysosomal enzymes. Cytoplasmic TAG hydrolysis, commonly termed lipolysis, is catalyzed by the sequential action of three major hydrolases, namely adipose triglyceride lipase, hormone-sensitive lipase, and monoacylglycerol lipase. All three enzymes interact with numerous protein binding partners that modulate their activity, cellular localization, or stability. Deficiencies of these auxiliary proteins can lead to derangements in neutral lipid metabolism and energy homeostasis. In this review, we summarize the composition and the dynamics of the complex lipolytic machinery we like to call "lipolysome".

Enzymatic characterization of a soluble aggregate induced by N-terminal extension to a lipolytic enzyme

A self-assembling peptide (27PEP) was isolated from an open reading frame (ORF). The ORF consisted of an unknown functional domain and a catalytic (lipolytic and phospholipolytic) domain (MPlaG) on metagenomic fosmid clone. This extension of 27 amino acids prior to the N-terminus of the catalytic domain (27PEP-MPlaG), starting at Met₂₆₁, produced an aggregate of high molecular weight (> 700 kDa). Compared with MPlaG, 27PEP-MPlaG showed the same temperature and pH effect for maximum activity but was stable in the presence of inhibitors such as EDTA and PMSF. The 27PEP-MPlaG exhibited lower specific activity than that of MPlaG, but when pre-incubated for 30 min at temperatures between 4 and 100 °C, its activity increased at temperatures greater than 40 °C under alkaline conditions and eventually reached the specific activity level of MPlaG at 60 °C. We experimentally determined that the aggregate caused by 27PEP was dissociated at elevated temperatures resulting in an active catalytic monomer. The 27PEP-indued aggregation may be attractive as application tool for improving or engineering of biocatalysts and biomaterials.

Different effects of growth hormone and fasting on the induction patterns of two hormone-sensitive lipase genes in red seabream Pagrus major

Growth hormone (GH) increases phosphorylation and mRNA levels of hormone-sensitive lipase (HSL) in the livers of some marine teleosts. The hepatic GH-HSL axis appears to play important roles in fasting-induced lipolysis. However, it is not known whether GH exerts similar effects on HSL in fish adipose tissues. Functional differentiation of two fish-specific HSL isoforms (HSL1 and HSL2) also remains unclear. The present study seeks to address two unanswered questions about fish lipolysis using red seabream (Pagrus major): (1) Does GH increase phosphorylation and mRNA levels of HSL in adipose tissue? (2) How do GH and fasting affect mRNA levels of two HSL isoform genes in the liver and adipose tissue? To this end, we first cloned HSL1 and HSL2 cDNAs and investigated their tissue distribution. Transcripts of both HSLs and HSL1 proteins were abundant in the visceral adipose tissue, gonads, and liver, suggesting the important role of HSL in adipose tissue lipolysis. HSL2 transcript levels were 20-65% those of HSL1 except in the skin, and HSL2 proteins were not detected by our in-house antisera. Ex vivo administration of GH increased HSL1 phosphorylation, non-esterified fatty acid (NEFA) release, and levels of HSL1 and HSL2 mRNA in both the liver and visceral adipose tissue. Hepatic HSL2 mRNA was particularly sensitive to GH administration and sometimes exceeded HSL1 mRNA levels with up to 13-fold induction. In contrast, fasting for 4 and 7d increased HSL1 mRNA levels, but had only marginal effects on HSL2 mRNA levels in both adipose tissue or liver. We concluded that GH would increase HSL mRNAs during adipose tissue lipolysis in red seabream; however, GH and fasting result in different induction ratio of two HSL isoform genes, suggesting that other hormone(s) also contributes to fasting-induced lipolysis.

Structural insights of a hormone sensitive lipase homologue Est22

Hormone sensitive lipase (HSL) catalyzes the hydrolysis of triacylglycerols into fatty acids and glycerol, thus playing key roles in energy homeostasis. However, the application of HSL serving as a pharmaceutical target and an industrial biocatalyst is largely hampered due to the lack of high-resolution structural information. Here we report biochemical properties and crystal structures of a novel HSL homologue esterase Est22 from a deep-sea metagenomic library. Est22 prefers short acyl chain esters and has a very high activity with substrate p-nitrophenyl butyrate. The crystal structures of wild type and mutated Est22 with its product p-nitrophenol are solved with resolutions ranging from 1.4 Å to 2.43 Å. The Est22 exhibits a α/β-hydrolase fold consisting with a catalytic domain and a substrate-recognizing cap domain. Residues Ser188, Asp287, and His317 comprise the catalytic triad in the catalytic domain. The p-nitrophenol molecule occupies the substrate binding pocket and forms hydrogen bonds with adjacent residues Gly108, Gly109, and Gly189. Est22 exhibits a dimeric form in solution, whereas mutants D287A and H317A change to polymeric form, which totally abolished its enzymatic activities. Our study provides insights into the catalytic mechanism of HSL family esterase and facilitates the understanding for further industrial and biotechnological applications of esterases.

DAG tales: the multiple faces of diacylglycerol--stereochemistry, metabolism, and signaling

The neutral lipids diacylglycerols (DAGs) are involved in a plethora of metabolic pathways. They function as components of cellular membranes, as building blocks for glycero(phospho)lipids, and as lipid second messengers. Considering their central role in multiple metabolic processes and signaling pathways, cellular DAG levels require a tight regulation to ensure a constant and controlled availability. Interestingly, DAG species are versatile in their chemical structure. Besides the different fatty acid species esterified to the glycerol backbone, DAGs can occur in three different stereo/regioisoforms, each with unique biological properties. Recent scientific advances have revealed that DAG metabolizing enzymes generate and distinguish different DAG isoforms, and that only one DAG isoform holds signaling properties. Herein, we review the current knowledge of DAG stereochemistry and their impact on cellular metabolism and signaling. Further, we describe intracellular DAG turnover and its stereochemistry in a 3-pool model to illustrate the spatial and stereochemical separation and hereby the diversity of cellular DAG metabolism.

Characterization of the bovine gene LIPE and possible influence on fatty acid composition of meat

LIPE is an intracellular neutral lipase, which is capable of hydrolyzing a variety of esters and plays a key role in the mobilization of fatty acids from diacylglycerols. The objectives of this study were to characterize the genetic polymorphism of bovine LIPE gene and to evaluate the possible association between three SNPs in the coding regions of this gene with the fatty acid composition of meat in a cattle population. Forty-three unrelated animals from different cattle breeds were re-sequenced and 21 SNPs were detected over approximately 2600 bp, five of these SNPs were novel. Three SNPs were selected, on the basis of evolutionary conservation, to perform validation and association studies in a crossbred cattle population. Our results may suggest a possible association of SNP1 with contents of oleic acid and total monounsaturated fatty acids (p < 0.01), and SNP2 and SNP3 with Heneicosylic acid content (p < 0.01), may be helpful to improve the quality of meat and improve health.

Structural and functional analyses of a bacterial homologue of hormone-sensitive lipase from a metagenomic library

Intracellular mobilization of fatty acids from triacylglycerols in mammalian adipose tissues proceeds through a series of lipolytic reactions. Among the enzymes involved, hormone-sensitive lipase (HSL) is noteworthy for its central role in energy homeostasis and the pathogenic role played by its dysregulation. By virtue of its broad substrate specificity, HSL may also serve as an industrial biocatalyst. In a previous report, Est25, a bacterial homologue of HSL, was identified from a metagenomic library by functional screening. Here, the crystal structure of Est25 is reported at 1.49 Å resolution; it exhibits an α/β-hydrolase fold consisting of a central β-sheet enclosed by α-helices on both sides. The structural features of the cap domain, the substrate-binding pocket and the dimeric interface of Est25, together with biochemical and biophysical studies including native PAGE, mass spectrometry, dynamic light scattering, gel filtration and enzyme assays, could provide a basis for understanding the properties and regulation of hormone-sensitive lipase (HSL). The increased stability of cross-linked Est25 aggregates (CLEA-Est25) and their potential for extensive reuse support the application of this preparation as a biocatalyst in biotransformation processes.

The microbial diversity of Polar environments is a fertile ground for bioprospecting

The term bioprospecting has been adopted for systematic searches in nature for new bioactive compounds, genes, proteins, microorganisms and other products with potential for commercial use. Much effort has been focused on microorganisms able to thrive under harsh conditions, including the Polar environments. Both the lipid and protein cellular building blocks of Polar microorganisms are shaped by their adaptation to the permanently low temperatures. In addition, strongly differing environments, such as permafrost, glaciers and sea ice, have contributed to additional functional diversity. Emerging massive-parallel sequencing technologies have revealed the existence of a huge, hitherto unseen diversity of low-abundance phylotypes--the rare biosphere--even in the Polar environments. This realization has further strengthened the need to employ cultivation-independent approaches, including metagenomics and single-cell genomic sequencing, to get comprehensive access to the genetic diversity of microbial communities for bioprospecting purposes. In this review, we present an updated snapshot of recent findings on the molecular basis for adaptation to the cold and the phylogenetic diversities of different Polar environments. Novel approaches in bioprospecting are presented and we conclude by showing recent bioprospecting outcomes in terms of new molecules patented or applied by some biotech companies.

The resurgence of Hormone-Sensitive Lipase (HSL) in mammalian lipolysis

The ability to store energy in the form of energy-dense triacylglycerol and to mobilize these stores rapidly during periods of low carbohydrate availability or throughout the strong metabolic demand is a highly conserved process, absolutely essential for survival. In the industrialized world the regulation of this pathway is viewed as an important therapeutic target for disease prevention. Adipose tissue lipolysis is a catabolic process leading to the breakdown of triacylglycerols stored in fat cells, and release of fatty acids and glycerol. Mobilization of adipose tissue fat is mediated by the MGL, HSL and ATGL, similarly functioning enzymes. ATGL initiates lipolysis followed by the actions of HSL on diacylglycerol, and MGL on monoacylglycerol. HSL is regulated by reversible phosphorylation on five critical residues. Phosphorylation alone, however, is not enough to activate HSL. Probably, conformational alterations and a translocation from the cytoplasm to lipid droplets are also involved. In accordance, Perilipin functions as a master regulator of lipolysis, protecting or exposing the triacylglycerol core of a lipid droplet to lipases. The prototype processes of hormonal lipolytic control are the β-adrenergic stimulation and suppression by insulin, both of which affect cytoplasmic cyclic AMP levels. Lipolysis in adipocytes is an important process in the management of body energy reserves. Its deregulation may contribute to the symptoms of type 2 diabetes mellitus and other pathological situations. We, herein, discuss the metabolic regulation and function of lipases mediating mammalian lipolysis with a focus on HSL, quoting newly identified members of the lipolytic proteome.

Lipolysis - a highly regulated multi-enzyme complex mediates the catabolism of cellular fat stores

Lipolysis is the biochemical pathway responsible for the catabolism of triacylglycerol (TAG) stored in cellular lipid droplets. The hydrolytic cleavage of TAG generates non-esterified fatty acids, which are subsequently used as energy substrates, essential precursors for lipid and membrane synthesis, or mediators in cell signaling processes. Consistent with its central importance in lipid and energy homeostasis, lipolysis occurs in essentially all tissues and cell types, it is most abundant, however, in white and brown adipose tissue. Over the last 5years, important enzymes and regulatory protein factors involved in lipolysis have been identified. These include an essential TAG hydrolase named adipose triglyceride lipase (ATGL) [annotated as patatin-like phospholipase domain-containing protein A2], the ATGL activator comparative gene identification-58 [annotated as α/β hydrolase containing protein 5], and the ATGL inhibitor G0/G1 switch gene 2. Together with the established hormone-sensitive lipase [annotated as lipase E] and monoglyceride lipase, these proteins constitute the basic "lipolytic machinery". Additionally, a large number of hormonal signaling pathways and lipid droplet-associated protein factors regulate substrate access and the activity of the "lipolysome". This review summarizes the current knowledge concerning the enzymes and regulatory processes governing lipolysis of fat stores in adipose and non-adipose tissues. Special emphasis will be given to ATGL, its regulation, and physiological function.

Vimentin is a functional partner of hormone sensitive lipase and facilitates lipolysis

Lipolysis involves a number of components including signaling pathways, droplet-associated proteins, and lipases such as hormone-sensitive lipase (HSL). We used surface enhanced laser desorption/ionization time-of-flight mass spectroscopy to identify cellular proteins that might interact with HSL and potentially influence lipolysis. Using recombinant HSL as bait on protein chips, clusters of proteins of 14.7-18.9, 25.8-26.8, 36.1, 44.3-49.1, and 53.7 kDa were identified that interact with HSL, particularly when lysates were examined from beta-agonist treated mouse adipocytes. The ability to detect these interacting proteins was markedly diminished when the adipocytes were treated with insulin. A very similar pattern of proteins was identified when anti-HSL IgG was used as the bait. Following immunocapture, the identification of the prominent 53.7 kDa protein was carried out by tryptic digestion and MS analysis and determined to be vimentin. The interaction of HSL with vimentin, and its hormonal dependence, was confirmed by coimmunoprecipitation. beta-Agonist stimulated lipolysis and the rate of HSL translocation were impaired in vimentin null adipocytes, even though normal amounts of lipases and droplet-associated proteins are expressed. The current studies provide evidence that vimentin participates in lipolysis through direct, hormonally regulated interactions with HSL.

Quarternary structure and enzymological properties of the different hormone-sensitive lipase (HSL) isoforms

BACKGROUND

Hormone-sensitive lipase (HSL) is a key enzyme in the mobilization of energy in the form of fatty acids from intracellular stores of neutral lipids. The enzyme has been shown to exist in different isoforms with different molecular masses (84 kDa, 89 kDa and 117 kDa) expressed in a tissue-dependent manner, where the predominant 84 kDa form in adipocytes is the most extensively studied.

METHODOLOGY/PRINCIPAL FINDINGS

In this study we employed negative stain electron microscopy (EM) to analyze the quarternary structure of the different HSL isoforms. The results show that all three isoforms adopt a head-to-head homodimeric organization, where each monomer contains two structural domains. We also used enzymatic assays to show that despite the variation in the size of the N-terminal domain all three isoforms exhibit similar enzymological properties with regard to psychrotolerance and protein kinase A (PKA)-mediated phosphorylation and activation.

CONCLUSIONS/SIGNIFICANCE

We present the first data on the quaternary structure and domain organization of the three HSL isoforms. We conclude that despite large differences in the size of the N-terminal, non-catalytic domain all three HSL isoforms exhibit the same three-dimensional architecture. Furthermore, the three HSL isoforms are very similar with regard to two unique enzymological characteristics of HSL, i.e., cold adaptation and PKA-mediated activation.

Spatiotemporal regulation of early lipolytic signaling in adipocytes

Hormone-sensitive lipase (HSL) is a key enzyme regulating the acute activation of lipolysis. HSL functionality is controlled by multiple phosphorylation events, which regulate its association with the surface of lipid droplets (LDs). We determined the progression and stability of HSL phosphorylation on individual serine residues both spatially and temporally in adipocytes using phospho-specific antibodies. Within seconds of beta-adrenergic receptor activation, HSL was phosphorylated on Ser-660, the phosphorylated form appearing in the peripheral cytosol prior to rapid translocation to, and stable association with, LDs. In contrast, phosphorylation of HSL on Ser-563 was delayed, the phosphorylated protein was predominantly detected on LDs, and mutation of the Ser-659/Ser-660 site to Ala significantly reduced subsequent phosphorylation on Ser-563. Phosphorylation of HSL on Ser-565 was observed in control cells; the phosphorylated protein was translocated to LDs with similar kinetics to total HSL, and the degree of phosphorylation was inversely related to phospho-HSL(Ser-563). These results describe the remarkably rapid, sequential phosphorylation of specific serine residues in HSL at spatially distinct intracellular locales, providing new insight into the complex regulation of lipolysis.

Adipose triglyceride lipase and the lipolytic catabolism of cellular fat stores

Fatty acids (FAs) are essential components of all lipid classes and pivotal substrates for energy production in all vertebrates. Additionally, they act directly or indirectly as signaling molecules and, when bonded to amino acid side chains of peptides, anchor proteins in biological membranes. In vertebrates, FAs are predominantly stored in the form of triacylglycerol (TG) within lipid droplets of white adipose tissue. Lipid droplet-associated TGs are also found in most nonadipose tissues, including liver, cardiac muscle, and skeletal muscle. The mobilization of FAs from all fat depots depends on the activity of TG hydrolases. Currently, three enzymes are known to hydrolyze TG, the well-studied hormone-sensitive lipase (HSL) and monoglyceride lipase (MGL), discovered more than 40 years ago, as well as the relatively recently identified adipose triglyceride lipase (ATGL). The phenotype of HSL- and ATGL-deficient mice, as well as the disease pattern of patients with defective ATGL activity (due to mutation in ATGL or in the enzyme's activator, CGI-58), suggest that the consecutive action of ATGL, HSL, and MGL is responsible for the complete hydrolysis of a TG molecule. The complex regulation of these enzymes by numerous, partially uncharacterized effectors creates the "lipolysome," a complex metabolic network that contributes to the control of lipid and energy homeostasis. This review focuses on the structure, function, and regulation of lipolytic enzymes with a special emphasis on ATGL.

Characterization and importance of the dimer interface of human calcium-activated nucleotidase

Human calcium-activated nucleotidase (CAN) exists as both a membrane-bound form in the endoplasmic reticulum and pre-Golgi intermediate membranes and as a secreted, soluble form. Although the wild-type human enzyme hydrolyzes ADP poorly, engineered soluble human proteins (SCANs) hydrolyze ADP much more efficiently, making them potentially useful therapeutic proteins for treatment of human clotting pathologies. According to the crystal structure and the recently identified dimeric nature of the soluble nucleotidase, the dimer interface contains a central core of hydrophobic residues. Previously, we demonstrated that the mutation of glutamic acid 130 (located in the dimer interface) to tyrosine increased both the tendency to form dimers and the ADPase activity. In the present study, we investigated the importance of the dimeric state for enzymatic activity and biological function in this nucleotidase by mutating isoleucine 170, which is located in the center of the hydrophobic core of the dimer interface. The results of analytical ultracentrifugation, chemical cross-linking, and tryptophan fluorescence analyses demonstrated that mutation of isoleucine 170 to either positively or negatively charged amino acids (lys or glu) disrupted the calcium-dependent dimerization in soluble CAN. Furthermore, these mutations decreased maximal ADPase activity for both the soluble and membrane-bound enzymes. Although not as critical as the hydrophobic interactions centered at isoleucine 170, the role of hydrophilic interactions in dimer formation was also demonstrated. Thus, mutation of aspartic acid 228 to threonine (D228T) decreased both the tendency to form dimers and ADPase activity, while double mutation of D228T/K224N largely restored the ability to form dimers and the ADPase activity, further indicating that the nucleotidase activity of CAN is linked to its quaternary structure. Since ADPase activity of the soluble form is crucial for its potential development as a therapeutic protein, these findings have implications for engineering the soluble human calcium-activated nucleotidase for clinical applications. In addition, future comparison of monomeric (I170K and I170E mutants) and dimeric (wild-type) crystal structures of SCAN will advance our understanding of its enzymatic mechanism and aid in engineering efforts.

Interaction of the adipocyte fatty acid-binding protein with the hormone-sensitive lipase: regulation by fatty acids and phosphorylation

Adipocyte fatty acid-binding protein (AFABP/aP2) forms a physical complex with the hormone-sensitive lipase (HSL) and AFABP/aP2-null mice exhibit reduced basal and hormone-stimulated lipolysis. To identify the determinants affecting the interaction fluorescence resonance energy transfer (FRET) imaging was used in conjunction with a mutagenesis strategy to evaluate the roles AFABP/aP2 fatty acid binding and HSL phosphorylation have in complex formation as well as determine the HSL binding site on AFABP/aP2. The nonfatty acid binding mutant of AFABP/aP2 (R126Q) failed to form a FRET-competent complex with HSL either under basal or forskolin-stimulated conditions, indicating that lipid binding is required for association. Once bound to HSL and on the surface of the lipid droplet, YFP-AFABP/aP2 (but not YFP-HSL) exhibited energy transfer between the fusion protein and BODIPY-C12-labeled triacylglycerol. Serine to alanine mutations at the two PKA phosphorylation sites of HSL (659 and 660), or at the AMPK phosphorylation sites (565), blocked FRET between HSL and AFABP/aP2. Substitution of isoleucine for lysine at position 21 of AFABP/aP2 (K21I), but not 31 (K31I), resulted in a non-HSL-binding protein indicating that residues on helix alphaI of AFABP/aP2 define a component of the HSL binding site. These results indicate that the ligand-bound form of AFABP/aP2.interacts with the activated, phosphorylated HSL and that the association is likely to be regulatory; either delivering FA to inhibit HSL (facilitating feedback inhibition) or affecting multicomponent complex formation on the droplet surface.

Structural study of metastable amyloidogenic protein oligomers by photo-induced cross-linking of unmodified proteins

Oligomers of amyloidogenic proteins are believed to be key effectors of cytotoxicity and cause a variety of amyloid-related diseases. Dissociation or inhibition of formation of the toxic oligomers is thus an attractive strategy for the prevention and treatment of these diseases. In order to develop reagents capable of inhibiting protein oligomerization, the structures and mechanisms of oligomer formation must be understood. However, structural studies of oligomers are difficult because of the metastable nature of the oligomers and their existence in mixtures with monomers and other assemblies. A useful method for characterization of oligomer size distributions in vitro is photo-induced cross-linking of unmodified proteins (PICUP) (Fancy and Kodadek, 1999). By providing "snapshots" of dynamic oligomer mixtures, PICUP enables quantitative analysis of the relations between primary and quaternary structures, offering insights into the molecular organization of the oligomers. This chapter discusses the photochemical mechanism; reviews the scope, usefulness, and limitations of PICUP for characterizing metastable protein assemblies; and provides detailed experimental instructions for performing PICUP experiments.

Human hormone-sensitive lipase (HSL): expression in white fat corrects the white adipose phenotype of HSL-deficient mice

In white adipose tissue (WAT), hormone-sensitive lipase (HSL) can mediate lipolysis, a central pathway in obesity and diabetes. Gene-targeted HSL-deficient (HSL-/-) mice with no detectable HSL peptide or activity (measured as cholesteryl esterase) have WAT abnormalities, including low mass, marked heterogeneity of cell diameter, increased diacylglycerol content, and low beta-adrenergic stimulation of adipocyte lipolysis. Three transgenic mouse strains preferentially expressing human HSL in WAT were bred to a HSL-/- background. One, HSL-/- N, expresses normal human HSL (41.3 +/- 9.1% of normal activity); two express a serine-to-alanine mutant (S554A) initially hypothesized to be constitutively active: HSL-/- ML, 50.3 +/- 12.3% of normal, and HSL-/- MH, 69.8 +/- 15.8% of normal. In WAT, HSL-/- N mice resembled HSL+/+ controls in WAT mass, histology, diacylglyceride content, and lipolytic response to beta-adrenergic agents. In contrast, HSL-/- ML and HSL-/- MH mice resembled nontransgenic HSL-/- mice, except that diacylglycerol content and perirenal and inguinal WAT masses approached normal in HSL-/- MH mice. Therefore, 1) WAT expression of normal human HSL markedly improves HSL-/- WAT biochemically, physiologically, and morphologically; 2) similar levels of S554A HSL have a low physiological effect despite being active in vitro; and 3) diacylglycerol accumulation is not essential for the development of the characteristic WAT pathology of HSL-/- mice.

Disruption of dimerization and substrate phosphorylation inhibit factor inhibiting hypoxia-inducible factor (FIH) activity

HIF (hypoxia-inducible factor) is an alphabeta transcription factor that modulates the hypoxic response in many animals. The cellular abundance and activity of HIF-alpha are regulated by its post-translational hydroxylation. The hydroxylation of HIF is catalysed by PHD (prolyl hydroxylase domain) enzymes and FIH (factorinhibiting HIF), all of which are 2-oxoglutarate- and Fe(II)-dependent dioxygenases. FIH hydroxylates a conserved asparagine residue in HIF-alpha (Asn-803), which blocks the binding of HIF to the transcriptional co-activator p300, preventing transcription of hypoxia-regulated genes under normoxic conditions. In the present paper, we report studies on possible mechanisms for the regulation of FIH activity. Recently solved crystal structures of FIH indicate that it is homodimeric. Site-directed mutants of FIH at residues Leu-340 and Ile-344, designed to disrupt dimerization, were generated in order to examine the importance of the dimeric state in determining FIH activity. A single point mutant, L340R (Leu-340-->Arg), was shown to be predominantly monomeric and to have lost catalytic activity as measured by assays monitoring 2-oxoglutarate turnover and asparagine hydroxylation. In contrast, the I344R (Ile-344-->Arg) mutant was predominantly dimeric and catalytically active. The results imply that the homodimeric form of FIH is required for productive substrate binding. The structural data also revealed a hydrophobic interaction formed between FIH and a conserved leucine residue (Leu-795) on the HIF substrate, which is close to the dimer interface. A recent report has revealed that phosphorylation of Thr-796, which is adjacent to Leu-795, enhances the transcriptional response in hypoxia. Consistent with this, we show that phosphorylation of Thr-796 prevents the hydroxylation of Asn-803 by FIH.

Role of the N terminus in enzyme activity, stability and specificity in thermophilic esterases belonging to the HSL family

A superposition between the structures of Alicyclobacillus acidocaldarius esterase 2 (EST2) and Burkholderia cepacia lipase, the latter complexed with a phosphonate inhibitor, allowed us to hypothesize for the EST2 N terminus a role in restricting the access to the active site and therefore in modulating substrate specificity. In order to test this hypothesis we generated by site-directed mutagenesis some truncated versions of EST2 and its double mutant M211S/R215L (S/L) at the N terminus. In parallel, an analysis of the Sulfolobus solfataricus P2 genome allowed us to identify a gene coding for a putative esterase of the HSL family having a natural deletion of the corresponding region. The product of this gene and the above-mentioned EST2 mutants were expressed in Escherichia coli, purified and characterised. These studies support the notion that the N terminus affects substrate specificity other than several other enzyme parameters. Although the deletions afforded a tenfold and 550-fold decrease in catalytic efficiency towards the best substrate pNP-hexanoate at 50 degrees C for EST2 and S/L, respectively, the analysis of the specific activities with different triacylglycerols with respect to pNP-hexanoate showed that their ratios were higher for deleted versus non-deleted enzymes, on all tested substrates. In particular, the above ratios for glyceryl tridecanoate were 30-fold and 14-fold higher in S/L and EST2 deleted forms, respectively, compared with their full-length versions. This behaviour was confirmed by the analysis of the S.solfataricus esterase, which showed similar specific activities on pNP-hexanoate and triacylglycerols; in addition, higher activities on the latter substrates were observed in comparison with EST2, S/L and their deleted forms. Finally, a dramatic effect on thermophilicity and thermostability in the EST2 deleted forms was observed. This is the first report highlighting the importance of the "cap" domain in the HSL family, since the N terminus partly contributes to the building up of this structure.

Metabolism of lipids in human white adipocyte

Adipose tissue is considered as the body's largest storage organ for energy in the form of triacylglycerols, which are mobilized through lipolysis process, to provide fuel to other organs and to deliver substrates to liver for gluconeogenesis (glycerol) and lipoprotein synthesis (free fatty acids). The release of glycerol and free fatty acids from human adipose tissue is mainly dependent on hormone-sensitive lipase which is intensively regulated by hormones and agents, such as insulin (inhibition of lipolysis) and catecholamines (stimulation of lipolysis). A special attention is paid to the recently discovered perilipins which could regulate the activity of the lipase hormono-sensible. Most of the plasma triacylglycerols are provided by dietary lipids, secreted from the intestine in the form of chylomicron or from the liver in the form of VLDL. Released into circulation as non-esterified fatty acids by lipoprotein lipase, those are taken up by adipose tissue via specific plasma fatty acid transporters (CD36, FATP, FABPpm) and used for triacylglycerol synthesis. A small part of triacylglycerols is synthesized into adipocytes from carbohydrates (lipogenesis) but its regulation is still debated in human. Physiological factors such as dieting/fasting regulate all these metabolic pathways, which are also modified in pathological conditions e.g. obesity.

Expression of human hormone-sensitive lipase (HSL) in postmeiotic germ cells confers normal fertility to HSL-deficient mice

Hormone-sensitive lipase (HSL, Lipe, E.C.3.1.1.3) is a multifunctional fatty acyl esterase that is essential for male fertility and spermatogenesis and that also plays important roles in the function of adipocytes, pancreatic beta-cells, and adrenal cortical cells. Gene-targeted HSL-deficient (HSL-/-) male mice are infertile, have a 2-fold reduction in testicular mass, a 2-fold elevation of the ratio of esterified to free cholesterol in testis, and unique morphological abnormalities in round and elongating spermatids. Postmeiotic germ cells in the testis express a specific HSL isoform. We created transgenic mice expressing a normal human testicular HSL cDNA from the mouse protamine-1 promoter, which mediates expression specifically in postmeiotic germ cells. Testicular cholesteryl esterase activity was undetectable in HSL-/- mice, but in HSL-/- males expressing the testicular transgene, activity was 2-fold greater than normal. HSL transgene mRNA became detectable in testes between 19 and 25 days of age, coinciding with the first wave of postmeiotic transcription in round spermatids. In contrast to nontransgenic HSL-/- mice, HSL-/- males expressing the testicular transgene were normal with respect to fertility, testicular mass, testicular esterified/free cholesterol ratio, and testicular histology. Their cauda epididymides contained abundant, normal-appearing spermatozoa. We conclude that human testicular HSL is functional in mouse testis and that the mechanism of infertility in HSL-deficient males is cell autonomous and resides in postmeiotic germ cells, because HSL expression in these cells is in itself sufficient to restore normal fertility.

Lysosomal acid lipase and atherosclerosis

PURPOSE OF REVIEW

Atherosclerosis remains the leading cause of death in the developed countries. In addition to lipid-lowering drugs - statins, dietary control, and exercise, new approaches are needed for the treatment and prevention of atherosclerosis. This review will focus on the role(s) of lysosomal acid lipase and its use as an enzyme therapy to reduce atherosclerotic lesions in a mouse model and to examine the molecular basis supporting this novel strategy and its mechanism of effect.

RECENT FINDINGS

Administration of human lysosomal acid lipase via tail vein into mice with atherosclerosis eliminates early aortic and coronary ostial lesions and reduces lesional size in advanced disease. The reduction of advanced lesional area is related to decreases in foamy macrophages, collagen positive areas, and necrotic areas. Compared with sham-treated mice, the human lysosomal acid lipase-treated mice also have reduced levels of plasma cholesteryl esters, and reduced levels of hepatic cholesterol and triglycerides.

SUMMARY

These studies indicate that administrated lysosomal acid lipase affects the atherogenesis by at least two mechanisms: (1) direct targeting of lesional macrophages with resultant decreases in cholesteryl esters and triglyceride in the lysosomes of macrophages in the lesions; (2) systemic effects that mediate the liver to reduce the hepatic cholesteryl ester and triglyceride release, possibly leading to reduced production of VLDL and LDL.

Using oxidative crosslinking and proximity labeling to quantitatively characterize protein-protein and protein-Peptide complexes

The quantitative analysis of protein-protein and protein-peptide complexes is of fundamental importance in biochemistry. We report here that nickel-catalyzed proximity biotinylation and Ru(II)(bpy)(3)(2+)-mediated oxidative crosslinking can be used to measure the equilibrium dissociation constant and stoichiometry of protein complexes. Only small amounts of protein are required, neither of the binding partners must be immobilized on a surface, and no special instrumentation is necessary. This chemistry should provide a useful complement to existing methods for the analysis of protein-protein and protein-peptide interactions.

Hormone-sensitive lipase--new roles for an old enzyme

Although described initially as an intracellular adipocyte-specific triacylglycerol lipase, it is now clear that HSL (hormone-sensitive lipase) is expressed in multiple tissues and plays a number of roles in lipid metabolism, including that of a neutral cholesteryl ester hydrolase. The major isoform is a single polypeptide with a molecular mass of approx. 84 kDa and which comprises three major domains: a catalytic domain, a regulatory domain encoding several phosphorylation sites and an N-terminal domain involved in protein-protein and protein-lipid interactions. The activity of HSL is regulated acutely by several mechanisms, including reversible phosphorylation by a number of different protein kinases, translocation to different sites within the cell and interaction with a number of proteins, some of which may serve to direct the inhibitory products of HSL away from the protein. It is also apparent from work with HSL null mice that more than one enzyme species may be classified as a hormone-sensitive lipase. The possible presence of HSL in macrophages remains controversial, and the role of the protein in pancreatic beta-cells has yet to be fully elucidated. Altered expression of HSL in different cell types may be associated with a number of pathological states, including obesity, atherosclerosis and Type II diabetes.

β-ADRENERGICRECEPTORS ANDREGULATION OFENERGYEXPENDITURE: A Family Affair

The family of adrenergic receptors (ARs) expressed in adipocytes includes three sibling betaARs and two alphaAR cousins. Together they profoundly influence the mobilization of stored fatty acids, secretion of fat-cell derived hormones, and the specialized process of nonshivering thermogenesis in brown adipose tissue. The two types of fat cells that compose adipose tissue, brown and white, are structurally and functionally distinct. Studies on the mechanisms by which individual betaAR regulates these cell-specific functions have recently uncovered new signal transduction cascades involved in processes traditionally ascribed to adenylyl cyclase/cAMP/protein kinase A system. They illustrate how betaAR signaling can orchestrate a coordinated set of intracellular responses for fine control of metabolic balance.

Angiotensin II activates cholesterol ester hydrolase in bovine adrenal glomerulosa cells through phosphorylation mediated by p42/p44 mitogen-activated protein kinase

In adrenal glomerulosa cells, the stimulation of aldosterone biosynthesis by angiotensin II (Ang II) occurs via activation of the Ca2+ messenger system, increased expression of the steroidogenic acute regulatory protein, and enhanced transfer of cholesterol to the inner mitochondrial membrane. We examined here whether Ang II affects the activity of cholesterol ester hydrolase (CEH), also named hormone-sensitive lipase, the enzyme recruiting cholesterol from intracellular pools, in bovine adrenal glomerulosa cells. In bovine adrenal tissue, CEH activity was detected with characteristics similar to those reported in other tissues (Michaelis constant = 46.3 +/- 6.7 microM, n = 3; maximal velocity = 1 nmol/mg.min). This activity was significantly enhanced in isolated bovine glomerulosa cells challenged for 2 h with 10 nM Ang II (to 149 +/- 11% of controls, n = 3). Similarly, 25 microM forskolin raised CEH activity to 151 +/- 5% of controls (n = 3). This increase in activity of CEH was not due to an increase in the amount of enzyme protein but was associated with an increased phosphorylation of the enzyme to 337 +/- 33% of controls (n = 9, P < 0.0001). Potassium ion (K+) and forskolin also stimulated [32P]orthophosphate incorporation, although to a lesser extent (to 157 +/- 18% and 186 +/- 25% of controls, respectively). On SDS-PAGE, the majority of this radioactivity was associated with a species of 172 kDa, corresponding to a CEH dimer. Both Ang II-induced CEH phosphorylation and pregnenolone production were significantly reduced (to 47 +/- 6% and 50 +/- 8% of controls with Ang II alone, respectively) in the presence of PD098059, an inhibitor of p42/p44 MAPK. Indeed, Ang II challenge led to a rapid 32P incorporation into p42/p44 MAPK. These results demonstrate that, in addition to its known effects on intramitochondrial cholesterol transfer, Ang II also promotes aldosterone biosynthesis by rapidly increasing cholesterol supply to the outer mitochondrial membrane.

The presence of a catalytically inactive form of hormone-sensitive lipase is associated with decreased lipolysis in abdominal subcutaneous adipose tissue of obese subjects

Hormone-sensitive lipase (HSL)-L is a key enzyme in the mobilization of fatty acids from triglyceride stores in adipocytes. A shorter variant of HSL (HSL-S) was detected in humans. This one is generated through in-frame skipping of exon 6 during the processing of HSL mRNA and results in a protein devoid of lipase activity. The role of HSL-S is unknown. The aims of this study were to identify both HSL variants in adipose tissue biopsies and to determine if the presence of HSL-S is correlated to the lipolytic capacity of adipocytes. The study was performed in human abdominal subcutaneous adipocytes from two groups of seven obese subjects. In the group of subjects with both HSL proteins (L+S) group, two immunoreactive bands (80 and 88 kDa) were detected, whereas only the 88-kDa protein was detected in the group with only the wild-type HSL-protein (L group). In the L+S group, the HSL activity was 20% lower (P < 0.05) and the (S/S(+)) HSL mRNA ratio was twofold higher than in the L group (P < 0.05). The maximally lipolytic capacities measured from isolated adipocytes incubated with norepinephrine or other lipolytic agents were 40% lower in the L+S group (P < 0.05). These results suggest that the presence of the truncated HSL protein is associated with an impaired adipocyte lipolysis.

Letting lipids go: hormone-sensitive lipase

PURPOSE OF REVIEW

Despite their pathophysiological importance, the molecular mechanisms and enzymatic components of lipid mobilization from intracellular storage compartments are insufficiently understood. The aim of this review is to evaluate the role of hormone-sensitive lipase in this process.

RECENT FINDINGS

Hormone-sensitive lipase exhibits a broad specificity for lipid substrates such as triglycerides, diglycerides, cholesteryl esters, and retinyl esters and the enzyme is in a wide variety of tissues. The high enzyme activity in adipose tissue was considered rate-limiting in the degradation of stored triglycerides. This view of a single enzyme controlling the catabolism of stored fat was challenged by recent findings that in hormone-sensitive lipase deficient mice adipose tissue triglycerides were still hydrolyzed and that these animals were leaner than normal mice. These results indicated that in adipose tissue hormone-sensitive lipase cooperates with other yet unidentified lipases to control the mobilization of fatty acids from cellular depots and that this process is coordinately regulated with lipid synthesis. Induced mutant mouse lines that overexpress or lack hormone-sensitive lipase also provided evidence that hormone-sensitive lipase-mediated cholesteryl ester hydrolysis is involved in steroid-hormone production in adrenals and affects testis function. Finally, hormone-sensitive lipase deficiency in mice results in a lipoprotein profile characterized by low triglyceride and VLDL levels and increased HDL cholesterol concentrations.

SUMMARY

The 'anti-atherosclerotic' plasma lipoprotein profile and the fact that hormone-sensitive lipase deficient animals become lean identifies the inhibition of hormone-sensitive lipase as a potential target for the treatment of lipid disorders and obesity.

Hormone-sensitive lipase: control of intracellular tri-(di-)acylglycerol and cholesteryl ester hydrolysis

Hormone-sensitive lipase (HSL) is an intracellular neutral lipase that is capable of hydrolyzing triacylglycerols, diacylglycerols, monoacylglycerols, and cholesteryl esters, as well as other lipid and water soluble substrates. HSL activity is regulated post-translationally by phosphorylation and also by pretranslational mechanisms. The enzyme is highly expressed in adipose tissue and steroidogenic tissues, with lower amounts expressed in cardiac and skeletal muscle, macrophages, and islets. Studies of the structure of HSL have identified several amino acids and regions of the molecule that are critical for enzymatic activity and regulation of HSL. This has led to important insights into its function, including the interaction of HSL with other intracellular proteins, such as adipocyte lipid binding protein. Accumulating evidence has defined important functions for HSL in normal physiology, affecting adipocyte lipolysis, steroidogenesis, spermatogenesis, and perhaps insulin secretion and insulin action; however, direct links between abnormal expression or genetic variations of HSL and human disorders, such as obesity, insulin resistance, type 2 diabetes, and hyperlipidemia, await further clarification. The published reports examining the regulation, and function of HSL in normal physiology and disease are reviewed in this paper.

Characterization of the functional interaction of adipocyte lipid-binding protein with hormone-sensitive lipase

Hormone-sensitive lipase (HSL) is an intracellular lipase that plays an important role in the hydrolysis of triacylglycerol in adipose tissue. HSL has been shown to interact with adipocyte lipid-binding protein (ALBP), a member of the family of intracellular lipid-binding proteins that bind fatty acids and other hydrophobic ligands. The current studies have addressed the functional significance of the association and mapped the site of interaction between HSL and ALBP. Incubation of homogeneous ALBP with purified, recombinant HSL in vitro resulted in a 2-fold increase in substrate hydrolysis. Moreover, the ability of oleate to inhibit HSL hydrolytic activity was attenuated by co-incubation with ALBP. Co-transfection of Chinese hamster ovary cells with HSL and ALBP resulted in greater hydrolytic activity than transfection of cells with HSL and vector alone. Deletional mutations of HSL localized the region of HSL that interacts with ALBP to amino acids 192-200, and site-directed mutagenesis of individual amino acids in this region identified His-194 and Glu-199 as critical for mediating the interaction of HSL with ALBP. Interestingly, HSL mutants H194L and E199A, each of which retained normal basal hydrolytic activity, failed to display an increase in hydrolytic activity when co-transfected with wild type ALBP. Therefore, ALBP increases the hydrolytic activity of HSL through its ability to bind and sequester fatty acids and via specific protein-protein interaction. Thus, HSL and ALBP constitute a functionally important lipolytic complex.

Direct interaction between the Arabidopsis disease resistance signaling proteins, EDS1 and PAD4

The Arabidopsis EDS1 and PAD4 genes encode lipase-like proteins that function in resistance (R) gene-mediated and basal plant disease resistance. Phenotypic analysis of eds1 and pad4 null mutants shows that EDS1 and PAD4 are required for resistance conditioned by the same spectrum of R genes but fulfil distinct roles within the defence pathway. EDS1 is essential for elaboration of the plant hypersensitive response, whereas EDS1 and PAD4 are both required for accumulation of the plant defence-potentiating molecule, salicylic acid. EDS1 is necessary for pathogen-induced PAD4 mRNA accumulation, whereas mutations in PAD4 or depletion of salicylic acid only partially compromise EDS1 expression. Yeast two-hybrid analysis reveals that EDS1 can dimerize and interact with PAD4. However, EDS1 dimerization is mediated by different domains to those involved in EDS1-PAD4 association. Co-immunoprecipitation experiments show that EDS1 and PAD4 proteins interact in healthy and pathogen-challenged plant cells. We propose two functions for EDS1. The first is required early in plant defence, independently of PAD4. The second recruits PAD4 in the amplification of defences, possibly by direct EDS1-PAD4 association.

Identification of the self-interaction of rat TCTP/IgE-dependent histamine-releasing factor using yeast two-hybrid system

To further understand the biological function of translationally controlled tumor protein (TCTP), also known as IgE-dependent histamine-releasing factor (HRF), the yeast two-hybrid system was used to screen interacting molecules. We isolated cDNA clones coding for TCTP/HRF, suggesting that it may have a self-interacting property. Domain mapping of the interaction revealed that the C-terminal region of residue 126-172 is involved in self-interaction. The self-interacting property of TCTP/HRF was further supported by FPLC gel-filtration chromatography and coimmunoprecipitation analysis from transfected COS-7 cells. Our data suggests that TCTP/HRF may have a potential to self-interact through the C-terminal region, and the self-interaction property may be related to its biological function.

Structure-function relationships of hormone-sensitive lipase

Research into the structure-function relationships of lipases and esterases has increased significantly during the past decade. Of particular importance has been the deduction of several crystal structures, providing a new basis for understanding these enzymes. The generated insights have, together with cloning and expression, aided studies on structure-function relationships of hormone-sensitive lipase (HSL). Novel phosphorylation sites have been identified in HSL, which are probably important for activation of HSL and lipolysis. Functional and structural analyses have revealed features in HSL common to lipases and esterases. In particular, the catalytic core with a catalytic triad has been unveiled. Furthermore, the investigations have given clear suggestions with regard to the identity of functional and structural domains of HSL. In the present paper, these studies on HSL structure-function relationships and short-term regulation are reviewed, and the results presented in relation to other discoveries in regulated lipolysis.