Fumiko Hirose

CGI-58 facilitates lipolysis on lipid droplets but is not involved in the vesiculation of lipid droplets caused by hormonal stimulation

A lipid droplet (LD)-associated protein, perilipin, is a critical regulator of lipolysis in adipocytes. We previously showed that Comparative Gene Identification-58 (CGI-58), a product of the causal gene of Chanarin-Dorfman syndrome, interacts with perilipin on LDs. In this study, we investigated the function of CGI-58 using RNA interference. Notably, CGI-58 knockdown caused an abnormal accumulation of LDs in both 3T3-L1 preadipocytes and Hepa1 hepatoma cells. CGI-58 knockdown did not influence the differentiation of 3T3-L1 adipocytes but reduced the activity of both basal and cAMP-dependent protein kinase-stimulated lipolysis. In vitro studies showed that CGI-58 itself does not have lipase/esterase activity, but it enhanced the activity of adipose triglyceride lipase. Upon lipolytic stimulation, endogenous CGI-58 was rapidly dispersed from LDs into the cytosol along with small particulate structures. This shift in localization depends on the phosphorylation of perilipin, because phosphorylated perilipin lost the ability to bind CGI-58. During lipolytic activation, LDs in adipocytes vesiculate into micro-LDs. Using coherent anti-Stokes Raman scattering microscopy, we pursued the formation of micro-LDs in single cells, which seemed to occur in cytoplasmic regions distant from the large central LDs. CGI-58 is not required for this process. Thus, CGI-58 facilitates lipolysis in cooperation with perilipin and other factors, including lipases.

The transactivating function of peroxisome proliferator-activated receptor γ is negatively regulated by SUMO conjugation in the amino-terminal domain

Peroxisome proliferator‐activated receptor (PPAR)‐γ2, a member of the nuclear hormone receptor superfamily, plays a key role in adipocyte differentiation. Its amino‐terminal region carries a ligand‐independent gene‐activating function, AF‐1, and is composed of activation as well as repression domains. We have found PPARγ2 and its isoform, PPARγ1, to be modified by small ubiquitin‐related modifier (SUMO)‐1 in vivo, at a lysine residue in the repression domain. In reporter assays, a sumoylation‐defective K107R mutant of PPARγ2 exhibited much stronger transactivation than the wild‐type, comparable with that of a mutant deleted for the repression domain. A close inverse correlation was observed between the levels of sumoylation and transactivation by PPARγ2, in analyses employing PPARγ2 forms with mutations in the sumoylation motif and a dominant‐negative mutant of the SUMO conjugating enzyme, Ubc9. Studies with phosphorylation‐defective mutants suggested that phosphorylation at S112 of PPARγ2 promotes K107 sumoylation, and this latter exerts the more potent repressive effects. The K107R mutant PPARγ2, when infected into NIH3T3 cells with a viral vector, promoted differentiation into adipocytes more efficiently than the wild‐type. These observations provide evidence that sumoylation is involved in negative regulation of the transactivating function of PPARγ2.

hDREF Regulates Cell Proliferation and Expression of Ribosomal Protein Genes

ABSTRACT Although ribosomal proteins (RPs) are essential cellular constituents in all living organisms, mechanisms underlying regulation of their gene expression in mammals remain unclear. We have established that 22 out of 79 human RP genes contain sequences similar to the human DREF (DNA replication-related element-binding factor; hDREF) binding sequence (hDRE) within 200-bp regions upstream of their transcriptional start sites. Electrophoretic gel mobility shift assays and chromatin immunoprecipitation analysis indicated that hDREF binds to hDRE-like sequences in the RP genes both in vitro and in vivo. In addition, transient luciferase assays revealed that hDRE-like sequences act as positive elements for RP gene transcription and cotransfection of an hDREF-expressing plasmid was found to stimulate RP gene promoter activity. Like that of hDREF, expression of RP genes is increased during the late G1 to S phases, and depletion of hDREF using short hairpin RNA-mediated knockdown decreased RP gene expression and cell proliferation in normal human fibroblasts. Knockdown of the RPS6 gene also resulted in impairment of cell proliferation. These data suggest that hDREF is an important transcription factor for cell proliferation which plays roles in cell cycle-dependent regulation of a number of RP genes.

Aspects of the regulatory mechanisms of PPAR functions: Analysis of a bidirectional response element and regulation by sumoylation

Peroxisome proliferator-activated receptors (PPARs) constitute a subfamily of nuclear receptor superfamily. A wide variety of compounds including hypolipidemic agents, antidiabetic drugs, and long-chain fatty acids are the potential ligands of PPARs. To approach the regulatory mechanisms of PPARs, we studied on two subjects in this work. First, we identified a functional PPAR-binding site in the spacer region between the PEX11α and perilipin genes, which are arranged in tandem on the mouse genome. By gene reporter assays and in vivo as well as in vitro binding assays, we show that these genes are regulated tissue-selectively through this common binding site: The PEX11α gene is activated by PPARα in the liver, whereas the perilipin gene by PPARγ in the adipose tissue. As the second subject, we found that PPARγ2 is conjugated with small ubiquitin-related modifier (SUMO) at a specific lysine residue in the amino-terminal region. By site-directed mutagenesis combined with gene reporter assays and sumoylation analyses, we show that sumoylation represses the ligand-independent transactivating function carried by this region, and hence negatively regulates the whole transactivating competence of PPARγ2. In addition, phosphorylation at a specific site in the amino-terminal region represses the transactivation by PPARγ2 possibly through enhancing sumoylation.

Active involvement of micro-lipid droplets and lipid-droplet-associated proteins in hormone-stimulated lipolysis in adipocytes.

Summary The regulation of lipolysis in adipocytes involves coordinated actions of many lipid droplet (LD)-associated proteins such as perilipin, hormone sensitive lipase (HSL), adipose triglyceride lipase (ATGL), and its activator protein, CGI-58. Here, we describe the cellular origin and physiological significance of micro LDs (mLDs) that emerge in the cytoplasm during active lipolysis, as well as the roles of key lipolytic proteins on mLDs in differentiated 3T3-L1 adipocytes. Multiplex coherent anti-Stokes Raman scattering (CARS) microscopy demonstrated that mLDs receive the fatty acid (FA) moiety of triglyceride from pre-existing LDs during lipolysis. However, when FA re-esterification was blocked, mLDs did not emerge. Time-lapse imaging of GFP-tagged LD-associated proteins and immunocytochemical analyses showed that particulate structures carrying LD-associated proteins emerged throughout the cells upon lipolytic stimulation, but not when FA re-esterification was blocked. Overall lipolysis, as estimated by glycerol release, was significantly lowered by blocking re-esterification, whereas release of free FAs was enhanced. ATGL was co-immunoprecipitated with CGI-58 from the homogenates of lipolytically stimulated cells. Following CGI-58 knockdown or ATGL inhibition with bromoenol lactone, release of both glycerol and FA was significantly lowered. AICAR, an activator of AMP-activated protein kinase, significantly increased FA release, in accordance with increased expression of ATGL, even in the absence of CGI-58. These results suggest that, besides on the surface of pre-existing central LDs, LD-associated proteins are actively involved in lipolysis on mLDs that are formed by FA re-esterification. Regulation of mLDs and LD-associated proteins may be an attractive therapeutic target against lipid-associated metabolic diseases.

Perilipin, a critical regulator of fat storage and breakdown, is a target gene of estrogen receptor-related receptor α

Perilipin is a protein localized on lipid droplet surfaces in adipocytes and steroidogenic cells, playing a central role in regulated lipolysis. Expression of the perilipin gene is markedly induced during adipogenesis. We found that transcription from the perilipin gene promoter is activated by an orphan nuclear receptor, estrogen receptor-related receptor (ERR)alpha. A response element to this receptor was identified in the promoter region by a gene reporter assay, the electrophoretic-gel mobility-shift assay and the chromatin immunoprecipitation assay. Peroxisome proliferator-activated receptor gamma coactivator (PGC)-1alpha enhanced, whereas small heterodimer partner (SHP) repressed, the transactivating function of ERRalpha on the promoter. Thus, the perilipin gene expression is regulated by a transcriptional network controlling energy metabolism, substantiating the functional importance of perilipin in the maintenance of body energy balance.

Human DNA Replication-related Element Binding Factor (hDREF) Self-association via hATC Domain Is Necessary for Its Nuclear Accumulation and DNA Binding

We previously demonstrated that hDREF, a human homologue of Drosophila DNA replication-related element binding factor (dDREF), is a DNA-binding protein predominantly distributed with granular structures in the nucleus. Here, glutathione S-transferase pulldown and chemical cross-linking assays showed that the carboxyl-terminal hATC domain of hDREF, highly conserved among hAT transposase family members, possesses self-association activity. Immunoprecipitation analyses demonstrated that hDREF self-associates in vivo, dependent on hATC domain. Moreover, analyses using a series of hDREF mutants carrying amino acid substitutions in the hATC domain revealed that conserved hydrophobic amino acids are essential for self-association. Immunofluorescence studies further showed that all hDREF mutants lacking self-association activity failed to accumulate in the nucleus. Self-association-defective hDREF mutants also lost association with endogenous importin β1. Moreover, electrophoretic gel-mobility shift assays revealed that the mutations completely abolished the DNA binding activity of hDREF. These results suggest that self-association of hDREF via the hATC domain is necessary for its nuclear accumulation and DNA binding. We also found that ZBED4/KIAA0637, another member of the human hAT family, also self-associates, again dependent on the hATC domain, with deletion resulting in loss of efficient nuclear accumulation. Thus, hATC domains of human hAT family members appear to have conserved functions in self-association that are required for nuclear accumulation.

Genetic screening for modifiers of the DREF pathway in Drosophila melanogaster: identification and characterization of HP6 as a novel target of DREF.

The DNA replication-related element-binding factor (DREF) regulates cell proliferation-related gene expression in Drosophila. By genetic screening, taking advantage of the rough eye phenotype of transgenic flies that express DREF in the eye discs, we identified 24 genes that suppressed and 12 genes that enhanced the rough eye phenotype when heterozygous for mutations. Five genes, HP6, pigeon, lace, X box binding protein 1 and guftagu were found to carry replication-related element (DRE) sequences in their 5′-flanking regions. Of these, the HP6 gene carries two sequences that match seven out of eight nucleotides of DRE and two additional sequences that match six out of eight nucleotides of DRE in the 5′-flanking region. Band mobility shift assays using Drosophila Kc cell nuclear extracts demonstrated DREF binding to two of these sites and chromatin immunoprecipitation using anti-DREF antibodies confirmed that this occurs in vivo. Knockdown of DREF in Drosophila S2 cells decreased the HP6 mRNA level. The results, taken together, indicate that DREF directly regulates expression of the HP6 gene. HP6 mRNA was detected throughout development by RT-PCR with highest levels in adult males. In addition, immunostaining analyses revealed colocalization of HP6 and DREF in nuclei at the apical tips in the testes.

Long‐term expression of the lamin A mutant associated with dilated cardiomyopathy induces senescence

Mutation of the lamin A gene (LMNA) causes a diverse range of diseases referred to as laminopathies. Because most laminopathies have a dominant inheritance pattern and progress gradually, cultured cells stably expressing mutant lamin A at the same level as endogenous wild‐type cells are required for chronological analysis. In this study, we showed that an expression system involving a lentiviral vector that carries the human metallothionein gene basal promoter ensures stable and basal‐level expression of proteins and is thus suitable for investigating the properties of lamin A mutants. The small ubiquitin‐related modifier (SUMO) modification (SUMOylation)‐defective E203G mutant that is associated with familial dilated cardiomyopathy exhibited abnormal subnuclear distribution and inhibited normal localization of WT lamin A in a dominant‐negative manner. Low‐level and long‐term expression of the E203G mutant resulted in multinucleated giant cells, aberrant lipid droplet accumulation in the cytoplasm and premature senescence. Expression of another SUMOylation‐defective mutant (K201R) did not induce any phenotypes observed in cells expressing E203G. These results indicate that the E203G mutant may inhibit the normal functions of wild‐type lamin A in a dominant‐negative manner, but a defect in SUMOylation itself may not be involved in disease pathogenesis.