Estudio del factor de transcripción SREBP1 en estados de resistencia a insulina.

RESUMEN Los factores de transcripción pertenecientes a la familia SREBP (sterol regulatory element binding protein) regulan la homeostasis lipídica del organismo controlando la expresión de numerosos enzimas necesarios para la síntesis de colesterol, ácidos grasos, triglicéridos y fosfolípidos. Esta familia está constituida por tres isoformas que intervienen en procesos específicos: SREBP1c que está implicado en la síntesis de ácidos grasos, en la homeostasis del tejido adiposo y en el metabolismo glucídico inducido por insulina principalmente en el hígado. SREBP2 es relativamente específico de la síntesis de colesterol y la isoforma SREBP1a que, aunque su expresión in vivo es muy reducida parece estar implicada tanto en la síntesis de ácidos grasos como de colesterol [1]. La alteración en el metabolismo de los ácidos grasos es un factor común a muchos fenómenos característicos de la diabetes tipo 2 (DM2), como son la resistencia periférica a insulina, el aumento de la gluconeogénesis hepática, pérdida de secreción de insulina inducida por glucosa y obesidad [2]. Por tanto, genes implicados en la homeostasis lipídica parecen buenos candidatos para condicionar la susceptibilidad a DM2 [3-5]. En este trabajo se ha examinado si variantes en el gen SREBF1 (codifica las proteínas SREBP1a y SREBP1c) o variantes en el gen INSIG2 (insulin induced gen 2), implicado en la regulación de SREBP1, están asociadas con DM2 o con fenotipos relacionados con esta enfermedad. Para ello se analizó el gen SREBF1 por SSCP en un grupo de pacientes con DM2 y controles sanos, identificando distintas mutaciones y polimorfismos. Para el estudio de INSIG2, se realizó una búsqueda en las bases de datos del genoma humano y se seleccionaron los SNP que potencialmente podrían modificar la expresión de esta proteína. Tras la identificación de estas variantes, se realizó un estudio genético de asociación con DM2 encontrándose distintas variantes en ambos genes que condicionaban el riesgo de desarrollar la enfermedad. El estudio se completó con la caracterización molecular de dichas variantes para evaluar su efecto sobre la expresión y/o función de estas proteínas y por tanto su posible implicación en patologías metabólicas humanas. BIBLIOGRAFÍA 1.- Eberle D, Hegarty B, Bossard P, Ferre P, Foufelle F. : SREBP transcription factors: master regulators of lipid homeostasis. Biochimie 86(11):839-48, 2004. Review. 2.-Weyer C, Bogardus C, Mott DM, Pratley RE: The natural history of insulin secretory dysfunction and insulin resistance in the pathogenesis of type 2 diabetes. J Clin Invest 104 :787 794,1999 3.-Sinha R, Dufour S, Petersen KF, LeBon V, Enoksson S, Ma YZ, Savoye M, Rothman DL, Shulman GI, Caprio S: Assessment of skeletal muscle triglyceride content by (1)H nuclear magnetic resonance spectroscopy in lean and obese adolescents: relationships to insulin sensitivity, total body fat, and central adiposity. Diabetes 51 :1022 1027,2002 4.-Lupi R, Del Guerra S, Fierabracci V, Marselli L, Novelli M, Patane G, Boggi U, Mosca F, Piro S, Del Prato S, Marchetti P: Diabetes: lipotoxicity in human pancreatic islets and the protective effect of metformin. Diabetes51 (Suppl 1) :S134 S117,2002 5.-Lam TK, van de Werve G, Giacca A: Free fatty acids increase basal hepatic glucose production and induce hepatic insulin resistance at different sites. Am J Physiol Endocrinol Metab 284 :E281 E290,2003 __________________________________________________________________________________________________The sterol regulatory element-binding protein (SREBP) family of transcription factors controls cholesterol and lipid metabolism and plays critical roles during adipocyte differentiation and insulin-dependent gene expression. This family consists of three different SREBP proteins, SREBP1a and SREBP1c/ADD1 encoded by SREBF1 gene, and SREBP2, encoded by SREBF2 gene. Dysregulated fatty acid metabolism is an underlying factor of the most T2D traits such as peripheral insulin resistance, increased hepatic gluconeogenesis, loss of glucose-induced insulin secretion and habitually obesity and seems to be an early marker of insulin resistance. Thus SREBF1 and INSIG2, a gene encoding INSIG2 protein, involved in SREBPs regulation, seem to be attractive candidates for susceptibility genes to metabolic disorders such as dislipidemia, obesity and type 2 diabetes (T2D). We performed a molecular screening of SREBF1 gene by SSCP in T2D patients, describing 18 variants within this gene. We further genotyped the most frequent variants in a case-control study in Spanish T2D patients and non diabetic non-insulin-resistant controls. In addition we performed different in vitro approaches to characterize the functional implications of variants identified in SREBF1 gene. In this study we found a modest but significant increased risk of T2D associated with a single variant and several haplotypes within SREBF1 gene. In addition we have identified a common deletion in 3´untranslated region and a rare new variant in 5´untranslated region of SREBF1 gene that modifies SREBP1c expression in vitro and could be involved in severe phenotypes of insulin resistance. To investigate whether mutations in INSIG2 gene might also contribute to diabetes, six single-nucleotide polymorphisms (SNPs) in the human INSIG2 gene were tested in Spanish subjects for association with T2D. Association analysis and functional studies showed that rs7589375:T alele caused a decrease of INSIG2 promoter activity which could lead to a dysregulation of SREBPs and ultimately of the lipid metabolism. Taking together these results our conclusion is that genetic variants in SREBF1 and INSIG2 genes might influence type 2 diabetes risk in Spanish population

Role of the MAPK/cJun NH2-Terminal Kinase signaling pathway in starvation-induced autophagy

Autophagy is required for cellular homeostasis and can determine cell viability in response to stress. It is established that MTOR is a master regulator of starvation-induced macroautophagy/autophagy, but recent studies have also implicated an essential role for the MAPK8/cJun NH2-terminal kinase 1 signal transduction pathway. We found that MAPK8/JNK1 and MAPK9/JNK2 were not required for autophagy caused by starvation or MTOR inhibition in murine fibroblasts and epithelial cells. These data demonstrate that MAPK8/9 has no required role in starvation-induced autophagy. We conclude that the role of MAPK8/9 in autophagy may be context-dependent and more complex than previously considered

A rare missense mutation in a type 2 diabetes patient decreases the transcriptional activity of human sterol regulatory element binding protein-1

11 pages, 3figures, 1 table.-PMID: 16429400 [PubMed]Sterol regulatory element binding protein 1 (SREBP-1) transcription factors play a key role in energy homeostasis by regulating genes involved in both carbohydrate and lipid metabolism, and in adipocyte differentiation. The 5' end of the mRNA-encoding SREBP-1 exists in two forms, designated 1a and 1c. The divergence results from the use of two transcription start sites that produce two separate 5' exons, each of which is spliced to a common exon 2. Mutations in the sterol regulatory element binding protein gene (SREBF)-1 may contribute to insulin resistance states. However, the variants described to date do not affect the SREBP function. In this study, we investigated the functional consequences of a novel missense mutation common to both SREBP-1 isoforms identified in a Spanish Type 2 diabetic patient (c.677C>T, SREBP-1a p.T226M; c.605C>T, SREBP-1c p.T202M). Using reporter gene analysis and electrophoretic mobility shift assays, we found that this variant impaires the transcriptional activity and reduces DNA binding ability despite its comparable protein stability to the wild-type SREBP-1. This decreased activity impaires the expression of known downstream targets, such as the LDL receptor and fatty acid synthase genes. Our findings suggest that the threonine residue and/or surrounding region play an important role in the SREBP-1 functionThis study was supported by grants from Instituto de Salud Carlos III, Red de Centros (RCMN) C03/08, and from Ministerio de Educación y Ciencia (SAF2003-01262). S.V. is supported by a fellowship from Consejo Superior de Investigaciones Científicas I3P-BPD2001-1.Peer reviewe

Increased Endoplasmic Reticulum Stress and Decreased Proteasomal Function in Lafora Disease Models Lacking the Phosphatase Laforin

12 pages, 6 figures, 1 table.-- PMID: 19529779 [PubMed].[Background] Lafora progressive myoclonus epilepsy (Lafora disease; LD) is a fatal autosomal recessive neurodegenerative disorder caused by loss-of-function mutations in either the EPM2A gene, encoding the dual specificity phosphatase laforin, or the EPM2B gene, encoding the E3-ubiquitin ligase malin. Previously, we and others have shown that both proteins form a functional complex that regulates glycogen synthesis by a novel mechanism involving ubiquitination and proteasomal degradation of at least two proteins, glycogen synthase and R5/PTG. Since laforin and malin localized at the endoplasmic reticulum (ER) and their regulatory role likely extend to other proteins unrelated to glycogen metabolism, we postulated that their absence may also affect the ER-unfolded protein response pathway.[Methodology/Principal Findings] Here, we demonstrate that siRNA silencing of laforin in Hek293 and SH-SY5Y cells increases their sensitivity to agents triggering ER-stress, which correlates with impairment of the ubiquitin-proteasomal pathway and increased apoptosis. Consistent with these findings, analysis of tissue samples from a LD patient lacking laforin, and from a laforin knockout (Epm2a-/-) mouse model of LD, demonstrates constitutive high expression levels of ERstress markers BIP/Grp78, CHOP and PDI, among others.[Conclusions/Significance] We demonstrate that, in addition to regulating glycogen synthesis, laforin and malin play a role protecting cells from ER-stress, likely contributing to the elimination of unfolded proteins. These data suggest that proteasomal dysfunction and ER-stress play an important role in the pathogenesis of LD, which may offer novel therapeutic approaches for this fatal neurodegenerative disorder.This work was supported by grants from the Fundación Marató TV3, the Fundación La Caixa, the Spanish Ministry of Education and Science (SAF2008-01907) and the European Commission (LSHM-CT-2004-005272).Peer reviewe

Diet-induced obesity mediated by the JNK/DIO2 signal transduction pathway

The cJun N-terminal kinase (JNK) signaling pathway is a key mediator of metabolic stress responses caused by consuming a high-fat diet, including the development of obesity. To test the role of JNK, we examined diet-induced obesity in mice with targeted ablation of Jnk genes in the anterior pituitary gland. These mice exhibited an increase in the pituitary expression of thyroid-stimulating hormone (TSH), an increase in the blood concentration of thyroid hormone (T4), increased energy expenditure, and markedly reduced obesity compared with control mice. The increased amount of pituitary TSH was caused by reduced expression of type 2 iodothyronine deiodinase (Dio2), a gene that is required for T4-mediated negative feedback regulation of TSH expression. These data establish a molecular mechanism that accounts for the regulation of energy expenditure and the development of obesity by the JNK signaling pathway

An alternative splicing program promotes adipose tissue thermogenesis

Alternative pre-mRNA splicing expands the complexity of the transcriptome and controls isoform-specific gene expression. Whether alternative splicing contributes to metabolic regulation is largely unknown. Here we investigated the contribution of alternative splicing to the development of diet-induced obesity. We found that obesity-induced changes in adipocyte gene expression include alternative pre-mRNA splicing. Bioinformatics analysis associated part of this alternative splicing program with sequence specific NOVA splicing factors. This conclusion was confirmed by studies of mice with NOVA deficiency in adipocytes. Phenotypic analysis of the NOVA-deficient mice demonstrated increased adipose tissue thermogenesis and improved glycemia. We show that NOVA proteins mediate a splicing program that suppresses adipose tissue thermogenesis. Together, these data provide quantitative analysis of gene expression at exon-level resolution in obesity and identify a novel mechanism that contributes to the regulation of adipose tissue function and the maintenance of normal glycemia

Hyper- and hypo- nutrition studies of the hepatic transcriptome and epigenome suggest that PPARα regulates anaerobic glycolysis

Diet plays a crucial role in shaping human health and disease. Diets promoting obesity and insulin resistance can lead to severe metabolic diseases, while calorie-restricted (CR) diets can improve health and extend lifespan. In this work, we fed mice either a chow diet (CD), a 16 week high-fat diet (HFD), or a CR diet to compare and contrast the effects of these diets on mouse liver biology. We collected transcriptomic and epigenomic datasets from these mice using RNA-Seq and DNase-Seq. We found that both CR and HFD induce extensive transcriptional changes, in some cases altering the same genes in the same direction. We used our epigenomic data to infer transcriptional regulatory proteins bound near these genes that likely influence their expression levels. In particular, we found evidence for critical roles played by PPARα and RXRα. We used ChIP-Seq to profile the binding locations for these factors in HFD and CR livers. We found extensive binding of PPARα near genes involved in glycolysis/gluconeogenesis and uncovered a role for this factor in regulating anaerobic glycolysis. Overall, we generated extensive transcriptional and epigenomic datasets from livers of mice fed these diets and uncovered new functions and gene targets for PPARα

Adipose tissue mTORC2 regulates ChREBP-driven de novo lipogenesis and hepatic glucose metabolism

Adipose tissue de novo lipogenesis (DNL) positively influences insulin sensitivity, is reduced in obesity, and predicts insulin resistance. Therefore, elucidating mechanisms controlling adipose tissue DNL could lead to therapies for type 2 diabetes. Here, we report that mechanistic target of rapamycin complex 2 (mTORC2) functions in white adipose tissue (WAT) to control expression of the lipogenic transcription factor ChREBPbeta. Conditionally deleting the essential mTORC2 subunit Rictor in mature adipocytes decreases ChREBPbeta expression, which reduces DNL in WAT, and impairs hepatic insulin sensitivity. Mechanistically, Rictor/mTORC2 promotes ChREBPbeta expression in part by controlling glucose uptake, but without impairing pan-AKT signalling. High-fat diet also rapidly decreases adipose tissue ChREBPbeta expression and insulin sensitivity in wild-type mice, and does not further exacerbate insulin resistance in adipose tissue Rictor knockout mice, implicating adipose tissue DNL as an early target in diet-induced insulin resistance. These data suggest mTORC2 functions in WAT as part of an extra-hepatic nutrient-sensing mechanism to control glucose homeostasis

Lafora progressive myoclonus epilepsy: NHLRC1 mutations affect glycogen metabolism

11 páginas, 8 figuras, 1 tabla.Lafora disease is a fatal autosomal recessive form of progressive myoclonus epilepsy. Patients manifest myoclonus and tonic-clonic seizures, visual hallucinations, intellectual, and progressive neurologic deterioration beginning in adolescence. The two genes known to be involved in Lafora disease are EPM2A and NHLRC1 (EPM2B). The EPM2A gene encodes laforin, a dual-specificity protein phosphatase, and the NHLRC1 gene encodes malin, an E3-ubiquitin ligase. The two proteins interact with each other and, as a complex, are thought to regulate glycogen synthesis. Here, we report three Lafora families with two novel pathogenic mutations (C46Y and L261P) and two recurrent mutations (P69A and D146N) in NHLRC1. Investigation of their functional consequences in cultured mammalian cells revealed that malin(C46Y), malin(P69A), malin(D146N), and malin(L261P) mutants failed to downregulate the level of R5/PTG, a regulatory subunit of protein phosphatase 1 involved in glycogen synthesis. Abnormal accumulation of intracellular glycogen was observed with all malin mutants, reminiscent of the polyglucosan inclusions (Lafora bodies) present in patients with Lafora disease.Peer reviewe

Inactivation of nuclear GSK3 beta by Ser(389) phosphorylation promotes lymphocyte fitness during DNA double-strand break response

Variable, diversity and joining (V(D)J) recombination and immunoglobulin class switch recombination (CSR) are key processes in adaptive immune responses that naturally generate DNA double-strand breaks (DSBs) and trigger a DNA repair response. It is unclear whether this response is associated with distinct survival signals that protect T and B cells. Glycogen synthase kinase 3 beta (GSK3 beta) is a constitutively active kinase known to promote cell death. Here we show that phosphorylation of GSK3 beta on Ser(389) by p38 MAPK (mitogen-activated protein kinase) is induced selectively by DSBs through ATM (ataxia telangiectasia mutated) as a unique mechanism to attenuate the activity of nuclear GSK3 beta and promote survival of cells undergoing DSBs. Inability to inactivate GSK3 beta through Ser(389) phosphorylation in Ser(389)Ala knockin mice causes a decrease in the fitness of cells undergoing V(D)J recombination and CSR. Preselection-Tcrb repertoire is impaired and antigen-specific IgG antibody responses following immunization are blunted in Ser(389)GSK3 beta knockin mice. Thus, GSK3 beta emerges as an important modulator of the adaptive immune response.We thank Dr T. Honjo and Dr K. Otsu for the generation of the original AID deficient mice and the p38 flox/flox mice, respectively. We thank C. Charland for flow cytometry analysis and cell sorting, the Vermont Cancer Center DNA Sequencing Facility and the University of Vermont College of Med. Microscopy Imaging Center for their services. We thank Dr D.R. Green and Dr R.C. Budd for helpful discussion regarding the mechanisms of cell death and reagents. This work was supported by NIH grant R01 AI051454 (M.R. and T.M.T.), P20 GM103496 (T.M.T.) NIH grant R37 GM41052 (M.S.K.) and Lake Champlain Cancer Research Organization (M.R.).S