Hei Sook Sul
Professor & Chair, Doris H. Calloway Chair in Human Nutrition
There are two types of adipose tissue. White adipose tissue (WAT) accumulates triglycerides (TAG) as the main energy storage form. Obesity from increased WAT mass with increased TAG storage to enlarge cell size, as well as with increased adipocyte number, has become an epidemic. Obesity is associated various diseases, including type 2 diabetes and cardiovascular diseases. In contrast to WAT, brown adipose tissue (BAT) dissipates energy as heat via non-shivering thermogenesis. Moreover, in certain conditions, subcutaneous WAT can become more like BAT by so-called beiging to perform thermogenesis. Thus, increasing BAT or BAT-like tissues may represent a means to obesity prevention/therapeutics. The broad aims of our research in the Sul Lab are to understand a) regulation of TAG synthesis and breakdown, b) recruitment and differentiation of adipocytes, and 3) development and function of BAT or BAT-like tissues.
Transcriptional regulation of lipogenic genes by feeding/insulin
Enzymes involved in lipogenesis are transcriptionally activated in response to feeding/insulin treatment in a coordinate manner. We are investigating FAS as a model system. We examined the cis- and trans-factors that bring about activation of FAS upon feeding/insulin treatment. We originally defined the insulin response sequence of the FAS gene at -65 that contains a core E-box where USF binds. We demonstrated in vivo role of USF at -65 E-box by generating transgenic mice containing various deletions and mutations of the FAS promoter region. We also showed that USF binding at the -65 E-box is required for SREBP binding to -150SRE during feeding/insulin treatment. By direct interaction, USF recruits SREBP to bind SRE for transcriptional activation of FAS and other lipogenic enzymes in response to insulin/feeding. By tandem affinity purification and MS/MS sequencing, we identified not only various components of the USF/SREBP complex but also their posttranslational modifications (phosphorylation and acetylation). We found that during feeding/insulin treatment, USF-1 recruits and is phosphorylated by DNA-PK, which is first dephosphorylated/activated by PP1. Phosphorylation of USF-1 allows recruitment and acetylation by P/CAF, resulting in the FAS promoter activation. In fasting, USF-1 is deacetylated by HDAC9 causing the promoter inactivation. Thus, in DNA-PK deficient SCID mice, feeding induced USF-1 phosphorylation/acetylation and FAS activation leading to lipogenesis are impaired, resulting in decreased hepatic and circulating triglyceride levels with decreased adiposity. Our study demonstrate that DNA-PK mediates the feeding/insulin-dependent lipogenic gene activation, which represents as a new insulin-signaling pathway.Moreover, we found that, by binding to USF-1, Brg1/Brm-associated factor (BAF) 60c functions as a specific chromatin remodeling component for lipogenic gene transcription in liver. In response to insulin/feeding, BAF60c is phosphorylated at S247 by atypical PKCζ/λ, which causes translocation of BAF60c to the nucleus and allows a direct interaction of BAF60c with USF-1, which is phosphorylated by DNA-PK and acetylated by P/CAF. Thus, BAF60c is recruited to form the lipoBAF complex to remodel chromatin structure for activation of lipogenic genes in response to insulin/feeding. Consequently, we found BAF60c promotes lipogenesis in vivo and increases triglyceride levels, demonstrating its role in metabolic adaption to activate lipogenic program in response to feeding/insulin.In addition, we found that USF1 directly interacts also with Med17, a subunit of the Mediator complex and insulin/feeding causes phosphorylation of Med17 at S53 by CK2. This occurs only in the absence of T570 phosphorylation via PKA-P38 MAPK pathway that occurs in fasting. More recently,we have been examining epigenetic regulation of lipogenic genes. Thus, we identified specific histone modifying enzymes to study their function in lipogenesis and their posttranslational modification and signaling pathways in response to feeding/insulin treatment.
Novel enzymes in lipid metabolism in adipose tissue
My laboratory identified several novel enzymes that play critical role in TAG metabolism; mitochondrial glycerol 3-phosphate acyltransferase (GPAT) that catalyzes the first-rate limiting step in TAG synthesis, desnutrin which is the bona fide adipocyte TAG lipase, and AdPLA (adipocyte specific phospholipase A2) that increases PGE2 levels to suppress lipolysis.
First, we originally identified several proteins that are transcriptionally activated during feeding/insulin treatment in a coordinate manner. By overexpression/purification/reconstitution, we identified one such protein as mitochondrial GPAT in early 90s. This was the first enzyme in glycerophospholipid biosynthesis cloned in mammalian system. We further characterized the enzyme by extensive site-directed mutagenesis and identified critical amino acid residues for catalytic activity, as well as substrate binding. Subsequently other enzymes in glycerophospholipid biosynthesis including AGPAT and DGAT were identified by other laboratories due to the presence of homology region, making it possible to elucidate function and regulation of mammalian TAG biosynthesis.
Hydrolysis of TAG (lipolysis) providing fatty acids for use by other tissues as energy source is a unique function of white adipocytes. In fasted condition, increase in circulating catabolic hormones, catecholamines and glucocorticoids, stimulate lipolysis, whereas in the fed condition, anabolic hormone, insulin decreases lipolysis. We identified two novel enzymes that play crucial role in lipolysis in adipocytes. We first identified an adipocyte-specific TAG lipase that is induced during fasting/glucocorticoids and we named it desnutrin. Two other laboratories subsequently identified this enzyme as the TAG lipase in adipocytes and thus desnutrin/ATGL/PNPLA2 is now accepted to be the bona fide TAG lipase, whereas previously known HSL functions as DAG lipase. During characterization of desnutrin as a TAG lipase via adenoviral overexpression and mutational analysis, we found that desnutrin is phosphorylated at S406 by AMPK, which activates the TAG lipase activity, representing a new mode of regulation of lipolysis during energy shortage by sensing energy state of the cell. Transgenic mice overexpressing desnutrin in adipose tissue that we generated showed reduced adiposity with increased oxygen consumption and body temperature. Our adipose tissue-specific desnutrin knockout mice showed the unique phenotype of their BAT converting into WAT, greatly impairing thermogenesis. These in vivo mouse models demonstrate the critical role of desnutrin-catalyzed lipolysis in the maintenance of BAT phenotype. We also found that PPARais activated specifically by fatty acids released by desnutrin acting as PPARa ligand directly or indirectly by conversion to another metabolite. This is a new exciting area since increasing BAT that can dissipate energy in place of the WAT that stores fat would be a future preventive or therapeutic strategy of ever-increasing obesity problem.
Phospholipase A2 (PLA2) superfamily of enzymes that catalyze hydrolysis fatty acids from the membrane phospholipids’ sn-2 position enriched with unsaturated fatty acids. Thus, PLA2 can release arachidonic acid and is regarded as the first rate-limiting step in eicosanoid biosynthesis. PLA2 may be expressed in specific tissues for production of local lipid mediators in order to regulate tissue-specific function. We recently identified a novel adipocyte specific phospholipase A2 that we named AdPLA. We identified and characterized AdPLA by overexpression and purification. AdPLA expression is under the nutritional control, very low in the fasted state but induced by feeding/ insulin. We observed AdPLA knockout mice that we generated exhibit a drastic decrease in adiposity and are protected from diet-induced and genetic obesity. In this regard, we detected PGE2 as the major prostaglandin produced in adipocytes and EP3 as the major PGE2 receptor in this cell type. We found that the PGE2 produced upon AdPLA induction in fed state plays a dominant inhibitory role in lipolysis through binding to the Gai-coupled EP3 to reduce cAMP levels, decreasing PKA mediated phosphorylation of HSL. Our study reveals a novel adipocyte-specific PLA2 which, through AdPLA/PGE2/cAMP pathway, plays a major role of in regulating lipolysis in an autocrine/paracrine manner.
Pref-1 secreted from adipose precursors to control adipogenesis
In an attempt to identify genes that regulate adipocyte differentiation, over the years, we have cloned/identified several novel molecules that can control adipogenesis. We cloned Pref-1 (Preadipocyte factor-1), a transmembrane protein with six EGF-repeats at the extracellular domain. Pref-1 is highly expressed in 3T3-L1 preadipocytes; its expression is extinguished during adipose conversion, and is not found in mature adipocytes. When constitutively expressed, Pref-1 blocks adipocyte differentiation in vitro, whereas absence of Pref-1 enhances differentiation. We found that processing of cell-associated Pref-1 by TACE generates a soluble Pref-1 of 50 kD corresponding to its ectodomain. And, only the 50 kD soluble form of Pref-1 is active as an inhibitor of adipocyte differentiation and a membrane form of Pref-1 mutated at the cleavage site was not functional. We generated Pref-1 knockout mice as well as transgenic mice that ectopically overexpress Pref-1 in adipose tissue: overexpression of Pref-1 in adipose tissue using aP2 promoter in transgenic mice caused these mice to be lean, but diabetic due to ectopic fat storage in other tissues. Conversely, ablation of Pref-1 gene caused an increase in adipose mass and insulin resistance. These in vivo experiments unequivocally demonstrate the inhibitory role of Pref-1 in adipogenesis and its effect on glucose/insulin homeostasis. Pref-1 is now widely used as the preadipocyte marker. Recently we found that the soluble Pref-1 interacts with fibronectin to inhibit adipocyte differentiation. Upon binding to fibronectin, Pref-1 increases MEK/ERK phosphorylation in a time- and dose-dependent manner to inhibit adipogenesis. We also found that Sox9 is an important target of Pref-1 to suppress expression ofC/EBPb and C/EBPd, preventing adipocyte differentiation. Furthermore, by inducing Sox9, Pref-1 promotes chondrogenic induction of mesenchymal cells but prevents chondrocyte maturation as well as osteoblast differentiation, confirmed by in vivo evidence in Pref-1 null and Pref-1 transgenic mice. Thus, Sox9 is a Pref-1 target and Pref-1 directs multipotent mesenchymal cells to the chondrogenic lineage, but inhibits differentiation into adipocytes as well as osteoblasts and chondrocytes.Recently, to better understand the origin and development of white adipose tissue, we generated transgenic mouse models for transient or permanent marking of cells using the Pref-1 promoter, and to ablate them in an inducible manner. We found that Pref-1 marked cells are very early adipose precursors, prior to expression of Zfp423 or PPARg,and these cells retain proliferative capacity. In addition, Pref-1 marked cells establishadipose precursors as mesenchymal, but not endothelial or pericyte in origin. During embryogenesis, Pref-1 marked cells first appear in the dorsal mesentericregion as early as E10.5, which become lipid-laden adipocytes at E17.5 in the subcutaneous region, whereas visceral WAT develops after birth. Ablation of Pref-1 marked cells prevents not only embryonic WAT development but also later adult adipose expansion upon high fat feeding, demonstrating the requirement of Pref-1 expressing cells for adipogenesis.
Factors that regulate BAT gene program and thermogenesis
With the recent evidence for the presence of significant functional BAT in human adults, we are examining brown adipocyte transcription and differentiation. Primarily, uncoupling protein 1 (UCP1) mediates non-shivering thermogenesis in brown fat (BAT) in response to cold exposure. By high-throughput screening using UCP1 promoter, we identify several transcription factors may play critical role in thermogenesis as well as brown adipogenesis. One such gene that we have characterized is Zfp516. Zfp516 can activate UCP1 and other BAT-enriched genes, including PGC1α and Cox8b. Zfp516 is selectively expressed in BAT but not in most other tissues and, through the cAMP-CREB/ATF2 pathway, Zfp516 itself is induced by cold or sympathetic stimulation also. Zfp516 binds to its response element at the promoter regions to activate UCP1 and other BAT genes. Thus, Zfp516 can drive a brown adipogenic program and ablation of Zfp516 prevents BAT development in mice. Moreover, ectopic expression of Zfp516 in adipose tissue of mice promotes browning of subcutaneous white fat increasing body temperature and energy expenditure. We are now studying additional transcription factors and its interacting histone modifying enzymes to understand the major transcriptional components and epigenetic factors for activation of BAT gene program during different nutritional or environmental conditions. In addition, we recently identified novel factors that affect thermogenic capacity of BAT and browning of subcutaneous fat and in the process of dissecting their function at the molecular level.
Overall, our research on enzymes in TAG metabolism in adipose tissue and those molecules that regulate WAT and BAT program, provide not only better understanding of adipose tissue function and development, but also future targets for prevention/therapeutics for obesity/diabetes.
For more information, please visit the Sul Lab page
Selected recent publications
Gulyaeva O, Dempersmier J, Sul H. S. Genetic and epigenetic control of adipose developmentBiochim Biophys Acta Mol Cell Biol Lipids. 1864, 3-12, 2019.
Joshi PA, Waterhouse PD, Kasaian K, Fang H, Gulyaeva O, and Sul H. S, Boutros P, Khokha R. "PDGFRα+ stromal adipocyte progenitors transition into epithelial cells during lobulo-alveologenesis in the murine mammary gland Nature Comm. 10, 1760, 2019.
Gulyaeva O, Nguyen H, Sambeat A, Heydari K, Sul H.S. Sox9-Meis1 inactivation is requried for adipogenesis, advancing Pref-1+to PDGFRa+cells. Cell Reports25, 002-1017, 2018.
Sambeat, A., Gulyaeva, O., Dempersmier, J., and Sul, H. S. Epigenetic Regulation of Brown adipose tissue program and thermogenesis. Trends Endo. Met. 28, 19-31, 2017.
Viscarra, J., Wang, Y., Kim, S.-J., Hong, I, and Sul, H. S. Transcriptional activation of de novo fatty acid synthesis by insulin requires MED17 phosphorylation by CK2. Science Signaling 10, 467, 2017.
Kim, S.-J., Tang, T., Abbott, M., Viscarra, J. A., Wang, Y., and Sul, H. S. AMPK phosphorylates desnutrin/ATGL and HSL to regulate lipolysis and fatty acid oxidation within adipose tissue. Mol. Cell. Biol. 36, 1961-1976, 2016.
Sambeat, A.,Gulyaeva, O., Dempersmier, J., Tharp, K., Stahl, A., Paul, S.M., and Sul, H.S. LSD1 interacts with Zfp516 to promote UCP1 transcription and Brown Fat Program. Cell Reports15, 2536-2549, 2016.
Wang, Y., Viscarra, J. A., Kim, S.-J., and Sul, H. S. Transcriptional regulation of lipogenesis. Nat.Rev. Mol. Cell. Biol.16, 678-689, 2015.
Perry, R. J., Camporez, J-P. G., Kursawe, R., Titchenell, P.M., Zhang, D., Perry, C.J., Jurczak, M.J., Han, M.S., Zhang, X-M., Ruan, H-B., Yang, X., Caprio, S., Kaech, S.M., Sul, H.S., Birnbaum, M.J., Davis, R.J., Cline, G.W., Petersen, K. F., and Shulman, G. I. Hepatic acetyl CoA regulates insulin action and links adipose tissue inflammation with hepatic insulin resistance and type 2 diabetes. Cell160, 745-758, 2015.
Dempersmier,J., Sambeat,A, Gulyaeva,O., Paul, S. M., Hudak, C., Raposo, H., Kwan, H.-Y., Kang, C., Wong, R. H., and Sul, H. S.Cold-inducible Zfp516 promotes browning of white Fat and is required for brown fat development. Molecular Cell57, 235-246, 2015.
Hudak, C. S., Gulyaeva, O., Park, S.-M., Lee, L., Kang, C., and Sul, H. S. Pref-1 marks early mesenchymal precursors required for adipose tissue development and expansion. Cell Reports8, 678-687, 2014.
Tang, T., Abbott, M. J., Ahmadian, M., Lopes, A. B., Wang,Y., and Sul, H. S. Desnutrin/ATGL Activates PPARdto Promote Mitochondrial function and Insulin Secretion in Isletb cells.Cell Metabolism18, 883-895, 2013.
Wang, Y., Wong, R. H., Tang, T., Hudak, C. S., Yang, D., Duncan, R. E., and Sul, H. S. Phosphorylation and recruitment of BAF60c in chromatin remodeling for lipogenesis in response to insulin. Molecular Cell49, 283-297, 2013.
Ahmadian, M., Abbott,M. J., Tang,T., Hudak,C. S. S., Kim,Y., Bruss, M., Hellerstein, M. K., Lee,H.-Y., Samuel, V. T., Shulman,G. I., Wang,Y., Duncan, R. E., Kang,C., and Sul, H. S. Adipose-specific ablation of desnutrin/ATGL promotes a brown-to-white adipose phenotype; Regulation by AMPK. Cell Metabolism 13,739-748, 2011.
Wang, Y., Zhao, L, Smas, C. M., and Sul, H. S. Pref-1 interacts with fibronectin to inhibit adipocyte differentiation. Mol. Cell. Biol.30, 3480-3492, 2010.
Duncan, R. E., Wang, Y., Ahmadian, M., Lu, J., Sarkadi-Nagy, E., and Sul, H. S. Characterization of desnutrin functional domains: Critical residues for triacylglycerol hydrolysis in cultured cells.J. Lipid Res. 51, 309-317, 2009.
Sul, H. S. Pref-1: role in adipogenesis and mesenchymal cell fate. Mol. Endo. 23. 1717-1725, 2009.
Jacobs F. M. J., van der Linden, A. J. A., Wang, Y., von Oerthel, L., Sul, H. S., Burbach, J. P. H., and Smidt, M. P. Identification of Dlk1, Ptpru and Klhl1 as novel Nurr1 target genes in meso-diencephalic dopamine neurons.Development136, 2363-2373, 2009.
Wang, Y., and Sul, H. S. Pref-1 regulates mesenchymal cell commitment and differentiation through Sox9.Cell Metabolism9, 287-302, 2009.
Ahmadian, M., Duncan, R. E., Varady, K. A., Frasson, D., Hellerstein, M. K., Birkenfeld, A. L., Samuel, V. T., Shulman, G. I., Wang, Y., Kang, C., and Sul, H. S. Adipose overexpression of desnutrin promotes fatty acid use and attenuates diet-induced obesity.Diabetes58, 855-866, 2009.
Wong, R. H. F., Chang, I., Hudak, C. S. S., Hyun, S., Kwan, H.-Y., and Sul, H. S. A role of DNA-PK for the metabolic gene regulation in response to insulin. Cell136, 1056-1072, 2009.
Jaworski K., Ahmadian, M., Duncan, R., Sarkadi-Nagy E., Varady, K. A., Hellerstein, M. K., Lee, H.-Y., Samuel, V. T., Shulman, G. I., Kim, K.-H., de Val, S., Kang, C., and Sul, H. S. AdPLA ablation increases lipolysis and prevents obesity induced by high-fat feeding or leptin deficiency. Nature Medicine15, 159-168, 2009.
Villena, J.A., Choi, C. S., Wang, Y., Kim, S., Hwang, Y.-J., Kim, Y.-B., Cline, G., Shulman, G. I., and Sul, H. S. Resistance to high-fat diet-induced obesity but exacerbated insulin resistance in mice overexpressing preadipocyte factor-1 (Pref-1). A new model of partial lipodystrophy. Diabetes57, 3258-3266, 2008.
Duncan, R. E., Sarkadi-Nagy, E., Jaworski, K., Ahmadian, M., and Sul, H. S. Identification and functional characterization of adipose-specific phospholipase A2(AdPLA).J. Biol. Chem.283, 25428-25436, 2008.