Our Research: Faculty/Research Laboratories

Ian Sweet, PhD

Research Associate Professor, Department of Medicine, Division of Metabolism, Endocrinology and Nutrition

Email Address: isweet@u.washington.edu

Division of Metabolism, Endocrinology and Nutrition Website:  http://depts.washington.edu/metab/

Ian Sweet, PhD

Background:

Dr. Sweet received his PhD from the University of Washington in Bioengineering in 1993. He received post-doctoral training in Biochemistry at the University of Pennsylvania in the lab of Dr. Franz Matschinsky until 1996.  Dr. Sweet’s laboratory focuses on metabolic basis for disease with particular emphasis on diabetes, cancer and cardiovascular disease.  The research has utilized sophisticated methods that have been engineered to non-invasively assess energetics and function in a diverse variety of cells and tissue important to these diseases. The wide range and applicability of the experimental approaches and research have resulted in funding by multiple sources including NIH (National Institute of Diabetes, Digestive and Kidney, National Eye Institute, and National Cancer Institute), National Science Foundation, Juvenile Diabetes Research Foundation, Merck Inc, and the Seattle Foundation.  As Director of the Cell Function Analysis Core of the Diabetes Research Center, the methods developed by Dr. Sweet’s group are available to other researchers as services carried out by two full time technicians in the laboratory.

For more information on the DERC Cell Analysis Core, visit their website athttp://depts.washington.edu/diabetes/biomedcore/ifac.index.html.

Focus:

Dr. Sweet’s research focuses on regulation and impairment of insulin secretion in Type 2 Diabetes; pancreatic beta cell death; metabolic basis of inflammation and T cell calcium metabolism in Type 1 Diabetes.

Regulation and impairment of insulin secretion in Type 2 diabetes
It is well established that inadequate insulin secretion by the pancreatic beta cell is a major cause for Type 2 Diabetes.  Accordingly we focus on elucidating the factors regulating glucose stimulation of insulin secretion, and identifying those that mediate the loss of islet function in the progression of the disease.  Specifically, calcium is the major intracellular signal whose transport across the plasma membrane is essential for insulin secretion to occur.  We are trying to identify the critical process stimulated by calcium that enables exocytosis of secretory granules to occur, and facilitates the further control of glucose-stimulated insulin release by potentiators such as acetylcholine, GLP-1 and other incretins, and fatty and amino acids.  Clues to its identity include a very high usage of energy that correlates with rates of insulin secretion, and dual control by both calcium and a metabolic factor downstream of electron transport.   Importantly, we have shown that its activity, assessed by the rate of calcium-sensitive oxygen consumption, is highly correlated with control of blood sugar in vivo in a rat model of Type 2 diabetes. Therefore, this process, which we have termed the Ca2+/metabolic coupling process (CMCP), is operational in vivo, and its impairment may contribute to the progression of hyperglycemia in Type 2 diabetes.  Identification of the proteins involved in the CMCP will lead to a greater understanding of the etiology of Type 2 Diabetes, and may be candidates for treatments aimed at increasing insulin secretion in patients with this disease.  Recently we have obtained evidence supporting the dual control of the CMCP by calcium and cytochrome c and ultimately insulin secretionThis exciting line of research could have major implicatinos for the understanding and treatment of diabetes.

Pancreatic beta cell death
High rates of pancreatic beta cell death are a central problem in all aspects of diabetes including Type 1, Type 2 and in the performance of islet transplantation.  The development of therapeutic strategies to prevent or slow the rate of cell death would be greatly facilitated by a fundamental characterization of beta cell population dynamics in vivo, and a precise understanding of the intracellular mechanisms mediating apoptosis, a primary cause leading to loss of functional beta cell mass. Major obstacles in the pursuit of these goals are a lack of methods to quantify and assess the progression of factors that mediate cell death.  Thus, our laboratory has endeavored to develop both in vivo and in vitro methods to quantify the amount of functional beta cells.  In developing an method to non-invasively assess beta cell mass In vivo, the method that appears to be the most promising is based on the use of Positron Emission Tomography, which boasts an exquisitely high sensitivity.  Unfortunately, its spatial resolution is poor, and is unable to resolve an individual pancreatic islet, without the signal being contaminated by greater than 98% contribution from non-beta cells.  Therefore we must rely on chemical specificity to obtain the necessary signal to noise for accurate quantification.  Because the identification of a ligand-receptor pair with such high cellular specificity is unprecedented, a systematic approach to the search is warranted.  We have thus developed in vitro screening criteria with which to evaluate candidate beta cell imaging agents that has correctly predicted in vivo response.  We continue to evaluate molecules in vitro and in vivo to define the optimal properties of a successful beta cell mass marker.

In vitro assessment of beta cell death is also proven difficult.  There has been particular need for such a method in the assessment of islet quality in the transplantation of human islets.  There is a wide range of viability and yield from donor organ to organ and being able to rule out islet quality as a factor in graft failure would have great utility.  To do this, we have focused on measures of electron transport, as an optimal “vital sign” of isolated islets.  This is based on the fact that electron transport is the site of generation of reactive oxygen species, energy for the cell to meet cell requirements, and critical pro- and anti-apoptotic signals.  We use glucose-stimulated oxygen consumption and cytochrome c reduction as parameters that 1.  are specific for beta cells and 2. more importantly these parameters correlate with the ability of transplanted islets to lower glucose in a diabetic model of a mouse.  Thus, it seems feasible that this method could become a Gold-standard in the assessment of islet quality.

Metabolic basis of inflammation
Chronically high levels of blood glucose and free fatty acids as seen in type 2 diabetes and obesity are associated with increased risk of cardiovascular disease.  In endothelial cells, excess free fatty acids activate the pro-inflammatory IKK beta-NF-kB pathway via a mechanism that involves Toll-like receptor 4 signaling, and this effect in turn causes cellular insulin resistance and impaired nitric oxide production. Exposure of endothelial cells to excess glucose also induces inflammation and insulin resistance, but the underlying mechanisms remain to be established. Most current hypotheses are based on increased glucose utilization by various metabolic pathways leading to accumulation of pro-inflammatory intermediates or byproducts (such as reactive oxygen species).  Candidate metabolic pathways include glycolysis, the pentose shunt, hexosamine biosynthesis, the tricarboxylic acid cycle, and the coupled mitochondrial processes of electron transport and oxidative phosphorylation. Work is currently being undertaken to determine how metabolic fluxes in primary endothelial cells respond to glucose concentrations above the physiologic range and relate metabolic flux to glucose-induced IKK beta activity.

Differentiation of Beta Cells From Stem Cells

Producing large numbers of functional beta cells would allow transplantation  of the cells into diabetic patients.  Our role in this endeavor is to develop methods to assess and improve protocols being used to induce stem cells to differentiate into insulin-producing cells.  We have developed a method to measure oxygen consumption by cells during periods of hours and days, a time period when growth of cells makes a bigger contribution to changes in total metabolic rate than alterations in bioenergetics.  Continuous profiles of oxygen consumption are an integration of the metabolic rate per cell times the number of cells.  In order to separate these two effects, we are working on an approach to simultaneously measuring oxygen consumption and cell growth. Surprisingly, despite the central importance to cell growth and death to many biomedical fields, accurate determination of cell number is difficult, and growth rates are typically estimated with surrogate biomarkers such incorporation of thymidine and Ki-67 staining.  Our results to date show a strong correlation between the rates of change in oxygen consumption and growth characteristics.  Using a lifetime imaging camera, that operates by measuring the extremely reproducible phosphorescence decay following a short excitatory pulse, large and intermediate changes in oxygen consumption were seen in a fast and slow growing cell line (melanoma and INS-1 cells), whereas no significant change was observed by islets (Fig. 1).  This method is unique in its ability to accurately quantify cell growth over such relatively short periods of time.  Nonetheless, because in general physiologically significant changes in growth rate over the course of a day, which may be only 10-20%, require very stable detection schemes.  It is necessary to keep in mind that even a 20% change in one day results in nearly a 4-fold increase in cell number in the course of a week. To this setup, we have incorporated automated control of the positioning of the imaging window to alternately assess oxygen sensors placed by each of the inflow and outflow ports.  This system can be run in two ways.  The first is where the plated cells are identically treated, but in one chamber they are exposed to a test agent, and the other chamber serves as the control.  In this way, unwanted changes in temperature or media composition effects each chamber equally, and cannot contribute to any differences observed as a response to the test compound.  Another setup would involve the perifusion of different cell populations with the same composition.  The 2-chamber system will eventually be expanded to 4 or more chambers.

Representative Publications:

Sweet IR, Cook DL, Lernmark A, Greenbaum CJ, Wallen AR, Marcum ES, Stekhova SA, Krohn KA. Systematic screening of potential beta-cell imaging agents. Biochem Biophys Res Commun. 314: 976-83, 2004.

Sweet IR, Gilbert M, Scott S, Todorov I, Jensen R, Nair I, Al-Abdullah I, Rawson J, Mullen Y, Kandeel F, Ferreri K. Glucose-stimulated increment in oxygen consumption rate as a standardized test of human islet quality. Amer. J. Trans. 8:183-192, 2008.

Gilbert M, Jung S-R, Sweet IR. Increased potency of calcium derived from L-type calcium channels compared to calcium from the endoplasmic reticulum on oxygen consumption and insulin secretion. J. Biol. Chem. 283:24334-42, 2008.

Sweet IR, Gilbert M, Maloney E, Hockenbery DM, Schwartz MW, Kim F. Endothelial inflammation induced by excess glucose is linked to intracellular accumulation of glucose-6-phosphate. Diabetalogia 52:921-3, 2009.

Jung S-R, Reed BJ, Sweet IR. A highly energetic process couples calcium influx through L-type calcium channels to insulin secretion in pancreatic beta cells. Am J Physiol. 297:E717-27, 2009

Jung S-R, Kuok IT, Couron D, Rizzo N, Margineantu DH, Hockenbery DM, Kim F, Sweet IR. Cytochrome c reduction is a critical regulatory co-factor mediating insulin secretion by pancreatic islets.  J. Biol. Chem. 286:17422-34, 2011

  • Sweet IR, Gustafson SS, Ramkrishna D. Population balance modeling of bubbling fluidized bed reactors-I. Well-stirred dense phase. Chemical Eng. Sci. 42:341-351, 1987.

    Vong RJ, Larson TV, Zoller WH, Covert DS, Charlson RJ, Sweet I, Peterson R, Miller T, Oloughlin JF, Stevenson MN. Rainwater chemistry near an isolated SO2 emission source. ACS Symposium Series. 349: 204-212, 1987.

    Weigle DS, Sweet IR, Goodner CJ. A kinetic analysis of hepatocyte responses to a glucagon pulse: Mechanism and metabolic consequences of differences in response decay times. Endocrinology 121:732-737, 1987.

    Goodner CJ, Sweet IR, Harrison HC Jr. Rapid reduction and return of surface insulin receptors after exposure to brief pulses of insulin in perifused rat hepatocytes. Diabetes 37:1316-1323, 1988.

    Koerker DJ, Sweet IR, Baskin DG. Insulin binding to individual rat skeletal muscles. Am. J. Physiol. 259:E517-E523, 1990.

    Kroll K, Deussen A, Sweet IR. A model of the transport and metabolism of adenosine and S-adenosylhomocysteine (SAH) in the heart. Circ. Res. 71:590-604, 1992.

    Sweet IR, Matschinsky FM. Mathematical model of ß-cell glucose metabolism and insulin release. I. Glucokinase as glucosensor hypothesis. Am. J. Physiol. 268:E775-E788, 1995.

    Sweet IR, Peterson L, Kroll K, Goodner CJ, Berry MA, Graham M. Effect of glucose on uptake of radiolabeled glucose, 2-DG and 3-O-MG by the perfused rat liver. Am J Physiol. 271:E384-96, 1996.

    Sweet IR, Najafi H, Li G, Berner DK, Matschinsky FM. Effect of a glucokinase inhibitor on energy production and insulin release in pancreatic islets. Am. J. Physiol. 271:E606-E625, 1996.

    Matschinsky FM, Sweet IR. Annotated questions and answers about glucose metabolism and insulin secretion of beta-cells. Diabetes Reviews. 4:130-144, 1996.

    Sweet IR, Najafi H, Li G, Grodberg J, Matschinsky FM. Measurement and modeling of glucose-6-phosphatase in pancreatic islets. Am. J. Physiol. 272:E696-E711, 1997.

    Sweet IR, Matschinsky FM. Are there kinetic advantages to GLUT2 in pancreatic glucose sensing? Diabetologia 40:112-119, 1997.

    P. Kesavan, Wang L, Davis E, Cuesta A, Sweet IR, Niswender K, Magnuson MA, Matschinsky FM. Structural instability of mutant beta-cell glucokinase: implications for the molecular pathogenesis of maturity onset diabetes of the young (type-2). Biochem. J. 322:57-63, 1997.

    Doliba NM, Sweet IR, Babsky A, Doliba N, Forster RE, Osbakken M. Simultaneous measurement of oxygen consumption and 13C16O2 production from 13C-pyruvate in diabetic rat heart mitochondria. Adv. Exp. Med Biol. 428:269-275, 1997.

    Davis EA, Cuesta-Munoz A, Raoul M, Buettger C, Sweet IR, Moates M, Magnuson MA, Matschinsky FM. Mutants of glucokinase cause hypoglycaemia- and hyperglycaemia syndromes and their analysis illuminates fundamental quantitative concepts of glucose homeostasis. Diabetologia 42:1175-86, 1999.

    Sweet IR, DL Cook, RW Wiseman, CJ Greenbaum, Å Lernmark, S Matsumoto, JC Teague, KA Krohn. Dynamic perifusion to maintain and assess isolated pancreatic islets. Diabetes Tech. Ther. 4:67-76, 2002.

    Chessler SD,Simonson W, Sweet IR, Hammerle LP. Expression of the vesicular inhibitory amino acid transporter in pancreatic islet cells: distribution of the transporter within rat islets. Diabetes 51:1763-71,2002.

    Matsumoto S, Qualley SA, Goel S, Hagman DK, Sweet IR, Poitout V, Strong DM, Robertson RP, Reems JA. Effect of the two-layer (University of Wisconsin solution-perfluorochemical plus O2) method of pancreas preservation on human islet isolation, as assessed by the Edmonton Isolation Protocol. Transplantation 74:1414-9, 2002.

    Sweet IR, G Khalil, AR Wallen, M Steedman, KA Schenkman, JA Reems, SE Kahn, JB Callis. Continuous measurement of oxygen consumption by pancreatic islets. Diabetes Tech. Ther. 4: 661-672, 2002.

    Sweet IR, DL Cook, E DeJulio, AR Wallen, G Khalil,, JB Callis, JA Reems. Regulation of ATP/ADP in pancreatic islets. Diabetes 53:401-409, 2004.

    Sweet IR, Cook DL, Lernmark A, Greenbaum CJ, Wallen AR, Marcum ES, Stekhova SA, Krohn KA. Systematic screening of potential beta-cell imaging agents. Biochem Biophys Res Commun. 314: 976-83, 2004.

    Sweet IR, Cook DL, Lernmark Å, Greeenbaum CJ, Krohn KA. Non-invasive imaging of beta cell mass: a quantitative analysis. Diabetes Tech Ther, 6: 652-659, 2004.

    Hampe CS, Wallen AR, Binder K, Schlosser M, Ziegler M, Sweet IR. Quantitative evaluation of a monoclonal antibody and its fragment as potential markers for pancreatic beta cell mass. Exp Clinical Endo Diabetes, 113: 381-7, 2005.

    Niswender CM, Willis BS, Wallen A, Sweet IR, Jetton TL, Thompson BR, Wu C, Lange AJ, McNight GS. Dre recombinase-dependent expression of a constitutively active mutant allele of the catalytic subunit of protein kinase A. Genesis. 43: 109-119, 2005.

    Sweet IR, Gilbert, Sabek O, Fraga DW, Gaber AO, Reems JA. Glucose stimulation of cytochrome c reduction and oxygen consumption as assessment of human islet quality. Transplantation 80: 1003-1011, 2005.

    Nunemaker CS, Wasserman DH, McGuinness OP, Sweet IR, Teague JC, Satin LS. Insulin secretion in the conscious mouse is biphasic and pulsatile. Am J Physiol Endocrinol Metab. 290: E523-9 2006.

    Suckow AT, Sweet IR, Yserloo BV, Rutledge EA, Hall T, Waldrop M, Chessler SD. Identification and characterization of a novel isoform of the vesicular GABA transporter with glucose-regulated expression in rat islets. J Mol Endo, 36: 187-99, 2006.

    Sweet IR, Gilbert M. Contribution of calcium influx in mediating glucose-stimulated oxygen consumption in pancreatic islets. Diabetes 55: 3509-19, 2006.

    Weydt, P, Pineda VV, Torrence AE, Libby RT, Satterfield TF, Lazarowski ER, Gilbert M, Morton GJ, Bammler TK, Stramd AD, Cui L, Beyer RP, Easley CN, Smith AC, Krainc D, Luquet S, Sweet IR, Schwartz MW, La Spada AR. Thermoregulatory and metabolic defects in Huntington’s disease transgenic mice implicate PGC-1a in Huntington’s disease neurodegeneration Cell Metabolism, 4: 349-62, 2006.

    Schwartz PS, Manion MK, Emerson CB, Fry JS, Schulz CM, Sweet IR, Hockenbery DM. 2-Methoxy antimycin reveals a unique mechanism for Bcl-xL inhibition. Molecular Cancer Therapeutics, 6: 2073-80, 2007.

    Sweet IR, Gilbert M, Scott S, Todorov I, Jensen R, Nair I, Al-Abdullah I, Rawson J, Mullen Y, Kandeel F, Ferreri K. Glucose-stimulated increment in oxygen consumption rate as a standardized test of human islet quality. Amer. J. Trans. 8:183-192, 2008.

    Sweet IR, Yanay O, Waldron L, Gilbert M, Fuller J, Tupling T, Lernmark A, Osborne B. Treatment of Diabetic Rats with Encapsulated Islets. J. Cell Mol. Biol. 12:2644-50, 2008.

    Raffo A, Hancock K, Polito T, Witkowski P, Ferrara C, Freeby M, Maffei A, Leibel R, Goland R, Sweet IR, Harris PE. Role of vesicular monoamine transporter type 2 in insulin secretion and glucose metabolism revealed by its specific antagonist tetrabenazine. J of Endocrinology, 198:41-9, 2008.

    Gilbert M, Jung S-R, Sweet IR. Increased potency of calcium derived from L-type calcium channels compared to calcium from the endoplasmic reticulum on oxygen consumption and insulin secretion. J. Biol. Chem. 283:24334-42, 2008.

    Cummings BP, Digitale EK, Stanhope KL, Graham JL, Baskin DG, Reed BJ, Sweet IR, Griffen SC, Havel PJ. Development and Characterization of a Novel Rat Model of Type 2 Diabetes Mellitus: The UC Davis Type 2 Diabetes Mellitus (UCD-T2DM) Rat. Am. J. Physiol. 295:R1782-93, 2008.

    Jung S-R, Reed BJ, Sweet IR. A highly energetic process couples calcium influx through L-type calcium channels to insulin secretion in pancreatic beta cells. Am J Physiol. (Epub) 2009.

    Sweet IR, Gilbert M, Maloney E, Hockenbery DM, Schwartz MW, Kim F. Endothelial inflammation induced by excess glucose is linked to intracellular accumulation of glucose-6-phosphate. Diabetalogia 52:921-3, 2009.

    Bollyky PL, Bice JB, Sweet IR, Falk BA, Gebe JA, Clark AE, Gersuk VH, Aderem A, Hawn TR, Nepom GT. The toll-like receptor signaling molecule Myd88 contributes to pancreatic beta-cell homeostasis in response to injury. PLoS ONE 4:e5063. 2009.

    Maloney E, Sweet IR, Hockenbery DM, Pham M, Rizzo NO, Tateya S, Handa P, Schwartz MW, Kim F.  Activation of NF-kB by palmitate in endothelial cells. A key role for NADPH Oxidase-derived superoxide in response to TLR4 Activation. Arterioscler Thromb Vasc Biol 29:1370-5, 2009

    Jung S-R, Reed BJ, Sweet IR. A highly energetic process couples calcium influx through L-type calcium channels to insulin secretion in pancreatic beta cells. Am J Physiol. 297:E717-27, 2009

    Linton JD, Holzhausen LC, Babai N, Song H, Miyagishima KJ, Stearns GW, Lindsay K, Wei J, Chertov AO, Peters TA, Caffe R, Pluk H, Seeliger M, Tanimoto N, Fong K, Bolton L, Kuok DL, Sweet IR, Bartoletti TM, Rad RA,Travis GA, Zagotta WN, Van der Zee CEEM, Sampath AP,  Sokolov M, Thoreson WB, Hurley JB.  Flow of Energy in the Outer Retina in Darkness and in Light.  Proc. Natl Acad Sci 107:8599-604, 2010 (PMCID: PMC2889335)

    Houamed K, Sweet IR, Satin LS.  BK channels mediate a novel ionic mechanism that regulates glucose-dependent electrical activity and insulin secretion in mouse pancreatic beta-cells. J. Physiology (London) 15:3511-23, 2010

    Sahni J, Tamura R, Sweet IR, Scharenberg A.  TRPM7 regulates quiescent/proliferative metabolic transitions in lymphocytes. Cell Cycle  9:3565-74, 2010

    Jung S-R, Kuok IT, Couron D, Rizzo N, Margineantu DH, Hockenbery DM, Kim F, Sweet IR. Cytochrome c reduction is a critical regulatory co-factor mediating insulin secretion by pancreatic islets.  J. Biol. Chem. 286:17422-34, 2011

    Stoll EA, Mikheev A, Sweet IR, Bielas JH, Zhang J, Rostomily RC, Horner PJ. Identification of a unique metabolic signature in aging neural stem cells. J. Biol. Chem. 286:34700-11, 2011

    Chertov AO, Holzhausen L, Kuok D, Parker E, Lindsay K, Sadilek M, Sweet IR, Hurley JB.  Roles of glucose in photoreceptor survival. J. Biol. Chem. 286:38592-601, 2011

    Meabon JS, Lee A, Meeker KD, Bekris LM, Fujimura RK,Yu C-E, Watson GS, Pow DV, Sweet IR, Cook DG.  Differential Expression of the Glutamate Transporter GLT-1 in Pancreas. Journal of Histochemsitry & Cytochemistry 60:139-51 2012

    Chen W, Lisowski M, Sweet IR, Shen AQ.  A 3-dimensional sensor for the measurement of oxygen consumption by single isolated pancreatic islets. PLoS One, 7:e33070, 2012.  (Sweet and Shen are co-Senior authors on this paper.)

    Han CY, Umemoto T, Omer M, Hartigh LJD, Buller CL, Sweet IR, Pennathur S, Abel ED and Chait A.  NADPH oxidase-derived reactive oxygen species increases expression of monocyte chemotactic factor genes in cultured adipocytes. J. Biol. Chem 287:10379-93. 2012

    Suckow AT, Zhang C, Egodage S, Comoletti D, Taylor P, Sweet IR, Chessler SD. Transcellular neuroligin-2 interactions enhance insulin secretion and are integral to pancreatic and B-cell function. J. Biol. Chem In Press. 2012

Current Collaborations:

Within the Diabetes and Obesity Center of Excellence, The Diabetes Research Center and its Affiliated Members
Karin Bornfeldt, PhD
Steven Chessler, MD, PhD (University of California at San Diego)
Vincinzo Cirulli, MD PhD
David Cummings, MD
Christiane Hampe, PhD
Peter Havel, DVM, PhD  (University of California at Davis)
David Hockenbery, MD (Fred Hutchinson Cancer Research Center)
James Hurley, PhD
Francis Kim, MD
Jerry Nepom, MD, PhD (Benaroya Research Institute)
William Osborne, PhD
Leslie Satin, PhD (University of Michigan)
Andrew Scharenberg, MD (Children’s Research Center)

Lab Members:

Austin Rountree
Wenjing Wang. MD PhD

Thesis Committees:

Ken Lindsay  (Department of Biochemistry)