D R Clemmons

University of North Carolina at Chapel Hill, Chapel Hill, NC, United States

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Publications (256)1433.95 Total impact

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    David R Clemmons
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    ABSTRACT: Insulin-like growth factor-I (IGF-I) is closely related to insulin but has distinct metabolic actions. IGF-I is an important stimulant of protein synthesis in muscle, but it also stimulates free fatty acid use. The administration of IGF-I to patients with extreme insulin resistance results in improvement in glycemic control, and IGF-I is associated with lowering glucose and enhancing insulin sensitivity in Type 1 and Type 2 diabetes. However, patients with diabetes are also sensitive to stimulation of side effects in response to IGF-I. IGF-I coordinately links growth hormone and insulin actions and has direct effects on intermediary metabolism.
    Endocrinology and metabolism clinics of North America 06/2012; 41(2):425-43, vii-viii. · 3.56 Impact Factor
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    ABSTRACT: We have previously shown that the association of integrin-associated protein (IAP) with tyrosine phosphatase non-receptor type substrate-1 (SHPS-1) regulates the response of cells, including osteoclasts, osteoblasts, smooth muscle and retinal endothelial cells, to IGF-I. Here we sought to: (1) determine whether the regulation of IGF-I responsiveness by the association of IAP with SHPS-1 is a generalised response of endothelial cells; (2) identify the mechanism by which this association contributes to changes in endothelial cell responses to IGF-I; and (3) determine whether inhibition of this association alters pathophysiological changes occurring in vivo. Endothelial cells were maintained in 5 mmol/l glucose and at hyperglycaemic levels, and exposed to an anti-IAP antibody that disrupts the association between IAP and SHPS-1. A rodent model of diabetes with endothelial cell dysfunction was used to investigate the role of the association of IAP with SHPS-1 in endothelial cell function in vivo. Endothelial cells maintained in 5 mmol/l glucose showed constitutive cleavage of the extracellular domain of IAP (which contains the SHPS-1 binding site), with no association between IAP and SHPS-1 being detected. In contrast, hyperglycaemia inhibited IAP cleavage, allowing IAP to associate with SHPS-1 and IGF-I to stimulate SHPS-1 tyrosine phosphorylation. Exposure to the anti-IAP antibody inhibited IGF-I-stimulated tube formation and increased permeability. In the rodent model, basal IAP-SHPS-1 association was not detected in retinal extracts from normal rats, but was fully restored in rats with diabetes. The anti-IAP antibody inhibited the association of IAP with SHPS-1, and reduced retinal vascular permeability and leucocyte adherence to levels similar to those in non-diabetic rats. The antibody also significantly inhibited the aberrant neovascularisation induced by hypoxia. Our results demonstrate that the increased association of IAP with SHPS-1 contributes to the pathophysiological changes in the endothelium that are induced by hyperglycaemia and hypoxia.
    Diabetologia 12/2011; 55(3):835-44. · 6.49 Impact Factor
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    ABSTRACT: Global deletion of the Igfbp2 gene results in the suppression of bone turnover. To investigate the role of insulin-like growth factor-binding protein-2 (IGFBP-2) in regulating osteoclast differentiation, we cultured Igfbp2(-/-) bone marrow cells and found a reduction in the number of osteoclasts and impaired resorption. Addition of full-length IGFBP-2 restored osteoclast differentiation, fusion, and resorption. To determine the molecular domains of IGFBP-2 that were required for this effect to be manifest, Igfbp2(-/-) bone marrow cells were transfected with constructs in which the heparin-binding (HBD) or the IGF-binding domains of IGFBP-2 were mutated. We found that both domains were necessary for osteoclastogenesis because expression of the mutated forms of either domain failed to support the formation of functionally mature osteoclasts. To discern the mechanism by which IGFBP-2 regulates osteoclast formation, PTEN abundance and phosphorylation status as well as AKT responsiveness to IGF-I were analyzed. Igfbp2(-/-) cells had elevated levels of PTEN and phospho-PTEN compared with controls. Expression of wild-type IGFBP-2 reduced the level of PTEN to that of wild-type cells. Cells expressing the IGF-binding mutant showed suppression of PTEN and phospho-PTEN equivalent to the wild-type protein, whereas those expressing the IGFBP-2 HBD mutant showed no PTEN suppression. When the ability of IGF-I to stimulate AKT activation, measured by Thr³⁰⁸ and Ser⁴⁷³ phosphorylation, was analyzed, stimulation of Ser⁴⁷³ in response to IGF-I in preosteoclasts required the presence of intact IGFBP-2. This effect was duplicated by the addition of a CK2 inhibitor that prevents the phosphorylation of PTEN. In contrast, in fully differentiated osteoclasts, stimulation of Thr³⁰⁸ phosphorylation required the presence of intact IGFBP-2. We conclude that IGFBP-2 is an important regulator of osteoclastogenesis and that both the heparin- and the IGF-binding domains of IGFBP-2 are essential for the formation of fully differentiated and functional osteoclasts.
    Journal of bone and mineral research: the official journal of the American Society for Bone and Mineral Research 10/2011; 27(2):390-400. · 6.04 Impact Factor
  • David R. Clemmons
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    ABSTRACT: The insulin-like growth factor-I receptor (IGF-IR) is present in many types of tumor cells. Additionally, some tumors show upregulation of IGF-IR number or constitutive activation. There is a great deal of interest in determining whether drugs that target the IGF receptors, such as anti-IGF-IR antibodies, can inhibit tumor cell proliferation or induce tumor cell apoptosis. The studies in cells in culture and those conducted using xenografts of tumors in animals have shown that anti-IGF-IR antibodies are very effective in inhibiting tumor cell growth. The initial human trials have not shown definitive evidence of tumor shrinkage in a large number of patients; however, subsets of patients particularly those with sarcomas have shown some responsiveness. Current studies are seeking to determine whether administration of this antibody in combination with traditional chemotherapy and/or other antibodies targeting other types of growth factor receptors can lead to improved responses. Additionally, targeting downstream signaling elements in conjunction with the IGF-IR is also being utilized as an alternative approach. The larger trials that are ongoing are likely to resolve these issues and determine the ultimate efficacy of this agent in cancer chemotherapy. This chapter focuses on antibodies directed toward the IGF-1R. The use of tyrosine kinase inhibitors as an alternative approach is discussed elsewhere in this book.
    09/2011: pages 193-213;
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    ABSTRACT: IGF-I is structurally related to proinsulin and when administered to human subjects it enhances insulin sensitivity. However because of its growth promoting properties and its relationship to growth hormone, it has been proposed as a etiologic factor in the development of diabetic complications. This review discusses recently published data regarding the ability of hyperglycemia to sensitize cells that are capable of dedifferentiating to the growth promoting effects of IGF-I. Under normoglycemic conditions vascular smooth muscle and endothelial cells are cystostatic and stimulation of the IGF-I receptor activates the adaptor protein IRS-1 which leads to PI-3 kinase pathway activation. Following exposure to hyperglycemia these cell types undergo a signaling switch whereby an entirely different mechanism is utilized to activate both the PI-3 kinase and the MAP pathways. This leads to increased cell proliferation and migration. This molecular mechanism involves the coordinate regulation of signaling molecules and scaffolding proteins. Activation of this alternative signaling mechanism is directly linked to the stimulation of pathophysiologic processes that are involved in the pathogenesis of both diabetic retinopathy and atherosclerosis. Inhibition of activation of these intermediates has been shown to attenuate glucose induced pathophysiologic changes and results in the inhibition of both atherosclerotic lesion progression and diabetic retinopathy. In summary, hyperglycemia induces a signaling switch in vascular endothelial and smooth muscle cells that results in enhanced sensitivity to the growth promoting effects of IGF-I. This may be an important variable for determining the progression of atherosclerosis in poorly controlled diabetes and in the development of retinopathy.
    Current diabetes reviews 06/2011; 7(4):235-45.
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    ABSTRACT: Insulin-like growth factor-1 (IGF-1) plays a central role in cellular growth, differentiation, survival, and cell cycle progression. It is expressed early during development and its effects are mediated through binding to a tyrosine kinase receptor, the insulin-like growth factor-1 receptor (IGF-1R). In the circulation, the IGFs bind to IGF-binding proteins (IGFBPs), which determine their bioavailability and regulate the interaction between the IGFs and IGF-1R. Studies in animal models and in humans have established critical roles for IGFs in skeletal growth and development. In this review we present new and old findings from mouse models of the IGF system and discuss their clinical relevance to normal and pathological skeletal physiology.
    Journal of bone and mineral research: the official journal of the American Society for Bone and Mineral Research 12/2010; 25(12):2543-52. · 6.04 Impact Factor
  • David R Clemmons
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    ABSTRACT: Measurement of serum growth hormone (GH) and insulin-like growth factor-I (IGF-) is used to monitor the degree of improvement that occurs following treatment of patients with acromegaly. Improvement in GH assay sensitivity has led to changes in the definition of normal GH however many studies that assess the predictive value of GH were conducted in an era where assays were less sensitive. Other problems that have occurred with GH measurements include utilization of different standards and failure to prove commutability of commonly accepted standard. GH reference ranges vary in their quality and are not stratified for age, sex or body mass index. IGF-I measurements are associated with similar problems. They do not use a common standard that has been proven to be commutable and results can vary widely when the same specimens are assayed in different laboratories. Although age and sex stratified reference ranges exist, these do not always have adequate numbers of subjects and BMI adjusted ranges are not available. These problems have led to significant discordance in a significant number of patients wherein the IGF-I and GH values may yield a discrepant prediction of disease stabilization. In these cases in general the IGF-I values correlate better with the presence of persistent symptoms. Patients who fail to suppress GH to normal but have a normal IGF-I have to be monitored carefully for recurrence but usually do not require further therapy if they are asymptomatic. For the long term assessment of outcome and clinical disease activity measurement of both hormones is recommended.
    Clinica chimica acta; international journal of clinical chemistry 11/2010; 412(5-6):403-9. · 2.54 Impact Factor
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    ABSTRACT: Hyperglycemia has been shown to induce the p66shc expression leading to increased reactive oxygen species (ROS) generation and apoptosis. In the present study, we demonstrated that hyperglycemia induced p66shc expression in vascular smooth muscle cells. This induction was associated with an increase in apoptosis as assessed by the increase of capspase-3 enzymatic activity, cleaved caspase-3 protein, and the number of dead cells. The ability of IGF-I to inhibit apoptosis was also attenuated. Further studies showed that hyperglycemia-induced p66shc inhibited IGF-I-stimulated phosphoinositide (PI)-3 kinase and AKT activation. Mechanistic studies showed that knockdown of p66shc enhanced IGF-I-stimulated SHPS-1/p85, p85/SHP-2, and p85/Grb2 association, all of which are required for PI-3 kinase/AKT activation. These responses were attenuated by overexpression of p66shc. IGF-I-stimulated p85 and AKT recruitment to the cell membrane fraction was altered in the same manner. Disruption of p66shc-Src interaction using either a blocking peptide or by expressing a p66shc mutant that did not bind to Src rescued IGF-I-stimulated PI-3 kinase/AKT activation as well as IGF-I-dependent cell survival. Although the highest absolute level of ROS was detected in p66shc-overexpressing cells, the relative increase in ROS induced by hyperglycemia was independent of p66shc expression. Taken together, our data suggest that the increase in p66shc that occurs in response to hyperglycemia is functioning to inhibit IGF-I-stimulated signaling and that the incremental increase in SMC sensitivity to IGF-I stimulation that occurs in response to p66shc induction of ROS is not sufficient to overcome the inhibitory effect of p66shc on Src kinase activation.
    Endocrinology 08/2010; 151(8):3611-23. · 4.72 Impact Factor
  • Growth Hormone & Igf Research - GROWTH HORM IGF RES. 01/2010; 20.
  • L. A. Maile, P. Dunbar, D. Clemmons
    Growth Hormone & Igf Research - GROWTH HORM IGF RES. 01/2010; 20.
  • Growth Hormone & Igf Research - GROWTH HORM IGF RES. 01/2010; 20.
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    ABSTRACT: Age-related osteoporosis is accompanied by an increase in marrow adiposity and a reduction in serum insulin-like growth factor-1 (IGF-1) and the binding proteins that stabilize IGF-1. To determine the relationship between these proteins and bone marrow adiposity, we evaluated the adipogenic potential of marrow-derived mesenchymal stromal cells (MSCs) from mice with decreased serum IGF-1 due to knockdown of IGF-1 production by the liver or knock-out of the binding proteins. We employed 10-16-week-old, liver-specific IGF-1-deficient, IGFBP-3 knock-out (BP3KO) and acid-labile subunit knock-out (ALSKO) mice. We found that expression of the late adipocyte differentiation marker peroxisome proliferator-activated receptor gamma was increased in marrow isolated from ALSKO mice. When induced with adipogenic media, MSC cultures from ALSKO mice revealed a significantly greater number of differentiated adipocytes compared with controls. MSCs from ALSKO mice also exhibited decreased alkaline-phosphatase positive colony size in cultures that were stimulated with osteoblast differentiation media. These osteoblast-like cells from ALSKO mice failed to induce osteoclastogenesis of control cells in co-culture assays, indicating that impairment of IGF-1 complex formation with ALS in bone marrow alters cell fate, leading to increased adipogenesis.
    Journal of Biological Chemistry 12/2009; 285(7):4709-14. · 4.65 Impact Factor
  • Jan Frystyk, Pamela Freda, David R Clemmons
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    ABSTRACT: For almost three decades, the measurement of circulating IGF-I has constituted a highly important biochemical tool in the management of GH disorders. In fact, in acromegaly the importance of circulating IGF-I has increased following the introduction of the GH receptor antagonist pegvisomant, as the use of this drug makes it impossible to use circulating GH as a monitor of disease activity. In addition, determination of circulating IGF-I constitutes a valuable scientific tool in various research areas, from epidemiological investigations through clinical trials and experimental studies. The multiple facets of IGF-I physiology and patho-physiology may explain why numerous endocrine laboratories have invested in IGF-I assays, by means of either in-house assays or commercial kits. However, despite its widespread use, the measurement of IGF-I is by no means trivial. On the contrary, the pronounced binding of IGF-I to the high-affinity IGF-binding proteins (IGFBPs) constitutes a notorious source of error, which has necessitated the development of methods that more or less successfully circumvent interference from the IGFBPs. Furthermore, there are some unsolved issues with the international standardization of the different IGF-I assays and there is no consensus regarding the procedures used when collecting and storing samples for measurement of circulating IGF-I. The aim of this review is to discuss the current state of the art of IGF-I immunoassays and to present the current analytical problems with IGF-I measurements. Finally, we would like to suggest an agenda that may be used when trying to produce internationally accepted uniform requirements for future IGF-I assays.
    Growth hormone & IGF research: official journal of the Growth Hormone Research Society and the International IGF Research Society 10/2009; 20(1):8-18. · 2.35 Impact Factor
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    ABSTRACT: The Acromegaly Consensus Group reconvened in November 2007 to update guidelines for acromegaly management. The meeting participants comprised 68 pituitary specialists, including neurosurgeons and endocrinologists with extensive experience treating patients with acromegaly. EVIDENCE/CONSENSUS PROCESS: Goals of treatment and the appropriate imaging and biochemical and clinical monitoring of patients with acromegaly were enunciated, based on the available published evidence. The group developed a consensus on the approach to managing acromegaly including appropriate roles for neurosurgery, medical therapy, and radiation therapy in the management of these patients.
    The Journal of clinical endocrinology and metabolism 03/2009; 94(5):1509-17. · 6.50 Impact Factor
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    ABSTRACT: Insulin-like growth factor-I (IGF-I) and IGF binding proteins (IGFBPs) are trophic factors for cartilage and have been shown to be chondroprotective in animal models of osteoarthritis (OA). IGFBP-5 is degraded in joint fluid and inhibition of IGFBP-5 degradation has been shown to enhance the trophic effects of IGF-I. To determine the identity of IGFBP-5 protease activity in human OA joint fluid. OA joint fluid was purified and the purified material was analyzed by IGFBP-5 zymography. Both crude joint fluid and purified material contained a single band of proteolytic activity that cleaved IGFBP-5. Immunoblotting of joint fluid for complement 1s (C1s) showed a band that had the same Mr estimate, e.g., 88 kDa. In gel tryptic digestion and subsequent peptide analysis by LC-MS/MS showed that the band contained human C1s. A panel of protease inhibitors was tested for their ability to inhibit IGFBP-5 cleavage by the purified protease. Three serine protease inhibitors, FUT175 and CP-143217 and CB-349547 had IC50's between 1 and 6 microM. Two other serine protease inhibitors had intermediate activity (e.g., IC50's 20-40 microM) and MMP inhibitors had no detectible activity at concentrations up to 300 microM. Human OA fluid contains a serine protease that cleaves IGFBP-5. Zymography, immunoblotting and LC-MS/MS analysis indicate that C1s is the protease that accounts for this activity.
    Osteoarthritis and Cartilage 11/2008; 17(4):547-55. · 4.26 Impact Factor
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    ABSTRACT: Serum insulin-like growth factor (IGF) -1 is secreted mainly by the liver and circulates bound to IGF-binding proteins (IGFBPs), either as binary complexes or ternary complexes with IGFBP-3 or IGFBP-5 and an acid-labile subunit (ALS). The purpose of this study was to genetically dissect the role of IGF-1 circulatory complexes in somatic growth, skeletal integrity, and metabolism. Phenotypic comparisons of controls and four mouse lines with genetic IGF-1 deficits-liver-specific IGF-1 deficiency (LID), ALS knockout (ALSKO), IGFBP-3 (BP3) knockout, and a triply deficient LID/ALSKO/BP3 line-produced several novel findings. 1) All deficient strains had decreased serum IGF-1 levels, but this neither predicted growth potential or skeletal integrity nor defined growth hormone secretion or metabolic abnormalities. 2) IGF-1 deficiency affected development of both cortical and trabecular bone differently, effects apparently dependent on the presence of different circulating IGF-1 complexes. 3) IGFBP-3 deficiency resulted in increased linear growth. In summary, each IGF-1 complex constituent appears to play a distinct role in determining skeletal phenotype, with different effects on cortical and trabecular bone compartments.
    The FASEB Journal 11/2008; 23(3):709-19. · 5.70 Impact Factor
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    ABSTRACT: We examined pituitary tumor volumes in patients treated with pegvisomant for 18 months or longer, and in whom the tumors were monitored for at least 3 years. We present details on 9 of 304 patients in clinical trials with pegvisomant who experienced tumor growth within the first year of treatment. Magnetic resonance images prior to start of pegvisomant and at last follow-up were examined in 43 patients (14% of participating patients). Twenty-nine had received prior radiation therapy (18% of irradiated patients) and all but five received somatostatin analogs between periods of pegvisomant treatment. At follow-up, the median tumor volume was 0.6 cc (range 0.0-19.7 cc), in comparison with 1.6 cc (0.0-19.7 cc) at baseline (P<0.001). Twenty-five patients, of which 23 received radiation therapy, had tumor volume reduction. Seventeen patients had no significant change. One patient, who had not received radiation therapy, had an increase in tumor volume from 1.61 to 1.93 cc. Of the nine patients with tumor growth, six had progressive growth before initiating pegvisomant. Two patients with stable tumors while on octreotide experienced enlargement after octreotide discontinuation but remained stable on long-term pegvisomant therapy. The present data indicate that pegvisomant does not promote tumor growth and suggest that the nine observed cases of tumor progression, which occurred within 8 months after commencing pegvisomant, are likely rebound expansions after discontinuation of somatostatin analogs and/or the natural history of aggressively growing pituitary tumors. Continued long-term surveillance of tumor volume, particularly in non-irradiated patients, is recommended.
    European Journal of Endocrinology 08/2008; 159(5):517-23. · 3.14 Impact Factor
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    ABSTRACT: IGF-binding protein-2 (IGFBP-2) is a 36-kDa protein that binds to the IGFs with high affinity. To determine its role in bone turnover, we compared Igfbp2(-/-) mice with Igfbp2(+/+) colony controls. Igfbp2(-/-) males had shorter femurs and were heavier than controls but were not insulin resistant. Serum IGF-I levels in Igfbp2(-/-) mice were 10% higher than Igfbp2(+/+) controls at 8 wk of age; in males, this was accompanied by a 3-fold increase in hepatic Igfbp3 and Igfbp5 mRNA transcripts compared with Igfbp2(+/+) controls. The skeletal phenotype of the Igfbp2(-/-) mice was gender and compartment specific; Igfbp2(-/-) females had increased cortical thickness with a greater periosteal circumference compared with controls, whereas male Igfbp2(-/-) males had reduced cortical bone area and a 20% reduction in the trabecular bone volume fraction due to thinner trabeculae than Igfbp2(+/+) controls. Serum osteocalcin levels were reduced by nearly 40% in Igfbp2(-/-) males, and in vitro, both CFU-ALP(+) preosteoblasts, and tartrate-resistant acid phosphatase-positive osteoclasts were significantly less abundant than in Igfbp2(+/+) male mice. Histomorphometry confirmed fewer osteoblasts and osteoclasts per bone perimeter and reduced bone formation in the Igfbp2(-/-) males. Lysates from both osteoblasts and osteoclasts in the Igfbp2(-/-) males had phosphatase and tensin homolog (PTEN) levels that were significantly higher than Igfbp2(+/+) controls and were suppressed by addition of exogenous IGFBP-2. In summary, there are gender- and compartment-specific changes in Igfbp2(-/-) mice. IGFBP-2 may regulate bone turnover in both an IGF-I-dependent and -independent manner.
    Endocrinology 06/2008; 149(5):2051-61. · 4.72 Impact Factor
  • David R. Clemmons
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    ABSTRACT: Acromegaly is usually caused by somatotroph adenomas of the pituitary gland that secrete excess growth hormone (GH). This state of GH excess stimulates IGF-I, a peptide hormone that stimulates cell growth. This chronic stimulation of tissue growth leads to multiple symptoms and signs that comprise the acromegalic condition, characterized by tissue overgrowth leading to significant morbidity. The diagnosis can be confirmed by measuring GH suppression following oral glucose administration and measurement of serum IGF-I concentration. Early diagnosis is facilitated by an awareness of the presenting features of the disease and the availability of high-quality, specific, and sensitive diagnostic tests.
    03/2008: pages 141-150;
  • V. E. DeMambro, C. J. Rosen, D. R. Clemmons
    Growth Hormone & Igf Research - GROWTH HORM IGF RES. 01/2008; 18.

Publication Stats

15k Citations
1,433.95 Total Impact Points

Institutions

  • 1980–2012
    • University of North Carolina at Chapel Hill
      • • Department of Medicine
      • • Department of Pathology and Laboratory Medicine
      • • Department of Pediatrics
      Chapel Hill, NC, United States
  • 2011
    • Maine Medical Center Research Institute
      Scarborough, Maine, United States
  • 2009
    • Aarhus University Hospital
      Aarhus, Central Jutland, Denmark
  • 2001
    • Sahlgrenska University Hospital
      Goeteborg, Västra Götaland, Sweden
    • University of California, Los Angeles
      • Department of Pediatrics
      Los Angeles, CA, United States
  • 2000
    • University of California, Berkeley
      Berkeley, California, United States
    • Inonu University
      Malatia, Malatya, Turkey
  • 1998
    • Oregon Health and Science University
      • Department of Pediatrics
      Portland, OR, United States
    • Kolling Institute of Medical Research
      Sydney, New South Wales, Australia
    • University of Michigan
      • Department of Biology
      Ann Arbor, MI, United States
  • 1994–1998
    • University of Illinois, Urbana-Champaign
      • Department of Animal Sciences
      Urbana, IL, United States
  • 1992–1998
    • National Institutes of Health
      • • Section on Molecular and Cell Biology
      • • National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
      Bethesda, MD, United States
    • The University of Western Ontario
      London, Ontario, Canada
  • 1996
    • Beth Israel Deaconess Medical Center
      Boston, Massachusetts, United States
  • 1991–1995
    • East Carolina University
      • Department of Medicine
      Greenville, NC, United States
  • 1989–1994
    • University of Washington Seattle
      • • Department of Medicine
      • • Department of Pediatrics
      Seattle, WA, United States
  • 1993
    • Sydney Children's Hospital
      Sydney, New South Wales, Australia
    • Washington University in St. Louis
      • Department of Biochemistry and Molecular Biophysics
      Saint Louis, MO, United States
    • Stanford University
      • Department of Pediatrics
      Stanford, CA, United States
  • 1992–1993
    • University of Maryland, Baltimore
      • Department of Medicine
      Baltimore, MD, United States
  • 1988
    • University of Florida
      Gainesville, Florida, United States