Oct 7, 1991 - Based on recent studies of the abnormal physiology and biochemistry of the glycogen synthesis in skeletal muscle of non-insulin-dependent ...
Rapid Publication Impaired Expression of Glycogen Synthase mRNA in Skeletal Muscle of NIDDM Patients HENRIK VESTERGAARD, CHRISTIAN BJ0RB/EK, PER H. ANDERSEN, JENS F. BAK, AND OLUF PEDERSEN
Based on recent studies of the abnormal physiology and biochemistry of the glycogen synthesis in skeletal muscle of non-insulin-dependent diabetes mellitus (NIDDM) patients and their first-degree relatives, the key enzyme of this pathway, glycogen synthase (GS), is considered a candidate gene in the pathogenesis of insulin resistance. Comparing matched groups of 14 NIDDM patients with 14 control subjects, we found that impaired insulin-stimulated nonoxidative glucose metabolism of peripheral tissue (P < 0.02) and reduced total GS activity (P < 0.05) of vastus lateralis muscle from patients with NIDDM were accompanied by a 39% reduction (P < 0.02) in the steady state level of GS mRNA per microgram DNA of muscle. In both diabetic and control subjects, the mRNA expression of GS was unaffected after euglycemic-hyperinsulinemic clamp for 4 h. With single-stranded conformation polymorphism analysis of the entire coding sequence of the GS gene, we were unable to detect any genetic variants in a subset of eight NIDDM patients. We conclude that abnormal pretranslational regulation of the GS gene may contribute to impaired glycogen synthesis of muscle in NIDDM. Our studies give no evidence for structural changes in the coding region of the GS gene, and it is unknown if the decreased mRNA expression is due to impaired transcription or accelerated degradation of the transcript. Diabetes 40:1740-45, 1991
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everal recent studies applying the glucoseinsulin clamp in combination with indirect calorimetry have shown that the dominant abnormality in the impaired glucose metabolism in skeletal muscle of patients with non-insulin-dependent diabetes mellitus (NIDDM) is insulin-resistant nonoxidative glucose metabolism (1-4). In a study of NIDDM patients with 13C nuclear magnetic resonance spectroscopy under hyperinsulinemic-hyperglycemic
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conditions (5), it was directly demonstrated that the reduced nonoxidative glucose metabolism was due to a severe reduction in the glycogen synthesis rate in skeletal muscle. Glycogen synthase (GS) is the key enzyme that regulates synthesis of glycogen. In muscle, GS exists mainly in an active dephosphorylated form and a less active phosphorylated form (6,7). The two forms of the enzyme are interconverted by protein kinase and phosphatase reactions (6-8) with glucose-6-phosphate (G6P) allosterically activating the phosphatase, whereas insulin appears to act covalently by reducing the kinase activities and increasing the activity of the phosphatase (9,10). Multiple defects have been described in the activation of GS in skeletal muscle of patients with NIDDM, including reduced total GS activity, decreased GS activity in the presence of a physiological concentration of G6P, and insulin-resistant in vivo activation of GS (1-4). Similar findings together with impaired GS phosphatase activity have been shown in nondiabetic insulinresistant individuals (11). We tested the hypothesis that the molecular mechanisms responsible for the insulinresistant glycogen synthesis in muscle of NIDDM patients might involve altered expression or polymorphism of the GS gene. RESEARCH DESIGN AND METHODS
Fourteen subjects with NIDDM and 14 nondiabetic subjects participated in the study (Table 1). All were white and sedentary. The control subjects had normal glucose tolerance and blood pressure and no family history of diabetes. Individuals with NIDDM, as defined by the National Diabetes Data Group (12), were recruited from
From the Steno Diabetes Centre, Copenhagen; the Hagedorn Research Laboratory, Copenhagen; and the Division of Endocrinology and Metabolism, University Clinic of Internal Medicine, Aarhus Amtssygehus, Aarhus, Denmark. Address correspondence and reprint requests to Henrik Vestergaard, MD, Steno Diabetes Centre, Niels Steensensvej 2, DK-2820 Gentofte, Denmark. Received for publication 16 September 1991 and accepted in revised form 7 October 1991.
DIABETES, VOL. 40, DECEMBER 1991
H. VESTERGAARD AND ASSOCIATES
continuous (0.25-|xCi/min) infusion, whereas in the NIDDM subjects, the primer was increased in proportion to the increase in fasting plasma glucose concentration; Diabetic Control the continuous infusion of labeled glucose was the same as in the control subjects (0.25 ^Ci/min). The clamp 4/10 8/6 n (F/M) Age (yr) 52 (40-64) 48 (34-65) was performed by continuous infusion of 2 mU Body mass index (kg/m2) 27.3 ± 1.0 25.2 ± 1.2 insulin • kg" 1 • min~1 (Actrapid, Novo Nordisk, BagsHbA1c (%) 8.1 ± 0.6 5.4 ± 0.1* vaerd, Denmark), and euglycemia was maintained by an Fasting plasma glucose (mM) 11.3 ± 1.2 5.2±0.1t variable infusion of 20% glucose at a rate determined by Fasting plasma insulin (pM) 100 ± 10 50 ± 6 * the measurement of plasma glucose levels at 5- to Steady state plasma insulin during euglycemic- hyperinsulinemic 10-min intervals. Total glucose disposal rate (ftd) was clamp (pM) 1090 ± 120 950 ± 70 calculated from the plasma concentrations of [3-3H]glucose and plasma glucose with Steele's non-steady state Values are means ± SE. equations (14). In these calculations, the distribution *P
