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STUDIES OF AUTOPHAGY IN RELATION TO INSULIN RESISTANCE, MITOCHONDRIAL FUNCTION,
AND LIPID ACCUMULATION IN HUMAN SKELETAL MUSCLE
Rikke K. Henriksen1, Jonas M. Kristensen1, Stine J. Petersson1, Nils J. Færgeman2, and Kurt Højlund1
1Department
of Endocrinology, Section for Molecular Diabetes & Metabolism, Odense University
Hospital and Institute of Clinical Research, University of Southern Denmark, Denmark.
2 The lipid-hormone group, Institute of Biochemistry and Molecular Biology, University of Southern
Denmark, Denmark
Background: Insulin resistance in human skeletal muscle is linked to mitochondrial dysfunction
and lipid accumulation. Recently, abnormalities in a cellular process autophagy have been
implicated in the pathogenesis of insulin resistance in liver and adipose tissue. Autophagy is a
catabolic process responsible for the degradation of long-lived proteins, damaged organelles such
as defective mitochondria, and lipid droplets (LDs). We hypothesize a role for defective autophagy
in relation to insulin resistance, mitochondrial dysfunction, lipid accumulation, and exercise
resistance in human skeletal muscle in obesity and type 2 diabetes (T2D).
Methods: Skeletal muscle biopsies from age- and gender-matched patients with T2D and nondiabetic, glucose-tolerant obese and lean individuals were used for studying autophagy-related
genes and proteins by qRT-PCR and immunoblotting. The individuals were examined before and
after 1) stimulation with physiological concentrations of insulin, 2) endurance training for 10
weeks, and 3) acute exercise (65-70% of VO2max for 1-h). All individuals were metabolically
characterized by euglycemic hyperinsulinemic clamp using glucose tracer and indirect calorimetry.
Expected results: Measures of mRNA levels, protein content and activity of protein complexes
involved in autophagy (ULK1, ATG7, LC3, ATG5/ATG12-complex, mTOR/S6K, p62, AMPK, FoxO1/3,
BECLIN1-hVPS34, and BNIP3/BNIP3L) in the biopsies are expected to identify novel abnormalities
in relation to obesity and T2D. These abnormalities can then be related to measures of
mitochondrial respiration, lipid droplet formation, and studied in muscle cell cultures and mice
models to test the effect of gene manipulation of autophagy-related genes on glucose
metabolism, insulin signaling, mitochondria function, and LD formation.
Perspectives: Our studies are expected to reveal whether dysfunctional mitochondria and
accumulation of LDs in insulin-resistant human skeletal muscle are caused by autophagic defects
in obesity and T2D. Moreover, it may answer whether defective autophagy is implicated in the
potential attenuated response to acute exercise and endurance training observed in insulinresistant conditions.