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Journal of Agricultural and Food Chemistry
Dietary (-)-epigallocatechin-3-gallate supplementation counteracts aging-associated skeletal muscle insulin resistance and fatty liver in senescence-accelerated mouse Hung-Wen Liua, Yin-Ching Chanb, Ming-Fu Wangb, Chu-Chun Weia, and Sue-Joan Changa* a
Department of Life Sciences, National Cheng Kung University, Tainan, Taiwan
b
Department of Food and Nutrition, Providence University, Taichung, Taiwan
*Corresponding author: Sue-Joan Chang Department of Life Sciences, National Cheng Kung University No. 1, University Road, Tainan, Taiwan Email:
[email protected] Fax: +886-6-2742583 Phone: +886-6-2757575, ext. 65542
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Abstract Aging is accompanied by pathophysiological changes including insulin resistance and fatty liver. Dietary supplementation with (-)-epigallocatechin-3-gallate (EGCG) improves insulin sensitivity and attenuates fatty liver disease. We hypothesized that EGCG could effectively modulate aging-associated changes in glucose and lipid metabolism in senescence-accelerated mice (SAM) prone 8 (SAMP8). Higher levels of glucose, insulin and free fatty acid, inhibited Akt activity, and decreased glucose transporter 4 (GLUT4) expression were observed in SAMP8 mice compared to the normal aging group, SAM resistant 1 mice. EGCG supplementation for 12 weeks successfully decreased blood glucose and insulin levels via restoring Akt activity and GLUT4 expression and stimulating AMPKα activation in skeletal muscle. EGCG up-regulated genes involved in mitochondrial biogenesis and subsequently restored mitochondrial DNA copy number in skeletal muscle of SAMP8 mice. Decreased adipose triglyceride lipase and increased sterol regulatory element binding proteins-1c (SREBP-1c) and carbohydrate responsive element binding protein at mRNA levels were observed in SAMP8 mice in accordance with hepatocellular ballooning and excess lipid accumulation. Prevented hepatic lipid accumulation by EGCG was mainly attributed to down-regulation of mTOR and SREBP-1c-mediated lipid biosynthesis via suppressing the positive regulator, Akt, and activating the negative regulator, AMPKα, in liver. EGCG beneficially modulates glucose and lipid homeostasis in skeletal muscle and liver leading to alleviation of aging-associated metabolic disorders.
Keywords: Aging; EGCG; Insulin resistance; Senescence accelerated mouse
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Journal of Agricultural and Food Chemistry
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Introduction
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Aging-related physiological changes including fat redistribution, mitochondrial
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dysfunction, increased inflammatory status, decreased antioxidant capacity, and
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defects in the insulin signaling pathway are highly associated with insulin resistance1-3.
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In the insulin resistant state, a marked increase in the rate of lipolysis is due to lack of
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insulin-inhibition of hormone-sensitive lipase in adipose tissue, thereby contributing
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to elevated levels of plasma free fatty acids (FFA)4. Increased blood FFA entering
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hepatocytes and subsequently re-esterifying into triglycerides (TG) results in
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excessive accumulation of TG in liver1. In skeletal muscle, elevated blood FFA
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leading to excess accumulation of intramyocellular lipids results in down-regulation
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of IRS1/PI3K (insulin receptor substrate 1/phosphatidylinositol-3-kinases) pathways
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and decreased insulin-mediated glucose uptake5. In search of therapeutic targets and
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strategies for aging-associated metabolic disorders, animal models with similar
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characteristics to human are essential.
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Senescence-accelerated mice (SAM) prone 8 (SAMP8), developed by selective
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inbreeding of the AKR/J strain, is a well-studied murine model for aging-associated
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disorders6. Three longer lived SAM resistant (SAMR) strains with normal phenotype
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have been developed along with SAMP strains as their corresponding control7.
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SAMP8 mice showed a shorter lifespan with specific aging-associated disorders such
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as memory disturbances and amyloid-β deposition at older age, compared to SAMR1
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mice8. In addition to brain aging and Alzheimer's disease, impaired glucose
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homeostasis and pancreas alterations have been observed in SAMP8 mice as
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compared with SAMR19. Insulin resistance is considered as the most important factor
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in the development of non-alcoholic fatty liver disease (NAFLD)10. SAMP8 mice
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with fatty liver demonstrate the relationship between insulin resistance and NAFLD
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during aging11. However, the underlying mechanisms responsible for aging-associated
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metabolic disorders including insulin resistance and fatty liver have not yet been fully
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elucidated in SAMP8 mice.
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(-)-Epigallocatechin-3-gallate (EGCG), the most abundant catechin component in
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green tea, revealed anti-diabetic effects through reversing defects in the insulin
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signaling pathway including suppressed protein kinase C (PKC) activation, decreased
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IRS1 serine phosphorylation12, and promoted glucose transporter 4 (GLUT4)
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translocation in skeletal muscle13. In addition to anti-diabetic effects, attenuation of
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fatty liver by EGCG was through counteracting the activity of PI3K /Akt and nuclear
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factor-κB (NFκB) in NAFLD rat model14. Inhibition of NFκB pathway and activation
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of longevity factor, sirtuin1 (SIRT1), by long-term administration of EGCG were
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considered as the mechanism to extend lifespan in healthy rats15. Therefore, the
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purpose of the present study was to evaluate whether EGCG attenuates
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aging-associated metabolic disorders through modulation of insulin signaling pathway,
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mitochondrial function, and lipid metabolism in skeletal muscle and liver using
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SAMR1 and SAMP8 models.
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Materials and Methods
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Materials
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EGCG (TEAVIGO®, DSM), a highly purified extract from green tea leaves (Camellia
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sinensis) containing >94% EGCG and
