The novel roles of liver for compensation of insulin resistance in human growth hormone transgenic rats.
Chronic excess of GH is known to cause hyperinsulinemia and insulin resistance. We developed human GH transgenic (TG) rats, which were characterized by high plasma levels of human GH and IGF-I. These TG rats showed higher levels of plasma insulin, compared with control littermates, whereas plasma glucose concentrations were normal. Insulin-dependent glucose uptake into adipocytes and muscle was impaired, suggesting that these rats developed insulin resistance. In contrast, insulin-independent glucose uptake into hepatocytes from TG rats was significantly increased, and glycogen and lipid levels in livers of TG rats were remarkably high. Because the role of liver in GH-induced insulin resistance is poorly understood, we studied insulin signaling at early stages and insulin action in liver and primary cultures of hepatocytes prepared from TG rats. There was no difference in insulin receptor kinase activity induced by insulin between TG and control rats; however, insulin-dependent insulin receptor substrate-2 tyrosine phosphorylation, glycogen synthase activation, and expression of enzymes that induce lipid synthesis were potentiated in hepatocytes of TG rats. These results suggest that impairment of insulin-dependent glucose uptake by GH excess in adipose tissue and muscle is compensated by up-regulation of glucose uptake in liver and that potentiation of insulin signaling through insulin receptor substrate-2 in liver experiencing GH excess causes an increase in glycogen and lipid synthesis from incorporated glucose, resulting in accumulation of glycogen and lipids in liver. This novel mechanism explains normalization of plasma glucose levels at least in part in a GH excess model.
Cho
Yoshitake
Y
Department of Animal Sciences , Graduate School of Agriculture and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan.
Ariga
Miyako
M
Uchijima
Yasunobu
Y
Kimura
Kumi
K
Rho
Jeung-Yon
JY
Furuhata
Yasufumi
Y
Hakuno
Fumihiko
F
Yamanouchi
Keitaro
K
Nishihara
Masugi
M
Takahashi
Shin-Ichiro
S
D000273:Adipose Tissue / Q000378:metabolism; D000818:Animals; D030801:Animals, Genetically Modified; D005260:Female; D005947:Glucose / Q000378:metabolism*; D006003:Glycogen / Q000378:metabolism; D006006:Glycogen Synthase / Q000378:metabolism; D022781:Hepatocytes / Q000378:metabolism; D019382:Human Growth Hormone / Q000502:physiology*; D055504:Insulin Receptor Substrate Proteins; D007333:Insulin Resistance; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D050356:Lipid Metabolism; D008099:Liver / Q000502:physiology*; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology; D010750:Phosphoproteins / Q000378:metabolism; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D011972:Receptor, Insulin / Q000378:metabolism; D014443:Tyrosine / Q000378:metabolism
C526292:IRS1 protein, human; C526297:IRS2 protein, human; D055504:Insulin Receptor Substrate Proteins; D047908:Intracellular Signaling Peptides and Proteins; C526296:Irs1 protein, rat; C526299:Irs2 protein, rat; D010750:Phosphoproteins; D012333:RNA, Messenger; D019382:Human Growth Hormone; D014443:Tyrosine; D006003:Glycogen; D006006:Glycogen Synthase; D019869:Phosphatidylinositol 3-Kinases; D011972:Receptor, Insulin; D005947:Glucose