Troglitazone Induces GLUT4 Translocation in L6 Myotubes

  1. Shin Yonemitsu,
  2. Haruo Nishimura,
  3. Mitsuyo Shintani,
  4. Ryou Inoue,
  5. Yuji Yamamoto,
  6. Hiroaki Masuzaki,
  7. Yoshihiro Ogawa,
  8. Kiminori Hosoda,
  9. Gen Inoue,
  10. Tatsuya Hayashi and
  11. Kazuwa Nakao
  1. Department of Medicine and Clinical Science, Kyoto University Graduate School of Medicine, Kyoto, Japan

    Abstract

    A number of studies have demonstrated that insulin resistance in the skeletal muscle plays a pivotal role in the insulin resistance associated with obesity and type 2 diabetes. A decrease in GLUT4 translocation from the intracellular pool to the plasma membranes in skeletal muscles has been implicated as a possible cause of insulin resistance. Herein, we examined the effects of an insulin-sensitizing drug, troglitazone (TGZ), on glucose uptake and the translocation of GLUT4 in L6 myotubes. The prolonged exposure (24 h) of L6 myotubes to TGZ (105 mol/l) caused a substantial increase in the 2-deoxy-[3H]d-glucose (2-DG) uptake without changing the total amount of the glucose transporters GLUT4, GLUT1, and GLUT3. The TGZ-induced 2-DG uptake was completely abolished by cytochalasin-B (10 μmol/l). The ability of TGZ to translocate GLUT4 from light microsomes to the crude plasma membranes was greater than that of insulin. Both cycloheximide treatment (3.5 × 106 mol/l) and the removal of TGZ by washing reversed the 2-DG uptake to the basal level. Moreover, insulin did not enhance the TGZ-induced 2-DG uptake additively. The TGZ-induced 2-DG uptake was only partially reversed by wortmannin to 80%, and TGZ did not change the expression and the phosphorylation of protein kinase B; the expression of protein kinase C (PKC)-λ, PKC-β2, and PKC-ζ; or 5′AMP-activated protein kinase activity. α-Tocopherol, which has a molecular structure similar to that of TGZ, did not increase 2-DG uptake. We conclude that the glucose transport in L6 myotubes exposed to TGZ for 24 h is the result of an increased translocation of GLUT4. The present results imply that the effects of troglitazone on GLUT4 translocation may include a new mechanism for improving glucose transport in skeletal muscle.

    Footnotes

    • Address correspondence and reprint requests to Haruo Nishimura, MD, PhD, Department of Diabetes and Endocrinorosy, Osakahu Saiseikai Nakatsu Hospital 2-10-39 Shibata, Kitaku, Osaka 530-0012, Japan. E-mail: hnis{at}kuhp.kyoto-u.ac.jp.

      Received for publication 14 June 1999 and accepted in revised form 24 January 2001.

      2-DG, 2-deoxy-[3H]d-glucose; α-MEM, α-minimal essential medium; AMPK, 5′AMP-activated protein kinase; CPM, crude plasma membrane; DTT, dithiothreitol; FBS, fetal bovine serum; KRPH, HEPES-buffered Krebs-Ringer phosphate; LM, light microsome; LPS, liposaccharide; PCR, polymerase chain reaction; PI, phosphatidylinositol; PKB, protein kinase B; PKC, protein kinase C; PMSF, phenylmethylsulfonyl fluoride; PPAR, peroxisome proliferator–activated receptor; RT, reverse transcription; TGZ, troglitazone.

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