Rubiscolin-6 activates opioid receptors to enhance glucose uptake in skeletal muscle
Timothy Sean Kairupan a,b,1, Kai-Chun Cheng a,1, Akihiro Asakawa a, Haruka Amitani a, Takakazu Yagi c, Koji Ataka a, Natasya Trivena Rokot a, Nova Hellen Kapantow d, Ikuo Kato e, Akio Inui a,*
a Department of Psychosomatic Internal Medicine, Kagoshima University Graduate School of Medical and Dental Sciences, Kagoshima, Japan
b Faculty of Medicine, Sam Ratulangi University, Manado, Indonesia
c Department of Orthodontics and Dentofacial Orthopedics, Kagoshima University Graduate School of Medical and Dental Sciences, Kagoshima, Japan
d Faculty of Public Health, Sam Ratulangi University, Manado, Indonesia
e Department of Medical Biochemistry, Kobe Pharmaceutical University, Kobe, Japan
Rubiscolin-6 is an opioid peptide derived from plant ribulose bisphosphate carboxylase/oxygenase (Rubisco). It has been demonstrated that opioid receptors could control glucose homeostasis in skeletal muscle independent of insulin action. Therefore, Rubiscolin-6 may be involved in the control of glucose metabolism. In the present study, we investigated the effect of rubiscolin-6 on glucose uptake in skeletal muscle. Rubiscolin-6-induced glucose uptake was measured using the fluorescent indicator 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) amino]-2-deoxyglucose (2-NBDG) in L6 and C2C12 cell lines. The protein expressions of glucose transporter 4 (GLUT4) and AMP-activated protein kinase (AMPK) in L6 cells were observed by Western blotting. The in vivo effects of rubiscolin-6 were characterized in streptozotocin (STZ)-induced diabetic rats. Rubiscolin-6 induced a concentrationdependent increase in glucose uptake levels. The increase of phospho-AMPK (pAMPK) and GLUT4 expressions were also observed in L6 and C2C12 cells. Effects of rubiscolin-6 were blocked by opioid receptor antagonists and/or associated signals inhibitors. Moreover, Rubiscolin-6 produced a dose-dependent reduction of blood glucose and increased GLUT4 expression in STZ-induced diabetic rats. In conclusion, rubiscolin-6 increases glucose uptake, potentially via an activation of AMPK to enhance GLUT4 translocation after binding to opioid receptors in skeletal muscle.
Keywords: AMPK, Glucose uptake, GLUT4, Opioid receptor, Rubiscolin-6