Essential role of ubiquitin-proteasome system in normal regulation of insulin secretion.
Insulin secretion from pancreatic beta-cells occurs by sequential cellular processes, including glucose metabolism, electrical activity, Ca2+ entry, and regulated exocytosis. Abnormalities in any of these functions can impair insulin secretion. In the present study, we demonstrate that inhibition of proteasome activity severely reduces insulin secretion in the mouse pancreatic beta-cell line MIN6-m9. Although no significant effects on glucose metabolism including ATP production were found in the presence of proteasome inhibitors, both glucose- and KCl-induced Ca2+ entry were drastically reduced. As Ca2+-ionophore-induced insulin secretion was unaffected by proteasome inhibition, a defect in Ca2+ entry through voltage-dependent calcium channels (VDCCs) is the likely cause of the impaired insulin secretion. We found that the pore-forming alpha-subunit of VDCCs undergoes ubiquitination, which does not decrease but slightly increases expression of the alpha-subunit protein at the plasma membrane. However, electrophysiological analysis revealed that treatment with proteasome inhibitors results in a severe reduction in VDCC activity in MIN6-m9 cells, indicating that VDCC function is suppressed by proteasome inhibition. Furthermore, insulin secretion in isolated mouse pancreatic islets was also decreased by proteasome inhibition. These results demonstrate that the ubiquitin-proteasome system plays a critical role in insulin secretion by maintaining normal function of VDCCs.
Department of Cellular and Molecular Medicine, Graduate School of Medicine, Chiba University, Kyoto 606-8507, Japan; Division of Cellular and Molecular Medicine, Kobe University Graduate School of Medicine, Kobe 650-0017, Japan.
Miho
M
Kawaguchi
Department of Cellular and Molecular Medicine, Graduate School of Medicine, Chiba University, Kyoto 606-8507, Japan; Department of Experimental Therapeutics, Translational Research Center, Kyoto University Hospital, Kyoto 606-8507, Japan.
Kohtaro
K
Minami
Department of Diabetes and Clinical Nutrition, Kyoto University Graduate School of Medicine, Kyoto 606-8507, Japan.
Kazuaki
K
Nagashima
Department of Cellular and Molecular Medicine, Graduate School of Medicine, Chiba University, Kyoto 606-8507, Japan; Division of Cellular and Molecular Medicine, Kobe University Graduate School of Medicine, Kobe 650-0017, Japan; Department of Experimental Therapeutics, Translational Research Center, Kyoto University Hospital, Kyoto 606-8507, Japan. Electronic address: seino@med.kobe-u.ac.jp.
Susumu
S
Seino
D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000235:genetics / Q000378:metabolism; D002460:Cell Line; D005786:Gene Expression Regulation; D005947:Glucose; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000378:metabolism*; D051379:Mice; D011189:Potassium Chloride; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D061988:Proteasome Inhibitors; D025801:Ubiquitin / Q000378:metabolism*
D015220:Calcium Channels; D007328:Insulin; D061988:Proteasome Inhibitors; D025801:Ubiquitin; D011189:Potassium Chloride; D046988:Proteasome Endopeptidase Complex; D005947:Glucose; D002118:Calcium