title: Cyclic AMP signaling stimulates proteasome degradation of thioredoxin interacting protein (TxNIP) in pancreatic beta-cells.
abstract: Thioredoxin interacting protein (TxNIP) functions as an effector of glucotoxicity in pancreatic beta-cells. Exendin-4 (Ex-4), a long-term effective GLP-1 receptor agonist, reduces TxNIP level in pancreatic beta-cells. Mechanisms underlying this reduction, however, remain largely unknown. We show here that Ex-4, 8-bromo-cAMP, the cAMP promoting agent forskolin, as well as activators of protein kinase A (PKA) and exchange protein activated by cAMP (Epac), all attenuated the effect of high glucose (20mM) on TxNIP level in the pancreatic beta-cell line Ins-1. Forskolin and Ex-4 also reduced TxNIP level in cultured primary rat islets. This repressive effect is at least partially mediated via stimulating proteasome-dependent TxNIP degradation, since the proteasomal inhibitor MG132, but not the lysosomal inhibitor chloroquine, significantly blocked the repressive effect of forskolin. Furthermore, forskolin enhanced TxNIP ubiquitination. Both PKA inhibition and Epac inhibition partially blocked the repressive effect of forskolin on TxNIP level. In addition, forskolin and Ex-4 protected Ins-1 cells from high glucose-induced apoptotic activity, assessed by measuring caspase 3 activity. Finally, knockdown of TxNIP expression led to reduced caspase 3 expression levels and blunted response to forskolin treatment. We suggest that proteasome-dependent TxNIP degradation is a novel mechanism by which Ex-4-cAMP signaling protects pancreatic beta cells.
authors.affiliation: Division of Cell and Molecular Biology, Toronto General Research Institute, University Health Network, Canada; Banting and Best Diabetes Centre, Faculty of Medicine, University of Toronto, Canada; Dept of Medicine, University of Toronto, Canada.
authors.forename: Weijuan
authors.initials: W
authors.lastname: Shao
authors.forename: Zhiwen
authors.initials: Z
authors.lastname: Yu
authors.forename: I George
authors.initials: IG
authors.lastname: Fantus
authors.forename: Tianru
authors.initials: T
authors.lastname: Jin
mesh_terms: D000818:Animals; D017209:Apoptosis; D002352:Carrier Proteins / Q000378:metabolism*; D018797:Cell Cycle Proteins; D002460:Cell Line; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D000077270:Exenatide; D005947:Glucose / Q000494:pharmacology; D050417:Insulin-Secreting Cells / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D010455:Peptides / Q000494:pharmacology; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D015398:Signal Transduction; D014688:Venoms / Q000494:pharmacology
chemical_list: D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D010455:Peptides; C493796:TXNIP protein, rat; D014688:Venoms; D005576:Colforsin; D000077270:Exenatide; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D046988:Proteasome Endopeptidase Complex; D005947:Glucose