title: Exendin-4 protects pancreatic beta cells from palmitate-induced apoptosis by interfering with GPR40 and the MKK4/7 stress kinase signalling pathway.
abstract: OBJECTIVE
  The mechanisms of the protective effects of exendin-4 on NEFA-induced beta cell apoptosis were investigated.
  
  
  METHODS
  The effects of exendin-4 and palmitate were evaluated in human and murine islets, rat insulin-secreting INS-1E cells and murine glucagon-secreting alpha-TC1-6 cells. mRNA and protein expression/phosphorylation were measured by real-time RT-PCR and immunoblotting or immunofluorescence, respectively. Small interfering (si)RNAs for Ib1 and Gpr40 were used. Cell apoptosis was quantified by two independent assays. Insulin release was assessed with an insulin ELISA.
  
  
  RESULTS
  Exposure of human and murine primary islets and INS-1E cells, but not alpha-TC1-6 cells, to exendin-4 inhibited phosphorylation of the stress kinases, c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK), and prevented apoptosis in response to palmitate. Exendin-4 increased the protein content of islet-brain 1 (IB1), an endogenous JNK blocker; however, siRNA-mediated reduction of IB1 did not impair the ability of exendin-4 to inhibit JNK and prevent apoptosis. Exendin-4 reduced G-protein-coupled receptor 40 (GPR40) expression and inhibited palmitate-induced phosphorylation of mitogen-activated kinase kinase (MKK)4 and MKK7. The effects of exendin-4 were abrogated in the presence of the protein kinase A (PKA) inhibitors, H89 and KT5720. Knockdown of GPR40, as well as use of a specific GPR40 antagonist, resulted in diminished palmitate-induced JNK and p38 MAPK phosphorylation and apoptosis. Furthermore, inhibition of JNK and p38 MAPK activity prevented palmitate-induced apoptosis.
  
  
  CONCLUSIONS
  Exendin-4 counteracts the proapoptotic effects of palmitate in beta cells by reducing GPR40 expression and inhibiting MKK7- and MKK4-dependent phosphorylation of the stress kinases, JNK and p38 MAPK, in a PKA-dependent manner.
authors.affiliation: Department of Emergency and Organ Transplantation, Section of Internal Medicine, Endocrinology, Andrology and Metabolic Diseases, University of Bari Aldo Moro, Piazza Giulio Cesare, 11, 70124, Bari, Italy.
authors.forename: Annalisa
authors.initials: A
authors.lastname: Natalicchio
authors.forename: Rossella
authors.initials: R
authors.lastname: Labarbuta
authors.forename: Federica
authors.initials: F
authors.lastname: Tortosa
authors.forename: Giuseppina
authors.initials: G
authors.lastname: Biondi
authors.forename: Nicola
authors.initials: N
authors.lastname: Marrano
authors.forename: Alessandro
authors.initials: A
authors.lastname: Peschechera
authors.forename: Emanuele
authors.initials: E
authors.lastname: Carchia
authors.forename: Maura Roberta
authors.initials: MR
authors.lastname: Orlando
authors.forename: Anna
authors.initials: A
authors.lastname: Leonardini
authors.forename: Angelo
authors.initials: A
authors.lastname: Cignarelli
authors.forename: Piero
authors.initials: P
authors.lastname: Marchetti
authors.forename: Sebastio
authors.initials: S
authors.lastname: Perrini
authors.forename: Luigi
authors.initials: L
authors.lastname: Laviola
authors.forename: Francesco
authors.initials: F
authors.lastname: Giorgino
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D002460:Cell Line; D002478:Cells, Cultured; D000077270:Exenatide; D006801:Humans; D015151:Immunoblotting; D050417:Insulin-Secreting Cells / Q000187:drug effects* / Q000378:metabolism*; D048670:MAP Kinase Kinase 4 / Q000235:genetics / Q000378:metabolism*; D048688:MAP Kinase Kinase 7 / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D010168:Palmitates / Q000494:pharmacology*; D010455:Peptides / Q000494:pharmacology*; D051381:Rats; D043562:Receptors, G-Protein-Coupled / Q000235:genetics / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction; D014688:Venoms / Q000494:pharmacology*
chemical_list: C473036:Ffar1 protein, mouse; D010168:Palmitates; D010455:Peptides; D043562:Receptors, G-Protein-Coupled; D014688:Venoms; D000077270:Exenatide; D048670:MAP Kinase Kinase 4; D048688:MAP Kinase Kinase 7