title: Fluoxetine impairs insulin secretion without modifying extracellular serotonin levels in MIN6 β-cells.
abstract: BACKGROUND
  Pancreatic β-cells synthetize and store Serotonin (5-Hydroxytriptamine, 5HT) which is co-released with insulin. It has been proposed that extracellular 5HT binds to specific cell surface receptors and modulate insulin secretion. On the other hand, Selective Serotonin Reuptake Inhibitor (SSRI) fluoxetine seems to reduce Glucose-Stimulated Insulin Secretion (GSIS). However, it is unknown whether this effect results from changes in extracellular 5HT concentration owed to the blockade of 5HT transporter (SERT) or from non-5HT dependent actions. The aims of this work were: 1) to quantify extracellular 5HT levels and GSIS in β-cell lines, 2) to determine whether extracellular 5HT levels and GSIS are changed by fluoxetine or 5-Hydroxytryptophan (5HTP, the immediate 5HT biosynthetic precursor), and 3) to quantify the expression of Slc6a4 gene (encoding SERT) in β-cell lines in relation to other genes involved in 5HT system.
  
  
  METHODS
  β-cell lines MIN6 and RINm5f were subjected to GSIS protocols, after treatment with fluoxetine, 5HTP or 5HT. Insulin and 5HT were quantified by ELISA and HPLC, respectively. Relative mRNA expression was quantified by RT-qPCR.
  
  
  RESULTS
  MIN6 β-cells secretes 5HT in response to glucose, showing a sharp increase in 5HT release when cells were preloaded with 5HTP. Treatment with 5HT or fluoxetine reduces GSIS. Fluoxetine fails to further increases 5HTP-induced elevation of secreted 5HT. MIN6 β-cells express both isoforms of Tryptophan Hydroxylase (Tph1 and Tph2), and have high expression levels of L-Dopa decarboxylase (Ddc), both enzymes involved in 5HT biosynthetic pathway, but do not express the 5HT transporters Slc6a4 or Slc6a3 (the Dopamine-5HT transporter) genes.
  
  
  CONCLUSIONS
  The inhibitory effect of fluoxetine on β-cell glucose stimulated insulin secretion is not mediated by blockage of 5HT transporter through SERT.
authors.affiliation: Department of Nutrition, Diabetes and Metabolism, School of Medicine, Pontificia Universidad Católica de Chile.
authors.forename: L R
authors.initials: LR
authors.lastname: Cataldo
authors.affiliation: Department of Nutrition, Diabetes and Metabolism, School of Medicine, Pontificia Universidad Católica de Chile.
authors.forename: V A
authors.initials: VA
authors.lastname: Cortés
authors.affiliation: Department of Nutrition, Diabetes and Metabolism, School of Medicine, Pontificia Universidad Católica de Chile.
authors.forename: M L
authors.initials: ML
authors.lastname: Mizgier
authors.affiliation: Laboratorio de Hemostasia, School of Medicine, Pontificia Universidad Católica de Chile.
authors.forename: E
authors.initials: E
authors.lastname: Aranda
authors.affiliation: Laboratorio de Hemostasia, School of Medicine, Pontificia Universidad Católica de Chile.
authors.forename: D
authors.initials: D
authors.lastname: Mezzano
authors.affiliation: Department of Nutrition, Diabetes and Metabolism, School of Medicine, Pontificia Universidad Católica de Chile.
authors.forename: P
authors.initials: P
authors.lastname: Olmos
authors.affiliation: Department of Nutrition, Diabetes and Metabolism, School of Medicine, Pontificia Universidad Católica de Chile.
authors.forename: J E
authors.initials: JE
authors.lastname: Galgani
authors.affiliation: Institute for Research in Dental Sciences, Faculty of Dentistry, Universidad de Chile.
authors.forename: J
authors.initials: J
authors.lastname: Suazo
authors.affiliation: Department of Nutrition, Diabetes and Metabolism, School of Medicine, Pontificia Universidad Católica de Chile.
authors.forename: J L
authors.initials: JL
authors.lastname: Santos
mesh_terms: D000818:Animals; D001142:Aromatic-L-Amino-Acid Decarboxylases / Q000378:metabolism; D045744:Cell Line, Tumor; D050483:Dopamine Plasma Membrane Transport Proteins / Q000378:metabolism; D005473:Fluoxetine / Q000494:pharmacology*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D050417:Insulin-Secreting Cells / Q000166:cytology / Q000378:metabolism*; D051379:Mice; D012701:Serotonin / Q000378:metabolism*; D050486:Serotonin Plasma Membrane Transport Proteins / Q000378:metabolism; D014365:Tryptophan Hydroxylase / Q000378:metabolism
chemical_list: D050483:Dopamine Plasma Membrane Transport Proteins; D007328:Insulin; D050486:Serotonin Plasma Membrane Transport Proteins; C493215:Slc6a3 protein, mouse; C493231:Slc6a4 protein, mouse; D005473:Fluoxetine; D012701:Serotonin; C487801:Tph1 protein, mouse; C487802:Tph2 protein, mouse; D014365:Tryptophan Hydroxylase; D001142:Aromatic-L-Amino-Acid Decarboxylases