title: Sam68 mediates leptin-stimulated growth by modulating leptin receptor signaling in human trophoblastic JEG-3 cells.
abstract: BACKGROUND
  Sam68, a member of the signal transduction and activation of RNA metabolism (STAR) family of RNA-binding proteins, has been previously implicated as an adaptor molecule in different signaling systems, including leptin receptor (LEPR) signaling. LEPR activation is known to stimulate JAK-STAT, MAPK and PI3K signaling pathways, thus mediating the biological effects of leptin in different cell types, including trophoblastic cells. We have recently found that leptin stimulation also promotes the overexpression and tyrosine phosphorylation of Sam68 in human trophoblastic JEG-3 cells, suggesting a role for Sam68 in leptin signaling and action in these cells. In the present work, we have studied the participation of Sam68 in the main signaling pathways activated by LEPR to increase growth and proliferation in trophoblastic JEG-3 cells.
  
  
  METHODS
  We used an antisense strategy to down-regulate Sam68 expression in these cells, and we studied LEPR signaling by immunoprecipitation and poly-U affinity precipitation and by analyzing phosphorylation levels of signaling proteins by immunoblot. The effect of leptin on protein synthesis and proliferation was studied by ³[H]-leucine and ³[H]-thymidine incorporation.
  
  
  RESULTS
  Sam68 knockdown impaired leptin activation of JAK-STAT, PI3K and MAPK signaling pathways in JEG-3 cells. We have also found that leptin-stimulated Sam68 tyrosine phosphorylation is dependent on JAK-2 activity, since the pharmacological inhibitor AG490 prevents the phosphorylation of Sam68 in JEG-3 cells. Finally, the trophic and proliferative effect of leptin in trophoblastic cells is dependent on Sam68 expression, since its down-regulation impaired the leptin-stimulated DNA and protein synthesis.
  
  
  CONCLUSIONS
  These data demonstrate that Sam68 participates in the main signaling pathways of LEPR to mediate the trophic and proliferative effect of leptin in human trophoblastic cells.
authors.affiliation: Department of Clinical Biochemistry, Virgen Macarena University Hospital, University of Seville, Seville 41071, Spain.
authors.forename: F
authors.initials: F
authors.lastname: Sánchez-Jiménez
authors.forename: A
authors.initials: A
authors.lastname: Pérez-Pérez
authors.forename: C
authors.initials: C
authors.lastname: González-Yanes
authors.forename: C L
authors.initials: CL
authors.lastname: Varone
authors.forename: V
authors.initials: V
authors.lastname: Sánchez-Margalet
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D016375:Antisense Elements (Genetics); D002460:Cell Line; D049109:Cell Proliferation; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D055785:Gene Knockdown Techniques; D006801:Humans; D053614:Janus Kinase 2 / Q000378:metabolism; D020738:Leptin / Q000378:metabolism / Q000494:pharmacology; D010766:Phosphorylation; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D054411:Receptors, Leptin / Q000378:metabolism / Q000502:physiology*; D050796:STAT3 Transcription Factor / Q000378:metabolism; D015398:Signal Transduction; D014327:Trophoblasts / Q000378:metabolism
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D016375:Antisense Elements (Genetics); D004268:DNA-Binding Proteins; C489559:KHDRBS1 protein, human; D020738:Leptin; D016601:RNA-Binding Proteins; D054411:Receptors, Leptin; D050796:STAT3 Transcription Factor; C494086:STAT3 protein, human; C507924:JAK2 protein, human; D053614:Janus Kinase 2