title: Imatinib induces apoptosis by inhibiting PDGF- but not insulin-induced PI 3-kinase/Akt survival signaling in RGC-5 retinal ganglion cells.
abstract: OBJECTIVE
  Platelet-derived growth factor (PDGF) and insulin promote the survival of neuronal cells, including retinal ganglion cells (RGCs), via activation of phosphoinositide 3-kinase (PI 3-kinase)/Akt signaling. Of importance, recent studies have shown that imatinib inhibition of PDGF receptors induces retinal toxicity in some patients. To date, the extent of activation and the functional significance of insulin-induced PI 3-kinase/Akt signaling remain unclear in the context of dysregulated PDGF receptor signaling in retinal cells. In the present study, we tested the hypothesis that the pro-survival effect of insulin-induced PI 3-kinase/Akt signaling is compromised by imatinib inhibition of PDGF receptor signaling in RGCs.
  
  
  METHODS
  RGC-5 cells were subjected to acute and long-term treatments with imatinib, a PDGF receptor tyrosine kinase inhibitor. Afterwards, the changes in RGC phenotype and apoptotic markers were assessed by fluorescence and phase contrast microscopy and caspase-3/poly(ADP-ribose) polymerase (PARP) cleavage, respectively. In addition, imatinib regulation of PDGF- and insulin-induced PI 3-kinase/Akt survival signaling was determined by immunoblot analyses, immunoprecipitation, and in vitro PI 3-kinase assays.
  
  
  RESULTS
  Treatment of RGC-5 cells with imatinib for up to 48 h resulted in apoptosis, which was not rescued by insulin supplementation. The apoptotic phenotype was associated with upregulation of cleaved caspase-3 and cleaved poly(ADP-ribose) polymerase. Time dependency experiments revealed that imatinib-mediated apoptosis was preceded by early and sustained abrogation of PDGF-induced increases in PDGF receptor tyrosine phosphorylation and phosphotyrosine-associated PI 3-kinase activity. In addition, imatinib inhibited PDGF-induced downstream phosphorylation of Akt, GSK-3beta, and p70S6kinase. However, imatinib exposure did not affect insulin-induced insulin receptor substrate (IRS)-associated PI 3-kinase activity and the downstream phosphorylation of Akt, GSK-3beta, and p70S6kinase.
  
  
  CONCLUSIONS
  Together, these data indicate that disruption of PDGF receptor signaling compromises the pro-survival effect of insulin-induced IRS-dependent PI 3-kinase/Akt signaling in RGCs, and that the maintenance of PDGF-induced PI 3-kinase/Akt signaling is critical for the survival of retinal neuronal cells.
authors.affiliation: Department of Medicine, Penn State Hershey Heart & Vascular Institute, Penn State College of Medicine, Hershey, PA 17033, USA.
authors.forename: Swarajit K
authors.initials: SK
authors.lastname: Biswas
authors.forename: Yan
authors.initials: Y
authors.lastname: Zhao
authors.forename: Lakshman
authors.initials: L
authors.lastname: Sandirasegarane
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D001549:Benzamides; D053148:Caspase 3 / Q000378:metabolism; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism; D000071679:Glycogen Synthase Kinase 3 beta; D006801:Humans; D000068877:Imatinib Mesylate; D007328:Insulin / Q000494:pharmacology*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D019000:Phosphotyrosine / Q000378:metabolism; D010879:Piperazines / Q000494:pharmacology*; D010982:Platelet-Derived Growth Factor / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D011065:Poly(ADP-ribose) Polymerases / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D011743:Pyrimidines / Q000494:pharmacology*; D051381:Rats; D011972:Receptor, Insulin / Q000378:metabolism; D012165:Retinal Ganglion Cells / Q000166:cytology / Q000187:drug effects* / Q000201:enzymology; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects*
chemical_list: D001549:Benzamides; D007328:Insulin; D010879:Piperazines; D010982:Platelet-Derived Growth Factor; D011743:Pyrimidines; D019000:Phosphotyrosine; D000068877:Imatinib Mesylate; D011065:Poly(ADP-ribose) Polymerases; D019869:Phosphatidylinositol 3-Kinases; D011972:Receptor, Insulin; C000605506:GSK3B protein, human; D000071679:Glycogen Synthase Kinase 3 beta; C000605508:Gsk3b protein, rat; D051057:Proto-Oncogene Proteins c-akt; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D038362:Glycogen Synthase Kinase 3; D053148:Caspase 3
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