title: PTK6 inhibition promotes apoptosis of Lapatinib-resistant Her2(+) breast cancer cells by inducing Bim.
abstract: BACKGROUND
  Protein tyrosine kinase 6 (PTK6) is a non-receptor tyrosine kinase that is highly expressed in Human Epidermal Growth Factor 2(+) (Her2(+)) breast cancers. Overexpression of PTK6 enhances anchorage-independent survival, proliferation, and migration of breast cancer cells. We hypothesized that PTK6 inhibition is an effective strategy to inhibit growth and survival of Her2(+) breast cancer cells, including those that are relatively resistant to Lapatinib, a targeted therapy for Her2(+) breast cancer, either intrinsically or acquired after continuous drug exposure.
  
  
  METHODS
  To determine the effects of PTK6 inhibition on Lapatinib-resistant Her2(+) breast cancer cell lines (UACC893R1 and MDA-MB-453), we used short hairpin ribonucleic acid (shRNA) vectors to downregulate PTK6 expression. We determined the effects of PTK6 downregulation on growth and survival in vitro and in vivo, as well as the mechanisms responsible for these effects.
  
  
  RESULTS
  Lapatinib treatment of "sensitive" Her2(+) cells induces apoptotic cell death and enhances transcript and protein levels of Bim, a pro-apoptotic Bcl2 family member. In contrast, treatment of relatively "resistant" Her2(+) cells fails to induce Bim or enhance levels of cleaved, poly-ADP ribose polymerase (PARP). Downregulation of PTK6 expression in these "resistant" cells enhances Bim expression, resulting in apoptotic cell death. PTK6 downregulation impairs growth of these cells in in vitro 3-D Matrigel(TM) cultures, and also inhibits growth of Her2(+) primary tumor xenografts. Bim expression is critical for apoptosis induced by PTK6 downregulation, as co-expression of Bim shRNA rescued these cells from PTK6 shRNA-induced death. The regulation of Bim by PTK6 is not via changes in Erk/MAPK or Akt signaling, two pathways known to regulate Bim expression. Rather, PTK6 downregulation activates p38, and pharmacological inhibition of p38 activity prevents PTK6 shRNA-induced Bim expression and partially rescues cells from apoptosis.
  
  
  CONCLUSIONS
  PTK6 downregulation induces apoptosis of Lapatinib-resistant Her2(+) breast cancer cells by enhancing Bim expression via p38 activation. As Bim expression is a critical biomarker for response to many targeted therapies, PTK6 inhibition may offer a therapeutic approach to treating patients with Her2 targeted therapy-resistant breast cancers.
authors.affiliation: Division of Hematology and Medical Oncology, Department of Medicine, Tisch Cancer Institute, Icahn School of Medicine at Mount Sinai, 1468 Madison Avenue, New York, NY, USA. sun.park@mssm.edu.
authors.forename: Sun Hee
authors.initials: SH
authors.lastname: Park
authors.affiliation: Division of Hematology and Medical Oncology, Department of Medicine, Tisch Cancer Institute, Icahn School of Medicine at Mount Sinai, 1468 Madison Avenue, New York, NY, USA. Koichi.Ito@mssm.edu.
authors.forename: Koichi
authors.initials: K
authors.lastname: Ito
authors.affiliation: Division of Hematology and Medical Oncology, Department of Medicine, Tisch Cancer Institute, Icahn School of Medicine at Mount Sinai, 1468 Madison Avenue, New York, NY, USA. Wolcott@mednet.ucla.edu.
authors.forename: William
authors.initials: W
authors.lastname: Olcott
authors.affiliation: Division of Hematology and Medical Oncology, Department of Medicine, Tisch Cancer Institute, Icahn School of Medicine at Mount Sinai, 1468 Madison Avenue, New York, NY, USA. igor.katsyv@mssm.edu.
authors.forename: Igor
authors.initials: I
authors.lastname: Katsyv
authors.affiliation: Division of Hematology and Medical Oncology, Department of Medicine, Tisch Cancer Institute, Icahn School of Medicine at Mount Sinai, 1468 Madison Avenue, New York, NY, USA. gwynethhn@gmail.com.
authors.forename: Gwyneth
authors.initials: G
authors.lastname: Halstead-Nussloch
authors.affiliation: Division of Hematology and Medical Oncology, Department of Medicine, Tisch Cancer Institute, Icahn School of Medicine at Mount Sinai, 1468 Madison Avenue, New York, NY, USA. hanna.irie@mssm.edu.|Department of Oncological Sciences, Tisch Cancer Institute, Icahn School of Medicine at Mount Sinai, 1468 Madison Avenue, New York, NY, USA. hanna.irie@mssm.edu.
authors.forename: Hanna Y
authors.initials: HY
authors.lastname: Irie
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology; D017209:Apoptosis / Q000187:drug effects* / Q000235:genetics*; D051017:Apoptosis Regulatory Proteins / Q000235:genetics / Q000378:metabolism*; D000072224:Bcl-2-Like Protein 11; D001943:Breast Neoplasms / Q000235:genetics* / Q000378:metabolism*; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects; D004195:Disease Models, Animal; D019008:Drug Resistance, Neoplasm / Q000235:genetics*; D004789:Enzyme Activation / Q000187:drug effects; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D000077341:Lapatinib; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D009363:Neoplasm Proteins / Q000235:genetics* / Q000378:metabolism; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D011505:Protein-Tyrosine Kinases / Q000235:genetics* / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D011799:Quinazolines / Q000494:pharmacology*; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000378:metabolism*; D023041:Xenograft Model Antitumor Assays; D048051:p38 Mitogen-Activated Protein Kinases / Q000378:metabolism
chemical_list: D000970:Antineoplastic Agents; D051017:Apoptosis Regulatory Proteins; D000072224:Bcl-2-Like Protein 11; D000077341:Lapatinib; D008565:Membrane Proteins; D009363:Neoplasm Proteins; D047428:Protein Kinase Inhibitors; D011505:Protein-Tyrosine Kinases; D011518:Proto-Oncogene Proteins; D011799:Quinazolines; D018719:Erb-b2 Receptor Tyrosine Kinases; D048051:p38 Mitogen-Activated Protein Kinases; C000606775:BCL2L11 protein, human; C508053:ERBB2 protein, human; C088375:PTK6 protein, human