title: Resistance to BRAF inhibition in BRAF-mutant colon cancer can be overcome with PI3K inhibition or demethylating agents.
abstract: OBJECTIVE
  Vemurafenib, a selective inhibitor of BRAF(V600), has shown significant activity in BRAF(V600) melanoma but not in less than 10% of metastatic BRAF(V600) colorectal cancers (CRC), suggesting that studies of the unique hypermethylated phenotype and concurrent oncogenic activation of BRAF(mut) CRC may provide combinatorial strategies.
  
  
  METHODS
  We conducted comparative proteomic analysis of BRAF(V600E) melanoma and CRC cell lines, followed by correlation of phosphoinositide 3-kinase (PI3K) pathway activation and sensitivity to the vemurafenib analogue PLX4720. Pharmacologic inhibitors and siRNA were used in combination with PLX4720 to inhibit PI3K and methyltransferase in cell lines and murine models.
  
  
  RESULTS
  Compared with melanoma, CRC lines show higher levels of PI3K/AKT pathway activation. CRC cell lines with mutations in PTEN or PIK3CA were less sensitive to growth inhibition by PLX4720 (P = 0.03), and knockdown of PTEN expression in sensitive CRC cells reduced growth inhibition by the drug. Combined treatment of PLX4720 with PI3K inhibitors caused synergistic growth inhibition in BRAF-mutant CRC cells with both primary and secondary resistance. In addition, methyltransferase inhibition was synergistic with PLX4720 and decreased AKT activation. In vivo, PLX4720 combined with either inhibitors of AKT or methyltransferase showed greater tumor growth inhibition than PLX4720 alone. Clones with acquired resistance to PLX4720 in vitro showed PI3K/AKT activation with EGF receptor (EGFR) or KRAS amplification.
  
  
  CONCLUSIONS
  We show that activation of the PI3K/AKT pathway is a mechanism of both innate and acquired resistance to BRAF inhibitors in BRAF(V600E) CRC and suggest combinatorial approaches to improve outcomes in this poor prognosis subset of patients.
authors.affiliation: Department of Gastrointestinal Medical Oncology, The University of Texas MD Anderson Cancer Center, and Graduate School of Biomedical Sciences, University of Texas, Houston, TX 77030, USA.
authors.forename: Muling
authors.initials: M
authors.lastname: Mao
authors.forename: Feng
authors.initials: F
authors.lastname: Tian
authors.forename: John M
authors.initials: JM
authors.lastname: Mariadason
authors.forename: Chun C
authors.initials: CC
authors.lastname: Tsao
authors.forename: Robert
authors.initials: R
authors.lastname: Lemos
authors.forename: Farshid
authors.initials: F
authors.lastname: Dayyani
authors.forename: Y N Vashisht
authors.initials: YN
authors.lastname: Gopal
authors.forename: Zhi-Qin
authors.initials: ZQ
authors.lastname: Jiang
authors.forename: Ignacio I
authors.initials: II
authors.lastname: Wistuba
authors.forename: Xi M
authors.initials: XM
authors.lastname: Tang
authors.forename: William G
authors.initials: WG
authors.lastname: Bornman
authors.forename: Gideon
authors.initials: G
authors.lastname: Bollag
authors.forename: Gordon B
authors.initials: GB
authors.lastname: Mills
authors.forename: Garth
authors.initials: G
authors.lastname: Powis
authors.forename: Jayesh
authors.initials: J
authors.lastname: Desai
authors.forename: Gary E
authors.initials: GE
authors.lastname: Gallick
authors.forename: Michael A
authors.initials: MA
authors.lastname: Davies
authors.forename: Scott
authors.initials: S
authors.lastname: Kopetz
mesh_terms: D000818:Animals; D000970:Antineoplastic Agents / Q000494:pharmacology*; D001374:Azacitidine / Q000494:pharmacology; D045744:Cell Line, Tumor; D049109:Cell Proliferation; D003110:Colonic Neoplasms / Q000235:genetics* / Q000378:metabolism*; D004195:Disease Models, Animal; D019008:Drug Resistance, Neoplasm / Q000235:genetics*; D006801:Humans; D007211:Indoles / Q000494:pharmacology*; D008745:Methylation / Q000187:drug effects; D051379:Mice; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D009154:Mutation; D051059:PTEN Phosphohydrolase / Q000235:genetics; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D000081082:Phosphoinositide-3 Kinase Inhibitors; D048493:Proto-Oncogene Proteins B-raf / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D013449:Sulfonamides / Q000494:pharmacology*; D000077484:Vemurafenib
chemical_list: D000970:Antineoplastic Agents; D001374:Azacitidine; D007211:Indoles; D020928:Mitogen-Activated Protein Kinases; D051059:PTEN Phosphohydrolase; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D048493:Proto-Oncogene Proteins B-raf; D051057:Proto-Oncogene Proteins c-akt; D013449:Sulfonamides; D000077484:Vemurafenib