Response and determinants of cancer cell susceptibility to PI3K inhibitors: combined targeting of PI3K and Mek1 as an effective anticancer strategy

Cancer Biol Ther. 2008 Feb;7(2):307-15. doi: 10.4161/cbt.7.2.5334. Epub 2007 Nov 21.


While small molecule inhibitors of the phosphatidylinositide-3-kinase (PI3K) are expected to impact the development of new cancer therapy, the tumor types and underlying cellular pathways determining inhibitor response remain poorly defined. In this report, we have studied anti-proliferative effects of the PI3K inhibitors WAY-266176 and WAY-266175 in a panel of histologically diverse cancer cells. Inactivation of PI3K caused potent growth suppression in some cells (MDA468, BT549, MDA361, MCF7, LNCap, PC3MM2) but minimal suppression in others (MDA231, MDA435, DU145, HCT116, A549), which correlated with a differential down-regulation of cyclin D1, c-Myc, and induction of apoptosis. A heightened PI3K/AKT/mTOR signaling was linked to the sensitive phenotype but did not generally predict inhibitor response. Interestingly, the resistant cells all displayed an elevated phospho-ERK that remained elevated after serum deprivation. In HCT116 cells, activation mutations in the PI3K catalytic subunit PIK3CA and Ki-Ras correlated with a resistant phenotype, which was partially sensitized by homologous replacement with the wild-type Ki-Ras but not by deletion of cellular PTEN. Depletion of Mek1 via siRNA in resistant cells enhanced PI3K inhibitor-induced growth suppression. Moreover, a profoundly augmented growth suppression and apoptosis were achieved in resistant cells by combination treatment with WAY-266176/WAY-266175 and Mek1 kinase inhibitor CI-1040 or UO126. The combination therapy efficiently inhibited mitogenic signaling and reduced expression of cyclin D1 and c-Myc. Our results identify deregulation of the Ras/Raf/Mek/ERK pathway as a dominant determinant in cancer cell resistance to PI3K inhibitors and highlight combined targeting of PI3K and Mek1 as an effective anticancer strategy.

MeSH terms

  • Animals
  • Apoptosis / drug effects
  • Biomarkers, Tumor / analysis
  • Cell Line, Tumor
  • Drug Resistance, Neoplasm
  • Drug Therapy, Combination
  • Enzyme Inhibitors / pharmacology*
  • Enzyme Inhibitors / therapeutic use*
  • Gene Expression Regulation, Neoplastic
  • Humans
  • Inhibitory Concentration 50
  • MAP Kinase Kinase 1 / antagonists & inhibitors
  • MAP Kinase Kinase 1 / metabolism*
  • Mice
  • Mice, Nude
  • Neoplasms / drug therapy*
  • Neoplasms / genetics
  • Neoplasms / metabolism
  • PTEN Phosphohydrolase / metabolism
  • Phosphatidylinositol 3-Kinases / metabolism*
  • Phosphoinositide-3 Kinase Inhibitors
  • Proto-Oncogene Proteins c-akt / antagonists & inhibitors
  • RNA Interference
  • Time Factors
  • Xenograft Model Antitumor Assays


  • Biomarkers, Tumor
  • Enzyme Inhibitors
  • Phosphoinositide-3 Kinase Inhibitors
  • Proto-Oncogene Proteins c-akt
  • MAP Kinase Kinase 1
  • PTEN Phosphohydrolase
  • PTEN protein, human