Parallel PI3K, AKT and mTOR inhibition is required to control feedback loops that limit tumor therapy

PLoS One. 2018 Jan 22;13(1):e0190854. doi: 10.1371/journal.pone.0190854. eCollection 2018.

Abstract

Targeting the PI3K pathway has achieved limited success in cancer therapy. One reason for the disappointing activity of drugs that interfere with molecules that are important player in this pathway is the induction of multiple feedback loops that have been only partially understood. To understand these limitations and develop improved treatment strategies, we comprehensively characterized molecular mechanisms of PI3K pathway signaling in bladder cancer cell lines upon using small molecule inhibitors and RNAi technologies against all key molecules and protein complexes within the pathway and analyzed functional and molecular consequences. When targeting either mTORC1, mTOR, AKT or PI3K, only S6K1 phosphorylation was affected in most cell lines examined. Dephosphorylation of 4E-BP1 required combined inhibition of PI3K and mTORC1, independent from AKT, and resulted in a robust reduction in cell viability. Long-term inhibition of PI3K however resulted in a PDK1-dependent, PIP3 and mTORC2 independent rephosphorylation of AKT. AKT rephosphorylation could also be induced by mTOR or PDK1 inhibition. Combining PI3K/mTOR inhibitors with AKT or PDK1 inhibitors suppressed this rephosphorylation, induced apoptosis, decreased colony formation, cell viability and growth of tumor xenografts. Our findings reveal novel molecular mechanisms that explain the requirement for simultaneous targeting of PI3K, AKT and mTORC1 to achieve effective tumor growth inhibition.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adaptor Proteins, Signal Transducing / metabolism
  • Animals
  • Antineoplastic Agents / administration & dosage*
  • Antineoplastic Combined Chemotherapy Protocols
  • Cell Cycle Proteins
  • Cell Line, Tumor
  • Cell Survival / drug effects
  • Chick Embryo
  • Enzyme Inhibitors / administration & dosage*
  • Feedback, Physiological / drug effects
  • Heterocyclic Compounds, 3-Ring / administration & dosage
  • Humans
  • Imidazoles / administration & dosage
  • Mechanistic Target of Rapamycin Complex 1 / antagonists & inhibitors
  • Phosphoinositide-3 Kinase Inhibitors*
  • Phosphoproteins / metabolism
  • Phosphorylation / drug effects
  • Protein Kinase Inhibitors / administration & dosage*
  • Proto-Oncogene Proteins c-akt / antagonists & inhibitors*
  • Quinolines / administration & dosage
  • Ribosomal Protein S6 Kinases, 70-kDa / metabolism
  • Signal Transduction / drug effects
  • TOR Serine-Threonine Kinases / antagonists & inhibitors*
  • Urinary Bladder Neoplasms / drug therapy*
  • Urinary Bladder Neoplasms / metabolism*
  • Urinary Bladder Neoplasms / pathology
  • Xenograft Model Antitumor Assays

Substances

  • Adaptor Proteins, Signal Transducing
  • Antineoplastic Agents
  • Cell Cycle Proteins
  • EIF4EBP1 protein, human
  • Enzyme Inhibitors
  • Heterocyclic Compounds, 3-Ring
  • Imidazoles
  • MK 2206
  • Phosphoinositide-3 Kinase Inhibitors
  • Phosphoproteins
  • Protein Kinase Inhibitors
  • Quinolines
  • MTOR protein, human
  • Mechanistic Target of Rapamycin Complex 1
  • Proto-Oncogene Proteins c-akt
  • Ribosomal Protein S6 Kinases, 70-kDa
  • TOR Serine-Threonine Kinases
  • ribosomal protein S6 kinase, 70kD, polypeptide 1
  • dactolisib

Grants and funding

This work was supported by the Fritz Thyssen Foundation, Postdoctoral Fellowship for Anuja Sathe.