Therapeutic relevance of the protein phosphatase 2A in cancer

Oncotarget. 2016 Sep 20;7(38):61544-61561. doi: 10.18632/oncotarget.11399.

Abstract

Chromosomal Instability (CIN) is regarded as a unifying feature of heterogeneous tumor populations, driving intratumoral heterogeneity. Polo-Like Kinase 1 (PLK1), a serine-threonine kinase that is often overexpressed across multiple tumor types, is one of the key regulators of CIN and is considered as a potential therapeutic target. However, targeting PLK1 has remained a challenge due to the off-target effects caused by the inhibition of other members of the polo-like family. Here we use synthetic dosage lethality (SDL), where the overexpression of PLK1 is lethal only when another, normally non-lethal, mutation or deletion is present. Rather than directly inhibiting PLK1, we found that inhibition of PP2A causes selective lethality to PLK1-overexpressing breast, pancreatic, ovarian, glioblastoma, and prostate cancer cells. As PP2A is widely regarded as a tumor suppressor, we resorted to gene expression datasets from cancer patients to functionally dissect its therapeutic relevance. We identified two major classes of PP2A subunits that negatively correlated with each other. Interestingly, most mitotic regulators, including PLK1, exhibited SDL interactions with only one class of PP2A subunits (PPP2R1A, PPP2R2D, PPP2R3B, PPP2R5B and PPP2R5D). Validation studies and other functional cell-based assays showed that inhibition of PPP2R5D affects both levels of phospho-Rb as well as sister chromatid cohesion in PLK1-overexpressing cells. Finally, analysis of clinical data revealed that patients with high expression of mitotic regulators and low expression of Class I subunits of PP2A improved survival. Overall, these observations point to a context-dependent role of PP2A that warrants further exploration for therapeutic benefits.

Keywords: PLK1; PP2A; chromosomal instability; mitotic checkpoint; synthetic dosage lethality.

MeSH terms

  • Antineoplastic Agents / pharmacology
  • Antineoplastic Agents / therapeutic use
  • Apoptosis / drug effects
  • Cantharidin / pharmacology
  • Cantharidin / therapeutic use
  • Cell Cycle Proteins / antagonists & inhibitors
  • Cell Cycle Proteins / genetics
  • Cell Cycle Proteins / metabolism*
  • Chromosomal Instability / drug effects*
  • Enzyme Inhibitors / pharmacology
  • Enzyme Inhibitors / therapeutic use
  • Gene Knockdown Techniques
  • Genes, Tumor Suppressor / drug effects*
  • HCT116 Cells
  • Humans
  • Mitosis / drug effects
  • Mutation
  • Neoplasms / drug therapy*
  • Neoplasms / genetics
  • Neoplasms / pathology
  • Phosphorylation
  • Polo-Like Kinase 1
  • Protein Phosphatase 2 / antagonists & inhibitors*
  • Protein Phosphatase 2 / genetics
  • Protein Phosphatase 2 / metabolism
  • Protein Serine-Threonine Kinases / antagonists & inhibitors
  • Protein Serine-Threonine Kinases / genetics
  • Protein Serine-Threonine Kinases / metabolism*
  • Proto-Oncogene Proteins / antagonists & inhibitors
  • Proto-Oncogene Proteins / genetics
  • Proto-Oncogene Proteins / metabolism*
  • RNA Interference
  • RNA, Small Interfering / metabolism
  • Retinoblastoma Binding Proteins / metabolism
  • Ubiquitin-Protein Ligases / metabolism

Substances

  • Antineoplastic Agents
  • Cell Cycle Proteins
  • Enzyme Inhibitors
  • Proto-Oncogene Proteins
  • RB1 protein, human
  • RNA, Small Interfering
  • Retinoblastoma Binding Proteins
  • Ubiquitin-Protein Ligases
  • Protein Serine-Threonine Kinases
  • Protein Phosphatase 2
  • Cantharidin