DDK Promotes Tumor Chemoresistance and Survival via Multiple Pathways

Neoplasia. 2017 May;19(5):439-450. doi: 10.1016/j.neo.2017.03.001. Epub 2017 Apr 25.

Abstract

DBF4-dependent kinase (DDK) is a two-subunit kinase required for initiating DNA replication at individual origins and is composed of CDC7 kinase and its regulatory subunit DBF4. Both subunits are highly expressed in many diverse tumor cell lines and primary tumors, and this is correlated with poor prognosis. Inhibiting DDK causes apoptosis of tumor cells, but not normal cells, through a largely unknown mechanism. Firstly, to understand why DDK is often overexpressed in tumors, we identified gene expression signatures that correlate with DDK high- and DDK low-expressing lung adenocarcinomas. We found that increased DDK expression is highly correlated with inactivation of RB1-E2F and p53 tumor suppressor pathways. Both CDC7 and DBF4 promoters bind E2F, suggesting that increased E2F activity in RB1 mutant cancers promotes increased DDK expression. Surprisingly, increased DDK expression levels are also correlated with both increased chemoresistance and genome-wide mutation frequencies. Our data further suggest that high DDK levels directly promote elevated mutation frequencies. Secondly, we performed an RNAi screen to investigate how DDK inhibition causes apoptosis of tumor cells. We identified 23 kinases and phosphatases required for apoptosis when DDK is inhibited. These hits include checkpoint genes, G2/M cell cycle regulators, and known tumor suppressors leading to the hypothesis that inhibiting mitotic progression can protect against DDKi-induced apoptosis. Characterization of one novel hit, the LATS2 tumor suppressor, suggests that it promotes apoptosis independently of the upstream MST1/2 kinases in the Hippo signaling pathway.

MeSH terms

  • Adenocarcinoma / drug therapy
  • Adenocarcinoma / genetics*
  • Adenocarcinoma / pathology
  • Adenocarcinoma of Lung
  • Apoptosis / drug effects
  • Cell Cycle Proteins / antagonists & inhibitors
  • Cell Cycle Proteins / genetics*
  • DNA-Binding Proteins / genetics
  • Drug Resistance, Neoplasm / genetics*
  • E2F Transcription Factors / genetics
  • Gene Expression Regulation, Neoplastic / drug effects
  • Hepatocyte Growth Factor / genetics
  • Humans
  • Lung Neoplasms / drug therapy
  • Lung Neoplasms / genetics*
  • Lung Neoplasms / pathology
  • Phosphorylation
  • Promoter Regions, Genetic
  • Protein-Serine-Threonine Kinases / genetics*
  • Proto-Oncogene Proteins / genetics
  • Retinoblastoma Binding Proteins / genetics
  • Signal Transduction / drug effects
  • Tumor Suppressor Protein p53 / genetics
  • Tumor Suppressor Proteins / genetics
  • Ubiquitin-Protein Ligases / genetics

Substances

  • Cell Cycle Proteins
  • DBF4 protein, human
  • DNA-Binding Proteins
  • E2F Transcription Factors
  • Proto-Oncogene Proteins
  • RB1 protein, human
  • Retinoblastoma Binding Proteins
  • TP53 protein, human
  • Tumor Suppressor Protein p53
  • Tumor Suppressor Proteins
  • macrophage stimulating protein
  • Hepatocyte Growth Factor
  • Ubiquitin-Protein Ligases
  • CDC7 protein, human
  • LATS2 protein, human
  • Protein-Serine-Threonine Kinases