Cyclin G2 promotes cell cycle arrest in breast cancer cells responding to fulvestrant and metformin and correlates with patient survival

Cell Cycle. 2016 Dec;15(23):3278-3295. doi: 10.1080/15384101.2016.1243189. Epub 2016 Oct 18.

Abstract

Definition of cell cycle control proteins that modify tumor cell resistance to estrogen (E2) signaling antagonists could inform clinical choice for estrogen receptor positive (ER+) breast cancer (BC) therapy. Cyclin G2 (CycG2) is upregulated during cell cycle arrest responses to cellular stresses and growth inhibitory signals and its gene, CCNG2, is directly repressed by E2-bound ER complexes. Our previous studies showed that blockade of HER2, PI3K and mTOR signaling upregulates CycG2 expression in HER2+ BC cells, and that CycG2 overexpression induces cell cycle arrest. Moreover, insulin and insulin-like growth factor-1 (IGF-1) receptor signaling strongly represses CycG2. Here we show that blockade of ER-signaling in MCF7 and T47D BC cell lines enhances the expression and nuclear localization of CycG2. Knockdown of CycG2 attenuated the cell cycle arrest response of E2-depleted and fulvestrant treated MCF7 cells. These muted responses were accompanied by sustained inhibitory phosphorylation of retinoblastoma (RB) protein, expression of cyclin D1, phospho-activation of ERK1/2 and MEK1/2 and expression of cRaf. Our work indicates that CycG2 can form complexes with CDK10, a CDK linked to modulation of RAF/MEK/MAPK signaling and tamoxifen resistance. We determined that metformin upregulates CycG2 and potentiates fulvestrant-induced CycG2 expression and cell cycle arrest. CycG2 knockdown blunts the enhanced anti-proliferative effect of metformin on fulvestrant treated cells. Meta-analysis of BC tumor microarrays indicates that CCNG2 expression is low in aggressive, poor-prognosis BC and that high CCNG2 expression correlates with longer periods of patient survival. Together these findings indicate that CycG2 contributes to signaling networks that limit BC.

Keywords: CCNG2; CDK10; Cell Cycle Arrest; Estrogen Deprivation; Fulvestrant; IGF-1R; Insulin; Metformin; RAF/MEK/MAPK pathway; Tamoxifen-Resistant.

MeSH terms

  • Breast Neoplasms / pathology*
  • Cell Cycle Checkpoints / drug effects*
  • Cell Line, Tumor
  • Cell Nucleus / drug effects
  • Cell Nucleus / metabolism
  • Cyclin G2 / metabolism*
  • Cyclin-Dependent Kinases / metabolism
  • Disease-Free Survival
  • Estradiol / analogs & derivatives*
  • Estradiol / pharmacology
  • Estrogens / pharmacology
  • Extracellular Signal-Regulated MAP Kinases / metabolism
  • Female
  • Fulvestrant
  • Gene Knockdown Techniques
  • Humans
  • Metformin / pharmacology*
  • Mitogen-Activated Protein Kinase Kinases / metabolism
  • Neoplasm Recurrence, Local / pathology
  • Phosphorylation / drug effects
  • Protein Transport / drug effects
  • Proto-Oncogene Proteins c-raf / metabolism
  • RNA Interference
  • Receptors, Estrogen / metabolism
  • Retinoblastoma Protein / metabolism
  • Signal Transduction / drug effects
  • Survival Analysis
  • Up-Regulation / drug effects

Substances

  • Cyclin G2
  • Estrogens
  • Receptors, Estrogen
  • Retinoblastoma Protein
  • Fulvestrant
  • Estradiol
  • Metformin
  • Proto-Oncogene Proteins c-raf
  • CDK10 protein, human
  • Cyclin-Dependent Kinases
  • Extracellular Signal-Regulated MAP Kinases
  • Mitogen-Activated Protein Kinase Kinases