Induction of the Ras activator Son of Sevenless 1 by environmental pollutants mediates their effects on cellular proliferation

Biochem Pharmacol. 2011 Jan 15;81(2):304-13. doi: 10.1016/j.bcp.2010.10.003. Epub 2010 Oct 13.


TCDD (2,3,7,8-tetrachlorodibenzodioxin), a highly persistent environmental pollutant and a human carcinogen, is the ligand with the highest affinity for the Aryl Hydrocarbon Receptor (AhR) that induces via the AhR, xenobiotic metabolizing enzyme genes as well as several other genes. This pollutant elicits a variety of systemic toxic effects, which include cancer promotion and diverse cellular alterations that modify cell cycle progression and cell proliferation. Large-scale studies have shown that the expression of Son of Sevenless 1 (SOS1), the main mediator of Ras activation, is one of the targets of dioxin in human cultured cells. In this study, we investigated the regulation of the previously uncharacterized SOS1 gene promoter by the AhR and its ligands in the human hepatocarcinoma cell line, HepG2. We found that several environmental pollutants (AhR ligands) induce SOS1 gene expression by increasing its transcription. Chromatin immunoprecipitation experiments demonstrated that the AhR binds directly and activates the SOS1 gene promoter. We also showed that dioxin treatment leads to an activated Ras-GTP state, to ERK activation and to accelerated cellular proliferation. All these effects were mediated by SOS1 induction as shown by knock down experiments. Our data indicate that dioxin-induced cellular proliferation is mediated, at least partially, by SOS1 induction. Remarkably, our studies also suggest that SOS1 induction leads to functional effects similar to those elicited by the well-characterized oncogenic Ras mutations.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cell Proliferation / drug effects*
  • Dose-Response Relationship, Drug
  • Environmental Pollutants / toxicity*
  • Extracellular Signal-Regulated MAP Kinases / genetics
  • Extracellular Signal-Regulated MAP Kinases / metabolism
  • Hep G2 Cells
  • Hepatocytes / cytology
  • Hepatocytes / drug effects
  • Hepatocytes / physiology
  • Humans
  • Phosphorylation
  • Polychlorinated Dibenzodioxins / administration & dosage
  • Polychlorinated Dibenzodioxins / toxicity*
  • Promoter Regions, Genetic
  • RNA, Messenger / genetics
  • RNA, Messenger / metabolism
  • RNA, Small Interfering
  • Receptors, Aryl Hydrocarbon / metabolism
  • SOS1 Protein / genetics
  • SOS1 Protein / metabolism*
  • ras Proteins / genetics
  • ras Proteins / metabolism


  • Environmental Pollutants
  • Polychlorinated Dibenzodioxins
  • RNA, Messenger
  • RNA, Small Interfering
  • Receptors, Aryl Hydrocarbon
  • SOS1 Protein
  • Extracellular Signal-Regulated MAP Kinases
  • ras Proteins