Apr 20th, 1:00 PM - 4:00 PM
Regulation of Cyclin E Turnover by the Aryl Hydrocarbon Receptor
Dr. Kristen Mitchell
The aryl hydrocarbon receptor (AhR) is a cytosolic, ligand-activated receptor that mediates the toxicity of halogenated aromatic hydrocarbons, including the potent environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Many of the toxic effects associated with TCDD exposure stem from aberrant cellular proliferation, yet the mechanism remains unclear. In a mouse model of liver regeneration induced by surgical partial hepatectomy, exposure to TCDD elicits a G1 cell cycle arrest in regenerating hepatocytes. This arrest coincides with decreased activity of cyclin-dependent kinase-2 (CDK2), which phosphorylates the retinoblastoma tumor suppressor protein and facilitates passage to S-phase of the cell cycle. Moreover, exposure to TCDD reduced the amount of CDK2-bound cyclin E, which is a positive regulator of CDK2 activity. The purpose of this study was to test the hypothesis that diminished levels of cyclin E in TCDD-treated mice are due to enhanced ubiquitin-proteasomal degradation of cyclin E protein. Mice were treated with 20 ug/kg TCDD or peanut oil control 24 hours prior to surgical removal of 70% of the liver. Mice were euthanized and livers were collected 0, 12, 24, 36, 48 and 96 hours post-surgery. Livers were homogenized, immunoprecipitated with an anti-cyclin E antibody, and probed for ubiquitin by western blot. Whereas results from initial experiments proved inconclusive, additional studies are in progress using a modified protocol to detect ubiquitination. Enhanced ubiquitination of cyclin E in liver homogenates from TCDD-treated mice would support the hypothesis that TCDD increases cyclin E protein degradation in vivo. This would direct future studies toward examining mechanisms by which exposure to TCDD influences the ubiquitin-proteasome pathway during cell cycle progression.