Mechanism of Hormonal Regulation of CAD Gene Expression in Human Breast Cancer Cells

The CAD gene is trifunctional and expresses carbamoylphosphate synthetaseaspartate carbamyltransferasedihydroorotase which are required for pyrimidine biosynthesis. TODD inhibited hormone-induced activation of CAD mRNA levels and pOAD promoter-reporter gene activity in MOF-7 and ZR-75 cells. Using fluorescence resonance energy transfer FRET it was shown that both E2 and TODD enhanced AhR-ER interactions. E2 also induced interactions between ER and Sp1 however cotreatment with TODD abrogated this effect. Results of this study demonstrate a unique model of AhR-ER crosstalk where the liganded AhR inhibits ER-Sp1 interactions and also recruits ER to Ah-responsive gene promoters such as CYP1A1. In addition CAD gene was also used as a model to study the mechanism underlying the crosstalk between ER and PPAR signaling pathways. PPAR ligand PGJ2 inhibited E2-induced OAD gene expression and also down regulated E2-mediated transactivation of CAD gene promoter constructs and this was reversed by PPAR antagonist TOO7 in MOF-7 cells. This suggests a possible inhibitory crosstalk between PPAR and ER signaling pathways in breast cancer cells. In addition, 1,1- Bis3indolyl-1-p-substitutedphenylmethanes containing p-t-butyl DIM-C-pPhtBu and p-phenyl DIM-C-pPhC6H5 groups a new class of PPAR agonists inhibited E2-mediated transactivation of CAD gene promoter constructs however this effect was PPAR-independent.