Abstract
Studies of structure-activity relationships (SARs) revealed that many of the responses elicited by halogenated aromatic hydrocarbons (HAHs) are mediated through the aryl hydrocarbon (Ah) receptor(AhR). In addition, SARs indicate a correlation between binding affinity for the AhR, levels of DNA-bound AhR complexes and induction potencies. However, SARs for alkyl-substituted PCDFs show a lack of correlation between binding affinity and enzyme induction. Therefore, the focus of this dissertation was to characterize the molecular and structural properties of AhR ligands in terms of the differential effects on gene expression. Rat hepatic cytosolic AhR is readily transformed with ligand to DNA-bound complex, an event which can then be utilized to investigate effects of ligand structure on AhR transformation. For a series of PCDD and PCDF congeners, their IC$sb{50}$ values for receptor binding and formation of the liganded AhR:DRE complexes were similar to their potencies as AhR agonists for CYP1A1-induced responses. Analysis of the DNA binding data showed no structure-dependent trend in the K$sb{rm D}$ values or in the half-lives of the transformed complexes. $alpha$-Naphthoflavone ($alpha$NF) inhibits TCDD-induced CYP1A1 gene expression possibly by forming cytosolic AhR complexes which fail to undergo transformation. This hypothesis was consistent with data obtained using $alpha$NF concentrations $<$10 $mu$M. However, 10 $mu$M $alpha$NF exhibited AhR agonist activity in several assays, and the results confirm that this compound is a partial AhR antagonist and weak agonist. In contrast, studies with a series of 6-substituted-3,4-benzocoumarins and 2-substituted phenanthridinones, compounds which also exhibited low AhR binding affinities, showed that these compounds were weak AhR agonists but inactive as partial AhR antagonists. The proteolytic digestion maps of the transformed and nuclear TCDD:AhR complexes bound to the DRE were comparable and independent of ligand-structure. In contrast, UV cross-linking experiments of nuclear extracts from cells treated with TCDD or 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) to a bromodeoxyuridine-substituted DRE (BrdU-DRE) resulted in ligand-dependent effect on formation of AhR:BrdU-DRE complexes and also ligand-dependent effects on proteolytic degradation products.
Santostefano, Michael Joseph (1994). Structure-activity relationships of aryl hydrocarbon (Ah) receptor ligands : mechanistic studies. Texas A&M University. Texas A&M University. Libraries. Available electronically from
https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -1554824.