Superfund Research Program
Activation Of Orphan Receptors By Chlorinated Hydrocarbons
Project Leader: David J. Waxman
Grant Number: P42ES007381
Funding Period: 1995 - 2000
A large number of industrial chemicals and environmental pollutants, including trichloroethylene (TCE), di(2-ethylhexyl)phthalate (DEHP), perfluorooctanoic acid (PFOA), and various phenoxyacetic acid herbicides, are 'non-genotoxic' rodent hepatocarcinogens whose actions are mediated by peroxisome proliferator-activated receptor alpha (PPAR). The human health risk of exposures to these chemicals is uncertain. Cell transfection studies were carried out to investigate the interactions of these chemicals with both PPAR and PPAR--a PPAR isoform that is highly expressed in multiple human tissues. With three environmental chemicals, TCE, perchloroethylene and DEHP, PPAR was found to be activated by metabolites, but not by the parent chemical. A decreased sensitivity of human PPAR compared to mouse PPAR to trans-activation was observed with some but not other peroxisome proliferators. Investigation of human and mouse PPAR revealed the transcriptional activity of this receptor to be stimulated by mono(2-ethylhexyl)phthalate, a DEHP metabolite that induces developmental and reproductive organ toxicities in rodents. This finding suggests that PPAR, which is highly expressed in human adipose tissue, where many lipophilic foreign chemicals tend to accumulate, as well as in colon, heart, liver, testis, spleen and hematopoietic cells, may be a target in human cells for a subset of industrial and environmental chemicals of the peroxisome proliferator class
Growth hormone (GH)-activated STAT5b inhibits by up to 80% the transcriptional activity of PPAR, and is proposed to serve as a mechanism whereby hormones and cytokines that activate STAT transcription factors can beneficially modulate the deleterious effects of PPAR-activating environmental chemicals. Investigation of the mechanism underlying this inhibitory cross-talk revealed that SOCS-3, a GH-inducible negative regulator of cytokine signaling to STAT5b, abolished the STAT5b inhibitory response. A constitutively active STAT5b mutant failed to inhibit PPAR activity, indicating that STAT5b does not induce synthesis of a more proximal PPAR inhibitor. STAT5b inhibition was not reversed by over-expression of PPAR's heterodimerization partner RXR or the nuclear receptor coactivators P300 and SRC-1, suggesting that STAT5b does not inhibit PPAR by competing for these limiting cellular cofactors. STAT5b did not inhibit a chimeric receptor comprised of yeast GAL4 DNA-binding domain linked to PPAR's ligand binding/AF-2 trans-activation domain, but did inhibit by ~ 80% transcription driven by PPAR's NH2-terminal ligand-independent AF-1 trans-activation domain in a GAL4-linked chimera. The conservation of this AF-1 trans-activation function in many nuclear receptors suggests that AF-1 may serve as an important target for inhibitory cross-talk between STAT transcription factors and xenochemical-activated nuclear receptors in a variety of signaling pathways.