Peroxisome proliferators alter the expression of estrogen-metabolizing enzymes.

Exposure to some peroxisome proliferator chemicals (PPC) leads to toxic effects on sex organ function possibly by alterations of steroid hormone metabolism. A systematic search for genes whose mRNA levels are modulated by the PPC WY-14643 (WY) was carried out in rat liver, a site of steroid hormone metabolism. The sequence of one up-regulated cDNA (2480 bp) was predicted to encode a protein of 735 amino acids with 82% identity to the porcine 17 beta-hydroxysteroid dehydrogenase type IV (HSD IV) originally isolated as a 17 beta-estradiol dehydrogenase. The rat HSD IV was localized to peroxisomes and was regulated by diverse PPC by two distinct mechanisms. Induction of HSD IV and acyl-CoA oxidase (ACO) proteins in rat liver at different treatment times and concentrations of gemfibrozil (GEM) and di-n-butyl phthalate (DBP) were almost identical, suggesting that HSD IV mRNA induction involves the peroxisome proliferator-activated receptor alpha, a regulator of ACO. In contrast, HSD IV protein levels were only weakly induced by WY, a strong inducer of ACO protein, even though the levels of both HSD IV and ACO mRNA were strongly stimulated by WY. Thus HSD IV protein levels were uniquely regulated pretranslationally by WY. In addition to HSD IV we also identified the male-specific alpha 2u-globulin as a PPC down-regulated gene. This prompted us to examine the expression of another male-specific gene, CYP2C11, that catalyzes the hydroxylations of estradiol at the 2 and 16 alpha positions. Cyp2C11 protein expression in rat liver was either decreased or completely abolished after a 3-week treatment by GEM or WY, respectively. Decreased expression of enzymes which inactivate estradiol including Cyp2C11, and the reported increased expression of aromatase may explain why male rats exposed to diverse PPC have higher serum estradiol levels. These higher estradiol levels in male rats have been thought to be mechanistically linked to Leydig cell hyperplasia and adenomas. Increased conversion of estradiol to the less active estrone by HSD IV induction may explain how exposure to the phthalate di-(2-ethylhexyl) phthalate leads to decreases in serum estradiol levels and suppression of ovulation in female rats.

Cloning genes responsive to a hepatocarcinogenic peroxisome proliferator chemical reveals novel targets of regulation.

To better understand the molecular basis of the hepatocyte proliferation and induction of hepatocellular adenomas by exposure to peroxisome proliferator chemicals (PPC), a systematic search for genes modulated by a PPC (WY-14643) in rat liver was carried out using the differential display technique. The fragments fell into two classes based on the time of initial and maximal induction by WY-14643. The class I genes (clones 5 and 30) were induced 3 h after a gavage exposure to WY-14643 with maximal expression at 24 h. The class II genes (clones 13 and 16) were induced after 24 h with maximal expression at 78 weeks. Expression of the class II genes was also increased after other treatments that cause cell proliferation. Clone 30 was identified as CYP4A2, previously shown to be regulated by PPC. Clone 13 was homologous to the mouse protein H gene, a component of the heterogeneous nuclear ribonucleoprotein particle important in mRNA splicing. Clone 16 was identified as cyclophilin-A, the receptor for the immunosuppressant drug cyclosporin A. The sequence of clone 5 was unique. These data demonstrate that WY-14643 increases the levels of a number of novel genes that are coordinately regulated with increases in chronic cell proliferation and fatty acid metabolism.

Identification of a cell-specific transcription activation domain within the human Ah receptor nuclear translocator.

In the presence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related chemicals, the Ah receptor nuclear translocator (Arnt) forms a heterodimeric complex with the ligand-bound Ah receptor, leading to recognition of dioxin-responsive elements within the enhancer of the CYP1A1 gene and transcription activation by an unknown mechanism. To understand the role of Arnt in transcription activation by the Ah receptor-Arnt heterodimer, we performed a deletion analysis of Arnt to locate domains that are directly involved in transcription activation. We showed that the C-terminal 34 amino acids of Arnt encode a transcription activation domain (TAD) that functions independently of other sequences in the Ah receptor complex when attached to the heterologous Gal4 DNA binding domain. Deletion of the C-terminal acidic-rich 14 amino acids completely abolishes activity. Sequences important in Arnt TAD function were independent of the glutamine-rich region which is an important structural feature in the TAD of other transcription factors. The strength of the Arnt TAD when compared with the strong TAD from the herpes simplex virus VP16 protein was cell-type specific. Both the Arnt and VP16 TAD were equally strong in COS-1 cells, but the Arnt TAD had weak activity in an Arnt-deficient mouse hepatoma cell line and was not needed for restoration of CYP1A1 activation. These results imply that for CYP1A1 activation the Ah receptor provides the dominant activation function for the heterodimer in hepatoma cells. The potential of the Arnt TAD to contribute to activation by the Ah receptor complex is likely determined by availability or activity of cell-specific factors with which the TAD interacts.

Rat 17 beta-hydroxysteroid dehydrogenase type IV is a novel peroxisome proliferator-inducible gene.

To better understand the molecular mechanisms of the pleiotropic responses induced by exposure to peroxisome proliferator chemicals (PPCs), we conducted a systematic search for genes whose mRNA levels are modulated by the PPC WY-14,643 (WY) in rat liver. The sequence of one up-regulated cDNA (2480 bp) was predicted to encode a protein of 735 aa with 82% identity to the porcine 17 beta-hydroxysteroid dehydrogenase type IV (HSD IV). Like the porcine enzyme, the rat HSD IV contains' a region homologous to yeast hydratase-dehydrogenase-epimerases and to sterol carrier proteins, indicating that the rat HSD IV has broad substrate specificity and contributes to cholesterol metabolism. The rat HSD IV was regulated by diverse PPCs via two distinct mechanisms. Induction of HSD IV and acyl-CoA oxidase (ACO) proteins in rat liver at different treatment times and concentrations of gemfibrozil and di-n-butyl phthalate were almost identical, indicating that HSD IV mRNA induction involves the peroxisome proliferator-activated receptor alpha, a regulator of ACO. In contrast, HSD IV protein levels were only weakly induced by WY, a strong inducer of ACO protein, even though the levels of HSD IV and ACO mRNA were strongly stimulated by WY and gemfibrozil. Thus, HSD IV protein levels were uniquely regulated pretranslationally by WY via a novel mechanism. Increased conversion of estradiol to the less-active estrone by HSD IV induction may explain how phthalate exposure leads to decreases in serum estradiol levels and suppression of ovulation.