title: Induction of hepatic xenobiotic metabolizing enzymes in female Fischer-344 rats following repeated inhalation exposure to decamethylcyclopentasiloxane (D5).
abstract: Decamethylcyclopentasiloxane (D5) is a cyclic siloxane with a wide range of commercial applications. The present study was designed to investigate the effects of D5 on the expression and activity of selected rat hepatic phase I and phase II metabolizing enzymes. Female Fischer-344 rats were exposed to 160 ppm D5 vapors (6 h/day, 7 days/week, for 28 days) by whole-body inhalation. Changes in the activity and relative abundance of hepatic microsomal cytochromes P450 (CYP1A, CYP2B, CYP3A, and CYP4A), epoxide hydrolase, and UDP-glucuronosyltransferase (UDPGT) were measured. Repeated inhalation exposure of rats to D5 increased liver size by 16% relative to controls by day 28. During a 14-day post-exposure period, liver size in D5-exposed animals showed significant recovery. Exposure to D5 did not change total hepatic P450, but increased the activity of hepatic NADPH-cytochrome c reductase by 1.4-fold. An evaluation of cytochrome P450 (CYP) enzymes in hepatic microsomes prepared from D5-exposed rats revealed a slight (1.8-fold) increase in 7-ethoxyresorufin O-deethylase (EROD) activity, but no change in immunoreactive CYP1A1/2 protein. A moderate increase (4.2-fold) in both 7-pentoxyresorufin O-depentylase (PROD) activity and immunoreactive CYP2B1/2 protein (3.3-fold) was observed. Testosterone 6beta-hydroxylase activity was also increased (2.4-fold) as was CYP3A1/2 immunoreactive protein. Although a small increase in 11- and 12-hydroxylation of lauric acid was detected, no change in immunoreactive CYP4A levels was measured. Liver microsomal epoxide hydrolase activity and immunoreactive protein increased 1.7- and 1.4-fold, respectively, in the D5-exposed group. UDPGT activity toward chloramphenicol was induced 1.8-fold, while no change in UDPGT activity toward 4-nitrophenol was seen. These results suggest that the profile for enzyme induction following inhalation exposure of female Fischer-344 rats to D5 vapors is similar to that reported for phenobarbital, and therefore D5 may be described as a weak "phenobarbital-like" inducer.
authors.affiliation: Dow Corning Corporation, Health and Environmental Sciences, Midland, Michigan 48686, USA. james.m.mckim@am.pnu.com
authors.forename: J M
authors.initials: JM
authors.lastname: McKim
authors.forename: S
authors.initials: S
authors.lastname: Choudhuri
authors.forename: P C
authors.initials: PC
authors.lastname: Wilga
authors.forename: A
authors.initials: A
authors.lastname: Madan
authors.forename: L A
authors.initials: LA
authors.lastname: Burns-Naas
authors.forename: R H
authors.initials: RH
authors.lastname: Gallavan
authors.forename: R W
authors.initials: RW
authors.lastname: Mast
authors.forename: D J
authors.initials: DJ
authors.lastname: Naas
authors.forename: A
authors.initials: A
authors.lastname: Parkinson
authors.forename: R G
authors.initials: RG
authors.lastname: Meeks
mesh_terms: D000280:Administration, Inhalation; D000818:Animals; D003577:Cytochrome P-450 Enzyme System / Q000187:drug effects*; D004790:Enzyme Induction / Q000187:drug effects*; D005260:Female; D066298:In Vitro Techniques; D008099:Liver / Q000187:drug effects / Q000201:enzymology*; D008862:Microsomes, Liver / Q000187:drug effects*; D051381:Rats; D011916:Rats, Inbred F344; D012833:Siloxanes / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D012833:Siloxanes; C114768:decamethylcyclopentasiloxane; D003577:Cytochrome P-450 Enzyme System