Toxicological Sciences 68, 304-313 (2002)
Copyright © 2002 by the Society of Toxicology
CARCINOGENICITY |
Downregulation of Lactoferrin by PPAR
Ligands: Role in Perturbation of Hepatocyte Proliferation and Apoptosis
Syngenta Central Toxicology Laboratory, Alderley Park, Macclesfield, SK10 4TJ, England, United Kingdom
PPAR
(peroxisome proliferator activated receptor ) is a transcription factor that mediates the rodent liver tumorigenic responses to peroxisome proliferators via regulation of genes that remain to be identified. Using microarray gene expression profiling of mRNA from wild type versus PPAR null mice, we detected a 3- to 7-fold downregulation of hepatic lactoferrin (LF) in response to the PP, diethylhexylphthalate (DEHP; 1150 mg/kg). Northern blot analyses confirmed a significant downregulation of LF mRNA by DEHP in wild type mouse liver. Since LF has been reported to repress tumor necrosis factor- (TNF-), LF downregulation by PPs may permit TNF- levels to rise, enhancing hepatocyte survival and proliferation. To test this hypothesis, we asked if exogenous LF could prevent the perturbation of hepatocyte growth by PPs but not by TNF-. In vitro, the PPs monoethylhexylphthalate (MEHP; 500 µM, the active metabolite of DEHP) and another PP, nafenopin (50 µM) or exogenous TNF- (5000 U/ml) induced hepatocyte proliferation and suppressed apoptosis. LF (200 µM) blocked the growth but not the peroxisome proliferation response to PPs but could not block the growth response to TNF-. Immunocytochemistry using specific antibodies to LF but also to transferrin (TF), a related gene previously shown to contain a PP response element (PPRE), demonstrated that both LF and TF are expressed in murine liver. Furthermore, both were downregulated by DEHP in both wild type and PPAR null mouse liver. These data suggest that the regulation of iron binding proteins by PPAR ligands plays a role in PP-mediated liver growth, but not in peroxisome proliferation.
Key Words: nongenotoxic carcinogenesis; lactoferrin; peroxisome proliferators; PPAR; hypolipidaemic drugs.
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