Downregulation of Lactoferrin by PPARalpha Ligands: Role in Perturbation of Hepatocyte Proliferation and Apoptosis

Abstract

PPARalpha (peroxisome proliferator activated receptor alpha) 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 PPARalpha 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-alpha (TNF-alpha), LF downregulation by PPs may permit TNF-alpha 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-alpha. In vitro, the PPs monoethylhexylphthalate (MEHP; 500 microM, the active metabolite of DEHP) and another PP, nafenopin (50 microM) or exogenous TNF-alpha (5000 U/ml) induced hepatocyte proliferation and suppressed apoptosis. LF (200 microM) blocked the growth but not the peroxisome proliferation response to PPs but could not block the growth response to TNF-alpha. 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 PPARalpha null mouse liver. These data suggest that the regulation of iron binding proteins by PPARalpha ligands plays a role in PP-mediated liver growth, but not in peroxisome proliferation.