Downregulation of lactoferrin by PPARalpha ligands: role in perturbation of hepatocyte proliferation and apoptosis.

Abstract

PPARα (peroxisome proliferator activated receptor α) 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 PPARα 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-α (TNF-α), LF downregulation by PPs may permit TNF-α 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-α. In vitro, the PPs monoethylhexylphthalate (MEHP; 500 μM, the active metabolite of DEHP) and another PP, nafenopin (50 μM) or exogenous TNF-α (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-α. 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 PPARα null mouse liver. These data suggest that the regulation of iron binding proteins by PPARα ligands plays a role in PP-mediated liver growth, but not in peroxisome proliferation.