ABSTRACT
Peroxisome proliferator-activated receptor-ß/δ (PPARß/δ) inhibits steatosis and inflammation, known risk factors for liver cancer. In this study, the effect of ligand activation of PPARß/δ in modulating liver tumorigenesis in transgenic hepatitis B virus (HBV) mice was examined. Activation of PPARß/δ in HBV mice reduced steatosis, the average number of liver foci, and tumor multiplicity. Reduced expression of hepatic CYCLIN D1 and c-MYC, tumor necrosis factor alpha (Tnfa) mRNA, serum levels of alanine aminotransaminase, and an increase in apoptotic signaling was also observed following ligand activation of PPARß/δ in HBV mice compared to controls. Inhibition of Tnfa mRNA expression was not observed in wild-type hepatocytes. Ligand activation of PPARß/δ inhibited lipopolysaccharide (LPS)-induced mRNA expression of Tnfa in wild-type, but not in Pparß/δ-null Kupffer cells. Interestingly, LPS-induced expression of Tnfa mRNA was also inhibited in Kupffer cells from a transgenic mouse line that expressed a DNA binding mutant form of PPARß/δ compared to controls. Combined, these results suggest that ligand activation of PPARß/δ attenuates hepatic tumorigenesis in HBV transgenic mice by inhibiting steatosis and cell proliferation, enhancing hepatocyte apoptosis, and modulating anti-inflammatory activity in Kupffer cells.
Subject(s)
Hepatitis B/complications , Liver Neoplasms/prevention & control , PPAR delta/drug effects , PPAR-beta/drug effects , Alanine Transaminase/blood , Animals , Apoptosis/drug effects , Cell Proliferation/drug effects , Cell Transformation, Neoplastic/drug effects , Cell Transformation, Neoplastic/metabolism , Hepatocytes/metabolism , Hepatocytes/pathology , Kupffer Cells/metabolism , Kupffer Cells/pathology , Ligands , Liver Neoplasms/metabolism , Male , Mice , Mice, Inbred C57BL , Mice, Transgenic , PPAR delta/physiology , PPAR-beta/physiology , Polymerase Chain Reaction , Thiazoles/pharmacologyABSTRACT
This study critically examined the role of PPARß/δ in colon cancer models. Expression of PPARß/δ mRNA and protein was lower and expression of CYCLIN D1 protein higher in human colon adenocarcinomas compared to matched non-transformed tissue. Similar results were observed in colon tumors from Apc(+/Min-FCCC) mice compared to control tissue. Dietary administration of sulindac to Apc(+/Min-FCCC) mice had no influence on expression of PPARß/δ in normal colon tissue or colon tumors. Cleaved poly (ADP-ribose) polymerase (PARP) was either increased or unchanged, while expression of 14-3-3ε was not influenced in human colon cancer cell lines cultured with the PPARß/δ ligand GW0742 under conditions known to increase apoptosis. While DLD1 cells exhibited fewer early apoptotic cells after ligand activation of PPARß/δ following treatment with hydrogen peroxide, this change was associated with an increase in late apoptotic/necrotic cells, but not an increase in viable cells. Stable over-expression of PPARß/δ in human colon cancer cell lines enhanced ligand activation of PPARß/δ and inhibition of clonogenicity in HT29 cells. These studies are the most quantitative to date to demonstrate that expression of PPARß/δ is lower in human and Apc(+/Min-FCCC) mouse colon tumors than in corresponding normal tissue, consistent with the finding that increasing expression and activation of PPARß/δ in human colon cancer cell lines inhibits clonogenicity. Because ligand-induced attenuation of early apoptosis can be associated with more late, apoptotic/necrotic cells, but not more viable cells, these studies illustrate why more comprehensive analysis of PPARß/δ-dependent modulation of apoptosis is required in the future.
