UC San Diego

The studies presented herein examine various aspects of paraoxonase 1 (PON1), specifically PON1's therapeutic potential and the hepatic expression of PON1. Bone marrow obtained from mice expressing a macrophage specific PON1- transgene was transplanted into atherosclerosis- susceptible mice lacking low density lipoprotein receptors (LDLR), then followed with gadolinium chloride (GdCl3) treatment, an agent that induces Kupffer cell apoptosis. GdCl3 treatment increased hepatic PON1 mRNA expression 9- fold in PON1-trangenic bone marrow recipient mice. After a 12 week cholesterol-enriched atherogenic diet feeding period, PON1-transgenic bone marrow recipient mice treated with GdCl3 exhibited a 50% reduction in atherosclerotic lesions; this protection was not seen in non-transgenic bone marrow recipients treated with GdCl3. These findings illustrate the important role of Kupffer cells in atherogenesis and suggest that GdCl3-facilitated replacement of Kupffer cells may enhance the efficacy of other HSC-based gene therapies. In additional studies, the regulation of PON1 expression by dietary bile acids was examined. It has been previously shown that a cholic-acid containing diet reduces the hepatic expression of PON1 in athero-susceptible C57BL/6 animals, but not in athero- resistant C3H/HeJ animals. When fed an atherogenic diet containing taurocholate, C3H/HeJ mice, in contrast to C57BL/6 mice, displayed a resistance to bile acid mediated repression of hepatic PON1 mRNA expression and HDL cholesterol. Further studies demonstrated that the absence of functional toll-like receptor 4 (TLR4s) in C3H/HeJ mice could not account for this response. However, mice with a genetic deletion of either fibroblast growth factor receptor 4 (FGFR4) or farnesoid X receptor (FXR) failed to repress PON1 and CYP7A1 in response to the taurocholate- containing diet. Additional studies demonstrated that FGF- 19, an FXR-inducible growth factor that binds specifically to FGFR4, decreased the expression of PON1 and CYP7A1 in HepG2 cells. The taurocholate containing diet increased the ileal expression of FGF-15, the murine homologue of FGF-19, in C57BL/6 and C3H/HeJ mice. Combined these data suggest that hepatic PON1 and CYP7A1 mRNA expression is repressed by bile acids via an FXR-mediated induction of FGF-15. The resistance of C3H/HeJ mice to bile acid- mediated repression of PON1 and CYP7A1 is, therefore, due to signaling events distal to the FGF-15/FGFR-4 association