Diminished hepatocellular proliferation in mice humanized for the nuclear receptor peroxisome proliferator-activated receptor alpha.
Lipid-lowering fibrate drugs function as agonists for the nuclear receptor peroxisome proliferator-activated receptor alpha (PPARalpha). Sustained activation of PPARalpha leads to the development of liver tumors in rats and mice. However, humans appear to be resistant to the induction of peroxisome proliferation and the development of liver cancer by fibrate drugs. The molecular basis of this species difference is not known. To examine the mechanism determining species differences in peroxisome proliferator response between mice and humans, a PPARalpha-humanized mouse line was generated in which the human PPARalpha was expressed in liver under control of the tetracycline responsive regulatory system. The PPARalpha-humanized and wild-type mice responded to treatment with the potent PPARalpha ligand Wy-14643 as revealed by induction of genes encoding peroxisomal and mitochondrial fatty acid metabolizing enzymes and resultant decrease of serum triglycerides. However, surprisingly, only the wild-type mice and not the PPARalpha-humanized mice exhibited hepatocellular proliferation as revealed by elevation of cell cycle control genes, increased incorporation of 5-bromo-2'-deoxyuridine into hepatocyte nuclei, and hepatomegaly. These studies establish that following ligand activation, the PPARalpha-mediated pathways controlling lipid metabolism are independent from those controlling the cell proliferation pathways. These findings also suggest that structural differences between human and mouse PPARalpha are responsible for the differential susceptibility to the development of hepatocarcinomas observed after treatment with fibrates. The PPARalpha-humanized mice should serve as models for use in drug development and human risk assessment and to determine the mechanism of hepatocarcinogenesis of peroxisome proliferators.
Cheung
Connie
C
Laboratory of Metabolism, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
Akiyama
Taro E
TE
Ward
Jerrold M
JM
Nicol
Christopher J
CJ
Feigenbaum
Lionel
L
Vinson
Charles
C
Gonzalez
Frank J
FJ
D000818:Animals; D000924:Anticholesteremic Agents / Q000494:pharmacology; D002273:Carcinogens / Q000494:pharmacology; D002455:Cell Division; D004261:DNA Replication / Q000187:drug effects; D005227:Fatty Acids / Q000378:metabolism; D022781:Hepatocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D006801:Humans; D051379:Mice; D008822:Mice, Transgenic; D010084:Oxidation-Reduction; D020025:Peroxisome Proliferators / Q000494:pharmacology; D011743:Pyrimidines / Q000494:pharmacology; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics / Q000502:physiology*; D013045:Species Specificity; D014157:Transcription Factors / Q000235:genetics / Q000502:physiology*
D000924:Anticholesteremic Agents; D002273:Carcinogens; D005227:Fatty Acids; D020025:Peroxisome Proliferators; D011743:Pyrimidines; D018160:Receptors, Cytoplasmic and Nuclear; D014157:Transcription Factors; C006253:pirinixic acid