Humans PPARa activation

Recent evidence confirms that species differences can involve more than one aspect of PPARa-mediated regulation of gene expression. The insensitivity of human liver to rodent peroxisome proliferators is associated with low levels of expression of PPARa in human liver. Marked species differences in the expression of PPARa mRNA have been demonstrated between rodent and human liver, with the latter expressing 1-10% of the levels found in mouse or rat liver (Palmer et al, 1994 Tugwood et al, 1996 Palmer etal, 1998). Using a sensitive and specific immuno/DNA binding assay. Palmer et al (1998) have shown that active PPARa protein is expressed at variable concentrations in human livers. The study compared 20 different human livers and found that those with the highest levels of PPARa protein expression contained less than 10% of the level in mice. Most of the samples (13/20) contained no detectable PPARa activity, but did... 

Induction of peroxisome proliferation following treatment with DEHP is not due to the parent compound, but to DEHP metabolites. Studies with MEHP in vitro have demonstrated that the proximate peroxisome proliferators are mono(2-ethyl-5-oxohexyl) phthalate (metabolite VI) and mono(2-ethyl-5-hydroxyhexyl) phthalate, (metabolite IX) and that for 2-ethylhexanol, the proximate proliferator is 2-ethylhexanoic acid (Elcombe and Mitchell 1986 Mitchell et al. 1985a). Similar findings were observed by Maloney and Waxman (1999), who showed that MEHP (but not DEHP) activated mouse and human PPARa and PPARy, while 2-ethylhexanoic acid activated mouse and human PPARa only, and at much higher concentrations. Based on its potency to induce enzyme activities, such as the peroxisomal fatty acid (3-oxidation cycle and carnitine acetyltransferase, DEHP might be considered a relatively weak proliferator. 

PPARa activator in humans—clofibric acid, fenofibric acid, pirinixic acid (WY-14643). 

MODE OF ACTION ANALYSIS AND HUMAN RELEVANCE OF LIVER TUMORS INDUCED BY PPARa ACTIVATION... 

Summary of the Mode of Action and Human Relevance of Liver Tumors Induced by PPARa Activation... 

There are several plausible explanations for the species-specific effects of PPARa activators (1) Full-length PPARa protein is expressed at levels at least 10-fold greater in rodent liver than in human liver. (2) Humans but not rodents express an inactive form of PPARa in the liver which inhibits the active PPARa. 

Table 17.3 summarizes PPARa MOA key events in responsive species (rats and mice summarized from Table 17.1) compared to Syrian hamsters, guinea pigs, Cynomolgus monkeys, and humans. Due to the relative paucity of data for key events, other endpoints associated with exposure to PPARa activators are included (i.e., liver weight to body weight, hypolipidemic effects). 

PPARa ligands do not induce cell proliferation or suppress apoptosis in human hepatocytes in vitro (Goll et al. 1999 Hasmall et al. 1999 Perrone et al. 1998 Williams and Perrone 1996). Many of these studies included a positive control to ensure that human hepatocytes were of sufficient quality to mount a positive growth response. In comparison, rat or mouse primary hepatocytes exposed to PPARa activators exhibit up to 8-fold induction in cell proliferation (summarized in (Klaunig et al. 2003)). There are no data on human hepatocyte proliferation in vivo, although in vivo and in vitro data from nonhuman primates show cell proliferation is not induced by PPARa activators [Table 17.3 and reviewed in Doull et al. (1999)]. In summary, available data suggest that PPARa activators are unlikely to alter apoptosis and proliferation in human hepatocytes. 

Molecular Basis of Species Differences. In the following section, the properties of PPARa and associated responses in the livers of rodents and primates are compared with an emphasis on human data, if available. The weight of evidence demonstrates that humans respond to PPARa activators differently than rodents in that many of the typical markers of PPARa activator exposure associated with hepatocarcinogenesis in rodents are absent in humans. Differences in the... 

Property Rodent (Rat/Mouse) Human Impact on Responsiveness to PPARa Activators in Humans Compared to Mice and Rats... 

Differences in Ligand Inducibility. Hiunan PPARa is not more sensitive than rodent PPARa to chemical activation. Most compounds activate the rodent receptor better or exhibit no differences between species. A munber of environmentally relevant chemicals and hypolipidemic agents were able to activate rat or mouse PPARa at lower concentrations or to higher absolute levels than hPPARa in side-by-side trans-activation studies. These PPARa activators include WY-14,643 (Keller et al. 1997 Maloney and Waxman 1999 Takacs and Abbott 2007), PFOA (Maloney and Waxman 1999), perfluorooctanesulfonate (Shipley et al. 2004 Takacs and Abbott 2007), and a number of phthalate ester metabolites [Bility et al. (2004) and summarized in Corton and Lapinskas (2005)]. Some PPARa activators show no differences between activation of the mouse and hiunan PPARa, including TCA, dichloroacetate, 2-ethylhexanoic acid (Maloney and Waxman 1999), a number of phthalates (Bility et al. 2004), clofibrate (Keller et al. 1993), and PFOA (Vanden Heuvel et al. 2006). Only perfluorooctanesulfonamide (Shipley et al. 2004) was shown to modestly activate the human but not the rodent PPARa at one lower dose (25 jM in human versus 34 pM mouse). Overall, the data indicate that hPPARa is no more sensitive than the mouse or rat PPARa to significant activation by environmentally relevant PPARa activators. 

The molecular basis for differences between mouse and human PPARa may be differences in the ability of the receptors to interact with transcriptional coactivators or to regulate miRNA cascades. Go-activators convey the transcriptional activation of the ligand-induced nuclear receptor to the transcriptional machinery. Elegant biochemical and crystallographic analyses have shown key interactions between co-activators and the ligand binding domains of nuclear receptors including PPAR family members (Li et al. 2008 Xu and Li 2008). The mouse and rat PPARa... 

NF-kB activation Yes Unknown NF-kB exists in humans but has not been measured in human liver or primary hepatocytes after exposure to PPARa activators. 

Liver tumors Yes Not likely Not measured in livers of humans exposed to PPARa activators no tumors in hamsters with expression of PPARa intermediate between mice/rats and humans. 

RELEVANCE OF PPARa ACTIVATOR-INDUCED RODENT LIVER TUMOR RESPONSE TO HUMANS ... 

The original PPARa agonists, which are characterized by a fibrate-like chemical structure, show a rather weak activity on human PPARa in cell-based transactivation... 

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