Humanized PPARa Mice

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. 

Even when expressed at levels similar to the mouse PPARa, human PPARa does not induce the cell-cycle machinery, cell proliferation, or liver cancer in mice. 

TABLE 17.4. Properties of Rodent (Rat and Mouse) PPARa Versus Human PPARa in Liver... 

The human PPARa does not possess aU of the functions of the rodent PPARa including the ability to regulate cell proliferation. Two mouse strains have been created which express the hPPARa in the absence of mPPARa ( humanized hPPARa mice). In the TRE-hPPARa mouse, PPARa is under the control of a liver-specific promoter and is preferentially expressed in hepatocytes (Gheung et al. 

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... 

In summary, our results define an important role for PPARa in the maintenance of cellular lipid and glucose homeostasis in vivo via the transcriptional control of target genes encoding mitochondrial and extra-mitochondrial fatty acid oxidation enzymes. We also demonstrate that gender-related mechanisms are involved in hepatic and cardiac lipid metabolism. Lastly, we propose that the PPARa -/- mouse may prove useful as a model of human diseases due to inborn and acquired alterations in cellular lipid metabolism. 

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... 

Following the cloning of mouse PPARa, three related nuclear receptors were cloned from a Xenopus cDNA library (15). Since all three receptors were capable of activating the acyl-CoA oxidase gene, these receptors were termed PPARa, [3, and y PPARy was subsequently cloned from mouse (16), hamster (17), and human (18) cells. There are two PPARy isoforms, yl and y2, in mouse (19) and human (20), which differ only in that y2 has an additional 30 N-terminal amino acid units. For the murine gene, the... 

Yu S, Cao W-Q, Kashireddy P, et al. (2001) Human peroxisome proliferator-activated receptor a (PPARa) supports the induction of peroxisome proliferation in PPARa-deficient mouse liver. The Journal of Biological Chemistry 45 42485 2491. 

Molecular explanations for the species differences in PP sensitivities include differences in PPAR expression levels and differences in PPAR transactivation capacities for target genes between species. The expression of PPARa in human hepatocytes was reported to be 10-fold lower than that in the rat or mouse [41,42], although others have not observed any differences in expres-... 

Lewis DFV, Jacobs MN, Dickins M, Lake BG. Molecular modelling of the peroxisome proliferator-activated receptor a (PPARa) from human, rat and mouse, based on homology with the human PPARr crystal structure. Toxicology 2002 176 51-7. 

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. 

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. 

Since the rodent effects of PPs ate mediated via PPARa and humans appear to be non-responsive to these adverse effects, species differences in PPARa expression levels provide a plausible explanation for the lack of hitman response. However, humans do respond to PPs by altering etqtression of enzymes that regulate serum cholesterol and lipid homeostasis. " In addition, human liver does contain a functional PPARa although the expression of PPARa in humans is around 10-fold lower when compared with responsive species such as rat and mouse."" "" In total, these data support a quantitative hypothesis whereby PPARa expression in humans is sufficient to mediate the beneficial effects of hypolipidaemic dmgs via regulation of genes for enzymes and lipid transporters. 

Ringseis, R., G. Wen, and K. Eder. 2012. Regulation of genes involved in carnitine homeostasis by PPARa across different species (rat, mouse, pig, cattle, chicken, and human). PPAR Research 2012 868317. 

Yang Q, Nagano T, Shah Y, Cheung C, Ito S, Gonzalez FJ (2008) The PPARa-humanized mouse a model to investigate species differences in liver tox-ieitymediated by PPARa. Toxicol Sci 101 132-139... 

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