Cytochrome P450 and xenobiotic receptor humanized mice

Abstract

Most xenobiotics that enter the body are subjected to metabolism that functions primarily to facilitate their elimination. Metabolism of certain xenobiotics can also result in the production of electrophilic derivatives that can cause cell toxicity and transformation. Many xenobiotics can also activate receptors that in turn induce the expression of genes encoding xenobiotic-metabolizing enzymes and xenobiotic transporters. However, there are marked species differences in the way mammals respond to xenobiotics, which are due in large part to molecular differences in receptors and xenobiotic-metabolizing enzymes. This presents a problem in extrapolating data obtained with rodent model systems to humans. There are also polymorphisms in xenobiotic-metabolizing enzymes that can impact drug therapy and cancer susceptibility. In an effort to generate more reliable in vivo systems to study and predict human response to xenobiotics, humanized mice are under development.

Figure 1

Serum concentrations of debrisoquine (DEB) (A) and 4-hydroxydebrisoquine (4-OH-DEB) (B) versus time curves for the wild-type, CYP2D6-humanized heterozygous, and homozygous mice after single oral administration of DEB (2.5 mg/kg). (C) DEB urinary metabolic ratio (MR) in wild-type and CYP2D6-humanized mice.

Figure 2

Impaired lactogenesis of CYP3A4-humanized mice associated with low estradiol levels. (A) Histological examination of mammary glands. In transgenic nursing mothers, it was sparsely filled with underdeveloped alveoli. In wild-type mice, the alveoli were fully distended by the accumulation of milk and minimal volume of adipose tissue (AD) was present. At higher magnification, the lumen (Lu) and epithelial cells (black arrow) of the alveoli are indicated. Scale bar: 50 μm. (B) Expression of milk protein genes in mammary glands as examined by RT-PCR. In both wild-type and CYP3A4-humanized virgin mouse mammary glands, whey acid protein (WAP) and β-casein were not detectable. During pregnancy and lactation, WAP and β-casein were abundant in wild-type mice and reduced or undetectable in transgenic mice. (C) Serum estradiol levels were significantly (*P < 0.05, n = 5 in each group) decreased in pregnant and lactating humanized mice. (D) Catabolism of testosterone and estradiol by CYP3A4. (E) Estradiol under enterohepatic circulation hydroxylated by intestinal CYP3A4.

Figure 2

Impaired lactogenesis of CYP3A4-humanized mice associated with low estradiol levels. (A) Histological examination of mammary glands. In transgenic nursing mothers, it was sparsely filled with underdeveloped alveoli. In wild-type mice, the alveoli were fully distended by the accumulation of milk and minimal volume of adipose tissue (AD) was present. At higher magnification, the lumen (Lu) and epithelial cells (black arrow) of the alveoli are indicated. Scale bar: 50 μm. (B) Expression of milk protein genes in mammary glands as examined by RT-PCR. In both wild-type and CYP3A4-humanized virgin mouse mammary glands, whey acid protein (WAP) and β-casein were not detectable. During pregnancy and lactation, WAP and β-casein were abundant in wild-type mice and reduced or undetectable in transgenic mice. (C) Serum estradiol levels were significantly (*P < 0.05, n = 5 in each group) decreased in pregnant and lactating humanized mice. (D) Catabolism of testosterone and estradiol by CYP3A4. (E) Estradiol under enterohepatic circulation hydroxylated by intestinal CYP3A4.

Figure 3

Differential peroxisome proliferation, replicative DNA synthesis, and regulation of genes involved in cell cycle control in livers of PPARα-humanized and wild-type mice treated with Wy-14643. (A) Immunohistochemical staining with anti-catalase antibody indicated that peroxisomes (brown granular structures) were obviously increased in the wild-type mice treated with Wy-14643. Magnification: 400×. (B) Labeling index of Wy-14643-induced incorporation of 5-bromo-2′-deoxyuridine (BrdU) into hepatocyte nuclei as examined by immunohistochemical analysis with anti-BrdU antibody. (C) Regulation of genes involved in cell cycle control as revealed by Northern hybridization. ACOX, peroxisome acyl-CoA oxidase; LCPT, liver carnitine palmitoyltransferase; PCNA, proliferating cellular nuclear antigen; CDK, cyclin-dependent kinase.

Figure 3

Differential peroxisome proliferation, replicative DNA synthesis, and regulation of genes involved in cell cycle control in livers of PPARα-humanized and wild-type mice treated with Wy-14643. (A) Immunohistochemical staining with anti-catalase antibody indicated that peroxisomes (brown granular structures) were obviously increased in the wild-type mice treated with Wy-14643. Magnification: 400×. (B) Labeling index of Wy-14643-induced incorporation of 5-bromo-2′-deoxyuridine (BrdU) into hepatocyte nuclei as examined by immunohistochemical analysis with anti-BrdU antibody. (C) Regulation of genes involved in cell cycle control as revealed by Northern hybridization. ACOX, peroxisome acyl-CoA oxidase; LCPT, liver carnitine palmitoyltransferase; PCNA, proliferating cellular nuclear antigen; CDK, cyclin-dependent kinase.