Differential regulation of the human CYP3A4 promoter in transgenic mice and rats

Mechanism of scrapie prion precipitation with phosphotungstate anions

The phosphotungstate anion (PTA) is widely used to facilitate the precipitation of disease-causing prion protein (PrP^Sc) from infected tissue for applications in structural studies and diagnostic approaches. However, the mechanism of this precipitation is not understood. In order to elucidate the nature of the PTA interaction with PrP^Sc under physiological conditions, solutions of PTA were characterized by NMR spectroscopy at varying pH. At neutral pH, the parent [PW₁₂O₄₀]^3– ion decomposes to give a lacunary [PW₁₁O₃₉]^7– (PW₁₁) complex and a single orthotungstate anion [WO₄]^2– (WO₄). To measure the efficacy of each component of PTA, increasing concentrations of PW₁₁, WO₄, and mixtures thereof were used to precipitate PrP^Sc from brain homogenates of scrapie prion-infected mice. The amount of PrP^Sc isolated, quantified by ELISA and immunoblotting, revealed that both PW₁₁ and WO₄ contribute to PrP^Sc precipitation. Incubation with sarkosyl, PTA, or individual components of PTA resulted in separation of higher-density PrP aggregates from the neuronal lipid monosialotetrahexosylganglioside (GM1), as observed by sucrose gradient centrifugation. These experiments revealed that yield and purity of PrP^Sc were greater with polyoxometalates (POMs), which substantially supported the separation of lipids from PrP^Sc in the samples. Interaction of POMs and sarkosyl with brain homogenates promoted the formation of fibrillar PrP^Sc aggregates prior to centrifugation, likely through the separation of lipids like GM1 from PrP^Sc. We propose that this separation of lipids from PrP is a major factor governing the facile precipitation of PrP^Sc by PTA from tissue and might be optimized further for the detection of prions.

A double transgenic mouse model expressing human pregnane X receptor and cytochrome P450 3A4

Cytochrome P450 3A4 (CYP3A4), the most abundant human cytochrome P450 in liver, participates in the metabolism of approximately 50% of clinically used drugs. The pregnane X receptor (PXR), a member of the nuclear receptor superfamily, is the major activator of CYP3A4 transcription. However, because of species differences in response to PXR ligands, it is problematic to use rodents to assess CYP3A4 regulation and function. The generation of double transgenic mice expressing human PXR and CYP3A4 (TgCYP3A4/hPXR) would provide a solution to this problem. In the current study, a TgCYP3A4/hPXR mouse model was generated by bacterial artificial chromosome transgenesis in Pxr-null mice. In TgCYP3A4/hPXR mice, CYP3A4 was strongly induced by rifampicin, a human-specific PXR ligand, but not by pregnenolone 16alpha-carbonitrile, a rodent-specific PXR ligand. Consistent with CYP3A expression, hepatic CYP3A activity increased approximately 5-fold in TgCYP3A4/hPXR mice pretreated with rifampicin. Most antihuman immunodeficiency virus protease inhibitors are CYP3A substrates and their interactions with rifamycins are a source of major concern in patients coinfected with human immunodeficiency virus and Mycobacterium tuberculosis. By using TgCYP3A4/hPXR mice, human PXR-CYP3A4-mediated rifampicin-protease inhibitor interactions were recapitulated, as the metabolic stability of amprenavir, nelfinavir, and saquinavir decreased 52, 53, and 99%, respectively, in the liver microsomes of TgCYP3A4/hPXR mice pretreated with rifampicin. In vivo, rifampicin pretreatment resulted in an approximately 80% decrease in the area under the serum amprenavir concentration-time curve in TgCYP3A4/hPXR mice. These results suggest that the TgCYP3A4/hPXR mouse model could serve as a useful tool for studies on CYP3A4 transcription and function in vivo.

Identification of New Human Pregnane X Receptor Ligands among Pesticides Using a Stable Reporter Cell System

Pregnane X receptor (PXR, NR1I2) is activated by various chemically unrelated compounds, including environmental pollutants and drugs. We proceeded here to in vitro screening of 28 pesticides with a new reporter system that detects human pregnane X receptor (hPXR) activators. The cell line was obtained by a two-step stable transfection of cervical cancer HeLa cells. The first transfected cell line, HG5LN, contained an integrated luciferase reporter gene under the control of a GAL4 yeast transcription factor-binding site. The second cell line HGPXR was derived from HG5LN and stably expressed hPXR ligand-binding domain fused to GAL4 DNA-binding domain (DBD). The HG5LN cells were used as a control to detect nonspecific activities. Pesticides from various chemical classes were demonstrated, for the first time, to be hPXR activators: (1) herbicides: pretilachlor, metolachlor, and alachlor chloracetanilides, oxadiazon oxiconazole, and isoproturon urea; (2) fungicides: bupirimate and fenarimol pyrimidines, propiconazole, fenbuconazole, prochloraz conazoles, and imazalil triazole; and (3) insecticides: toxaphene organochlorine, permethrin pyrethroid, fipronil pyrazole, and diflubenzuron urea. Pretilachlor, metolachlor, bupirimate, and oxadiazon had an affinity for hPXR equal to or greater than the positive control rifampicin. Some of the newly identified hPXR activators were also checked for their ability to induce cytochrome P450 3A4 expression in a primary culture of human hepatocytes. HGPXR, with HG5LN as a reference, was grafted onto nude mice to assess compound bioavailability through in vivo quantification of hPXR activation. Altogether, our data indicate that HGPXR cells are an efficient tool for identifying hPXR ligands and establishing pesticides as hPXR activators.

Transgenic modifications of the rat genome

The laboratory rat (R. norvegicus) is a very important experimental animal in several fields of biomedical research. This review describes the various techniques that have been used to generate transgenic rats: classical DNA microinjection and more recently described techniques such as lentiviral vector-mediated DNA transfer into early embryos, sperm-mediated transgenesis, embryo cloning by nuclear transfer and germline mutagenesis. It will also cover techniques associated to transgenesis such as sperm cryopreservation, embryo freezing and determination of zygosity. The availability of several technologies allowing genetic manipulation in the rat coupled to genomic data will allow biomedical research to fully benefit from the rat as an experimental animal.