Drug metabolism by cultured human hepatocytes: how far are we from the in vivo reality?

The investigation of metabolism is an important milestone in the course of drug development. Drug metabolism is a determinant of drug pharmacokinetics variability in human beings. Fundamental to this are phenotypic differences, as well as genotypic differences, in the expression of the enzymes involved in drug metabolism. Genotypic variability is easy to identify by means of polymerase chain reaction-based or DNA chip-based methods, whereas phenotypic variability requires direct measurement of enzyme activities in liver, or, indirectly, measurement of the rate of metabolism of a given compound in vivo. There is a great deal of phenotypic variability in human beings, only a minor part being attributable to gene polymorphisms. Thus, enzyme activity measurements in a series of human livers, as well as in vivo studies with human volunteers, show that phenotypic variability is, by far, much greater than genotypic variability. In vitro models are currently used to investigate the hepatic metabolism of new compounds. Cultured human hepatocytes are considered to be the closest model to the human liver. However, the fact that hepatocytes are placed in a microenvironment that differs from that of the cells in the liver raises the question of to what extent drug metabolism variability observed in vitro actually reflects that in the liver in vivo. This issue has been examined by investigating the metabolism of the model compound, aceclofenac (an approved analgesic/anti-inflammatory drug), both in vitro and in vivo. Hepatocytes isolated from programmed liver biopsies were incubated with aceclofenac, and the metabolites formed were investigated by HPLC. The patients were given the drug during the course of clinical recovery, and the metabolites, largely present in urine, were analysed. In vitro and in vivo data from the same individual were compared. There was a good correlation between the in vitro and in vivo relative abundance of oxidised metabolites (4'-OH-aceclofenac + 4'-OH-diclofenac; Spearman's rho = 0.855), and the hydrolysis of aceclofenac (diclofenac + 4'-OH-aceclofenac + 4'-OH-diclofenac; rho = 0.691), while the conjugation of the drug in vitro was somewhat lower than in vivo. Globally, the metabolism of aceclofenac in vitro correlated with the amount of metabolites excreted in urine after 16 hours (rho = 0.95). Overall, although differing among assays, the in vitro/in vivo metabolism data for each patient were surprisingly similar. Thus, the variability observed in vitro appears to reflect genuine phenotypic variability among the donors.     

Altered adhesion features and signal transduction in NRK-49F cells transformed by down-regulation of lysyl oxidase

Lysyl oxidase (LOX) down-regulation induced an oncogenic phenotype in NRK-49F. This event was accompanied by a constitutive activation of ras oncogene and down-regulation of PDGF beta receptor, among other important phenotypic and molecular modifications. In the present paper we show that ras activation is not accompanied by a constitutive activation of the MAP kinases as expected. Surprisingly, even if MAPK-independent, ras activation was accompanied by a constitutive Ser(63) and Ser(73) phosphorylation of c-jun, a further downstream target of ras. Although rare, this ras alternative pathway has been described. Since ras alone is seldom able to trigger cell transformation and the transformed phenotype showed clearly an abnormal adhesion pattern, we investigated the main molecules involved in cell-cell adhesion. In fact, we found that beta-catenin was up-regulated, escaping the glycogen synthase kinase-3 beta (GSK-3 beta) control, through unclear mechanisms. Its nuclear accumulation was accompanied by an up-regulation of cyclin D1, as classically described in the activation of the Wnt/beta-catenin signal pathway. We believe that the resulting up-regulation of cyclin D1 acted in synergy with ras to induce the cell transformation.     

Penetratin and related cell-penetrating cationic peptides can translocate across lipid bilayers in the presence of a transbilayer potential

Fluorescent-labeled derivatives of the Antennapedia-derived cell-penetating peptide penetratin, and of the simpler but similarly charged peptides R(6)GC-NH(2) and K(6)GC-NH(2), are shown to be able to translocate into large unilamellar lipid vesicles in the presence of a transbilayer potential (inside negative). Vesicles with diverse lipid compositions, and combining physiological proportions of neutral and anionic lipids, are able to support substantial potential-dependent uptake of all three cationic peptides. The efficiency of peptide uptake under these conditions is strongly modulated by the vesicle lipid composition, in a manner that suggests that more than one mechanism of peptide uptake may operate in different systems. Remarkably, peptide uptake is accompanied by only minor perturbations of the overall barrier function of the lipid bilayer, as assessed by assays of vesicle leakiness under the same conditions. Fluorescence microscopy of living CV-1 and HeLa cells incubated with the labeled peptides shows that the peptides accumulate in peripheral vesicular structures at early times of incubation, consistent with an initial endosomal localization as recently reported, but gradually accumulate in the cytoplasm and nucleus during more extended incubations (several hours). Our findings indicate that these relatively hydrophilic, polybasic cell-penetrating peptides can translocate through lipid bilayers by a potential- and composition-dependent pathway that causes only minimal perturbation to the overall integrity and barrier function of the bilayer.     

Ultracytochemistry as a tool for the study of the cellular and subcellular localization of membrane-bound guanylate cyclase (GC) activity. Applicability to both receptor-activated and receptor-independent GC activity

Membrane-bound guanylate cyclase activity was detected by ultracytochemistry at the electron microscope level in several mammalian tissues. The technique used in these studies allows the detection of active enzyme at the membrane site where it is located. In a few cases, such as normal and regenerating peripheral nerves and placenta, membrane-bound guanylate cyclase could be detected in the absence of stimulators of enzyme activity. However, in the majority of these studies membrane-bound guanylate cyclase was investigated following stimulation with natriuretic peptides, guanylin, or the Ca²⁺ sensor proteins, S100B and S100A1. In general, membrane-bound guanylate cyclase was localized to plasma membranes, in accordance with the functional role of this enzyme. Yet, in secretory cells the enzyme activity was localized on intracellular membranes, suggesting a role of membrane-bound guanylate cyclase in secretory processes. Finally, S100B and S100A1 were found to colocalize with membrane-bound guanylate cyclase on photoreceptor disc membranes and to stimulate enzyme activity at these sites in dark-adapted retinas in a Ca²⁺-dependent manner. The results of these analyses are discussed in relation to the proposed functional role(s) of this enzyme.     

Isolated durum wheat and potato cell mitochondria oxidize externally added NADH mostly via the malate/oxaloacetate shuttle with a rate that depends on the carrier-mediated transport

We investigated whether and how mitochondria from durum wheat (Triticum durum Desf.) and potato (Solanum tuberosum), isolated from etiolated shoots and a cell suspension culture, respectively, oxidize externally added NADH via the mitochondrial shuttles; in particular, we compared the shuttles and the external NADH dehydrogenase (NADH DHExt) with respect to their capacity to oxidize external NADH. We found that external NADH and NADPH can be oxidized via two separate DHExt, whereas under conditions in which the activities of NAD(P)H DHExt are largely prevented, NADH (but not NADPH) is oxidized in the presence of external malate (MAL) and MAL dehydrogenase, in a manner sensitive to several non-penetrant compounds according to the occurrence of the MAL/oxaloacetate (OAA) shuttle. In durum wheat mitochondria and potato cell mitochondria, the rate of NADH oxidation was limited by the rate of a novel carrier, the MAL/OAA antiporter, which is different from other carriers thought to transport OAA across the mitochondrial membrane. No NAD(P)H oxidation occurred arising from the MAL/Aspartate and the alpha-glycerophosphate/dihydroxyacetonphosphate shuttles. We determined the kinetic parameters of the enzymes and the antiporter involved in NADH oxidation, and, on the basis of a kinetic analysis, we showed that, at low physiological NADH concentrations, oxidation via the MAL/OAA shuttle occurred with a higher efficiency than that due to the NADH DHExt (about 100- and 10-fold at 1 microm NADH in durum wheat mitochondria and in potato cell mitochondria, respectively). The NADH DHExt contribution to NADH oxidation increased with increasing NADH concentration.     

Molecular cytogenetic assignment of genes to bovine chromosome 5

Seven genes were assigned by molecular cytogenetic methods to bovine chromosome 5. To accomplish this, specific primers were either publicly available or were designed from highly conserved regions of the publicly available mammalian gene sequences. The identity of the amplified segments was verified by sequencing and alignment with the published sequences. The optimized primers that amplified the desired bovine genes were used for screening a bovine bacterial artificial chromosome library. The positive clones were localized to a specific band of bovine chromosome 5 by fluorescence in situ hybridization. The genes HOXC4, SP1 and IGFBP6 were localized to band q21, COL2A1 was localized to bands q21-q23, IGF1 was localized to band q26, MB to band q31 and the gene CYP2D6 was localized to band q35. The cytogenetic assignment of SP1, IGFBP6, COL2A1, IGF1, MB and CYP2D6 is first reported here and the assignment of HOXC4 refines the previous assignment of this gene. The identification and localization of these genes further support the development of the human to bovine comparative map through characterizing the homologous segments conserved in the evolution of these species. This information will be useful for the future localization of genes that affect economically important traits in bovines.     

Polarization of the C. elegans zygote proceeds via distinct establishment and maintenance phases

Polarization of the C. elegans zygote along the anterior-posterior axis depends on cortically enriched (PAR) and cytoplasmic (MEX-5/6) proteins, which function together to localize determinants (e.g. PIE-1) in response to a polarizing cue associated with the sperm asters. Using time-lapse microscopy and GFP fusions, we have analyzed the localization dynamics of PAR-2, PAR-6, MEX-5, MEX-6 and PIE-1 in wild-type and mutant embryos. These studies reveal that polarization involves two genetically and temporally distinct phases. During the first phase (establishment), the sperm asters at one end of the embryo exclude the PAR-3/PAR-6/PKC3 complex from the nearby cortex, allowing the ring finger protein PAR-2 to accumulate in an expanding 'posterior' domain. Onset of the establishment phase involves the non-muscle myosin NMY-2 and the 14-3-3 protein PAR-5. The kinase PAR-1 and the CCCH finger proteins MEX-5 and MEX-6 also function during the establishment phase in a feedback loop to regulate growth of the posterior domain. The second phase begins after pronuclear meeting, when the sperm asters begin to invade the anterior. During this phase (maintenance), PAR-2 maintains anterior-posterior polarity by excluding the PAR-3/PAR-6/PKC3 complex from the posterior. These findings provide a model for how PAR and MEX proteins convert a transient asymmetry into a stably polarized axis.     

Circular dichroism study of ribonuclease A mutants containing the minimal structural requirements for dimerization and swapping

Four residues Pro19, Leu28, Cys31 and Cys32 proved to be the minimal structural requirements in determining the dimeric structure and the N-terminal segment swapping of bovine seminal ribonuclease, BS-RNase. We analyzed the content of secondary and tertiary structures in RNase A, P-RNase A, PL-RNase A, MCAM-PLCC-RNase A and MCAM-BS-RNase, performing near and far-UV CD spectra. It results that the five proteins have very similar native conformations. Thermal denaturation at pH 5.0 of the proteins, studied by means of CD measurements, proved reversible and well represented by the two-state ND transition model. Thermodynamic data are discussed in the light of the structural information available for RNase A and BS-RNase.