Direct determination of functional activity of cytochrome P-4501A1 and NADPH DT-diaphorase in hepatoma cell lines using noninvasive scanning laser cytometry

Resorufin-based responsive probes for fluorescence and colorimetric analysis

The fluorescence imaging technique has attracted increasing attention in the detection of various biological molecules in situ and in real-time owing to its inherent advantages including high selectivity and sensitivity, outstanding spatiotemporal resolution and fast feedback. In the past few decades, a number of fluorescent probes have been developed for bioassays and imaging by exploiting different fluorophores. Among various fluorophores, resorufin exhibits a high fluorescence quantum yield, long excitation/emission wavelength and pronounced ability in both fluorescence and colorimetric analysis. This fluorophore has been widely utilized in the design of responsive probes specific for various bioactive species. In this review, we summarize the advances in the development of resorufin-based fluorescent probes for detecting various analytes, such as cations, anions, reactive (redox-active) sulfur species, small molecules and biological macromolecules. The chemical structures of probes, response mechanisms, detection limits and practical applications are investigated, which is followed by the discussion of recent challenges and future research perspectives. This review article is expected to promote the further development of resorufin-based responsive fluorescent probes and their biological applications.

Persistence and dioxin-like toxicity of carbazole and chlorocarbazoles in soil

Halogenated carbazoles have recently been detected in soil and water samples, but their environmental effects and fate are unknown. Eighty-four soil samples obtained from a site with no recorded history of pollution were used to assess the persistence and dioxin-like toxicity of carbazole and chlorocarbazoles in soil under controlled conditions for 15 months. Soil samples were divided into two temperature conditions, 15 and 20 °C, both under fluctuating soil moisture conditions comprising 19 and 44 drying-rewetting cycles, respectively. This was characterized by natural water loss by evaporation and rewetting to -15 kPa. Accelerated solvent extraction (ASE) and cleanup were performed after incubation. Identification and quantification were done using high-resolution gas chromatogram/mass spectrometer (HRGC/MS), while dioxin-like toxicity was determined by ethoxyresorufin-O-deethylase (EROD) induction in H4IIA rat hepatoma cells assay and multidimensional quantitative structure-activity relationships (mQSAR) modelling. Carbazole, 3-chlorocarbazole and 3,6-dichlorocarbazole were detected including trichlorocarbazole not previously reported in soils. Carbazole and 3-chlorocarbazole showed significant dissipation at 15 °C but not at 20 °C incubating conditions indicating that low temperature could be suitable for dissipation of carbazole and chlorocarbazoles. 3,6-Dichlorocarbazole was resistant at both conditions. Trichlorocarbazole however exhibited a tendency to increase in concentration with time. 3-Chlorocarbazole, 3,6-dibromocarbazole and selected soil extracts exhibited EROD activity. Dioxin-like toxicity did not decrease significantly with time, whereas the sum chlorocarbazole toxic equivalence concentrations (∑TEQ) did not contribute significantly to the soil assay dioxin-like toxicity equivalent concentrations (TCDD-EQ). Carbazole and chlorocarbazoles are persistent with the latter also toxic in natural conditions.

Human NAD(P)H:quinone oxidoreductase inhibition by flavonoids in living cells

Procedures for assessing enzyme inhibition in living cells are an important tool in the study of the relevance of enzyme-catalyzed reactions and interactions in the human body. This paper presents the effects of flavonoids on NAD(P)H:quinone oxidoreductase 1 (NQO1) activity, by a newly developed method to measure NQO1 inhibition in intact cells. The principle of this method is based on the resorufin reductase activity of NQO1. The change in fluorescence in time was used to determine NQO1 activity in intact Chinese hamster ovary (CHO) cells genetically engineered to overexpress human NQO1. Applying this method to determine the inhibitory effects of reported in vitro NQO1 inhibitors (dicoumarol, 7,8-dihydroxyflavone, chrysin) showed that for all inhibitors tested, the IC50 in intact cells was at least 3 orders of magnitude higher than the IC50 in cell lysates. This result demonstrates that in vitro studies with purified NQO1 or with extracts from disrupted tissues are of limited value for obtaining insight into the situation in living cells. Possible factors underlying this discrepancy are being discussed. For the first time, we determined NQO1 inhibition by flavonoids in cells without disruption of the cells or addition of cofactors, enabling the assessment of enzymatic activity and the interaction of modulators of enzymatic activity in an intracellular situation.

Feasibility of a simple double-layered coculture system incorporating metabolic processes of the intestine and liver tissue: application to the analysis of benzo[a]pyrene toxicity

A simple double-layered coculture system using Caco-2 cell and Hep G2 cell, which mimic metabolic processes occurring in humans such as absorption through the intestine and cytochrome P450 1A1/2 involving biotransformation in both the intestine and liver cells, was used to investigate the toxicity of model chemical, benzo[a]pyrene (B[a]P). It was found that both Caco-2 and Hep G2 cells can metabolize B[a]P to toxic metabolites including B[a]P-7,8-hydrodiol (7,8-diol), an immediate precursor to the highly-reactive ultimate toxicant of B[a]P, B[a]P-7,8-hydrodiol-9,10-epoxide (BPDE), possibly mediated by cytochrome P450 1A1/2 activity. However, in a double-layered coculture system, no significant reduction of Hep G2 cell viability was found, although an approximately 50% reduction in viability was observed in pure Hep G2 cells. HPLC analysis showed that Caco-2 cells transfer B[a]P and its toxic metabolites back to the apical side, thus decreasing the concentrations of toxic metabolites including B[a]P-7,8-hydrodiol (7,8-diol) in cocultured Hep G2 cells. These results appear to be correlated with in vivo data on the effects of orally administered B[a]P, that is, low (10%) bioavailability in the rats and almost no acute lethal toxicity in rats or mice. As such, the simple double-layered coculture system can provide more accurate information regarding the toxic actions of the hazardous chemicals in humans than a pure culture system, as it also gives the final toxicity as a result of many complicated phenomena such as selective permeation in the intestine and biotransformation in the intestine and liver.

Probing enhanced cytochrome P450 2B1/2 activity in rat hepatocyte spheroids through confocal laser scanning microscopy

Cytochrome P450 (CYP450) enzymes are essential for xenobiotic metabolism. Although CYP450s are found in many tissues, CYP2B1/2 are primarily expressed in the rat liver. The constitutive expression in vivo of CYP2B1/2 is low but it is induced in the presence of various drugs such as phenobarbital (PB). In this study, CYP2B1/2 activity in cultured hepatocytes was assessed in situ with the introduction of a fluorogenic substrate, pentoxyresorufin. The product of 7-pentoxyresorufin-O-dealkylation (PROD), which is catalyzed specifically by CYP2B1/2, was detected using confocal laser scanning microscopy (CLSM). Primary hepatocytes cultured as monolayers on collagen-coated surfaces exhibited background PROD activity and minimal PB inducibility after 4 days in culture. In contrast, rat hepatocytes organized in compacted aggregates, or spheroids, exhibited higher levels of PROD activity and retained their ability for PB induction. The results from the CLSM analysis were verified by RT-PCR and Western immunoblotting analysis. Furthermore, CLSM in conjunction with image processing techniques and three-dimensional reconstruction revealed the localization of enhanced PROD activity in the center of spheroids. The results support the use of CLSM as a powerful tool for investigating CYP2B1/2 activity in cultured rat hepatocytes.

The role of actin filaments and microtubules in hepatocyte spheroid self-assembly

Cultured rat hepatocytes self-assemble into three-dimensional structures or spheroids that exhibit ultrastructural characteristics of native hepatic tissue and enhanced liver-specific functions. The spheroid formation process involves cell translocation and changes in cell shape, indicative of the reorganization of the cytoskeletal elements. To elucidate the function of the cytoskeleton, hepatocytes undergoing spheroid formation were treated with drugs that disrupt the different cytoskeletal components. Cytochalasin D, which targets the actin filaments, caused inhibition of spheroid formation. The role of microtubules in this process was assessed by incubating the cells with taxol or nocodazole. Perturbation of microtubules had minimal effects on spheroid assembly. Scanning electron micrographs showed no morphological differences between spheroids formed in control cultures and those formed in the presence of taxol or nocodazole. In addition, the effects of those agents on hepatocyte functions were investigated. Albumin secretion and cytochrome P450 2B1/2 activities of hepatocytes were comparable in spheroids formed in the presence of taxol or nocodazole to those formed in control cultures. The levels of these liver-specific activities were lower in cytochalasin D--treated cultures where only dispersed cells or cell clumps were found but spheroids had not found. Thus, hepatocytes require an intact actin network to self-assemble efficiently into functional tissue-like structures. Perturbation of the microtubule lattice does not impair the formation process. Events that transpire during hepatocyte spheroid self-assembly exhibit striking similarities to processes commonly observed in tissue morphogenesis. The results provide insight into the mechanisms that cells employ to organize into tissues and can contribute to our understanding of how to control the cellular assembly in tissue engineering and clinical applications.

Receding cytochrome P450 activity in disassembling hepatocyte spheroids

Primary rat hepatocytes can self-assemble to form multicellular spheroids when plated onto Primaria petri dishes or suspended in stirred vessels. These spheroids exhibit prolonged viability, enhanced liver-specific functions and differentiated ultrastructure compared to monolayer cultures. Upon transfer to collagen coated surface, or upon the addition of fetal bovine serum (FBS) to the culture, these spheroids began to disassemble and spread on the surface. The dynamics of cytochrome P450 CYP1A1/2 activity in the course of spheroid disassembly was examined in situ by detection of the fluorescent product, resorufin, of ethoxyresorufin O-dealkylation. Optical sectioning of the disassembling spheroids by confocal microscopy demonstrated that hepatocytes that reverted to monolayer exhibited markedly lower CYP1A1/2 activity than those that remained in a multilayered structure. This occurred whether the disassembly was caused by incubation with FBS-containing medium or by cultivation on a collagen-coated surface. When spheroids were cultured on the surface of agar, the disassembly process was retarded even in the presence of FBS. However, even in those intact spheroids, the exposure to FBS markedly decreased CYP1A1/2 activity. The decreased CYP1A1/2 activity was correlated to a diminished smooth endoplasmic reticulum as seen in the transmission electron micrograph. The results clearly demonstrate that the disassembly of hepatocyte spheroids led to decreased CYP1A1/2 activity. Furthermore, FBS contained a factor that caused CYP1A1/2 to decrease even in intact spheroids.

Enhanced cytochrome P450 IA1 activity of self-assembled rat hepatocyte spheroids

Primary rat hepatocytes can self-assemble to form multicellular spheroids when plated onto Primaria petri dishes. Spheroids have been observed to exhibit enhanced liver-specific functions and differentiated ultrastructure compared to monolayer cultures on dry collagen. With confocal scanning laser microscopy, CYP1A1 activity was evaluated in situ by detecting resorufin. This highly fluorescent molecule is the P450-mediated product of 7-ethoxyresorufin O-dealkylation (EROD). Significantly higher P450 activity was observed in spheroids compared to monolayers on collagen upon induction with 50 microM beta-naphthoflavone (BNF), a CYP1A inducer. This was confirmed by measuring microsomal EROD activity. The distribution of CYP1A1 activity within spheroids was heterogeneous, with higher activity localized to the hepatocytes in the interior. During the process of spheroid formation, cells were initially seen to attach and spread out as a monolayer. This stage was associated with relatively low CYP1A1 activity. As cells formed multicellular structures and aggregated into spheroids, the level of CYP1A1 activity increased over time. At least a fivefold higher fluorescence intensity was observed in spheroids compared to that of monolayers maintained on collagen. The higher P450 activity within spheroids may be associated with their ability to maintain a greater degree of differentiation compared to monolayers. These studies demonstrate the potential of hepatocyte spheroids as a model system for investigating drug metabolism, tissue engineering, and tissue self-assembly.