Time for a consistent approach to preparing and culturing hepatocytes?

Micro‑dimpled surface atelocollagen maintains primary human hepatocytes in culture and may promote their functionality compared with collagen coat culture

Primary human hepatocytes (PHHs) are the gold standard for drug development procedures; however, maintaining functional PHHs in vitro is challenging in conventional collagen-coated cultures. In the present study, we developed a new scaffold comprising high amounts (≥1 mg/cm²) of atelocollagen exposed to ultraviolet radiation to induce cross-linking and improve stability. Scanning and transmission electron microscopy revealed a micro-dimpled surface (MDS) scaffold composed of randomly arranged atelocollagen fibrils. The scaffold was therefore designated as MDS atelocollagen. PHHs cultured on MDS atelocollagen were round with a compact cytoplasm and exhibited enhanced levels of albumin (ALB) secretion and cytochrome P450 (CYP) 3A4 activity. The expression of hepatocyte-related genes, such as serum proteins, drug metabolism-related CYPs, and nuclear receptors, was enhanced in cells cultured on MDS atelocollagen, but not in those cultured on conventional atelocollagen. Moreover, the abnormal gene expression of cell adhesion molecules observed in conventional atelocollagen culture was suppressed when the cells were grown on MDS atelocollagen, thereby suggesting a cell behavior similar to that of in vivo hepatocytes. These results suggest that MDS atelocollagen functionally preserves PHHs while conserving the simplicity of conventional PHH atelocollagen-coated cultures.

Primary hepatocytes and their cultures for the testing of drug-induced liver injury

Drug-induced liver injury is a major reason for discontinuation of drug development and withdrawal of drugs from the market. Intensive efforts in the last decades have focused on the establishment and finetuning of liver-based in vitro models for reliable prediction of hepatotoxicity triggered by drug candidates. Of those, primary hepatocytes and their cultures still are considered the gold standard, as they provide an acceptable reflection of the hepatic in vivo situation. Nevertheless, these in vitro systems cope with gradual deterioration of the differentiated morphological and functional phenotype. The present paper gives an overview of traditional and more recently introduced strategies to counteract this dedifferentiation process in an attempt to set up culture models that can be used for long-term testing purposes. The relevance and applicability of such optimized cultures of primary hepatocytes for the testing of drug-induced cholestatic liver injury is demonstrated.

Primary hepatocyte cultures for pharmaco-toxicological studies: at the busy crossroad of various anti-dedifferentiation strategies

Continuously increasing understanding of the molecular triggers responsible for the onset of diseases, paralleled by an equally dynamic evolution of chemical synthesis and screening methods, offers an abundance of pharmacological agents with a potential to become new successful drugs. However, before patients can benefit of newly developed pharmaceuticals, stringent safety filters need to be applied to weed out unfavourable drug candidates. Cost effectiveness and the need to identify compound liabilities, without exposing humans to unnecessary risks, has stimulated the shift of the safety studies to the earliest stages of drug discovery and development. In this regard, in vivo relevant organotypic in vitro models have high potential to revolutionize the preclinical safety testing. They can enable automation of the process, to match the requirements of high-throughput screening approaches, while satisfying ethical considerations. Cultures of primary hepatocytes became already an inherent part of the preclinical pharmaco-toxicological testing battery, yet their routine use, particularly for long-term assays, is limited by the progressive deterioration of liver-specific features. The availability of suitable hepatic and other organ-specific in vitro models is, however, of paramount importance in the light of changing European legal regulations in the field of chemical compounds of different origin, which gradually restrict the use of animal studies for safety assessment, as currently witnessed in cosmetic industry. Fortunately, research groups worldwide spare no effort to establish hepatic in vitro systems. In the present review, both classical and innovative methodologies to stabilize the in vivo-like hepatocyte phenotype in culture of primary hepatocytes are presented and discussed.

Cytochromes P450 catalyze the reduction of α,β-unsaturated aldehydes

The metabolism of α,β-unsaturated aldehydes, e.g., 4-hydroxynonenal, involves oxidation to carboxylic acids, reduction to alcohols, and glutathionylation to eventually form mercapturide conjugates. Recently, we demonstrated that P450s can oxidize aldehydes to carboxylic acids, a reaction previously thought to involve aldehyde dehydrogenase. When recombinant cytochrome P450 3A4 was incubated with 4-hydroxynonenal, O(2), and NADPH, several products were produced, including 1,4-dihydroxynonene (DHN), 4-hydroxy-2-nonenoic acid (HNA), and an unknown metabolite. Several P450s catalyzed the reduction reaction in the order (human) P450 2B6 ≅ P450 3A4 > P450 1A2 > P450 2J2 > (mouse) P450 2c29. Other P450s did not catalyze the reduction reaction (human P450 2E1 and rabbit P450 2B4). Metabolism by isolated rat hepatocytes showed that HNA formation was inhibited by cyanamide, while DHN formation was not affected. Troleandomycin increased HNA production 1.6-fold while inhibiting DHN formation, suggesting that P450 3A11 is a major enzyme involved in rat hepatic clearance of 4-HNE. A fluorescent assay was developed using 9-anthracenealdehyde to measure both reactions. Feeding mice a diet containing t-butylated hydroxyanisole increased the level of both activities with hepatic microsomal fractions but not proportionally. Miconazole (0.5 mM) was a potent inhibitor of these microsomal reduction reactions, while phenytoin and α-naphthoflavone (both at 0.5 mM) were partial inhibitors, suggesting the role of multiple P450 enzymes. The oxidative metabolism of these aldehydes was inhibited >90% in an Ar or CO atmosphere, while the reductive reactions were not greatly affected. These results suggest that P450s are significant catalysts of the reduction of α,β-unsaturated aldehydes in the liver.

Cytochromes P450 catalyze oxidation of alpha,beta-unsaturated aldehydes

We sought to establish whether heme-thiolate monooxygenases oxidize, alpha,beta-unsaturated aldehydes generated during lipid peroxidation. Several recombinant P450s co-expressed with NADPH:P450 oxidoreductase were surveyed for aldehyde oxidation activity with anthracene-9-carboxaldehyde and 4-hydroxy-trans-2-nonenal (HNE). Murine P4502c29, human P4503A4, human P4502B6, and rabbit P4502B4 were good catalysts of aldehyde oxidation to carboxylic acids. Other P450s (e.g., P4501A2, 2E1, and 2J2) did not oxidize these aldehydes. P4502c29 and P4503A4 displayed K(m)/S(0.5) values of approx. 1-20microM. The product measured by HPLC that co-migrates with authentic 4-hydroxynonenoic acid (HNA) had a mass spectrum identical to the standard. Using P4502c29, HNE was a mixed-competitive inhibitor of anthracene-9-carboxaldehyde oxidation, suggesting that both aldehydes are substrates for P4502c29. Specific inhibitors of aldehyde dehydrogenases and P450 were used to assess their role in the metabolism of HNE in primary rat hepatocytes. Inhibitors of aldehyde dehydrogenase (cyanamide) inhibited HNA formation by 60% and together cyanamide and miconazole (P450) caused over 85% inhibition of HNA formation. P450s are significant participants in metabolism of endogenous and exogenous unsaturated aldehydes in primary rat hepatocytes.

Experimental models for evaluating enzyme induction potential of new drug candidates in animals and humans and a strategy for their use

Experimental models that have application for evaluating enzyme induction potential have been described in order of increasing complexity. The main focus was on models that have had wide application thus far. However, many new models are currently being developed that may have future applications in evaluating enzyme induction potential. A strategy to evaluate the enzyme induction potential of drug candidates was outlined. This scheme uses a combination of new and established techniques to evaluate data in a stepwise manner that is appropriate to the drug's current stage of development.