Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME.

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Caspase-3 feeds back on caspase-8, Bid and XIAP in type I Fas signaling in primary mouse hepatocytes.

The TNF-R1 like receptor Fas is highly expressed on the plasma membrane of hepatocytes and plays an essential role in liver homeostasis. We recently showed that in collagen-cultured primary mouse hepatocytes, Fas stimulation triggers apoptosis via the so-called type I extrinsic signaling pathway. Central to this pathway is the direct caspase-8-mediated cleavage and activation of caspase-3 as compared to the type II pathway which first requires caspase-8-mediated Bid cleavage to trigger mitochondrial cytochrome c release for caspase-3 activation. Mathematical modeling can be used to understand complex signaling systems such as crosstalks and feedback or feedforward loops. A previously published model predicted a positive feedback loop between active caspases-3 and -8 in both type I and type II FasL signaling in lymphocytes and Hela cells, respectively. Here we experimentally tested this hypothesis in our hepatocytic type I Fas signaling pathway by using wild-type and XIAP-deficient primary hepatocytes and two recently characterized, selective caspase-3/-7 inhibitors (AB06 and AB13). Caspase-3/-7 activity assays and quantitative western blotting confirmed that fully processed, active p17 caspase-3 feeds back on caspase-8 by cleaving its partially processed p43 form into the fully processed p18 species. Our data do not discriminate if p18 positively or negatively influences FasL-induced apoptosis or is responsible for non-apoptotic aspects of FasL signaling. However, we found that caspase-3 also feeds back on Bid and degrades its own inhibitor XIAP, both events that may enhance caspase-3 activity and apoptosis. Thus, potent, selective caspase-3 inhibitors are useful tools to understand complex signaling circuitries in apoptosis.