Generating hepatic cell lineages from pluripotent stem cells for drug toxicity screening

Abstract

Hepatotoxicity is an enormous and increasing problem for the pharmaceutical industry. Early detection of problems during the drug discovery pathway is advantageous to minimize costs and improve patient safety. However, current cellular models are sub-optimal. This review addresses the potential use of pluripotent stem cells in the generation of hepatic cell lineages. It begins by highlighting the scale of the problem faced by the pharmaceutical industry, the precise nature of drug-induced liver injury and where in the drug discovery pathway the need for additional cell models arises. Current research is discussed, mainly for generating hepatocyte-like cells rather than other liver cell-types. In addition, an effort is made to identify where some of the major barriers remain in translating what is currently hypothesis-driven laboratory research into meaningful platform technologies for the pharmaceutical industry.

Figure 1

Liver injury. The liver is the primary organ for metabolic biotransformation of xenobiotics, including drugs, and is consequently a frequent target for a variety of hepatotoxic insults. Chemically-mediated toxicity can affect any cell-type and mimic any naturally occurring disease process. Frequently, the initial toxic insult is followed by involvement of the immune system with a resulting inflammatory reaction.

Figure 2

The multi-tiered defence response against chemically-induced toxicity within the liver. Diagram to show the range of defense responses adopted by the liver in reaction to increasing severity of chemical stress. Initial defence is provided through constitutive expression of antioxidant proteins and low molecular weight scavengers (e.g. glutathione), as well as phase II drug metabolizing enzymes and transporters. Subsequently these defences are bolstered through a transcriptionally induced up-regulation of defence proteins, principally orchestrated through the factor Nrf2. Finally, the cell is scheduled to undergo apoptotic self-destruction as an alternative to necrotic cell death, which carries with it the added risk of an inflammatory response.

Figure 3

The drug discovery pathway. Schematic overview of both the pre-clinical and clinical phases of drug development. ESC-derived liver cells are highly desirable as cell models during lead optimization.

Figure 4

The human liver and its composite cell-types. Diagram to show the organization of different cell-types in the liver. Hepatocytes are arranged as anastomosing plates, one cell thick. Biliary epithelial cells line the bile duct and contribute to bile secretion. Bile collects in canaliculi which run along the surface of hepatocytes. Hepatic sinusoids are small blood vessels lined by discontinuous endothelial and Kupffer cells (specialized macrophages of hematopoietic origin). The sinusoids are separated from the hepatocytes by the space of Disse, which contains stellate cells. In the absence of liver injury, stellate cells at this location are considered quiescent but are thought to activate in response to inflammatory stimuli when they migrate into the liver parenchyma and deposit the excessive extracellular matrix characteristic of liver fibrosis.

Figure 5. Schematic diagram representing hepatic differentiation in vivo correlated to in vitro differentiation of human pluripotent stem cells

Liver development in vivo and hepatocyte differentiation in vitro are depicted with examples of gene expression associated with each developmental or differentiation stage. Exogenous factors are shown that are typically added in vitro to induce differentiation.