How to Keep Hepatocytes Happy

purple fishThe liver plays a central role in metabolising and clearing drugs and other xenobiotics from the body and, because of this role, is particularly susceptible to damage. More than 900 drugs have been linked to more or less severe liver damage, and hepatotoxicity is the most common reason for drugs being withdrawn from the market. Animal studies and even early stage clinical trials often fail to pick up liver toxicity and, although many drugs cause low level injury that shows up only in biochemical tests for liver function, drug-induced liver injury accounts for 5% of hospital admissions and more than 50% of cases of acute liver failure. As well as imposing a risk to patients, the recall of marketed products results in substantial financial costs to the pharmaceutical industry. As an ageing population uses more and more prescription drugs, as well as over-the counter medicines, the chances of drug-induced liver injury are likely to increase.

Both in vitro and in vivo tests to evaluate the potential for liver toxicity are widely used and a team led by researchers at Massachusetts General Hospital have now developed an innovative method of culturing liver cells that they believe will be more predictive than existing in vitro methods. Freshly cultured liver cells rapidly lose their metabolic competence under standard culture conditions and earlier studies had suggested that animal-derived serum, which is commonly used in cell cultures, may interfere with the metabolism of cultured hepatocytes. Since one of the key stresses involved in moving cells from an in vivo environment into cell culture is a ten-fold drop in oxygen levels, the researchers hypothesised that a high-oxygen, serum-free culture medium might provide a better environment for growing hepatocytes. Experiments showed that both human and rat hepatocytes grown with endothelial cells in a serum-free culture with 95% oxygen quickly resumed normal metabolic activity, including gene expression and cell function. These cultured cells successfully predicted the clearance rates for both rapidly and slowly cleared drugs and maintained a high level of metabolic activity for several weeks. Although oxygen levels had been thought to affect cell survival, they had not previously been recognised to influence gene expression or metabolic function of cultured hepatocytes. The authors believe that the new culture method provides an environment that is more similar to that of hepatocytes in an intact liver and will keep cells viable for longer, thus providing a more predictive system for evaluating hepatotoxicity.

The study is published in the online early edition of PNAS.

P-Glycoprotein – A Close-up View

P-glycoprotein (Pgp), with its ability to transport a wide range of xenobiotic compounds – including drug molecules – across cell membranes, is the bane of medicinal chemists. Pgp, which probably evolved as a defense mechanism against toxic substances, is an ATP-dependent integral membrane protein that is particularly highly expressed in cells of the gut and kidney, and also in capillary endothelial cells that make up the blood-brain barrier. This means that as well as preventing absorption of drugs from the gut after oral dosing, Pgp can limit entry of drugs into the brain. Expression of Pgp by tumour cells also results in decreased accumulation of anti-cancer drugs in the cells, and contributes to multi-drug resistance in chemotherapy.

crystal structure of PGPA team led by scientists at the Scripps Research Institute have now succeeded in solving the X-ray crystallographic structure of murine Pgp, a result that they hope will help chemists to design more effective drugs. The 3.8Å structure of the apo protein revealed an internal cavity of ca 6000Å3, with a 30Å separation of the two nucleotide-binding domains. Two additional structures with bound inhibitors showed that hydrophobic and aromatic amino acids form distinct drug-binding sites which are capable of stereo-recognition. The apo and drug-bound Pgp structures are open to the cytoplasm and the inner leaflet of the lipid bilayer for drug entry, representing initial stages of the transport cycle. The overall structure of Pgp is very similar to that of the bacterial protein, MsbA, which transports lipids out of bacteria, suggesting that Pgp may work in a similar way. In the bacterial transporter, binding of ATP changes the accessibility of the carrier from cytoplasmic (inward) facing to extracellular (outward) facing so that substances caught inside the cavity are ejected from the cell. The cavities of both transporters are lined with hydrophobic amino acids, but Pgp contains a larger number of highly varied amino acids which perhaps explains its broader substrate specificity.

The study, which is published in full in the journal Science, should help chemists to better understand, if not tame, the beast.