Putting Peps in Your Heps

The liver is a special organ that performs a whole host of essential functions. The liver stores iron, vitamins and minerals; it detoxifies alcohol, drugs, and other chemicals that accumulate in our bloodstreams, and it produces bile (used to dissolve fats so that they can be degraded), and blood-based proteins like clotting factors and albumin. The liver also stores sugar in the form of glycogen. All of these tasks are undertaken by a single cell type, the hepatocyte (otherwise known as a liver cell).


When your liver fails, you get really sick. This was greatly illustrated to me by one of my colleagues where I teach whose wife suffered extensive liver damage as a result of her battle with lupus (short for systemic lupus erythematosus, an autoimmune disease). Now that this dear lady has had a liver transplant, she is a new person. What a difference a healthy liver makes.

What can regenerative medicine do for patients with failing livers? Human pluripotent stem cells, either embryonic stem cells or induced pluripotent stem cells, can be directed to differentiate into liver cells in culture, but the liver cells made by these cells are very immature. They express proteins commonly found in fetal liver cells (for example, alpha-fetoprotein) and they also lack key enzymes associated with adult cells (such as cytochrome P450s). Rashid and others in the Journal of Clinical Investigation (2010; 120: 3127-3136) showed this. The development of three-dimensional culture systems have increased the maturity of such cells, but there is still a long way to go (see T Takebe and others, Nature 2013; 499:481-484 and J Shan and others, Nature Chemical Biology 2013; 9: 514-520).

Two papers from the journal Cell Stem Cell might show a way forward to making mature liver cells for regenerative liver treatments without destroying embryos or even using and pluripotent stem cell lines. These papers utilize the procedure known as “direct reprogramming,” otherwise known as “direct lineage conversion.” Direct reprogramming requires the forced overexpression of particular genes that causes the cells to switch their cell types.

In the first of these papers, Pengyu Huang and his colleagues from the Chinese Academy of Sciences in Shanghai, China overexpressed a three-gene combination in mouse embryonic fibroblasts that converted the cells into hepatocytes at an efficiency of 20% after 14 days in culture. This gene combination, known as 3TF (HNF4/HNF1A/FOXA3), converted the mouse embryonic skin cells into mature liver cells that made blood proteins and drug-processing enzymes. The only problem was that these mature cells could not grow in culture because they were mature. Therefore, Huang and others infected these cells with a virus called SV40, which drove the cells to divide. Now these cells could be grow in culture and expanded for further experiments.

When transplanted into the livers of mice with failing livers, the induced liver cells made by Huang and others restored proper liver function and allowed the mice to survive.

A second paper by Yuanyuan Du and others from the Peking-Tsinghua Center for Life Sciences at Peking University in Beijing, China, used a large gene combination to make mature liver cells from human skin fibroblasts. This gene combination included eight genes (HNF1A/HNF4A/HNF6/ATF5/PROX1/CEBPA/p53 ShRNA/C-MYC) that converted the human skin cells into liver cells after 30 days in culture at an efficiency of nearly 80%. Again, these cells metabolized drugs as they should, made blood proteins, took up cholesterol, and stored glycogen. Du and others compared the gene expression profile of these human induced hepatocytes or “hiHeps” to the gene expression profile of liver cells taken from liver biopsies. While there were differences in gene expression, there was also significant overlap and a large overall similarity. In fact the authors state, “these results indicate that hiHeps show a similar expression profile to primary human hepatocytes.”

Next, Du and others used three different mouse models of liver failure in all three cases, the hiHeps were capable of colonizing the damaged liver of the mouse and regenerating it. Mind you, the hiHeps did not do as good a job as human primary hepatocytes, but they still worked pretty well. This shows that this direct reprogramming protocol, as good as it is, can still be optimized and improved.

These studies show that the production of highly functional human hepatocyte-like cells using direct reprogramming is feasible and represents an exciting step towards the production of a supply source of cells for drug development, and therapies for liver disease.

The Use of Stem Cells in Drug Development

Why is it that one person can have surgery and wake up, eat a full lunch and show no ill effects while others are sick for several days after receiving general anesthesia?

The fact is that we all process drugs differently, and these differences are a function of the genetic diversity between all of us. These differences stem from 1) different targets; 2) different liver enzyme activities; and 3) different levels of absorption, excretion and distribution.

A few examples might be illustrative. It is fairly well established that a particular type of blood pressure medicines called “ACE inhibitors” do not work terribly well in African-Americans (see Park IU, Taylor AL. Ann Fam Med. 2007 Sep-Oct;5(5):444-52). The reason for this is that the target of ACE inhibitors, the enzyme angiotensin converting enzyme, which is mercifully abbreviated ACE, works on a substrate that already exists at low concentrations in most African-American patients. Thus a target difference causes differential responses to particular blood pressure medicines.

As a second example, two liver enzymes that degrade drugs, Cyp2C19 and Cyp2D6 are encoded by genes that are subject to genetic variation. In 3-10% of whites, the Cyp2D6 enzyme does not completely function and the drugs processed by this enzyme, a blood pressure medicine called debrisoquine and a heart medicine called sparteine, show impaired degradation. Thus these patients are in danger of overdosing on these drugs at normal dosages, since they are degraded and excreted at such low rates. Other people, however, have a version of Cyp2D6 that is hyperactive. This variant is most commonly found in Ethiopians and Saudi Arabians that consequently, drugs degraded by this enzyme, such as tricyclic antidepressants (e.g., nortriptyline) must be dosed at two the three times the normal concentration. Also, some drugs are given as prodrugs, which are inactive until the liver activates them. In individuals with the overactive Cyp2D6 enzyme variant, a prodrug, such as codeine is overactivated and at normal doses, causes severe side effects (stomach pains). Thus a distinct enzyme difference causes different clinical outcomes with the same drugs (see JK Hicks, et al., Clin Pharmacol Ther. 2013 May;93(5):402-8).

So then, how do we test for drug safety and efficacy given these variations in drug metabolism?

Stem cell technology has the ability to improve drug testing in a multitude of ways. Drug safety can be tested with stem cells as can drug efficacy without feeding them to human volunteers.

Now scientists from the University of Edinburgh have shown that stem cell-based drug tests are almost ready for the prime time. David Hay from the Medical Research Centre for Regenerative Medicine at the University at the University of Edinburgh and his colleagues have generated cell in the laboratory that reach the gold standard required by the pharmaceutical industry to test drug safety.

In this study, the Hay laboratory made liver cells from H9 human embryonic stem cells and from 33D6 human induced pluripotent stem cells. Since is the liver is the main organ that biochemically processes drugs in our bodies (a phenomenon known as biotransformation), testing drug safety in cultured liver cells makes good sense.

Next, Hay and his colleagues found that these pluripotent-derived liver cells were equally effective in drug safety tests as frozen human liver tissue extracted from cadavers. Such livers are in short supply and the results researchers derive from them varies wildly according to the genetic make-up of the donor. Thus frozen liver tissue is not optimal for such drug testing protocols.

However, these drug-testing protocols that use stem cell-based protocols can provide reproducible drug safety results and can also be adapted for individuals with particular genetic compositions who process drugs differently from other people.

David Hay explained it this way: “Differing genetic information plays a key role in how patients’ livers process drugs. We are now able to efficiently produce human liver cells in the laboratory from different people model the functional differences in human genetics.”

Hay and others hope to generate liver cells that contain distinct DNA sequences that will reflect the genetic variations in metabolism found in the population. These cultured liver cells from human pluripotent stem cells can be used to identify differences in drug biotransformation.

These laboratory-generation liver cells could also be used to screen certain drugs that need close monitoring in order to optimize the efficacy of patient treatment, and the safety of these treatment regimes.

Hay and his colleagues are working with Edinburgh BioQuarter in order to form a spin-off company that will commercialize this research and its clinical ramifications.