Liver Failure and Stem Cells
In this short guide we are going to look at liver failure and if stem cells can help. You will learn about liver failure, the symptoms and causes, diagnosis and treatment, how stem cells can help, directions in which research is going, about hepatocyte transplantation, autologous bone marrow stem cells, mesenchymal stem cells, mobilizing stem cells with G-CSF, hepatic stem cells, artificial livers, drug testing and clinical studies that are being carried out. This is an informational guide only and should not be used as a substitute for medical advice.
About Liver Failure
The liver is the largest solid organ in the body. It is also the largest blood reservoir in the body too. One quarter of all the blood leaving the heart is directed to the liver. As blood circulates through this organ a number of key functions that are critical for sustaining life are carried out. The liver metabolizes nutrients transported from the intestines, removes waste products from out body, filters toxic substances and drugs from our blood and helps to maintain the levels of blood sugar, fat and hormones. The liver also plays a part in the immune response against infection.
Eighty percent of the liver is made up of liver cells called hepatocytes. These cells have an average lifespan of just 150 days. This means that the liver has to constantly renew itself under normal conditions. Hepatocytes are the powerhouses in the liver and they perform most of the critical metabolic functions. The hepatocytes secrete glucose and proteins into the blood and bile into the small channels, called canaliculi, that run between the cells. Bile is stored in the gall bladder and is necessary for the digestion and absorption of fats. Other important cell types include the bile duct cells, endothelial cells, stellate cells, pit cells and macrophages (called Kupffer cells in the liver) that remove debris, pathogens and damaged blood vessels.
The liver is a very strong organ and it can withstand a great deal of abuse whilst still continuing to function. When the liver reaches a state of failure, the damage and function is said to be irreversible and its effectiveness in maintaining so many of the body’s essential functions is diminished beyond repair. People who develop liver failure can die within months of their diagnosis.
Liver failure is increasing and it is estimated that the number of people with this condition will triple over the next 20 years. The Center for Disease Control and Prevention estimates that 30 million people in the United States suffer from a liver disorder and 27,000 people die each year from liver disease. The most common cause of chronic liver failure is viral hepatitis and alcohol. Liver disease is the fourth leading cause of death in the world. The most common cause is a condition that is known as fatty liver disease but Hepatitis B and C are also major causes of chronic liver disease.
Acute versus Chronic Liver Disease
There are over 100 different kinds of liver disease. The causes for most of these include alcohol, viruses, obesity, genetics, autoimmune diseases, drugs, toxins and cancer. Liver failure can be the result of an acute condition that develops in a matter of days or a chronic conditions that happens slowly over time. In children, the causes are different and the leading causes of acute liver failure include acetominophen toxicity, metabolic disorders and autoimmune disease. Chronic liver failure in children is most often caused by a blockage in the tubes that connect the gall bladder to the liver. This is known as biliary astresia. Acute liver failure in adults, however is most often caused by viral hepatitis. The main causes of chronic liver failure in adults is cirrhosis which is often caused by alcohol or hepatitis C. A lifestyle of excess which is characterized by obesity and high triglycerides stresses out the liver and contributes to a condition known as fatty liver which has now become the most common cause of liver failure in adults.
Cirrhosis of the liver caused by alcohol is the leading cause of chronic liver disease worldwide. Alcohol abuse over a period of time can induce permanent changes in the liver. This can be characterized by scar tissue or fibrosis and inflammation in a significant portion of the liver. Although this damage is permanent, the liver is a large organ and some portions may still remain unaffected. If the disease progresses, more and more of the liver becomes scarred and the capacity for regeneration in the healthy portion of the organ diminishes. When this happens, liver failure is the result. The rise in liver disease and cirrhosis in the younger demographic is particularly alarming and it is thought to be associated with binge drinking.
Hepatitis is inflammation of the liver that is most commonly caused by a viral infection. The hepatitis A virus is found in contaminated food or water. The liver usually recovers from this but the hepatitis A virus infection can be fatal. Hepatitis B is a virus that is transmitted through blood and bodily fluids from mother to baby, by sexual contact and drug use. The incidences of new cases of Hepatitis B is falling today and this is due to the successful vaccination programs that are available around the world. The hepatitis C virus causes a serious and incurable liver disease and is becoming more prevalent worldwide. This virus is normally transmitted by blood or drug use (needle sharing) and the infection leads to chronic inflammation of the liver and cirrhosis. The hepatitis C infection can develop without symptoms and progress to a chronic stage before a person becomes aware that they are carrying the virus.
There are some other causes of liver failure and these can be from overexposure to environmental toxins such as polyvinyl chloride and carbon tetrachloride, or from complications arising from other diseases such as diabetes mellitus, heart failure and kidney failure.
Diagnosis and Treatment
The liver itself is very much like skin, blood and bone and it can regenerate itself in response to regular wear and tear and injury. At end stage liver failure, no matter the cause, the structure and function of the liver is irreversibly compromised and the organ is no longer able to regenerate itself. When this happens, patients may experience such symptoms as nausea, vomiting, reduced appetite, jaundice, low grade fever, abdominal pain and fatigue.
Diagnosing liver disease often requires using liver enzymes as markers of hepatocyte death or by measuring the bilirubin levels or abnormalities in the blood clotting. Whether the liver failure is a result of acute or chronic causes, end-stage liver disease means that there isn’t long left before the organ fails.
The liver is a very tough organ and it can tolerate and recover from extensive abuse. Even with this in mind, there are no signs to say that it is failing until it is too late. Once the line has been crossed from chronic liver disease to end-stage liver disease or liver failure, the options become limited. At present, there is no liver dialysis that can rehabilitate liver function in the way that kidney failure is treated. The only effective treatment that there is for liver failure is a liver transplant but there are a number of drawbacks to this including the risk of infection, the risks associated with surgery and the shortage of donors. It is estimated that for every donor organ there are 10 patients on a waiting list and many people end up dying from liver failure while waiting on a donor organ.
Can Stem Cells Help?
The use of stem cells for treating liver failure is still very much in the experimental stages but the possibility of harnessing stem cells to churn out limitless numbers of hepatocytes for transplant therapy is something that researchers are continuing to look into. As of 2012, adult liver stem cells in humans have not been conclusively isolated so while researchers are continuing to work towards the goal of using stem cell therapies to cure liver disease, they are also looking at the possibility of getting stem cells from other tissue sources such as bone marrow, peripheral blood, fat, skin, amniotic, embryonic and induced. They are hoping that these will be able to be used to treat various types of liver failure.
Researchers looking for ways to treat liver failure have begun with transplanting hepatocytes. It was hoped that this strategy would be able to reverse inborn defects in the liver metabolism, bridge patients to whole organ transplantation or even someday replace whole organ transplant. Although dozens of patients with acute liver failure have received hepatocyte transplants from cadeveric donors, with some improvement in liver function, the effects were short lived and there was no overall survival benefit. The major challenges with this approach include a shortage of cadaveric donors and the immunosuppression of the patients. These are essentially the same challenges as there are with whole organ transplants.
Researchers are looking to try and solve the donor problem by learning to grow hepatocytes from different types of stem cells. In order to do this, they need to identify the signals that specifically stimulate hepatocyte generation and improve their methods for isolating, expanding and storing hepatocytes. They are hoping that transplanting these stem cells will become a viable future therapy for treating liver failure. The expansion issue is a big one because hepatocytes do not divide easily in culture and it is estimated that a massive number of hepatocytes will be needed to see any clinical benefit. The storage issue is another problem that researchers are looking at because at the moment hepatocytes can’t be stored for a long time in culture before their lose their ability to function normally.
Autologous Bone Marrow Stem Cells
In 2000, researchers were able to show that hepatocytes could grown in the body from non-liver cell sources. This is called transdifferentiation and was originally observed in female patients who were given bone marrow transplants from male donors. The new liver cells in the female patients could be traced to the donor bone marrow cells of the male donors. Today, autologous (from the patient) bone marrow stem cells are the only stem cells that have been used clinically to treat liver disease. The major advantage with using these cells is that because they come fro the patient the risk of rejection when they are transplanted is very low. In clinical trials that were held between 2005 and 2010, autologous bone marrow cells were used as sources of cell therapy for a variety of liver diseases. Most of these trials were small and contained one to forty patients and showed a measure of improvement in liver function.
Researchers are still unclear as to how the bone marrow stem cells actually contribute to liver regeneration. They think that it may be that the cells are transdifferentiating into hepatocytes or that they are producing soluble factors that promote regeneration or repair. There is also the possibility that the bone marrow stem cells may be fusing with resident hepatocytes to direct their regeneration. The other option is that cell fusion is happening. The cell fusion mechanism occurs during normal development; for example when myoblasts (muscle cells) form myotubes (muscles) or when phagocytes in the bone marrow form osteoclasts that remodel bone.
In terms of benefits to the liver, it is still hard to know whether whole bone marrow or particular types of stem cells in the bone marrow are responsible. Sorted populations such as CD133+ bone marrow cells have been used with some success in patients that have to have an extensive amount of their liver removed but unsorted autologous bone marrow stem cells have also contributed to better liver function in patients with cirrhosis and hepatocellural carcinoma (HCC).
Mesenchymal Stem Cells
Mesenchymal stem cells (MSCs) are found throughout the body in tissue such as bone, muscle, cartilage and fat. Other sources of these cells include bone marrow, cord blood, placenta, fetal liver tissue and dental papilla. MSCs are among the most multipotent stem cells that remain in the body after birth. This means that these cells are still able to make a variety of different cells types. Researchers are looking to see if MSCs can transdifferentiate into hepatocytes in vitro or if they can be coaxed into making hepatocytes ex vitro for transplantation. Preliminary trials have shown that patients with liver cirrhosis have benefited from autologous bone marrow derived MSCs, but larger trials will be necessary to predict whether or not this direction is worth pursuing and how exactly mesenchymal stem cells are having these effects.
Mobilizing Stem Cells With G-CSF
G-CSF is a growth factor that can be used to stimulate CD34+ hematopoietic stem cells to exit the bone marrow and enter the blood. This will make it easier to collect the stem cells and separate them from other blood cells before they are delivered back into the patient as autologous transplants. Small clinical trials have tested this strategy in patients with end stage liver disease as a result of alcohol or hepatitis B virus induced cirrhosis. The procedure has been proven safe and in some cases benefits have been seen for as long as 12 months. G-CSF can also be administered on its own with the intention of stimulating endogenous liver stem cells in the body to help repair damage. Clinical trials are being carried out to evaluate this strategy also.
Hepatic Stem Cells
We already know that the liver is capable of regeneration and repair and researchers are looking into the possibility that this is due to liver stem cells. It isn’t easy to isolate or characterize these cells in humans. Normal hepatocytes have an intrinsic capacity to replace themselves and a lot of researchers think that these are the cells that account for how the liver repairs itself. Researchers have also recently identified a liver stem cell in mice and these can produce hepatocytes and other liver cells when transplanted into recipient mice. A similar population of hepatic progenitor cells have been isolated from human liver tissue but so far only in vitro. While this sounds promising, pre-clinical studies have shown that human hepatic progenitor cells produce tumors in mice and this means that there is a lot of additional work that must be done before these cells can be used in clinical trials.
One of the main concerns about transplanting cells into a liver that is undergoing failure is that the environment of the liver might not allow the graft to take. The possibility of creating an artificial liver, similar to a dialysis machine for kidney failure, that would perform all the functions of a normal liver might replace the need for transplants or provide a bridge to therapy for patients waiting on transplants.
Artificial livers loaded with fresh hepatocytes that were obtained from human or pig livers have been tested in phase I clinical trials. Despite providing some respite for patients, these artificial liver bioreactors have not provided any overall survival benefit or reduced the need for transplants. In addition to this, obtaining massive amounts of fresh hepatocytes to load into the bioreactors is an ongoing issue.
Researchers at the McGowan Institute for Regenerative Medicine in Pittsburg are trying an alternative approach. They are co-opting part of the body, in this case the lymph node, to act as a reservoir for transplanted hepatocytes. The hepatocytes grow well in the lymph nodes, and the resulting hepatiasized lymph nodes are even able to rescue mice with lethal liver failure. One of the issues with this approach for humans is the risk of hepatocyte rejection given the donor population of hepatocytes. Researchers are hoping that induced pluripotent stem cells created from the patients themselves could be used to generate the hepatocytes needed for the transplant and this will remove the issue with graft rejection.
In an international collaboration, researchers from Japan, the USA and Italy are attempting to create “neolivers” by reseeding livers stripped down to a three dimensional scaffold with fresh hepatocytes. They have had some success testing this approach in rats and are now optimizing their seeding strategies and making sure that the stripping technique does not compromise the ability of the neoliver to be infiltrated with blood vessels.
Pharmaceutical companies require large numbers of hepatocytes in order to identify which chemicals could be potential new drugs. This procedure is invaluable because 50 percent of drugs are taken off the market because of toxicity to the liver. At the moment, hepatocytes that are left over from transplants or hepatocyte cell lines derived from liver cancers are being used as sources for drug testing but variation and functionality are issues that can confuse the results. This is where stem cells could be of tremendous use. Stem cells could theoretically generate limitless numbers of hepatoccytes and induced pluripotent stem cells could even provide patient-specific hepatocytes to verify that a drug therapy would not harm the patient’s liver.
There are dozens of open NIH registered clinical trials that are investigating stem cells as an intervention for liver disease. The majority of these trials are in phase I or phase II and are focused on the safety and/or efficacy of stem cell treatments for diseases such as liver or biliary cirrhosis, liver fibrosis and liver cancer. To reduce the chance of transplant rejection, particular populations from the bone marrow, such as CD34+, CD133+, endothelial progenitors and mononuclear cells, are being used as autologous transplants. Mesenchymal stem cells sourced from bone marrow or umbilical cord are also being investigated and researchers are hoping htat the properties of these cells will work towards regenerating the liver and/or mitigating inflammation within the liver. There are also a number of trials that are investigating the potential of the growth factor G-CSF to mobilize autologous stem cells for transplant in patients with liver disease.
As research into the option of using stem cells to treat end stage liver disease continues to move forward, researchers and clinicians continue to struggle with a number of challenges. These include the methods for differentiating and scaling up the production of hepatocytes from stem cells because such large numbers of cells are required for transplants, drug testing and in vitro models. In addition to this, researchers are looking at different routes for transplanting the stem cells. A number of different routes for transplanting stem cells or their progeny into the liver have already been tested in humans such as through the portal vein, the hepatic artery or splenic artery. The ideal route however has yet to be determined. In the case of arterial transplantation, the high pressure in the arteries could contribute to low engraftment and in patients with cirrhosis of the liver there can be a strange reversal of flow in the portal vein which re-routes the transplanted cells to the spleen instead of the liver.
For researchers, functionality, storage, engraftment and safety are the key issues that need to be looked and and more work is being done here before the full potential of using stem cells to treat end stage liver failure can be achieved.
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