US researchers may have moved a step further towards gaining a deeper understanding of the role of stem cells in liver cancer.
A team of experts from Penn State College of Medicine and the University of Southern California used a unique approach that involves study of individual cells, and became the first ever researchers to show a population of cancer stem cells in the liver prior to tumour formation.
Writing about their work in the journal Stem Cells, the researchers say that their findings suggest a potential link between liver stem cells and liver cancer.
Lead researcher C. Bart Rountree and colleagues have revealed that they used a liver-specific PTEN (phosphatase and tensin homolog deleted on chromosome 10) mouse model to study the microenvironment of the liver.
"The PTEN knock-out mouse is one model of chronic liver injury that ultimately leads to liver cancer. During chronic injury, liver stem cells proliferate, and at times of healthy liver, the liver stem cells are very rare. We were initially looking for what is driving liver stem cell proliferation during chronic liver injury," Rountree said.
"We started investigating liver stem cells in many different liver injury models with the idea we may be able to help people with liver disease, but we discovered that some cells we isolated were malignant. It was quite a surprise for us because there were not any tumors in the mice when we isolated the cells," Rountree said.
The liver is the only organ in the body that is able to fully regenerate itself. Its cells, including hepatocytes and cholangiocytes, can divide and repopulate themselves.
In cases of chronic liver injury, including by a virus or alcoholism, the hepatocytes lose the ability to make more of themselves. In such settings, liver stem cells proliferate and can make either of the cell types.
However, patients with chronic injury also develop liver cancer, opening the possibility that the stem cells are involved in tumour formation.
"There's been a groundswell of interest in understanding the role of specific stem cells in the development of liver cancer. There is a cancer stem cell lurking out there that may be very bad. It has stem cell properties and is malignant, resistant to chemotherapy. These properties make it harder to treat these cancers," Rountree said.
"What we ended up doing was shifting our understanding of liver stem cells and their role in malignancy. All work previously done was looking at patients, animal models or cell lines after the tumor already developed. What we did was identify malignant stem cells before there is evidence of the primary tumor. This gave us a new perspective on not only what the potential of stem cells for therapy is, but also in terms of what's driving cancer formation. Imagine treating a cancer before a primary malignancy forms," Rountree added.
For their study, the researchers created ten cell lines to study using a single-cell isolation technique. They separated cells that make a unique surface protein called CD133 by placing them in a liquid medium, and running through a flow cytometer.
Once identified, a robot took a single CD133-positive cell and placed it in a single drop into one well of a culture dish. Doing that several hundred times, the cell lines were established.
The researchers said that, when expanded up, the single cells were found to have stem cell characteristics, having markers of both hepatocytes and cholangiocytes.
When the researchers injected the lines into a mouse with a deficient immune system, the tumours then formed.
According to Rountree, there is interest in targeting these stem cells with malignant potential.
"Can we target these cells in patients with hepatitis B or C, either before or after their cancer forms? The broader implication is very powerful. If you look at a patient with chronic injury and find a way to specifically target cells with malignant potential, you may be able to prevent liver cancer in the first place," Rountree said.