By: Susan Kim
With a high regenerative ability and hundreds of metabolically complex functions, the liver is an extraordinary organ. A living donor can donate 40-60% of their healthy liver to a transplant recipient and expect their liver to be completely regenerated within eight weeks.
The liver is responsible for processing and distributing anything we eat or drink. It deserves a trophy for being one of the hardest working and resilient organs, but it can be injured to functional and morphological irreversibility if a harmful threat, such as heavy alcohol drinking or diet-induced obesity, is sustained. Perhaps due to the liver cell’s ability to regenerate quickly and efficiently, an individual can go often years or even decades without realizing they have chronic liver disease before symptoms appear.
Dr. Natasha Snider, Assistant Professor in the Department of Cell Biology and Physiology at UNC-Chapel Hill, is interested in studying the molecular mechanisms that stimulate liver injury and disease development, as well as those that contribute to stress resistance. Currently, there are no therapies for the liver diseases that Dr. Snider and her team studies, including alcoholic and nonalcoholic fatty liver disease. The only intervention for patients with permanent liver damage is a liver transplant.
Unfortunately, a liver transplant is not a viable option for patients with severe liver disease who are actively drinking. To be on the transplant list, you need to be sober for at least 6 months, but short-term 90-day mortality is very high in these patients.
Dr. Snider’s work is focused on hepatocytes, the major liver cell type that accounts for 80% of the liver mass. Injury to the hepatocytes is often the first event that triggers a cascade of immune responses leading to chronic disease. Hepatocyte-targeted therapies have the potential to intervene early in the disease process to halt disease progression or even reverse the damage.
There are predictive stages of liver disease progression. If an insult to hepatic injury is not removed, a patient with fatty liver disease and alcoholic fatty liver disease can develop fibrosis, which is an accumulation of scar tissue. Persistence of the injury can cause cirrhosis, or hardened tissue on the liver, where most of the liver function is lost at that point. If this is left untreated, the end stage liver disease is hepatocellular carcinoma, liver cancer, which has a low survival rate.
Hepatocytes, as well as other types of cells, express the cell surface protein ecto-5’-nucleotidase (CD73), which metabolizes extracellular adenosine monophosphate (AMP) to adenosine. Dr. Snider discovered that mice engineered to lack NT5E, the gene that encodes CD73, were protected against hepatocellular injury (Snider et al. Hepatology 2013). Based on the results, it’s easy to conclude that not having the enzyme at all is beneficial. However, CD73’s classical enzymatic activity drastically decreases soon after liver injury.
“If you’re killing the enzyme’s activity soon after you injure the liver, why is it bad to have this protein?” Dr. Snider mentioned as a question her lab is addressing. Under stress conditions, proteins can acquire new functions; in the case of enzymes these are referred to as “moonlighting enzymes.” “We are testing the hypothesis that CD73 is a receptor for some kind of signaling molecule that is released during injury,” said Dr. Snider. In search of this unknown endogenous ligand to make a distinction between CD73 protein and CD73 enzyme, the lab is doing in vitro screening on purified human CD73 and adding different molecules to see what binds.3 The molecules that do bind can be validated in mouse and human hepatocyte ex vivo models, and mouse in vivo models.
Dr. Snider also discovered a novel splice variant, NT5E-2, that encodes a protein called CD73S (S for “short”), which is a shorter and enzymatically-inactive isoform of the widely studied CD73. This non-catalytic isoform is notably up-regulated in human cirrhosis and liver cancer. The significance and function of this novel CD73 isoform is being studied in the Snider lab, as are the other potential “moonlighting” functions of CD73.
“We think what [CD73S] might be doing is making the hepatocytes more resistant to dying, and because it is expressed during later stages of liver disease (the so-called “regenerative” phase) where you are more likely to develop cancer, so we think that it may be helping the cancer develop and grow,” Dr. Snider said. One of the hallmark characteristics of cancer is resistance to cell death. There is already interest in drug discovery for CD73 because its involvement in cancers such as breast cancer. However, if an enzyme inhibitor were designed for CD73 it would not necessarily be targeting the CD73S isoform, which has no enzymatic activity.
An invaluable research component of Dr. Snider’s work are human patient samples. These are provided by her UNC collaborators Dr. Ramon Bataller and Dr. HJ Kim, who have strong interests and programs in understanding the molecular mechanisms of disease in the patients that they see. There are, of course, challenges in working with patient materials. With alcoholic patients, the biological specimen vary due to drinking patterns, genetics, any drugs present, and the part of the liver in which the sample originated.
However, research progresses despite inevitable hurdles. And while human specimens provide single snapshots of what things look like in a disease setting, when combined with powerful and dynamic cellular and animal models, they can help Dr. Snider and her team piece together the puzzle to understand how to make hepatocytes more robust to withstand injury so patients may be able to receive a better prognosis.