Both reports appear in the Feb. 7 issue of Science.

The first study, conducted by researchers at Genentech Inc., unfolds almost as a love story between two types of cells: certain liver cells and the endothelial cells that line the blood vessels.

The liver has a unique ability to regenerate. "It's really a marvel," says Dr. Adrian Di Bisceglie, medical director of the American Liver Foundation. "You can take three-quarters of somebody's liver, and within two weeks it grows back to its original size."

Needless to say, scientists have studied this phenomenon with great interest and have, in fact, identified various growth factors that contribute to the process.

Now, the Genentech researchers have discovered an additional pathway that plays a part in liver growth.

After injecting toxin into laboratory mice to damage their livers, the researchers observed that the liver cells released a certain growth factor -- vascular endothelial growth factor A, or VEGF-A -- that sent commands to nearby endothelial cells. The endothelial cells then responded by releasing growth factors which, in turn, caused the liver cells to multiply, thus helping the liver to regenerate.

"It's an intimate communication between the cell populations," says Dr. Leonard I. Zon, author of an accompanying editorial and a professor of pediatrics at Children's Hospital and Harvard Medical School in Boston. "Here I am the injured hepatocyte [liver cell]. I start making growth hormones that stimulate the endothelial cells to make me better."

Scientists had known that liver cells could tell endothelial cells to multiply. This new relationship is a surprise, Zon says.

In a second experiment, the researchers neutralized the growth factor and then induced liver damage. They found that neutralizing the growth factor either before or after inducing damage greatly increased liver cell death. This strongly suggests that the growth factor has a role in protecting the liver.

"Our idea is that, by taking advantage of endogenous mechanisms, we can increase this mechanism to have a protective [or growth-stimulating] effect," says Dr. Napoleone Ferrara, senior author of the study and a senior investigator at Genentech in San Francisco. "It's just initial clinical data, but this could have beneficial effects in liver damage whether due to drugs, alcohol or perhaps viral hepatitis."

Sometime in the distant future, this could be used not only to prevent liver disease -- say, immediately after someone has been exposed to a toxin -- but to try to rejuvenate the liver after damage has already occurred, Zon says.

The second Science article, written by researchers in Germany, details the discovery of a new class of compounds that attack the hepatitis B virus in an entirely new way. Hepatitis B, which is transmitted via bodily fluids, is a leading cause of cirrhosis and liver cancer.

"It's a new class of compounds, but it's also acting in a novel way as well," Di Bisceglie says. "It interferes with a new part of the hepatitis B life cycle."

The discoveries are exciting but aren't likely to translate into tangible gains anytime soon.

"These are both very important discoveries of the mechanisms of how things happen and they have potential implications for treatment but, in both instances, there's a big gap between this and actual real treatment," Di Bisceglie notes.

More information

The American Liver Foundation has information on all aspects of liver health. The National Institute of Diabetes and Digestive and Kidney Diseases has pages on hepatitis B and liver cancer.