BioWorld International Correspondent
LONDON - The Epstein-Barr virus (EBV), notorious for its role in several human cancers, stimulates cells to multiply out of control by booting key molecules out of the nucleus that normally act as brakes on cell division.
The discovery will trigger a search for new types of anticancer drugs. If researchers can show that a similar mechanism occurs in other malignant tumors, too, it is possible that such drugs could be widely applicable as cancer treatments, and not limited just to those cancers caused by EBV.
Eiji Hara, group leader of the Cell Cycle Group at the Paterson Institute for Cancer Research in Manchester, UK, told BioWorld International: "We have found that, in EBV-infected cells, the normal way in which the cell suppresses tumor growth is inactivated by a specific mechanism in which molecules that usually suppress cell replication are exported from the nucleus. If we can find a drug that can specifically block this export, then in theory we could develop new treatments for types of cancer in which this mechanism is important."
Hara, whose work is funded by Cancer Research UK, the largest cancer research charity in the UK, and his colleagues are keen to collaborate with groups that have expertise in drug screening with the aim of finding such compounds.
A paper describing the group's work is published in the July 21, 2003, Journal of Cell Biology. Its title is "Epstein-Barr virus LMP1 blocks p16INK4a/RB-pathway by promoting nuclear export of E2F4/5."
About 90 percent of the population is infected with EBV. Normally, the immune system keeps the virus under control, but in people whose immune systems are suppressed, such as those receiving immunosuppressive drugs following organ transplants or those infected with HIV, it can cause cancers such as Hodgkin's lymphoma and Burkitt's lymphoma. It also is associated with nasopharyngeal carcinoma and gastric cancer.
Hara, together with colleagues in Wales, Germany, Australia, France and Japan, set out to determine how EBV subverts the cell's normal control mechanisms and turns on uncontrolled cell replication.
Research by others had already shown that EBV encodes several genes that act as oncogenes in human cells, although how those genes transform cells remained poorly understood. One of these genes encodes a protein called LMP1, which has been shown to damp down expression of a human tumor suppressor gene called p16. The protein encoded by p16 normally acts as a brake to stop uncontrolled cell growth.
"We knew that LMP1 inactivates the p16 brake, but no one knew exactly how," Hara said.
Two years ago, the group published a paper in Nature showing that the expression of p16 is regulated by the transcription factor known as Ets2. As they report now, their latest experiments showed that when LMP1 is expressed in a human fibroblast cell, Ets2 is inactivated.
Transcription factors bind to the piece of DNA that regulates the expression of its associated gene. Their place of action is therefore in the nucleus. Hara's group went on to show that, if LMP1 is expressed in a cell, Ets2 disappeared from the nucleus and could be found only in the cytoplasm.
The next part of their study examined the control of p16 expression in more detail. Progress through the cell cycle is regulated by a series of cyclin-dependent kinases (CDKs), which phosphorylate and inactivate proteins of the retinoblastoma (Rb) family. P16 binds to and inactivates these CDKs, and consequently activates the Rb family of proteins. Although the level of p16 expression is very low in normal, healthy, growing cells, p16 expression rises significantly if the cell expresses oncogenic proteins, for example. In that way, p16 appears to act as a safeguard against neoplasia.
Hara and his colleagues knew from previous work by others that, for the p16 brake to work, proteins of the Rb family first needed to bind to two further transcription factors, called E2F4 and E2F5.
"We found from our experiments that if LMP1 is expressed in the cell, it not only inactivates Ets2, but it also inactivates E2F4 and E2F5. This means that when LMP1 is expressed, both upstream and downstream regulators of p16 are inactivated," Hara said. "This seems to be why LMP1 has a very strong effect."
As with Ets2, the group showed that E2F4 and E2F5 were similarly inactivated by being forced out of the nucleus.
"As far as we know," Hara said, "LMP1 does not appear to induce the nuclear export of other proteins. So if we could understand how LMP1 induces its specific nuclear export of Ets2, E2F4 and E2F5, then we might be able to identify a specific drug that would block this process."
Normal human cells also have a protein analogous in function to LMP1, and Hara speculated that protein may cause cancer if it becomes activated by mutation or by an unknown mechanism. "We now want to find out if the gene encoding this protein is aberrantly activated in human cancers not linked to EBV infection," he said.