Out-of-Control Enzyme May Be Reason that Some Cells Become Cancer

"Colorectal [cancer] is the second leading cause of cancer mortality, and the molecular pathways [by which it develops] remain incompletely understood,” said Dr. Zigang Dong, director of the University of Minnesota Hormel Institute (Austin, MN, USA). "In this study, we provided evidence showing that TOPK promotes transformation [of normal cells to cancerous ones] in colorectal carcinoma.”

The researchers first frequently observed that members of this enzyme network are overactive in the cells of several human cancers. The function of all these enzymes is to activate other enzymes and proteins, which makes them suitable for passing along signals.

For example, a cancer-causing substance or a hormone may find its way from the bloodstream to the outer membrane of a cell. After its arrival, it initiates a chain of reactions, or signals, inside the cell. Some of these signals take the form of certain enzymes activating others. Eventually, the "news” reaches the genetic material inside the cell nucleus, where changes resulting in uncontrolled growth--cancer--or some other cell behavior are made.

It was thought that some form of an enzyme called MEK, which belongs to the family of signaling enzymes, was the culprit. But to cause cancer, an enzyme or other agent is expected to be active all the time, similar to a light with no off switch. MEK, however, is never active all the time in nature; only if kept artificially active can it lead to cancer. Therefore, researchers theorized that a related enzyme was activating others in the network and keeping the signaling process going.

Dr. Dong and his colleagues evaluated the role of TOPK. This enzyme is not very active in normal tissues, but it is quite active in cancerous cells in blood, breast, prostate and colorectal tumors, among others. The onset of cancer could trigger the permanent turning-on of TOPK. One piece of evidence for this is that whereas TOPK is similar to MEK in structure, TOPK has characteristics that suggest it is easier to keep turned on all the time. The team performed several experiments to determine whether high TOPK activity could lead to cancer.

When the researchers modified mouse skin cells to generate excessive amounts of the enzyme and then injected the cells into other mice, those mice developed tumors. The control mice had none. After they had "silenced” the gene for TOPK in human colorectal cancer cells, growth of the cells was significantly slowed compared to control colorectal cancer cells. The researchers also discovered that TOPK and a related enzyme (not MEK) activated each other in growing human colorectal cancer cells.

Taken together, these and other findings suggested that TOPK is, in effect, the major cancer-causing member of the network for which researchers have been looking for, according to Dr. Dong. Moreover, the fact that TOPK appears active principally in cancer cells--not healthy ones--means it could make a good target for chemotherapy.

How TOPK gets turned on is not known, but it appears to happen as a result of abnormal activity in cells that stems from genetic mutations in specific well-known genes. Such genes, called oncogenes, typically do not cause any trouble until mutations occur in them.

The study was published in the July 2007 issue of the journal Gastroenterology.


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