By Dean Haycock
Special To BioWorld Today
To become cancerous, a cell must, like a fugitive, flee from authorities. It must run roadblocks established to prevent its escape. In cancer biology, ¿escape¿ equals uncontrolled proliferation. Cell biologists call the biochemical roadblocks that regulate cell division ¿checkpoints.¿ Obviously, cancer cells have ways of skirting checkpoints.
Colon cancer cells are known, for example, to be able to get around a checkpoint called the retinoblastoma (Rb) tumor suppressor, even though Rb is known to be intact in these malignant cells. How do they avoid the Rb checkpoint? The answer appears to involve a tumor-suppressor gene known as adenomatous polyposis coli (APC). Most human colon cancers have a mutation in the APC gene. This mutation results in a loss of functional APC protein.
Loss of functional APC leads to overproduction of a protein called beta-catenin. Even in colon and other cancers with normal APC genes, mutant forms beta-catenin have been detected. So, scientists have long suspected beta-catenin of being a driving factor in colon cancer, and perhaps others. But they did not know how it might aid and abet cancer cells fleeing from control checkpoints.
Colon Cancer Cells Produce Extra Cyclin D1
With the publication of the paper ¿Beta-catenin regulates expression of cyclin D1 in colon cancer cells,¿ in the April 1 issue of Nature, the story becomes clearer. The paper shows how beta-catenin enables colon cancer cells to get around the Rb molecular checkpoint. Osamu Tetsu, postdoctoral fellow, and Frank McCormick, professor in the Cancer Research Institute of the University of California School of Medicine, in San Francisco, found that high levels of beta-catenin lead to the synthesis of another protein, cyclin D1. Cyclin D1 is a well-studied regulator of cell division. It also previously was known to be overexpressed in many cases of colon cancer.
Cyclin D1 prompts cells to enter what biologists call the ¿S-phase¿ of the cell cycle, in which DNA is duplicated in preparation for cell division. So, colon cancer cells appear to sidestep the Rb checkpoint by producing excess cyclin D1, which pushes them toward proliferation. When checkpoints such as Rb are negated, the defining feature of cancer ¿ uncontrolled cell growth and proliferation ¿ is realized.
¿We need to put this new discovery into the whole context of cancer, first of all in mice and then hopefully in humans,¿ McCormick told BioWorld Today. ¿Showing this connection in cell lines and culture is the first step. The next step is going to involve complex biological systems. But it is a new and, I think, unexpected angle, so we are very excited about it.¿
Tetsu and McCormick began their project by searching the GenBank database for specific human genes involved in controlling cell growth. They specifically looked for genes activated by beta-catenin. They knew that beta-catenin interacts with transcription factors of a type known as TCF/LEF. Genes controlling cell growth that bound members of this type of transcription factor, therefore, received highest priority in the search. The pair identified four candidate target genes. Cyclin D1 was one of them. It became the focus of their experiments because it was known to be overexpressed in colon cancer. The experiments that indicated the influence of beta-catenin on cyclin D1 were conducted in colon cancer cell lines.
By highlighting one of the consequences of the APC mutation, the work may be expected to stimulate efforts to develop drugs that interfere with beta-catenin¿s influence on cyclin D1 expression. This approach also may have potential for developing treatments for hepatocellular carcinoma, gastric cancer, prostate cancer and melanoma ¿ all of which have been reported to involve beta-catenin in some way.
Ras Pathway¿s Contribution Explored
¿We know that cyclin D1 is the protein partner of an enzyme called cdk4 [a cyclin-dependent kinase, an enzyme that phosphorylates substrates],¿ McCormick said. ¿We know that many pharmaceutical companies have been targeting cdk4 as a potential anticancer drug. In other cancers, cyclin D1 is up-regulated by different mechanisms, or cdk4 is hyperactive through different mechanisms. If our discovery turns out to hold water in the context of real tumors in animals or in humans, it would suggest that basically all colon cancers would be good targets for this drug strategy.¿
¿We are trying to assess the total biological significance [of the finding] in the context of colon cancer, because that is the kind of cancer in which we know for sure that high levels of beta-catenin have a major causal role in the development of the cancer,¿ he added.
McCormick noted that cyclin D1 responds to several signals. ¿Another signal which is supposed to turn on cyclin D1 comes from the Ras pathway, which is activated in about 50 percent of human cancers,¿ he said. ¿We are trying to figure out what the relative contribution of Ras compared with beta-catenin would be in a human colon-cancer cell line.¿
The researchers are also now attempting to activate the beta-catenin pathway in a mouse model of colon cancer, to see if they can suppress colon cancer by knocking out cyclin D1. ¿There are mice available which lack cyclin D1 completely, and other mice which have hyperactive beta-catenin,¿ McCormick said. ¿We are going to cross them, and see if loss of cyclin D1 suppresses colon cancer."