Normal Cells Sequester Telomerase But Cancer Cells Do Not

Researchers have found that while the telomerase enzyme complex that maintains the telomeres capping each chromosome is sequestered within the nucleolus during most of the cell cycle and released into the nucleus only just before cell division, telomerase is not sequestered in constantly reproducing tumor cells and continually interacts with the chromosomes. These findings were reported in the August 17, 2002, online edition of Nature Cell Biology.

During most of the cell cycle of cultured human fibroblasts, telomerase is attached to the outer region of the nucleolus, based on localization studies of the telomerase reverse transcriptase protein tagged with green fluorescent protein. However, just before cell division, when one would expect telomerase to be active at the telomeres, it suddenly appears throughout the nucleus.

Cancer cells, on the other hand, have evolved a mechanism to mobilize telomerase continually so they can maintain continual cell division while keeping the telomeres intact. The study's senior author, Dr. Kathleen Collins from the department of molecular and cell biology at the University of California, Berkeley (USA), explained, "Normal somatic cells keep most of the telomerase away from the DNA, whereas cancer cells let all of it go. This should help the transformed (cancer) cells use telomerase much more efficiently.”

A related finding was that cells exposed to ionizing radiation immediately sequester telomerase. This prevents the enzyme from adding telomere caps to exposed bits of DNA, which would hamper the cell's efforts to repair the damage.



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