What controls the reproduction of cancer cells?

In normal tissues, the rate of cell division is offset by the rate of cell death. Unlike most normal cells, HeLa cells keep growing because they have a genetic imbalance that heavily favors cell reproduction over cell death. Henrietta Lacks was infected with the human papillomavirus, which stimulates cell division in cervical cells. In addition, an enzyme called telomerase, which keeps DNA intact and prevents cell death, is overexpressed in HeLa cells. This combination of traits—increased cell division plus decreased apoptosis—leads to the extraordinary growth rate of HeLa cells.

Because HeLa cells came from a tumor, you might expect that they are still cancer cells, and that they can form a tumor. But such is not the case. They are just cell division machines. What do you think they are missing that tumor cells have?

Future directions

Knowledge of which genes are abnormal in tumors has led to the development of drugs targeted to the specific proteins that are made from these genes. In some instances, such as breast cancers expressing HER2, the results in the clinic have been extraordinary. Patients are living longer, and even being cured by these targeted drugs. Less attention has been given to the other part of the cell population equation—cell death. Many of the widely used drugs that affect the cell cycle in general (see Figure 11.26) work not just by stopping the cell cycle but by damaging cells, triggering apoptosis. But as you may know, these drugs have widespread side effects, stopping cell division and triggering apoptosis in normal cell as well as tumor cells. We know about the molecular events in apoptosis. Perhaps targeted drugs that stimulate this process only in tumor cells could be effective.