Human papillomaviruses are considered to be the cause of approximately five percent of all cancers worldwide. They primarily cause cervical cancer but are also responsible for many cancers of the head and neck, the reproductive tract and the anus.
A couple of years ago, researchers were able to uncover the mechanism that the viruses use to turn cells cancerous. Two viral proteins, E6 and E7, disable two crucial cancer brakes in infected cells, thus being responsible for cancer developing.
“E6 and E7 drive cancer growth by preventing senescence, a cell aging process that is associated with irreversible cessation of cell growth,” says Felix Hoppe-Seyler of the German Cancer Research (Deutsches Krebsforschungszentrum, DKFZ) in Heidelberg. When E6 and E7 are blocked, the cancer cells stop growing. “However, our knowledge about the functions of E6 and E7 is mostly based on results from cell culture experiments, where oxygen saturation is high,” Hoppe-Seyler explains. “But many tumors have regions where oxygen is deficient because of inadequate supply by blood vessels. So we wanted to know what happens in a state of hypoxia.”
When the scientists lowered the oxygen concentration in the Petri dish to levels commonly found in oxygen-deficient tumor tissue, the cancer cells reduced the production of E6 and E7 and stopped growing. However, they did not induce senescence, but instead entered a dormant state. When oxygen supply was increased again, the dormant cancer cells awakened and promptly resumed cell division.
The sleeper cells that can form in tumor regions with low oxygen levels are more resistant to chemotherapy, which preferably targets dividing cells. Additionally, they escape immune defense because they no longer produce HPV proteins that could be recognized by immune cells.
Until now, E6 and E7 have been regarded as ideal molecular targets for targeted treatment of HPV-induced tumors, which currently is a field of intensive research. However, even these targeted drugs would be ineffective against the dormant cells because they lack the crucial targets.
Hoppe-Seyler said: “For patients with HPV-induced tumors, the sleeper cells are a latent danger. If a tumor shrinks, for example in the wake of successful treatment, and if surviving sleeper cells get reconnected to blood vessels and oxygen supply, they might cause the disease to relapse. Assessing the relevance of the findings, he added: “In our efforts to develop novel therapies, we cannot solely focus on E6 and E7 as targets. We also need to develop strategies for eliminating the sleeper cells.”