Cancer cell intravasation may occur earlier than thought, says TSRI researchers

Invasive tumors may send out tumor cells much earlier than previously believed, reported researchers from The Scripps Research Institute (TSRI) in La Jolla, CA, in searching for answers as to why some cancers can reoccur years after remission. These escaped cells can then seed secondary tumors and remain undetected for years, until recurrence is found. Researchers published these results in a recent issue of the journal Cell Reports.

In addition, their results demonstrated that these “escaped” tumor cells enter the blood stream via the vasculature deep within the dense tumor core, which is in sharp contrast to what researchers and clinicians currently thought—that metastatic cells originate at the tumor’s invasive borders.

“The actual process of cancer cell dissemination via hematogenous routes is a relatively under-studied process, but we finally have an answer as to where it takes place,” said Elena Deryugina, PhD, TSRI assistant professor, who led the study in a long-term collaboration with William Kiosses, PhD, TSRI staff scientist.

Traditionally, the spread of cancer cells has been thought to begin as the solid tumor enters stage 2. During stages 0 and 1, the tumor is thought to be relatively confined. Scientists hold that the tumor can initiate tumor cell ‘sending’ to distant organs only after extensive invasion into the adjacent stroma during stage 3. Stage 4 is characterized by the presence of secondary tumors, or metastases.

Drs. Daryugina and Kiosses studied this traditional view of how cancer cells spread in the body. They used cancer cells generated from human fibrosarcoma and carcinoma tumors, and observed that primary tumors were capable of sending out cells very early, and independently of cancer invasion into the adjacent tissue. These findings corroborate frequent findings of secondary tumors earlier than predicted, and may also explain why patients with early stage tumors are still at risk for developing metastatic disease.

“These metastases may have been seeded when the primary tumor was even too small to be visualized,” said Dr. Deryugina.

Drs. Deryugina and Kiosses along with fellow resesarchers developed animal models that allowed for microscopic analysis of tumor cell dissemination, which allowed them to ‘see through’ dense primary tumors, something that had not been possible before. They adapted mouse ear and chick embryo models to examine developing tumors through thin layers of tissue.

After tagging human tumor cells with a fluorescent protein, they used high-resolution confocal microscopy techniques that had been developed by Dr. Kiosses to construct 3-D maps of all the blood vessels contained in entire tumors, from the dense core out to the invasive outer borders.

They mapped the location of each tumor cell relative to the center of the closest blood vessel, and were able to analyze the escape process or intravasation, and demonstrate where these cells entered the blood vessels.

Most did so within the tumor core, rather than in the outer invasive borders. According to Dr. Deryugina, the newly formed vessels in the tumor core—which are permeable enough to let tumor cells pass—are perfect conduits for the escaping tumor cells. At the tumor border, blood vessel walls have had longer to mature, and therefore, tumor cells can no longer easily break through them and ‘hop a ride.’

Thus, the timeframe for the onset of cancer cell dissemination may need revision.

Finally, these researchers found that EGFR levels may predict which tumor cells would intravasate by regulation of the ability of the tumor to make blood vessels that allow for the cancer cells’ escape.

“Therefore, the data indicate the importance of harnessing the EGFR activity early on in cancer patients,” said Dr. Deryugina.

They next plan to assess the roles of the various cell types within primary tumors in supporting intratumoral cancer cell intravasation.

This study was supported by the National Institutes of Health (grants R01CA105412, R01CA129484, and R01CA157792).