Breast Cancer Cell Re-Dissemination from Lung Metastases-A Mechanism for Enhancing Metastatic Burden

Abstract

Although metastatic disease is the primary cause of mortality in cancer patients, the mechanisms leading to overwhelming metastatic burden are still incompletely understood. Metastases are the endpoint of a series of multi-step events involving cancer cell intravasation, dissemination to distant organs, and outgrowth to metastatic colonies. Here we show, for the first-time, that breast cancer cells do not solely disseminate to distant organs from primary tumors and metastatic nodules in the lymph nodes, but also do so from lung metastases. Thus, our findings indicate that metastatic dissemination could continue even after the removal of the primary tumor. Provided that the re-disseminated cancer cells initiate growth upon arrival to distant sites, cancer cell re-dissemination from metastatic foci could be one of the crucial mechanisms leading to overt metastases and patient demise. Therefore, the development of new therapeutic strategies to block cancer cell re-dissemination would be crucial to improving survival of patients with metastatic disease.

Keywords: TMEM doorways; dissemination; intravasation; metastasis.

Figure 1

Cancer cells disseminate from lung metastases to tertiary sites. (A) Experimental design. E0771 Dendra2-expressing cancer cells are injected into the tail veins of C57BL mice to generate lung metastases. Approximately three weeks later, lung metastases are exposed to UV light for photoconversion. Three days after photoconversion, blood, brain, and bone marrow were collected and analyzed for the presence of red-fluorescent cancer cells. (B–D) Representative images of cancer cells detected in the blood, brain, and bone marrow of mice. Top panels are images of disseminated cancer cells (DCCs) with non-photoconverted Dendra2 (note green-fluorescence) from control mice (no photoconversion), whereas the bottom panels show images of DCCs from mice with photoconverted lungs (note green- and red-fluorescence). Scale bar 10 µm.

Figure 2

The roads of metastasis. (A) Cartoon representing multi-directional pathways of metastasis formation. Pathway A. Cancer cells can leave the primary tumor, circulate, and eventually return to its origin, i.e., primary tumor [10]. Pathway B. Cancer cells can leave the primary tumor and extravasate at the secondary site (e.g., lungs) where they form metastatic foci [4,5]. Pathway C. From a metastatic focus, cancer cells can also re-intravasate and re-seed back to the primary tumor (if still in place), or travel and seed tertiary metastatic sites, Pathway D [12,13,14,39], enhancing metastatic burden. (B) Schematic representation showing that cancer cell re-dissemination from metastatic foci drastically enhances metastatic burden, which may contribute to cancer patient mortality. Primary tumor that, according to the classic concept of metastasis, produce only three metastatic foci (light blue circles), could, after only two rounds or re-dissemination originating from these three metastatic foci, result in 27 new metastatic foci (yellow circles), provided that the dissemination rate is constant. Thus, cancer cell re-dissemination from metastatic foci may be a new mechanism of enhancing metastatic burden.