Stromal-cell and cancer-cell exosomes leading the metastatic exodus for the promised niche

Exosomes are thought to play an important role in metastasis. Luga and colleagues have described the production of exosomes by stromal cells such as cancer-associated fibroblasts that are taken up by breast cancer cells and are then loaded with Wnt 11, which is associated with stimulation of the invasiveness and metastasis of the breast cancer cells. Previous studies have shown that exosomes produced by breast cancer cells are taken up by stromal fibroblasts and other stromal cells, suggesting that exosomes are agents of cross-talk between cancer and stromal cells to stimulate metastasis. Imaging of exosomes by labeling with fluorescent proteins will enlighten the process by which exosomes enhance metastasis, including premetastatic niche formation.

Do stromal cells in the tumor microenvironment also produce exosomes that stimulate cancer metastasis? We previously showed that cancer cell-associated stromal cells are necessary for metastasis to occur [7]. Luga and colleagues now report in a recent issue of Cell that stromal cells indeed produce exosomes and that fi broblast-secreted exosomes promote breast cancer cell (BCC) motility and metastasis by mobilizing autocrine Wnt11-induced planar cell polarity in the cancer cells [8]. Wnt11 is tethered to the fi broblast-derived exosomes within BCCs. Exosome stimulation of BCC invasion and metastasis was shown to be dependent on Wnt11 produced in the BCCs. Cancer-associated fi broblast (CAF)-derived exosomes were internalized by BCCs and then loaded with Wnt11 in orthotopic mouse models of breast cancer. Luga and colleagues report that exosomes secreted from human breast CAFs stimulate BCC protrusive activity, motility, and metastasis. Th ese proper ties are dependent on the exosome tetraspanin, Cd81, which is critical for exosome-stimu lated BCC metastasis [8]. Th ese are intriguing fi ndings on how stromal cells promote metastasis via exosomes.
Previous suggestions indicated that recruited bone marrow progenitor cells generated a premetastatic niche to which the cancer cells metastasize [9][10][11]. Secreted factors in the bone marrow may be important to mobilize cells to form the pre metastatic niche. Exosomes, derived from cancer cells, have recently been shown to have an important role in premetastatic niche formation [9][10][11]. However, Luga and colleagues do not suggest how stromal cell-derived exosomes play a role in the formation of a premetastatic niche [8].
Investigating premetastatic niche formation requires the possibility for dynamic imaging of exosomes in vivo.
To image the fate of cancer cell-derived exosomes in orthotopic nude mouse models of breast cancer, we used GFP-tagged CD63, which is a general marker of exosomes [12]. BCCs were imaged to transfer their own exosomes to other cancer cells and normal lung tissue cells in culture. In orthotopic nude mouse models, BCCs secreted exosomes into the tumor microenvironment. Tumor-derived exosomes were incorporated into tumorassociated cells at a metastatic site, including CAFs (Figure 1), and in the circulation. Th ese results suggest that tumor-derived exosomes may contribute to forming a niche to promote tumor growth and metastasis. Our results demonstrate the usefulness of GFP imaging to investigate the role of exosomes in cancer metastasis [12][13][14][15].
Both cancer cell-derived or stromal cell-derived exosomes are thus able to alter the tumor environment and may participate in forming a distant metastatic niche to promote metastasis. Dynamic imaging of exosomes derived from cancer or stromal cells in metastatic models may hence help us to understand the mechanism of Abstract Exosomes are thought to play an important role in metastasis. Luga and colleagues have described the production of exosomes by stromal cells such as cancer-associated fi broblasts that are taken up by breast cancer cells and are then loaded with Wnt 11, which is associated with stimulation of the invasiveness and metastasis of the breast cancer cells. Previous studies have shown that exosomes produced by breast cancer cells are taken up by stromal fi broblasts and other stromal cells, suggesting that exosomes are agents of cross-talk between cancer and stromal cells to stimulate metastasis. Imaging of exosomes by labeling with fl uorescent proteins will enlighten the process by which exosomes enhance metastasis, including premetastatic niche formation.

© 2010 BioMed Central Ltd
Stromal-cell and cancer-cell exosomes leading the metastatic exodus for the promised niche Robert M Hoff man* 1,2

V I E W P O I N T
*Correspondence: Email: all@anticancer.com 1 AntiCancer, Inc., 7917 Ostrow Street, San Diego, CA 92111, USA Full list of author information is available at the end of the article cancer metastasis. Imaging of exosomes may also be useful to predict the location of future metastasis in real time.
Th is viewpoint demonstrates the importance of exosome cross-talk between cancer cells and stromal cells. Luga and colleagues demonstrate the production of exosomes by stromal cells such as CAFs that are taken up by BCCs, which in turn promote their invasive and metastatic activity [8]. Suetsugu and colleagues demonstrate production of exosomes by BCCs that are taken up by CAFs [10], the reciprocal of what was observed by Luga and colleagues [8]. With the use of fl uorescentprotein in vivo imaging [13][14][15], further understanding of exosomes and their relationship to metastasis, including niche formation, will surely be enlightened.