Macrophages play paradoxical roles in cancer: They can be tumoricidal, but in many cancers, macrophages promote metastasis. There has been growing evidence that macrophages can modulate cell behavior via unconventional cell contact-mediated communication in development and homeostasis. We have recently extended these paradigms by discovering that macrophages can also engage in unconventional cell contact- mediated communication with tumor cells within the tumor microenvironment, and that these interactions contribute to metastasis. By visualizing and manipulating highly migratory melanoma cells and their microenvironment in vivo, we unexpectedly found that tumor-associated macrophages transfer cytoplasmic contents to melanoma cells in a cell contact-dependent manner. Remarkably, 70-80% of melanoma cells that received macrophage cytoplasm disseminated from the primary tumor in both zebrafish and murine models. We are now ideally positioned to identify key component(s) that are transferred from macrophages to tumor cells for metastasis, and how this transfer occurs. Mitochondria are dynamic organelles that perform a variety of essential cellular functions. Mitochondria have been shown to transfer to tumor cells in vivo, restoring their respiration and ability to form tumors. While these elegant ?proof of principle? studies demonstrated that mitochondrial lateral transfer can occur in the tumor microenvironment, the donor stromal cell(s) were not identified, the transfer mechanism was not defined, and the fates and functions of mitochondria in tumor cells were not characterized. Excitingly, we have found that primary human macrophages can transfer mitochondria to human breast cancer cells and melanoma cells, two cancers in which macrophages have been shown to play a pro-tumorigenic role. Tumor cells that receive macrophage mitochondria either by spontaneous mitochondrial transfer in coculture, or by direct injection of purified macrophage mitochondria, exhibit increased proliferation. Surprisingly, we find that after mitochondrial transfer occurs, the transferred mitochondria remain as a spatially distinct population from the host mitochondrial network. Furthermore, we found that high levels of local reactive oxygen species accumulate at transferred mitochondria, suggesting an intriguing hypothesis that transferred mitochondria may provide a signal to tumor cells, rather than providing excess mitochondrial function as has been previously described. To test this hypothesis, we propose to:
(Aim 1) Understand how mitochondria dynamically reorganize for mitochondrial transfer to tumor cells and (Aim 2) Determine how mitochondrial transfer mechanistically induces cancer cell proliferation. Taken together, these experiments will reveal whether macrophage mitochondrial transfer can instruct breast cancer and melanoma cells to become more robust and metastatic. Our goals are to define how immune cells function in the tumor microenvironment, and to provide a basis for developing future immunotherapies that limit metastasis.

Public Health Relevance

Macrophages, a component of our immune system, have a paradoxical role in cancer ? they can promote metastasis. We previously discovered that macrophages spill their internal contents into cancer cells during metastasis, but what is the functionally important information in the contents? We recently discovered that mitochondria are transferred, and we seek to answer how thieving even small amounts of mitochondria can drive big changes in cancer cell metastasis.

National Institute of Health (NIH)
National Cancer Institute (NCI)
Method to Extend Research in Time (MERIT) Award (R37)
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Tumor Microenvironment Study Section (TME)
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Willis, Kristine Amalee
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University of Utah
Schools of Medicine
Salt Lake City
United States
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